chapter 10 data link control protocols. agenda definition functions attributes concepts types...
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TRANSCRIPT
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