Patent Publication Number: US-6335938-B1

Title: Multiport communication switch having gigaport and expansion ports sharing the same time slot in internal rules checker

Description:
FIELD OF THE INVENTION 
     This invention relates to data communication systems, and more particularly, to a dynamic mechanism for distributing time slots assigned to expansion and high-speed ports of a communication switch. 
     BACKGROUND ART 
     A multiport communication switch may be provided in a data communication network to enable data communication between multiple network stations connected to various ports of the switch. To support cascade operations of multiple communication switches, each of them may contain an expansion port that provides data transferring between the switches. A logical connection may be created between receive ports and transmit ports of the switch to forward received frames to appropriate destinations. Based on frame headers, a frame forwarding arrangement selectively transfers received frames to a destination station. 
     All receive ports including the expansion port should be assigned with a time slot, during which data associated with frames received from the ports are transferred to logic circuitry that determines a destination station. To increase the efficiency of the bandwidth utilization it would be convenient to dynamically distribute time slots assigned to the expansion port and a high-speed receive port, such as a gigabit port for receiving data from a gigabit physical layer device, depending on relative data traffic at the ports. 
     DISCLOSURE OF THE INVENTION 
     The invention provides a novel method of data processing in a multiport data switching system having a decision making engine for controlling data forwarding between receive ports and at least one transmit port. The receive ports include an expansion port for receiving data packets from another switching system, and a high-speed port for receiving data packets at a rate higher than data rates at regular receive ports. In accordance with the method of the present invention, data blocks representing received data packets are placed in a plurality of data queues corresponding to the plurality of the receive ports. The data queues are transferred in successive time slots to logic circuitry for determining at least one transmit port. The time slots assigned to each of the plurality of receive ports includes expansion port time slots assigned to the expansion port and high-speed time slots assigned to the high-speed port. The expansion and high-speed time slots are dynamically distributed between the expansion and high-speed ports in accordance with relative data traffic at the ports. 
     In a preferred embodiment of the invention, requests for expansion time slots and high-speed time slots in each cycle of data queue transferring are counted. Non-requested high-speed port time slots may be allocated to the expansion port, if the number of the high-speed port requests is less than the number of the high-speed time slots. Similarly, non-requested expansion port time slots may be allocated to the high-speed port, if the number of the expansion port requests is less than the number of the expansion port time slots. 
     In accordance with the present invention, the decision making engine includes a plurality of queuing devices corresponding to the plurality of the receive ports for queuing data blocks representing the data packets received by the corresponding receive ports. To identify at least one selected transmit port for each data packet, logic circuitry receives the data blocks from the plurality of queuing devices in successive time slots assigned to each of the plurality of receive ports. A scheduler interacts with the plurality of queuing devices for dynamically distributing time slots assigned to the expansion and high-speed ports between these ports in accordance with relative data traffic at the ports. 
     Various objects and features of the present invention will become more readily apparent to those skilled in the art from the following description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a packet switched network including a multiple port switch according to an embodiment of the present invention. 
     FIG. 2 is a block diagram of the multiple port switch of FIG.  1 . 
     FIG. 3 is a block diagram illustrating in detail the switching subsystem of FIG.  2 . 
     FIG. 4 is a block diagram of an internal rules checker. 
     FIG. 5 is a diagram illustrating time slots assigned to various ports of the switch. 
     FIG. 6 is a flow chart illustrating dynamic distribution of the time slots assigned to gigabit and expansion ports between these ports in accordance with the present invention. 
