Chapter 7 Data Link Control

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William Stallings

Data and Computer

Communications

Chapter 7

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Flow Control

 _{Ensuring the sending entity does not}

overwhelm the receiving entity

 _{Preventing buffer overflow}  _{Transmission time}

 _{Time taken to emit all bits into medium}  _{Propagation time}

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Stop and Wait

 _{Source transmits frame}

 _{Destination receives frame and replies}

with acknowledgement

 _{Source waits for ACK before sending next}

frame

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Fragmentation

 _{Large block of data may be split into small}

frames

 _{Limited buffer size}

 _{Errors detected sooner (when whole}

frame received)

 On error, retransmission of smaller

frames is needed

 _{Prevents one station occupying medium}

for long periods

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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)

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Sliding Window Enhancements

 _{Receiver can acknowledge frames without}

permitting further transmission (Receive Not Ready)

 _{Must send a normal acknowledge to resume}  _{If duplex, use piggybacking}

 _{If no data to send, use acknowledgement}

frame

 _{If data but no acknowledgement to send,}

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Error Detection

 _{Additional bits added by transmitter for}

error detection code

 _Parity

 _{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}

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Cyclic Redundancy Check

 _{For a block of}_k_{bits transmitter generates}

n bit sequence

 _Transmit_k+n_{bits which is exactly divisible}

by some number

 _{Receive divides frame by that number}  _{If no remainder, assume no error}

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Error Control

 _{Detection and correction of errors}  _{Lost frames}

 _{Damaged frames}

 _{Automatic repeat request}  _{Error detection}

 _{Positive acknowledgment}

 _{Retransmission after timeout}

 _{Negative acknowledgement and}

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Automatic Repeat Request (ARQ)

 _{Stop and wait}  _{Go back N}

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Stop and Wait

 _{Source transmits single frame}  _{Wait for ACK}

 _{If received frame damaged, discard it}  _{Transmitter has timeout}

 _{If no ACK within timeout, retransmit}

 _{If ACK damaged,transmitter will not recognize} it

 _{Transmitter will retransmit}

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Stop and Wait

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Stop and Wait - Pros and Cons

 _Simple

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Go Back N (1)

 _{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}

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Go Back N - Damaged Frame

 _{Receiver detects error in frame}_i  _{Receiver sends rejection-}_i

 _{Transmitter gets rejection-}_i

 _{Transmitter retransmits frame}_i_{and all}

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Go Back N - Lost Frame (1)

 _Frame_i_lost

 _{Transmitter sends}_i+1

 _{Receiver gets frame}_i+1_{out of sequence}  _{Receiver send reject}_i

 _{Transmitter goes back to frame}_i_and

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Go Back N - Lost Frame (2)

 _Frame_i_{lost and no additional frame sent}

 _{Receiver gets nothing and returns neither}

acknowledgement nor rejection

 Transmitter times out and sends

acknowledgement frame with P bit set to 1

 _{Receiver interprets this as command which}

it acknowledges with the number of the

next frame it expects (frame i )

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Go Back N - Damaged

Acknowledgement

 _{Receiver gets frame}_i_{and send}

acknowledgement (i+1) which is lost

 _{Acknowledgements are cumulative, so}

next acknowledgement (i+n) may arrive before transmitter times out on frame i

 _{If transmitter times out, it sends}

acknowledgement with P bit set as before

 _{This can be repeated a number of times}

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Go Back N - Damaged Rejection

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Go Back N -

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Selective Reject

 _{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

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Selective Reject

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High Level Data Link Control

 _HDLC

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HDLC Station Types

 _{Primary station}

 _{Controls operation of link}

 _{Frames issued are called commands}

 _{Maintains separate logical link to each}

secondary station

 _{Secondary station}

 _{Under control of primary station}  _{Frames issued called responses}  _{Combined station}

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HDLC Link Configurations

 _Unbalanced

 _{One primary and one or more}

secondary stations

 _{Supports full duplex and half duplex}  _Balanced

 _{Two combined stations}

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HDLC Transfer Modes (1)

