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NETE0510 Data-link Layer and Protocols. Dr. Supakorn Kungpisdan [email protected] Outline. Encoding Techniques Data Link Layer Fundamental Direct-link Protocols BSC, DDCMP, HDLC, PPP. Signal Encoding Techniques. Encoding Schemes. Bit rate and Baud rate.

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nete0510 data link layer and protocols

NETE0510Data-link Layer and Protocols

Dr. Supakorn Kungpisdan

[email protected]

NETE0510: Communication Media and Data Communications

outline
Outline
  • Encoding Techniques
  • Data Link Layer Fundamental
  • Direct-link Protocols
    • BSC, DDCMP, HDLC, PPP

NETE0510: Communication Media and Data Communications

signal encoding techniques
Signal Encoding Techniques

NETE0510: Communication Media and Data Communications

encoding schemes
Encoding Schemes

NETE0510: Communication Media and Data Communications

bit rate and baud rate
Bit rate and Baud rate
  • Bit rate or data rate: number of bits transmitted over a period of time (bit per second)
  • Baud rate: number of symbols transmitted over a period of time (baud)

NETE0510: Communication Media and Data Communications

non return to zero level nrz l
Non-return to Zero-Level (NRZ-L)
  • two different voltages for 0 and 1 bits
  • voltage constant during bit interval
    • no transition I.e. no return to zero voltage
    • such as absence of voltage for zero (0 V), constant positive voltage for one (+5V)
    • more often, negative voltage (-5 V) for one value and positive (+5 V) for the other

NETE0510: Communication Media and Data Communications

problems of nrz l
Problems of NRZ-L
  • Long sequence of 0s or 1s signal leads to “baseline wander”
    • Receiver maintains baseline of detecting 0 or 1
    • Too many 0s or 1s cause this baseline to change
  • Clock synchronization
    • Frequent transitions of signals are necessary to enable clock recovery
    • Every clock cycle the sender transmits a bit and the receiver recovers a bit
    • Slightly difference in clocks at either sender and receivers may cause a problem

NETE0510: Communication Media and Data Communications

non return to zero inverted nrzi
Non-return to Zero Inverted (NRZI)
  • nonreturn to zero inverted on ones
  • constant voltage pulse for duration of bit
  • data encoded as presence or absence of signal transition at beginning of bit time
    • transition (low to high or high to low) denotes binary 1
    • no transition denotes binary 0
  • example of differential encoding since have
    • data represented by changes rather than levels
    • More example of different encoding: Different Manchester
    • more reliable detection of transition rather than level
    • easy to lose sense of polarity

NETE0510: Communication Media and Data Communications

manchester encoding
Manchester Encoding
  • has transition in middle of each bit period
  • transition serves as clock and data  no need to send clock
  • low to high represents one (1), high to low represents zero (0)
  • used by IEEE 802.
  • Both 0s and 1s result in a transition to the signal, the clock can be effectively recovered at the receiver

NETE0510: Communication Media and Data Communications

differential manchester encoding
Differential Manchester Encoding
  • midbit transition is clocking only
  • transition at start of bit period representing 0
  • no transition at start of bit period representing 1
    • this is a differential encoding scheme
  • used by IEEE 802.5 (Token Ring)

NETE0510: Communication Media and Data Communications

problems of manchester encoding
Problems of Manchester Encoding
  • Double the rate at which signal transition are made on the link  the receiver has half the time to detect each pulse of the signal.
    • Bit rate is half of baud rate  encoding is only 50% efficient

NETE0510: Communication Media and Data Communications

4b 5b
4B/5B
  • Used together with other encoding techniques
  • Used in 100Base-TX transmission
  • The idea is to insert extra bits into the bit stream so as to break up long sequences of 0s and 1s
  • Every 4 bits of actual data are encoded in a 5-bit code  4B/5B
  • Each 5-bit code has no more than one leading 0s and no more than two trailing 0s.
  • No pair of 5-bit codes results in more than three consecutive 0s  solve problem of many consecutive 0s
  • Then the 5-bit codes are then transmitted using the NRZI encoding
    • NRZI already solved the problem of consecutive 1s so 4B/5B results in 80% efficiency

