Patent Publication Number: US-5633870-A

Title: Method and apparatus for controlling data flow through an ATM interface

Description:
FIELD OF THE INVENTION 
     The present invention relates to Asynchronous Transfer Mode (ATM) communication, and more specifically, to a method and apparatus for controlling the flow of data between memory on a host computer and an ATM network. 
     BACKGROUND OF THE INVENTION 
     In an ATM network, data flows from memory on a host computer (&#34;host memory&#34;) through an ATM interface to the ATM network. The ATM interface must package the stream of data it receives from the host memory into ATM cells prior to delivering the data to the ATM network. The stream of data may include data that is to be sent on numerous different channels within the ATM network. Therefore, the ATM interface must process and package the data based on the channels on which the data is to be transmitted. 
     FIG. 3 illustrates a typical ATM interface 100 that connects a host computer 101 with an ATM network 103. ATM interface 100 may either reside on the host computer 101 or on an interface card which plugs into a slot in the host computer 101. ATM interface 100 includes a DMA unit 102, a local buffer 104, a segmentation unit 106 and cell timers 108. 
     The DMA unit 102 is coupled to a host memory 116 on the host computer 101 over a system bus 114. The DMA unit 102 moves data from the host memory 116 to the local buffer 104. The local buffer 104 temporarily holds the data until the data is retrieved by the segmentation unit 106. The local buffer 104 couples the DMA unit 102 to the segmentation unit 106. The segmentation unit 106 is coupled between the local buffer 104 and the ATM network 103. Segmentation unit 106 retrieves the data from local buffer 104, packages the data into ATM cells, and transmits the ATM cells to ATM network 103. 
     The DMA unit 102, local buffer 104 and segmentation unit 106 constitute the data path between the host memory 116 and the ATM network 103. This same data path is used for all data sent from the host computer 101 through ATM interface 100 to the ATM network 103. However, the parameters which control the data flow between the host computer 101 and the ATM network 103 may be different for each channel. Thus, segmentation unit 106 may have to process data for one channel based on different parameters than data for another channel. 
     For example, each of the connections in an ATM network is typically set up with a desired data transmission rate. ATM interface 100 attempts to transmit cells for each connection at a rate which matches the desired transmission rate for the connection. In the illustrated ATM interface 100, the transmission rates for the various channels are controlled by cell timers 108. Each channel has a cell timer. Each cell timer transmits a cell transmit request to segmentation unit 106 to indicate that a cell should be transmitted on the channel that corresponds to the cell timer. Each cell timer is configured to transmit such requests at a rate that corresponds to the desired transmission rate associated with its channel. The various cell timers 108 compete with each other to request the segmentation unit 106 to transmit cells for their channels. 
     The efficiency of ATM interface 100 may be adversely affected if DMA unit 102 and segmentation unit 106 do not work together to ensure a smooth flow of data. For example, the bandwidth of the ATM network 103 will be underutilized if the segmentation unit 106 spends time trying to transmit data from a channel for which insufficient data is currently stored in local buffer 104. As mentioned above, the cell timers 108 transmit requests to the segmentation unit 106 to transmit cells for their corresponding channels. Once a request is received by segmentation unit 106, the segmentation unit 106 must retrieve a number of cell parameters for that particular channel. However, when a cell timer makes a request, the cell timer does not know if there is enough data stored in local buffer 104 to construct a cell for its channel. The segmentation unit 106 will waste time if upon reading those cell parameters, it is discovered that the local buffer 104 does not contain sufficient data for the channel to construct a cell for the channel. 
     Lack of cooperation between the segmentation unit 106 and DMA unit 102 may also lead to inefficiency in data transfers between the host memory 116 and local buffer 104. Specifically, each channel is allotted some fixed amount of memory in the local buffer 104. If the DMA unit 102 fills the memory for each channel randomly, the DMA unit 102 may begin to fill the local buffer memory for a channel even though the local buffer memory for that channel is almost full. When this occurs, the DMA unit 102 is only able to move a small amount of data into the local buffer. The DMA unit 102 would then have to save all the parameters for that channel and retrieve new parameters for another channel before beginning another data movement. When the DMA unit 102 is only able to move a small amount of data before switching to another channel, the DMA unit 102 is less efficient. 
