Variable Bit Rate Bandwidth Allocation in ATM Networks

Figure 7-2 Input buffering.

C. Central Buffering

Central buffering uses a shared memory inside the switch and combines the individual output queues of the output queuing into a central queue. The behavior of central buffering is very similar to output buffering. Figure 7-3 Output buffering. The mean waiting time is the same as the output buffering. However, central buffering offers a better utilization of the available buffer space due to complete sharing. Thus, it requires a smaller buffer size than the buffer size required for output buffering for the same cell loss ratio. However, a more complicated control logic is necessary to maintain the order of cells when delivering cells to output Figure 7-4 Central buffering.

II. Routing Models for ATM Switching Networks

The switching networks can be classified into two categories, blocking and non-blocking, in terms of their internal blocking characteristics. An N×N non-blocking switching network allows N input-output pairs to be connected simultaneously, given that these pairs are disjoint. The best known non-blocking switch is the crossbar switch. A typical 4×4 crossbar switch is shown in Figure 7-5. It consists of an array of 4×4 cross-point switches representing each input-output pair. The throughput of the crossbar switch approaches to unity under ideal conditions, in which 4 disjoint routing requests are presented simultaneously to the switch at every connection interval. Because of its ideal throughput characteristics, a crossbar switch is used as a reference model for a performance study of other switching networks. In practice, due to the high cost associated with very large number of cross- points, crossbar switching networks are not suitable for a very large network structures. Therefore, alternative switching network models with an acceptable degree of blocking probability were introduced. The output blocking in ATM switching networks is unavoidable due to the arrival of cells at different inputs destined for the same output port in the same time slot even if the switching network is internally non-blocking such as the crossbar switch. Additionally, the blocking switching networks such as the Banyan network also suffer from internal path conflicts. As a result, both output and internal blocking require buffering of cells in ATM switching networks. Buffering of packets which cannot be transferred to an output port during a current time slot can be done either at an input port or an output port as discussed in Section 7.1.