NETE0510: Communication Media and Data Communications

4b 5b encoding
4B/5B Encoding
  • 4-bit data is encoded into 5-bit code
  • “One leading 0, two trailing 0s” rule
  • 16 codes used, left the other 16 codes for other purposes
  • Code 00000  line is dead
  • Code 1111 line is idle
  • Code 00100  halt
  • 7 of them violates the rule
  • The other 6 represent control symbols

NETE0510: Communication Media and Data Communications

outline1
Outline
  • Encoding Techniques
  • Data Link Layer Fundamental
  • Direct-link Protocols
    • BSC, DDCMP, HDLC, PPP

NETE0510: Communication Media and Data Communications

data link layer
Data Link Layer
  • Because of transmission errors, or because the receiver may need to regulate data rate, it is necessary to have a layer of control communication devices
  • Need layer of logic above Physical to manage exchange of data over a link
    • Frame synchronization: beginning and end of each frame must be recognizable
    • Flow control: sender must not send frames at a rate faster that the receiver can absorb them
    • Error control: correct bit errors
    • Addressing: identify involving communication parties
    • Control and data: receiver can distinguish control information from transmitted data
    • Link management: initiation, maintenance, and termination

NETE0510: Communication Media and Data Communications

flow control
Flow Control
  • ensure sending entity does not overwhelm receiving entity
    • by preventing buffer overflow
  • influenced by:
    • transmission time
      • time taken to emit all bits into medium
    • propagation time
      • time for a bit to traverse the link

NETE0510: Communication Media and Data Communications

stop and wait
Stop and Wait
  • source transmits frame
  • destination receives frame and replies with acknowledgement (ACK)
  • source waits for ACK before sending next
  • destination can stop flow by not send ACK
  • works well for a few large frames
  • Stop and wait becomes inadequate if large block of data is split into small frames

NETE0510: Communication Media and Data Communications

sliding windows flow control
Sliding Windows Flow Control
  • allows multiple numbered frames to be in transit
  • receiver has buffer W long
  • transmitter sends up to W frames without ACK
  • ACK includes number of next frame expected
  • sequence number is bounded by size of field (k)
    • frames are numbered modulo 2k
    • giving max window size of up to 2k - 1
  • must send a normal acknowledge to resume
  • Piggybacking: insert acknowledgement number field in data frame so that a frame can contain both sequence number and ACK numbers

NETE0510: Communication Media and Data Communications

sliding window diagram
Sliding Window Diagram

NETE0510: Communication Media and Data Communications

error control
Error Control
  • detection and correction of errors such as:
    • lost frames
    • damaged frames
  • common techniques use:
    • error detection
    • positive acknowledgment (ACK)
    • retransmission after timeout
    • negative acknowledgement (NAK) & retransmission

NETE0510: Communication Media and Data Communications

automatic repeat request arq
Automatic Repeat Request (ARQ)
  • collective name for such error control mechanisms, including:
  • stop and wait
  • go back N
  • selective reject (selective retransmission)

NETE0510: Communication Media and Data Communications

stop and wait1
Stop and Wait
  • source transmits single frame
  • wait for ACK
  • if received frame damaged, discard it
    • transmitter has timeout
    • if no ACK within timeout, retransmit
  • if ACK damaged,transmitter will not recognize it
    • transmitter will retransmit
    • receive gets two copies of frame
    • use alternate numbering and ACK0 / ACK1

NETE0510: Communication Media and Data Communications

stop and wait2
Stop and Wait
  • see example with both types of errors
  • pros and cons
    • simple
    • inefficient

NETE0510: Communication Media and Data Communications

go back n
Go Back N
  • based on sliding window
  • if no error, ACK as usual
  • use window to control number of outstanding frames
  • if error, reply with rejection
    • discard that frame and all future frames until error frame received correctly
    • transmitter must go back and retransmit that frame and all subsequent frames

NETE0510: Communication Media and Data Communications

go back n handling
Go Back N - Handling
  • Damaged Frame
    • error in frame i so receiver rejects frame i
    • transmitter retransmits frames from i
  • Lost Frame
    • frame i lost and either
      • transmitter sends i+1 andreceiver gets frame i+1 out of seq and rejects frame i
      • or transmitter times out and send ACK with P bit set which receiver responds to with ACK i
    • transmitter then retransmits frames from i

