1 Data Link Layer Lesson 6 NETS2150/2850. 2 Position of the data-link layer McGraw-Hill © The McGraw-Hill Companies, Inc., 2004.

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Data Link Layer

Lesson 6NETS2150/2850

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Position of the data-link layer

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LLC and MAC sublayers

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IEEE Standards for LANs

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The Data Link Layer The principles behind data link layer

services: error detection, correction reliable data transfer, flow control sharing a broadcast channel: multiple

access (see later) link layer addressing (see later)

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Lesson Outline Introduction Flow control

Stop and wait Sliding Window

Error Detection Parity Check CRC

Error Correction Stop and wait ARQ Sliding Window ARQ

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Link Layer: IntroductionSome terminology: End systems and routers are

nodes communication channels

that connect adjacent nodes along communication path are links wired links wireless links

L2-PDU is a frame, encapsulates a packet

“link”

data-link layer has responsibility of transferring frame from one node to adjacent node over a link Adapted from Kurose &

Ross book

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Link Layer Services

Framing, link access: encapsulate packet into frame, adding

header, trailer control channel access if shared medium ‘physical addresses’ used in frame headers

to identify source, dest • different from IP address!

Reliable delivery between adjacent nodes seldom used on low bit error link (fibre,

some twisted pair) crucial for wireless links: high error rates

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Link Layer Services (2) Flow Control:

pacing between adjacent sending and receiving nodes

Error Detection: errors caused by signal attenuation, noise receiver detects presence of errors

Error Correction: receiver identifies and corrects bit error(s)

Half-duplex and full-duplex transmission with half duplex, nodes at both ends of link can

transmit, but not at same time

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Adapters Communicating

link layer implemented in “adapter” (aka NIC) E.g. Ethernet card,

PCMCIA card, 802.11 card

sending side: encapsulates packet in a

frame adds error checking bits,

flow control, etc.

receiving side looks for errors, flow

control, etc extracts packet, passes

to rcving node adapter is semi-

autonomous Contains link & physical

layers

sendingnode

frame

rcvingnode

packet

frame

adapter adapter

link layer protocol

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Flow control refers to a set of Flow control refers to a set of procedures used to restrict the amount procedures used to restrict the amount of data that the sender can send before of data that the sender can send before

waiting for acknowledgmentwaiting for acknowledgment

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Flow Control Ensuring the sending entity does not

overwhelm the receiving entity Preventing buffer overflow

Two schemes: Stop and Wait Sliding Window

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Stop and Wait Source transmits frame Destination receives frame and

replies with acknowledgement Source waits for ACK before sending

next frame Destination can stop flow by not

send ACK Works well for a few large frames

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Normal operation

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Stop-and-Wait, lost ACK frame

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Numbering frames prevents the Numbering frames prevents the retaining of duplicate framesretaining of duplicate frames

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Stop and Wait Utilisation

Transmission time (tframe) Time taken to emit all bits into medium

Propagation time (tprop) Time for a bit to traverse the link

Link utilisation is the fraction of time a link is used for data

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Stop and Wait Utilisation (2)

Light in vacuum

= 300 m/s

Light in fibre

= 200 m/s

Electricity

= 200 m/s

Speed of Signals

RTT

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Utilization: Examples Satellite Link: Propagation Delay,

tprop = 270 msFrame Size = 4000 bitsData rate = 56 kbps tframe = 4/56 = 71 ms

= tprop/tframe = 270/71 = 3.8U = 1/(2 +1) = 0.12

Short Link: 1 km, tprop = 5 s,Rate=10 Mbps, Frame size= 500 bytes

tframe = 4k/10M= 400 s

=tprop/tframe=5/400=0.012 U=1/(2 +1)=0.98

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Fragmentation Large block of data may be split into small

frames. Why? Limited buffer size at rcvr Errors can be detected sooner On error, only retransmission of smaller

frames is needed Prevents one station occupying medium for

long periods However, stop and wait is inadequate to

cater for long-delay links!!

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Sliding Window Flow Control Allow multiple frames to be in transit Receiver has buffer W-frame long Transmitter can send up to W frames without

ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of

sequence # field (k-bit) Frames are numbered modulo 2k

Max window size 2k - 1

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Sender and Receiver windows

Last frame acknowledged

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Example Sliding Window

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Sliding Window Enhancements Receiver can acknowledge frames

without permitting further transmission (using Receive Not Ready or RNR frame) Must send a normal acknowledge to

resume (Receive Ready or RR frame) Use piggybacking

When need to ACK and send data, use same data frame to carry ACK number

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Sliding Window Utilisation

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Sliding Window Utilisation (2)

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Model of Frame Transmission

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Data can be corrupted during transmission. For reliable

communication, errors must be detected and corrected.

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Types of ErrorTypes of Error

Single-Bit Error

Burst Error

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In a single-bit error, only one bit in the data unit has changed.

