Chapter7 - DATA LINK CONTROL PROTOCOLS docx
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DATA LINK CONTROL
PROTOCOLS
2
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
3
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
4
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
5
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)
6
Introduction
7
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
8
Application Environments
The data link topology may be
Point-to-point circuit
Direct physical connection
Operates on an end-to-end basis
9
Application Envirionments
10
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
11
Applications Environments
12
Application Environments
13
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
14
Character-oriented Protocols
Simplex protocols
Half-duplex protocols
Full-duplex protocols
15
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
16
Character-oriented Protocols –
Simplex protocols
Kermit:
Synchronous transmission (normally)
Point-to-point data link
Stop and Wait ARQ
17
Character-oriented Protocols –
Simplex protocols
18
Character-oriented Protocols –
Simplex protocols
Kermit:
Frame format
19
Character-oriented Protocols –
Simplex protocols
Kermit:
Frame format
20
Character-oriented Protocols –
Simplex protocols
Kermit
Protocol operation
21
Character-oriented Protocols –
Half-duplex Protocols
Most character-oriented protocols operate in the half-
duplex, stop and wait mode
The best known is BSC
22
Character-oriented Protocols –
Half-duplex Protocols
BSC:
Synchronous transmission
Connection-oriented protocol
Multipoint network/ multidrop bus network
23
Character-oriented Protocols –
Half-duplex Protocols
BSC
24
Character-oriented Protocols –
Half-duplex Protocols
BSC:
Frame format
Data
supervisory
25
Character-oriented Protocols –
Half-duplex Protocols
BSC
