Abstract:
A method and a communication circuit for directing control of communication signals in a concatenated payload in a communication circuit is disclosed. The method and apparatus includes receiving a multiplex order of the concatenated payload in M communication signals, dividing the M communication signals by three to determine a number Y, and determining the control of the M communication signals by designating the first signal of the M communication signals as a control signal, designating the second signal through a Yth signal of the M communication signals as being controlled by the immediately preceding signal thereto, and designating each Y+1th signal of the M signals through the Mth communication signal as being controlled by a signal Y positions prior thereto. The method and communication circuit includes communication signals that are synchronous transport signals. In an embodiment, the first signal of the M communication signals is a control signal read and write capability for frequency difference buffering using increment/decrement technology.

Description:
This application claims the benefit of provisional Patent Application No. 60/211,815 filed Jun. 15, 2000. 
   CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is related to patent application Ser. No. 09/477,166, filed Jan. 4, 2000, and entitled “METHOD AND APPARATUS FOR A REARRANGEABLY NON-BLOCKING SWITCHING MATRIX,” having A. N. Saleh, D. E. Duschatko and L. B. Quibodeaux as inventors. This application is assigned to Cisco Technology, Inc., the assignee of the present invention, and is hereby incorporated by reference, in its entirety and for all purposes. 
   This application is related to patent application Ser. No. 09/232,395 now U.S. Pat. No. 6,724,757, filed Jan. 15, 1999, and entitled “A CONFIGURABLE NETWORK ROUTER,” having H. M. Zadikian, A. N. Saleh, J. C. Adler, Z. Baghdasarian, and V. Parsi as inventors. This application is assigned to Cisco Technology, Inc., the assignee of the present invention, and is hereby incorporated by reference, in its entirety and for all purposes. 
   This application is related to U.S. Pat. No. 6,735,197 filed Jun. 30, 2000, and entitled “CONCATENATION DETECTION ACROSS MULTIPLE CHIPS,” having Douglas E. Duschatko, Lane Byron Quibodeaux, Robert A. Hall, Andrew J. Thurston as inventors. This application is assigned to Cisco Technology, Inc., the assignee of the present invention, and is hereby incorporated by reference, in its entirety and for all purposes. 
   This application is related to patent application Ser. No. 09/607,912 filed Jun. 30, 2000, and entitled “PATH AIS INSERTION FOR CONCATENATED PAYLOADS ACROSS MULTIPLE PROCESSORS,” having Douglas E. Duschatko, Lane Byron Quibodeaux, Robert A. Hall, Andrew J. Thurston as inventors. This application is assigned to Cisco Technology, Inc., the assignee of the present invention, and is hereby incorporated by reference, in its entirety and for all purposes. 
   This application is related to patent application Ser. No. 09/608,461 filed Jun. 30, 2000, and entitled “CHANNEL ORDERING FOR COMMUNICATION SIGNALS SPLIT FOR MATRIX SWITCHING,” having Douglas E. Duschatko, Lane Byron Quibodeaux, Robert A. Hall, Andrew J. Thurston as inventors. This application is assigned to Cisco Technology, Inc., the assignee of the present invention, and is hereby incorporated by reference, in its entirety and for all purposes. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to data communications, and, more particularly, efficiency in determining control signals in data communication circuits that include concatenated payloads. 
   2. Description of the Related Art 
   A data communications network is the interconnection of two or more communicating entities (i.e., data sources and/or sinks) over one or more data links. A data communications network allows communication between multiple communicating entities over one or more data communications links. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, these demands are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation, and capacity for future growth. 
