CIS 321Data Communications & Networking

Chapter 11 – Data Link Control and Protocols

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Introduction

Protocol – set of rules governing communication specific to one or more layers of the OSI model

Data link protocols define the rules devices use to implement data link layer functions

Contain rules for line discipline, flow control, and error control

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Data Link Layer Functions

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Flow Control

Coordinates amount of data sent before receiving acknowledgement

Purpose: prevent overwhelming receiver Buffer overflow

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Error Control

Error detection and error correction Receiver informs sender of any frames lost or

damaged and coordinates retransmission of those frames by the sender Usually handled via automatic repeat request (ARQ)

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Flow and Error Control Mechanisms

Stop-and-Wait ARQ Go-back-N ARQ Selective-Repeat ARQ

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11.2 Stop-and-Wait ARQ

Sender keeps copy of last frame sent and waits for ACK for that frame

Next frame cannot be sent until ACK has been received

Frames are numbered alternately 0 and 1 Damaged or lost frames are resent Repeats until EOT is sent

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Stop-and-Wait Normal Operation

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Lost or Damaged Frame

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Lost Acknowledgement

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Delayed Acknowledgement

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Stop-and-Wait

Advantage: simplicity; each frame is checked and ACK’d before next frame is sent Numbering of frames prevents duplication

Disadvantage: inefficiency; slow Frame and ACKs use entire bandwidth If distance is long between devices, time spent

waiting can be significant

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Bidirectional Transmission

Possible if two parties have separate channels for full-duplex transmission or share same channel for half-duplex transmission

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Piggybacking

Method of combining data frame and acknowledgement

Saves bandwidth due to less overhead from separate data frame and ACK frame into one frame

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Piggybacking

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Sliding Window

Sender may transmit several frames before needing an ACK

Much more efficient; receiver may use a single ACK to confirm multiple frames

Sliding window refers to upper and lower limit on number of frames that may be transmitted before ACK is required

Frames must be numbered to allow receiver to identify which frame is acknowledged

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11.3 Go-Back-N ARQ

Allows for more efficient transmission – send multiple frames before requiring an ACK

Specify a window or range of sequence numbers of frames that may be received

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Sliding Window

Receiver includes number of next frame it expects to receive in ACK

Sender then knows all previous frames through that number have been received

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Sender Window

Window contains 0 to 2m -1 frames

Window shrinks as frames are sent out

Once ACK arrives, window expands equal to number of frames acknowledged by ACK

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Receiver Window

Size of window is always 1 Receiver expects the next

ordered frame (must always be in order)

Any frame arriving out of order is discarded

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Control Variables

Sender : S – sequence number of recently sent frame; SF - sequence number of first frame in window; SL – sequence number of last frame in window

Window size is W = SL - SF + 1 Receiver : R – sequence number of next frame expected

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Go-Back-N Operation

Timers: sender sets a timer for each frame (none for receiver)

Acknowledgement – receiver sends positive ACK; silent on damaged or out of order frames

Resending Frames – sender sends set of frames from damaged up to last one sent and ACK’d

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11.4 Selective Repeat ARQ

Go-Back-N less efficient since all out of order or damaged frames must be resent in order

Selective Repeat is a more efficient method, yet required more processing

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Sender and Receiver Windows

Sender window and control variables are same as Go-Back-N

Receiver window is same size; looks for range of sequence numbers Requires two control variables to define window

boundaries: RF and RL

Also defines a negative acknowledgement (NAK) to report sequence number of damaged frame

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Selective Repeat ARQ

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Selective Repeat ARQ, Lost Frame

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Comparisons between Go-back-n and Selective-Reject

Sending only specific damaged or lost frames requires complexity of sorting and more storage is required in select-reject

Go-back-n is typically used due to simplicity

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11.5 HDLC

High-level Data Link Control – protocol supporting half-duplex and full-duplex communication over point-to-point and multipoint links

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HDLC Modes of Communication

Relationship between two devices involved in an exchange

Defines who controls the link Two modes:

Normal response mode (NRM) Asynchronous balanced mode (ABM)

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Normal Response Mode

Refers to standard primary-secondary relationships

Used for all exchanges in unbalanced configurations

Primary can issues commands Secondary must have permission from primary

before responding or sending data

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NRM

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Asynchronous Balanced Mode

All stations are equal Stations in point-to-point configurations act as both

primary and secondary

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HDLC Frames

Three types; each functions as an envelope to transmit a specific type of message

Information frames (I-frames) – transports user data and control info relating to user data

Supervisory frames (S-frames) – used to transport control info for data link layer flow and error controls

Unnumbered frames (U-frames) – used for system mgmt and link mgmt

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HDLC Frame

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Flag Field

Marks the beginning and end of frame and provides synchronization with 01111110

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Address Field

Contains address of secondary station that is originator or destination of the frame

If created by primary, contains a to address If created by secondary, contains a from address May be one byte or several bytes long

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Control Field

One- or two-byte segment for flow management Fields differ depending on frame type First or first and second bits identify type of frame All three frame types contain a poll/final (P/F) bit;

used to identify whether frame was sent by primary to a secondary or from secondary to primary

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Information Field

Contains user’s data in an I-frame; network management in a U-frame; not included in an S-frame

Often used in a data frame to acknowledge receipt of another separate frame – called piggybacking

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FCS Field

Error detection field Stores either two- or four-byte CRC

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Different Control Frames

I-frame – used for user information and transport and piggyback acknowledgements

S-frame – used for acknowledgement, flow control, and error control when piggybacking is not appropriate

U-frame – used to exchange session management and control information between devices

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Data Transparency

To address the possibility that a bit pattern may match flag field indicator and be misinterpreted, bit stuffing may be used

Anytime 5 consecutive 1s are encountered, a redundant 0 is inserted

Identifies that the current sequence is not a flag

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Bit Stuffing and Removal

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Coming Up… Ch 12

Point-to-Point Access: PPP (brief)

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Credits

All figures obtained from publisher-provided instructor downloadsData Communications and Networking, 3rd edition by

Behrouz A. Forouzan.  McGraw Hill Publishing, 2004