     When there is data in the host memory 116 that is to be transmitted through ATM interface 100 to the ATM network 103, the host computer 101 sends signals to the ATM interface 100 indicating the data to be transmitted. When these signals arrive, the ATM interface 100 may not be able to immediately perform the specified transfer. For example, the DMA unit 102 of the ATM interface 100 may be busy transferring data from one channel, the segmentation unit 106 may be busy transmitting a cell from another channel, while signals from the host computer 101 indicate that data is to be transferred from host memory 116 to local buffer 104 for yet another channel. Under these conditions, the ATM interface 100 cannot immediately perform the data transfer for the host memory 116, but must remember that the data is to be transferred. Consequently, when DMA unit 102 becomes available, the appropriate data may be transferred from the host memory 116 to the local buffer 104. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for controlling the flow of data between a host computer and an ATM network is provided. The apparatus resides on an ATM interface and includes a state machine for each channel supported by the ATM interface. The state machine moves from state to state when (1) an operation is scheduled for transferring data for the channel from the host computer to a local buffer, (2) the host computer indicates that data for the channel is ready to be transferred from the host computer to the local buffer, (3) all of the data in the host computer that is to be sent on the channel has been copied into the local buffer, (4) a portion of the local buffer that is allocated for the channel becomes full before all of the data in the host computer that is to be sent on the channel has been copied into the local buffer, (5) the local buffer contains an mount of data for the channel that exceeds a first predetermined threshold, or (6) the local buffer contains an amount of data for the channel that falls below a second predetermined threshold. 
     The ATM interface includes a DMA unit and a segmentation unit that operate responsive to the states of the various state machines to avoid inefficient transfer operations. Specifically, the DMA unit does not attempt to move data for a channel from the host computer to the local buffer if the state of the state machine for the channel indicates that (1) there is no more data in the host computer for the channel or (2) there is no more room in the local buffer to receive data for the channel from the host computer. The segmentation unit does not attempt to transmit cells for a channel when the state of the state machine for the channel indicates that the local buffer does not contain enough data for the channel to construct a cell for the channel. 
     According to one aspect of the invention, an apparatus for controlling a flow of data between a host computer and a network is provided. The apparatus includes a local buffer, a timer, a channel flow controller and a segmentation unit. The local buffer stores data to be transmitted onto the network. 
     The timer corresponds to a channel. The timer is configured to generate cell transmit requests. The channel flow controller is coupled to the timer. The channel flow controller generates an enable signal associated with the channel. The channel flow controller generates the enable signal when the local buffer contains enough data for the channel for a cell to be constructed for the channel. 
     The segmentation unit is coupled to the timer, the channel flow controller and the local buffer. The segmentation unit reads data from the local buffer and transmits the data in cells onto the network on the channel in response to the cell transmit requests from the timer. The segmentation unit responds to the cell transmit requests from the timer if and only if the channel flow controller is generating the enable signal. Because the enable signal is generated when the local buffer contains enough data for a cell to be constructed for the channel, the segmentation unit does not waste time attempting to transmit cells for a channel when there is not enough data to create cells for the channel. 
     The apparatus further includes a DMA unit that transfers data from the host computer to the local buffer. Rather than perform the transfers randomly, the DMA unit performs transfers by servicing entries stored in a scheduler queue. Each entry in the scheduler queue identifies a channel for which the DMA unit is to transfer data from the host computer to the local buffer. An entry for a channel is placed in the scheduler queue when the amount of data for the channel in the scheduler queue falls below a predetermined threshold. Because the DMA unit only services the channels identified in the scheduler queue, the DMA unit never begins a transfer operation for channels whose portion of the local buffer is almost full. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 illustrates an exemplary computer system network incorporating the ATM network interface circuit which utilizes the method and apparatus of data transfer coordination of the present invention; 
     FIG. 2 is a simplified system diagram illustrating the architecture of an ATM network interface circuit in accordance with a preferred embodiment of the present invention; 
     FIG. 3 illustrates a typical ATM interface for connecting a host computer with an ATM network; 
     FIG. 4 is a block diagram illustrating the transmit circuitry of a network interface circuit according to one embodiment of the invention; and 
     FIG. 5 illustrates a channel flow control state machine used by the transmit circuitry to control the flow of data for each channel from the host computer to the ATM network according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates an exemplary computer system network incorporating the asynchronous transfer mode (ATM) network interface circuit of the present invention. The computer system network 10 includes host computer systems (not shown) which incorporate one or more of the ATM network interface circuits (NIC) 12. The NICs 12 are coupled through a local ATM switch 14 to a public ATM switch 16 to enable asynchronous transfer of data between host computer systems coupled to the network 10. Alternately, the NICs 12 can be coupled directly to the public ATM switch 16. As shown in FIG. 1, the computer system network 10 may also include computer systems which incorporate the use of a Local Area Network (&#34;LAN&#34;) emulation 15 which serves as a gateway for connecting other networks such as Ethernet or token ring networks 17 which utilize the ATM network as a supporting framework. 
