Principles of Computer System Design (Saltzer and Kaashoek)

Interesting Properties of Networks

The design of communication networks is dominated by three intertwined considerations: (1) a trio of fundamental physical properties, (2) the mechanics of sharing, and (3) a remarkably wide range of parameter values. The first dominating consideration is the trio of fundamental physical properties:

1. The speed of light is finite. Using the most direct route, and accounting for the velocity of propagation in real-world communication media, it takes about 20 milliseconds to transmit a signal across the 2,600 miles from Boston to Los Angeles. This time is known as the propagation delay, and there is no way to avoid it without moving the two cities closer together. If the signal travels via a geostationary satellite perched 22,400 miles above the equator and at a longitude halfway between those two cities, the propagation delay jumps to 244 milliseconds, a latency large enough that a human, not just a computer, will notice. However, communication between two computers in the same room may have a propagation delay of only 10 nanoseconds. That shorter latency makes some things easier to do, but the important implication is that network systems may have to accommodate a range of delay that spans seven orders of magnitude.

2. Communication environments are hostile. Computers are usually constructed of incredibly reliable components, and they are usually operated in relatively benign environments. But communication is carried out using wires, glass fibers, or radio signals that must traverse far more hostile environments ranging from under the floor to deep in the ocean. These environments endanger communication. Threats range from a burst of noise that wipes out individual bits to careless backhoe operators who sever cables that can require days to repair.

3. Communication media have limited bandwidth. Every transmission medium has a maximum rate at which one can transmit distinct signals. This maximum rate is determined by its physical properties, such as the distance between transmitter and receiver and the attenuation characteristics of the medium. Signals can be multilevel, not just binary, so the data rate can be greater than the signaling rate. However, noise limits the ability of a receiver to distinguish one signal level from another. The combination of limited signaling rate, finite signal power, and the existence of noise limits the rate at which data can be sent over a communication link. Different network links may thus have radically different data rates, ranging from a few kilobits per second over a long-distance telephone line to several tens of gigabits per second over an optical fiber. Available data rate thus represents a second network parameter that may range over seven orders of magnitude.