data link protocols asynchronous protocols synchronous protocols –character-oriented protocols...

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.

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 layerData link protocol is a set of specifications used to implement the data link layer

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

Data Link Protocol Categories

Asynchronous Protocols

XMODEM Frame

- half-duplex stop-and-wait ARQ protocol- half-duplex stop-and-wait ARQ protocol

SOH –Start of Header (1 byte)CRC - Cyclic Redundancy CheckSOH –Start of Header (1 byte)CRC - Cyclic Redundancy Check

Signal from receiver side - ACK - NAK - CAN (cancel)

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

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

Bit-oriented protocols- the frame or packet is interpreted as a series of bits

Character-Oriented Protocols

Binary Synchronous Communication (BSC)

Supports half-duplex transmissionUses stop-and-wait ARQ flow control and error correction Supports half-duplex transmissionUses stop-and-wait ARQ flow control and error correction

- the best-known character-oriented protocol- the best-known character-oriented protocol

Data Frame

BSC Frame with Header

Multiblock Frame

Simple Data Frame

SYN – 0010110STX – 0000010ETX – 0000011

SOH – 0000001ITB – 0011111

Multiframe Transmission

ETB – 0010111ETB – 0010111

BSC Control Frames

BSC Control Frames cont…

SDLC – Synchronous Data Link ControlHDLC – High-Level Data Link ControlLAPs – Link Access ProtocolsLAN – Local Area Nelwork

IBM ISO ITU-T IEEE

Bit-Oriented Protocols

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 multipointHDLC does not support balanced multipoint

Figure 11-15

HDLC ModesWho controls the link

NRM – Normal Response Mode ( unbalanced configuration)ARM – Asynchronous Response ModeABM – Asynchronous Balanced Mode

* Mode for symmetrical & balanced can be set using U-frames

HDLC Frame Types

Unnumbered frameUnnumbered frame

Information frameInformation frame

Supervisory frameSupervisory frame

HDLC Flag F& FCS field

HDLC Address Field

HDLC Control Field

Code (S-frame) – carry coded flow and error control informationCode (U-frame) – identify the type of U-frame and its functionCode (S-frame) – carry coded flow and error control informationCode (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 NRMSet NRM (extended)Set ARMSet ARM (extended)

Set NRMSet NRM (extended)Set ARMSet ARM (extended)

Unnumbered infoDisconnectRequest disconnectSet initialization modeUnnumbered poll

Unnumbered infoDisconnectRequest disconnectSet initialization modeUnnumbered poll

Frame rejectExchange IDResetRequest information mode

Frame rejectExchange IDResetRequest information mode