   The synchronous optical network (SONET) protocol is among those protocols designed to employ an optical infrastructure and is widely employed in voice and data communications networks. SONET is a physical transmission vehicle capable of transmission speeds in the multi-gigabit range, and is defined by a set of electrical as well as optical standards. 
   In some networks, network nodes store data which they use for proper operation. In SONET, data between adjacent nodes are transmitted in modules called STS&#39;s (synchronous transport signals). Each STS is transmitted on a link at regular time intervals (for example, 125 microseconds). See GR-253 (GR-253 : Synchronous Optical Network  ( SONET )  Transport Systems , Common Generic Criteria, Issue 2 [Bellcore, December 1995] (hereinafter referred to as GR-253 Specification) incorporated herein by reference for all purposes. An STS-1 is a Synchronous Transport Signal-level 1 is the basic module in SONET and is defined as a specific sequence of 810 bytes (6480 bits) including overhead bytes and an envelope capacity for transporting payloads. In general, the higher-level signals, the STS-N signals, are lower-level modules that are multiplexed together and converted to an OC-N or STS-N signal. An STS-N frame is a sequence of N×810 bytes wherein N is a predetermined number. An STS-N is formed by byte-interleaving of STS-1 and STS-M modules, wherein M is less than N. 
   In some systems, such as certain ISDN and ATM systems, multiple STS-1 payloads are transported as super rate payloads. To accommodate such a payload an STS-Nc module is formed by linking N constituent STS-1s together in fixed phase alignment. The payload is then mapped into a single STS-Nc Synchronous Payload Envelope (SPE) for transport. Network equipment supporting the multiplexing, switching or transport of STS-Nc SPES treat an STS-Nc SPE as a single entity. When an STS-Nc SPE is treated as a single entity, concatenation indicators are present in the second through the Nth STS payload pointers which show that the STS-1s in the STS-Nc are linked together. 
   STS-Ncs can exist in many different combinations in an STS-M payload. One problem with concatenated STS signals includes connecting an combination of STS-Ncs within an STS-M payload in a manner that is a working combination of STS-Ncs. 
   Furthermore, an efficient method of connecting multiple STS-1s in an STS-M payload is needed. 
   SUMMARY OF THE INVENTION 
   Accordingly, a method for connecting STS-1s in an STS-M payload provides a fixed formula for connecting any permissible combination of STS-Nc in a multiple STS payload. 
   According to an embodiment of the invention, a method and apparatus for hookup STS-1s for routing of control signals includes designating a step size, Y as an STS step size, designating a size of a total STS payload as M, and designating the Y STS step size as M divided by three. Further the method includes, for a first set of Y channels other than a first channel, in the multiple STS payload, designating a previous channel as a control channel, and for a second set of Y channels, designating control signals for each channel within the second set of Y channels as a channel Y positions before the given channel. 
   The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
       FIG. 1A  is a block diagram of an exemplary router. 
       FIG. 1B  is a block diagram of a network including a number of the routers of FIG.  1 A. 
       FIG. 2  is a block diagram of the signal paths and functional blocks of the router of FIG.  1 A. 
       FIG. 3  is a block diagram of the control paths of the router of FIG.  1 A. 
       FIG. 4  illustrates the major components of one of the line cards. 
       FIG. 5  illustrates a view of a switching matrix that includes clock/data recovery units and connections to the line cards. 
       FIG. 6  illustrates a standard frame of the synchronous optical network protocol. 
       FIGS. 7A and 7B  illustrate interleaved STS-N possibilities for concatenated payloads. 
   