     FIG. 2 is a simplified system diagram illustrating the architecture of the ATM NIC 12 in accordance with a preferred embodiment of the present invention. The ATM NIC 12 interfaces a host computer system 48 coupled through system bus 38 to the network media 40 operating in accordance with the ATM protocol. 
     The ATM NIC 12 shown includes a System Bus interface 20, a Generic Input/Output (&#34;GIO&#34;) interface 24, a System and ATM Layer Core 22, a Local Slave interface 26, a transmit (TX) FIFO 28, a receive (RX) FIFO 30, a Media interface 32, an External Buffer Memory Interface 34 and a clock synthesis circuit 36. 
     Together, the elements 20-36 of NIC 12 cooperate to transfer data between the host computer 48 and the other computers in the network through multiple, dynamically allocated channels in multiple bandwidth groups. Collectively, the elements of the network interface circuit 12 function as a multi-channel intelligent direct memory access (DMA) controller coupled to the System Bus 38 of the host computer system 48. In a preferred embodiment, multiple transmit and receive channels are serviced as virtual connections utilizing a full duplex 155/622 Mbps physical link. Multiple packets of data, subscribed to different channels over the System Bus 38 to the external buffer memory 42, via the External Buffer Memory Interface 34, are segmented by the System and ATM Layer Core 22 into transmit cells for transmission to the Media 40 through Media interface 32. The Core 22 also comprises reassembly logic to facilitate reassembly of the received cells to packets. 
     The TX and RX FIFOS 28, 30, coupled between the Core 22 and the Media Interface 32, are used to stage the transmit and receive cell payloads of the transmit and receive packets respectively. The Media Interface 32 transmits and receives cells to the Media 40 of the network, driven by clock signals provided by Clock Synthesis Circuit 36. Preferably the Media, and therefore the Media interface 32, conforms to the Universal Test and Operations Physical Interface for ATM (&#34;UTOPIA&#34;) standard, as described by the ATM Forum Ad Hoc specification. To conform to the UTOPIA specification, the clock synthesis circuit 36 provides either a clock signal of 20 MHz or 40 MHz to enable the Media interface 32 to support a byte stream at 20 MHz for 155 Mbps or a 16 bit stream at 40 MHz for a 622 Mbps data stream. 
     In the present embodiment, the Media Interface 32 receives 52-byte data cells each having a 4-byte cell header and a 48-byte payload from the TX FIFO 28. The Media Interface 32 inserts a checksum as a fifth byte to the cell header into each cell prior to providing the 53-byte data cell to the Media 40. Conversely, when the Media Interface 32 receives cells from the Media 40, it examines the checksum in the fifth byte of each cell to determine if the checksum is correct. If so, the byte representing the checksum is stripped from the cell and the cell is forwarded to the RX FIFO 30. Otherwise, the entire cell is dropped. 
     The System Bus Interface 20 and GIO interface 24 insulate the host computer system 48 from the specifics of the transfer to the Media 40. Furthermore, the Core 22 is insulated from the specifics of the system bus 38 and host specifics. In the present preferred embodiment, the System Bus is an S-Bus, as specified in the Institute of Electronics and Electrical Engineers (&#34;IEEE&#34;) standard 1496 specification. The System Bus Interface 20 is configured to communicate in accordance with the specifications of the system bus, in the present illustration, the S-Bus. It is contemplated that the System Bus Interface 20 can be configured to conform to different host computer system busses. The System Bus Interface 20 is also configured to transfer and receive data in accordance with the protocols specified by the GIO interface 24. The GIO interface 24 provides a singular interface through which the Core 22 communicates with the host computer. Thus, the Core 22 does not change for different embodiments of the NIC 12 which interface to different host computer systems and busses. 
     The host computer system 48 includes host memory 49 which contains data packets and pointers to the packets being transmitted and received. As noted previously, the NIC 12 also shields the cell delineation details of asynchronous transfer from the applications running on the host computer system. For present purposes, it is assumed that software running on the host computer system 48 manage transmit and receive data using wrap around transmit and receive rings with packet interfaces as is well known in the art. 
