Link Layer
Introduction
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o Unless
accurately received by
a 2
electricity
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nd
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device, a
signal TX over
a wire is
a waste of
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o With TX alone, we can put a signal to the
line, but we have
-no way of
controlling which of several devices attached to that line will
receive
it
-no way of
knowing if the intended receiver is ready and able to receive it
-No way of
keeping a second device from TX at the same time
o Communication requires at least 2 devices
working together:
-Sender
-Receiver
o Even such a basic arrangement requires
great deal of coordination
o For Example, in Half Duplex TX,
it is essential that only one device TX at a time
o If both device TX, the signals will
collide leaving nothing on the line but Noise
o The
coordination of half-duplex
TX is a
part of a
procedure called Line
Discipline,
which
is one of the functions included in the second layer of OSI Model, the data
link
layer
o In addition to Line Discipline, the most
important functions in the data link layer are
Flow
Control and Error Control
o Collectively these functions are called
Data Link Control
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Line Discipline:
- Coordinates
the link systems, which device can send and when it can send?
Flow Control:
-The amount of
data that can be sent before the receiving acknowledgement
-It also
provides the receiver's acknowledgement for frames received intact and
so
is linked to error control
Error Control:
-Means Error
detection and Correction
-It allows the
receiver to inform the sender of any frames lost or damaged in TX
and
coordinates Retransmission of those frames by the sender
Line Discipline
o How
efficient the system
is, no device
in it should
be allowed to
transmit until that
device
has the evidence that the intended receiver is:
-able to receive
-is prepared to
accept the TX
o What if the Rx device does not expect a
transmission or is busy
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