3. the data link layer - university of...
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Computer and Data Networks, 3. Date Link Layer 1 ©Dr.Z.Sun
3. The Data link layer
Computer and Data Networks, 3. Date Link Layer 2 ©Dr.Z.Sun
Outline
n Link layer servicesn Framingn Error correction and detection codes n Error and flow control: Slide window protocolsn HDLCn SLIP and PPP
Computer and Data Networks, 3. Date Link Layer 3 ©Dr.Z.Sun
Functions of the Data Link Layer
Relationship between packets and frames.
Computer and Data Networks, 3. Date Link Layer 4 ©Dr.Z.Sun
Link layer services
n Unacknowledged connectionlessn Acknowledged connectionlessn Acknowledged connection-oriented
n Framingn Error controln Flow control
Computer and Data Networks, 3. Date Link Layer 5 ©Dr.Z.Sun
Framing methods:
Four methods:
n Character count
n Flag bytes with bytes stuffing
n Starting & ending flags with bit stuffing
n Physical layer coding violations
Computer and Data Networks, 3. Date Link Layer 6 ©Dr.Z.Sun
Framing – Character count
A character stream. (a) Without errors. (b) With one error.
Computer and Data Networks, 3. Date Link Layer 7 ©Dr.Z.Sun
Framing – Flag bytes and byte stuffing
(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after stuffing.
Computer and Data Networks, 3. Date Link Layer 8 ©Dr.Z.Sun
Framing – bit stuffing
Bit stuffing(a) The original data.(b) The data as they appear on the line.(c) The data as they are stored in receiver’s memory after destuffing.
Computer and Data Networks, 3. Date Link Layer 9 ©Dr.Z.Sun
Error correction codes
n Codeword = message + redundantn If two codewords are a Hamming
distance d apart, it will require d single-bit error to convert one into the other
n To detect d errors, you need a distance of d+1 code
n To correct d errors, you need a distance 2d+1 code
n Example: parity bit (even or odd)Hamming method: (m+r+1)<=2r
n power of 2 (1,2,4, ...) check bits n the rest (3,5,6,7,9, ...) data bitsn example bit 11 checked by (1,2,8)
Computer and Data Networks, 3. Date Link Layer 10 ©Dr.Z.Sun
Assume Xi are data bits and Ri data bits
n X3 corresponds to R1 and R2n X5 corresponds to R1 and R4n X6 corresponds to R2 and R4n X7 corresponds to R1, R2 and R4n X9 corresponds to R1 and R8n X10 corresponds to R2 and R8n X11 corresponds to R1, R2 and R8
n R1 corresponds to {X3, X5, X7, X9, X11}
n R2 corresponds to {X3, X6, X7, X10, X11}
n R4 corresponds to {X5, X6, X7}n R8 corresponds to {X9, X10,
X11}
n Example: m = 1101101 => ?? 1 ? 101 ? 101 => 11 1 0 101 0 101
n If the receiving node received: 11 1 0 1 11 1 0 101
=> R1 = 1, R2 = 0, R4= 1, R8 =0 => 0(1) + 1(2) + 1(4) + 0(8) = 66This implies that bit 6 is incorrect, as it is checked by R2 and R4
Computer and Data Networks, 3. Date Link Layer 11 ©Dr.Z.Sun
Error detection codes
Polynomial code (cyclic redundancy code or CRC code)
n Let r be the degree of generator polynomial G(x). Append r zeros to the low-order end of the frame M(x) =>xrM(x).
n Divide the bit string corresponding to G(x) into the bit string corresponding toxrM(x) using modulo 2 division
n Subtract the remainder from the bit string corresponding to xrM(x) using modulo 2 subtraction. The results is the check-summed frame called polynomial T(x).
Computer and Data Networks, 3. Date Link Layer 12 ©Dr.Z.Sun
Stop-and-wait and slide window protocol
n Stop-and-wait ARQ: the sender sends a frame and waits for acknowledgement before sending any further frame. There are two sorts of errors: sending and acknowledgement.
n How about efficiency (stop and wait or window size is 1)?Satellite link is 50 kbit/s, round trip time 500 ms, frame size 1000 bits, frame transmission time 20 ms.Efficiency = 20/(20+500) =1/26
n Slide window protocol allows the sender sends more frames up to a window size before stopping and waiting for acknowledgment
Computer and Data Networks, 3. Date Link Layer 13 ©Dr.Z.Sun
Go Back N and Selective repeat
n Pipe line technique: channel capacity is B bit/s, frame size L, round-trip time R.
Channel Efficiency = L/(L+BR) If L< BR the efficiency will be less
than 50%.n Two techniques trade-off
bandwidth and buffer space Go back to N (receive window = 1)
only receives frame in order and retransmit all frames after errors.
Selective repeat buffers frames and retransmit only the errored frame.
Computer and Data Networks, 3. Date Link Layer 14 ©Dr.Z.Sun
Window size and Negative acknowledgment
n NAK acknowledges when frame damaged or unexpected (potential lost frame)
n The maximum window size should be at most half of the range of the sequence number (Max_Seq+1)/2
n Trade-off risk of unnecessary retransmission, or idle for a long period after errors
n Standard deviation of acknowledgment is small, the timer can be set “tight”. Otherwise “loose”.
Computer and Data Networks, 3. Date Link Layer 15 ©Dr.Z.Sun
High-level Data link Control (HDLC)
n Bit oriented and bit stuffing
n 2x8 bits for framingn 8 bit address field identify
multiple terminalsn Control field can be used
for three types of frames: information, supervision and unnumbered.
n Data fieldn 16 bits for CRC error
check
Computer and Data Networks, 3. Date Link Layer 16 ©Dr.Z.Sun
Internet SLIP and PPP protocols
n Serial Line IP (SLIP): no error detection and correction, support only IP, need to know IP address in advance, no authentication, not aproved Internet standard.
n Point-to-Point protocol (PPP): handles no error detection, support multiple protocol, address can be negotiated at connection time, permit authentication, many improvement over SLIP
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PPP frame format (character oriented)
n Closely resemble HDLC frame formatn Character oriented
Default for unnumbered frame
0: Net layer protocol 1: other
Default is 2. Default is 1500.
Normally 2 but can be negotiated for 4.
Default
Computer and Data Networks, 3. Date Link Layer 18 ©Dr.Z.Sun
Summary
n Link layer servicesn Framingn Error control
• Error correction and Error detection n Flow control
• Stop-and-wait, slide window• Go back N, Selective repeat, Negative
Acknowledgementn Dial-up protocols for Internet access
• HDLC and PPP