Computer Network No.4 (data Link Layer) From Apcoms
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Faisal Amjad CPS 422
CPS 422 Computer Networks
THE PERSPECTIVE
DATA LINK LAYER
DATA LINK LAYER Addressing
Packetizing
Media Access Control
Flow Control
Error Control
Faisal Amjad CPS 422
Data can be corrupted during transmission, for reliable communication errors must be detected and corrected
ERROR DETECTION AND CORRECTION
Faisal Amjad CPS 422
TYPES OF ERROR
Faisal Amjad CPS 422
ERROR DETECTION
SINGLE – BIT ERRORS 1-BIT ERROR
0 0 0 0 0 0 1 0 SENT
0 0 0 0 1 0 1 0 RECEIVED
BURST ERRORS Length of Burst Error (5-bits) SENT
0 0 1 0 0 0 0 0 Bits corrupted by burst error
0 0 0 0 1 0 1 0 RECEIVED
Faisal Amjad CPS 422
One error detection mechanism could be the transmission of all data units, TWICE. Error detection uses the concept of “Redundancy”, which means adding extra bits to data for detecting errors at dest. The receiver puts the entire stream [data + redundancy] through a checking function If the bit stream passes checking criteria, the data unit is accepted and redundant bits discarded. Faisal Amjad CPS 422
1
ERROR DETECTION (Contd…)
Simple Parity Check
Types of error detection techniques
A redundant “Parity-bit” added to every data unit In “Even” parity, the total number of 1s including the parity bit must be even. In “Odd” parity, the total number of 1s including the parity bit must be Odd
o Parity Check Simple Two-Dimensional
o Cyclic Redundancy Check (CRC) o Checksum
Faisal Amjad CPS 422
Original data
1 0 1 0 0 1 1 1 0 0 0 1 0 1 0 0 1 0 1 1 1
Block of bits is organized in a table (rows and columns) Parity bits for each row and each column calculated and appended to the end of respective row/column.
1 0 0 0 1 0 1
1 0 1 0 0 1 1 0 1 0 0 0 1 0 1 1 0 0 1 0 1 1 1 0
Row parity
Two-Dimensional Parity Check
Faisal Amjad CPS 422
1 0 0 0 1 0 1 1 1 0 0 0 1 0 0 0 Column parity Transmitted data
1 0 1 0 0 1 1 0
1 0 0 0 1 0 1 1
1 0 0 0 1 0 1 1
1 0 0 0 1 0 0 0
Faisal Amjad CPS 422
Cyclic Redundancy Check (CRC) Based on “binary Division” A sequence of redundant bits, called the CRC Remainder or simply “CRC” is appended to the end of data unit. Resulting data unit becomes exactly divisible by a second, “predetermined” binary number called the “Divisor”.
Faisal Amjad CPS 422
0 0 1 0 1 1 1 0
Faisal Amjad CPS 422
CRC – Process A string of n 0s (zeros) appended to the data unit. The divisor is n+1 bits long. Elongated data unit is divided by the divisor, using binary division. Remainder of this process is the CRC. The CRC replaces the n 0s appended earlier at the beginning, and bit stream transmitted. The CRC may consist of all 0s. The receiver divides the received stream by the Divisor. If remainder is all 0s, no error occurred
Faisal Amjad CPS 422
2
CRC - Generation Pre-determined Divisor 1
1 1 1 1 0 1 1 0 1
1 0 0 1 1 0 1 0 1 1 1 1
1 1 0 0 0 1
Original Data + CRC
CRC - Checking
Extra 0s
0 1 1 0 0 1
1 1 1 1 1 0 0 1 0 0 1 1 Original Data
Quotient
0 0 0 0 0
0 1 1 0 1 1 0
Similar process – Remainder must be all 0s, for data to be correct 0 0 0 0 0 1 0 1
CRC Divisor is represented as a polynomial e.g. in our example its equivalent polynomial is x3 + x2 + 1 (1101) Remainder / CRC
1 0 0 1 0 0 1 0 0 1 0 0 0 0 1 Faisal Amjad CPS 422
Faisal Amjad CPS 422
CHECKSUM Steps at Sender
Original data
Faisal Amjad CPS 422
0 0 1 1 1 0 0
1 0 1 0 1 0 0 1 0 0 1 1 1 0 0 1 1 1 1 0 0 0 1 0
Sum
0 0 0 1 1 1 0 1
Checksum
Transmitted data
1 0 1 0 1 0 0
Receiver
Steps at Receiver The unit is divided into k sections, each n bits long All sections are added using “1’s Compliment” The resulting sum is complemented If the result is zero, data is accepted.
