Patent Publication Number: US-RE44487-E

Title: Command packet packing to mitigate CRC overhead

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention is related to the field of interconnects for electronic systems such as computer systems. 
     2. Description of the Related Art 
     Electronic components in systems (such as integrated circuits and other electrical devices) communicate with each other over defined interfaces such as links between the components. Data is usually transmitted over the links with reference to a clock signal. That is, data is driven and sampled on the link responsive to the clock signal. Recently, double data rate links have been defined in which data is driven/sampled according to both the rising and falling edges of the clock signal. 
     The frequency of the clock associated with a link, along with its width (in bits of data transferred) defines the bandwidth on the link (the amount of data transferred per unit time). One way to increase bandwidth (and also decrease latency, for larger transfers) is to increase the clock frequency. However, as the frequency increases, the error rate also increases as factors such as noise, clock uncertainty, skew, rise and fall times, etc. become bigger factors in the shorter clock cycle. Viewed in another way, data is present on the link for a shorter period of time, and the margin for error is smaller. At some point, the error rate increases to a level that impacts reliability. 
     To mitigate the increased error rate, error detection (and possibly correction) can be implemented on the link. For example, a cyclical redundancy check (CRC) is often implemented on links. For CRC, each data transfer on the link is followed by a CRC code that is generated from the data. The receiver can generate the same CRC code, and compare the generated CRC code to the received code to detect an error. The receiver can report the error to the transmitter, which can retransmit the data or take other corrective action. 
     For relatively large data transfers, the addition of the CRC code to the end of the transfer doesn&#39;t impact bandwidth very much (since the added code is small compared to the data transferred). However, the CRC code does impact latency, as the receiver generally must store the entire data covered by the CRC until the CRC is received. If the CRC validates that the data is correct, then the data can be forwarded. In many cases, the first data in the transfer is the most critical (e.g. in caching systems in which the data that is currently requested is transferred first, followed by the remainder of the cache block for storage). In such cases, the increase in latency is a decrease in performance. 
     Furthermore, in coherent systems, many of the transfers between components are relatively small messages (e.g. probes searching for the most recent copies of the data, probe responses, done indications for source and target, etc.). The impact of the CRC on these small messages is significant, increasing the size of the messages by as much as 100% (for a message that is the same size as the CRC code). Size increases of 50% or 33% are common as well. In coherent systems, a large number of the transfers over the links are these small messages, so the increased bandwidth consumed to add the CRC is significant. 
     SUMMARY 
     In one embodiment, a node is configured to couple to a link. The node comprises a transmit circuit configured to transmit packets on the link, each packet comprising one or more cells. The node further comprises a cell scheduler coupled to the transmit circuit and configured to schedule cells to be transmitted by the transmit circuit. The cell scheduler is configured to schedule at least one error detection cell to be transmitted, the error detection cell covering a preceding packet, and if a second packet is available for scheduling, the cell scheduler is configured to schedule cells of the second packet instead of the error detection cell. When subsequently scheduled, the error detection cell covers the preceding packet and the second packet. 
     In another embodiment, a node is configured to couple to a link and comprises a packet scheduler configured to schedule packets to be transmitted on the link; and an interface circuit coupled to the packet scheduler. The interface circuit is configured to transmit the packets on the link, and to generate error detection data covering the packets. The error detection data is transmitted between packets on the link, and the interface circuit is configured to cover up to N packets with one transmission of error detection data (where N is an integer greater than or equal to two). The number of packets covered with one transmission of error detection data is determined by the interface circuit dependent on an availability of packets to transmit. 
     In another embodiment, a node is configured to couple to a link and comprises a packet scheduler configured to schedule packets to be transmitted on the link. Coupled to the packet scheduler and configured to transmit the packets on the link, and interface circuit is configured to generate error detection data covering the packets. The interface circuit is configured to monitor an amount of bandwidth being consumed on the link and to dynamically vary a frequency of transmission of the error detection data on the link based on the amount of bandwidth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
         FIG. 1  is a block diagram of one embodiment of a computer system. 
         FIG. 2  is a block diagram of one embodiment of two nodes shown in  FIG. 1 , shown in greater detail. 
         FIG. 3  is a block diagram illustrating one embodiment of control fields for a control register. 
         FIG. 4  is a flowchart illustrating operation of one embodiment of a cell scheduler shown in  FIG. 2  to schedule a command cell. 
         FIG. 5  is a flowchart illustrating operation of one embodiment of a cell scheduler shown in  FIG. 2  to schedule a data cell. 
         FIG. 6  is a flowchart illustrating operation of one embodiment of a cell scheduler shown in  FIG. 2  to dynamically vary CRC operation. 
         FIG. 7  is a flowchart illustrating operation of one embodiment of a receiver. 
         FIG. 8  is a table illustrating one embodiment of control encodings on the interconnect. 
         FIG. 9  is a block diagram illustrating examples of the encodings shown in  FIG. 8 . 
