Abstract:
The Fiber Channel standard was created by the American National Standard for Information Systems (ANSI) X3T11 task group to define a serial I/O channel for interconnecting a number of heterogeneous peripheral devices to computer systems as well as interconnecting the computer systems themselves through optical fiber and copper media at gigabit speeds (i.e., one billion bits per second). Multiple protocols such as SCSI (Small Computer Serial Interface), IP (Internet Protocol), HIPPI, ATM (Asynchronous Transfer Mode) among others can concurrently utilize the same media when mapped over Fiber Channel. A Fiber Channel Fabric is an entity which transmits Fiber Channel frames between connected Node Ports. The Fiber Channel fabric routes the frames based on the destination address as well as other information embedded in the Fiber Channel frame header. Node Ports are attached to the Fiber Channel Fabric through links.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/147,113, filed Jun. 6, 2005 U.S. Pat. No. 7,801,118; which is a continuation of U.S. application Ser. No. 09/330,398, filed Jun. 11, 1999 issued on Jun. 7, 2005 as U.S. Pat. No. 6,904,053, which is a divisional of U.S. application Ser. No. 08/801,471, filed Feb. 18, 1997 now issued as U.S. Pat. No. 6,185,203, and are incorporated herein by reference as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to input/output channel and networking systems, and more particularly to a digital switch which switches Fibre Channel frames at link speeds of up to at least one gigabit per second (i.e., one billion bits per second). 
     BACKGROUND OF THE INVENTION 
     There is a never ending demand for increased computer system performance. A common limiting factor in computer system performance is the path from the main central processing unit (CPU) to storage, or the I/O path. The CPU usually requires data from attached storage many times faster than the I/O path. Fibre Channel is a standard which addresses this I/O bandwidth limitation. 
     Fibre Channel is an American National Standards Institute (ANSI) set of standards which describes a high performance serial transmission protocol which supports higher level storage and networking protocols such as HIPPI, IPI, SCSI, IP, ATM, FDDI and others. Fibre Channel was created to merge the advantages of channel technology with network technology to create a new I/O interface which meets the requirements of both channel and network users. Channel technology is usually implemented by I/O systems in a closed, structured and predictable environment where network technology usually refers to an open, unstructured and unpredictable environment. 
     Advantages of Fibre Channel include the following. First, it achieves high performance, which is a critical in opening the bandwidth limitations of current computer to storage and computer to computer interfaces at speeds up to 1 gigabit per second or faster. Second, utilizing fiber optic technology, Fibre Channel can overcome traditional I/O channel distance limitations and interconnect devices over distances of 6 miles at gigabit speeds. Third, it is high level protocol independent, enabling Fibre Channel to transport a wide variety of protocols over the same media. Fourth, Fibre Channel uses fiber optic technology which has very low noise properties. Finally, cabling is simple in that Fibre Channel typically replaces bulky copper cables with small lightweight fiber optic cables. 
     Fibre Channel supports three different topologies, point-to-point, arbitrated loop and fabric attached. The point-to-point topology attaches two devices directly. The arbitrated loop topology attaches devices in a loop. The fabric attached topology attaches a device directly to a fabric. 
     A Fibre Channel fabric is an entity which switches frames between connected devices. Fabric is a word which is synonymous with switch or router. The fabric must route the frame to the appropriate destination port or return a busy if the port is not available. 
     Because of the high link speeds, Fibre Channel fabrics face unique problems that are not present in current network switch design. Current network switches which support Ethernet, Fast Ethernet or Asynchronous Transfer Mode (ATM) protocols route incoming data at speeds up to ten to one hundred times slower than Fibre Channel fabrics. Current network switches also perform some incoming frame validation and network statistics collection. All these network switch features are more difficult to implement when the incoming frame rate is high, as in the case of Fibre Channel. 
     Route determination in network switches is usually performed by microprocessors. The requirement to route frames which are entering the fabric at speeds of up to one gigabit per second requires the fabric to route the frame in very little time. Routing depends not only on the incoming frame address but a host of other parameters and current state conditions as well. There are no currently available microprocessors which can in real time route sixteen lines of incoming frames with a link speed of 1 gigabit per second. 
     Frame validation creates another set of problems. In Fibre Channel fabrics frame validation must be performed at rates up to one hundred times faster than in Ethernet switches. 
     Statistics collection is also another function which must be performed in real time. Statistics collected are defined by the Fibre Channel fabric Management Information Base (MIB) and include the number of frames transmitted and received, the number of fabric rejects and fabric busies transmitted and received, etc. Gathering statistics for sixteen one gigabit per second ports creates new challenges. 
     Current fabric realizations use either fast microprocessors or digital signal processors to perform the route determination functions. Typically, processors are single instruction devices which serially decode the instructions and perform the specified function. Digital signal processors contain parallel functions and can perform several functions at one time. Still the problem exists to determine the route for many simultaneous incoming frames at one gigabit per second. Current fabric implementations perform routing on the order of tens of microseconds to hundreds of milliseconds. Ideally, routing should be accomplished in less than one microsecond. 
     Another problem with fabric realization is the support of the Arbitrated Loop topology. This topology has unique characteristics and requirements. Current fabric implementations do not support this topology. 
     Efficient support of both connection based classes of service (i.e., Class 1) and connectionless classes of service (i.e., Class 2 and 3) is also a challenge. A fabric must implement a different type of switch core to implement each class of service. Coordination between the different switch cores can be a burdensome task. Current fabric implementations support either a connection based or a connectionless switch core. This leads to inefficiencies, e.g., a connectionless switch core cannot switch Class 1 traffic if routes are not determined in frame time (i.e., less than one microsecond) and a connection switch core is very inefficient when routing Class 2 and Class 3 traffic. 
     Another problem with fabric realization is the interconnection or networking of fabrics. This is a problem due to the high speeds involved. Determining a network route is sometimes even more difficult than determining a local route. Destination addresses must be matched based not only on all bits matching but also matching a portion of the address. Route priorities should also be implemented to allow backup routes to a destination. 
     SUMMARY OF THE INVENTION 
     The present invention described and disclosed herein comprises a method and apparatus for transporting Fibre Channel frames between attached devices. The apparatus comprises logic which supports but is not limited to the following features: Transport of Class 1, Class 2 and Class 3 frames, Support for the Arbitrated Loop topology on each link, Support for Fabric point-to-point topology on each link, Route determination in frame arrival time, and Interconnection or Networking of Fabrics. 
     In one aspect of the invention, the apparatus comprises separate port control modules, one for each attached device, a central router module, a switch core module, a fabric control module and a brouter (bridge/router) module. In the preferred embodiment, the port control modules are connected to the router modules by separate route request connections and separate route response connections. Through this structure, route requests may be provided from the port control module to the router while simultaneously the router provides route request responses to the same port control module. Preferably, a common route request channel is utilized. Thus, apparatus is provided to return a route response to a previously requesting port while other ports are arbitrating and sending route requests to the centralized router. More generally, this apparatus provides for reading resource requests from multiple requesters while at the same time returning resource grant responses to previous requesters. 
     The router of the subject invention includes many advantageous aspects. In the preferred embodiment, the router includes multiple state machines arranged in series for pipeline operation. Specifically, in the preferred embodiment of the router, a hardware finite state machine operates on the route request and a hardware finite state machine provides the route response. Thus, in this embodiment, the router includes an input for receiving the output of the route request generator of the port control module, an output for sending a route request response to the route request response receiver in the port control module, a hardware finite state machine to receive the route request, and a hardware finite state machine to provide the route response, in combination with a route determination system. Through implementation in hardware, route responses may be made in less than two microseconds, which permits essentially real-time routing at gigahertz frequencies. 
     In yet another aspect of the router, it routes Fibre Channel frames to a destination port on the Fabric based on a selected portion of the incoming frame&#39;s destination address. In the preferred embodiment, Fibre Channel FCPH protocol rules are applied to an incoming frame to determine whether to route the frame or return a fabric reject or busy frames or to discard the frame. Validation of the routing of a Fibre Channel frame is based on the rules defined in the ANSI FCPH standards. In the preferred embodiment, route requests are serviced in a round robin manner from multiple ports. 
     In another embodiment an apparatus and method is provided to store blocked route requests until either the blocking condition resolves itself or a specified time period expires. Thus, a method for servicing route request from multiple attached devices where the routing is subject to blocked and unblocked conditions may be effective, where the method comprises the steps of servicing a route request which is not blocked, but saving a blocked route request in hardware, preferably in registers, and then servicing that request if the route changes from a blocked to an unblocked condition, in the preferred embodiment, prior to the expiration of a specified time period. In a more general sense, the invention manages the blocking and unblocking of multiple resource requests to a central resource. 
     In another embodiment an apparatus is provided to handle the scenario when a port input fifo is going to overflow with an incoming Fibre Channel frame. Generally, the incoming data stream is typically provided to an encoder/decoder, from which it is supplied to a buffer. In the event of a data overrun condition to the buffer, overrun prevention logic causes the setting of tag bits to a condition which may be recognized downstream as indicative of a buffer overflow condition. 
     In another embodiment an apparatus is provided to interleave accesses by the processor on the outgoing port bus in between outgoing frames or when the output fifo is full. 
     In another embodiment an apparatus is provided to pack requests in a register array in order of first arrival but allow the removal of the requests from anywhere in the array. 
     OBJECTS OF THE INVENTION 
     It is an object of this invention to provide a fibre channel fabric capable of operating at at least 1 gigabit speeds. 
     It is yet a further object of this invention to permit the establishment of a path through a fabric in real time at gigabit speeds. 
     It is yet a further object of this invention to provide 1 microsecond or less response time to fibre channel frames. 
     It is another object of this invention to determine in real time at gigabit speeds that no through path can be established through the fabric. 
     It is yet another object of this invention to provide a fibre channel fabric capable of simultaneously supporting Class 1, Class 2 and Class 3 service. 
     It is an object of this invention to provide a fibre channel switching fabric which supports arbitrated loop topology. 
     It is yet another object of this invention to provide systems and methods adapted for interconnection of multiple fabrics. 
     It is yet another object of this invention to provide a system which supports Fabric point-to-point topology on each link. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the use of a Fibre Channel Fabric. 
         FIG. 2  is a block diagram of a Fibre Channel Fabric. 
         FIG. 3  is a block diagram of the Fabric Control module. 
         FIG. 4  is a block diagram of the fabric Router. 
         FIG. 5  is a block diagram of the fabric Port Control. 
         FIG. 6  is a block diagram of the fabric Switch core 
         FIG. 7  is a block diagram of the Brouter Module. 
         FIG. 8  is a diagram of the Port Control FIFO Overrun Prevention Logic. 
         FIG. 9  is a diagram of the Port Control Process to Endec Arbitration Logic. 
         FIG. 10  is a more detailed description of the Port Control module. 
         FIG. 11  is a diagram of the main Port Control FSM. 
         FIG. 12  is a diagram of the Port Control PCFIFO module interface signals. 
         FIG. 13  is a diagram of the Router address matching module. 
         FIG. 14  is a diagram of the Router Route Request Unblock Determination module. 
         FIG. 15  is a detailed diagram of the Route Request Unblock Determination module circuit. 
         FIG. 16  is another detailed diagram of the Route Request Unblock Determination module circuit. 
         FIG. 17  is a diagram of the Blocked Route Request Table. 
         FIG. 18  is a diagram of the Router Control State Machine. 
         FIG. 19  is a diagram of the Blocked Route Request Port Register Array. 
         FIG. 20  is a diagram of both the Route State Table and the Route Determination modules. 
         FIG. 21  is a more detailed diagram of the Route Determination module. 
         FIG. 22  is a another more detailed diagram of the Route Determination module. 
         FIG. 23  is a still another more detailed diagram of the Route Determination module. 
         FIG. 24  is a diagram of the Port Control Route Request Interface module. 
         FIG. 25  is a diagram of the Port Control Route Response Interface module. 
         FIG. 26  is a diagram of the Router to Port Control Route Request State Machine. 
         FIG. 27  is a diagram of the Router to Port Control Route Response State Machine. 
         FIG. 28  is a diagram of the Port Control to Router Interface State Machine. 
         FIG. 29  is a diagram of the Hub Port Control module. 
         FIG. 30  is a diagram of the format of the Blocked Route Request Table entry. 
         FIG. 31  is a diagram of the format of the Route Request. 
         FIG. 32  is a diagram of the format of the Router to Port Control Response. 
         FIG. 33  is a diagram of the format of the Address Table entry. 
         FIG. 34  is a diagram of the format of the Route State Table entry. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     
       
