Time-Domain View

Can be used to represent both an analog and a digital signal.

Analog signal - signal intensity varies in a smooth fashion over time.

• No breaks or discontinuities in the signal.

• E.g. voice signal traveling over traditional phone line.

Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level.

• E.g. stream of 1 and 0 values represented as “low” and “high” signal

Two Graphical Views of an Electromagnetic Signal

Both are real in some way.

Think of it as energy that radiates from an antenna and is picked up by another antenna.

• Helps explain properties such as attenuation.

Can also view it as a “ray” that propagates between two points.

• Helps explain properties such as reflection and multipath

Transmission Channel Considerations

For wired networks, channel limits are an inherent property of the channel.

• Different types of fiber and copper have different properties.

As technology improves, these parameters change, even for the same wire.

• Electronics rule.

For wireless networks, limits are often imposed by policy.

• Can only use a certain part of the spectrum.

• Regulatory/business considerations.

Channel Capacity

Data rate - rate at which data can be communicated (bps).

• Channel Capacity – the maximum rate at which data can be transmitted over a given channel, under given conditions.

Bandwidth (signal theory) - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz).

Noise - average level of noise over the communications path.

Error rate - rate at which errors occur.

• Error = transmit 1 and receive 0; transmit 0 and receive 1.

Propagation Modes

Line-of-sight (LOS) propagation

Most common form of propagation.

Happens above ~ 30 MHz.

Subject to many forms of degradation.

Ground-wave propagation

More or less follows the contour of the earth.

For frequencies up to about 2 MHz, e.g. AM radio.

Sky wave propagation

Signal “bounces” off the ionosphere back to earth –can go multiple hops.

Used for amateur radio and international broadcasts.