- Connection Admission Control
Admission control, in the most primitive sense, is the simple practice of discriminating which traffic is admitted into a network in the first place [#REF|  . Admission control in ATM networks is known as "Connection Admission Control" (CAC).
Connection admission control is defined as ‘the set of actions taken by the network during the call set-up phase (or during the call renegotiation phase) to establish whether a VC/VP connection can be accepted’ [#REF|  [#REF|  .
Where is Connection Admission Control (CAC) used?
By definition, CAC can only exist in a connection oriented network. Connectionless networks such as the
Internetand SMDSnetworks have no CAC role. However, in any connection-oriented network, CAC is a major part of the strategy for controlling congestion (see congestion control), and in the case of circuit switching and CBR services (see Traffic Contract), it is the only mechanism [#REF|  .
What is the Role of Connection Admission Control?
In principle, in a connection-oriented network, the role of CAC is to decide whether there are sufficient free resources on the requested link to allow a new connection [#REF|  . A connection can only be accepted if sufficient resources are available to establish the connection end-to-end with its required
quality of service. The agreed quality of service of existing connections in the network must not be affected by the new connection [#REF|  .
If the network has the required resources, the CAC may allow a connection request to proceed; if not, the CAC will indicate this and notify the originator of the request that the request has been refused [#REF|  .
What criteria are used to make the decision?
When a connection is requested by an application, the application indicates to the network (in the form of a traffic descriptor-see
traffic contract) [#REF|  the:
* type of service required
* traffic parameters of each data flow in both directions (a set of parameters describing the source traffic characteristics)
* quality of service parameters requested in each direction
The CAC uses this information to determine the required
quality of serviceof the connection, and determines whether it has sufficient resources to accept the requested connection based on the current network status.
Strategy and Policy of Connection Admission Control for different services
The strategy employed by CAC varies according to the type of service (see
traffic contract), and in the case of VBR, ABR, and UBR services, becomes a complex issue and is still the subject of intensive research.
The policy for each connection type depends on the stochastic nature of the service. The following gives a brief indication of policies for each connection type [#REF|  .
In this case, the request for a connection, if granted, results in a link connection being allocated to the connection. In principle, a resource manager can be completely non-discriminatory in its allocation of free capacity to connection requests as they will all have the same effect on the state of the resource. There may, however, be some circumstances where CAC may refuse a new connection even when there is sufficient capacity for the connection. For example, circuits could be designated as low priority or high priority, and the CAC may choose to accept a high-priority connection while refusing a lower priority. In refusing the lower priority connection, it is effectively reserving the remaining capacity on the resource for high-priority connections. Prioritisation becomes important in service restoration after a failure where the resources become scarce and some connections must be dropped. Higher priority services can reasonably attract a premium tariff [#REF|  [#REF|  [#REF|  .
While the principles of CAC are basically similar to those for
circuit-switchedconnections, CAC is more likely to be discriminatory in CBR connections (see traffic contract). As the connections can have different bandwidths, they will have different impacts on the remaining capacity within the link. For example, CAC for CBR connections could employ a strategy of only accepting connections that take less than half of the remaining capacity. With such a strategy, some connections will be refused even if there is capacity available to resource the connection. This makes good commercial sense if several smaller connections are worth more than one large connection of the same capacity [#REF|  [#REF|  .
With VBR connections (see
traffic contract), the CAC is not only making judgment about the nature of future connection requests, but is also having to make a judgment on the future behavior of the connections it has already accepted onto the link based on the VBR parameters. This is based on statistical assessment as to whether accepting the new connection will take the likelihood of cell loss for the current traffic beyond acceptable limits [#REF|  [#REF|  .
The role of CAC in ABR services (see
traffic contract) depends on the ABR strategy employed. The use of the resource management protocol allows for the use of CAC; however, it is quite possible to use a policy of accepting all connections and simply regulating the rate of all the connections on the resource to ensure the resource does not become overloaded [#REF|  [#REF|  .
The role of CAC in basic UBR (see traffic contract) connections is similar to that in VBR connections in that it must decide according to the requested peak cell rate (see traffic contract) on whether to admit the connection [#REF|  [#REF|  .
Teletraffic engineering in broadband networks
 Ferguson P., Huston G., “Quality of Service: Delivering QoS on the Internet and in Corporate Networks”, John Wiley & Sons, Inc., 1998. ISBN 0-471-24358-2.
 Handel R., Manfred H., Schroder S., “ATM Networks: Concepts, Protocols, Applications“, Addison-Wesley Publishing company Inc., 1994. ISBN 0-201-42274-3.
 ITU-T: Recommendation I.371. ‘Traffic Control and Congestion Control in B-ISDN’. Geneva, 1993.
 Sexton M., Reid A., “Broadband Networking: ATM, SDH and SONET”, Artech House Inc., Boston, London, 1997. ISBN 0-89006-578-0.
 Hiroshi Saito, Teletraffic Technologies in ATM Networks, Artech House, 1993. ISBN 0-89006-622-1.
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