   The use of the same reference symbols in different drawings indicates identical items unless otherwise indicated. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The following is intended to provide a detailed description of an example of the invention and should not be taken to be limiting of the invention itself. Rather, any number of variations may fall within the scope of the invention which is defined in the claims following the description. 
   In addition, the division of the detailed description into separate sections is merely done as an aid to understanding and is in no way intended to be limiting. 
   An Exemplary Network Element 
     FIG. 1A  illustrates a router  100 . Router  100  includes an input/output section  110 , a node controller  120 , and a switching matrix  130 . Node controller  120  contains, for example, real time software and intelligent routing protocols (not shown). Router  100  supports interfaces including, but not limited to, optical signal interfaces (e.g., SONET), a user interface module  150 , and a management system  160 . Internal input signals  170  and internal output signals  180  may be electrical or optical in nature. 
     FIG. 1B  illustrates a network  190  that includes a number of nodes, network nodes  195 ( 1 )-(N). One or more of network nodes  195 ( 1 )-(N) can be a router such as router  100 . Network  190  can thus support the automatic provisioning, testing, restoration, and termination of virtual paths (exemplified by a virtual path  191 ) over a physical path (exemplified by a physical path  192 ) from one of network nodes  195 ( 1 )-(N) to another of network nodes  195 ( 1 )-(N). 
   In one embodiment, there are at least three types of processors in a router  100 . The lowest level, level-3, resides on the line card and is responsible for all real time aspects of the processing of the physical protocol (e.g., SONET). In a SONET implementation, every level-3 processor is responsible for a single optical signal (e.g., an OC48 signal) and, via a protocol processor, performs all required SONET/SDH section and line termination functions. The fast response time required from the level-3 processor makes a firmware implementation preferable. The firmware, which may be written in the “C” or “C++” programming languages, assembler, or other programming language, is preferably optimized for low latency and resource efficiency. Higher-level processing is implemented on a separate module, the shelf processor module, which is shared by several line cards. 
   The second level of processors, level-2, reside on a shelf and main matrix processor modules. The software on the shelf processor module is responsible for managing and controlling line cards. Only half the line cards supported are active at any one time in order to support 1+1 protection. A level-2 processor deals with tasks that require a reasonable response time (for example, on the order of milliseconds), but have no direct impact on the data path. In other words, missed events, such as hardware interrupts, do not result in bit errors. Some of the functions handled by the shelf processor include the periodic collection of maintenance data from the line cards, receiving and processing periodic keep-alive messages from those cards, shelf startup and configuration, proxy management, and other related functions. 
   The third processor level, level-1, resides on a system processor module and provides system-wide management and control services. In one embodiment, there are preferably two fully synchronous copies of the level-1 processor in the system, both of which are simultaneously active and, through a dedicated and redundant high-speed link, keep their run-time and stored databases fully synchronized. One of the two processors is designated the master and is responsible for all level-1 processing. An update message is sent to the second processor whenever a change is made to the database and before that change is effected. A periodic keep-alive mechanism allows either copy of the system controller to detect failures on the other copy. 
   Router  100  provides yet another type of processor, referred to herein as a route processor. Such a processor is dedicated to the path/route discovery and restoration functions. The route processor is responsible for receiving failure indications from the line cards, calculating a new route for failed connections, and sending reconfiguration requests to all affected nodes, including its own. 
   Hardware Architecture 
   In one embodiment, router  100  can be used, for example, as SONET/SDH line terminating equipment (LTE) capable of terminating the Section and Line overheads of received OC-48 signals, and cross-connects those signals according to provisioned input-output mappings. Some of the terminated signals can optionally be protected using any of the common protection schemes (1+1, 1:1, and 1:N). Overhead processing and generation is performed on the line card by a protocol processor. This protocol processor handles all aspects of the SONET protocol, including framing, insertion and extraction of embedded data channels, error checking, AIS detection, pointer processing, clock recovery, multiplexing/duplexing, and similar duties. 
   Signal Path 