         
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Table of Contents 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A. 
                 Definitions 
               
               
                   
                 B. 
                 Fibre Channel Fabric Model 
               
               
                   
                 C. 
                 Fabric Control Module 
               
               
                   
                 D. 
                 Fabric Router 
               
             
          
           
               
                   
                 1. 
                 Port Control Route Request Interface Module 
               
               
                   
                 2. 
                 Port Control Route Response Interface Module 
               
               
                   
                 3. 
                 Address Table 
               
               
                   
                 4. 
                 Address Match Module 
               
               
                   
                 5. 
                 Blocked Route Request Table 
               
               
                   
                 6. 
                 Blocked Route Request Port Register Array 
               
               
                   
                 7. 
                 Blocked Route Request Timer 
               
               
                   
                 8. 
                 Route Request Unblock Determination Module 
               
               
                   
                 9. 
                 Route Request Selector 
               
               
                   
                 10. 
                 Route Determination Module 
               
               
                   
                 11. 
                 Route State Table 
               
               
                   
                 12. 
                 Router Statistics Gathering Module 
               
               
                   
                 13. 
                 Router Control FSM 
               
             
          
           
               
                   
                 E. 
                 Port Control 
               
             
          
           
               
                   
                 1. 
                 Port Control Module 
               
               
                   
                 2. 
                 FIFO Overrun Prevention Logic 
               
               
                   
                 3. 
                 Processor/Data Arbitration Logic 
               
               
                   
                 4. 
                 Port Control Hub Module 
               
             
          
           
               
                   
                 F. 
                 Switch Core 
               
               
                   
                 G. 
                 Router Module 
               
               
                   
                 H. 
                 Other Documents 
               
               
                   
                   
               
             
          
         
       
