The QoS requirements of various traffic classes caused by numerous telecommunications services are different (Sec. 4.4.3). Therefore, an admission policy has to be specified for each traffic class to make a decision if there are enough transmission resources in the network, which can ensure the required QoS. The decision can be made in different ways; for example, as presented in [BeardFr01], according to the current network conditions; free network capacity, current transmission delays in the network, and so on. A possibility for application of separated admission policy for various traffic classes can be ensured by allocation of different logical transmission channels, provided by a multiple access scheme (Sec. 5.2.4), to various traffic classes, as is shown in Fig. 5.57. Besides reserved, idle and error states, the transmission channels can be allocated for different kinds of services that are divided into a number of classes. So, a CAC mechanism and a corresponding admission policy can be implemented separately for each service class, depending on the required QoS guarantees. The channels to be used by a particular service class can be allocated in a fixed manner, or the allocation can be organized dynamically, depending on the current traffic conditions in the network and priorities of particular service classes. There are numerous proposals for different channel (resource) allocation strategies, which can be classified in following five types [BeardFr01]