Bit-Oriented Protocols
- In character-oriented protocols, bits are grouped into predefined patterns forming characters
- By comparison, bit-oriented protocols can pack more information into shorter frames
A lot of bit-oriented protocols have been developed over the years:
- One of these HDLC is the design of the ISO and has become the basis for all bitoriented protocols in use today
- In 1975, IBM gave Synchronous Data Link Control (SDLC)
- In 1979, ISO answered with High Level Data Link Control (HDLC)
- Since 1981, ITU-T has developed a series of protocols called Link Access Protocols
- LAPs: LAPB, LAPD, LAPM, LAPZ etc. all based on HDLC
High Level Data Link Control (HDLC)
- 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
- 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)
Secondary Station
- The primary sends commands to the secondary stations
- A primary issues commands and a secondary issues responses
STATION TYPES (4)
Combined Station
- A combined station can both command and respond
- A combined station is one of a set of connected peer devices programmed to behave either as a prim
ary or as a secondary depending on the nature and the direction of the transmission
Configuration (1)
- Configuration refers to the relationship of the hardware devices on a link
- Primary , secondary and combined stations can be configured in three ways:
- Unbalanced Configuration
- Symmetrical Configuration
- Balanced Configuration
Configuration (2)
Unbalanced Configuration
- Also called Master/Slave Configuration
- One device is a primary and others are secondary
- Unbalanced configuration can be point to point if only two devices are involved
- Most of the times it is multipoint with one primary controlling several secondaries
Configuration (3)
Symmetrical Configuration
- Each physical station on a link consists of two logical stations, one a primary and the other a secondary
- Separate lines link the primary aspect of one physical station to the secondary aspect of another physical station
Configuration (4)
Balanced Configuration
- Both stations in a point-to-point topology are of combined type
- HDLC does not support balanced multipoint
Station Types & Configurations
Modes
- A mode in HDLC is the relationship b/w two devices involved in an exchange
- The mode describes who controls the link
- 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)
- Refers to the standard primary-secondary relationship
- Secondary device must have permission from primary device before transmitting
- Once permission has been granted, the secondary may initiate a response transmission of one or more frames containing data
Asynchronous Response Mode (ARM)
- A secondary may initiate a TX w/o permission from the primary whenever the channel is idle
- ARM does not alter the primary secondary relationship in any other way
- All transmissions from the primary still go to the secondary and are then relayed to the other devices
Asynchronous Balanced Mode (ABM)
- All stations are equal and therefore only combined stations connected in point-topoint are used
- 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)
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
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
- The second field of HDLC frame contains the address of the secondary station that is either the originator or the destination of the frame
- If a primary station creates Frame it includes a ‘To’ address and if a secondary creates the frame, it contains a ‘From’ address
- Can be of one byte or several bytes depending upon the network
- If the address field is only 1 byte, the last bit is always a 1
- 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
- Ending each intermediate byte with 0 indicates to the receiver that there are more address bytes to come
HDLC Control Field
- The control field is a one o two byte segment of the frame used for flow management
- The two byte case is called the Extended Mode
- 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
- P/F bit is a single bit with dual purpose
- It has meaning only when it is ‘1’ and it can mean Poll or Final
- When the frame is sent by a primary to secondary, it means POLL
- When the frame is sent by a secondary to a primary, it is FINAL
HDLC Control Field –EXTENDED
- Control field in the I-Frame and S-Frame is two bytes long to allow seven bits of sending and receiving sequence
- However the control field in the U-Frame is still one byte
- Information field contains the user’s data in an I-Frame and Network Management information in a U-Frame
- An S-Frame has no information field
- Its length can vary from one network to another but remains fixed within each network
- It is possible to send Control information in the information field of the I-Frame along with data.
- This process is called Piggybacking
- The FCS is HDLC’s error detection field
- It can contain a two- or four byte CRC
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, 11.5“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan