White Paper - QoS Bandwidth Management - White Paper

Introduction

As Voice over IP (VoIP) emerges as the future of voice communications, questions remain about its quality and security. Put simply, the actual capacity of an IP network to carry voice is poorly defined and not well understood. Congestion, packet loss and delays in an IP network can adversely affect the Quality of Service (QoS) for the subscribers. Therefore, mechanisms to manage bandwidth usage must be utilised to maximise the service quality.

The bandwidth calculation 'per call' is simple; however, meshed, router networks are not. It is not always clear how to use these figures to produce a deterministic and consistent VoIP service. Compounding this calculation is the differing 'per call' bandwidths resulting from a variety of codecs that subscribers may use. When all of these factors are mixed together in a real network, often only experimental techniques can really determine actual network capacity.

However, session controller technology can provide a pragmatic approach to this quality-related problem through the use of advanced bandwidth management techniques. The Newport Networks 1460 session controller addresses these issues by limiting the amount of traffic in the network, policing existing calls and rejecting new sessions as appropriate, thus underpinning call quality within the network.

Bandwidth Management

There are three main components within bandwidth management:

Session Admission Control (SAC). A model within the session controller holds the details of resources and capacities within the network. Whenever a new call is attempted, the model is examined, the appropriate resources reserved for that call and the total available resource decremented appropriately. If there are no resources available in the model, the call is rejected. Typically, this aspect of the bandwidth management capability is termed Session Admission Control.
Policing. The call establishment signalling protocols enable the subscribers' terminals to automatically negotiate the codec type that will be used for the duration of the call, and hence, the call's data rate. Each call is policed individually against the data rate of the negotiated codec. Any traffic significantly above the negotiated rate will be discarded. This, in turn, underpins the accurate and consistent view of network resources offered by the SAC function.
Anti-Tromboning (Media Release). This feature optimises the use of the access network. For example, in an IP-Centrex service environment, the call may be most effectively transported entirely within the subscribers' private network. Therefore, calls that are to remain within the subscribers' own private network should not be included in the access network accounting mechanisms.

Session Admission Control (SAC) and the 1460 Session Controller

The approach taken to guarantee quality is to model the network being used and then limit the resources used for calls. The limits are set in order to meet commercial objectives, or to meet network limits, such as access network bandwidth.

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