Telecommunications Fundamentals

Signals

• Analog

• Digital

• Both are two-state in data communications:

– Two frequencies for analog

– Two voltage levels for digital

Data encoding

• Each bit is encoded as voltage or frequency state

• Group of bits is encapsulated with start and stop and parity bits

Example – A = 01000001

Parity (error checking)

Parity – a way of checking to see if the signal has been corrupted (for example, a voltage spike on the line might shift all 0s to 1s).

Even parity – an extra bit is added to make the total number of 1s an even number.

Odd parity – an extra bit is added to make the total number of 1s odd.

An A with even parity would be 010000010 – there are two ones in A, so it is already an even number.

An A with odd parity would be 010000011 – there are two ones in A, so an additional 1 is added to come to an odd number – three.

If the receiving computer expects even parity, and finds an odd number of 1s, it knows there was a transmission problem.

A start and stop bit are usually added to show where the letter begins and ends, so the complete packet for a letter A with even parity would be:

10100000101

Modulation

• Encoded data is merged onto existing signal

• Principle devices for this are

– MODEM (modulator/demodulator)

– Multiplexer (MUX)

Communication media

	Strength	Problems
Twisted pair (STP and UTP)	Cheap to install, and large established base	Attenuation, cross talk, eavesdropping
Coax	Good speeds, easy to work with	Bulky and only moderate bandwidth
Optical fiber	Best bandwidth	Connections are difficult
microwave	Good bandwidth, crosses land that might be expensive to trench	Weather interference, eavesdropping
satellite	Good speeds, and great footprint	Expensive to install and repair
Radio (wifi)	No need for wires	Eavesdropping, limited speed, still limited availability

Communication Problems

Attenuation – weakening. Signals lose energy over time, and the voltage levels that show a 1, can lower enough to look like 0s.

EMI – electro-magnetic interference. Surrounding equipment throws off an electromagnetic field. This field generates a current that can interfere with other systems. This is a common problem in factories with larger electrical motors.

Corss-talk. If two wires are right next to each other, the current on one may induce a current on the other. We usually experience this as hearing some stranger’s voice on our phone.

Traits in communication media:

– Speed (T-1)

– Accuracy

– Privacy (can the signal be intercepted?)

– Cost (installation and maintenance)

– EMI resistance

– Weather resistance

– Theft resistance