3. The Data link layer

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Outline n n n n n n

Link layer services Framing Error correction and detection codes Error and flow control: Slide window protocols HDLC SLIP and PPP

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Functions of the Data Link Layer Relationship between packets and frames.

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Link layer services n n n

Unacknowledged connectionless Acknowledged connectionless Acknowledged connection-oriented

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Framing Error control Flow control

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Framing methods: Four methods: n

Character count

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Flag bytes with bytes stuffing

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Starting & ending flags with bit stuffing

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Physical layer coding violations

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Framing – Character count A character stream. (a) Without errors. (b) With one error.

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Framing – Flag bytes and byte stuffing (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

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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.

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Error correction codes Codeword = message + redundant n If two codewords are a Hamming distance d apart, it will require d singlebit 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) ?? 1 ? 101 ? 101 => 11 1 0 101 0 101

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If the receiving node received: 11 1 0 1 1 1 0 101

=> R1 = 1, R2 = 0, R4= 1, R8 =0 => 0(1) + 1(2) + 1(4) + 0(8) = 6 This implies that bit 6 is incorrect, as it is checked by R2 and R4 Computer and Data Networks, 3. Date Link Layer

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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 to xrM(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

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Stop-and-wait and slide window protocol n

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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. 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 Slide window protocol allows the sender sends more frames up to a window size before stopping and waiting for acknowledgment

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Go Back N and Selective repeat 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. n

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Window size and Negative acknowledgment n n n n

NAK acknowledges when frame damaged or unexpected (potential lost frame) The maximum window size should be at most half of the range of the sequence number (Max_Seq+1)/2 Trade-off risk of unnecessary retransmission, or idle for a long period after errors Standard deviation of acknowledgment is small, the timer can be set “tight”. Otherwise “loose”.

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High-level Data link Control (HDLC) n n n n

n n

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Bit oriented and bit stuffing 2x8 bits for framing 8 bit address field identify multiple terminals Control field can be used for three types of frames: information, supervision and unnumbered. Data field 16 bits for CRC error check ©Dr.Z.Sun

Internet SLIP and PPP protocols n

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Serial Line IP (SLIP): no error detection and correction, support only IP, need to know IP address in advance, no authentication, not a proved Internet standard. 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 n

Closely resemble HDLC frame format Character oriented Default is 2.

Default

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Default for unnumbered frame

Default is 1500.

Normally 2 but can be negotiated for 4.

0: Net layer protocol 1: other

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Summary n n n n

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Link layer services Framing Error control • Error correction and Error detection Flow control • Stop-and-wait, slide window • Go back N, Selective repeat, Negative Acknowledgement Dial-up protocols for Internet access • HDLC and PPP

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