Transcript Chapter 10 Data Link Control Protocols

Chapter 10
Data Link Control Protocols
Agenda
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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
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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:
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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
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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
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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
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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:
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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
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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
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Definition
Functions
Attributes
Concepts
Types
Conversion
Discussion
• Select proper protocol for the network of a
bank state you reasons