     
     A. DEFINITIONS 
     For expository convenience, the present invention is referred to as the Fibre Channel Fabric or Fabric, the lexicon being devoid of a succinct descriptive name for a system of the type hereinafter described. 
     The “Fibre Channel ANSI standard” describes the physical interface, transmission protocol and signaling protocol of a high-performance serial link for support of the higher level protocols associated with HIPPI, IPI, SCSI, IP, ATM and others. 
     The “Fibre Channel Fabric” comprises hardware and software that switches Fibre Channel frames between attached devices at speeds up to one gigabit per second. 
     The following discussions will be made clearer by a brief review of the relevant terminology as it is typically (but not exclusively) used. 
     “Fibre Channel” is an American National Standard for Information Systems (ANSI) standard which defines a high performance serial link for support of the higher level protocols associated with HIPPI, IPI, SCSI, IP, ATM, FDDI and others. 
     “FC-1” defines the Fibre Channel transmission protocol which includes the serial encoding, decoding, and error control. 
     “FC-2” defines the signaling protocol which includes the frame structure and byte sequences. 
     “FC-3” defines a set of services which are common across multiple ports of a node. 
     “FC-4” is the highest level in the Fibre Channel standards set. It defines the mapping between the lower levels of the Fibre Channel and the IPI and SCSI command sets, the HIPPI data framing, IP, and other Upper Level Protocols (ULPs). 
     “Fibre” is a general term used to cover all transmission media specified in the ANSI X3.230 “Fibre Channel Physical and Signaling Interface (FC-PH)” standard. 
     A “fabric” is an entity which interconnects various N_Ports attached to it and is capable of routing frames by using only the D_ID information in the FC-2 frame header. The word Fabric can be seen as a synonym with the word switch or router. 
     “Fabric topology” is a topology that uses the Destination Identifier (D_ID) embedded in the Frame Header to route the frame through a Fabric to the desired destination N_Port. 
     “Point-to-point topology” allows communication between N_Ports without the use of a Fabric. 
     A “circuit” is a bidirectional path that allows communication between two L_Ports. 
     “Arbitrated Loop topology” permits three or more L_Ports to using arbitration to establish a point-to-point circuit. When two L_Ports are communicating, the arbitrated loop topology supports simultaneous, symmetrical bidirectional flow. 
     “Link Control Facility” is a facility which attaches to an end of a link and manages transmission and reception of data. It is contained within each Port type. 
     “Port” is a generic reference to an N_Port or F_Port. 
     An “N_Port” is a hardware entity which includes a Link Control Facility. An “NL_Port” is an N_Port that contains Arbitrated Loop functions associated with Arbitrated Loop topology. 
     An “F_Port” is a generic reference to an F_Port or FL_Port. 
     An “FL_Port” is an F_Port that contains Arbitrated Loop functions associated with Arbitrated Loop topology. 
     An “L_Port” is an N_Port or F_Port that contains Arbitrated Loop functions associated with Arbitrated Loop topology. 
     A “Node” is a collection of one or more N_Ports controlled by a level above FC-2. 
     A “dedicated connection” is a communicating circuit guaranteed and retained by the Fabric for two given N_Ports. 
     A “connection” is the process of creating a Dedicated Connection between two N_Ports. 
     A “disconnection” is the process of removing a Dedicated Connection between two N_Ports. 
     A “frame” is an indivisible unit of information used by FC-2. 
     “Frame content” is the information contained in a frame between its Start-of-Frame and End-of-Frame delimiters, excluding the delimiters. 
     A “data frame” is a frame containing information meant for FC-4/ULP or the Link application. 
     “Payload” is the contents of the Data Field of a frame, excluding Optional Headers and fill bytes, if present. 
     “Source Identifier” or S_ID is the address identifier used to indicate the source Port of the transmitted frame. 
     “Destination Identifier” or D_ID is the address identifier used to indicate the targeted destination of the transmitted frame. 
     “Valid frame” is a frame received with a valid Start of Frame (SOF), a valid End of Frame (EOF), valid Data Characters, and proper Cyclic Redundancy Check (CRC) of the Frame Header and Data Field. 
     “Classes of Service” are different types of services provided by the Fabric and used by the communicating N_Ports. 
     “Class 1” service is a service which establishes a dedicated connection between communicating N_Ports. 
     “Class 2” service is a service which multiplexes frames at frame boundaries to or from one or more N_Ports with acknowledgement provided. 
     “Class 3” service is a service which multiplexes frames at frame boundaries to or from one or more N_Ports without acknowledgement. 
     “Intermix” is a service which interleaves Class 2 and Class 3 frames on an established Class 1 connection. 
     A “Gigabit Link Module” is a module which interfaces to the Endec through either a 10-bit or 20-bit interface and interfaces to the Fibre Channel link through either a copper or fiber interface. 
     An “Encoder/Decoder” or Endec is a device which implements the FC-1 layer protocol. 
     A “Router” is a module which determines the destination port from an address and other Fibre Channel frame parameters. 
     A “Port Control” is a module which reads in a Fibre Channel header, requests a route and forwards the frame to the switch core. 
     “Credit” is the login credit which represents the number of frames that may be transmitted before receiving an acknowledgement or R_RDY. 
     “Fabric Login Protocol” is when an N_Port interchanges Service Parameters with the Fabric by explicitly performing the Fabric Login protocol or implicitly through an equivalent method not defined in FC-PH. 
     “Application Specific Integrated Circuit” or (ASIC), an integrated circuit designed to perform a particular function by defining the interconnection of a set of basic circuit building blocks drawn from a library provided by the circuit manufacturer. 
     “FPGA” Field Programmable Gate Array, a gate array where the logic network can be programmed into the device after its manufacture. An FPGA consists of an array of logic elements, either gates or lookup table RAMs, flip-flops and programmable interconnect wiring. Most FPGAs are dynamically reprogrammable, since their logic functions and interconnect are defined by RAM cells. 
     “FIFO” a data structure or hardware buffer from which items are taken out in the same order they were put in. 
     “Bridge” a device which forwards traffic between network segments based on datalink layer information. These segments would have a common network layer address. 
     “Router” a device which forwards traffic between networks. The forwarding decision is based on network layer information and routing tables, often constructed by routing protocols. 
     “Brouter” a device which bridges some packets (i.e. forwards based on datalink layer information) and routes other packets (i.e. forwards based on network layer information). The bridge/route decision is based on configuration information. 
     “Hub” a device connecting several other devices. 
     “Serdes” serial encoder/decoder, converts the Fibre Channel serial interface to/from a 10 or 20 bit parallel interface. 
     “HIPPI” is a computer bus for use over fairly short distances at speeds of 800 and 1600 megabytes per second. HIPPI is described by the ANSI standard X3T9/88-127. 
     “SCSI” or Small Computer System Interface is a standard for system-level interfacing between a computer and intelligent devices including hard disks, tape drives, and many more. SCSI is described by the ANSI standard X3.131-1986 and by ISO/IEC 9316. 
     “ATM” or Asynchronous Transfer Mode is a method for the dynamic allocation of bandwidth using a fixed-size packet, also called a cell. 
     “SNMP” or Simple Network Management Protocol is an Internet Standard protocol defined in RFC 1157, developed to manage nodes on an IP network. 
     “MIB” or management information base is a database of managed objects accessed by network management protocols such as SNMP. 
     “Web” is the World-Wide Web, an Internet client-server distributed information retrieval system which originated from the CERN High-Energy Physics Laboratories in Geneva, Switzerland. 
     “Web Browser” is a program which allows a person to read information from the Web. The browser gives some means of viewing the contents of nodes (or “pages”) and of navigating from one node to another. 
     B. Fibre Channel Fabric Model 
     Referring to  FIG. 1 , a Fibre Channel Fabric is an entity which transports Fibre Channel frames between attached devices. The data transmission between the connected device port (i.e., N_Port) and the Fabric port (i.e., F_Port) is serial and consists of one or more frames. The transmission protocol and speeds along with the fabric functionality are defined in the American National Standard for Information Systems (ANSI) FCPH standard (see Other documents, section H, below). 
     The primary function of the Fabric is to receive frames from a source N_Port and route the frames to the destination N_Port whose address identifier is specified in the frames. Each N_Port is physically attached through a link to the Fabric or in the case of an Arbitrated Loop topology attached to the same loop. FC-2 specifies the protocol between the Fabric and the attached N_Ports. A Fabric is characterized by a single address space in which every N_Port has a unique N_Port identifier. 
     The Fabric model contains three or more F_Ports or FL_Ports. Each F_Port is attached to an N_Port through a link. Each F_Port is bidirectional and supports one or more communication models. The receiving F_Port responds to the sending N_Port according to the FC-2 protocol The Fabric optionally verifies the validity of the frame as it passes through the Fabric. The Fabric routes the frame to the F_Port directly attached to the destination N_Port based on the N_Port identifier (D_ID) embedded in the frame. The address translation and the routing mechanisms within the Fabric are transparent to N_Ports. 
     There are two Sub-Fabric models, a Connection based model and a Connectionless based model. The Connection based Sub-Fabric provides Dedicated Connections between F_Ports and the N_Ports attached to these F_Ports. A Dedicated Connection is retained until a removal request is received from one of the communicating N_Ports or an exception condition occurs which causes the Fabric to remove the Connection. The Connection based Sub-Fabric is not involved in flow control which is managed end-to-end by the N_Ports. If the Fabric is unable to establish a Dedicated Connection, it returns a busy or reject frame with a reason code. 
     A Connectionless Sub-Fabric is characterized by the absence of Dedicated Connections. The Connectionless Sub-Fabric multiplexes frames at frame boundaries between an F_Port and any other F_Port and between the N_Ports attached to them. 
     A given frame flows through the Connectionless Sub-Fabric for the duration of the routing. After the frame is routed, the Connectionless Sub-Fabric is not required to have memory of source, routing or destination of the frame. When frames from multiple N_Ports are targeted for the same destination N_Port in Class 2 or Class 3, congestion of frames may occur within the Fabric. Management of this congestion is part of the Connectionless Sub-Fabric and buffer-to-buffer flow control. 
       FIG. 1  shows a possible environment containing a Fibre Channel fabric. The fabric  1 ,  2  illustrated are connected with a mix of workstations  3 , disk arrays  4 , mainframe computers  5 , and Personal Computers (PC)  6 . Fabric interconnection is not limited to particular equipment or a network topology as illustrated in  FIG. 1 . Two types of fabric topologies are illustrated in  FIG. 1 , the direct fabric attached topology  9  and the arbitrated loop topology  7 . 
     The fabrics in  FIG. 1  are shown interconnected or networked through a link  8 . All links to the fabric can operate at either 266 Mbps, 533 Mbps or 1.063 Gbps speeds and operate over either copper or fiber media, or any other compatible media. 
       FIG. 2  shows a block diagram of the fabric. The fabric is composed of a fabric control module  54 , a router module  52 , multiple port control modules  51 ,  74 ,  75  a switch core module  53  and optionally one or more brouter modules  55 . As is understood in the art, the functions allocated to these respective devices may, in alternate embodiments, be allocated to different logical blocks. 
     The fabric control module  54  contains a processor and associated hardware. The fabric control module software performs but is not limited to the following functions: (1) Fabric power on self test, (2) Fabric configuration, (3) Broadcast, Simple Name, ARP and Directory services servers, (4) Fabric Loop Attached profile Extended link service command, (5) Management, (6) Network Management SNMP agent, (7) Web based fabric management, (8) Uninterruptable power supply monitoring and control, and (9) Brouter Module Configuration/Control. The Fabric Control module controls and configures the rest of the fabric but is not usually involved in the normal routing of frames. 
     The fabric Router  52  performs some or all of the following functions: (1) route address matching, (2) route determination based on the ANSI X3T11 rules, (3) route request blocking and unblocking, (4) switch core programming  63 , (5) statistics collection and (6) port control module route request/response handling  59 ,  60 ,  61 ,  62 ,  66 ,  67 ,  72 ,  73 . 
     The fabric Port Control modules (PCM)  51 ,  70 ,  74 ,  75  perform some or all of the following functions: (1) receive Fibre Channel frames from the fiber or copper media  56 ,  77 ,  78 , (2) perform frame validation, (3) send a route request to the router  59 ,  61 ,  66 ,  72 , (4) receives a route response from the router  60 ,  62 ,  63 ,  67 ,  73 , (4) forwards the frame to the switch core  57 ,  69 , and (5) either discards the frame, modifies the frame into a fabric reject (F_RJT) or fabric busy (F_BSY) frame or forwards the frame depending on the route response from the router. 
     The fabric switch core  53  is a nonblocking N×N matrix switch with 36 bit wide transmit and receive I/Os. The switch core switches frames from the PCMs  51 ,  70 ,  74 ,  75  to the destination PCMs or Brouter Module. 
     The Brouter Module  55  performs some or all of the following functions: protocol bridging and/or routing function between a Fibre Channel network and the network implemented by the Brouter Module. The Brouter Module “looks” like a Fibre Channel port to the rest of the switch. This is due to a protocol conversion function in the Brouter Module which converts the brouter networked frames to Fibre Channel frames. Converted Fibre Channel frames from the Brouter Module enter the fabric switch through an internal port control module  70 . Fibre Channel frames from the fabric switch core enter the Brouter Module through an internal path  76 . 
     C. Fabric Control Module 
       FIG. 2  shows the Fabric Control module (FCM)  54 . The FCM  54  serves some or all of the following functions: configures the fabric, collects and reports network management parameters and implements the fabric defined servers such as the Simple Name Server, Directory Services, etc. The FCM  54  configures the router  52 , the port control modules  51 ,  74 ,  75  and the brouter module  55 . 
       FIG. 3  shows the Fabric Control module (FCM) in more detail. The FCM is made up preferably of fast SRAM  82 , DRAM  83 , a DUART  84 , flash memory  85  (nonvolatile storage), a processor  81  and a Decode/DMA Control module  87 . The code for the processor is contained in the flash memory  85  and is copied to SRAM upon bootup. The interface to the brouter module  55  allows the FCM to communicate through legacy networks such as ethernet and fast ethernet, depending on the brouter module. 
     The FCM is attached to the rest of the fabric in two different manners: both in-band  80  to the fabric and out of band  79  to the fabric. The in-band connection is through the internal port control module. This connection allows the Fabric Control Module to communicate with both locally and remotely attached Fibre Channel compliant devices via Fibre Channel frames. The FCM connects out of band to the rest of the system for monitoring, initialization and control reasons. 
     D. Fabric Router 
     The Fabric Router  52  ( FIG. 2 ) receives route requests generated from the Port Control modules  59 ,  61 ,  66 ,  72 , determines the frame route, reports the route responses to the Port Control modules  60 ,  62 ,  67 ,  73 , programs the switch core to connect and disconnect the routes  63 , manages blocked route requests and collects the routing statistics. In the preferred embodiment, there is one central router contained in a fabric. The Router  52  connects and disconnects routes on a frame by frame basis. Since the router can determine a route in real time (i.e., Fibre Channel frame time) the Fabric can support Class 1 frames. The router is realized in hardware through either an FPGA or a custom ASIC. The router is composed of thirteen functional modules as illustrated in  FIG. 4 :
         (1) Port Control Route Request Interface (PCRRIM)  130     (2) Port Control Route Response Interface (PCRSPM)  144     (3) Address Table  132     (4) Address Match Module (ADM)  131     (5) Blocked Route Request Table Module (BRTBL)  133     (6) Blocked Route Request Port Register Array (BRRA)  134     (7) Blocked Route Request Timer (BRTMR)  135     (8) Route Request Unblock Determination Module (RRUNB)  136     (9) Route Request Selector (RRS)  137     (10) Route Determination Module (RDM)  138     (11) Route State Table (RST)  139     (12) Router Statistics Gathering Module (RST)  141     (13) Router Control FSM (RCFSM)  140 .       