         FIG. 10  is a table illustrating another embodiment of control encodings on the interconnect. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The description below refers to a computer system that implements error detection data on its links. However, any electronic system that communicates data on links between components can implement the description herein. Generally, a link may comprise a communication path from one component to another. The link may be bidirectional or unidirectional, in various embodiments. The link may have any width, and may have one or more clock cycles associated with it that clocks data on the link. Also, CRC will be used as an example of error detection data. Generally, CRC is generated as a combination of the data bits that are covered by the CRC, according to a predefined formula used by both the transmitter and the receiver. However, any error detection data may be used. Generally, the error detection data comprises redundant data that allows for detection (and optionally correction) of errors in the transmitted data. For example, error detection data may be parity based, or may be similar to various error detection/correction codes (ECC) used in memory systems, such those that correct single bit errors and detect double bit errors. ECCs that correct and detect larger numbers of errors may also be used. Error detection data can be based Hamming Codes or Reed-Solomon Codes, for example, or any other code defined to detect and/or correct errors. An instance of error detection data may be referred to as “covering” a set of data. The error detection data covers the data if an error in the data is detectable using the error detection data (although error detection data has limits, and thus a large enough error in the data may not be detected even though the error detection data covers the erroneous data). 
     Turning now to  FIG. 1 , an embodiment of a computer system  300  is shown. In the embodiment of  FIG. 1 , computer system  300  includes several processing nodes  312 A,  312 B,  312 C, and  312 D. Each processing node is coupled to a respective memory  314 A- 314 D via a memory controller  316 A- 316 D included within each respective processing node  312 A- 312 D. Additionally, processing nodes  312 A- 312 D include an interface circuit to communicate between the processing nodes  312 A- 312 D. For example, processing node  312 A includes interface circuit  318 A for communicating with processing node  312 B, interface circuit  318 B for communicating with processing node  312 C, and interface circuit  318 C for communicating with yet another processing node (not shown). Similarly, processing node  312 B includes interface circuits  318 D,  318 E, and  318 F; processing node  312 C includes interface circuits  318 G,  318 H, and  318 I; and processing node  312 D includes interface circuits  318 J,  318 K, and  318 L. Processing node  312 D is coupled to communicate with a plurality of input/output devices (e.g. devices  320 A- 320 B in a daisy chain configuration) via interface circuit  318 L. Other processing nodes may communicate with other I/O devices in a similar fashion. 
     Processing nodes  312 A- 312 D implement a packet-based interface for inter-processing node communication. In the present embodiment, the interface is implemented as sets of unidirectional links (e.g. links  324 A are used to transmit packets from processing node  312 A to processing node  312 B and links  324 B are used to transmit packets from processing node  312 B to processing node  312 A). Other sets of links  324 C- 324 H are used to transmit packets between other processing nodes as illustrated in  FIG. 1 . Generally, each set of links  324  may include one or more data lines, one or more clock lines corresponding to the data lines, and one or more control lines indicating the type of packet being conveyed. The link may be operated in a cache coherent fashion for communication between processing nodes or in a noncoherent fashion for communication between a processing node and an I/O device (or a bus bridge to an I/O bus of conventional construction such as the Peripheral Component Interconnect (PCI) bus or Industry Standard Architecture (ISA) bus). Furthermore, the link may be operated in a non-coherent fashion using a daisy-chain structure between I/O devices as shown. It is noted that a packet to be transmitted from one processing node to another may pass through one or more intermediate nodes. For example, a packet transmitted by processing node  312 A to processing node  312 D may pass through either processing node  312 B or processing node  312 C as shown in  FIG. 1 . Any suitable routing algorithm may be used. Other embodiments of computer system  300  may include more or fewer processing nodes then the embodiment shown in  FIG. 1 . 
     Generally, the packets may be transmitted as one or more bit times on the links  324  between nodes. A given bit time may be referenced to the rising or falling edge of the clock signal on the corresponding clock lines. That is, both the rising and the falling edges may be used to transfer data, so that the data rate is double the clock frequency (double data rate, or DDR). The packets may include request packets for initiating transactions, probe packets for maintaining cache coherency, and response packets for responding to probes and requests (and for indicating completion by the source/target of a transaction). Some packets may indicate data movement, and the data being moved may be included in the data movement packets. For example, write requests include data. Probe responses with dirty data and read responses both include data. Thus, in general, a packet may include a command portion defining the packet, its source and destination, etc. A packet may optionally include a data portion following the command portion. The data may be a cache block in size, for coherent cacheable operations, or may be smaller (e.g. for non-cacheable reads/writes). 
     Processing nodes  312 A- 312 D, in addition to a memory controller and interface logic, may include one or more processors. Broadly speaking, a processing node comprises at least one processor and may optionally include a memory controller for communicating with a memory and other logic as desired. One or more processors may comprise a chip multiprocessing (CMP) or chip multithreaded (CMT) integrated circuit in the processing node or forming the processing node, or the processing node may have any other desired internal structure. Any level of integration or any number of discrete components may form a node. Other types of nodes may include any desired circuitry and the circuitry for communicating on the links. For example, the I/O devices  320 A- 320 B may be I/O nodes, in one embodiment. 
     Memories  314 A- 314 D may comprise any suitable memory devices. For example, a memory  314 A- 314 D may comprise one or more RAMBUS DRAMs (RDRAMs), synchronous DRAMs (SDRAMs), DDR SDRAM, static RAM, etc. The address space of computer system  300  is divided among memories  314 A- 314 D. Each processing node  312 A- 312 D may include a memory map used to determine which addresses are mapped to which memories  314 A- 314 D, and hence to which processing node  312 A- 312 D a memory request for a particular address should be routed. In one embodiment, the coherency point for an address within computer system  300  is the memory controller  316 A- 316 D coupled to the memory storing bytes corresponding to the address. In other words, the memory controller  316 A- 316 D is responsible for ensuring that each memory access to the corresponding memory  314 A- 314 D occurs in a cache coherent fashion. Memory controllers  316 A- 316 D may comprise control circuitry for interfacing to memories  314 A- 314 D. Additionally, memory controllers  316 A- 316 D may include request queues for queuing memory requests. 