     FLOW CIRCUITRY OVERVIEW 
     Referring to FIG. 4, it is a block diagram illustrating transmit circuitry 200 of a network interface circuit 12 according to one embodiment of the invention. In addition to a DMA unit 202, a local buffer 204, a segmentation unit 206 and cell timers 208, transmit circuitry 200 includes a channel flow controller 210 and a set of AND gates 212. Channel flow controller 210 includes a scheduler 216 and a plurality of state machines 218. Each of state machines 218 corresponds to a particular channel supported by transmit circuitry 200. For each channel supported by transmit circuitry 200, channel flow controller 210 is configured to generate an enable signal based on the current state of the state machine 218 for the channel. Scheduler 216 and state machines 218 shall be described in greater detail below. 
     Channel flow controller 210 is coupled to AND gates 212 by a plurality of lines 214. Each of lines 214 corresponds to a particular channel supported by transmit circuitry 200. Channel flow controller 210 transmits the enable signal for each of the various channels over the line 214 that corresponds to the channel. 
     AND gates 212 correspond to the channels supported by transmit circuitry 200. The inputs of the AND gate for a given channel are coupled to the cell timer 208 for that channel and to the line 214 that is associated with that channel. If the line 214 associated with a channel carries an enable signal, then the cell transmit requests from the cell timer 208 associated with the channel pass through the corresponding AND gate 212 and are received by segmentation unit 206. If the line 214 associated with a channel does not carry an enable signal, then the cell transmit requests from the cell timer 208 associated with the channel do not pass through the corresponding AND gate 212 and are not received by segmentation unit 206. 
     FLOW CONTROL STATE MACHINE 
     Referring to FIG. 5, it illustrates a channel flow control state machine 300. Channel flow control state machine 300 includes a plurality of states, including a reset state 301, a host ready state 302, a segmentation state 303, a data available state 304, a scheduled state 305 and a host ready/segmentation state 306. State machine 300 moves from state to state in response to events that occur within transmit circuitry 200. According to one embodiment of the invention, channel flow controller 210 includes a state machine 300 for each channel supported by transmit circuitry 200. 
     By providing a state machine 300 for each ATM channel, channel flow controller 210 avoids the flow problems described above with reference to ATM interface 100. Specifically, channel flow controller 210 prevents the segmentation unit 206 from receiving requests to transmit cells for channels when there is not enough data in local buffer 204 to construct cells for the channels. Further, channel flow controller 210 prevents DMA unit 202 from performing DMA transfers from host memory 116 to local buffer 204 for a particular channel when the memory within local buffer 204 that is dedicated to storing data for the channel is almost full. In addition, channel flow controller 210 allows for deferred DMA transfers by providing a mechanism for remembering when data for a channel is ready for transfer from the host memory 116 into the local buffer 104. 
     INSUFFICIENT BUFFERED DATA CONDITIONS 
     State machine 300 is configured so that the enable signal for a channel is asserted only when (1) there is enough data in local buffer 204 for segmentation unit 206 to transmit a cell for the channel or (2) a DMA transfer for the channel is to be scheduled. Consequently, segmentation unit 206 does not waste time servicing cell transmit requests for channels when there is not enough data to transmit cells for the channels. 
     As mentioned above, channel flow controller 210 generates an enable signal to enable requests to the segmentation unit 206 for a particular channel based on the state of the state machine 300 associated with the channel. In the illustrated embodiment, channel flow controller 210 asserts an enable signal for the channel associated with state machine 300 when state machine 300 is in the segmentation state 303, the data available state 304, the host ready state 302 and the host ready/segmentation state 306. Channel flow controller 210 does not assert an enable signal for the channel associated with state machine 300 when state machine 300 is in the reset state 301 or the scheduled state 305. 
     For the purposes of explanation, it shall be assumed that state machine 300 corresponds to a channel &#34;1&#34; supported by transmit circuitry 200 and that state machine 300 is initially in scheduled state 305. As shall be described hereafter, when state machine 300 is in the scheduled state, the host computer 101 has requested DMA unit 202 to move a particular block of data for channel &#34;1&#34; from host memory 116 into the local buffer 204, but the local buffer 204 does not yet contain enough data for a cell to be constructed for channel &#34;1&#34;. 