     1. Port Control Route Request Interface Module (PCRRIM) 
     The Port Control Route Request Interface Module (PCRRIM)  130  of  FIG. 4  (and  FIG. 24  numeral  581 ) interfaces with the PCMs ( 51 ,  74 ,  75  of  FIG. 2 ) to read route requests and registers the route request for use by the internal router modules. The PCRRIM  FIG. 24  is composed of the following functional blocks: round-robin arbitration  582 , route request state machine  583 , registered route request  584 , and the port winning arbitration register  585 . The PCRRIM  581  is connected to each PCM (items  56 ,  77  and  78  of  FIG. 4 ) through a separate PCM requester signal  586 . The PCRRIM  581  is also connected to each PCM through a common shared route request data channel  588 . After a PCM captures an incoming frame and builds a route request the PCM raises the PCM route request signal  586 . The PCRRIM round robin arbitration block  582  will read all request signals and choose the requester in a round robin manner. This implements requester fairness, i.e., one requester will not be able to starve other concurrent PCM requesters. The round robin arbitration block  582  will notify the winning PCM requester via the route request state machine  583  by pulsing for one clock period the PCM acknowledge signal  587  back to the winning PCM. During the next four clocks the PCM sends the route request over the common route request channel  588  to the registered route request block  584 . The Route Request channel is implemented as an eight bit bus, but is not restricted to that size. The route request is thirty two bits and is shown in  FIG. 31 . The signals are described below. 
     
       
         
               
               
             
           
               
                   
               
               
                 Route Request 
                   
               
               
                 Field 
                 Description 
               
               
                   
               
             
             
               
                 SID Mismatch 
                 Indicates that the incoming frame SID does not match the 
               
               
                   
                 expected SID 
               
               
                 EOFrcvd 
                 Indicates that the entire frame including the EOF was 
               
               
                   
                 received 
               
               
                 Route Direct 
                 A flag to override the router address matching logic. This 
               
               
                   
                 is used to route frames from the fabric control module 
               
               
                   
                 out to a specific port without the use of the DID field 
               
               
                 Delimiter 
                 Is an encoded field which specifies the received frames 
               
               
                   
                 delimiter 
               
               
                 Destination 
                 The DID from the incoming frame. This field is valid 
               
               
                 Address 
                 only when the route direct flag is not set. 
               
               
                 Destination 
                 Only valid when the route direct flag is set, indicates the 
               
               
                 port to route to 
                 remote port to route the frame to. 
               
               
                   
               
             
          
         
       
     
     The winning PCM port number is registered  585  ( FIG. 24 ) and held for use by the internal router modules  589 . The PCRRIM is controlled by the Router Control FSM through the request serviced signal  591 . The PCRRIM will raise the request valid signal  590  whenever it has a valid route request from a PCM in its register  584 . The PCRRIM will halt any further route request reads from the PCM until the request serviced signal  591  is pulsed for one clock period by the Router Control FSM. 
       FIG. 26  shows the PCRRIM state machine. The state machine is described below. 
     
       
         
               
               
             
           
               
                   
               
               
                 State 
                 Description 
               
               
                   
               
             
             
               
                 IDLE 611 
                 Wait for a route request from a port control 
               
               
                 CMP_RR_VECT 612 
                 Route robin logic, compare the current select 
               
               
                   
                 vector with the port control. If a match occurs 
               
               
                   
                 the port control is currently requesting a route. 
               
               
                 SHIRT_RR_VECT 613 
                 Shift the current select vector. 
               
               
                 WAITCLK 614 
                 Signal the select port control module, wait one 
               
               
                   
                 clock before reading the route request channel 
               
               
                   
                 for the route request. 
               
               
                 LDWORD0, 1, 2, 3 
                 Read the route request from the route request 
               
               
                 615, 616, 617, 618 
                 channel. Since the route request channel is 8 
               
               
                   
                 bits wide and the route request is thirty two 
               
               
                   
                 bits, four clocks are needed to read the route 
               
               
                   
                 request. 
               
               
                 RTNAVAIL 619 
                 Wait until the Main Route Control FSM 
               
               
                   
                 signals that the route request is no longer 
               
               
                   
                 needed (RTACK) then return to idle and wait 
               
               
                   
                 for another route request from the port control 
               
               
                   
                 modules. 
               
               
                   
               
             
          
         
       
     
     2. Port Control Route Response Interface Module (PCRSPM) 
     As shown in  FIG. 4  the Port Control Route Response Interface Module (PCRSPM)  144  interfaces with all the PCMs  114 , the Route Determination module  138  and the Router Control FSM module  140 . The PCRSPM main function is to return route responses to the PCMs  114 . The PCRSPM  144  is independent of the PCRRIM  101  which enables the router  52  to concurrently receive route requests and send route responses. This separation in function adds parallelism to the router, permits pipelined operation of the router and increases its performance. 
     As shown in  FIG. 25  the PCRSPM is preferably composed of the following functional blocks: the route response state machine  602  and the route response register  603 . The PCRSPM registers the route response  608  from the Route Determination module when the load route response signal  607  is pulsed for one clock period by the Router Control FSM  140  ( FIG. 4 ). When the Router Control FSM  140  pulses the send route response signal  606  the route response state machine  602  will inform the PCM corresponding to the port vector  609  by pulsing the PCM response acknowledgement signal  604  and putting the route response on the common route response channel  605  for the next four clocks.  FIG. 32  shows the thirty two bit route response format. An eight bit common route response channel is shown but a thirty two bit wide channel can be used depending on the implementation. 
       FIG. 27  shows the PCRSPM state machine (item  602  of  FIG. 25 ). The state machine is described below. 
     
       
         
               
               
             
           
               
                   
               
               
                 State 
                 Description 
               
               
                   
               
             
             
               
                 IDLE 631 
                 Wait for main Router Control FSM to assert the return 
               
               
                   
                 route response signal. 
               
               
                 XMTRSP 632 
                 Acknowledge the main Router Control FSM that the 
               
               
                   
                 route response will be returned. Signal the specific port 
               
               
                   
                 control module the route response will be on the route 
               
               
                   
                 response data channel on the next two clocks. 
               
               
                 XMT_DT0 633 
                 Load the first eight bits of the route response on the 
               
               
                   
                 route response data channel. 
               
               
                 XMT_DT1 634 
                 Load the second eight bits of the route response on the 
               
               
                   
                 route response data channel, return to IDLE. 
               
               
                   
               
             
          
         
       
     
     3. Address Table 
     The Address Table  132  of  FIG. 4  is initially configured by the processor in the fabric control module  122 . The Address Table  132  contains entries against which the incoming Fibre Channel frame destination identifier (D_ID) is compared.  FIG. 33  shows the preferred address table entry format. The address entry contains a twenty four bit address mask register along with a twenty four bit address register. The incoming D_ID is ANDed with the address mask register and the result is compared to the address register. This allows a match to be performed on any number of bits in the address. This also implements routing based on any combination of the address domain (upper eight bits of the address field), area (middle eight bits of the address field) or port (lower eight bits of the address field) fields. Additional address fields include the destination port and the address priority fields. The destination port indicates which remote F_Port to route the frame to and the address priority field specifies a priority for this address table entry match. For any two address matches the address table entry match which is the highest priority will be used. This implements an alternate routing in case of port failure. 
     4. Address Match Module (ADM) 
     The Address Match module  13  (ADM) in  FIG. 4  ( FIG. 13  numeral  351 ) performs the comparison with the incoming frame D_ID address from the route request  105  with the Address Table contents  109 . The results are used by the Route determination module  138 . As shown in  FIG. 13  the ADM  351  has as an input the twenty-four bit address to match  352 , i.e., the incoming frame D_ID address from the route request, and returns the following responses: the remote match port  354 , the address matched indication  355  and the route to control module indication  353 . The ADM will match an incoming D_ID address to all the addresses in the address table in one clock. The ADM logic is implemented in combinatorial logic. The ADM performs the following checks for each address table entry: 
     Address Match indication=(address in table=(address mask &amp; D_ID)) 
     The results are then priority decoded based on address priority contained in the address table and the resulting address match signal and port are generated. There is one special mode which is implemented which will preemptively route all frames to the Fabric Control module except frames originating from the Fabric Control module. This allows the fabric control module to process all incoming frames which is useful when the fabric is functioning in certain environments. 
     5. Blocked Route Request Table (BRTBL) 
     The Blocked Route Request Table  133  (BRTBL) in  FIG. 4  functions to save blocked route requests. Preferably, it is realized by an array of registers. The BRTBL saves enough information to regenerate the route request once the blocking condition is cleared. The format of the blocked route request is shown in  FIG. 30 . The blocked route request contains the requesting PCM port, the matched destination PCM port, the block reason, whether an EOF delimiter was received by the requesting PCM, i.e., whether the entire frame was received before the PCM requested a route, the delimiter in the incoming frame, i.e., SOF type, whether there was an address match, whether to route to the fabric control port and whether a fabric reject (F_RJT) or fabric busy (F_BSY) should be generated. 
     As shown in  FIG. 4  the BRTBL reads the blocked route request from route request bus  107  when instructed to do so by the Route Control FSM  140 . As shown in  FIG. 17  a blocked route request is loaded upon a LOADFIFO  447  signal pulse by the Router Control FSM. Blocked route requests are cleared when the CLRFIFO  448  signal is pulsed by the Router Control FSM. The port input vector,  449 , selects which port location in the table to load or clear the blocked route request. There is one blocked route request entry for each PCM and the blocked route request is registered so certain fields are available  FIG. 4  numeral  116  to the Route Request Unblock Determination module  FIG. 4  numeral  136 . As shown in  FIG. 17 , the BRTBL  441  contains the registered blocked route request table  442  so certain fields in the blocked route request can be monitored by other router internal modules,  443 ,  444 ,  445 ,  446 . The signals which are monitored include whether the specific entry contains a blocked route request  444 , the block reason  443  which includes blocked due to the remote port busy or blocked due to the remote port in a class 1 connection with a port other than this one, and intermix is not support by the remote port. Other monitored fields include whether the blocked request frame is a Class 1 frame as indicated by the SOF delimiter. 
     6. Blocked Route Request Port Register Array (BRRA) 
     The Blocked Route Request Port Register Array  134  (BRRA) in  FIG. 4  reads in the requesting port  103  and saves it into a register array which keeps the PCM request order. This order is wired  118  to the Route Request Unblock Determination module  136 . The BRRA is shown in more detail in  FIG. 19 . When the LOADFIFO  483  signal from the Router Control FSM is pulsed for one clock period the requesting PCM port  482  is saved into position  0  numeral  489  of the register array. Register array entries are removed by the Route Request Unblock Determination module through the CLRFIFO  488  signal and DEQRQ_SEL  485  vector, i.e., when the CLRFIFO signal is pulsed for one clock period the BRRA will unload the register specified by the DEQRQ_SEL vector. 
     Position  0  numeral  489  contains the newest route request and position  16  numeral  490  contains the oldest route request. Register array contents are shifted by one, from the newest position to the oldest, when the LOADFIFO signal is pulsed to make room for the newest blocked route request port number. The shifting circuit must take into account ‘holes’ in the register array. The algorithm identifies the first free register array entry closest to position  0  and shifts all the entries from position  0  to the free register array entry. The shifting circuit creates a shift vector (STTMP) which is used to load the contents of the individual register array entries. The circuit is shown below in verilog for eight ports. 
     
