2 - Phys & D.L Layer and Errors

Question 1

P.L Responsibilities (6)

Answer 1

• provides the physical topology of the network
• provides the transfer medium
• encodes data into a transmission signal
• generarate/amplifies signal
• detects transmission errors
• detects arriving signal levels to sync speed and timing

Question 2

D.L.L Responsibilities (7)

Answer 2

• constructs data frames using the format of the P.L
• creates CRCs
• checks for arriving errors
• initiates and arbitrates the link to reduce interruption
• examines arriving and passing device addr fields
• retransmits data if error
• ACK of frame

Question 3

Metrics of Network Performance

Answer 3

• Latency
• B/W
• Throughput

Question 4

Causes of Transmission Errors

Answer 4

• Thermal Background
• Impulse Noise
• Distorted frequency
• Crosstalk
• Reduction of dynamic range and noise

Question 5

Character Stuffing

Answer 5

Putting in a character to overcome sync problems

Question 6

Bit Stuffing

Answer 6

Lower level character stuffing with 1s and 0s

Question 7

Encoding schemes (2)

Answer 7

• Non-return to Zero Level (NRZ-L)

- Non-return to Zero Inverted (NRZ-I), example of differential encoding

Question 8

Differential Encoding

Answer 8

Where the signal transition rather than the signal level indicates the value of each bit

Question 9

Codewords are constructed with (2)

Answer 9

• data

- check bits

Question 10

Hamming Distance

Answer 10

The XOR between two code words

Question 11

To detect delta errors..

Answer 11

a hamming distance of (delta +1) is required

Question 12

To correct delta errors..

Answer 12

a hamming distance of (2 x delta + 1) is required

Question 13

Polynomial Codes

Answer 13

Represented by a bit string with 1 for each power represented in the polynomial, and 0 otherwise.

Question 14

When do errors go undetected?

Answer 14

E(x) - num of errors
If E(x) is divisible by G(x) then errors can go undetected

Question 15

Burst Error

Answer 15

Where the error is a number of bits than are in sequence with each other

Question 16

Types of CRCs (4)

Answer 16

• CRC12
• CRC16
• CRC-CCITT
• CRC32