NETE0510: Communication Media and Data Communications

go back n handling1
Go Back N - Handling
  • Damaged Acknowledgement
    • receiver gets frame i, sends ack (i+1) which is lost
    • acks are cumulative, so next ack (i+n) may arrive before transmitter times out on frame i
    • if transmitter times out, it sends ack with P bit set
    • can be repeated a number of times before a reset procedure is initiated
  • Damaged Rejection
    • reject for damaged frame is lost
    • handled as for lost frame when transmitter times out

NETE0510: Communication Media and Data Communications

selective reject
Selective Reject
  • also called selective retransmission
  • only rejected frames are retransmitted
  • subsequent frames are accepted by the receiver and buffered
  • minimizes retransmission
  • receiver must maintain large enough buffer
  • more complex logic in transmitter
  • hence less widely used
  • useful for satellite links with long propagation delays

NETE0510: Communication Media and Data Communications

go back n vs selective reject
Go Back N vsSelective Reject

NETE0510: Communication Media and Data Communications

error detection
Error Detection
  • Two-dimensional parity
  • Internet checksum
  • Cyclic Redundancy Check

NETE0510: Communication Media and Data Communications

internet checksum
Internet Checksum
  • Not used in the link layer
  • checksum for the internet protocol:
    • Sum 16-bit data blocks
    • Take one complement of the result to produce the checksum
  • E.g. calculate checksum of 5 and 3
    • 5 (0101) + 3 (0011) = 8 (1000)
    • Checksum = -8 (0111)
    • Send 5, 3, -8 to receiver  send 010100110111
    • Calculate 5+3+(-8)

0101

0011

0111

1111

NETE0510: Communication Media and Data Communications

outline2
Outline
  • Encoding Techniques
  • Data Link Layer Fundamental
  • Direct-link Protocols
    • BSC, DDCMP, HDLC, PPP

NETE0510: Communication Media and Data Communications

bsc or bisync protocol
BSC (or BISYNC) Protocol
  • Binary Synchronous Communication
  • Byte-oriented approach: view each frame as a collection of bytes (characters) rather than a collection of bits
  • Support a particular character set: ASCII, EBCDIC, and IBM’s 6-bit Transcode

NETE0510: Communication Media and Data Communications

problem of bisync
Problem of BISYNC
  • ETX character may appear in the body of a frame
  • Solved by “character stuffing”  insert a special character called DLE (data-link-escape) character whenever it appears in the body

NETE0510: Communication Media and Data Communications

point to point ppp protocol
Point-to-point (PPP) Protocol
  • Designed to encapsulate IP inter-network data
  • Commonly run over dial-up modem links, but can be used on any leased line for point-to-point connections not supported by FR or ATM.
  • Have character stuffing
  • Several of the field sizes are negotiated rather than fixed  using LCP (Link Control Protocol)
  • Two sub protocols: LCP (Link Control Protocol) and NCP (Network Control Protocol) to negotiate options for a network-layer protocol running on top of PPP
    • IPCP (Internet Protocol Control Protocol) for IP
    • IPXCP for IPX, ATCP for AppleTalk

NETE0510: Communication Media and Data Communications

ppp frame format
PPP Frame Format
  • Flag: 011111110
  • Protocol: identify high-level protocol e.g. IP or IPX
  • Payload: size is negotiated, default at 150 bytes
  • Checksum: 2-4 bytes long

NETE0510: Communication Media and Data Communications

slide36
LCP
  • Negotiation of payload size is conducted by LCP
  • Frame payload size can be negotiated, 1500 byte by default
  • LCP sends control messages encapsulated in PPP frames
    • Such messages are denoted by an LCP identifier in the PPP Protocol field
    • Sizes are changed based on the information contained in those control messages

NETE0510: Communication Media and Data Communications

ppp cont d
PPP (cont’d)
  • PPP tends to be reserved for dialup or a mixed-vendor environment, whereas HDLC is the default for T1 serial connections

NETE0510: Communication Media and Data Communications

decnet s ddcmp
DECNET’s DDCMP
  • Byte-Counting Approach: include number of bytes contained in a frame as a field in the frame header
  • Transmission error can corrupt the COUNT field  framing error

NETE0510: Communication Media and Data Communications

high level data link control hdlc
High-level Data Link Control (HDLC)
  • Previously known as Synchronous Data Link Control (SDLC) protocol
  • Most popular data link control (L2) protocol
  • Form the basis of ISDN and FR protocols and services
  • specified as ISO 33009, ISO 4335
  • station types:
    • Primary - controls operation of link
      • Frames issued are called “commands”
    • Secondary - under control of primary station
      • Frames issued are called “responses”
    • Combined - issues commands and responses
  • link configurations
    • Unbalanced - 1 primary, multiple secondary
    • Balanced - 2 combined stations

