Internally Buffered Switches Recirculation Buffered Switches

BASICS OF PACKET SWITCHING 34 the cell loss rate can be reduced. This results in system throughput improve- ment. The disadvantage of recirculation switches is that they require a large switch to accommodate the recirculation ports. Also, recirculation may cause out-of-sequence errors. Some mechanisms are needed to preserve the cell sequencing among the cells in the same connection. The best-known recircu- w x w x lation switches are the Starlite switch 6 and the Sunshine switch 4 .

2.2.3 Buffering Strategies

In this subsection, we classify ATM switches according to their buffering strategies. Each type of switch is described, and its advantages and disadvan- tages are discussed.

2.2.3.1 Internally Buffered Switches

Internally buffered switches are Ž . those that employ buffers within switch elements SEs . An example of this Ž . switch type is the buffered banyan switch, as shown in Figure 2.15 a . These Fig. 2.15 Buffering strategies for ATM switches. SWITCH ARCHITECTURE CLASSIFICATION 35 buffers are used to store internally blocked cells so that the cell loss rate can be reduced. Scalability of the switch can be easily achieved by replicating the SEs. However, this type of switch suffers from low throughput and high transfer delay that is caused by the delay from multiple stages. To meet QoS requirements, some scheduling and buffer management schemes need to be installed at the internal SEs, which will increase the implementation cost.

2.2.3.2 Recirculation Buffered Switches

This type of switch is pro- posed to overcome the output port contention problem. As shown in Figure Ž . 2.15 b , the switch consists of both input routput ports and special ports called recirculation ports. When output port contention occurs, the switch allows the successful cell to go to the output port. Cells that have lost the contention are stored in a recirculation buffer and try again in the next time slot. In order to preserve the cell sequence, a priority value is assigned to each cell. In each time slot, the priority level of the cells losing contention is increased by one so that these cells will have a better chance to be selected in the next time slot. If a cell has reached its highest priority level and the cell still has not gotten through, it will be discarded to avoid out-of-sequence errors. The number of recirculation ports can be engineered to achieve accept- able cell loss rate. For instance, it has been shown that to achieve a cell loss rate of 10 y6 at 80 load and Poisson arrivals, the ratio of recirculation ports w x to input ports must be 2.5. The Starlite switch 6 is an example of this type of switch. The number of recirculation ports can be reduced dramatically by allowing more than one cell to arrive at the output port in each time slot. It has been shown that for a cell loss probability of 10 y6 at 100 load and Poisson arrivals, the ratio of the recirculation ports to the input ports is reduced to w x 0.1 by allowing three cells arriving at each output port. The Sunshine 4 switch is an example of this switch type.

2.2.3.3 Crosspoint-Buffered Switches