WO1995001029A1

WO1995001029A1 - Flow control method

Info

Publication number: WO1995001029A1
Application number: PCT/GB1994/001323
Authority: WO
Inventors: John Spencer Arnold
Original assignee: Gpt Limited
Priority date: 1993-06-21
Filing date: 1994-06-20
Publication date: 1995-01-05
Also published as: GB9412331D0; GB2281005A; ZA944435B; GB9312741D0; AU6976194A

Abstract

In an ATM network, traffic can be self-regulated to avoid congestion by a method where the 'congested' indication provided for in the Payload Type field of the standard cell format is set by a switch when the size of queues provided in the switch to statistically multiplexed switched inputs onto output ports suggests the approach of congestion and where the said 'congested' indication is subject to a check at the receiving port of the network and a message is sent to the source port if the proportion of cells carrying the 'congested' indication on a particular channel is relatively high and where, on receipt of such a message, the permitted bandwidth of the channel is reduced and, if no congestion messages are received over a specified period the permitted bandwidth is increased, said reduction and increase of bandwidth being in small increments between absolute limits.

Description

FLOW CONTROL METHOD

A concept for an intrinsically connectionless ATM network is described in Patent Application No. GB 2,255,257A "ATM Mini-Switch for Business Systems": addressing in this solution closely follows International Telegraph and Telephone Consultative Committee (CCITT) standards with the Destination Address (which defines the path through the network) carried in the 12-bit VPI field of the NNI format and the Source Address (which defines the "channel" at the destination) carried in the most-significant 12 bits of the VCI field, the least signⁱficant 4 bits being reserved to provide up to 16 Virtual Channels between source and destination or to provide source and/or destination addresses for 4-way multiplexers.

The above solution has the disadvantage that the number of addresses available is limited to 4,096 and a second solution is described in Patent Application No. GB 9300279.8, published as GB2274227A, which uses the available address space in the standard cell header format more efficiently to provide for 32,678 possible addresses (still retaining the reserved 4 bits) at the cost of ignoring the standard division between VPI and VCI fields.

The second patent application describes a signalling system which utilises the same routing mechanism on the destination address but where the source address is replaced by one of a small number, for example three, reserved values which are used to identify one of several types of equipment unit which may be connected to a peripheral port (user port) of the network; including at least an Exchange Terminator (ET) adjacent to the switch, a Network Terminator (NT) at the user end of the connection link, and a Multiplexer between ET and NT: the source address is displaced to the cell payload. By this means signalling messages may be transported between a network manager and peripheral units, between peripheral units on the one network port, and between peer units on different network ports.

A network of the kind described requires regulation of the traffic flows carried on shared bearers within the core of the network in order to ensure an equitable sharing of the available resource and a satisfactory quality of service. One of the important advantages of this type of network is that there is no centralised control; the network is fully distributed and this principle needs also to be applied to traffic management; the principal mechanism used is therefore self-regulation.

The CCITT standard cell header includes a 3-bit "Payload Type" field which carries a "congested" indication: all switches are required to set this condition in cells when conditions within the switch, for example excessively large queues, suggests the approach of congestion on relevant bearers. The way in which this indication is to be used in not specified.

An AMiS network, being a "connectionless" network does not carry channels recognisable to the core network and is thus totally dependent upon the operation of the self-regulating mechanism, thus a formal mechanism is necessary.

In order to ensure good statistical smoothing within the network, the 16 Virtual Chanels identified by the 4-bit reserved field (multiplexer ports are also treated as Virtual Channels) are monitored in the upstream direction in the ET and "throttled" to a default value which is a proportion of the bandwidth of core network bearers P_n, a typical value of which is 1/16.

At the destination port the ET will examine the Payload Type field for the "congested" indication. Persistence of this indication in a significant proportion of cells received on a Virtual Channel will result in a signalling message being returned to the peer ET using the last received source address in a cell carrying the "congested" indication. At the source ET receipt of this message results in the value of P_n being decremented by a value P^, typically 1/64. If no congestion messages are received in an interval T_d the value of P_n is incremented by a value P.,-, typically 1/256. The decrement and increment algorithms have limits set for P_n of typically 1/64 and 1/8 respectively, however these limits may be modified by instructions from the NT; for example if the user port is used for a Constant Bit Rate (CBR) service then P_n may be frozen at some intermediate value.

The mechanism described is illustrated in the accompanying single figure.

According to the present invention there is provided a method of self-regulating traffic to avoid congestion in an ATM network where the "congested" indication provided for in the Payload Type field of the standard ce⁷l format is set by a switch when the size of queues provided in the switch to statistically multiplexed switched inputs onto output ports suggests the approach of congestion and where the said "congested" indication is subject to a check at the receiving port of the network and a message is sent to the source port if the proportion of cells carrying the "congested" indication on a particular channel is relatively high and where, on receipt of such a message, the permitted bandwidth of the channel is reduced and, if no congestion messages are received over a specified period the permitted bandwidth is increased, said reductions and increases of bandwidth being in small increments between absolute limits.

