DATA LINK CONTROL PROTOCOLS. 2 Introduction Data link control layer – often abbreviated simply to...
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DATA LINK CONTROL PROTOCOLS
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Introduction Data link control layer – often abbreviated
simply to data link layer – is concerned with the transfer of data over a serial data link
The transmission mode may be either asynchronous or synchronous and based on either a character-oriented or a bit-oriented transmission control protocol
The data link layer is fundamental to the operation of all data communication applications
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Introduction
Depending on the application, the user service provided by the data link layer may be either a simple best-try (connectionless) service or a reliable (connection-oriented) service
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Introduction The connectionless service
means that although error check bits are used to detect errors, any frames that are found to contain transmission errors are simply discarded by the link layer protocol entity.
It is also referred to as an unacknowledged service
Retransmission becomes a function of a higher layer protocol
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Introduction The connection-oriented service
Data link protocol employs error and flow control procedures to provide a reliable service
To achieve this, prior to sending any data (information frames), a logical connection between the two data link layer protocol entities is established (using L_Connect service)
All data is transfer using a suitable retransmission and flow control protocol
When all data has been exchanged, the logical connection is clear (using L_Disconnect service)
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Introduction
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Application Environments In some instances the data link protocols
are located in the two communicating DTEs, and the protocol is said to operate on an end-to-end basis
In others, data link protocols operates over the local link connecting, for example, the DTE to a network, the protocol is said to have only local significance
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Application Environments
The data link topology may be Point-to-point circuit
Direct physical connection Operates on an end-to-end basis
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Application Envirionments
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Application Environments The data link topology may be
Multipoint or multidrop topology Single transmission line – known as a bus
or data highway – is used to connect all the computer together.
Normally used in applications that involve a single master (supervisory) computer communicating with a distributed community of slave computers
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Applications Environments
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Application Environments
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Character-oriented Protocols Character-oriented protocols
are in use in both point-to-point and multipoint applications
Are characterized by the selected transmission control characters used to perform the various transmission control function associated with link management, start of frame and end of frame – frame delimiting, error control and data transparency
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Character-oriented Protocols
Simplex protocols Half-duplex protocols Full-duplex protocols
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Character-oriented Protocols – Simplex protocols
Simplest since it allows a simplex – one direction only – transfer of data from one DTE to another
Point-to-point data link topology The most widely used protocols for
this function is Kermit
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Character-oriented Protocols – Simplex protocols
Kermit: Synchronous transmission (normally) Point-to-point data link Stop and Wait ARQ
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Character-oriented Protocols – Simplex protocols
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Character-oriented Protocols – Simplex protocols
Kermit: Frame format
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Character-oriented Protocols – Simplex protocols
Kermit: Frame format
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Character-oriented Protocols – Simplex protocols
Kermit Protocol operation
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Character-oriented Protocols – Half-duplex Protocols
Most character-oriented protocols operate in the half-duplex, stop and wait mode
The best known is BSC
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Character-oriented Protocols – Half-duplex Protocols
BSC: Synchronous transmission Connection-oriented protocol Multipoint network/ multidrop bus
network
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Character-oriented Protocols – Half-duplex Protocols BSC
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Character-oriented Protocols – Half-duplex Protocols
BSC: Frame format
Data supervisory
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Character-oriented Protocols – Half-duplex Protocols
BSC
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Character-oriented Protocols – Half-duplex Protocols
BSC
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Character-oriented Protocols – Half-duplex Protocols
BSC
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Character-oriented Protocols – Half-duplex Protocols
BSC Protocol operation
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Character-oriented Protocols – Half-duplex Protocols
BSC
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Character-oriented Protocols – Half-duplex Protocols BSC
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Character-oriented Protocols – Half-duplex Protocols
BSC Protocol
performance
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Character-oriented Protocols – Duplex Protocols
A few character-oriented protocols operate in full-duplex mode
Character-oriented, full duplex protocols ARPANET
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Character-oriented Protocols – Duplex Protocols
ARPANET
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Bit-oriented Protocols
All new data link protocols are bit-oriented protocols
Such protocols use defined bit patterns rather than transmission control characters to signal the start and end of frame – known as frame delimiting
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Network configuration Point-to-point with single primary and
secondary Multipoint with single primary and
multiple secondary Point-to-point with two primaries and
two secondaries
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Operational modes Normal response mode (NRM)
used in balanced configurations Slave stations (or secondaries) can
transmit only when specially instructed by the master (primary) station
The link may be point-to-point or multipoint (in this case only one primary station is allowed)
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Operational modes Asynchronous response mode (ARM)
Used in unbalanced configurations Allows a secondary to initiate a
transmission without receiving permission from the primary.
Normally used with point-to-point configurations and duplex links
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Operational modes Asynchronous balanced mode (ABM)
Used mainly on duplex point-to-point links
Each station has an equal status and performs both primary and secondary functions
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format Flag field:
a string of bits 01111110 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 indicating abort
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format Address field:
Depend on the mode operation In NRM, on multidrop line, every secondary
station is assigned a unique address. Whenever the primary station communicates with a secondary, the address field contains the address of the secondary
Not used in this way in ABM because only direct point-to-point links are involved. Instead, it is used to indicate the direction of commands and their associated responses
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format FCS field
16-bit CRC with G(x) = x16 + x12 + x5 + 1
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format Control field
Unnumbered frames used for such functions as link setup and
disconnect Information frames
carry the actual information or data and are normally referred to simply as I-frame
Supervisory frames used for error and flow control and hence
contain send and receive sequence numbers
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format Control field
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Frame format
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Protocol operation Link management
Before any information (data) may be transmitted, a logical connection between the two communicating parties must be established
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Protocol operation Data transfer
Error control uses a continuous RQ with either selective repeat or go-back-N retransmission strategy
Flow control is based on a window mechanism
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Protocol operation Data transfer
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
Protocol operation Data transfer
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
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Bit-oriented Protocols – HDLC (High-level Data Link Control)
User interface
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
LAPB Subset of HDLC that is used to control the transfer of I-frame across a point-to-point duplex data link that connects a computer to a public or private packet-switching network (X.25 networks)
LAPB is an extended version of an earlier subset LAPA (Link Access Procedure version A)
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
The computer is the DTE and the packet switching exchange is the data circuit-terminating equipment (DCE)
LAPB is used to control the transfer of information frames across the local DTE-DCE interface (local significance)
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
LAPB used asynchronous balanced mode with DTE and DCE and
All I-frame are treated as command frames
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
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Bit-oriented Protocols – Link Access Procedure version B (LAPB)
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Bit-oriented Protocols – Link Access Procedure – D channel (LAPD)
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 up
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Bit-oriented Protocols – 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,
SSAP)