     FIG. 2  is a block diagram of signal paths  200  within router  100 . The primary signal paths in router  100  include one or more groups exemplified by groups  210 ( 1 )-(N), group matrices  212 ( 1 )-(N), and a main matrix  214 . As depicted in  FIG. 1A , groups  210 ( 1 )-(N), and group matrices  212 ( 1 )-(N) are shown as having receive and transmit sections. Groups  210 ( 1 )-(N) each include line cards  220 ( 1 , 1 )-( 1 ,N), through line cards  220 (N, 1 )-(N,N). Signals from line cards  220 ( 1 , 1 )-(N,N) are sent to the corresponding group matrix. In one embodiment, two sets of the group matrix cards, group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N) are employed. Main matrix  214  is also mirrored in one embodiment by a redundant copy, a backup main matrix  218 , which together form switching matrix  130 . As shown in  FIG. 2 , the redundancy for group matrices  212 ( 1 )-(N) (i.e., group matrices  216 ( 1 )-(N)), is also provided on the transmit side. 
   It will be noted that the variable identifier “N” is used in several instances in  FIG. 2  (and subsequent use of other variables, such as “m,” “x,” “k,” and others) to more simply designate the final element (e.g., group matrix  212 (N), line card  220 (N,N), and so on) of a series of related or similar elements (e.g., group matrices  212 ( 1 )-(N), line cards  220 ( 1 , 1 )-(N,N), and so on). The repeated use of such variable identifiers is not meant to imply a correlation between the sizes of such series of elements. The use of such variable identifiers does not require that each series of elements has the same number of elements as another series delimited by the same variable identifier. Rather, in each instance of use, the variable identified by “N” (or “m,” “x,” “k,” and others) may hold the same or a different value than other instances of the same variable identifier. For example, group matrix  212 (N) may be the tenth group matrix in a series of group matrices, whereas line card  220 (N,N) may be the forty-eighth line card in a series of line cards. 
   Using signal paths  200  as an example, data enters the system at one of line cards  220 ( 1 , 1 )-(N,N). It is at this point, in a SONET-based system, that the Section and Line overheads are processed and stripped off by a protocol processor (not shown). The extracted SONET/SDH payload envelope is then synchronized with the system clock and sent to two different copies of a local matrix, depicted as group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N) in FIG.  1 A. In one embodiment, group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N) are used mainly as 2:1 reduction stages that select one of two optical signals and pass the selected optical signal to switching matrix  130 . This allows the implementation of a variety of protection schemes (including 1:N, or 0:1) without having to use any additional ports on main matrix  214 . All protect signals are terminated at group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N). In order to maximize bandwidth, it is preferable that only active signals be passed through to switching matrix  130 . 
   In one embodiment, switching matrix  130  is an errorless, rearrangeably non-blocking switching network. In one embodiment, switching matrix  130  is a 256×256 switching network that consists of three columns and 16 rows of 16×16 switching elements that allow any of their inputs to be connected to any of their outputs. A single copy of the matrix may be housed, for example, in a single rack that contains three shelves, one for each column (or stage) of the matrix. Each one of such shelves contains cards housing the 16 switching elements in each stage. The switching element itself includes, for example, a 16×16 crosspoint switch, with optical transceivers, and a microcontroller for controlling the crosspoint switch and providing operational feedback to the level-2 processor. Communications between the two processors may be carried, for example, over an Ethernet connection. The level-2 processor in turn communicates with the level-1 and route processors. 
   The switching elements in each matrix copy of the exemplary embodiment may be connected using fiber-optic cables, for example. While copper cabling may also be employed, such an option may not offer the speed and number of connections provided by an optical arrangement. After passing through the stages of switching matrix  130 , an optical signal may be routed to an I/O shelf that (optionally) splits the optical signal into two signals. One of the signals is sent to an active line card, while the other, when available, is sent to a backup card. 
   Line cards  220 ( 1 , 1 )-(N,N) receive optical signals from group matrices  212 ( 1 )-(N) and  216  ( 1 )-(N) which are in turn connected to two separate copies of the main matrix. Line cards  220 ( 1 , 1 )-(N,N) monitor both signals for errors and, after a user-defined integration period, switch to the backup signal if that signal exhibits better bit error rate (BER) performance than the prior active signal. This scheme, referred to herein as 1-plus-1, allows line cards  220 ( 1 , 1 )-(N,N) to select between the two copies of the group matrix without any level-1 or level-2 CPU intervention. This helps to ensure that such a switch can be made in 50 ms or less (per Bellcore&#39;s recommendations in GR-253 (GR-253:  Synchronous Optical Network  ( SONET )  Transport Systems , Common Generic Criteria, Issue 2 [Bellcore, December 1995], included herein by reference, in its entirety and for all purposes)). The selected signal is then processed by the transmit section of the protocol processor, which inserts all required transport overhead bytes into the outgoing stream. 
