System and Method for Avoiding Host Transmit Underruns in a Communication Network

1. Field of the Invention

This invention relates to communication networks, e.g., LANS. More particularly, the invention relates to a system and method for avoiding host transmit underruns using a frame adjustable threshold in a communication network.

2. Description of Prior Art

In a communication network in which a host system exchanges data with the network through a Peripheral Component Interconnect (PCI) bus and a Medium Access Control (MAC) device, a significant problem is presented when transmit underruns occur requiring the host system to retransmit a sequence of frames. Due to the poor performance of the PCI bus in some computers, MAC devices may have difficulty moving data from a host buffer to a transmit storage device, e.g., RAM, fast enough to keep data flowing to the network. Often delay states are introduced in this transfer by the PCI bus, adding time to the transfer. The additional data transfer time across the PCI Bus causes the data to be read out of the transmit storage device faster on the network side than data is provided to the input side of the transmit storage device. If the data being written into the transmit storage device is at a slower rate than transmitted to the network, a transmit underrun can occur.

Because of the possibility of transmit underruns, one technique is to preload data into the transmit storage device before beginning transmission on the network. A problem is presented in determining how much data to preload to the transmit storage device. If too much data is preloaded than is needed to prevent an underrun, the network performance will be degraded. If not enough data is preloaded, transmit underruns will occur. A solution to the problem is to determine the correct amount of data to preload on transmission. The amount of data to be loaded can vary from system-to-system depending on the PCI bus performance. The amount of data that is preloaded in the transmit storage device is referred to as a "transmit threshold".

The primary objective in frame transmission is to transmit each frame as soon as possible, by keeping the transmit threshold as low as possible. However, if the threshold is too low, the system may not be able to keep up with the demand for data to be transmitted, thereby incurring an underrun which must be retried with a higher threshold. For performance reason, underruns should be avoided by incurring a slightly higher frame latency rather than initiate a retry on a large number of frames.

Prior art related to transmit underruns in a communications system include the following:

U.S. Pat. No. 5,602,537 issued Feb. 11, 1997, filed May 13, 1994, discloses a technique for eliminating data transfer memory underruns. Once the amount of preloading in a FIFO reaches a predetermined threshold value or the end of frame data has been preloaded, a transmitter begins transmitting data from the memory.

U.S. Pat. No. 5,434,872 issued Jul. 18, 1995, discloses a dedicated transmit buffer with monitoring of the data in the frame to prevent underrun. The amount of data of a frame downloaded by the host to the transmit buffer is monitored to make a threshold determination of an amount of data of the frame resident in the transmit data buffer. A network interface controller includes logic for initiating transmission of the frame when a threshold determination indicates that a sufficient portion of the frame is resident in the transmit buffer, prior to the transfer of all of the data of the frame into the transmit buffer. The monitoring logic includes a threshold state which is programmable by the host computer for storing a threshold value. The threshold value may be set by the host system to optimize performance in a given setting.

U.S. Pat. No. 5,623,606 issued Apr. 22, 1997, filed Oct. 29, 1992, discloses a threshold amount of preloading that is set in a MAC device by host software. The MAC device has logic to determine if the threshold is met before starting transmission.

U.S. Pat. No. 4,942,553 issued Jul. 17, 1990, discloses detecting a fill or empty level of a FIFO. When a fill or empty level exceeds a first request level, notification is made to a Direct Memory Access (DMA) or a coprocessor. The fill or empty level is compared to a second request level, and when such level exceeds a second level request, second level notification to the CPU is generated. In most cases, DMA or coprocessor is able to obtain control of the bus before the request level for a CPU interrupt is reached thereby preventing a waste of CPU intervention as well as FIFO overrun/underrun.

U.S. Pat. No. 4,945,548 issued Jul. 31, 1990, discloses detecting impending overflow and/or underruns of an elasticity buffer. Write overflow or read underrun of a storage element is detected before any data corruption can occur by comparing input and output pointers. An error condition is detected if the input and output pointers overlap for a threshold, which can be shorter than a period required for writing or reading of a multi-bit data unit to or from the buffer. The overlap time period is determined by comparing the pointers at one or more sampling times corresponding to selected phases of a clock.