    
    
     BEST MODE CARRYING OUT THE INVENTION 
     FIG. 1 is a block diagram of an exemplary system in which the present invention may be advantageously employed. The exemplary system  10  is a packet switched network, such as an Ethernet (IEEE 802.3) network. The packet switched network includes integrated multiport switches (IMS)  12  that enable communication of data packets between network stations. The network may include network stations having different configurations, for example twelve ( 12 ) 10 megabit per second (Mb/s) or 100 Mb/s network stations  14  (hereinafter 10/100 Mb/s) that send and receive data at a network data rate of 10 Mb/s or 100 Mb/s, and a 1000 Mb/s (i.e., 1 Gb/s) network node  22  that sends and receives data packets at a network speed of 1 Gb/s. The gigabit node  22  may be a server, or a gateway to a high-speed backbone network. Hence, the switches  12  selectively forward data packets received from the network nodes  14  or  22  to the appropriate destination based upon Ethernet protocol. 
     Each switch  12  includes a media access control (MAC) module  20  that transmits and receives data packets to and from 10/100 Mb/s physical layer (PHY) transceivers  16  via respective shared media independent interfaces (MII)  18  according to IEEE 802.3u protocol. Each switch  12  also includes a gigabit MAC port  24  for sending and receiving data packets to and from a gigabit PHY  26  for transmission to the gigabit node  22  via a high speed network medium  28 . 
     Each 10/100 Mb/s network station  14  sends and receives data packets to and from the corresponding switch  12  via a media  17  and according to either half-duplex or full duplex Ethernet protocol. The Ethernet protocol ISO/IEC 8802-3 (ANSI/IEEE Std. 802.3, 1993 Ed.) defines a half-duplex media access mechanism that permits all stations  14  to access the network channel with equality. The 10/100 Mb/s network stations  14  that operate in full duplex mode send and receive data packets according to the Ethernet standard IEEE 802.3u. The full-duplex environment provides a two-way, point-to-point communication link enabling simultaneous transmission and reception of data packets between each link partner, i.e., the 10/100 Mb/s network station  14  and the corresponding switch  12 . 
     Each switch  12  is coupled to 10/100 physical layer (PHY) transceivers  16  configured for sending and receiving data packets to and from the corresponding switch  12  across a corresponding media independent interface (MII)  18 . A magnetic transformer  19  provides AC coupling between the PHY transceiver  16  and the corresponding network medium  17 . Each switch  12  also includes an expansion port  30  for transferring data between other switches according to a prescribed protocol. 
     FIG. 2 is a block diagram of the switch  12 . The switch  12  contains a decision making engine  40  that performs frame forwarding decisions, a switching subsystem  42  for transferring frame data according to the frame forwarding decisions, a buffer memory interface  44 , management information base (MIB) counters  48 , and MAC (media access control) protocol interfaces  20  and  24  to support the routing of data packets between the Ethernet (IEEE 802.3) ports serving the network stations  14  and  22 . The MIB counters  48  provide statistical network information in the form of management information base (MIB) objects to an external management entity controlled by a host CPU  32 , described below. 
     The external memory interface  44  enables external storage of packet data in a synchronous static random access memory (SSRAM)  36  in order to minimize the chip size of the switch  12 . In particular, the switch  12  uses the SSRAM  36  for storage of received frame data, and memory structures. The memory  36  is preferably either a Joint Electron Device Engineering Council (JEDEC) pipelined burst or Zero Bus Turnaround™ (ZBT)-SSRAM having a 64-bit wide data path and a 17-bit wide address path. The external memory  36  is addressable as upper and lower banks of 128K in 64-bit words. The size of the external memory  36  is preferably at least 1 Mbytes, with data transfers possible on every clock cycle through pipelining. Additionally the external memory interface clock operates at clock frequencies of at least 66 MHz, and, preferably, 100 MHz and above. 
     The switch  12  also includes a processing interface  50  that enables an external management entity such as a host CPU  32  to control overall operations of the switch  12 . In particular, the processing interface  50  decodes CPU accesses within a prescribed register access space, and reads and writes configuration and status values to and from configuration and status registers  52 . 
     The internal decision making engine  40 , referred to as an internal rules checker (IRC), makes frame forwarding decisions for data frames received from one source to at least one destination station. 
     The switch  12  also includes an LED interface  54  that clocks out the status of conditions per port and drives external LED logic. The external LED logic drives LED display elements that are human readable. 