   Regarding the signals described herein, both above and subsequently, those skilled in the art will recognize that a signal may be directly transmitted from a first logic block to a second logic block, or a signal may be modified (e.g., amplified, attenuated, delayed, latched, buffered, inverted, filtered or otherwise converted, etc.) between the logic blocks. Although the signals of the embodiments described herein are characterized as transmitted from one block to the next, other embodiments may include modified signals in place of such directly transmitted signals with the informational and/or functional aspect of the signal being transmitted between blocks. To some extent, a signal input at a second logic block may be conceptualized as a second signal derived from a first signal output from a first logic block due to physical limitations of the circuitry involved (e.g., there will inevitably be some attenuation and delay). Therefore, as used herein, a second signal derived from a first signal includes the first signal or any modifications to the first signal, whether due to circuit limitations or due to passage through other circuit elements which do not substantively change the informational and/or final functional aspect of the first signal. 
   Control Path 
     FIG. 3  illustrates a control path  300  of a router, such as router  100 . Control path  300  includes all non-payload-related flows within the system and the hardware and software necessary to the control of the signal paths illustrated in FIG.  2 . All major control flows are carried over an internal local area network (LAN), which is, for example, a collection of switched Ethernet segments. The structure of the internal LAN is hierarchical and can be created using a mixture of 10 Mbps and 100 Mbps Ethernet segments, for example. Higher-speed segments (e.g., gigabit Ethernet) can be used as well. 
   Groups 
   At the bottom of the hierarchy is what is referred to herein as a group matrix, or a Group Ethernet Repeater in a system using Ethernet communications, and depicted in  FIG. 3  as group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N). Each one of group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N), also referred to herein as a hub, a repeater, or concentrator, is a physical layer device and preferably supports a star network topology, such as the IEEE 802.3 10BASE-T networking standard. The redundant connections from line cards  220 ( 1 , 1 )-(N,N) in each of groups  310 ( 1 )-(N) are connected to two repeaters that reside on two separate copies of the group matrix module. Preferably, each one of line cards  220 ( 1 , 1 )-(N,N) supports two network ports (e.g., 10BASE-T Ethernet ports). The two sets of four signals from each port pass through a relay that selects one of them for connection to the LAN for purposes of redundancy. Groups  310 ( 1 )-(N) represent the first layer of the control bus hierarchy. Group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N) are each controlled by a shelf processor (not shown, for the sake of clarity) and communicate with one of the shelf switches described below via LAN connections. 
   Shelf Ethernet Switch 
     FIG. 3  also illustrates certain features of router  100  pertaining to the relationship between shelf switches  320 ( 1 )-(N) and  321 ( 1 )-(N), and groups  310 ( 1 )-(N). Groups  310 ( 1 )-(N) are again shown, with regard to the control functions thereof. In this depiction of groups  310 ( 1 )-(N), line cards  220 ( 1 )-(N,N) are shown as being attached to networking devices, indicated here as group matrices. Group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N) may be, for example, multi-port Ethernet hubs running at 10 Mbps. Each of line cards  220 ( 1 )-(N,N) feed signals into two of group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N). For example, line card  220 ( 1 , 1 ) feeds received information to group matrices  212 ( 1 ) and  216 ( 1 ). Group matrices  212 ( 1 )-(N) and  216 ( 1 )-(N) each feed a signal into shelf switches  320 ( 1 )-(N) and  321 ( 1 )-(N) of FIG.  2 . Shelf switches  320 ( 1 )-(N) and  321 ( 1 )-(N) are each controlled by a shelf processor (not shown for the sake of clarity) and communicate with one of the system switches (not shown, for the sake of clarity). 
   Shelf switches  320 ( 1 )-(N) and  321 ( 1 )-(N) are the next higher level of the control hierarchy in router  100 , and are located on the shelf processor module (exemplified by line racks ( 330 ( 1 )-(N)). Each copy of shelf switches  320  ( 1 )-(N) and  321 ( 1 )-(N) interconnects six connections from the three groups in each shelf, another connection from the shelf processor, and one connection from system switch  340  (and  341 ). Shelf switches  320 ( 1 )-(N) and  321 ( 1 )-(N) can be implemented, for example, using an 8-port Ethernet configured to handle 10 Mbps Ethernet traffic and a single-port, dual-rate switch (e.g., 10 Mbps/100 Mbps Ethernet). 
   System Switch 
   The next level of the hierarchy is the system switch, of which there are two copies in each router. These are shown as system switches  340  and  341  in FIG.  3 . This fully redundant scheme prevents failures on one switch from taking down the entire control bus. In one embodiment, a system switch manages connections from the following sources:
         1. High-speed connection(s) from shelf switches  320 ( 1 )-(N) and  321 ( 1 )-(N);   2. High-speed connection(s) to higher-level processors (e.g., redundant level-1 processors  350  and  351 , and redundant route processors  360  and  361 ); and   3. High-speed connection(s) to matrix shelf processors  370 ( 1 )-(N) and  371 ( 1 )-(N) which, in turn, control matrix cards  380 ( 1 , 1 )-( 1 ,N)), located in main matrix racks  390 ( 1 )-(N).       

