2 - Phys & D.L Layer and Errors Flashcards
P.L Responsibilities (6)
- 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
D.L.L Responsibilities (7)
- 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
Metrics of Network Performance
- Latency
- B/W
- Throughput
Causes of Transmission Errors
- Thermal Background
- Impulse Noise
- Distorted frequency
- Crosstalk
- Reduction of dynamic range and noise
Character Stuffing
Putting in a character to overcome sync problems
Bit Stuffing
Lower level character stuffing with 1s and 0s
Encoding schemes (2)
- Non-return to Zero Level (NRZ-L)
- Non-return to Zero Inverted (NRZ-I), example of differential encoding
Differential Encoding
Where the signal transition rather than the signal level indicates the value of each bit
Codewords are constructed with (2)
- data
- check bits
Hamming Distance
The XOR between two code words
To detect delta errors..
a hamming distance of (delta +1) is required
To correct delta errors..
a hamming distance of (2 x delta + 1) is required
Polynomial Codes
Represented by a bit string with 1 for each power represented in the polynomial, and 0 otherwise.
When do errors go undetected?
E(x) - num of errors If E(x) is divisible by G(x) then errors can go undetected
Burst Error
Where the error is a number of bits than are in sequence with each other
