CSN200 Introduction to Telecommunications, Fall 1999 Lecture-09 Error Sources, Detection and Correction
Error Sources, Detection and Correction
The Data Link Layer
[Ref: Chap-5 Fitzgerald & Dennis, Chap-5 p.142 Stallings]

Error Control In Networks:
• Network errors are in the form of corrupted data or lost data.
• Network errors occur naturally on all networks due to electrical noise and distortion
and must be detected and corrected by either hardware or software.
• Bit Error Rates (BERs) are calculated as the number of bits in error divided by the
number of bits transmitted. A BER of 1 in 100,000 might be shown as 1:10
5
or simply
as a BER of 10
-5
.
• Errors often occur in bursts where many bits in a sequence will be in error.

What are Network Errors and What Causes Errors?
Noise is any form of undesirable electrical signals introduced by equipment or natural
disturbances (lightening).
Noise has the effect of causing bits to change state, extra bits or missing bits resulting in
errors in a message.

Forms of Line Noise and Distortion:
• White Noise
Caused by the thermal agitation of electrons in all matter. Always present (like the
background hiss on radios) but not usually a problem unless the signal itself is very
weak.
• Impulse Noise
Noise spikes (large voltage fluctuations on the line) are a primary source of errors.

patterns, resulting in errors.
• Harmonic distortion
Distortion introduced by amplifiers on a circuit.
• Line outages
Failure of a communication circuit for a brief period of time resulting in a major loss of
data. Caused by telephone system equipment failure or power outages.

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CSN200 Introduction to Telecommunications, Fall 1999 Lecture-09 Error Sources, Detection and Correction
Error Prevention:
• Shielding (p.140)
Covering a cable or equipment with a grounded metallic conductor shields it from
electrical noise, and prevents it from generating any noise as well.
• Moving Cables
Relocating cables away from sources of noise (like power cables, fluorescent lights, and
electrical machinery)
• Changing Multiplexing Techniques - changing frequencies or guard bands.
• Improving Connection Quality
• Amplifiers and Repeaters
Amplifiers are used on analog phone lines to overcome attenuation. Digital Repeaters are
used on digital phone lines. Repeaters and amplifiers placed closer together will
improve signal quality.
• Equalization
Equalizers are devices that compensate for attenuation and delay distortion. They can be
built into leased phone lines and are built into most modems to improve error rates.
• Conditioning
Leased private lines can be conditioned (at a cost) using many of the techniques
described above, to lower error rates.

Error Detection:

• LRC: Longitudinal Redundancy Checking
LRC adds an additional character, the Block Check Character (BCC) to the end of the
message or block of data. The BCC uses parity on each bit position for each character in
the message; in other words, by determining parity on the first bit of each character and
setting the first bit of the BCC to that value; then determining parity on the second bit of
each character, etc.
Has an improved error detection rate of more than 98%.
Letter ASCII Parity Bit
D 1000100 1
A 1000001 1
T 1010100 0
A 1000001 1
BCC 1101111 1

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CSN200 Introduction to Telecommunications, Fall 1999 Lecture-09 Error Sources, Detection and Correction
• Polynomial Checking
• Checksum
A checksum is calculated by adding up the decimal value of each character in the
message, dividing the sum by 255 and using the remainder as the checksum to be
transmitted with the message.
The receiver calculates its own checksum and compares it with the one sent. If they
are the same, it assumes there are no errors.
It is able to detect errors about 95% of the time. (Still not very good).
• CRC: Cyclic Redundancy Checking (Best Method **)
This method is used for most network error detection today. An 8, 16, 24 or 32 bit
number is added to the end of the data. Known as CRC-8, CRC-16, CRC-24 or CRC-
32 based on the number of bits used.
All of the data in the message is processed according to a mathematical algorithm (A
polynomial is divided by a prime number resulting in a quotient and a remainder.