Tài liệu lecture 09: Error Sources, Detection and Correction doc

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 corr

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 1:105 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

Caused by telephone system equipment, lightening, fluorescent lights, poor electrical connections, etc A 1/100 second voltage spike would damage at least 144 bits if transmission was at 14,400 bps

• Cross-talk

Signals picked up on one circuit from adjacent circuits (We often hear this in voice

• Intermodulation Noise

A special type of crosstalk The signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal Similar to harmonics in music

• Echoes and Echo Suppression

Echo suppressers are used in the telephone system on long distance calls Echoes are caused by impedance mismatches in the telephone equipment Modems automatically disable echo suppressers so they can communicate in full duplex Echoes can

sometimes be strong enough to cause data errors

• Attenuation

Attenuation is the loss of power or signal strength as signals travel from a transmitting device to a receiving device Weaker signals are more subject to errors

• Attenuation Distortion

Not all signal frequencies are attenuated the same amount; high frequencies are

attenuated more than low frequencies This distortion can cause errors

• Delay distortion

Distortion caused by the fact that different frequencies travel through the cable at slightly different speeds

• Jitter

Phase jitter refers to small fluctuations in the phase of a carrier signal If some form of

phase modulation is being used by a modem (and most do) then this jitter will

incorrectly add to the natural phase shift used by the modem for sending different bit 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

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:

Error detection involves sending extra information with a message (frame, packet, block, etc.) which is used by the receiver to determine whether the data has been received

correctly and is complete Usually some type of calculation is performed on the data by the transmitter The receiver performs the same calculation on the received data and if the two results are identical it assumes the data is correct

Error Correction:

If the data has an error, a short message (NAK or Negative Acknowledgment) must be sent back to the transmitter requesting that the entire message be retransmitted

Forward Error Correction involves sending sufficient redundant information in the

original transmission that errors can be corrected at the receiver without requiring

Error Detection Methods:

• Parity Checking

The oldest, simplest and least effective method of error detection is parity checking One

additional bit is added to each byte transmitted, based on even (total number of 1 bits must be an even number) or odd (total number of 1 bits is an odd number) parity

Even parity - total number of '1's must be an even number Odd parity - total number of '1's must be an odd number

Detects single bit errors (or any odd number of errors) but cannot detect 2, 4 or 6 bits (any even number of errors) in error This results in a very poor error detection rate of

about 50%

Odd Parity ‘D’ 1000100 1

Even Parity ‘D’ 1000100 0

Odd Parity ‘D’ 1000101 1 <== bit 0 damaged (Parity violation detected)

Odd Parity ‘D’ 1000111 1 <== bits 0 and 1 damaged (Parity violation NOT detected)

• 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

• 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 8, 16, 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 Uses a "linear feedback shift register", normally implemented with hardware to

generate a unique 16 or 32 bit remainder based on the data that was sent ) The

remainder is transmitted as the CRC code The receiver processes the incoming data using the same algorithm and then compares it’s CRC with the one transmitted If they are the same, the data must be correct

• CRC-16 detects 99.99% errors; that is 1 in 10,000 errors goes undetected

• CRC-32 is extremely proficient in detecting errors; approximately 1 in 1014

errors

go undetected

If transmitting at 9600 bps, 24 hours a day, 365 days a year, it would take 3000 years for a single undetected error to occur

Ethernet Local Area Networks use a 32-bit CRC code

Error Correction via Retransmission:

Except for simple asynchronous transmission, most data is sent in frames or packets (Fig 5-12)

synchronously When a packet is received with errors in it, some method must be established to

correct the error

The simplest way of dealing with errors is to ask the sender to retransmit the original frame or packet This method is called ARQ (Automatic Repeat Request) Two variations of ARQ are:

• Stop and Wait ARQ (Fig 5-4)

• This is a very slow method, essentially a half-duplex communication method

• The sender must stop and wait after sending each packet of data for either an ACK

(positive acknowledgment) or a NAK (negative ack) to come back from the receiver If an error occurred, a NAK is sent and the packet is retransmitted The sender spends a lot of

time waiting

• Continuous ARQ (Fig 5-5)

• a much more efficient method of error handling which communicates in full duplex

• The sender does not wait for an acknowledgment before sending the next packet

• Acknowledgments must arrive at the sender before a certain number of packets have been

sent; this number is the window size

• The receiver sends back an ACK or a NAK every so often This ACK acknowledges all packets received since the last ACK It must include a packet sequence number so it can

identify which packets are being acknowledged

• When a NAK is received, the sender must retransmit packets starting from that packet

number Two methods are used:

• LAP-M: Link Access Protocol for Modems which requires that only the

packet containing the error needs to be re-transmitted (Most efficient)

• GO-BACK-N ARQ: Must re-transmit all packets starting from the one

with the error in it

• Sliding window methods are used by many file transfer protocols such as zmodem

Forward Error Correction:

Forward Error Correction adds sufficient redundancy to a message so it can detect and correct

errors at the receiver without having to retransmit If a system used 100% redundancy in a message, essentially everything would be sent twice Typical Forward Error Correction methods add 25% additional bits to a transmission V.32 and V.34 modems use Forward Error Correction (V.42)

Various methods exist:

Hagelbarger Code: corrects up to 6 consecutive bit errors provided such a group is followed by al least 19 valid bits before more errors occur

Bose-Chaudhuri code: can correct double-bit errors and detect up to 4 errors

Hamming Codes: See example on p.145 and Figure 5-6