This section discusses the feasible analytical models to dimension the lub link capacity to satisfy a required call blocking ratio of the circuit-switched type of traffic. 6.2.1 Erlang-B Formula The Erlang-B formula has been published by the Danish mathematician A. K. Erlang in 1917 [EHA48]. It is considered to be the classical model to evaluate the performance of a loss system which has finite capacity and limited number of channels (servers, trunks, etc.). The Erlang-B formula can be used to estimate the probability of a call being blocked and lost in a circuit-switched network. It is used under the following assumptions: 1. The network comprises of n identical channels. A call is accepted for service if any channel is idle. Every call needs only one channel. 2. If all channels are busy, a call is blocked and lost. It is called the Erlang loss model. 3. Furthermore it is assumed that the call arrive process is a Poisson process with rate 2 and the service times are exponentially distributed with intensity u (mean service time = 1/u). Therefore, the Erlang loss model is denoted as M/M/n loss system. In Erlang's loss model with Poisson arrival process, the offered traffic, denoted by A, is equivalent to the average number of call attempts per mean holding time as given in (6.1). 6.2 Dimensioning for Circuit-Switched Traffic The unit of the offered traffic A is Erlang, which is a dimensionless unit. (6.1) State transition diagram Figure 6.1 shows the state transition diagram in order to describe the above loss system with a limited number of channels (n), Poisson arrival process (2), and the exponential service times (u). (i-1)u (i+1)u Figure 6.1: State transition diagram for a system with a limited number of channels (n), Poisson arrival process (2), and exponential service times (1). The state of the system, depicted as i (i= 0, 1, 2, n), is defined as the number of busy channels. As the number of channels is limited to n, the number of states becomes n+1. In Figure 6.1, all states of the system are shown as cir