     Generally, interface circuits  318 A- 318 L may comprise a variety of buffers for receiving packets from the link and for buffering packets to be transmitted upon the link. Computer system  300  may employ any suitable flow control mechanism for transmitting packets. For example, in one embodiment, each interface circuit  318  stores a count of the number of each type of buffer within the receiver at the other end of the link to which that interface logic is connected. The interface logic does not transmit a packet unless the receiving interface logic has a free buffer to store the packet. As a receiving buffer is freed by routing a packet onward, the receiving interface logic transmits a message to the sending interface logic to indicate that the buffer has been freed. Such a mechanism may be referred to as a “coupon-based” system. 
     I/O devices  320 A- 320 B may be any suitable I/O devices. For example, I/O devices  320 A- 320 B may include devices for communicating with another computer system to which the devices may be coupled (e.g. network interface cards or modems). Furthermore, I/O devices  320 A- 320 B may include video accelerators, audio cards, hard or floppy disk drives or drive controllers, SCSI (Small Computer Systems Interface) adapters and telephony cards, sound cards, and a variety of data acquisition cards such as GPIB or field bus interface cards. Furthermore, any I/O device implemented as a card may also be implemented as circuitry on the main circuit board of the system  300  and/or software executed on a processing node. It is noted that the term “I/O device” and the term “peripheral device” are intended to be synonymous herein. 
     Furthermore, one or more processors may be implemented in a more traditional personal computer (PC) structure including one or more interfaces of the processors to a bridge to one or more I/O interconnects and/or memory. 
     In one embodiment, the links  324 A- 324 H are compatible with the HyperTransport™ (HT) specification promulgated by the HT consortium, specifically version 3. The protocol on the links is modified from the HT specification to support coherency on the links, as described above. However, other embodiments may implement any links and any protocol thereon. Furthermore, the CRC techniques described herein may be used for non-coherent links as well. 
     Turning now to  FIG. 2 , a block diagram of one embodiment of the processing nodes  312 A- 312 B is shown, illustrating additional details of one embodiment of the nodes. In the illustrated embodiment, the processing node  312 A comprises the interface circuit  318 A as shown in  FIG. 1  as well as a packet scheduler  24 , a history buffer  26 , and a processor core  28 . Particularly, the interface circuit  318 A includes a receiver circuit  10 , a transmit circuit  12 , a cell scheduler  14  (including CRC generator  16 ), a cell queue  18 , a busy counter  20 , and one or more control registers  22 . The receiver  10  is coupled to the link  324 B to receive packets from the processing node  312 B, and is coupled to the packet scheduler  24 . The control register  22  and the busy counter  20  are coupled to the cell scheduler  14 , which is coupled to the transmit circuit  12  and the cell queue  18 . The cell queue  18  is coupled to the transmit circuit  12  and the packet scheduler  24 . The transmit circuit  12  is coupled to the link  324 A to transmit packets to the processing node  312 B. The packet scheduler  24  is further coupled to the history buffer  26 , the processor core  28 , and various other packet sources. 
     The processing node  312 B comprises the interface circuit  318 D, as shown in  FIG. 1 , as well as a packet scheduler  40 , a data buffer  42 , and a command buffer  44 . The interface circuit  318 D comprises an acknowledgement (Ack) counter  30 , a CRC check circuit  32 , a cell queue  34 , a receive circuit  36 , and a transmit circuit  38 . The receive circuit  36  is coupled to the link  324 A, the cell queue  34 , and the CRC check circuit  32 . The CRC check circuit  32  is coupled to the Ack counter  30  (which is further coupled to the transmitter  38 ) and to the cell queue  34  (which is further coupled to the packet scheduler  40 , the data buffer  42 , and the command buffer  44 ). The transmit circuit  38  is coupled to the link  324 B. 
     The link  324 A is shown in greater detail in  FIG. 2 , and includes CAD[n:0], CTL[m:0], and CLK[m:0]. The CAD lines may carry packet bit times, and the CLK lines may comprise one or more clock lines to which the bit times are referenced. The CTL lines may indicate the type of data carried in a given bit time, as described in more detail below. There are n CAD lines, where n is an integer power of 2 multiple of 8 minus one. That is, n may be 7, 15, 31, etc. Thus, a power of 2 bytes may be transmitted on the CAD lines per bit time. There are m CLK and CTL lines, where m is equal to n+1 divided by 8. That is, there is one CLK line and one CTL line per byte of CAD. The CLK line is the clock reference for the corresponding 8 CAD lines. 
     While various embodiments may have varying widths, the minimum unit of transfer on the link may be defined as a cell. That is, no packet on the link is less than one cell in size, and packets are an integer multiple of cells long. A cell can be any size in various embodiments, but is 32 bits (4 bytes) in one embodiment. If the link is narrower than a cell, once a cell transmission begins, the remainder of that cell will be transmitted in succession. Thus, for example, a 32 bit cell and a 16 bit link may be implemented and each cell is transmitted as two successive bit times on the CAD lines. 