NETE0510: Communication Media and Data Communications

hdlc transfer modes
HDLC Transfer Modes
  • Normal Response Mode (NRM)
    • unbalanced config, primary initiates transfer, secondary may only transmit data in response to a command
    • used on multi-drop lines, e.g. host + terminals
  • Asynchronous Balanced Mode (ABM)
    • balanced config, either station initiates transmission, has no polling overhead, widely used
  • Asynchronous Response Mode (ARM)
    • unbalanced config, secondary may initiate transmit without permission from primary, rarely used

NETE0510: Communication Media and Data Communications

hdlc frame structure
HDLC Frame Structure
  • synchronous transmission of frames (no start-stop bits required)
  • single frame format used

trailer

header

NETE0510: Communication Media and Data Communications

flag fields and bit stuffing
Flag Fields and Bit Stuffing
  • delimit frame at both ends with 01111110 sequence
  • The sequence is transmitted any time that the link is idle to keep clock synchronized
  • “bit stuffing” used to avoid confusion with data containing flag sequence 01111110
    • 0 inserted after every sequence of five 1s
    • if receiver detects five 1s it checks next bit
      • if next bit is 0, it is deleted (was stuffed bit)
      • if next bit is 1 and seventh bit is 0, accept as the end-of-frame flag
    • if sixth and seventh bits 1, sender is indicating abort (error occurred)

NETE0510: Communication Media and Data Communications

flag fields and bit stuffing cont d
Flag Fields and Bit Stuffing (cont’d)
  • In both bit stuff and character stuffing, the size of a frame is dependent on the data being sent in the payload.
  • Not possible to make all the frames exactly the same size

NETE0510: Communication Media and Data Communications

address field
Address Field
  • identifies secondary station that sent or will receive frame
  • usually 8 bits long
  • may be extended to multiples of 7 bits
    • LSB indicates if is the last octet (1) or not (0)
  • not needed for point-to-point link
  • all ones address 11111111 is broadcast
    • Allow primary station to broadcast a frame for reception by all secondaries

NETE0510: Communication Media and Data Communications

control field
Control Field
  • different for different frame type
    • Information (I-) frame - data transmitted to user (next layer up)
      • Flow and error control piggybacked on information frames
    • Supervisory (S-) frame - ARQ when piggyback not used
    • Unnumbered (U-) frame - supplementary link control
  • first 1-2 bits of control field identify frame type

NETE0510: Communication Media and Data Communications

control field cont d
Control Field (cont’d)
  • use of Poll/Final bit depends on context
  • in command frame is P bit set to1 to solicit (poll) response from peer
  • in response frame is F bit set to 1 to indicate response to soliciting command
  • Sequence number in S- and I-frames usually 3 bits
    • can extend to 8 bits as shown below

NETE0510: Communication Media and Data Communications

information fcs fields
Information & FCS Fields
  • Information Field
    • in information and some unnumbered frames
    • must contain integral number of octets
    • variable length
  • Frame Check Sequence Field (FCS)
    • used for error detection
    • either 16 bit CRC or 32 bit CRC

NETE0510: Communication Media and Data Communications

hdlc operation
HDLC Operation
  • consists of exchange of information, supervisory and unnumbered frames
  • have three phases
    • initialization
      • by either side, set mode & seq
    • data transfer
      • with flow and error control
      • using both I & S-frames (RR, RNR, REJ, SREJ)
    • disconnect
      • when ready or fault noted

Selective reject

Go Back N reject

NETE0510: Communication Media and Data Communications

hdlc commands and responses
HDLC Commands and Responses

NETE0510: Communication Media and Data Communications

hdlc operation example
HDLC Operation Example

Set sync balanced mode

Send seq no

Receive seq no

busy

A must respond with RR or RNR

Unnumbered acknowledgement

Return from busy condition

Disconnect

NETE0510: Communication Media and Data Communications

hdlc operation example1
HDLC Operation Example

NETE0510: Communication Media and Data Communications

questions

Questions?

Next Lecture

LANs and Hi-speed LANs

NETE0510: Communication Media and Data Communications

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