       
         
               
             
               
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                  always @(F1_NULL or F2_NULL or F3_NULL or F4_NULL or 
               
               
                  F5_NULL or F6_NULL or F7_NULL or F8_NULL) begin 
               
               
                  // build fifo shift control word (indicates how to shift fifo) 
               
               
                  casex ({F8_NULL, F7_NULL, F6_NULL, F5_NULL, 
               
             
          
           
               
                   
                 F4_NULL, F3_NULL, F2_NULL, F1_NULL}) 
               
             
          
           
               
                   
                 8′b1xxxxxxx: STTMP = 8′b11111111; 
               
               
                   
                 8′b01xxxxxx: STTMP = 8′b01111111; 
               
               
                   
                 8′b001xxxxx: STTMP = 8′b00111111; 
               
               
                   
                 8′b0001xxxx: STTMP = 8′b00011111; 
               
               
                   
                 8′b00001xxx: STTMP = 8′b00001111; 
               
               
                   
                 8′b000001xx: STTMP = 8′b00000111; 
               
               
                   
                 8′b0000001x: STTMP = 8′b00000011; 
               
               
                   
                 8′b00000001: STTMP = 8′b00000001; 
               
               
                   
                 default: STTMP = 8′b00000000; 
               
             
          
           
               
                  endcase 
               
             
          
           
               
                 end 
                 // always 
               
             
          
           
               
                 where F1_NULL, .... , F8_NULL are true if register array position 
               
               
                 1 to 8 (respectively) are empty. 
               
               
                   
               
             
          
         
       
     
     The shifting vector is then used with the CLRFIFO signal  484  and the dequeue port signal (DEQRQ_SEL)  485  to clear the register array contents. 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
             
               
               
             
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 always @(posedge clk or negedge reset) begin 
               
             
          
           
               
                   
                  if (!reset) 
                 FIFO2 &lt;= NULLVALUE; 
               
             
          
           
               
                   
                  else if (LOADFIFO &amp;&amp; STTMP[1]) 
               
             
          
           
               
                   
                 FIFO2 &lt;= FIFO1; 
               
             
          
           
               
                   
                  else if (CLRFIFO &amp;&amp; DEQRQ_SEL == FIFO2) 
               
             
          
           
               
                   
                   
                 FIFO2 &lt;= NULLVALUE; 
               
               
                   
                  else 
                 FIFO2 &lt;= FIFO2; 
               
               
                   
                 end 
               
               
                   
                   
               
             
          
         
       
     
     7. Blocked Route Request Timer (BRTMR) 
     The Blocked Route Request Timer  135  (BRTMR) in  FIG. 4  implements one timer per PCM. The timer is enabled when a route request is blocked for the particular PCM. The timer is disabled when the blocked route request becomes unblocked. The BRTMR is controlled by the Route Control FSM which not only enables the timer but also indicates which timer to enable. Enabled timers are selected by the port from the incoming route request  104 . Disabled timers are selected by the port from the route request selector module  146 . The different timers are defined by the ANSI FCPH standard. When a timeout occurs the Route Request Unblock Determination module is signaled  119  to dequeue the blocked request as soon as possible. 
     8. Route Request Unblock Determination Module (RRUNB) 
     The Route Request Unblock Determination module  136  (RRUNB) in  FIG. 4  determines when and which blocked route request to unblock. The RRUNB reads information from the Blocked Route Request Table  116 , the Blocked Route Request Port Register Array  118  the Blocked Route Request Timer  119  and the Route State Table  124 . A more detailed view of the RRUNB is shown in  FIG. 14 ,  FIG. 15  and  FIG. 16 . 
     As shown in  FIG. 14  the RRUNB  361  reads information from several internal router modules and determines the most recent and highest priority blocked route request to dequeue from the Blocked Route Request Table. The RRUNB signals the port to dequeue  371  to both the Blocked Route Request Table and the Router Control FSM. The inputs to the RRUNB include the following information from the Route State Table: Port is currently busy signal  365  and the Port is currently in a class 1 connection signal  366 . The inputs to the RRUNB from the Blocked Route Request Table include the blocked route request indication, the destination port in which the blocked route request is waiting for, the block reason (whether waiting for the remote port to become free or both free and disconnected from a Class 1 route), and if the blocked route request is a Class 1 frame. 
       FIG. 16  shows part of the RRUNB circuit which generates intermediate terms necessary to calculate which blocked route requests to unblock. Each blocked route is waiting for certain conditions to clear from a destination port. The destination port vector  429 ,  431 ,  433 ,  435  is used to select which remote signal to look at  421 ,  422 ,  423 ,  424 , to generate the remote status  430 ,  432 ,  434 ,  436 . For example if a route request is blocked from port  1  the destination port which port  1  is waiting for is used to select the remote port busy signal. It is also used to select the “remote port is currently in a Class 1 connection signal”. 
       FIG. 15  shows another part of the RRUNB circuit. There are seventeen different DEQx_FLAGS, only two are shown for brevity, i.e., DEQ 0 _FLAG  381  and DEQ 16 _FLAG  382 . The DEQx_FLAG signals are generated according to the following circuit: 
     
       
         
               
             
               
               
             
           
               
                   
               
             
             
               
                 DEQ0_FLAG = Timeout indication for port 0 from RTMR || 
               
             
          
           
               
                   
                 ((!(remote port 0 busy) &amp;&amp; 
               
               
                   
                 (!(block reason == wait for remote port 0 Class 1 connected &amp;&amp; 
               
               
                   
                 (remote port 0 Class 1 connected)))) 
               
               
                   
                   
               
             
          
         
       
     
     The timeout indication is generated from the BRTMR module  362  in  FIG. 14 . The remote port  0  busy  430  and the remote port  0  Class 1 connected signals  434  are generated from the circuit described in  FIG. 16 . The block reason comes from the BRTBL  369 . There is one DEQ 0 _FLAG signal for every PCM. 
     As shown in  FIG. 15  each DEQ_FLAG  381 ,  382  signal is input into sixteen multiplexers  383 ,  384 , representing the number of potentially blocked route requests. Multiplexer numeral  383  uses the port number in the BRRA register array in position  0 , numeral  385 , and multiplexer  384  uses the port number in the BRRA register array in position  16  numeral  386 . For example if the contents of position  0  in the BRRA register array is port  4  then the DEQ 4 _FLAG is select by multiplexer  383  and output to the DEQIND 0  signal  387 . The DEQIND signals  387 ,  388  are used as inputs to the binary encoder block  389 . The binary encoder block  389  takes the highest DEQIND signal, DEQ 16 IND being higher than DEQ 0 IND and encodes the value to a select  390  which selects the position in the BRRA  392 ,  393  to dequeue  394 . For example if DEQ 16 IND signal is set then the port number contained in position  16  of the BRRA is output  394  from multiplexor  391 . 
       FIG. 15  also describes a similar circuit which accounts for blocked route requests for Class 1 frames. The resulting port derived from this circuit takes precedence to the circuit previously described. This allows priority dequeueing of blocked route requests for Class 1 frames. The circuit uses the DEQx_FLAGs  387 ,  388  generated from multiplexors identified by numeral  383  and  384 . The DEQx_FLAGs are ANDed with the remote port Class 1 connected signals generated in  FIG. 16  numerals  434 ,  436  to form the inputs  396 ,  397  to the multiplexors identified by numeral  398  and  399 . The multiplexors  398 ,  399  select the destination port contained in the BRRA array  400 ,  401 . The output signals  402 ,  403  are binary encoded  404  to take the highest input signal to select the position in the BRRA  406 ,  407  to dequeue  408 . 
     The inputs to multiplexor numeral  395  represent the oldest blocked route request  394  and the oldest blocked route request of a Class 1 frame  408 . Multiplexor  395  will give priority to the Class 1 frame port  408  before choosing the oldest non-Class 1 route request  394 . The resulting vector  409  is the blocked route request to dequeue. 
     This circuit can be used to unblock other types of resources besides Fibre Channel route requests. The circuit is implemented as combinatorial logic and selects the blocked route request within one clock. 
     9. Route Request Selector (RRS) 
     The Route Request Selector module  137  (RRS) in  FIG. 4  functions to select between the incoming route request from the PCRRIM module  108  or the BRTBL  115 . The resulting route request is output  110  to the Route Determination module. The RRS is controlled by the Route Control FSM  140 . 
     10. Route Determination Module (RDM) 
     The Route Determination module  138  (RDM) in  FIG. 4  applies rules defined in the ANSI Fibre Channel specifications to calculate how to route the incoming frame. The RDM receives the route request  110  from the RRS  137  along with route context for the source and destination ports  112  from the Route State Table  139 . The RRS outputs the route results  145 ,  111  to both the Router Control FSM  140  and the PCRSPM  144 . The RDM is implemented in combinatorial logic and applied the route rules in one clock. 
       FIG. 20  shows the RDM  501  in more detail. The RDM reads the route request from the RRS which includes the source requesting port  503 , the destination port  504 , the frame SOF delimiter  505 , the EOF received flag  506 , the route to port  0  (i.e., fabric controller) flag  507  and the timeout indication  508 . The RDM also reads in the route table context for both the source and destination ports  512  and reads in a test enable vector  513 . The test enable vector  513  turns off selected route rule checks for more flexibility when the router is implemented in an ASIC. The outputs from the RDM include the route results vector  509 ,  510  which indicates whether to route the frame or return an error, the reject/busy action/reason vector  10  which is valid when the RDM detects an error and the route back indication  511  which signals the port that the frame is in error and will routed back to the same port. Finally the updated source and destination port contexts are updated to reflect the RDM actions  514  and wired back to the route state table  502 . 
       FIG. 21  shows the RDM route selection logic in more detail. As mentioned earlier the inputs to the RDM include the route state context for both the source and destination ports  522  and the route request  523 . The RDM has prewired rules checks to detect five conditions: discard frame  525 , block the route request until the remote port is not busy  526 , return a fabric reject (F_RJT) frame  527 , return a fabric busy (F_BSY) frame  528 , wait until the frame is completely received  529 . If all of the four conditions are not detected then the frame should be routed successfully to the remote port The conditions mentioned above are derived from the ORing of multiple rules checks. For example the discard frame signal is derived from the ORing of five discard frame rules checks. An example rules check is shown below. 
     
