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LECTURE
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 HD LC be characterized by: Types
odes
HDLC can
- Station
- Configurations
- Response M
STATION TYPES
HDLC differentiates b/w 3 types of stations:
– Primary Station
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- 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 The primary is a device in point-to-point or multipoint line configuration that has complete control of the link
STATION TYPES (3)
o
Secondary Station o The primary sends commands to the secondary stations o A primary issues commands and a secondary issues responses
S TATION 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)
igured in three ways:
Unbalanced Configuration
Balanced Configuration
o Configuration refers to the relationship of the hardware devices on a link o Primary , secondary and combined stations can be conf
Symmetrical Configuration
Configuration (2) Unbalanced Configuration o Also called Master/Slave Configuratio o One device is a primary and others are
n secondary guration can be point to point if only two devices are
it is multipoint with one primary controlling several sec Config Sym tion
o Unbalanced confi involved o Most of the times ondaries uration (3) metrical Configura o Each physical station on a link consists of two logical stations, one a primary and the other a secondary o Separate lineslink the primary aspect of one physical station to the secondary aspect of another physical station
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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
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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
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
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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 o It can contain a two- or four byte CRC
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Link Access Procedures LAPB
- Link access procedure, balanced
LAPD
- Link access procedure for D- channel
LAPM
- Link access procedure for modems
Summary
Synchronous Protocols
- Bit-Oriented Protocols
- HDLC
Link Access Protocols (LAPs)
Reading Sections
Section11.4, 11. “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
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