     Channel flow controller 210 monitors local buffer 204 to detect when the DMA unit 202 has moved enough data into local buffer 204 for a cell to be constructed for channel &#34;1&#34;. When enough data for a cell to be constructed for channel &#34;1&#34; has been moved into local buffer 204, the channel flow control state machine 300 moves from the scheduled state 305 into a data available state 304. 
     As noted above, the channel flow controller 210 asserts an enable signal for channel &#34;1&#34; when state machine 300 is in the data available state 304. While the enable signal is asserted, cell transmit requests made by the cell timer 208 associated with channel &#34;1&#34; pass through the corresponding AND gate 212 and are received by segmentation unit 206. In response to such request signals, segmentation unit 206 constructs cells for channel &#34;1&#34; from the data contained in local buffer 204 and transmits the cells onto ATM network 103. 
     Channel flow controller 210 monitors DMA unit 202 to detect when DMA unit 202 has finished moving the block of data for channel &#34;1&#34; from host memory 116 to local buffer 204. Once the DMA unit 202 has moved the entire block of data for channel &#34;1&#34; from the host memory 116 to the local buffer 204, the channel flow control state machine 300 enters the segmentation state 303. While state machine 300 is in the segmentation state 303, requests to the segmentation unit 206 for the transmission of cells for channel &#34;1&#34; are still enabled. 
     While state machine 300 is in the segmentation state 303, channel flow controller 210 monitors local buffer 204 to detect when all of the data for channel &#34;1&#34; has been transmitted by segmentation unit 206 to ATM network 103. When the segmentation unit 206 has transmitted all the available data for channel &#34;1&#34;, the channel flow control state machine 300 moves into a reset state 301. While state machine 300 is in the reset state 301, the enable signal for channel &#34;1&#34; is not asserted. Consequently, requests for the transmission of cells on channel &#34;1&#34; do not pass through AND gates 2 12 to reach the segmentation unit 206. 
     If the portion of local buffer 204 that has been allocated for channel &#34;1&#34; data becomes full before the entire block of data for channel &#34;1&#34; has been transferred from the host memory 116, then state machine 300 moves from the data available state 304 to host ready/segmentation state 306. While state machine 300 is in the host ready/segmentation state 306, requests to the segmentation unit 206 for the transmission of cells for channel &#34;1&#34; are still enabled. However, DMA unit 202 ceases to transfer data for channel &#34;1&#34; into local buffer 204. 
     While state machine 300 is in the host ready/segmentation state 306, data is being transmitted from but not moved into the portion of local buffer 204 allocated to channel &#34;1&#34;. Consequently, the amount of data in the portion of local buffer 204 allocated to channel &#34;1&#34; will decrease. When the amount of data in the portion of local buffer 204 allocated to channel &#34;1&#34; fails below a predetermined threshold, state machine 300 enters the host ready state 302. 
     In the host ready state 302, there is not enough data in the local buffer 204 to transmit a cell for channel &#34;1&#34;, and there is data for channel &#34;1&#34; in host memory 116 waiting to be moved into local buffer 204. Under these conditions, a DMA transfer for channel &#34;1&#34; should be scheduled by segmentation unit 206. To cause segmentation unit 206 to schedule the desired DMA transfer, the enable signal for channel &#34;1&#34; is asserted in the host ready state 302 even though there is not enough data in the local buffer 204 to transmit a cell for channel &#34;1&#34;. Scheduling shall be described in greater detail below. 
     ALMOST-FULL BUFFER CONDITIONS 
     As explained above, it is inefficient for the DMA unit 202 to initiate a data transfer operation for a channel when the portion of the local buffer 204 that is allocated to the channel is almost full. To ensure that DMA unit 202 does not initiate inefficient data transfer operations, DMA unit 202 is configured to perform transfers based on the scheduler 216. 
     The scheduler 216 is a queue, where each entry in the queue identifies a channel number. According to one embodiment of the invention, DMA unit 202 is configured to sequentially read and process the entries in scheduler 216. The DMA unit 202 processes an entry by transferring data for the channel identified in the entry from the host memory 116 to the local buffer 204. For example, in response to reading an entry from the scheduler 216 that identifies channel &#34;1&#34;, DMA unit 202 would transfer data for channel &#34;1&#34; from host memory 116 into the portion of local buffer 204 that corresponds to channel &#34;1&#34;. 