     The packet scheduler  24  may schedule a packet for transmission on the link  324 A, and may transfer the packet&#39;s cells to the cell queue  18  (or may cause the packet&#39;s cells to be transferred to the cell queue  18 , if they are not stored in the packet scheduler  24 ). The cell scheduler  14  may schedule cells from the cell queue  18  for transmission through the transmit circuit  12 . Generally, cells of a packet may be scheduled during successive bit times, although some non-data movement packets may be scheduled within the data of a preceding data movement packet, in one embodiment. If a cell is scheduled, the cell scheduler  14  causes the cell queue  18  to output the cell to the transmit circuit  12 , which transmit the cell on the link  324 A (with appropriate CTL line encodings). If a cell is not scheduled, the cell scheduler  14  may schedule a no-operation (noop) command for transmission. The noop command may include buffer release fields, in one embodiment, to indicate that buffers are available in the node  312 A to receive packets from the node  312 B. The buffer release counts may be maintained by the interface circuit  318 A, or may be provided by the packet scheduler  24 , in various embodiments. 
     The cell scheduler  14  is configured to generate a CRC for each packet (using the CRC generator  14 ). By default, the cell scheduler  14  may append the CRC for a packet to the end of that packet, providing per-packet error detection on the link. In one embodiment, the CRC comprises one cell of data, although other embodiments may implement multiple cells of CRC. Subsequent to scheduling the last cell of the packet, the cell scheduler  14  may schedule the CRC cell(s). 
     In one embodiment, the interface circuit  318 A (and more particularly the cell scheduler  14 ) may be configured to pack two or more packets together and cover them with one CRC. That is, rather than transmit a first packet and its CRC, then a second packet with its separate CRC, the cell scheduler  14  may transmit the first packet, followed by the second packet, followed by one transmission of CRC that covers both packets. Some embodiments may be configured to pack more than two packets (e.g. up to N packets, where N is an integer greater than or equal to 2). By packing packets together for one CRC transmission, the overhead of CRC transmissions on the interconnect may be reduced, as compared to pure per-packet CRC. Particularly for small packets, the reduction of CRC overhead may significantly reduce bandwidth consumption on the link. In one embodiment, packets may be packed if a packet is available for transmission (e.g. in the cell queue  18 ) at or near the end of transmission of the last cell of the previous packet. Particularly, in one embodiment, the cell scheduler  14  may schedule the last cell of a packet and determine if the CRC or another packet is to be scheduled based on whether or not a cell or cells of the other packet are in the cell queue  18 . 
     In one specific embodiment, the cell scheduler  14  may be configured to pack packets that each include a command but exclude data (e.g. dataless packets, or command-only packets). Since there are frequently communications on a coherent link to maintain coherency that do not include data, the incidence of such packets may be high on a coherent link (although packing may also be implemented on a non-coherent link). On a coherent link, command-only packets may include probe requests, some probe responses, read requests, and done responses, for example. In one embodiment, command-only packets may comprise 1-3 cells, depending on the command and address size in the system. With a one cell CRC and two 1 cell commands, packing just two packets together for CRC transmission may result in a 25% reduction in bandwidth consumption (from 4 cells to 3 cells). 
     In one embodiment, the cell scheduler  14  may be configured to insert one or more partial CRC transmissions within a packet. Each partial CRC may cover the portion of the packet that precedes the insertion (cumulative to the beginning of the packet, even if other partial CRC insertions have been made). For example, a command and data packet may be lengthy, and forwarding of the first data transmitted may be important to performance. By inserting a partial CRC after the first data that would be forwarded, the first data may be validated with the partial CRC at the receiver and may be forwarded. If an error is detected in a later CRC, the forwarded data is still known to be valid and may be used. Thus performance may be improved by permitting the early forwarding of data, in some embodiments. 
     Various embodiments may implement both the CRC packing and the partial CRC insertion, or only one, as desired. 
     CRC packing and partial CRC insertion may be programmable in the control register  22 . Specifically, packing and/or insertion may be enabled. If more than one packet may be packed, the maximum number of packets to pack may be programmable as well. If multiple partial CRC insertion points are supported within a packet, which insertion points to use may be programmable. 
     In one embodiment, the packing and partial CRC insertion may be controlled dynamically. Generally, the cell scheduler  14  may monitor the amount of bandwidth being consumed on the link  324 A, and may adjust the CRC transmissions according to the bandwidth. In one embodiment, cells are transmitted each bit time on the link. If there are no cells to transmit, a noop command may be transmitted. Thus, the cell scheduler  14  may monitor noop commands as unconsumed bandwidth and other cells as consumed bandwidth (packet cells, CRC cells, etc.). If the bandwidth consumption is high (e.g. exceeds a threshold), CRC packing may be performed (if enabled) to reduce CRC bandwidth consumption. Additionally, partial CRC insertion may be temporarily stopped (or reduced, if multiple CRC insertions are performed). Bandwidth may be measured separately for CRC packing control and for partial CRC insertion control, if desired. 
     In the illustrated embodiment, the busy counter  20  may be used to track bandwidth. The counter may, e.g., be a saturating counter that is incremented for each non-noop cell and decremented for each noop cell. Thus, the busy counter is an approximation of recent bandwidth consumption. Based on the busy counter value, CRC packing and partial CRC insertion may be dynamically varied. 
