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

Simple Data Frame Data Frame BSC Frame with Header Multiblock Frame

SYN – 0010110 STX – 0000010 ETX – 0000011 SOH – 0000001 ITB – 0011111

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 I

nformation frame

S

upervisory frame

U

nnumbered 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