Data Link Protocols
• Asynchronous Protocols
• Synchronous Protocols
– Character-Oriented Protocols
– Bit-Oriented Protocols
Data Link Protocol Categories
A protocol in data communications is the set of rules
or specifications used to implement one or more layers
of the OSI model.
Data link protocol is a set of specifications
used to implement the data link layer
Data Link Protocol Categories
Asynchronous protocols
-
treat each character in a bit stream independently
use start and stop bits to frame the data units
inexpensive
used primarily in modems
Synchronous protocols
-
take the whole bit stream and chop it into characters
of equal size
- faster than asynchronous transmission
Asynchronous Protocols
XMODEM Frame
- half-duplex stop-and-wait ARQ protocol
SOH –Start of Header (1 byte)
CRC - Cyclic Redundancy Check
Signal from receiver side
- ACK
- NAK
- CAN (cancel)
• YMODEM
– similar to XMODEM
– Major differences
• Data unit = 1024 bytes
• CANs = 2
• Multiple files can be sent simultaneously
• ZMODEM
– Combining features of both XMODEM and YMODEM
• BLAST (Blocked asynchronous transmission)
– More powerful than XMODEM.
– Full-duplex
• KERMIT
– Most widely used asynchronous protocol
Synchronous Protocols
Character-oriented protocols
(or byte-oriented protocols)
- the frame or packet is interpreted as a series of characters
Bit-oriented protocols
- the frame or packet is interpreted as a series of bits
Character-Oriented
Protocols
Binary Synchronous
Communication (BSC)
- the best-known character-oriented protocol
Supports half-duplex transmission
Uses stop-and-wait ARQ flow control and error correction
Data Frame
Simple Data Frame
SYN – 0010110
STX – 0000010
ETX – 0000011
BSC Frame with Header
SOH – 0000001
ITB – 0011111
Multiblock Frame
Multiframe Transmission
ETB – 0010111
BSC Control Frames
BSC Control Frames cont…
Bit-Oriented Protocols
IBM
ISO
ITU-T
SDLC – Synchronous Data Link Control
HDLC – High-Level Data Link Control
LAPs – Link Access Protocols
LAN – Local Area Nelwork
IEEE
HDLC
 Supports half/full duplex communications over point to
point or multipoint links
 Characterized by: station types, configuration, response
modes
 Station Types:
 Primary – issues commands
 Secondary – issues responses
 Combined – issues both command & response
HDLC Configuration
HDLC does not support balanced multipoint
Figure 11-15
HDLC Modes
Who controls the link
NRM – Normal Response Mode ( unbalanced configuration)
ARM – Asynchronous Response Mode
ABM – Asynchronous Balanced Mode
* Mode for symmetrical & balanced can be set using U-frames
HDLC Frame Types
Information frame
Supervisory frame
Unnumbered frame
HDLC Flag F& FCS field
HDLC Address Field
HDLC Control Field
Code (S-frame) – carry coded flow and error control information
Code (U-frame) – identify the type of U-frame and its function
Use of P/F Field
U-Frame Control Field
U-Frame Control Field
Set NRM
Set NRM (extended)
Set ARM
Set ARM (extended)
Unnumbered info
Disconnect
Request disconnect
Set initialization mode
Unnumbered poll
Frame reject
Exchange ID
Reset
Request information
mode