     The switching subsystem  42 , configured for implementing the frame forwarding decisions of the IRC  40 , includes a port vector first in first out (FIFO) buffer  56 , a plurality of output queues  58 , a multicopy queue  60 , a multicopy cache  62 , a free buffer queue  64 , and a reclaim queue  66 . 
     The MAC unit  20  includes modules for each port, each module including a MAC receive portion, a receive FIFO buffer, a transmit FIFO buffer, and a MAC transmit portion. Data packets from a network station  14  are received by the corresponding MAC port and stored in the corresponding receive FIFO. The MAC unit  20  obtains a free buffer location (i.e., a frame pointer) from the free buffer queue  64 , and outputs the received data packet from the corresponding receive FIFO to the external memory interface  44  for storage in the external memory  36  at the location specified by the frame pointer. 
     The IRC  40  monitors (i.e., “snoops”) the data bus to determine the frame pointer value and the header information of the received packet, including source, destination, and virtual LAN (VLAN) address information. The IRC  40  uses the header information to determine which MAC ports will output the data frame stored at the location specified by the frame pointer. The decision making engine may thus determine that a given data frame should be output by either a single port, multiple ports, or all ports (i.e., broadcast). Also, the IRC  40  may decide that the frame should not be forwarded to any port. 
     For example, each data frame includes a header having source and destination address, where the decision making engine  40  may identify the appropriate output MAC port based upon the destination address. Alternatively, the destination address may correspond to a virtual address that the appropriate decision making engine identifies as corresponding to a plurality of network stations. In addition, the frame may include a VLAN tag header. The IRC  40  may also determine that the received data frame should be transferred to another switch  12  via the expansion port  30 . Hence, the internal rules checker  40  will decide whether a frame temporarily stored in the buffer memory  36  should be output to a single MAC port or multiple MAC ports. 
     The internal rules checker  40  outputs a forwarding decision to the switch subsystem  42  in the form of a forwarding descriptor. The forwarding descriptor includes a port vector identifying each MAC port that should receive the data packet, priority class identifying whether the frame is high priority or low priority, VLAN information, Rx port number, Opcode, and frame pointer. The port vector identifies the MAC ports to receive the data packet for transmission (e.g., 10/100 MAC ports  1 - 12 , Gigabit MAC port, and/or Expansion port). The port vector FIFO  56  decodes the forwarding descriptor including the port vector, and supplies the frame pointer to the appropriate output queues  58  that correspond to the output MAC ports to receive the data packet transmission. In other words, the port vector FIFO  56  supplies the frame pointer on a per-port basis. The output queues  58  fetch the data packet identified in the port vector from the external memory  36  via the external memory interface  44 , and supply the retrieved data packet to the appropriate transmit FIFO of the identified ports. If a data packet is to be supplied to a management agent, the frame pointer is also supplied to a management queue  68 , which can be processed by the host CPU  32  via the CPU interface  50 . 
     The multicopy queue  60  and the multicopy cache  62  keep track of the number of copies of the data packet that are fetched from the respective output queues  58 , ensuring that the data packet is not overwritten in the SSRAM  36  until the appropriate number of copies of the data packet have been output from the SSRAM  36 . Once the number of copies output corresponds to the number of ports specified in the port vector FIFO  56 , the frame pointer is forwarded to the reclaim queue  66 . The reclaim queue  66  stores frame pointers that can be reclaimed by the free buffer queue  64  as free pointers. After being returned to the free buffer queue  64 , the frame pointer is available for reuse by the MAC unit  20  or the gigabit MAC unit  24 . 
     FIG. 3 depicts the switch subsystem  42  of FIG. 2 in more detail according to an exemplary embodiment of the present invention. Other elements of the multiport switch  12  of FIG. 2 are reproduced in FIG. 3 to illustrate the connections of the switch subsystem  42  to these other elements. 
