Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1999-08-03
2001-05-15
Olms, Douglas W. (Department: 2732)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S229000
Reexamination Certificate
active
06233223
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to apparatus for use at a near end of a link, for controlling admission of connections on a communications on a communication link, and to corresponding methods, and to software for carrying out such methods.
BACKGROUND TO THE INVENTION
Connection Admission Control is one of a number of known techniques for managing and controlling traffic and congestion in connection-orientated networks. In particular, it is used in ATM (Asynchronous Transfer Mode) networks to provide quality of services (QOS) guarantees. It is not limited to use in ATM networks.
Connection Admission Control (CAC) procedures are used to decide if a request for an ATM connection can be accepted, based on the network capacity and the attributes of both the requested connection and existing connections. It is important that there is always enough bandwidth so that quality of service guarantees for the existing connections and the requested connections, can be met.
CAC procedures may be used at an access node at the edge of an ATM network to enable control of access to the entire route through the ATM network as route selection is made. A second level, may be used at each node along the selected route through the ATM network, to confirm that a respective link beyond that node, can admit the connection. This is also called flow control, but for the present purposes, it is intended to be encompassed by the term admission control.
A useful summary of known admission techniques is contained in an article by Perros and Khaled in IEEE communications magazine November 1996, “Call Admission Control Schemes, a Review”. Conventionally, when a CAC algorithm is used, for a link having numerous identified channels, one of these channels will be allocated to the information flow being admitted. As channels can be allocated from either end of the link, there is a risk of a channel being simultaneously allocated by both ends of different information flows. In this case, there is no mechanism to prevent or recover from this, and both information flows would be transmitted but neither would be received. Where the information flows are connections, they would both or either be dropped and the information would be lost.
This has been accepted up to now as the risk is usually very low, depending on the number of free channels, the mechanism for finding and allocating a free channel, and the delay (transmission delay and processing delay) in one side alerting the other side that a channel is no longer free.
However, if it desired to run a network at closer to maximum capacity, and as guarantees of quality of service become more important and more valuable, the inventors have regarded this risk of simultaneous allocation as a potential problem.
Another potential problem not addressed by conventional CAC schemes is where a change of speed, or change in some other characteristic of a connection, occurs after the connection has been admitted. There are many examples of this. In handling connections in the form of calls from the PSTN, a codec may assume a call to be voice and thus codable at a low bit rate, e.g. 8 kb/sec. Once set up and admitted to a network by a CAC algorithm, it turns out to be a fax or a modem call which needs the full 64 kb/sec. Thus, when in band DTMF (dual tone multiple frequency) tones are detected which may show it is not a voice call, the codec rate may change and thus one of the criteria used by the CAC alogorithm may change.
If the CAC algorithm showed the network to be close to the threshold for the maximum allowed bandwidth, it is possible that a change in an existing connection will exceed the threshold, and the change, despite having been admitted at first, will not be admitted by the CAC algorithm.
The connection would be dropped. This risk has been tolerated, but again, where it is desired to run a network close to maximum capacity, and as guarantees of quality of service become more important, the inventors have regarded this risk as a potential problem.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided apparatus for use at a near end of a link, for controlling admission of information flows to the link from the near end, the having a finite number of channels, at least some of which can be allocated by corresponding apparatus at a far end of the link, the near end apparatus comprising:
circuitry for maintaining a record at the near end, of which of the channels are free,
circuitry for synchronising, a corresponding record at the far end apparatus with the near end record by propagating changes in the near end record to the far end record, and
circuitry for allocating a free channel to one of the information flows such that a probability of the far end apparatus allocating the same channel before the far end record is synchronised, is controllable. By making the probability controllable, the risk of dropping a connection which would mean losing a call, can be managed. The risk can be set at a level which suits the environment and the commercial needs of the network operator.
PREFERRED FEATURES
Preferably, the probability is made controllable by controlling a proportion of the channels which can be allocated from both ends of the link
Preferably the circuitry for allocating a free channels is arranged to search for a free channel from those channels which are not allocatable by the far end apparatus, before searching from those allocatable from both ends of the link. This enables the risk of simultaneous allocation to be reduced.
Preferably the record is maintained as a sequential list of channel identifiers, and the circuitry for allocating a free channel is arranged to search the list sequentially from one end for a free channel, such that the one end is the opposite end of the list to that used by the corresponding circuitry at the far end. This enables the risk of simultaneous allocated to be reduced still further, since each end is searching different parts of the list as far as possible.
Preferably the sequential search into the list is continued until either a free channel is found or a predetermined limit is reached. Setting such a limit is a convenient way of controlling the proportion of the channels which are allocatable from both ends.
Preferably the information flows comprises connections.
Preferably the link comprises an ATM virtual circuit.
Preferably the apparatus at the near end of the link further comprises circuitry for determining if a maximum allowed bandwidth for a group of the channels, would be exceeded, after the channel allocating circuitry has allocated a free channel.
Preferably the probability is made controllable by controlling a proportion of the channels allocatable from both ends, and by waiting a period of controllable duration after allocating a free channel, before using this channel. This allows the risk to be reduced further by reducing the period when one end has started using the channel yet the other end's record is unsynchronised. The wait period may be controlled so as not to exceed limits on PDD (post dialing delay)
OTHER ASPECTS OF THE INVENTION
Another aspect of the invention provides apparatus for controlling admission of connections on a communication link, the apparatus comprising:
circuitry, for receiving a request for admission of a new connection
circuitry for receiving an indication of a change in traffic characteristics of an existing connection, and
circuitry for determining whether an equivalent bandwidth used would exceed a maximum allowed bandwidth, wherein the maximum allowed bandwidth is determined according to whether a new connection is being requested or whether a change in traffic characteristics of an existing connection is indicated
By treating changes differently from new requests, it is possible to reduce the risk of dropping an existing connection, without necessarily reducing the maximum allowable bandwidth for new connections, thus the bandwidth eficiency and quality of service can be independently maximised. This is particularly useful where there
Gibbs Graeme A
Gvozdanovic Rade
Sabry Martin
Tang Cho Y
Lee, Mann, Smith, McWilliams, Sweene & Ohlson
Nortel Networks Limited
Olms Douglas W.
Pizarro Ricardo
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