Nete0510 data link layer and protocols
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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

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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


Nete0510 data link layer and protocols

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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