o Information field
contains the user's
data in an
I-Frame and Network
Management
information
in a U-Frame
o An S-Frame has no information field
o Its
length can vary
from one network
to another but
remains fixed within
each
network
o It is possible to send Control
information in the information field of the I-Frame along
with
data.
o This process is called Piggybacking
|
AMAFHH
Friday, September 19, 2014
Bit-Oriented Protocols-Cont
Bit-Oriented Protocols-Cont
o
|
The second
field of HDLC frame contains the address of the secondary station
that
is either the originator or the destination of the frame
|
o If
a primary station
creates Frame it
includes a 'To'
address and if
a secondary
creates
the frame, it contains a 'From' address
o Can be of one byte or several bytes
depending upon the network
o If the address field is only 1 byte, the
last bit is always a 1
o If the address is of several bytes, all
bytes but the last one will end with 0 , and the
last
will end with a 1
o Ending each intermediate byte with 0
indicates to the receiver that there are more
address
bytes to come
|
Bit-Oriented Protocols-Cont
Asynchronous
Response Mode (ARM)
|
o A
secondary may initiate
a TX w/o
permission from the
primary whenever the
channel
is idle
o ARM does not alter the primary secondary
relationship in any other way
o All
transmissions from the
primary still go
to the secondary
and are then
relayed to
the
other devices
Asynchronous
Balanced Mode (ABM)
o All
stations are equal
and therefore only
combined stations connected
in point-to-
point
are used
o Either
combined station may
initiate TX with
the other combined
station w/o
permission
HDLC
Frames
|
Bit-Oriented Protocols-Cont
Configuration
(4)
Balanced
Configuration
o Both stations in a point-to-point
topology are of combined type
o HDLC does not support balanced multipoint
Station
Types & Configurations
|
Modes
o A mode in HDLC is the relationship b/w
two devices involved in an exchange
o The mode describes who controls the link
o HDLC supports 3 modes of communication
b/w stations:
-Normal
Response Mode (NRM)
-Asynchronous
Response Mode (ARM)
-Asynchronous
Balanced Mode (ABM)
|
Normal
Response Mode (NRM)
o Refers to the standard primary-secondary
relationship
o Secondary device must have permission
from primary device before transmitting
o Once permission has been granted, the
secondary may initiate a response transmission
of
one or more frames containing data
|
Bit-Oriented Protocols
Bit-Oriented Protocols
o In
character-oriented
protocols, bits are
grouped into predefined
patterns forming
characters
o By comparison, bit-oriented protocols can
pack more information into shorter frames
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