     As shown in FIG. 3, the MAC module  20  includes a receive portion  20   a  and a transmit portion  24   b . The receive portion  20   a  and the transmit portion  24   b  each include  12  MAC modules (only two of each shown and referenced by numerals  70   a ,  70   b ,  70   c , and  70   d ) configured for performing the corresponding receive or transmit function according to IEEE 802.3 protocol. The MAC modules  70   c  and  70   d  perform the transmit MAC operations for the 10/100 Mb/s switch ports complementary to modules  70   a  and  70   b , respectively. 
     The gigabit MAC port  24  also includes a receive portion  24   a  and a transmit portion  24   b , while the expansion port  30  similarly includes a receive portion  30   a  and a transmit portion  30   b . The gigabit MAC port  24  and the expansion port  30  also have receive MAC modules  72   a  and  72   b  optimized for the respective ports. The transmit portions  24   b  and  30   b  of the gigabit MAC port  24  and the expansion port  30   a  also have transmit MAC modules  72   c  and  72   d , respectively. The MAC modules are configured for full-duplex operation on the corresponding port, and the gigabit MAC modules  72   a  and  72   c  are configured in accordance with the Gigabit Proposed Standard IEEE Draft P802.3z. 
     Each of the receive MAC modules  70   a ,  70   b ,  72   a , and  72   b  include queuing logic  74  for transfer of received data from the corresponding internal receive FIFO to the external memory  36  and the rules checker  40 . Each of the transmit MAC modules  70   c ,  70   d ,  72   c , and  72   d  includes a dequeuing logic  76  for transferring data from the external memory  36  to the corresponding internal transmit FIFO. The queuing logic  74  uses the fetched frame pointers to store receive data to the external memory  36  via the external memory interface controller  44 . The frame buffer pointer specifies the location in the external memory  36  where the received data frame will be stored by the receive FIFO. 
     The external memory interface  44  includes a scheduler  80  for controlling memory access by the queuing logic  74  or dequeuing logic  76  by any switch port to the external memory  36 , and an SSRAM interface  78  for performing the read and write operations with the SSRAM  36 . In particular, the switch  12  is configured to operate as a non-blocking switch, where network data is received and output from the switch ports at the respective wire rates of 10, 100, or 1000 Mb/s. Hence, the scheduler  80  controls the access by different ports to optimize usage of the bandwidth of the external memory  36 . 
     Each receive MAC stores a portion of a frame in an internal FIFO upon reception from the corresponding switch port; the size of the FIFO is sufficient to store the frame data that arrives between scheduler time slots. The corresponding queuing logic  74  obtains a frame pointer and sends a write request to the external memory interface  44 . The scheduler  80  schedules the write request with other write requests from the queuing logic  74  or any read requests from the dequeuing logic  76 , and generates a grant for the requesting queuing logic  74  (or the dequeuing logic  76 ) to initiate a transfer at the scheduled event (i.e., slot). Sixty-four bits of frame data is then transferred over a write data bus  69 a from the receive FIFO to the external memory  36  in a direct memory access (DMA) transaction during the assigned slot based on the retrieved frame pointer. The frame is stored in the location pointed to by the free buffer pointer obtained from the free buffer pool  64 , although a number of other buffers may be used to store a frame, as will be described. 
     The rules checker  40  also receives the frame pointer and the header information (including source address, destination address, VLAN tag information, etc.) by monitoring (i.e., snooping) the DMA write transfer on the write data bus  69   a . The rules checker  40  uses the header information to make the forwarding decision and generate a forwarding instruction in the form of a forwarding descriptor that includes a port vector. The port vector has a bit set for each output port to which the frame should be forwarded. If the received frame is a unicopy frame, only one bit is set in the port vector generated by the rules checker  40 . The single bit that is set in the port vector corresponds to a particular one of the ports. 
     The rules checker  40  outputs the forwarding descriptor including the port vector and the frame pointer into the port vector FIFO  56 . The port vector is examined by the port vector FIFO  56  to determine which particular output queue, e.g. at least one of the queues  58  or the management queue  68 , should receive the associated frame pointer. The port vector FIFO  56  places the frame pointer into the top of the appropriate queue  58  and/or  68 . This queues the transmission of the frame. The output queue  68  is processed separately by the host CPU  32  via the CPU interface  50 . 