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A burst error means that 2 or more bits in the data unit have changed.

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Error Detection MethodsError Detection Methods

Parity Check

Cyclic Redundancy Check (CRC)

Checksum (see later)

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Error detection uses the concept of redundancy, which means adding

extra bits for detecting errors at the destination.

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Detection Methods

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In even parity check, a parity bit is added to every data unit so that the

total number of 1s is even (or odd for odd-parity).

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Example 1Example 1

Suppose the sender wants to send the word world. In ASCII the five characters are coded as

1110111 1101111 1110010 1101100 1100100

The following shows the actual bits sent

11101110 11011110 11100100 11011000 11001001

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Example 2Example 2

Now suppose the word world in Example 1 is received by the receiver without being corrupted in transmission.

11101110 11011110 11100100 11011000 11001001

The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted.

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Example 3Example 3

Now suppose the word world in Example 1 is corrupted during transmission.

11111110 11011110 11101100 11011000 11001001

The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

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Simple parity check can detect all Simple parity check can detect all single-bit errors. It can detect burst single-bit errors. It can detect burst errors only if the total number of errors only if the total number of errors in each data unit is odd.errors in each data unit is odd.

Even number of bit errors goes Even number of bit errors goes undetected!undetected!

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In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the

whole block.

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Two-dimensional parity (even)

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Example 4Example 4

Suppose the following block is sent:

10101001 00111001 11011101 11100111 10101010

However, it is hit by a burst noise of length 8, and some bits are corrupted.

10100011 10001001 11011101 11100111 10101010

When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded.

10100011 10001001 11011101 11100111 10101010

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Cyclic Redundancy Check

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r bits

r+1 bits

r bits

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Cyclic Redundancy Check

goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (divisor)

(modulo 2 arithmetic) receiver knows G, divides <D,R> by G.

If non-zero remainder: error detected! can detect all burst errors less than r+1 bits

widely used in practice (HDLC, ATM)

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CRC Example

Assume:D = 101110 (d bits)G = 1001 (r+1 bits)

What is the CRC?

Transmitted block is:

101110011 (d+r bits)

011 (r bits)

Stallings Section 6.3

CRC detect all burst errors r bits

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Error Control Deliver frames without error, in the

proper order to network layer Handles Lost frames

And damaged frames An automatic repeat request scheme:

Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and

retransmission

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Automatic Repeat Request (ARQ) Stop and wait ARQ Sliding window schemes

Go back N ARQ Selective reject (selective repeat) ARQ

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Stop and Wait ARQ

Source transmits single frame And waits for ACK

If received frame damaged, discard it Transmitter has timer If no ACK within timeout, retransmit

If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1 to distinguish

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Stop and Wait -Diagram

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Go Back N ARQ Based on sliding window If no error, ACK as usual with next

expected frame If error, reply with a rejection frame

Rcvr discards that frame and all future frames until errored frame received correctly

Transmitter must go back and retransmit that frame and all subsequent frames

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Go Back N - Diagram

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Selective Reject ARQ Only rejected frames are

retransmitted Subsequent frames are accepted by

the receiver and buffered Minimizes retransmission

Receiver must maintain large enough buffer

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Selective Reject -Diagram

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High Level Data Link Control (HDLC) The most important and widely

used DL protocol Also the basis for many other DL

protocols

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HDLC Frame Structure3 types of frame has same frame format: Information frame (I-frame) – carries

network layer data Flow and error control piggybacked on

information frames Supervisory frame (S-Frame) - ARQ when

piggyback not used (i.e. RR, RNR, REJ) Unnumbered frame (U-frame) - link control

frames

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Flag Fields Delimit frame at both ends

01111110 Receiver hunts for flag sequence to

synchronise (i.e. frame synchronisation) Bit stuffing used to avoid confusion with

data containing 01111110 0 inserted after every sequence of five 1s If receiver detects five 1s, it checks next bit

• If 0, it is deleted• If 1 and seventh bit is 0, accepts as flag• If sixth and seventh bits 1, sender is indicating abort

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Bit Stuffing

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Control Field Different for different frame type First one or two bits of control field

identify frame type

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Control Field

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Information Field

Only in I- and some U-frames Must contain integral number of

octets Variable length

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Frame Check Sequence (FCS) Field Error detection code (i.e. redundant

bits) 16-bit CRC

Optional 32-bit CRC

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HDLC Operation Three phases

Initialization (Connection setup)

Data transfer Disconnect

(Connection Termination)

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Examples of Operation (1)U-frames

I-frames S-frames

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Examples of Operation (2)

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Summary Flow control and error

control through the same scheme

HDLC a basis for many other data link layer protocols

Read Stallings sec. 6.2, 6.3, 6.5 and chap 7

Next: LAN Overview