4.1 Introduction

Connection-oriented network technologies such as time division multiplexing (TDM) and asynchronous transfer mode (ATM) have an implicit admission control capability which is used in establishing the path between sender and receiver, to ensure that there are sufficient resources for the connection. In contrast, IP is connectionless, and has no implicit admission control capability.

In Chapter 3, we described how to deploy the Differentiated Services architecture (Diffserv); Diffserv is by far the most widely deployed IP QOS architecture today, in both enterprise and SP networks. Diffserv effectively provides the capability to manage network capacity on a per-traffic type or class basis. In Chapter 1, we described that the SLAs for IP services are defined in terms of delay, jitter, packet loss rate, throughput, and availability; because the service level experienced by a particular class of traffic is dependent both upon how much capacity has been allocated to that class and upon the current offered load of that class, Diffserv enables differentiated delay, jitter, loss, throughput, and availability commitments to be supported for different classes of traffic.

Diffserv, however, supports no explicit mechanisms for admission control. In order to offer tightly bound service levels for real-time traffic and to assure consistent service within the SLA bounds, admission control mechanisms may be required to ensure that the actual load for a class does not exceed acceptable levels. Without admission control mechanisms, if there...