IBM Technical Disclosure Bulletin (TDB) published May 1991, pages 370-371, discloses a technique for detecting overrun and underrun conditions in a queue communications network while guaranteeing the data integrity of a First-In or First-Out (FIFO) buffer.

None of the prior art establishes an adjustable transmit threshold for a MAC device on a frame-by-frame basis using blocks of data instead of a byte count and providing a "retry" signal to a host system to readjust the transmit threshold according to the host system's performance level.

An object of the invention is an improved system and method for avoiding host system transmit underruns in a communication network.

Another object is an improved system and method for avoiding host system transmit underruns via a frame-based threshold in a communication network.

Another object is a system and method for optimizing a frame threshold for preloading data into an adapter to avoid transmit underruns in a communication network.

Another object is an improved interface and method of operation in a communication network for avoiding a host system transmit underrun via a host system descriptor using an adjustable frame-based threshold.

Another object is an improved Media Access Control (MAC) device and method providing a retry signal to a host system for re-adjusting a frame-based threshold to avoid transmit underruns to a network.

These and other objects, features and advantages are achieved in a communication network, e.g. a LAN including a host system coupled to the network through a peripheral component interconnect (PCI) bus, a medium access control (MAC) device, and a medium independent interface. The host system includes host software, device driver, host descriptors, and a host buffer for exchanging data with the network via the MAC device. The MAC device includes a PCI interface, processor and storage buffer, e.g., RAM, in which host data is moved by the processor in blocks from the host buffer to the MAC storage device. The block size is determined by the host driver and provided to the processor. Each frame has a descriptor which contains a field defining a threshold indicating the number of data blocks to transfer from the host system before transmission to the network. The threshold is specified for each frame by the device driver. When an underrun occurs in the storage device, the processor provides a "retry" status bit to the host descriptor. The device driver uses the "retry" bit to re-determine the transmit threshold for each frame to avoid underruns. The device driver uses an algorithm to monitor internal counters and determine the optimum threshold for frame transmissions to avoid underruns. Using the algorithm, the transmit threshold is raised or lowered to minimize the number of underruns and maximize the network's performance. The threshold is relayed to the MAC processor on a frame-by-frame basis and determines the number of blocks to transfer from the host buffer before starting transmission to the network.

The invention will be further understood from a detailed description of a preferred embodiment, taken in conjunction with an appended drawing, in which:

FIG. 1 is a block diagram of a host system for exchanging data with a communication network via a Peripheral Component Interconnect (PCI) bus, a Media Access Control (MAC) device, a medium independent interface and incorporating the principles of the present invention.

FIG. 1A is a representation of a device driver in the host of FIG. 1 and include counters for use in the flow diagram of FIG. 4.

FIG. 2 is a block diagram of a threshold descriptor used in the system of FIG. 1 in transferring data from the host system to the MAC device.

FIG. 3 (including FIG. 3a and FIG. 3b) is a table of a transmit frame descriptor parameter/status field describing bit number, bit name and bit description.

FIG. 4 (including FIG. 4a, FIG. 4b and FIG. 4c) is a flow diagram for adjusting a frame threshold in the descriptor of FIG. 2 to avoid a transmit underrun in the system of FIG. 1.

FIG. 5 is a timing diagram for determining the interval for adjusting the threshold according to the method shown in FIG. 4.

In FIG. 1, a Local Area Network (LAN) 10, for example, an Ethernet, token ring or the like includes a physical layer 12 coupled to a host system 14 through a Media Access Control (MAC) device 16 and a Peripheral Component Interconnect (PCI) bus 18. Blocks of data frames are exchanged between the host system and the network 10 using conventional protocols, described for example in, a text "Ethernet--Building A Communication Infrastructure" by H. Hegering et al, published by Addison-Wesley, Reading, Mass., 1994 (ISBN:0-201-62405-2) or "IBM's Token Ring Handbook" by G. C. Sackett, published by McGraw-Hill, Inc., New York, N.Y., 1993, or "Computer Networks" by Andrew S. Tannenbaum, published by Prentice-Hall, Englewood Cliffs, N.J., 1989.

The host 14 includes a device driver 20 in the form of software which serves as a link between the network operating system and the MAC device. The device driver presents the operating system with a set of functions which the host system can call upon the device driver to perform. The device driver translates these functions into specific sets of instructions which the MAC device uses to operate in transferring blocks of data in a sequence of frames from a host buffer 24 to the MAC device. The host system also includes host descriptors 22 for each frame of data as it is transferred from a host memory 15 to the network 10 via host buffers 24.

