Principles of Broadband Switching and Networking

9

PACKET SWITCHING AND INFORMATION TRANSMISSION

All communication networks comprise transmission systems and switching systems, even though their designs are usually treated as two separate issues. Communication channels are generally disturbed by noise from various sources. In circuit-switched networks, reliable communication requires error-tolerant transmission of bits over noisy channels. In packet-switched networks, however, not only can bits be corrupted with noise, but resources along connection paths are also subject to contention. Thus, quality of service (QoS) is determined by buffer delays and packet losses. The theme of this chapter is to show that transmission noise and packet contention actually have similar characteristics and can be tamed by comparable means to achieve reliable communication. The following analogies between switching and transmission are identified:

1. Buffering against contention is a process that is similar to the error correction of noise corrupted signals. A signal-to-noise ratio that represents the carried load of packet switches can be deduced from the Boltzmann model of packet distribution.
2. When deflection routing is applied to Clos networks, the loss probability decreases exponentially, which is similar to the exponential behavior of the error probability of binary symmetric channels with random channel coding. In information theory, this result is stated as the noisy channel coding theorem.
3. The similarity between Hall's condition of bipartite ...