1 0 1 0 1 0 0
Sender
o The unit is divided into k sections, each n bits long o All sections are added using “1’s Compliment” o The resulting sum is complemented which becomes the “Checksum” o Checksum is transmitted with the data. o o o o
Faisal Amjad CPS 422
Checksum - Generation
0 0 1 1 1 0 0
0 0 0 1 1 1 0 1
1 0 1 0 1 0 0 1 0 0 1 1 1 0 0 1 0 0 0 1 1 1 0 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
Sum Complement
ASSIGNMENT # 2 Write a note on the performance of fol: o o o o
Simple Parity check Two-Dimensional parity check CRC Checksum
Faisal Amjad CPS 422
3
CPS 422 Computer Networks
THE PERSPECTIVE
DATA LINK LAYER
DATA LINK LAYER Addressing
Packetizing
Media Access Control
Flow Control
Error Control
Faisal Amjad CPS 422
Data can be corrupted during transmission, for reliable communication errors must be detected and corrected
ERROR DETECTION AND CORRECTION
Faisal Amjad CPS 422
TYPES OF ERROR
Faisal Amjad CPS 422
ERROR DETECTION
SINGLE – BIT ERRORS 1-BIT ERROR
0 0 0 0 0 0 1 0 SENT
0 0 0 0 1 0 1 0 RECEIVED
BURST ERRORS Length of Burst Error (5-bits) SENT
0 0 1 0 0 0 0 0 Bits corrupted by burst error
0 0 0 0 1 0 1 0 RECEIVED
Faisal Amjad CPS 422
One error detection mechanism could be the transmission of all data units, TWICE. Error detection uses the concept of “Redundancy”, which means adding extra bits to data for detecting errors at dest. The receiver puts the entire stream [data + redundancy] through a checking function If the bit stream passes checking criteria, the data unit is accepted and redundant bits discarded. Faisal Amjad CPS 422
1
ERROR DETECTION (Contd…)
Simple Parity Check
Types of error detection techniques
A redundant “Parity-bit” added to every data unit In “Even” parity, the total number of 1s including the parity bit must be even. In “Odd” parity, the total number of 1s including the parity bit must be Odd
o Parity Check Simple Two-Dimensional
o Cyclic Redundancy Check (CRC) o Checksum
Faisal Amjad CPS 422
Original data
1 0 1 0 0 1 1 1 0 0 0 1 0 1 0 0 1 0 1 1 1
Block of bits is organized in a table (rows and columns) Parity bits for each row and each column calculated and appended to the end of respective row/column.
1 0 0 0 1 0 1
1 0 1 0 0 1 1 0 1 0 0 0 1 0 1 1 0 0 1 0 1 1 1 0
Row parity
Two-Dimensional Parity Check
Faisal Amjad CPS 422
1 0 0 0 1 0 1 1 1 0 0 0 1 0 0 0 Column parity Transmitted data
1 0 1 0 0 1 1 0
1 0 0 0 1 0 1 1
1 0 0 0 1 0 1 1
1 0 0 0 1 0 0 0
Faisal Amjad CPS 422
Cyclic Redundancy Check (CRC) Based on “binary Division” A sequence of redundant bits, called the CRC Remainder or simply “CRC” is appended to the end of data unit. Resulting data unit becomes exactly divisible by a second, “predetermined” binary number called the “Divisor”.
Faisal Amjad CPS 422
0 0 1 0 1 1 1 0
Faisal Amjad CPS 422
CRC – Process A string of n 0s (zeros) appended to the data unit. The divisor is n+1 bits long. Elongated data unit is divided by the divisor, using binary division. Remainder of this process is the CRC. The CRC replaces the n 0s appended earlier at the beginning, and bit stream transmitted. The CRC may consist of all 0s. The receiver divides the received stream by the Divisor. If remainder is all 0s, no error occurred
Faisal Amjad CPS 422
2
CRC - Generation Pre-determined Divisor 1
1 1 1 1 0 1 1 0 1
1 0 0 1 1 0 1 0 1 1 1 1
1 1 0 0 0 1
Original Data + CRC
CRC - Checking
Extra 0s
0 1 1 0 0 1
1 1 1 1 1 0 0 1 0 0 1 1 Original Data
Quotient
0 0 0 0 0
0 1 1 0 1 1 0
Similar process – Remainder must be all 0s, for data to be correct 0 0 0 0 0 1 0 1
CRC Divisor is represented as a polynomial e.g. in our example its equivalent polynomial is x3 + x2 + 1 (1101) Remainder / CRC
1 0 0 1 0 0 1 0 0 1 0 0 0 0 1 Faisal Amjad CPS 422
Faisal Amjad CPS 422
CHECKSUM Steps at Sender
Original data
Faisal Amjad CPS 422
0 0 1 1 1 0 0
1 0 1 0 1 0 0 1 0 0 1 1 1 0 0 1 1 1 1 0 0 0 1 0
Sum
0 0 0 1 1 1 0 1
Checksum
Transmitted data
1 0 1 0 1 0 0
Receiver
Steps at Receiver The unit is divided into k sections, each n bits long All sections are added using “1’s Compliment” The resulting sum is complemented If the result is zero, data is accepted.
1 0 1 0 1 0 0
Sender
o The unit is divided into k sections, each n bits long o All sections are added using “1’s Compliment” o The resulting sum is complemented which becomes the “Checksum” o Checksum is transmitted with the data. o o o o
Faisal Amjad CPS 422
Checksum - Generation
0 0 1 1 1 0 0
0 0 0 1 1 1 0 1
1 0 1 0 1 0 0 1 0 0 1 1 1 0 0 1 0 0 0 1 1 1 0 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
Sum Complement
ASSIGNMENT # 2 Write a note on the performance of fol: o o o o
Simple Parity check Two-Dimensional parity check CRC Checksum
Faisal Amjad CPS 422
3
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