   It will be noted that main matrix  214  includes matrix cards  380 ( 1 , 1 )-( 1 ,N), and that, more generally, main matrices  214  and  218  are included matrix racks  390 ( 1 )-(N). 
   System switches  340  and  341  are located in a management bay. As noted, the fully redundant switches manage connections from various router elements, such as I/O and matrix bays, level-1 processors, and route processors. Each of level-1 processors  350  and  351  and route processors  360  and  361  is preferably connected to system switches  340  and  341  using 100 Mbps Ethernet connections in a configuration that creates an expandable, efficient, and fully redundant control bus. 
   Physical Configurations and Modules 
   System Modules 
   Line Card 
     FIG. 4  illustrates the major components of one of line cards  220 ( 1 , 1 )-(N,N), exemplified in  FIG. 4  by a line card  400 . A line card integrates all the necessary hardware and software functions to properly terminate the physical layer. In a SONET implementation, a line card terminates the transport overhead (Section+Line) of a full duplex OC-48 signal. Other components on this card provide a redundant optical connection to the switch matrix, and a communication channel to other modules in the system. 
   Line card  400  receives optical signals from other network elements via a line-side optical receiver  405  and from the local router&#39;s system via a system-side optical receiver  406 . Each of these receivers implements an optical-to-electrical (O/E) conversion function. Line card  400  transmits optical signals to other network elements using a line-side optical transmitter  410  and to the group matrices using a system-side optical transmitter  411 . Each of these transmitters implements an electrical-to-optical (E/O) conversion function. It will be noted that line-side refers to the side of the line card coupled to other network elements and system-side refers to the side of the line card coupled to the group matrices. 
   Line-side optical receiver  405  is coupled to a protocol processor  420  which performs clock recovery multiplexing, demultiplexing, and SONET STE/LTE processing in both directions. Similarly, system-side optical receiver  406  is also coupled to protocol processor  420  to allow protocol processor  420  to receive optical signals. The processed electrical signals from protocol processor  420  are coupled to the transmitters  410  and  411 . The clock recovery functions are combined with demultiplexers and multiplexers to support reception and transmission of the optical data, respectively. The multiplexers serialize output data generated in protocol processor  420  by performing parallel-to-serial conversion on the parallel data. In contrast, de-multiplexers are used in protocol processor  420  to perform serial-to-parallel conversion on received data. 
   In order to add protection channels, line-side optical transmitter  410  is also coupled to a 1:2 broadcast unit  435 . To receive such optical signals, optical receiver  406  is also coupled to a 2:1 selector  436  in order to select the working channel before the optical signals leave the shelf and thus prevent the standby channel (also referred to herein as the protect channel) from using any bandwidth on switching matrix  130 . 
   Protocol processor  420  is coupled to a bus  445 . Protocol processor  420  interfaces the line card to two copies of the matrix in a 1+1 physical protocol. In a SONET implementation, protocol processor  420  provides both STE/LTE processing according to published industry standards. Also coupled to bus  445  are a memory  460  and a CPU  470 . Memory  460  should be fast enough for efficient operation of CPU  470 . 
   CPU  470  communicates with other of line cards  220 ( 1 , 1 )-(N,N) over a control bus (not shown) using a transceiver  480  that is coupled to CPU  470 . Transceiver  480 , is coupled to a transformer  485  which is coupled to a switch  490 . Switch  490  is coupled to the control bus. Switch  490  implements a 1:1 protection scheme for transceiver  480  and couples CPU  470  to two independent ports on the backplane (not shown). Each of the two ports connects to one copy of the hub of the group matrix. This allows the software on the line card to switch to the backup link when the software detects failures on the active link. 
   Preferably, CPU  470  includes numerous integrated peripherals including embedded SCC channels (e.g., in-band communications) and an Ethernet controller (for example, to support communications with other system modules). In one embodiment, CPU  470  provides an onboard communications processor module (not shown) that handles time-critical aspects of the protocols supported. 
   Hub 
   One or more hubs are also provided to support communication between the group matrices and system switches in router  100 . In an Ethernet communications environment, the hub&#39;s functions are carried out primarily by repeater interface controllers (RICs). Each RIC integrates the functions of a repeater, clock and data recovery unit (CDR), Manchester encoder/decoder, and transceiver. Each RIC has a set of registers that convey status information and allow a number of configuration options to be specified by the user using, for example, a microcontroller. 
   Shelf Processor Module 
   A shelf processor module provides, among other elements, a shelf processor and switch that interconnect the LAN segments from the groups and the shelf processor to a port on the shelf switch. 
   The shelf processor is responsible for the overall operation, management, and control of the shelf. 
   A network switch interconnects the lower speed inter-processor communication network segments in each shelf. In one embodiment, the network switch provides support for 10 Mbps and 100 Mbps segments. 
   In certain embodiments, the shelf processor is able to connect to two separate Ethernet segments. This can implement a 1:1 protection scheme that allows the shelf processor to recover from failures on the active segment by simply switching to the other segment. 
   System Switch 
   One embodiment of a system capable of interconnecting network segments in a switched configuration allows communications between shelf switches, higher-level (e.g., level-1) processors, and shelf-processors. In an Ethernet-based system, the system switch supports both 10 Mbps and 100 Mbps connections. The segments come from the shelf switching in the I/O shelf and the matrix switches, among others, and allow these elements to communicate. 
   Management Bay 
   The management bay can house, for example, the following modules:
         1. Level-1processors, or system controllers, and their associated storage devices;   2. Route processors;   3. Optional group and WAN cards,   4. System Ethernet switches; and   5. Synchronization modules.       

