1. Chap. 4 Data link layer 2

2. Connection control Connection control
defines reliable transmission between two adjacent nodes
three step
Connection establishment
Frame transmission
Connection termination

3. Connection-oriented service or connection mode
Connectionless service or connectionless mode

4. acknowledged service
unacknowledged service

5. Flow control Flow control
Ensuring the transmitter does not overwhelm capability of receiver
preventing buffer overflow
host controller
protocol controller
transmit buffer and receive buffer

6. Stop-and-wait flow control
Source transmits frame
Destination receives frame and replies with acknowledgement
Source waits for ACK before sending next frame
Destination can stop flow by not send ACK
Works well for a few large frames

7. Sliding Windows Flow Control
Allow multiple frames to be in transit
Receiver has buffer W long
Transmitter can send up to W frames without ACK
Each frame is numbered
ACK includes number of next frame expected
Sequence number bounded by size of field (k)
Frames are numbered modulo 2k

8. Error detection Error detection
Additional bits added by transmitter for error detection code
FCS (frame check sequence)

9. Parity bit
Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
Even number of bit errors goes undetected ex
ASCII code

10. Cyclic Redundancy Check
For data of k bits, transmitter generates FCS(frame check sequence) of n-k bits
Transmit frame of n bits which is exactly divisible by some number (modulo 2 operation)
Receive divides frame by that number
modulo 2 operation
exclusive-OR operation : 1^1 = 0, 1^0 = 1
Equation
T : frame of n bits
D : data of k bits
F : FCS of n-k bits
P : divisor of n-k+1 bits

11. Divisor
Denoted using polynomial
P=11001 => P(X)=X4+X3+1
CRC-16 : CAN
P(X)=X16+X15+X2+1
CRC-32 : IEEE 802.3
Example
Transmitter
ASCII data of ‘a’
Odd parity bit
Divisor of P(X)=X4+X3+1
Receiver
Frame is divided by divisor
If remainder is not 0, error is occurred

12. Error control Error control
If error is detected after error detection procedure, error recovery process is started
Sequence number is used for error recovery
Type of error
lost frame : if frames are out of sequence
damaged frame : if error is detected after receiving

13. Stop-and-wait ARQ ARQ (automatic repeat request)
Procedure for error recovery
Source transmits single frame
Wait for ACK
If received frame damaged, discard it
Transmitter has timeout
If no ACK within timeout, retransmit

14. If ACK is damaged, transmitter will not recognize
Transmitter will retransmit
Receive gets two copies of frame
Use ACK0 and ACK1

15. Go-back-N ARQ Based on sliding window
If no error, ACK as usual with next frame expected
Use window to control number of outstanding frames
If error, reply with rejection
Discard that frame and all future frames until error frame received correctly
Transmitter must go back and retransmit that frame and all subsequent frames

16. Selective Reject ARQ Also called selective retransmission
Only rejected frames are retransmitted
Subsequent frames are accepted by the receiver and buffered
Minimizes retransmission
Receiver must maintain large enough buffer
More complex login in transmitter