     The receive circuit  36  may receive the cells from the link  324 A. Based on encodings of the CTL lines and possibly decoding within the cell, the receive circuit  36  may determine that the cell is command or data in the packet (or an inserted command, a packed command, or partial CRC). The receive circuit  36  may store the received cells (and any decode/control data) in the cell queue  34 , and may also pass the cell to the CRC check circuit  32 . Eventually, a CRC cell is received and may be compared to the CRC generated by the CRC check circuit  32  to detect an error or validate the received cells. If there is no error, the CRC check circuit  32  may signal the cell queue  34  to forward the validated cells. Additionally, the CRC check circuit  32  may increment the Ack counter  30  if a complete packet has been received without error. The value of the Ack counter  30  may be transmitted (by the transmit circuit  38 , as an Ack packet on the link  324 B) to the processing node  312 A to acknowledge successfully received packets. The processing node  312 A may retain transmitted packets (e.g. via the history buffer  26 ) until they are known to be received accurately at the processing node  312 B. When the Ack packet is received (and is provided by the receiver  10  to the packet scheduler  24 ), the corresponding packets may be deleted. 
     Generally, the interface circuit  318 D may forward validated packet data to a target. The target may be internal (e.g. a processor or other circuitry within the node  312 B) or external. The first step in forwarding to the target may be to write the cells into a command buffer  44  or a data buffer  42  (depending on the whether the cells are command or data cells). The interface circuit  318 D may also provide information regarding the packet to the packet scheduler  40 , which may subsequently schedule the packet for delivery to the internal target or forwarding via another interface circuit (not shown in  FIG. 2 ). 
     If the CRC check circuit  32  does detect an error, an Ack packet signalling the error and providing the Ack count indicating the last known good packet may be sent by the transmit circuit  38 . The packet scheduler  24  may delete correctly-received packets from the history buffer  26 , and load the remaining packets back into the packet scheduler  24  to be retransmitted. Specifically, the packet scheduler  24  may drain, the interface circuit  318 A may reset the link, and the packet scheduler  24  may begin resending the packets noted in the history buffer  28 . In other embodiments, error handling may be performed in other fashions rather than automatically retransmitting the packets received in error. 
     Additionally, the transmit circuit  38  may be configured to transmit buffer release commands, which the receive circuit  10  may forward to the packet scheduler  24 . The transmit circuit  38  may further transmit packets scheduled by the packet scheduler  40  (received from a different source such as one of the internal sources or another interface circuit in the node  312 B), similar to the operation described for the packet scheduler  24  and the interface circuit  318 A. In fact, the nodes  312 A- 312 B may be symmetrical, in that the operations and circuitry described for each node may also be included in the other node. For simplicity, not all details have been shown in both nodes. Also, other interface circuits  318 B- 318 C and  318 E- 318 L may be symmetrical with the interface circuits  318 A and  318 D. 
     Generally, the packet scheduler  24  may receive packets from various internal sources (e.g. processor core  28  shown in  FIG. 2 , the memory controller  316 A shown in  FIG. 1 , etc.) as well as from external sources through the various interface circuits  318 A- 318 C. The packet scheduler  24  may store the packets directly, or may store indicators of the packets. The actual packets may be stored in distributed command and data buffers in the node  312 A (e.g. similar to the command and data buffers  42  and  44  in the processing node  312 B), and when a packet is scheduled, the packet data may be provided from the command and data buffers in which it is stored. 
     Turning now to  FIG. 3 , a block diagram of one embodiment of field in the control register(s)  22  is shown. Other embodiments may include additional and/or alternative data to that shown in  FIG. 3 . Some fields are described as bits, but may be multibit fields in other embodiments. 
     The fields in  FIG. 3  include a capability bit  50 , a pack enable bit  52 , an insert enable bit  54 , an insert control field  56 , a dynamic pack enable bit  58 , and a dynamic insert enable bit  60 . Optionally, one or more threshold fields may be included (e.g. threshold fields  62  and  64 ). 
     The capability bit  50  may be a read-only bit that reports whether or not the node is capable of implementing CRC packing or partial CRC insertion. Software may check the capability bit  50  to ensure that the node is capable of performing the operations prior to enabling the functions (using the pack enable bit  52  and the insert enable bit  54 , respectively). In other embodiments, separate capability bits may be provided for each type of operation. 
     The insert control field  56  may be coded to control which insert points in a packet are to have partial CRC insertions. In one embodiment, the data portion of the packet may be divided into N equal sized portions (or beats), where N is an integer greater than or equal to 2. For example, N may be 4 in one embodiment. Partial CRC insertion may be available between each set of beats, as well as between the command portion and the first beat. Specifically, in one embodiment, encodings may be supported that cause insertion between: (1) the first and second beats; (2) between the first and second beats and between the command and the first beat; (3) between the first and second beats, between the command and the first beat, and between the second and third beats; or (4) between the first and second beats, between the command and the first beat, between the second and third beats; and between the third and fourth beats. Another encoding of the insert control field  56  may be provided to indicate no insertion, and the insert enable bit  54  may be eliminated. In another embodiment, a one-hot encoding for each possible insertion point may be used to permit any combination of insertions, or other combinations of insertions may be supported. In yet another embodiment, only insertion between the first and second beats may be supported, and an insert enable bit  54  may be provided without an additional control field. 
     If more than two packets can be packed for one CRC transmission, there may be a pack control field to control the maximum number of packets to be packed. 
     The dynamic pack enable bit  58  and the dynamic insert enable bit  60  may enable dynamic variation of CRC packing and partial CRC insertions, respectively, according to the bandwidth consumption. In some embodiments, the threshold at which variation is invoked may be programmable, using one or more threshold fields  62  and  64 . For example, different thresholds may be supported for the CRC packing and the partial CRC insertion. Different thresholds may be used to control different amounts of CRC packing and/or partial CRC insertions. For example, as thresholds are exceeded, fewer and fewer partial CRC insertions may be made in a packet, beginning with insertions later in the packet and moving forward, removing the insertion between the first and second beats last (after removing the insertion between the command portion and the first beat). In other embodiments, the thresholds may be fixed. 
