Multiplex communications – Data flow congestion prevention or control – Flow control of data transmission through a network
Patent
1997-12-04
2000-09-19
Ngo, Ricky
Multiplex communications
Data flow congestion prevention or control
Flow control of data transmission through a network
370252, 370399, 370412, 370129, H04J 314, H04J 316, H04J 322, H04J 324
Patent
active
061222534
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an A synchronous Transfer Mode (ATM) network switch having a buffer for buffering the flow of ATM cells and a congestion control mechanism associated therewith. The invention also relates to a method of controlling congestion in an ATM network switch.
BACKGROUND TO THE INVENTION
Intelligent cell discard schemes have been proposed to allow better throughput in ATM cell switches. ATM cells may either carry individual data or be part of a segmented larger data packet in accordance with AAL5 (ATM Adaptation Layer 5). Individual data cells may be discarded at random when a switch is congested. However, for an AAL5 connection, all ATM cells which comprise a single packet must be delivered through the network to the destination without loss. If a single cell is lost within a packet, the destination will not be able to reconstitute (or reassemble) the packet, and all the other cells received within that packet will be worthless and must be discarded. It will be noted that packets may comprise dozens, or even hundreds of cells.
Intelligent cell discard schemes, often referred to as "Early packet (or Cell) Discard", detect the end of a packet by reading the 3-bit Payload Type (PT) field value of 0X1 (hexadecimal 1) in the ATM cell header, and then setting a flag. When the next user data cell arrives, with a PT value of 0X0 (hexadecimal 0), the level of congestion in the switch is assessed to determine whether there is capacity to accept the entire packet without cell loss. A flag is then set to indicate either acceptance of the whole packet, or that the whole packet is to be discarded. In this way, individual cells within a packet should not be discarded.
Switches become congested when the total input bandwidth exceeds the output bandwidth at a particular switch queuing point. In a typical switch, for example of the type described and claimed in our earlier UK Patent Applications 9507454.8, 9509483.5 and 9509484.3, congestion may occur on the receive side (input to the switch fabric) if the switch fabric is congested and will not allow further cells to be input. It may also occur on the transmit side (output from the switch fabric) where cells arrive faster than they can be transmitted.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an apparatus and a method for assessing current congestion level and determining when to discard a packet (or individual ATM cell).
An ATM network switch according to the invention includes a buffer for buffering the flow of ATM cells and a congestion control mechanism which includes means for allocating buffer space to each virtual connection and means for determining the amount of buffer space available for each virtual connection.
According to one embodiment of the invention, the congestion control mechanism includes means for storing a predetermined value Max, representing the maximum permissible number of ATM cells which may be stored in the buffer for each virtual connection (VC) through the switch, and a predetermined offset value K, which determines the maximum occupancy of the buffer for the VC; means responsive to the arrival of a new ATM cell at the buffer means for determining the total number of cells TSC currently stored in the buffer and the number of cells VcCnt stored for the respective VC for the cell; means for determining the remaining capacity BS of the buffer; calculating means for calculating an adjusted value Max' of Max in accordance with the formula: ##EQU1## and means for comparing Max' with VcCnt and for discarding the cell if VcCnt is greater than or equal to Max'.
According to another embodiment of the invention, the congestion control mechanism includes means for storing an EFCI (Explicit Forward Congestion Indication) Threshold value for each virtual connection (VC) through the switch, and a predetermined offset value K, which determines the maximum occupancy of the buffer for the VC; means responsive to the arrival of a new ATM cell at the buffer for determining the to
REFERENCES:
patent: 5550823 (1996-08-01), Irie et al.
patent: 5787073 (1998-07-01), Ikeda
patent: 5790522 (1998-08-01), Fichou et al.
patent: 5838677 (1998-11-01), Kozaki et al.
Gallagher Thomas A.
General Data Comm Inc.
Gordon David P.
Jacobson David S.
Ngo Ricky
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