Performance

When evaluating the suitability of a particular medium to a specific application, five factors should be kept in mind: cost, speed, attenuation, electromagnetic interference, and security.

• Cost. The cost of the materials, plus installation.

• Speed. The maximum number of bits per second that a medium can transmit reliably. Among other factors, speed varies with frequency (higher frequencies can transport more bits per second), with the physical size of the medium and/or transmission equipment, and with the conditioning of the conductor.

• Attenuation. The tendency of an electromagnetic signal to become weak or distorted over distance. During transmission, the signal's energy can become absorbed or dissipated by the medium itself. For example, a wire's resistance can leach energy from a signal and emit it in the form of heat.

• Electromagnetic interference (EMI). The susceptibility of the medium to external electromagnetic energy inadvertently introduced onto a link that interferes with the intelligibility of a signal. Familiar effects of EMI are static (audio) and snow (visual).

• Security. Protection against eavesdropping. How easy is it for an unauthorized device to listen in on the link? Some media, like broadcast radio and unshielded twisted-pair cable, are easily intercepted. Others, like fiber-optic cable, are more secure.

Table 5.3-1 compares the various media based on the qualities listed above.

Table 5.3-1 Transmission media performance

Medium

Cost

Speed

Attenuation

EMI

Security

UTP

Low

1-100 Mbit/s

High

High

Low

STP

Moderate

1-150 Mbit/s

High

Moderate

Low

Coax

Moderate

1 Mbit/s-1 Gbit/s

Moderate

Moderate

Low

Optical fiber

High

10 Mbit/s-2 Gbit/s

Low

Low

High

Radio

Moderate

1-10 Mbit/s

Low-high

High

Low

Microwave

High

1 Mbit/s-10 Gbit/s

Variable

High

Moderate

Satellite

High

1 Mbit/s-10 Gbit/s

Variable

High

Moderate

Cellular

High

9.6-19.2 kbit/s

Low

Moderate

Low

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