Controlling the flow of ATM cells in an ATM network

Details Controlling the flow of ATM cells in an ATM network Controlling the flow of ATM cells in an ATM network

1997-12-04

2000-09-05

Ngo, Ricky

Multiplex communications

Data flow congestion prevention or control

Control of data admission to the network

370253, 370395, 370465, H04J 116, H04J 314

Patent

active

061153583

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a method of controlling the flow of ATM cells in an ATM network, and to an ATM network switch in which the method is carried out.


BACKGROUND TO THE INVENTION

The ABR (Available Bit Rate) service in Asynchronous Transfer Mode (ATM) networks is intended to make the best use of remaining capacity after higher priority services such as CBR (Constant Bit Rate) and VBR (Variable Bit Rate) have been provided for. To achieve this, special ATM cells known as Resource Management or Rm cells are sent from the source, through the destination and back to the source to provide information on the congestion level in the switches in the path. This is used by the source to modify its transmission rate, the objective being to utilize link capacity fully while not losing any cells as a result of congestion. An Explicit Rate (ER) parameter is set in the Rm cells passing through the switch in the forwards and backwards directions, the ER being reduced as congestion occurs down to the Minimum Cell Rate (MCR) value to reduce the rate at which the source sends.
In addition, the Message Type field in the Rm cell includes Congestion Indication (CI) and No Increase (NI) bits which may be set respectively to signal to the source that a predetermined decrease in the transmission rate should be effected and that the source should not automatically increase the send rate--it is normally arranged to keep pushing the send rate up again after the ER has signalled a reduction, in order to ensure that the rate is not permanently depressed. The switch may generate a backwards Rm cell (known as BECN) to the source to indicate congestion, the rate of these being limited to 10 per second per connection.
The Explicit Rate or ER parameter thus dictates the maximum rate at which the source is allowed to send in cells per second. MCR dictates the minimum rate at which a source may always send. ER must therefore not be set less than MCR.


SUMMARY OF THE INVENTION

The present invention provides a method of controlling the flow of ATM cells in an ATM network, comprising signalling to the source of cells on a connection to reduce the transmission rate when congestion occurs in a switch in the network, the signalling being carried out by way of insertion into Resource Management Rm cells of a new Explicit Rate (ER) value for the connection, wherein the ER value is obtained at a given point in the network for each cell arriving at said point, said point including a buffer for the cells, by: predetermined time interval to give an ABR Output Bandwidth value (AOB); and
Preferably, the method comprises: number of cells stored in the buffer; buffer; and the remaining capacity BS of the buffer; and calculating the ER value as a function of AOB, N.sub.act VcCnt, TSC and BS.
More preferably, the ER value is calculated from the following formula: ##EQU1## wherein MAX is a predetermined function reflecting the delay from the source of ABR cells in the network to the said point, and K.sub.er is a predetermined offset value which determines the occupancy of the buffer for the VC on which the cell is being transmitted.
Preferably, N.sub.act is a value which is incremented when a cell is written into the buffer on a VC having a VcCnt of less than 16, and the formula is: ##EQU2##
Preferably, the method includes multiplying the number of ABR cells output during the predetermined period by a predetermined constant to give the AOB value.
The invention further provides an ATM network switch comprising a buffer means for buffering the flow of ATM cells and means for calculating the Explicit Rate (ER) value for a connection thereon, the calculating means comprising: predetermined time interval to give an ABR Output Bandwidth value (AOB); and
Preferably the switch comprises: predetermined number of cells stored in the buffer; means for determining the number of cells VcCnt stored in the buffer for the cell's VC; buffer and the remaining capacity BS of the buffer; a function of AOB, N.sub.act, VcCnt, TSC and BS.
More prefer

REFERENCES:
patent: 5400329 (1995-03-01), Tokura et al.
patent: 5515359 (1996-05-01), Zheng
patent: 5787071 (1998-07-01), Basso et al.
patent: 5889761 (1999-03-01), Yamato
patent: 5940370 (1999-08-01), Curtis et al.

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