Friday, September 19, 2014

Bit-Oriented Protocols-Cont

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

o    The FCS is HDLC's error detection field

o    It can contain a two- or four byte CRC





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
o    The control field is a one o two byte segment of the frame used for flow management
o    The two byte case is called the Extended Mode

Control fields differ depending on the frame type:
-    If the control field is a 0, the frame is an I-Frame
-    If the first bit is 1 and the second bit is a 0 , it is S-Frame
-    If both first and second bits are 1's, it is U-Frame

o    P/F bit is a single bit with dual purpose
o    It has meaning only when it is '1' and it can mean Poll or Final
o    When the frame is sent by a primary to secondary, it means POLL
o    When the frame is sent by a secondary to a primary, it is FINAL

HDLC Control Field -EXTENDED

o    Control  field  in  the  I-Frame  and  S-Frame  is  two  bytes  long  to  allow  seven  bits  of
sending and receiving sequence
o    However the control field in the U-Frame is still one byte

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
HDLC defines 3 types of Frames:
-    Information Frames (I-Frames)
-    Supervisory Frames (S-Frames)
-    Unnumbered Frames(U-Frames)

I-Frames are used to transport user data and control information relating to user data
S-Frames are used only to transport control information
U-Frame are reserved for System Management
Each frame in HDLC may contain up to six fields
-A beginning Flag Field
-An address field
-A control field
-An information Field
-A frame check sequence (FCS)
-An ending Flag Field


The  flag  field  of  an  HDLC  frame  is  an  8-bit  sequence  with  a  bit  patter  01111110  that
identifies both the beginning and the ending of the of a frame
It serves as a Synchronization pattern for the receiver
Fig. shows placement of 2 flag fields in an I-Frame

HDLC Address Field


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

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