     Turning not to  FIG. 4 , a flowchart is shown illustrating operation of one embodiment of the cell scheduler  14  in response to scheduling a command cell (that is, a cell that is part of the command portion of a packet). Specifically, in this embodiment, the command cell is part of a dataless packet, although other embodiments may support CRC packing for packets that include data as well as command portions. The blocks are shown in a particular order for ease of understanding. However, other orders may be used. Furthermore, blocks may be performed in parallel by combinatorial logic circuitry in the cell scheduler  14 . Blocks, combinations of blocks, or the flowchart as a whole may be pipelined over multiple clock cycles. 
     If the scheduled command cell is not the last command cell in the packet (decision block  70 , “no” leg), the cell scheduler  14  may schedule the next cell in the command (block  72 ). If the scheduled command cell is the last command cell in the packet (decision block  70 , “yes” leg), and either the CRC packing is not enabled via the enable bit  52  (decision block  74 , “no” leg), the maximum number of packets have been packed (decision block  76 , “yes” leg), or the dynamic control is enabled and indicates that another command should not be packed (decision block  78 , “no” leg), the cell scheduler  14  may schedule the CRC cell (block  80 ). Additionally, if there is not another command (Cmd 2 ) available to be scheduled (decision block  82 , “no” leg), the cell scheduler  14  may schedule the CRC cell (block  80 ). When the CRC cell is scheduled and transmitted, the accumulated CRC maintained by the CRC generator  16  may be reinitialized to begin accumulating CRC for the next packet. 
     If another command is available (decision block  82 , “yes” leg), the first cell of the next command (Cmd 2 ) may be scheduled (block  84 ). That is, the decision of whether or not to pack a command is made at the time that the last cell of the current command is scheduled, in this embodiment. The cell scheduler  14  may not wait for another command, instead appending the CRC without packing if a second command is not available to be packed. Other embodiments may make the determination of whether or not to pack at or near the time of the scheduling of the last cell of the current packet, or at any other time, as desired. 
     In one embodiment, if a noop command is being transmitted (not packed with any previous command) and a command is scheduled by the packet scheduler  24 , the cell scheduler  14  may pack the command with the noop command even if packing is not enabled (or is dynamically disabled currently). In this case, the reduction of latency for the command may be realized by packing the command with the noop command, rather than transmitting a separate CRC for the noop command prior to the newly scheduled command. 
     While the above description refers to the operation of the cell scheduler  14 , the operation of the interface circuit  318 A as a whole may be similar even if the internal structure of the interface circuit  318 A differs from the embodiment of  FIG. 2 . For example, packets may be received for transmission, packing may be determined based on the availability of another packet to transmit, control may be dynamic based on bandwidth consumption, etc. 
     Turning now to  FIG. 5 , a flowchart is shown illustrating operation of one embodiment of the cell scheduler  14  in response to scheduling a data cell (that is, a cell that is part of the data portion of a packet). The blocks are shown in a particular order for ease of understanding. However, other orders may be used. Furthermore, blocks may be performed in parallel by combinatorial logic circuitry in the cell scheduler  14 . Blocks, combinations of blocks, or the flowchart as a whole may be pipelined over multiple clock cycles. 
     If the scheduled data cell is the last data cell in the packet (decision block  90 , “yes” leg), the cell scheduler  14  may schedule the CRC cell for transmission (block  92 ). In this case, the CRC cell is the CRC covering the entire packet, as accumulated over the cells of the packet in previous cycles. The CRC may also be reinitialized in this case to prepare for the next packet If the scheduled data cell is not the last data cell in the packet (decision block  90 , “no” leg), and either the partial CRC insertion is not enabled via the enable bit  54  (decision block  94 , “no” leg), the packet is not at an insert point (decision block  96 , “no” leg), or the dynamic control is enabled and indicates that partial CRC insertion should not be performed at this point (decision block  98 , “no” leg), the cell scheduler  14  may schedule the next data cell (block  100 ). Otherwise, the cell scheduler  14  may schedule a CRC cell (block  92 ). In this case, the CRC cell is a partial CRC insertion covering the packet up to the current point. The CRC is not reinitialized in this case. The data packet may not be at an insert point, as indicated by decision block  96 , if each beat includes more than one cell and the current cell is not the end of a beat, or if the current insert point is not enabled via the insert control field  56 . 
     While the above description refers to the operation of the cell scheduler  14 , the operation of the interface circuit  318 A as a whole may be similar even if the internal structure of the interface circuit  318 A differs from the embodiment of  FIG. 2 . For example, packets may be received for transmission, partial CRC insertion may be performed if enabled, control may be dynamic based on bandwidth consumption, etc. 
     Turning now to  FIG. 6 , a flowchart is shown illustrating operation of one embodiment of the cell scheduler  14  to monitor bandwidth and dynamically vary CRC packing and/or partial CRC insertion. The blocks are shown in a particular order for ease of understanding. However, other orders may be used. Furthermore, blocks may be performed in parallel by combinatorial logic circuitry in the cell scheduler  14 . Blocks, combinations of blocks, or the flowchart as a whole may be pipelined over multiple clock cycles. 
