6.9: Detail- Communication System Requirements

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Paul Penfield, Jr.
Massachusetts Institute of Technology via MIT OpenCourseWare

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The model of a communication system that we have been developing is shown in Figure 6.1. The source is assumed to emit a stream of symbols. The channel may be a physical channel between different points in space, or it may be a memory which stores information for retrieval at a later time, or it may even be a computation in which the information is processed in some way.

Naturally, different communication systems, though they all might be well described by our model, differ in their requirements. The following table is an attempt to illustrate the range of requirements that are reasonable for modern systems. It is, of course, not complete.

The systems are characterized by four measures: throughput, latency, tolerance of errors, and tolerance to nonuniform rate (bursts). Throughput is simply the number of bits per second that such a system should, to be successful, accommodate. Latency is the time delay of the message; it could be defined either as the delay of the start of the output after the source begins, or a similar quantity about the end of the message (or, for that matter, about any particular features in the message). The numbers for throughput, in MB (megabytes) or kb (kilobits) are approximate.

Table 6.1: Various Communication Systems
	Throughput (per second)	Maximum Latency	Errors Tolerated?	Bursts Tolerated?
Computer Memory	many MB	microseconds	no	yes
Hard disk	MB or higher	milliseconds	no	yes
Conversation	??	50 ms	yes; feedback error control	annoying
Telephone	20 kb	100 ms	noise tolerated	no
Radio broadcast	??	seconds	some noise tolerated	no
Instant message	low	seconds	no	yes
Compact Disc	1.4 MB	2 s	no	no
Internet	1 MB	5 s	no	yes
Print queue	1 MB	30 s	no	yes
Fax	14.4 kb	minutes	errors tolerated	yes
Shutter telegraph station	??	5 min	no	yes
E-mail	N/A	1 hour	no	yes
Overnight delivery	large	1 day	no	yes
Parcel delivery	large	days	no	yes
Snail mail	large	days	no	yes