Friday, September 19, 2014

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

A lot of bit-oriented protocols have been developed over the years:

o    One  of  these  HDLC  is  the  design  of  the  ISO  and  has  become  the  basis  for  all  bit-
oriented protocols in use today
In 1975, IBM gave Synchronous Data Link Control (SDLC)
In 1979, ISO answered with High Level Data Link Control (HDLC)

o    Since 1981, ITU-T has developed a series of protocols called Link Access Protocols
o    LAPs: LAPB, LAPD, LAPM, LAPZ etc. all based on HDLC
HDLC is basis for all protocols, so we will study it in detail:

High Level Data Link Control (HDLC)
o    Bit-oriented data link protocol designed for:
-    Full Duplex and Half Duplex
-    Point-to-point And Multipoint Links

Characterization of HDLC
HDLC can be characterized by:
-Station Types
-Configurations
-Response Modes

STATION TYPES (1)
HDLC differentiates b/w 3 types of stations:
-    Primary Station
-    Secondary Station
-    Combined Station

STATION TYPES (2)

Primary Station

o    Primary station works in the same way as primary devices in the discussion of flow
control
o    The  primary  is  a  device  in  point-to-point  or  multipoint  line  configuration  that  has
complete control of the link
STATION TYPES (3)

Secondary Station
o    The primary sends commands to the secondary stations
o    A primary issues commands and a secondary issues responses

STATION TYPES (4)

Combined Station
o    A combined station can both command and respond
o    A combined station is one of a set of connected peer devices programmed to behave
either as a primary or as a secondary depending on the nature and the direction of the
transmission

Configuration (1)
o    Configuration refers to the relationship of the hardware devices on a link
o    Primary , secondary and combined stations can be configured in three ways:

¾   Unbalanced Configuration
¾   Symmetrical Configuration
¾   Balanced Configuration

Configuration (2)
Unbalanced Configuration
o    Also called Master/Slave Configuration
o    One device is a primary and others are secondary
o    Unbalanced  configuration  can  be  point  to  point  if  only  two  devices  are
involved
o    Most   of   the   times   it   is   multipoint   with   one   primary   controlling   several
secondaries
Configuration (3)
Symmetrical Configuration
o    Each physical station on a link consists of two logical stations, one a primary
and the other a secondary
o    Separate lines link the primary aspect of one physical station to the secondary
aspect of another physical station

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