     If the cell scheduler  14  schedules a new packet for transmission (decision block  110 , “yes” leg), the cell scheduler  14  may increment the busy counter  20  (block  112 ). If the cell scheduler  14  schedules a noop packet (NOP packet—decision block  114 , “yes” leg), the cell scheduler  14  may decrement the busy counter  20  (block  116 ). As mentioned previously, the busy counter  20  may be a saturating counter. That is, increments to the counter may saturate at all binary ones in the counter  20  (and subsequent increments do not change the value until a decrement occurs and reduces the value) and decrements to the counter may saturate at all binary zeros in the counter  20  (and subsequent decrements do not change the value until an increment occurs and increases the value). The value may be treated as an unsigned integer. A noop packet may be defined to be a noop packet that does not include any buffer release counts, in one embodiment, because such noop packets may be the only mechanism for transmitting the counts. It is noted that blocks  110 - 116  are independent of the remainder of the flowchart, and may represent monitoring the bandwidth consumption for this embodiment. 
     If the busy counter exceeds a threshold (decision block  118 , “yes” leg), bandwidth consumption is considered high. Accordingly, partial CRC insertion may be disabled and CRC packing may be enabled, to conserve bandwidth (block  120 ). On the other hand, if the busy counter does not exceed the threshold (decision block  118 , “no” leg), bandwidth consumption is considered low. Partial CRC insertion may be enabled to permit earlier forwarding of data and CRC packing may be disabled to improve packet forwarding latency, in such a case, since bandwidth is available and thus can be used for additional CRC transmissions without impacting performance (block  122 ). 
     Blocks  118 - 122  may represent part of the decision blocks  78  and  90  in  FIGS. 4 and 5 , respectively. That is, if dynamic control is enabled, blocks  118 - 122  may determine whether or not dynamic control indicates that CRC packing or partial CRC insertion is permissible. Block  120  may correspond to decision block  78 , “yes” leg and decision block  90 , “no” leg. Block  122  may correspond to decision block  78 , “no” leg and decision block  90 , “yes” leg. 
     As mentioned previously, in some embodiments, more than one threshold may be supported. For example, there may be thresholds corresponding to each possible combination of partial CRC insertions. As bandwidth consumption increases, the number of partial CRC insertions may be decreased, beginning with those that add the least overall value (e.g. those nearer the end of the data packet, followed by the insertion between the command portion and the data portion, and lastly the insertion between the first and second data beats). Similarly, if more than two packets may be packed for CRC, various thresholds may be used to increase (as the thresholds are exceeded) the number of packets packed for CRC. 
     While the above description refers to the operation of the cell scheduler  14 , the operation of the interface circuit  318 A as a whole may be similar even if the internal structure of the interface circuit  318 A differs from the embodiment of  FIG. 2 . For example, bandwidth consumption may be monitored and dynamic control may be based on the detected bandwidth consumption. 
     The embodiment of  FIG. 6  also illustrates a general mechanism for varying the frequency of CRC transmissions, dependent on the amount of bandwidth being consumed. As the bandwidth consumption increases, the frequency of CRC transmission is lowered (e.g. by increasing CRC packing and/or by reducing partial CRC insertions). As the bandwidth consumption decreases, the frequency of CRC transmission is increased (e.g. by inhibiting CRC packing and/or increasing partial CRC insertions). 
     Turning now to  FIG. 7 , a flowchart is shown illustrating operation of one embodiment of the interface circuit  318 D (and more specifically the receiver circuit  36  and the CRC check circuit  32 , for the illustrated embodiment, although other embodiments may employ different structures) for handling packed CRC and partial CRC insertions. The blocks are shown in a particular order for ease of understanding. However, other orders may be used. Furthermore, blocks may be performed in parallel by combinatorial logic circuitry in the interface circuit  318 D and/or receiver circuit  36 /CRC check circuit  32 . Blocks, combinations of blocks, or the flowchart as a whole may be pipelined over multiple clock cycles. 
     The receiver circuit  36  may decode the control data from the control lines CTL[n:0] (possibly over several bit times if a cell is received over several bit times) to determine what type of cell is received (block  130 ). If the cell is a command cell or data cell of a packet (decision block  132 , “yes” leg), the cell may be placed in the cell queue  34  for CRC validation (block  134 ). Command cells may be partially decoded, e.g. to identify the virtual channel associated with the packet. The CRC check circuit  32  may accumulate the CRC from the received cell(s) (block  136 ). Alternatively, the cells may be placed in the cell queue  34  and the CTL data may decoded after being enqueued. 
     If the cell is not a command or data cell, the cell is a CRC cell (decision block  132 , “no” leg). The CRC check circuit  32  may compare the CRC cell to the accumulated CRC to determine if an error has been detected (block  138 ). The CRC may be either a partial CRC or a full CRC. If an error is not detected (decision block  140 , “no” leg), the command/data cells covered by the CRC may be forwarded from the cell queue  34  (block  142 ). If the forwarding is a partial forward, due to no error detected in a partial CRC, the interface circuit  318 D may record which data has been forwarded (block  144 ). The information may be used if an error is later detected in the packet, so that previously forwarded data is not forwarded again when the packet is retransmitted. If the forwarding is the end of the packet, the Ack counter  30  may be updated (block  146 ), and the CRC accumulated by the CRC check circuit  32  may be reinitialized for the next packet. 
     If an error is detected (decision block  140 , “yes” leg), the CRC check circuit  32  may update the Ack counter  30  to indicate the last successfully received packet (block  148 ), and the transmit circuit  38  may signal an error with the Ack count to the node  312 A, for error processing (block  150 ). 