     As shown in FIG. 3, each of the transmit MAC units  70   d ,  70   e ,  70   f ,  72   d , and  72   c  have an associated output queue  58   a ,  58   b ,  58   c ,  58   d , and  58   e , respectively. Preferably, each of the output queues  58  has a high priority queue for high priority frame pointers, and a low priority queue for low priority frame pointers. The high priority frame pointers are used for data frames that require a guaranteed access latency, e.g., frames for multimedia applications or management MAC frames. The frame pointers stored in the FIFO-type output queues  58  are processed by the dequeuing logic  76  for the respective transmit MAC units. At some point in time, the frame pointer reaches the bottom of an output queue  58 , for example the output queue  58   e  for the gigabit transmit MAC  72   c . The dequeuing logic  76  for the transmit gigabit port  24   b  takes the frame pointer from the corresponding gigabit port output queue  58   e , and issues a request to the scheduler  80  to read the frame data from the external memory  36  at the memory location specified by the frame pointer. The scheduler  80  schedules the request, and issues a grant for the dequeuing logic  76  of the transmit gigabit port  24   b  to initiate a DMA read. In response to the grant, the dequeuing logic  76  reads the frame data (along the read bus  69   b ) in a DMA transaction from the location in external memory  36  pointed to by the frame pointer, and stores the frame data in the internal transmit FIFO for transmission by the transmit gigabit MAC  72   c . If the frame pointer specifies a unicopy transmission, the frame pointer is returned to the free buffer queue  64  following writing the frame data into the transmit FIFO. 
     A multicopy transmission is similar to the unicopy transmission, except that the port vector has multiple bits set, designating the multiple ports from which the frame will be transmitted. The frame pointer is placed into each of the appropriate output queues  58  and transmitted by the appropriate transmit MAC units  20   b ,  24   b , and/or  30   b.    
     The free buffer pool  64 , the multicopy queue  60 , the reclaim queue  66 , and the multicopy cache  62  are used to manage use of frame pointers and reuse of frame pointers once the frame has been transmitted to its designated output port(s). In particular, the dequeuing logic passes frame pointers for unicopy frames to the free buffer queue  64  after the buffer contents have been copied to the appropriate transmit FIFO. 
     For multicopy frames, the port vector FIFO  56  supplies multiple copies of the same frame pointer to more than one output queue  58 , each frame pointer having a unicopy bit set to zero. The port vector FIFO also copies the frame pointer and the copy count to the multicopy queue  60 . The multicopy queue writes the copy count to the multicopy cache  62 . The multicopy cache is a random access memory having a single copy count for each buffer in external memory (i.e., each frame pointer). 
     Once the dequeuing logic  76  retrieves the frame data for a particular output port based on a fetched frame pointer and stores the frame data in the transmit FIFO, the dequeuing logic checks if the unicopy bit is set to 1. If the unicopy bit is set to 1, the frame pointer is returned to the free buffer queue  64 . If the unicopy bit is set to zero indicating a multicopy frame pointer, the dequeuing logic  76  writes the frame pointer with a copy count of minus one (−1) to the multicopy queue  60 . The multicopy queue  60  adds the copy count to the entry stored in the multicopy cache  62 . 
     When the copy count multicopy cache  62  for the frame pointer reaches zero, the frame pointer is passed to the reclaim queue  66 . Since a plurality of frame pointers may be used to store a single data frame in multiple buffer memory locations, the frame pointers are referenced to each other to form a linked-list chain of frame pointers to identify the entire stored data frame. The reclaim queue  66  traverses the chain of buffer locations identified by the frame pointers, and passes the frame pointers to the free buffer queue  64 . 
