Defining Throughput

Defining Throughput

Remember the previous post "Bandwidth and throughput". it turns out to be difficult to define precisely the throughput of a switch. Intuitively, we might think that if a switch has n inputs that each support a link speed of si , then the throughput would just be the sum of all the si . This is actually the best possible throughput that such a switch could provide, but in practice almost no real switch can guarantee that level of performance. One reason for this is simple to understand. Suppose that, for some period of time, all the traffic arriving at the switch needed to be sent to the same output. As long as the bandwidth of that output is less than the sum of the input bandwidths, then some of the traffic will need to be either buffered or dropped. With this particular traffic pattern, the switch could not provide a sustained throughput higher than the link speed of that one output. However, a switch might be able to handle traffic arriving at the full link speed on all inputs if it is distributed across all the outputs evenly; this would be considered optimal.

Another factor that affectsthe performance of switches is the the size of packets arriving on the inputs. For an ATM switch, this is normally not an issue because all “packets” (cells) are the same length. But for  Ethernet switches or IP routers, packets of widely varying sizes are possible. Some of the operations that a switch must perform have a constant overhead per packet, so a switch is likely to perform differently depending on whether all arriving packets are very short, very long, or mixed. For this reason, routers or switches that forward variable-length packets are often characterized by a packet per second (pps) rate as well as a throughput in bits per second. The pps rate is usually measured with minimum-sized packets.

The first thing to notice about this discussion is that the throughput of the switch is a function of the traffic to which it is subjected. One of the things that switch designersspend a lot of their time doing is trying to come up with traffic models that approximate the behavior of real data traffic. It turns out that it is extremely difficult to achieveaccurate models.

A traffic model attempts to answer several important questions: (1) When do packets arrive? (2) What outputs are they destined for? And (3) how big are they? Traffic modeling is a wellestablished science that has been extremely successful in the world of telephony, enabling telephone companies to engineer their networks to carry expected loads quite efficiently. This is partly because the way people use the phone network does not change that much over time: The frequency with which calls are placed, the amount of time taken for a call, and the tendency of everyone to make calls on Mother’s Day have stayed fairly constant for many years.4 By contrast, the rapid evolution of computer communications, where a new application like Napster can change the traffic patterns almost overnight, has made effective modeling of computer networks much more difficult. To give you a sense of the range of throughputs that designers need to be concerned about, a high-end router used in the Internet at the time of writing might support 10 OC-192 links for a throughput of approximately 100 Gbps. A 100- Gbps switch, if called upon to handle a steady stream of 64-byte packets, would need a packet per second

rate of

100 × 109 ÷ (64 × 8)

= 195 × 106 pps