       
         
               
             
           
               
                   
               
             
             
               
                 // discard frame if local SOFc1 received and local port is in a class 1 connection 
               
               
                 wire DISFRM4 = TEN[2] &amp;&amp; DELIM == SOFn1 &amp;&amp; SRC_CSTATE == 
               
               
                 Connected; 
               
               
                   
               
             
          
         
       
     
     The TEN[2] term above selects a bit from the test enable vector. Turning the bit off will disable the above rules check. The rule above will assert the DISFRM4 signal if the incoming frame contains an SOFn1 delimiter and the incoming port is not already in a Class 1 connection. 
     As shown in  FIG. 21  all potential rules check results  531  are encoded and selected by using the rules checks  525 ,  526 ,  527 ,  528 ,  529  as the multiplexor selector. The routing result selected is then output  532  to both the Router Control FSM and the PCRSPM. All rules checks are completed within one clock period. 
     Finally  FIG. 22  shows how the preencoded fabric reject  544  and fabric busy responses  548  are selected by the fabric reject  542  and fabric busy  546  rules checks. The result  551  is output to the PCRSPM module to be included in the route response. 
     11. Route State Table (RST) 
       FIG. 4  shows the Route State Table (RST)  139 . The function of the RST is to keep the current context for each port. The RST interfaces with the Route Determination Module (RDM)  138 , the Route Request Unblock Determination Module (RRUNB)  136  and the processor in the Fabric Control module  121 .  FIG. 20  shows the RST  502  in relation to the RDM  501 . The RST is controlled by the Router Control FSM which signals the RST  515  to either output the source and destination context  512  or save the updated source and destination context  514 . The RST outputs certain context fields into the RRUNB  FIG. 4  numeral  124  to assist in route request unblocking calculation. 
     The RST contains a context entry for each port. The context entry is shown in  FIG. 34 . There are two parts to the route context: a static portion which is updated by the processor in the Fabric Control module  FIG. 2  numeral  54  and a dynamic portion updated by the RDM module  FIG. 4  numeral  138 . The processor updates the static portion upon infrequent events such as power up and fabric login. The RDM updates the dynamic portion on a per frame basis. In current commercially available fabrics a processor manages all of the route state table fields, the current embodiment uses a register memory in the RST and the RDM to update the context. The table below lists the context fields. 
     
       
         
               
               
             
           
               
                   
               
               
                 Signal 
                 Description 
               
               
                   
               
             
             
               
                 Destination Port 
                 If a route exists this specifies the remote port. 
               
               
                 Connected To 
               
               
                 Class 1 Destination 
                 If this port is in a Class 1 connection this field 
               
               
                 Port 
                 specifies the remote port. 
               
               
                 Timer State 
                 If this port is waiting for a route and a timer is 
               
               
                   
                 enabled, this field specifies the timer. 
               
               
                 Class 1 Connection 
                 This field specifies whether this port is currently 
               
               
                 State 
                 in a Class 1 connection. 
               
               
                 Port Busy 
                 This field specifies whether this port is currently 
               
               
                   
                 routing a frame to a remote port. 
               
               
                 Port State 
                 This field specifies the link state, whether 
               
               
                   
                 initializing, offline, online, or error. 
               
               
                 Class Supported 
                 This field specifies the Classes of service 
               
               
                   
                 supported by this port. 
               
               
                 Loop Port Indication 
                 This field specifies whether this port is a loop 
               
               
                   
                 port or a point to point port. 
               
               
                 Port Speed 
                 This field specifies the link speed for this port. 
               
               
                 Intermix Support 
                 This flag specifies support for Intermix for this 
               
               
                   
                 port. 
               
               
                 FLOGI occurred 
                 This field specifies whether a FLOGI/ACC 
               
               
                   
                 exchange occurred. 
               
               
                   
               
             
          
         
       
     
     12. Router Statistics Gathering Module (RSG) 
       FIG. 4  shows the Router Statistics Gathering Module (RSG)  141 . The RSG gathers fabric generated statistics. The RSG is enabled by the Router Control FSM  140  and has as inputs the source and destination ports, the route result and the frame Class  142 . The RSG is implemented in hardware because of the requirement of collecting statistics at gigabit rates. 
     13. Router Control FSM (RCFSM) 
       FIG. 4  shows the Router Control FSM (RCFSM)  140 . The RCFSM controls the entire router through control signals to the internal router modules  147 . The RCFSM state diagram is shown in  FIG. 18 . 
     The RCFSM is triggered from idle by one of three events: a processor request to read or write a router data structure  470 , a blocked route request becoming unblocked  471  or an incoming route request received from a port control module signal  472 . The three events are prioritized in case of multiple simultaneous events. The priorities from high to low include: 1) processor request, 2) a blocked route request becoming unblocked and 3) an incoming route request. When a processor updates any of the router fields the router must be in a quiescent state, i.e., not updating any data structure. When a processor requests access to a router data structure the processor signals the RCFSM by asserting the BLKCTLREQ signal. If in idle the RCFSM enters the RTBLKED state  452  and waits until the processor has finished its access. While in the RTBLKED state the RCFSM signals it is in this state by asserting the BLKCTLACK signal. The router processor interface logic will hold off the processor access via a WAIT signal until the BLKCTLACK signal is enabled. 
     The remaining RCFSM diagram states and description is discussed below. Refer to  FIG. 18  for the state diagram and to  FIG. 4  for the module description. 
     
       
         
               
               
             
           
               
                   
               
               
                 State 
                 Description 
               
               
                   
               
             
             
               
                 DEQROUTE 467 
                 Program RRS 137 to use the newly unblocked 
               
               
                   
                 route request as an input 115 
               
               
                 CLR_FIFO 468 
                 Signal the BRTBL 133 to remove the blocked 
               
               
                   
                 route 
               
               
                 DECODERRSP 455 
                 Wait one clock for the RDM 138 to apply routing 
               
               
                   
                 rules checks to the route request 110 
               
               
                 RTOK 456 
                 The RDM 138 has determined the route is ok. 
               
               
                   
                 Signal the RST 139 to update the route table, 
               
               
                   
                 signal the RSG 141 to collect statistics for this 
               
               
                   
                 route and select the destination port from the 
               
               
                   
                 ADM 131 results. 
               
               
                 RTBSY 459 
                 The RDM 138 has determined to return a fabric 
               
               
                   
                 busy (F_BSY) frame to the sending port. Signal 
               
               
                   
                 the RST 139 to update the route table, signal the 
               
               
                   
                 RSG 141 to collect statistics and assign the 
               
               
                   
                 destination port from the source port (i.e., route 
               
               
                   
                 F_BSY back to the same port). 
               
               
                 RTRJT 460 
                 The RDM 138 has determined to return a fabric 
               
               
                   
                 reject (F_RJT) frame to the sending port. Signal 
               
               
                   
                 the RST 139 to update the route table, signal the 
               
               
                   
                 RSG 141 to collect statistics and assign the 
               
               
                   
                 destination port form the source port (i.e., route 
               
               
                   
                 F_RJT back to the same port). 
               
               
                 RTDISCARD 461 
                 The RDM 138 has determined that the port 
               
               
                   
                 control module should discard the frame. Signal 
               
               
                   
                 the RSG 141 to collect statistics. 
               
               
                 RTWAIT_EOF 462 
                 The RDM 138 has determined that the port 
               
               
                   
                 control module should wait until the entire frame 
               
               
                   
                 is received before resubmitting the route request. 
               
               
                 RTBLK 463 
                 The RDM 138 has determined to block the route 
               
               
                   
                 request. The BRTBL 133 and the BRRA 134 are 
               
               
                   
                 signaled to save the route request and save the 
               
               
                   
                 port requesting the route. 
               
               
                 PGMSW 457 
                 Program the switch core 123 to make a path from 
               
               
                   
                 the source to the destination port. 
               
               
                 RTNRSP 458 
                 Signal the PCRRSPM 144 to return a route 
               
               
                   
                 request complete indication. 
               
               
                 LDRTSTATE 464 
                 Signal the RST 139 to update its context and 
               
               
                   
                 signal the BRTMR 104 to enable a blocked route 
               
               
                   
                 request timer. 
               