     Various embodiments may encode the CTL lines for a cell in various fashions. For example,  FIG. 8  is one embodiment of encoding the CTL lines for a 32 bit cell (thus having four CTL lines per cell, in this embodiment). Additionally, this embodiment packs up to two dataless packets for CRC coverage. 
     The CTL lines are coded b‘1111’ for a command cell, and b‘0000’ for a data cell, unless otherwise indicated in the table. The CRC for a dataless packet (command-only packet) is b‘0011’. The CTL line coding of b‘1100’ has two meanings. It may either indicate the data CRC is being provided (not a partial CRC insertion, but the final CRC appended to the end of the packet) or it may indicate the last cell of the first command for packed commands. Thus, if this CTL coding was preceding by a command cell, the cell is the last cell of the first command and if this CTL coding was preceded by a data cell, the cell is the data CRC. Furthermore, the encodings for data CRC and command CRC differ. 
     The CTL line coding of b‘0111’ also has two meanings. It is either the first cell of a command packet inserted in the data portion of another packet, or is the first cell of the second command of a pair of packed commands. Thus, the meaning of this coding may be discerned based on whether or not the preceding coding was b‘1100’. Additionally, the coding for the first cell of a packed command differs from that of other command cells, which may aid identifying the first cell. Additionally, the coding for the last cell of the first command differs from other command encodings. 
     The CTL coding of b‘1001’ may be used for a command cell that precedes a partial CRC insertion after the command portion of the packet. Alternatively, the coding b‘1010’ may be used for a partial CRC insertion between data beats of a packet. Thus, the coding that indicates that the next cell is CRC may differ for the insertion between the command and data portions, as compared to insertions between data beats. Finally, the CTL code of b‘1011’ may indicate a partial CRC cell. Thus, the coding for a partial CRC cell may differ from the codings for a command CRC and a data CRC, as well as differing from other data cells. 
     The CTL codings indicated in the table of  FIG. 8  are selected to reduce the incidence of errors on the CTL lines that can cause errors in the reception of packets. That is, if an error occurs on a CTL line, the receive circuit  36  may have a higher likelihood of detecting an invalid sequence of CTL codings, and thus may detect an error for the packet transmission. 
       FIG. 9  is a set of example packets and the corresponding CTL codings for each cell, for the encodings shown in  FIG. 8 . Each cell is labeled, and the CTL coding precedes the cell in the example. Generally, the cells of a command are labeled “Cmd” followed by a number (which is the same for all cells of the command), followed by a lower case letter to distinguish different cells of the same command. The cells of data are labeled “Data” followed by a number (which is the same for all cells of the same data, and is the same as the corresponding command), followed by a lower case letter to distinguish different cells of the same data. 
     A single (unpacked) dataless command packet having two cells (Cmd 0 a and Cmd 0 b) and corresponding CRC (CRC 0 ) is shown at reference numeral  160 . Reference numeral  162  is an example of a packet having two command cells (Cmd 0 a and Cmd 0 b) and two data cells (Data 0 a and Data 0 b). The corresponding CRC follows the data cells (CRC 0 ). 
     A pair of packed command packets is illustrated at reference numeral  164 , with each command packet having two cells. The command cells for the first packet (Cmd 0 a and Cmd 0 b) are followed by the command cells for the second packet (Cmd 1 a and Cmd 1 b) and the CRC that covers both packets (CRC 0 +1). The CTL coding for Cmd 0 b is thus b‘1100’ to indicate the last cell of the first command, and the CTL code for Cmd 1 a is b‘0111’ to indicate the first cell of the second command. Other command cells have the regular command cell coding of b‘1111’. The CRC cell is also coded b‘0011’ to indicate the command CRC. Reference numeral  166  is an example of a pair of packed single cell commands, and thus no cells have the regular command cell coding. 
     Reference numeral  168  illustrates a command and data packet (Cmd 0  and Data 0 ) having two command-only packets inserted in the data portion of the packet (Cmd 1  and its CRC cell CRC 1 , and Cmd 2  and its CRC cell CRC 2 ). As illustrated, the CTL coding of b‘0111’ is used for the first cell of each of commands Cmd 1  and Cmd 2 , to signify insertion in the data. In each case, the b‘0111’ coding is not preceded by the b‘1100’ coding, and thus the commands may be detected as insertions in the data portion of the packet, and not packed commands. 
     Lastly, reference numeral  170  is an example of a packet having partial CRC insertions (CRC 0 a, CRC 0 b, and CRC 0 c), illustrating the signalling of the next cell as partial CRC, except for the final CRC (CRC 0 ). 
       FIG. 10  is another embodiment of the coding of the CTL lines for a 32 bit cell and four CTL lines per cell. In this embodiment, each bit position of the CTL lines corresponding to the cell (bits  0  to  3 ) is assigned a different meaning. The LCTL and HCTL codings for bit positions  0  and  2  may be the same as that defined in the HT specification. These bits determine whether a cell is a command cell or data. Bit position  1  may be defined for command cells to be a ˜Pack bit. That is, if the bit is 0, the next cell transmitted will be another command cell. If the bit is a 1, the next cell transmitted will be the CRC for the command. Bit position  1  may be defined for data cells to be an Insert bit. If the bit is 0, the next cell is another data cell (or an inserted command cell). If the bit is 1, the next cell is a partial CRC insertion cell. Bit position  3  is reserved. 
     Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.