Page 25 - Optical Communications Essentials

Basic Concepts of Communication Systems

Basic Concepts of Communication Systems 15

TABLE 1.4. Commonly Used SONET and SDH Transmission Rates
SONET level Electrical level SDH level Line rate, Mbps Common rate name
OC-1 STS-1 — 51.84 —
OC-3 STS-3 STM-1 155.52 155Mbps
OC-12 STS-12 STM-4 622.08 622Mbps
OC-48 STS-48 STM-16 2,488.32 2.5Gbps
OC-192 STS-192 STM-64 9,953.28 10Gbps
OC-768 STS-768 STM-256 39,813.12 40Gbps

Original Amplified
signal signal
Attenuated
signal

Transmission line
Amplifier
Point 1 Point 2 Point 3

Figure 1.11. Amplifiers periodically compensate for energy losses along a channel.

1.9. Decibels
Attenuation (reduction) of the signal strength arises from various loss mecha-
nisms in a transmission medium. For example, electric power is lost through
heat generation as an electric signal flows along a wire, and optical power is
attenuated through scattering and absorption processes in a glass fiber or in an
atmospheric channel. To compensate for these energy losses, amplifiers are used
periodically along a channel to boost the signal level, as shown in Fig. 1.11.
A convenient method for establishing a measure of attenuation is to refer-
ence the signal level to some absolute value or to a noise level. For guided
media, the signal strength normally decays exponentially, so for convenience
one can designate it in terms of a logarithmic power ratio measured in deci-
bels (dB). In unguided (wireless) media, the attenuation is a more complex
function of distance and the composition of the atmosphere. The dB unit is
defined by

P

Power ratio in dB 10 log 2 (1.1)
P
1
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.