               
                 LD_RT 453 
                 Signal the RRS 137 to read the route request 108 
               
               
                   
                 that was just read from the PCRRIM 130. 
               
               
                 SOFOREOF 454 
                 Signal the PCRRIM 130 to fetch another route 
               
               
                   
                 request since the current request is registered in 
               
               
                   
                 the RRS 137 module. Load the route results from 
               
               
                   
                 the RDM 138 into the PCRRSPM 144 (in case 
               
               
                   
                 delimiter is an EOF). Go to the DECODERRSP 
               
               
                   
                 455 state if the delimiter in the route request is an 
               
               
                   
                 SOF otherwise go to the EOFDELIM 465 state. 
               
               
                 EOFDELIM 465 
                 Signal the RRS 137 to use the destination port 
               
               
                   
                 from the route context in the RST 139. 
               
               
                 DISTIMER 466 
                 Signal the switch core to disconnect the path from 
               
               
                   
                 the specified source port to the destination port, 
               
               
                   
                 signal the RST 139 to update the route table 
               
               
                   
                 context to reflect the disconnected path and signal 
               
               
                   
                 the PCRRSPM 144 to return a route request 
               
               
                   
                 complete indication. 
               
               
                   
               
             
          
         
       
     
     E. Port Control 
       FIG. 2  shows the Port Control (PC) locations  51 ,  70 ,  74 ,  75 , within the fabric block diagram. Preferably, there is one PC per port or link. The PC interfaces with the fabric attached device through either copper or fiber media  56 ,  77 ,  78 . The PC interfaces to the switch core through transmit  58  and receive  57  data buses and control signals. The PC interfaces to the router through route request  59 ,  61 ,  66 ,  72  and route response  60 ,  62 ,  67 ,  73  buses and control signals. Finally the PC interfaces to the Fabric Control module through a processor interface bus  65 . 
       FIG. 5  shows the Port Control in more detail. Frames are received from the fiber or copper link  151  and enter the Endec  153 . The Endec implements the 8 B/10 B encoding/decoding, the loop port state machine and fabric/point-to-point state machine functions and outputs thirty two bit data words with two bits of parity and tag information to the receive FIFO  155 . The PC contains a module which guards against a receive FIFO overrun  154  condition. Once the receive FIFO  155  starts filling, the Port Control Module (PCM)  156  reads the frame header, requests a route from the router  163 ,  164  and forwards the frame to the switch core  161 ,  162 . The PCM is configurable by the processor  170  in the Fabric Control module. The Port Control also receives frames from the switch core  165 ,  166  to be transmitted by the Endec  153 . 
     Port Control Module (PCM) 
       FIG. 10  shows the Port Control Module (PCM) in more detail. The PCM is responsible for reading a portion of the received header from the input FIFO  250 , building a route request for the router  262 ,  263 ,  264 ,  260 , receiving the route response from the router  265 ,  266 ,  261  and either forwarding the frame to the switch core  249  or building a fabric reject (F_RJT) or fabric busy (F_BSY) frame and forwarding those to the switch core. The PCM also performs miscellaneous functions such as receive frame validation against parity errors, short frames, frames too large, tag errors and other checks. 
     The PCM is composed of the following four modules:
         (1) Port Control FIFO Module (PCFIFO)  247     (2) Port Control to Router I/F Module (PCRTIF)  234     (3) Port Control Main Control FSM (PCFSM)  232     (4) Port Control Configuration/Counter Module (PCCFG)  233         

     1. Port Control FIFO Module (PCFIFO) 
       FIG. 10  shows the Port Control FIFO module (PCFIFO)  247 . The PCFIFO buffers several words of the incoming frame with internal registers. The registers include four general input registers (fifo_reg 0   237 , fifo_reg 1   238 , fifo_reg 2   239 , fifo_reg 3   240 ), five special input registers (sof_reg  241 , rctldid_reg  242 , type_reg  243 , param_reg  244 , eof_reg  245 ) and a main input and output register (EDATA_OUTR  236  and SW_DATAIN  246 ). The input register (EDATA_OUTR) gates the data in from the input FIFO  250  by asserting the FIFOREQ_signal. The output register sends the data to the switch core by asserting the SWACK_signal  249 . The general and special input registers are loaded from the EDATA_OUTR register. The general and special registers also are connected to a multiplexor which feeds the SW_DATAIN register  246 . The special registers allow the PCFIFO to build fabric reject (F_RJT) and fabric busy (F_BSY) frames and to insert special EOF delimiters when the route response  261  specifies to do so. 
     The received destination address (D_ID) along with the SOF delimiter is wired to the PCRTIF module  254  to build the route request  260 . Finally the PCFIFO is controlled by the PCFSM  232 . 
     The PCFIFO module performs certain frame validations. These validations include parity and tag field checking and regeneration, CRC, invalid transmit word and link down while receiving frame validations. When the frame validations fail the PCFIFO automatically inserts the appropriate EOF delimiter  251 , either an EOFa, EOFni or EOFdti. 
     The PCFIFO will build a fabric frame reject (F_RJT) when the route response from the router specifies to do so  261 . The PCFIFO builds the fabric reject by changing certain fields in the frame header  241 ,  242 ,  244 . Since the entire header is not yet in the PCFIFO internal registers a counter is implemented to indicate when to insert the modified header fields. The frame fields which are modified include the R_CTL field  242 , the parameter field  244  and potentially the EOF delimiter  245 . In addition if there was a payload associated with the frame it is discarded. 
     The PCFIFO will also build a fabric busy (F_BSY) frame when the route response from the router response specifies to do so  261 . The PCFIFO modifies the R_CTL field  242 , the type field  243  and potentially the EOF delimiter  245 . As in the F_RJT frame modification the payload for the F_BSY frame is discarded. 
     2. Port Control Main Control Module (PCFSM) 
       FIG. 10  shows the Port Control Main Control Module (PCFSM) module  232  (PCM). The PCFSM controls the other modules which compose the PCM  252 ,  258 ,  272 . The PCFSM is triggered by a frame being received from the input FIFO.  FIG. 11  shows the PCFSM state diagram and is described in detail below. 
     
       
         
               
               
             
           
               
                   
               
               
                 State 
                 Description 
               
               
                   
               
             
             
               
                 IDLE 301 
                 Wait until the first three words of a frame are received 
               
               
                   
                 from the input FIFO. This is the first state after a 
               
               
                   
                 system reset. 
               
               
                 CLRSOF 302 
                 A frame has been received. Reset the EOF register and 
               
               
                   
                 start the route request signal if the frame is not a short 
               
               
                   
                 frame. 
               
               
                 ROUTEFRM 303 
                 In this state the PCFSM signals the PCRTIF to send a 
               
               
                   
                 route request (RREQ) to the router. The PCFSM will 
               
               
                   
                 loop in this state until a route response (RRACK) is 
               
               
                   
                 received back from the router. 
               
               
                 XMTFRM 304 
                 Transmit the frame through the Port Control from the 
               
               
                   
                 input receive FIFO to the switch core. 
               
               
                 RTNRJTBSY 308 
                 The router has determined that a fabric reject (F_RJT) 
               
               
                   
                 or fabric busy (F_BSY) frame should be returned. The 
               
               
                   
                 SOF delimiter is modified along with the R_CTL 
               
               
                   
                 field. 
               
               
                 WAITEOF 306 
                 Wait until an EOF is received. The Port Control 
               
               
                   
                 usually implements cut through routing, i.e., when a 
               
               
                   
                 frame is received it is forwarded to the remote before 
               
               
                   
                 the end of frame is received. Certain conditions dictate 
               
               
                   
                 that the frame should be received in its entirety before 
               
               
                   
                 being forwarded. An example condition includes the 
               
               
                   
                 remote port speed is lower than the source port. 
               
               
                 DISCRT 305 
                 Signal the PCRTIF to send a route disconnect request 
               
               
                   
                 (RREQ) and loop in this state until a route 
               
               
                   
                 disconnected response (RRACK) signal is received. 
               
               
                 UPDATE_CDT 
                 This one clock state is entered into after transmitting or 
               
               
                 306 
                 discarding a frame. If a Class 2 or Class 3 frame was 
               
               
                   
                 operated on the Endec CREDIT_signal is pulsed. The 
               
               
                   
                 EOF register (RESET_EOF) is cleared, the frame 
               
               
                   
                 counter is cleared and the EOF in received FIFO 
               
               
                   
                 counter is decremented. 
               
               
                 WAITEOF1 309 
                 Wait until an EOF is received from the Endec due to a 
               
               
                   
                 F_RJT/F_BSY frame being returned. An EOF must 
               
               
                   
                 be received so as to not cause a transmitter underrun 
               
               
                   
                 at the remote Endec. 
               
               
                 XMT_FRJTBSY 
                 Wait until an EOF is transmitted which signals that the 
               
               
                 10 
                 F_RJT or F_BSY EOF was transmitted. While in the 
               
               
                   
                 XMT_FRJTBSY state assert either the xmt_frjt or 
               
               
                   
                 xmt_fbsy signal to the PCFIFO module to specify 
               
               
                   
                 which frame to transmit. 
               
               
                   
               
             
          
         
       
     
     3. Port Control Configuration/Counter Module (PCCFG) 
       FIG. 10  shows the Port Control Configuration/Counter (PCCFG) module  233 . The PCCFG maintains counters and provides the processor interface  271  to the Port Control Module. The PCCFG contains an EOF received counter  267 , a current frame count register  268  and a port control configuration register  269 . The EOF received counter keeps track of the number of full frames received by the Endec contained in the receive frame FIFO. The current frame count register monitors the current number of words received on a per frame basis. This counter is used to detect short and long frames. Finally the port control configuration register contains miscellaneous information/configuration information used by the Port Control module. 
     The port control configuration register fields are described below. 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Field 
                 Bit Location 
                 Description 
               
               
                   
               
             
             
               
                 Max Frame Size 
                 9:0 
                 Indicates the maximum receive 
               
               
                   
                   
                 frame size in words. 
               