     As discussed above, the internal rules checker (IRC)  40  monitors (i.e., “snoops”) the data bus to determine the frame pointer value and the header information of the received frame (including source, destination, and VLAN address information). The IRC  40  uses the frame pointer value and the associated header information to determine which MAC ports will output the data frame stored at the location specified by the frame pointer. 
     As shown in FIG. 4, the IRC  40  may contain multiple rules queues  102  having frame pointers and frame header information for frames received by the receive ports of the IMS  12 . A single rules queue  102  is assigned to each receive port of the IMS  12 . In particular, rules queues  1  to  12  are provided for 10/100 MAC ports  1  to  12  configured to receive data from the corresponding 10/100 Mb/s network stations  14 , a rules queue  13  may be arranged to support the gigabit MAC port  24  capable of receiving data from the gigabit network node  22 , and a rules queue  14  may be assigned to the expansion port  30 . Each rules queue  102  may hold frame headers in a synchronous random access memory (SRAM) having four 40-byte entries and may store frame pointers in a SRAM having four 13-bit entries. 
     The IRC  40  monitors the data bus  68  to place in each rules queue  102  the header information and frame pointers transferred by the queuing logic  74  of the corresponding receive module to the external memory  36 . An IRC scheduler  104  controls the transfer of data held in each rules queue  102  from the corresponding rules queue  102  to IRC logic circuits such as ingress rules logic  106 , source address (SA) lookup logic  108 , destination address (DA) lookup logic  110  and egress rules logic  112  to produce a forwarding descriptor supplied to the port vector FIFO  56 . 
     The ingress rules logic  106  detects whether a frame was received with an error and checks for preset DA and VLAN information. If an error is detected or the frame address information does not match with allocated DA addresses or VLAN data, the ingress rules logic  106  produces a forwarding descriptor with a null port vector. This forwarding descriptor is transferred directly to the port vector FIFO  56  without performing SA and DA lookup operations and egress rules operations. 
     The SA and DA lookup logic circuits  108  and  110  search an IRC MAC address table  114  for entries associated with the MAC source and destination addresses for the corresponding frame. If source and destination address data of a frame match with the address table entries, the egress rules logic  112  checks each transmit port in the port vector list produced by the DA lookup logic circuit  110  to remove or mask the disabled ports, the ports that do not belong to a required VLAN, and the port, from which the frame is received. As a result, the egress rules logic  112  generates a forwarding descriptor including a port vector identifying each MAC port that should receive the corresponding frame. 
     The IRC logic circuitry performs sequential processing of data held in rules queues  102 . The data from each rules queue is transferred to the IRC logic circuitry in successive time slots. The IRC scheduler  104  provides arbitration between the rules queues  102  to allocate a time slot, during which data from a given rules queue will be transferred to the IRC logic circuitry. In particular, when a rules queue  102  has data to be processed by the IRC logic circuitry, the rules queue  102  sends to the IRC scheduler  104  a request for the time slot. In response, the IRC scheduler  104  produces grant signals supplied to the rules queue  102  to enable it to transfer its data to the IRC logic circuitry during the allocated time slot. 
     Each rules queue  102  is initially assigned with at least one time slot in each scheduling cycle of the IRC scheduler  104 . FIG. 5 illustrates an exemplary scheduling cycle of the IRC scheduler  104  having 25 time slots for the rules queues  102 . Each time slot may be equal to 5 clock cycles. One time slot may be assigned to each rules queue representing the 10/100 MAC ports  1 - 12 , 10 time slots may be assigned to the rules queue representing the gigabit MAC port  24 , and 3 time slots may be assigned to the rules queue that supports the expansion port  30 . 
     As shown in FIG. 5, the time slots in the scheduling cycle are arranged as follows: G 1 G 2 G 3 E 4 G 5 G 6 G 7 E 8 G 9 G 10 G 11 G 12 E, where G indicates the time slots assigned to the gigabit MAC port  24 , E indicates the time slots assigned to the expansion port  30 , and numerals  1  to  12  indicate the time slots assigned to the 10/100 MAC ports  1  to  12 , respectively. 
