1. Field of the Invention
This invention relates to interfacing a node, including an end node such as a work station, with a LAN (Local Area Network) link in a communication network. More particularly, this invention relates to maintaining data frame continuity without extensive data buffering in an environment where data rate capability of a link is much higher than the data rate capability of a node attached to the link.
2. Description of Related Art
One technique for increasing the data capacity a work station may use in a LAN is to decrease the number of stations that share the data capacity of that LAN. In FIG. 1, work stations 10 and 12, requiring very high capacity (for example, file servers) can be attached to a dedicated switch port in a dedicated capacity concentrator 14. Concentrator 14 contains a data transfer unit 13, such as a crossbar switch and ports 15. Ports 15 connect to communication links that, in turn, are connected to work stations or other LAN networks. Ports 15 would contain extensive buffering and communication media controls to implement the communication media protocol.
Ports 15A and 15B are connected as full duplex links to work stations 10 and 12, respectively. Port 15C is connected to shared capacity concentrator 16. Concentrator 16 has work stations 18, 19 and 20 connected to it in a ring network configuration. If the capacity of the ring network 16 is 100 Mbps (mega bits per second) and there are three work stations sharing the capacity of the ring, each station effectively has an average useable data rate of 33.3 Mbps. In contrast, each of the stations 10 and 12 has its own full duplex communication link, and uses, simultaneously, the entire 100 Mbps capacity in each direction of the full duplex link (effectively 200 Mbps capacity).
A Local Area Network with single full duplex 100 Mbps links between crossbar switches in the network is taught in an article by M. D. Schroeder et al, entitled "Autonet: A High-Speed, Self-Configuring Local Area Network Using Point-to-Point Links," published in IEEE Journal On Selected Areas In Communications, Vol. 9, No. 8, October 1991 (pp. 1318-1335).
Schroeder et al, in the above-cited article, recognize that in a network with high capacity data links, the data may back up through the network if a crossbar switch cannot keep up with the data rate. Autonet provides 4 kB (kilo byte) FIFO (First In First Out) buffers at the receiving switch ports in the crossbar switches and flow control for the links. The flow control between sending and receiving crossbar switch stations is accomplished by controls in the receiving switch station issuing start/stop signals to the sending switch station.
This problem of data backup on high-speed data links is greatly accentuated at the link interface to user work stations. Usually, a work station has an internal bus that has a much lower capacity than the data link. For example, a personal computer, as a work station, has an internal bus with a typical capacity of approximately 20 Mbps in contrast to a high-speed data link with a capacity of 100-200 Mbps. In this situation, data frames must be buffered at the work station during both receive and transmit operations. Since data frames are lost if a buffer overflows or underflows, and since data frames are typically 4 kB (kilo Bytes) in length, large buffers, 64 kB to 256 kB, are required at the work station. In a communication network switching node, or in an end node (personal computer system or work station), this equates to having to add expensive memory chips on each communication adapter card.