ic. 6.4 Dimensioning for Transport Network Performance RT or NRT Departures Arrivals Server process (deterministic service rate) (a) Single-service system Packet scheduling: Non-preemptive priority Arrivals Departures Server process (deterministic service rate) (b) Multi-service system Figure 6.43: System Models (AAL2 packet level) In case (a), there is only one service type, either RT or NRT traffic. It is used for the scenario where only voice traffic or only packet-switched traffic is transmitted over the lub. As seen in Figure 6.42, the arrival traffic to the AAL2 buffer is a superposition of the FP PDUs streams of all users. The service rate of the AAL2 queue is deterministic due to the shaping applied at the AAL2 layer. Thus the resulting queuing delay in the AAL2 buffer strongly depends on the characteristics of the arrival traffic and the distributions of the FP PDU sizes. Due to the stochastic arrival process, traffic can be very bursty in nature and the AAL2 queuing becomes significant. Case (b) is a more realistic scenario as usually the networks will hold multiple services simultaneously. It is applied for the scenario of both RT traffic and NRT traffic being transmitted on the lub link, for example the conversational voice traffic mixed with the web traffic. In this case, the system model consists of two AAL2 queues each serving different type of traffic, and one server with deterministic service rate, but the RT traffic is taken higher priority over NRT traffic. The low priority queue is mainly subject to the lub congestion situation, and therefore its queuing delays are the main issue for the lub dimensioning. Thus the major goal of applying queuing models is to estimate the distribution of the low priority queue, and in turn to allocate the bandwidth accordingly for satisfying a required delay bound for the NRT traffic. Here, the queuing delay of the low priority queue depends not only on the properties of the arrival traffic of the low priority queue, but also the characteristics of the arrival t