 _{Normal Response Mode (NRM)}  _{Unbalanced configuration}

 _{Primary initiates transfer to secondary}  _{Secondary may only transmit data in}

response to command from primary

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HDLC Transfer Modes (2)

 _{Asynchronous Balanced Mode (ABM)}  _{Balanced configuration}

 _{Either station may initiate transmission}

without receiving permission

 _{Most widely used}

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HDLC Transfer Modes (3)

 _{Asynchronous Response Mode (ARM)}  _{Unbalanced configuration}

 _{Secondary may initiate transmission}

without permission form primary

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Frame Structure

 _{Synchronous transmission}  _{All transmissions in frames}

 _{Single frame format for all data and}

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Flag Fields

 _{Delimit frame at both ends}  _01111110

 _{May close one frame and open another}  _{Receiver hunts for flag sequence to}

synchronize

 _{Bit stuffing used to avoid confusion with data} containing 01111110

 _{0 inserted after every sequence of five 1s}  _{If receiver detects five 1s it checks next bit}  _{If 0, it is deleted}

 _{If 1 and seventh bit is 0, accept as flag}  _{If sixth and seventh bits 1, sender is}

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Bit Stuffing

 _{Example with}

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Address Field

 _{Identifies secondary station that sent or}

will receive frame

 _{Usually 8 bits long}

 _{May be extended to multiples of 7 bits}

 _{LSB of each octet indicates that it is the}

last octet (1) or not (0)

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Control Field

 _{Different for different frame type}

 _{Information - data to be transmitted to}

user (next layer up)

 _{Flow and error control piggybacked on}

information frames

 _{Supervisory - ARQ when piggyback not}

used

 _{Unnumbered - supplementary link control}  _{First one or two bits of control filed identify}

frame type

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Poll/Final Bit

 _{Use depends on context}  _{Command frame}

 _{P bit}

 _{1 to solicit (poll) response from peer}  _{Response frame}

 _{F bit}

 _{1 indicates response to soliciting}

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Information Field

 _{Only in information and some}

unnumbered frames

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Frame Check Sequence Field

 _FCS

 _{Error detection}  _{16 bit CRC}

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HDLC Operation

 _{Exchange of information, supervisory and}

unnumbered frames

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Other DLC Protocols (LAPB,LAPD)

 _{Link Access Procedure, Balanced (LAPB)}  _{Part of X.25 (ITU-T)}

 _{Subset of HDLC - ABM}

 _{Point to point link between system and packet} switching network node

 _{Link Access Procedure, D-Channel}  _{ISDN (ITU-D)}

 _ABM

 _{Always 7-bit sequence numbers (no 3-bit)}

 _{16 bit address field contains two sub-addresses}  _{One for device and one for user (next layer}

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Other DLC Protocols (LLC)

 _{Logical Link Control (LLC)}  _{IEEE 802}

 _{Different frame format}

 _{Link control split between medium access layer}

(MAC) and LLC (on top of MAC)

 _{No primary and secondary - all stations are peers}  _{Two addresses needed}

 _{Sender and receiver}

 _{Error detection at MAC layer}  _{32 bit CRC}

 _{Destination and source access points (DSAP,}

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Other DLC Protocols

(Frame Relay) (1)

 _{Streamlined capability over high speed}

packet witched networks

 _{Used in place of X.25}

 _{Uses Link Access Procedure for}

Frame-Mode Bearer Services (LAPF)

 _{Two protocols}

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Other DLC Protocols

(Frame Relay) (2)

 _ABM

 _{7-bit sequence numbers}  _{16 bit CRC}

 _{2, 3 or 4 octet address field}

 _{Data link connection identifier (DLCI)}  _{Identifies logical connection}

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Other DLC Protocols (ATM)

 _{Asynchronous Transfer Mode}

 _{Streamlined capability across high speed}

networks

 _{Not HDLC based}

 _{Frame format called “cell”}  _{Fixed 53 octet (424 bit)}

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Required Reading

 _{Stallings chapter 7}

 _{Web sites on HDLC, frame relay, Ethernet}