Chapter 10

Data Link Control Protocols

Agenda

• Definition

• Functions

• Attributes

• Concepts

• Types

• Conversion

Definition

• Protocols: set of rules or guidelines for communication between DTEs

• Data link protocol: rules for operating the circuit and sending message over it

Functions

• Communication startup

• Character identification and framing

• Message identification

• Line control

• Error control

• Termination

Data Link Protocols Attributes

• Transparency (DTE – any bit pattern as data)

• Code independence

• Efficiency (control codes)

Protocol Concepts

• Line Access– Contention– Polling – Token passing

• Message format

Contention• Carrier sense multiple access with collision

detection (CSMA/CD) by XEROX

• For:– Point-to-point circuits– Traffic light for multipoint circuits– Fast speed of circuit– Office application & LAN

• Not for– Manufacturing due to inconsistent response

time

Polling

• Characteristics– One master and slavers– Special polling character

• Types– Roll call polling (polling list)– Fast select polling (several terminals)– Hub polling (station to station)– Token passing (token)

• Token (particular sequence of bits, Free or busy)

Message Format

• Start of header (SOH)– Header (Address, sequence, date and time)– Start of text (STX)

• Text

• End of text (ETX) or text length

• Trailer– Block check character (BCC)– Synchronization character (SYN)

Asynchronous Data Link Control Protocols - Characteristics

• Start/stop bits

• Parity checking

• Character oriented

• Less expensive and less complicated equipment

• For personal computers

Asynchronous Data Link Control Protocols - I

• XMODEM (simple, less reliable error checking)

• XMODEM-CRC (more reliable)

• XMODEM-1K (more efficient)

• YMODEM (reliable, multiple files transfer)

• YMODEM-G (fast)

• ZMODEM (fast, good failure recovery)

• X.PC (pack switching network, multiple sessions on one circuit)

Asynchronous Data Link Control Protocols - II

• KERMIT (reliable, fast file transfer, PC & mainframe)

• Serial Line Internet Protocol (SLIP)– Full-duplex– IP over asynchronous dial-up or leased lines– No error correction

• Point-to-point Protocol (PPP)– PC to a TCP/IP network– Full-duplex for synchronous and asynchronous

transmission– Authentication, compression, error correction, & packet

sequencing

Synchronous Data Link Protocols - Classification

• Character-oriented protocols– Special character for start and end of message– Binary Synchronous Communication Protocol (BSC or

BISYNC)

• Byte-count-oriented protocols– Special character for start of the header, count field,

message, block check character (BCC)– DEC’s Digital Data Communication Message Protocol

(DDCMP)

• Bit-oriented protocols– Use flag character for start and end of message– IBM’s Synchronous Data Link Control (SDLC)– ISO’s High-Level Data Link control (HDLC)

Binary Synchronous Communications

• By IBM• For 6-bit transcode (SBT), ASCII, EBCDIC• SYN at start and middle of transmission• Point to point and multipoint (polling)• ARQ approach for error checking (ACK1, ACK0, NAK)• Pros:

– Transparency and non-transparency modes– Efficient, understandable, and widely used– Point-to-point & multipoint operations

• Cons:– Code dependent– Half-duplex protocol– Cumbersome for transparency mode

Digital Data Communications Message Protocol (DDCMP)

• Usage– Digital Network Architecture (DNA)– High-bandwidth and high-latency (satellite) links

• Data message and control message• Error checking

– Use block check character– Unique and increasing sequence number for sending message– ACK indicates the last message received

• Pro:– Efficient at transparency mode– Variable length of data– Only one control character: SOH– Easy implement for full-duplex and message sequence

High-level Data Link Control (HDLC) - I

• Operating modes– Normal response mode (NRM) for a primary node and

one or more secondary modes on a circuit (polling)– Asynchronous balanced mode (ABM) for nodes with

peers (most used for efficiency with no polling in full duplex)

– Asynchronous response mode (ARM) for primary and secondary nodes with same transmission right (rarely used)

• Frames– Supervisory or S format: control acknowledge– Unnumbered or U format: (operation mode, start, termination)– Information or I format: (data)

High-level Data Link Control (HDLC) - I

• Flag– 01111110– Bit stuffing 0 after five 1s by hardware– For synchronization

• Other fields– Address field– Control field: type of frame, sequence number for

information frames– Information field: multiple of 8 bits– Frame check sequence (FCS) field: error checking

• Frame flow– Initialization phase, data transfer phase, and disconnect

phase

Other Data Link Control Protocols

• Link access procedure, balanced (LAPB)– Full-duplex, point-to-point mode– For X.25 DTE and packet switching network

• Link access procedure, D-channel (LAPD)– ISDN network

• Link access procedure for frame –mode bearer service (LAPF)– High-speed packet switching network on low error rate

digital circuits

• Asynchronous transfer mode– Data transfer on high s-speed, digital, error free

network

Protocol Conversion

• Methods– Dedicated hardware protocol converters– Add-in circuit board protocol converters– Software protocol converters – Gateway

• Functions– Code translation– Control characters, format, and error checking

Points to Remember

• Definition

• Functions

• Attributes

• Concepts

• Types

• Conversion

Discussion

• Select proper protocol for the network of a bank state you reasons