In FIG. 2 a 32 bit wide transmit frame descriptor 22' is shown for each frame transmitted to the network. The transmit frame descriptor links multiple data buffers together in the host memory 15, the contents which form the frame. The number, length, and host memory locations of each of these buffers is contained in the transmit frame descriptor 22' which contains the following fields:

A parameter/status field 22(a) contains the MAC frame status after the frame has been transferred from host memory to the MAC device. The status contents of this field are provided by the MAC device and used by the device driver software to determine the status of the transmitted frame. The parameter contents of the field are set by the device driver to indicate information to the MAC on how the data movement should be handled.

A buffer count field 22(b) contains the number of data buffers used to contain the frame being transmitted. This field is set by the device driver before transmission starts and can contain any value from 1 to 255.

A forward pointer field 22(c) contains the host memory address of the next transmit frame descriptor queued for transmission. This field is set by the device driver and is used by the MAC. Using this pointer, the next frame queued for transmission can be located in the host memory.

A data buffer address field 22(d) contains the host memory starting address of the data buffer that contains all or part of a frame to be transmitted. This field is set by the device driver and is used by the MAC device for locating the frame content. Each buffer associated with the descriptor has a data buffer address field and a buffer byte count field located in the descriptor.

A buffer byte count field 22(e) contains the number of bytes in the associated buffer that are to be transferred from the buffer by the MAC. This field is set by the device driver and used by the MAC. Each buffer associated with the descriptor has a data buffer address field and a buffer byte count field located in the descriptor.

A data buffer address and byte count field 22(f) contains other frame or partial frames to be transmitted. The buffer structure is built by the host software and is used by the MAC device as the source of the data to be transmitted.

FIG. 3 reflects the transmit frame descriptor status at the time the frame has been moved from the host into the MAC and not after the frame has been transmitted to the medium or network. In FIG. 3, bit 13 is used by the MAC device as a "retry bit" to report a transmit underrun to the device driver when the MAC device cannot provide data to the network at a rate required to maintain the transmission rate. A frame transmit threshold field is generated by the device driver in the last 8-bits of the parameter/status field.

Returning to FIG. 1, the MAC device is coupled to the bus 18 by means of an interface 26 under the control of a processor 28 for transferring data from the host buffer 24 to a transmit storage device, e.g. RAM 30. Data is moved in logical blocks. Block size is a parameter set by the device driver, as is the frame threshold field of the host descriptor. The threshold field is specified for each frame. Data is transferred from the RAM 30 to a medium independent interface 34 under the control of a transmit logic for output to the network 10. The unit 32 is further described in a copending application, Ser. No. 08/982,726, filed Dec. 2, 1997, entitled "A System and Method for Automatic Retry of a Transmit, Independent of a Host Processor, After an Underrun Occurs in a LAN" (RA9-97-062) assigned to the same assignee as that of the present invention and fully incorporated herein by reference.

More specifically, the transmitted frame from the host to the network, the descriptors and data buffers associated with that frame are built up by the device driver in the host memory. The frame may require more than one host buffer. The number of buffers required depends on the length of the frame and the number of bytes placed in each buffer. Information regarding the number of buffers, including those with zero data bytes, the starting host memory address of each buffer, and the number of bytes in each buffer are contained in the frame descriptor. The order in which the buffers are transmitted is implied by the descriptor.

The transmit block threshold which is the last 8-bits of the parameter/status field is set by the host software to indicate to the processor 28 how many blocks of transmit frame data to transfer to the transmit RAM 30 before initiating a transmit. This field is initialized to a non-zero value by the host software. Data is transferred by the processor 28 in the following manner: The first block transferred is the same size as the transmit block size set in the block-sized register. Subsequent blocks are transferred using a block size determined by the device driver. The block size is conveyed to the processor 28 via write register (not shown). The processor 28 will initiate transmission of a frame once the number of blocks indicated in the transmit threshold field has been transferred from host memory to the transmit RAM 30. If the transmit RAM underrun occurs, the processor will retransmit the frame by transferring the frame data again from host memory to the transmit RAM, but transmission will not begin until the transmit threshold plus one block has been transferred.