               
                 LISM Mode 
                 10 
                 The Port is currently going through 
               
               
                   
                   
                 loop initialization indication 
               
               
                 Clear Interrupt 
                 11 
                 Clear the bad parity notification 
               
               
                   
                   
                 interrupt. 
               
               
                 Pulse CDT_line 
                 12 
                 Pulse the Endec credit line. 
               
               
                 Clear Interrupt 
                 13 
                 Clear interrupt latch 
               
               
                 Enable Remote 
                 14 
                 Enable preemptive remote port 
               
               
                 Port Routing 
                   
                 routing. 
               
               
                 This Port Number 
                 19:16 
               
               
                 EOF Counter 
                 26:24 
                 Counter value for the number of EOF 
               
               
                   
                   
                 delimiters received from the Endec. 
               
               
                 Frame Discarded 
                 27 
                 Frame was discarded by the Port 
               
               
                   
                   
                 Control. 
               
               
                 Frame Too Short 
                 28 
                 A frame which was less than eight 
               
               
                 Detected 
                   
                 words in length was received. 
               
               
                 Frame Too Big 
                 29 
                 A frame which was greater than the 
               
               
                 Received 
                   
                 specified maximum frame size (bits 9 
               
               
                   
                   
                 to 0) was received. 
               
               
                 Tag Error 
                 30 
                 A tag error was detected (i.e., a tag 
               
               
                 occurred 
                   
                 of either 00 or 11). 
               
               
                 Parity Error 
                 31 
                 Clear parity interrupt indication 
               
               
                 occurred 
                   
                 register. 
               
               
                   
               
             
          
         
       
     
     4. Port Control to Router Interface Module (PCRTIF) 
       FIG. 10  shows the Port Control to Router Interface Module (PCRTIF)  234 . The PCRTIF builds route requests for the router  260 , signals the router that a valid request is present  262 , waits for a router response valid signal (RTPCREQ)  263  and receives the router response  261 . The PCRTIF builds the route request from the D_ID field, the SOF delimiter and some miscellaneous signals from both the PCFIFO  254  and the PCCFG  273  modules. The route request is transmitted over a shared command channel bus  264  to the router. This command channel bus is shared by all the PCMs. The route response is received over a different shared response channel bus (RT_DATA)  266  which is also shared by all the PCMs. By implementing different buses or channels for the route request and route response the router can simultaneously read route requests along with returning route responses. 
     FIFO Overrun Prevention Logic (FOPL) 
       FIG. 5  shows the FIFO Overrun Prevention Logic (FOPL)  154  within the Port Control area. The purpose of the FOPL is to handle the case where the FIFO  155  is full and frames are received by the Endec  153 . Since the frame arrival rate is extremely fast at gigabit link data rates, the FOPL must act in real time. An additional situation the FOPL must handle is when the frame arrives and is being routed to the remote port and the back end of the frame overruns the FIFO. Still another situation is where multiple frames overrun the FIFO. The FOPL operates on the TAG bits  154  not the data bits  171 . The Endec takes gigabit serial transmission from the link side, decodes the transmission and outputs thirty two bit words to the port control FIFO. Along with the thirty two bit words are a two bit tag field and a two bit parity field. The tag and parity field additions are a common interface characteristics. Tag bits are bits attached to the thirty two bit words to indicated delimiters such as the SOF or EOF. When the FIFO is full and a frame is received from the Endec the FOPL sets the tag bits to an illegal value. When the FIFO enters the not full condition the next word will contain the illegal tag bits. The illegal tag bits will signal the Port Control module to abort the frame with the appropriate EOF delimiter. 
       FIG. 8  shows the FOPL in more detail. The FOPL  201  interfaces with the Endec tag bits  202 , the Endec receive frame DMA request signal  203 , and the Endec receive frame DMA acknowledgement  204  signal. The FOPL interfaces with the FIFO by supplying the tag bits and through the FIFONOTEMPTY  206  signal. During normal operation the FOPL will set the FIFO tag bits  205  to the value of the Endec tag bits  202 . When the FIFO is full, i.e., when the FIFONOTEMPTY signal  206  is deasserted, the FOPL will output an illegal value for the tag bits  205  going to the FIFO. If the overflow word is the last word to be received the FOPL will wait until the FIFONOTEMPTY signal  206  is asserted and then output a word with bad tag bits by asserting the FIFOWRITE  207  signal. This last scenario handles the case where the last word received overflows the FIFO and there are no other words to receive. 
     Processor/Data Arbitration Logic (PDAL) 
       FIG. 5  shows the Processor/Data Arbitration Logic (PDAL)  157  within the Port Control area. Since the Endec  153  multiplexes the transmit bus with the internal register configuration bus, logic is needed to arbitrate between processor accesses  168  and frames being transmitted from the switch core  166 . This logic must manage processor accesses to the Endec which are slower than transmit data word dma&#39;s. In other words if a frame is currently being transmitted, processor accesses to the Endec must be held off until either the frame transmission is complete or the internal Endec transmit FIFO is full, allowing enough time for a processor access before a transmitter underrun occurs. 
     The PDAL acts as the arbitrator between processor accesses and transmit data to the Endec. The PDAL accomplishes this by keeping track of when the switch core is transmitting frames to the Endec and inserting processor accesses between frames or when the Endec&#39;s internal transmit FIFO is full.  FIG. 9  shows the PDAL in more detail. The PDAL interfaces to the Endec through the Endec transmit frame DMA request  209  signal, the chip select  208  and the wait  207  signals. The PDAL interfaces with the switch core through the transmit frame dma request signal  212 . The PDAL interfaces with a bus transceiver  225  through an enable signal  216 . The PDAL interfaces with the processor from the Fabric Control module through the chip select  214 , wait  215  and write  222  signals. Finally the PDAL interfaces to the Router Module through the route busy  226  signal. The processor will only access the Endec transmit/configuration bus  224  when the wait signal  215  is deasserted. The PDAL uses two conditions to create the processor wait signal. The first condition is that there are no frames being transmitted to the Endec. This condition is indicated by the route busy signal  226  from the router being deasserted. The second condition is the transmit frame dma request signal  209  deasserted, indicating that the internal Endec transmit FIFO is full. The second condition creates enough time for a processor access to the Endec internal registers before the Endec&#39;s internal transmit FIFO empties. 
     Port Control Hub Module 
       FIG. 29  shows the Port Control Hub Module (PCHM). The PCHM extends the functionality of the Port Control Module by adding several Fibre Channel Arbitrated Loop Hub ports. This has the affect of leveraging a single switch port over multiple attached devices  705 . All attached devices  705  are logically on a single loop connected to the switch through an internal Endec  700 . The internal Endec is connected on the loop by both a transmit  701  and receive  702  serdes modules. The output of the serdes module is a gigabit serial stream of data. The loop is repeated by commercially available 1.0625 Gbit/sec Channel Repeater/Hub Circuits  703  such as Vitesses&#39; VSC7120. (See, e.g., Vitesse Semiconductor Corporation “1996 Communications Products Data Book”). The repeater/hub circuits contain a monolithic Clock Recovery Unit (CRU), a digital Signal Detect Unit (SDU) and a Port Bypass Circuit (PBC). The repeater/hub circuits allow devices to attach and detach without interrupting the loop. The repeater/hub circuits are connected to a Gigabit Interface Converter (GBIC) module  704  which supports either copper or fiber media via a plug in module. All repeater/hub circuits are controlled by the fabric control processor through a register  705 . This allows the fabric control module to monitor the state of each port and integrate the status with the general switch network management. 
     The integral hub provides many advantages over standalone hubs. These advantages include:
         Leveraging the redundant power supplies and fans usually resident in the fabric   Segmenting loops to allow for increased performance per loop and greater immunity from loop failure   Allowing for hot pluggable hub boards   Leveraging the switches SNMP network management capability for greater control and monitoring of the loop.       

     F. Switch Core 
       FIGS. 2 and 6  shows the Switch Core. The switch core implements a nonblocking N×N matrix switch. The input to the switch core comes from the individual Port Control modules  FIG. 2  numerals  57 ,  69  and  FIG. 9   183 ,  186 . The output from the switch core is wired to the Endec  FIG. 2  numeral  58 ,  FIG. 9  numeral  220  and the Brouter Module  FIG. 2  numeral  76 . The switch core is paths are setup and torn down by the router  FIG. 2  numeral  63 . 
     G. Brouter Module 
       FIG. 2  numeral  55  and  FIG. 7  show the Brouter Module. The Brouter Module receives frames from the switch core  76  and transmits frames to the internal Port Control module  70 . The Brouter Module is responsible for converting Fibre Channel frames to frames of the connected network  68 . The Brouter Module looks to the rest of the fabric like a Port Control module. The Brouter module sends and receives frames which adhere to the Fibre Channel protocol. The frames are converted within the Brouter module to other network frames such as Ethernet, Fast Ethernet, or Gigabit Ethernet and are transmitted out to the network connection  68 . 
     Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it may be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. 
     H. Other Documents 
     ANSI X3.230-1994, “Fibre Channel Physical and Signaling Interface (FC-PH)”. 
     ANSI X3.297-1996, “Fibre Channel Physical and Signaling Interface (FC-PH-2)”. 
     ANSI X3.303-1996, “Fibre Channel Physical and Signaling Interface (FC-PH-3)”. 
     ANSI X3.272-1996, “Fibre Channel Arbitrated Loop (FC-AL)”. 
     ANSI X3T11 Project #1133-D, “Fibre Channel Arbitrated Loop 2 (FC-AL2)”. 
     ANSI X3T11/95-41, “Fibre Channel Fabric Generic Requirements (FC-FG), Rev 3.2” 
     ANSI X3T11 Project 1134-D “(FC-G52)”. 
     ANSI X3T11 Project 959-D “Fibre Channel Switch Topology (FC-SW)”. 
     ANSI X3T11 Project 1235-DT, “Fibre Channel Fabric Loop Attachment (FC-FLA) Rev 2.2” 
     FCA “N_Port to F_Port Interoperability Profile, Rev 1.0”