     Segmentation unit 206 is configured to insert entries into scheduler 216 under certain conditions. Specifically, the segmentation unit 206 inserts an entry that identifies a particular channel into the scheduler 216 when (1) state machine 300 is in the host ready state 302 and (2) the segmentation unit 206 detects that space within local buffer 204 that is allocated to a particular channel is almost empty. Specifically, when state machine 300 is in the host ready state 302, segmentation unit 206 still receives requests to transmit cells for channel &#34;1&#34; even though local buffer 204 does not contain enough data to construct a cell for channel &#34;1&#34;. When segmentation unit 206 receives a request to service channel &#34;1&#34;, segmentation unit 206 detects that state machine 300 is in the host ready state 302 and therefore knows to schedule a DMA operation for channel &#34;1&#34; by inserting an entry for channel &#34;1&#34; into scheduler 216 rather than attempt to transmit a cell for channel &#34;1&#34;. 
     At the same time that the segmentation unit 206 inserts an entry for channel &#34;1&#34; into the scheduler 216, the channel flow control state machine 300 moves from the host ready state 302 to the scheduled state 305. While the channel flow control state machine 300 remains in the scheduled state 305, the segmentation unit 206 does not to schedule any more DMA activity for channel &#34;1&#34; and transmit requests for channel &#34;1&#34; are not enabled. 
     DMA unit 202 sequentially services the entries in scheduler 216. Eventually, the entry for channel &#34;1&#34; will move to the head of scheduler 216 to be serviced by DMA unit 202. To service the entry for channel &#34;1&#34;, DMA unit 202 will moved data for channel &#34;1&#34; from host memory 116 to local buffer 204. 
     State machine 300 remains in the scheduled state 305 until DMA unit 202 has moved enough data from host memory 116 to the local buffer 204 for segmentation unit 206 to construct a cell for channel &#34;1&#34;. When channel flow controller 210 detects that the channel &#34;1&#34; portion of local buffer 204 contains enough data to form a cell, state machine 300 moves from the scheduled state 305 to the data available state 304, as described above. 
     As explained above, the DMA unit 202 performs transfer operations for the channels identified in scheduler 216. Because the channels identified in scheduler 216 are channels for which little or no data remains in local buffer 204, DMA unit 202 only begins data movement for the channels whose portion of the local buffer 204 is empty or almost empty. Consequently, DMA unit 202 does not waste time initiating transfers for channels whose portion of the local buffer 204 is almost full. 
     DEFERRED TRANSFERS 
     As mentioned above, DMA unit 202 services the channels identified in the entries of scheduler 216. While DMA unit 202 is performing a data transfer for the channel at the head of scheduler 216, host computer 101 may request a data transfer from host memory 116 to local buffer 204 for a different channel. Because DMA unit 202 is busy with a different data transfer operation, it cannot immediately service the requests of the host computer 101. 
     Consequently, the channel flow controller 210 is configured to &#34;remember&#34; that the host computer 101 has data ready for a channel by causing the state machine for the channel to enter a host ready state whenever there is data in the host memory 116 that needs to be moved to the local buffer 204 for that channel. Specifically, when channel flow controller 210 detects a request made by host computer 101 to transfer data from host memory 116 to local buffer 204, the channel flow control state machine that corresponds to the channel identified in the request assumes a host ready state. 
     In the illustrated embodiment, each state machine has two host ready states: host ready state 302 and host ready/segmentation state 306. State machine 300 assumes host ready/segmentation state 306 in response to a request to transfer data for channel &#34;1&#34; when state machine 300 is currently in the segmentation state 303. State machine 300 assumes host ready state 302 in response to a request to transfer data for channel &#34;1&#34; when state machine 300 is currently in the reset state 301. State machine 300 also assumes host ready state 302 when the portion of the local buffer associated with channel &#34;1&#34; is almost empty and state machine 300 is currently in host ready/segmentation state 306. 
     In the host ready state 302, requests to the segmentation unit 206 are enabled even though there is not enough data in local buffer 204 to construct a cell for channel &#34;1&#34;. When the segmentation unit 206 services the request, the segmentation unit 206 detects that state machine 300 is in the host ready state 302 and inserts an entry that identifies channel &#34;1&#34; into the scheduler 216. When the entry identifying channel &#34;1&#34; has been inserted into scheduler 216, the channel flow control state machine 300 enters the scheduled state 305. 