After each frame is transferred from host memory to the transmit RAM, the transmit descriptor parameters/status field is updated by the processor 28. If an underrun occurs, the processor 28 indicates this to the device driver 20 via the "retry" status bit in the host descriptor. This bit indicates that an underrun occurred and a "retry" was necessary. The device driver uses the "retry" bit to determine the value to which the transmit threshold should be set in the host descriptor. The device driver maintains internal counters to determine the optimum threshold, as will now be described in FIG. 4 taken in conjunction with FIG. 5.

In FIG. 4, counters are incremented with each timer tick from a timer (not shown) associated with the device driver. The counters shown in FIG. 1A, are maintained over two time intervals: One for determining whether the threshold should be raised, and the other for determining whether the threshold should be lowered.

The timing intervals are shown in FIG. 5 along a time axis in which the timer ticks are indicated for lowering or raising the threshold. The interval or timer ticks for lowering the threshold is long while the interval for raising the threshold is short. The raise interval is a sub-interval of the lower threshold interval. The intervals are monitored by the counters checking for timer ticks. The counters used in the threshold determining process are as follows:

(1) TXUpFrameCount--Frames transmitted in the "raise threshold" interval.

(2) TXUpRetryCount--Frames retried in the "raise threshold" interval.

(3) TXUpTimerTicks--Time spent in the "current raise threshold" interval.

(4) TXDownFrameCount--Frames transmitted in the "lower threshold" interval.

(5) TXDownRetryCount--Frames retried in the "lower threshold" interval.

(6) TXDownTimerTicks--Time spent in the "current lower threshold" interval.

Returning to FIG. 4, the frame threshold is adjusted on a frame-by-frame basis in accordance with the following process:

The process is entered in a step 113 in which the transmit status is tested to determine if a frame has been transmitted. A "no" condition returns the process to the initial condition. A "yes" condition initiates a step 114 to increment counter (1) TxUpFrameCount and counter (4) TXDownFrameCount.

In a step 116, the transmit status is tested to determine whether the frame transmission has been posted with a "retry" or "underrun" condition. A "no" condition returns the process to the "start" state. A "yes" condition increments the counter (2) TxUpRetryCount and the counter (4) TxDownRetryCount in a step 116.

In a step 117, the time ticks are checked and the counter (6) TxDownTimerTicks is incremented in a step 118, after which the counter (6) is compared to a timer tick limit set for the lower threshold interval. A "no" condition returns the process to checking timer ticks in step 117. A "yes" condition initiates a test 200 to determine if the counter (5) TxDownRetryCount is greater than zero. A "no" condition initiates a step 210 to lower the threshold by one block. A "yes" condition for the test 200 advances the process to the step 220 in which the counters (4), (5) and (6) are reset in a step 220, after which the process returns to step 117 in checking the timer ticks.

In a step 230, the timer ticks are used to increment the counter (3) TxUpTimerTicks in a step 240, after which the counter is compared to the limit for the "raise threshold" interval in a step 250. A "no" condition returns the process to checking time ticks in the step 230. A "yes" condition initiates a test to determine if counter (2) TxUpRetryCount is greater than the limit. A "yes" condition raises the threshold by one block in a step 270. A "no" condition advances the process to a step 280 to reset the counters (1), (2) and after which the process returns to step 230 in checking timer ticks.

Summarizing the process increments the transmit up frame counter and the transmit down frame counter for every frame transmitted. The transmit up retry counter and the transmit down retry counter are incremented for every frame completed with a retry/underrun condition. At the end of the "raise threshold interval" if a certain percentage (e.g., 1% of the frames transmitted were retried), the threshold is raised by one block to not exceed the size of the frame. At the end of this interval, the relevant counters are reset. At the end of the "lower threshold interval" if no frames were retried, the threshold is lowered by one block to a minimum of one block. At the end of this lower threshold interval, relevant counters are reset. Using this process, the transmit threshold is raised or lowered during operation to minimize the number of underruns and maximize the performance. The process shown in FIGS. 4 and 5 determine and adjusts the threshold which is relayed to the processor 28 by way of the host descriptor on a frame-by-frame basis.

While the invention has been shown and described in conjunction with a preferred embodiment, various changes may be made in the spirit and scope of the system and method without departing from the spirit and scope of the invention, as defined in the appended claims, in which: