o The Rx device must be able to sort frames
and insert the retransmitted frame
into
the proper place
The
selective reject ARQ differs from Go Back n in the following ways:
o The Rx device must contain sorting logic
to enable it to reorder frames
received
out of sequence
o Sending
device must contain
a searching mechanism
that allows it
to
find
and select only the requested frame for retransmission
o Selective Reject ARQ
o A
buffer in the
receiver must keep
all previously received
frames on
hold
until all retransmissions have been stored
o To
avoid selectivity, ACK
number, like NAK
numbers must refer
to
frame
received instead of next expected frame
o A smaller window size is required because
of this added complexity
Selective Reject ARQ-Lost Frame
Lost
ACK/NAK are treated exactly in the same way as by Go Back n
Selective
Reject ARQ vs Go Back n
o Although
retransmitting only specific
damaged or lost
frames may seems
more
efficient
than resending all the frames
o Because
of the complexity
of sorting and
storage required by
the receiver and
extra
logic needed by sender to select specific frames for retransmission,
selective
reject
ARQ is EXPENSIVE and not often used
o Selective reject gives better performance
but in practice it is usually it is discarded
in
favor of go-back-n for simplicity of implementation
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Protocols
Protocol: Set of rules
or conventions for executing a particular task
Protocol in Data Comm.: Set
of rules or
specifications used to
implement one or
more
layers
of the OSI Model
Example: EIA 232-D
interface is a protocol used at the physical layer in the OSI Model
Protocols
Data
Link Protocols:
Set of specifications used to implement the data link layer
Data
link protocols contain rules for:
-Line
Discipline
-Flow Control
-Error Control
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