Note that a "throttle" is provided at the source end for each possible value of the 4-bit VCI/MUX field; similarly, persistence of the "congested" indication is separately checked for each value of the VCI/MUX -^ld at the destination end. This solution is not perfect since the source, the same VCI/MUX value may be used to several destina s not all of which may be congested and, at the destination, the sa... i/CI/MUX field value may be received from several sources. Nevertheless this mechanism is considered to be adequate for the majority of uses of network ports.

In order to cater for network ports where tighter discrimination is considered to be necessary a "disable self-regulation" message may be sent from NT to ET on the same port; in this case, "throttling" is held at the default value P^, typically 1/16 of the core bearer rate, and regulation is taken over by the NT. The NT is, in fact, a sub-layer of an adaption unit and collaborates closely with the Segmentation And Reassembly (SAR) sub-layer. The SAR sub-layer is required to recognise "Virtual Calls"; a Virtual Call is set-up on receipt of a cell with an unrecognised Source + VCI/MUX address and is terminated by receipt of a cell carrying the "end-of-message" indication in the Payload Type field. The NT/SAR sub-layers are therefore able to provide self-regulation of Virtual Calls but at the price of increased complexity and hence cost; thus regulation at the ET is preferred for the majority of cases.

Each individual switch in such a network will interpret the Destination Address to a switch output port, thus successive cells between the same source and destination will follow the same path; this ensures that sequence integrity is retained. The interpreter table at each switch input port (or common to all the inputs on the one switch) is set up and maintained by a Resource Manager which is an application run on a PC on any network port. In order to determine optimum routings the Resource Manager must have access to information on the traffic carried by each core network bearer. The queue size indication used to set the "congestion" indication into the cell Payload Type field is not a good indication for this purpose since the relationship between traffic and queue size is highly non-linear and provides little useful guidance under normal operating conditions with relatively light loads. Instead, the running average occupancy of each switch output bearer is monitored and held in the switch control memory where it is available for polling by the Resource Manager; for this purpose a signalling address is allocated to each switch.

A conventional wisdom in traditional telecommunications systems is that bandwidth is an expensive commodity which must be conserved; a broadband system will, however, use optical fibre interconnections at least in the core network and for fibre technology bandwidth is a relatively cheap commodity (and the cost is reducing as the technology matures). Thus, in order to provide the network with resilience to short-term traffic peaks, the target load under normal operating conditions should be in the region of 40-60%. The Resource Manager software provides a set of tools which will enable high occupancy bearers to be easily identified and paths to be re-routed; the software suite will also provide for storing approved changes which will be implemented during low traffic periods, say at night, since path changes can result in temporary loss of cells and loss of sequence integrity.

The self-regulating mechanism previously described can only operate effectively in a well-balanced network thus it is important that a periodic check is made by the Resource Manager; the self-regulating mechanism will however tend to distort the results. Because of the non-linear relationship between queue size and bearer occupancy, this will only be a problem if the network becomes heavily loaded or seriously unbalanced and for this case a broadcast message is sent to all ETs to instruct them to turn off the self-regulator and set the bandwidth to the default value P_d.

Claims

1. A method of self-regulating traffic to avoid congestion in an Asynchronous Transfer Mode (ATM) network where the "congested" indication provided for in the Payload Type field of the standard cell format is set by a switch when the size of queues provided in the switch to statistically multiplexed switched inputs onto output ports suggests the approach of congestion and where the said "congested" indication is subject to a check at the receiving port of the network and a message is sent to the source port if the proportion of cells carrying the "congested" indication on a particular channel is relatively high and where, on receipt of such a message, the permitted bandwidth of the channel is reduced and, if no congestion messages are received over a specified period the permitted bandwidth is increased, said reduction and increases of bandwidth being in small increments between absolute limits.

2. An ATM network using the self-regulating mechanism of Claim 1 of the "direct-connectionless" type where the absolute source and destination address is carried in the header address field of each cell and where the destination address is interpreted in each switch in the path to a switch output port address so that successive cells between the same source and destination will follow the same path so retaining sequence integrity and where a Resource Manager is provided which has signalling access to all switches in the network, said switches providing means to determine the running average occupancy of each switch port and to hold this information for polling by the Resource Manager and where the said Resource Manager tests the network periodically for excessive bearer loading, makes recommends on the re-routing of paths through the network and stores approved recommendations for implementation at a time when the traffic in the network is low.

3. An ATM network using the self-regulating mechanism of Claim 1 of the "connection-mode" type where the header address field carries a "Virtual Path" identifier and a "Virtual Channel" identifier, said identifiers being labels which identify respectively a path or a channel on a single bearer and where the identifier is translated to a new unique value on the outgoing bearer of a switch using values held in a table in each switch, said table values being set up according to information supplied by signalling messages at the commencement of a call and where the values are determined by a routing algorithm utilising information supplied by the user on the characteristics of the service required when the call demand is originated and the average occupancy of relevant network bearers, said bearer occupancies being maintained in an up-to-date state by polling, from time-to-time, all switches in the network for the running average occupancy on ports of said switches.