     Thus, the first time slot (G) in the scheduling cycle may be assigned to the rules queue  102  that represents the gigabit MAC port  24 , the second time slot ( 1 ) may be assigned to rules queue  102  that supports the MAC port  1 , the third time slot (G) may be assigned to rules queue  102  representing the gigabit MAC port  24 , etc. Finally, the last time slot (E) in the scheduling cycle may be assigned to the rules queue  102  supporting the expansion port  30 . 
     Each individual port has a priority in accessing the time slots assigned to that port. Hence, when a rules queue  102  for a given port requests the time slot assigned to that port, its request is granted, even if the other rules queues request time slots. However, when no data are supplied to the internal rules checker from a port assigned with a current time slot, the bandwidth allocated to that port would be wasted, whereas processing of frame headers from the overloaded ports might be delayed because the bandwidth allocated to them is not sufficient. 
     In accordance with the present invention, the IRC scheduler  104  is provided with a system for dynamically distributing time slots assigned to the gigabit MAC port  24  and the expansion port  30  between these ports depending on the relative data traffic at the ports. Hence, as illustrated in FIG. 5, a time slot G assigned to the gigabit port  24  may be allocated to the expansion port  30 . Similarly, a time slot E assigned to the extension port  30  may be allocated to the gigabit port  24 . 
     In each scheduling cycle, the IRC scheduler determines the number Rg of requests for a time slot from the rules queue for the gigabit port  24  and the number Re of time slot requests from the rules queue for the expansion port  30 . Based on the numbers Rg and Re of requests, and the numbers Ng and Ne of time slots G and E respectively assigned to the gigabit and expansion ports, the IRC scheduler  104  determines the availability of time slots G and E not requested by the corresponding ports assigned with these time slots. 
     If the number Rg of time slot requests from the gigabit port  24  is higher than the number Ng of the time slots G assigned to the gigabit port  24 , the IRC scheduler  104  allocates the non-requested expansion port time slots E to the gigabit port  24 . The time slots E requested by the expansion port  30  are allocated to the port  30 . 
     Similarly, if the number Re of time slot requests from the expansion port  30  is higher than the number Ne of time slots E assigned to the expansion port  30 , the IRC scheduler allocates the non-requested gigabit port time slots G to the expansion port  30 . The time slots G requested by the gigabit port  24  are allocated to the port  24 . 
     Referring to FIG. 6, the IRC scheduler  104  counts the number Rg of gigabit port requests and the number Re of expansion port requests (block  202 ). The IRC scheduler  104  compares the count Rg with the number of time slots G assigned to the gigabit port  24  (block  204 ), to determine whether any non-requested time slots G are available. Then, the IRC scheduler  104  determines whether the number of requests from the expansion port  30  is more than the number of time slots E assigned to the expansion port  30  (block  206 ). If so, the IRC scheduler  104  allocates the non-requested gigabit port time slots G to the expansion port  30  (block  208 ). The gigabit port time slots G requested by the gigabit port  24  are allocated to the port  24 . 
     If the IRC scheduler  104  determines that no non-requested gigabit time slots G are available, the scheduler  104  compares the number of expansion port requests Re with the number of time slots E assigned to the expansion port  30 , to check whether any non-requested expansion port time slots E are available (block  210 ). If the IRC scheduler  104  determines that the number of requests from the gigabit port  24  is more than the number of time slots G assigned to the port  24  (block  212 ), the scheduler  104  allocates the non-requested time slots E to the gigabit port  24  (block  214 ). The time slots E requested by the expansion port  30  are allocated to the port  30 . 
     Thus, expansion port time slots E and gigabit port time slots G are dynamically distributed between the expansion port  30  and the gigabit port  24  in accordance with relative data traffic at these ports. 
     In this disclosure, there are shown and described only the preferred embodiments of the invention, but it is to be understood that the invention is capable of changes and modifications within the scope of the inventive concept as expressed herein.