# Data encoding

##       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

# 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

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.