   All of the above modules are fully redundant and communicate with the rest of router  100  over redundant control buses. The placement of individual modules within the rack is not addressed in this document, since there are no architectural preferences, or restrictions, on such choices. 
   Level-1 Processor/System Controller 
   A system controller (also referred to herein as a level-1 processor) provides overall control of router  100 . The system controller also communicates with the system switches. The system controller includes a bus such as an all-purpose bus (APB), which in turn provides access to several bus and communications controllers. Among the controllers interfaced to the APB is a bus bridge, a peripheral interface, and an I/O interface. The I/O interface may provide functionality such as 10 Mbps/100 Mbps Ethernet communications. The I/O interface also supports peripherals such as keyboards, mice, floppy drives, parallel ports, serial ports, and the like. The bus bridge allows communications between the system controller&#39;s processor and other devices. The peripheral interface allows communications with peripherals such as hard disks. The system controller performs various functions, such as communicating with the route processor(s) to determine how the matrix should be configured, managing the router&#39;s resources, and similar duties. 
   APB may also be connected to a dual-channel serial communication controller (SCC), for example, which can be used to communicate with one or more remote Operations Systems (OS) using, for example, the X.25 protocol. For more OS links and higher link speeds, the user can optionally install one or more WAN Interface Modules in the management bay. Such modules, which preferably handle all real-time aspects of the OS link, including layer-2 of the OSI stack, communicate with the system controller. 
   Main Matrix Bay 
   Switching matrix  130  is based on a rearrangeably non-blocking switching matrix and can consist, for example, of switch nodes arranged in a staged array. For example, switching matrix  130  configured as a 256×256 switching matrix consists of 48 nodes arranged in an array of 16 rows by 3 columns, with each column containing one stage. All 48 nodes in the switch matrix are substantially similar. Each node is preferably a crossbar device, such as a 16×16 crossbar device that allows any of its 16 inputs to be connected to any of its 16 outputs, regardless of the crossbar&#39;s current state. 
     FIG. 5  illustrates a simplified view of switching matrix  130 , including connections to the line cards. The depiction of switching matrix  130  in  FIG. 5  shows certain other details, such as clock/data recovery units (CDRs)  500 ( 1 , 1 )-( 6 , 256 ) and line cards  510 ( 1 , 1 )-( 16 , 16 ). A CDR recovers clock and data information from a serial bitstream by recovering the clocking signal from the incoming bitstream (e.g., using a phase-locked loop (PLL)), and then recovering the data using the clock thus recovered. 
   It will be noted that line cards  510 ( 1 , 1 )-( 16 , 16 ) correspond loosely to line cards  220 ( 1 , 1 )-(N,N), as depicted in FIG.  2 . It will also be noted that line cards  510 ( 1 , 1 )-( 16 , 16 ) are each shown as being divided into a receive section and a transmit section as shown in  FIG. 5 , again in a fashion similar to that depicted in FIG.  2 . Also depicted in  FIG. 5  are switch nodes  520 ( 1 , 1 )-( 16 , 3 ) and a switching matrix control circuit  530 . More generically, the control function represented by switching matrix control circuitry  530  is depicted in  FIG. 3  as matrix shelf processors  370 ( 1 )-(N) and  371 ( 1 )-(N). As previously noted, switch nodes  520 ( 1 , 1 )-( 16 , 3 ) and their related CDRs are divided into three stages, which are depicted in  FIG. 5  as matrix first stage  540 , matrix center stage  550 , and matrix third stage  560 . It will be noted that matrix first stage  540 , matrix center stage  550 , and matrix third stage  560  correspond to the matrix stages represented by switch nodes  1100 ( 1 , 1 )-( 16 , 1 ), switch nodes  1100 ( 1 , 2 )-( 16 , 2 ), and switch nodes  1100 ( 1 , 3 )-( 16 , 3 ). It will also be noted that the transmit side of line cards  510 ( 1 , 1 )-( 16 , 16 ) each include CDR functionality. 
   SONET Frame 
     FIG. 6  illustrates a standard frame of the synchronous optical network (SONET) protocol, exemplified here by a SONET frame  1500 . SONET frame  1500  is divided horizontally into ninety columns and is divided vertically into nine rows. The first three columns of SONET frame  1500  contain overhead bytes used for framing, communications, and other purposes. The remaining 87 columns contain data and are collectively referred to as payload. The overhead bytes include an A 1  byte  1502 , an A 2  byte  1504 , a J 0 /Z 0  byte  1506 , a B 1  byte  1510 , an E 1  byte  1512 , an F 1  byte  1514 , a D 1  byte  1520 , a D 2  byte  1522 , a D 3  byte  1524 , an H 1  byte  1530 , an H 2  byte  1532 , an H 3  byte  1534 , an H 4  byte  1536 , a B 2  byte  1540 , a K 1  byte  1542 , a K 2  byte  1544 , a D 4  byte  1550 , a D 5  byte  1551 , a D 6  byte  1552 , a D 7  byte  1553 , a D 8  byte  1554 , a D 9  byte  1555 , a D 10  byte  1556 , a D 11  byte  1557 , a D 12  byte  1558 , an S 1 /Z 1  byte  1570 , an M 1 /Z 1  byte  1572 , and an E 2  byte  1574 . Also included in SONET frame  1500  is payload data, represented here by payload bytes  1590 - 1598 . It will be noted that each of payload bytes  1590 - 1598  includes 87*48 bytes of data for an OC-48 SONET frame (except payload bytes  1593 , which includes 86*48 bytes of data (due to the existence of H 4  byte  1536 )). 
   Concatenated Payloads 
   For a SONET system to function as an OC-192 system, data payloads may be concatenated for transmission. Accordingly, integrated circuits, such as ASICs, are coupled to transmit the data, for example, through a router, such as a wavelength router. 
   Referring now to Table 1, below, a typical STS SPE payload pointer for a SONET system is shown in bits  7  through  16 . The table shows bits seven through sixteen are designated either “I” for an increment or “D” for decrement. These bits are typically designated as the pointer value to indicate the offset between the pointer word and the first byte of the STS SPE. In a concatenated payload, in which more than one STS-1 is used to carry an SPE, these bits are used to carry a concatenation indicator in the second through the nth STS-1. Thus, the concatenation detection requires the detection of the pointer word value to serially pass from the nth STS-1 to the first STS-1. 
   