     STATE MACHINE OVERVIEW 
     Table 1 summarizes the states provided by each of state machines 218. Table 2 summarizes the events which cause a state machine to move from one state to another. By using state machines 218 that function to control the flow of data through transmit circuitry 200 in the described manner, inefficient data transfer conditions are avoided. Specifically, DMA unit 202 does not attempt to fill local buffer 204 for channels when little room is left in local buffer 204 for the channels, and segmentation unit 206 does not attempt to transmit cells for channels when there is not enough data in local buffer 204 to construct cells for the channels. 
     
                       TABLE 1                                                     
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STATE         CONDITIONS                                                  
______________________________________                                    
RESET 301     Local buffer 204 does not contain                           
              enough data to transmit a cell for the                      
              channel.                                                    
              Host memory 116 does not currently                          
              have data for the channel.                                  
              A DMA transfer has not been scheduled                       
              for the channel.                                            
              Cell transmit requests are not enabled                      
              for the channel.                                            
HOST READY 302                                                            
              Local buffer 204 does not contain enough                    
              data to transmit a cell for the channel.                    
              Host memory 116 has data for the channel.                   
              A DMA transfer has not been scheduled for                   
              the channel.                                                
              Cell transmit requests are enabled.                         
SEGMENTATION 303                                                          
              Local buffer 204 contains enough data to                    
              transmit a cell for the channel.                            
              Host memory 116 does not currently have                     
              data for the channel.                                       
              A DMA transfer has not been scheduled for                   
              the channel.                                                
              Cell transmit requests are enabled for                      
              the channel.                                                
DATA AVAILABLE 304                                                        
              Local buffer 204 contains enough data to                    
              transmit a cell for the channel.                            
              Data for the channel is being transferred                   
              from host memory 116 to the local buffer                    
              204.                                                        
              Cell transmit requests are enabled for                      
              the channel.                                                
SCHEDULED 305 Local buffer 204 does not contain enough                    
              data to transmit a cell for the channel.                    
              Host memory 116 currently has data for                      
              the channel.                                                
              A DMA transfer has been scheduled for                       
              the channel.                                                
              Cell transmit requests are not enabled for                  
              the channel.                                                
HOST READY/   Local buffer 204 contains enough data to                    
SEGMENTATION 306                                                          
              transmit a cell for the channel.                            
              Host memory 116 currently has data for                      
              the channel.                                                
              A DMA transfer has not been scheduled for                   
              the channel.                                                
              Cell transmit requests are enabled for                      
              the channel.                                                
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                                           TABLE 2                                 
__________________________________________________________________________
FROM STATE:  TO STATE:    EVENT:                                          
__________________________________________________________________________
HOST READY 302                                                            
             SCHEDULED 305                                                
                          Segmentation unit 206 places                    
                          an entry for the channel in                     
                          scheduler 216.                                  
SCHEDULED 305                                                             
             DATA AVAILABLE 304                                           
                          DMA unit 202 moves enough                       
                          data into local buffer 204 to                   
                          construct a cell for the channel.               
DATA AVAILABLE 304                                                        
             SCHEDULED 305                                                
                          The amount of data contained                    
                          in local buffer 204 for the                     
                          channel falls below the amount                  
                          required to construct a cell for                
                          the channel.                                    
DATA AVAILABLE 304                                                        
             SEGMENTATION 303                                             
                          DMA unit 202 finishes                           
                          transferring all of the data for                
                          the channel from host memory                    
                          116 to local buffer 204.                        
DATA AVAILABLE 304                                                        
             HOST READY/  The portion of the local buffer                 
             SEGMENTATION 306                                             
                          allocated to the channel                        
                          becomes full.                                   
HOST READY/  HOST READY 302                                               
                          The amount of data contained                    
SEGMENTATION 306          in local buffer 204 for the                     
                          channel falls below the amount                  
                          required to construct a cell for                
                          the channel.                                    
SEGMENTATION 303                                                          
             RESET 301    The amount of data contained                    
                          in local buffer 204 for the                     
                          channel falls below the amount                  
                          required to contruct a cell for                 
                          the channel.                                    
SEGMENTATION 303                                                          
             HOST READY/  The host computer 101                           
             SEGMENTATION 306                                             
                          indicates that data for the                     
                          channel is available for transfer               
                          from host memory 116.                           
RESET 301    HOST READY 302                                               
                          The host computer 101                           
                          indicates that data for the                     
                          channel is available for transfer               
                          from host memory 116.                           
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     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.