     
       
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
                 
                 
               Byte 0 
             
             
               H1 byte 
               H2 byte 
               H3 byte 
               of payload 
             
             
                 
             
           
           
             
               1 2 3 4 5 6 7 8 
               9 10 11 12 13 14 15 16 
               . . . . 
               . . . 
             
             
               N|N|N|N|—|—|I|D 
               I|D|I|D|I|D|I|D| 
               Negative 
               Positive 
             
             
                 
                 
               Stuff Byte 
               Stuff Byte 
             
             
                 
             
           
        
       
     
   
   According to the GR-253 specification, an STS-N module can be formed by byte interleaving lower-level modules, such as STS-1s and STS-Ms. Those STS-Ns that are formed by byte-interleaving lower level modules must follow SONET rules dictating that before byte-interleaving to form an STS-N, the transport overhead byte positions of all constituent STS-1s and STS-Ms must be frame aligned. According to the GR-253 Specification, alignment of the STS-1s and STS-Ms is accomplished by adjusting the STS Payload Pointers to reflect the new relative positions of the STS SPEs. An example given in the GR-253 specification is to logically interleave any STS-1 inputs in sets of three consecutive STS-1s to form an STS-3 module, and then interleave those STS-3 modules and other STS-M inputs to form an STS-N. Another interleaving includes interleaving multiple STS-1 inputs to form an STS-N. However the STS is formed, the output byte sequence must follow the sequence dictated in the GR-253 Specification. 
   Referring to  FIGS. 7A and 7B , possible byte interleaving is demonstrated in order  700 ,  710  and  750 . As shown, the order  700  provides an interleaving of four STS-3s, order  710  provides an ordering of twelve STS-1s, and order  750  provides an ordering of three STS-3s, an STS-3c, an STS-12C, and a block  760  indicating an unspecified STS-1 and STS-Mc combination equivalent to 24 STS-1s. 
   When an SPE payload is concatenated, there are multiple payloads with one set of path overhead. At issue is how to hook up the STS-Ns, whether STS-1, STS-3 or STS-variable, such that the first STS-N has control over subsequent STS-Ns. In a concatenated payload, the subsequent STS-Ns are not independent. More specifically, the subsequent STS-Ns do not have independent read and write capabilities, but rely on the first STS-N to provide control. Furthermore, the control signals must reach the first STS-N within a predetermined time. 
   The control of the concatenated payload within an embodiment of the invention minimizes wire, routing and logic resources. Further, the combinations created are supported without additional logic required, thereby providing an efficient solution for concatenated payload control determinations. The control signals include read and write signals that transmit increment and decrement signals as well as frequency difference buffering. As those skilled in the communication art will appreciate, control signals are important for maintaining appropriate frequency levels A typical SONET communication system, for example, employs frequency difference buffering to ensure appropriate levels. 
   According to an embodiment of the invention, a formula for hookup of STS-1s for routing of control signals includes designating a step size, Y as an STS step size, designating a size of a total STS payload as M, and designating the Y STS step size as M divided by three. The M will be evenly divisible by three in a according to the GR-253 Specification. Further the method includes, for a first set of Y channels other than a first channel, in the multiple STS payload, designating a previous channel as a control channel, and for a second set of Y channels, designating control signals for each channel within the second set of Y channels as a channel Y positions before the given channel. 
   More particularly, Table 2, below, illustrates an application of the formula to an OC-48 concatenated combination of STS-1s in an STS-M. As shown, the formula applies to designate the control channel, or control STS-1. As shown, those STS-1s that do not have a designated control channel by applying the Y/3 formula are designated to follow the control of the STS-1 immediately preceding the STS-1. 
   
     
       
             
           
         
             
               TABLE 2 
             
             
                 
             
           
           
             
               (STS-1, CONTROL): (1,1) (2,1) (3,2) (4,3) (5,4) (6,5) (7,6) (8,7) (9,8) 
             
             
               (10,9) (11,10) (12,11) (13,12) (14,13) (15,14) (16,15) (17,1) (18,2) (19,3) 
             
             
               (20,4) (21,5) (22,6) (23,7) (24,8) (25,9) (26,10) (27,11) (28,12) (29,13) 
             
             
               (30,14) (31,15) (32,16) (33,17) (34,18) (35,19) (36,20) (37,21) (38,22) 
             
             
               (39,23) (40,24) (41,25) (42,26) (43,27) (44,28) (45,29) (46,30) (47,31) 
             
             
               (48,32) 
             
             
                 
             
           
        
       
     
   
   As shown in Table 2, the formula applies to the connection of STS-1s in an STS-M to create an efficient routing of control signals for all types of concatenation combinations, including but not limited to the concatenation of mixing shown in  FIGS. 7A and 7B . As shown in Table 2, applying the formula, the STS step size for hookup is first determined. In Table 2, the STS size of the full concatenated payload is 48. Accordingly, applying Y=M/3, the STS step size is determined by applying M=48, therefore, Y=48/3, which produces a step size of 16. Next, applying the rule that the first STS-1 channel controls itself, the rule applies to each subsequent STS1 to provide a control from the next previous STS-1, until the step size 16 is complete. Thereafter, the control for the next STS-1, is the next STS-1, i.e., number 17, is designated as the first STS-1, and thereafter, each following STS-1 receives control from the STS-1 Y positions prior thereto. 
   While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from this invention and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention.