Chapter3 Data Link Layer

7/27/2019 Chapter3 Data Link Layer

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The Data Link Layer

Chapter 3


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Data Link Layer Design Issues

Main topics: the algorithms for achieving reliable, efficient

communication between two adjacentmachines at the datalink layer.

By adjacent, we mean the two machines are physically

connected by a communication channel (e.g., a coaxial cable

or a phone line) that acts conceptually like a wire.

The essential property of a channel that makes it ``wire-like''

is that the bits are delivered in exactly the same order in

which they are sent.

Why need data link layer software ?

1. Communication circuits make errors occasionally,

2. they have a finite data rate,

3. there is a nonzero propagation delay, and

4. the adjacent machines have finite and maybe different processing speeds.


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Functions of the Data Link Layer

Provide service interface to the network layer

Dealing with transmission errors

Regulating data flow: slow receivers notswamped by fast senders


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Relationship between packets and frames


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Services Provided to Network Layer

Virtual communication. Actual communication.

The principal service: transferring data from the network layer on the source

machine to the network layer on the destination machine.

Three possible services:

1. Unacknowledged connectionless service.

2. Acknowledged connectionless service.

3. Connection-oriented service.


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Placement of the data link protocol

An example: a subnet consisting of routers connected by point-to-point lines.1. When a frame arrives at a router, the hardware verifies the checksum and passes the frame to

the data link layer software.

2. The data link layer software, which may be embedded in a chip on the network adaptorboard, checks to see if this is the frame expected. and if so, gives the packet in the payload

field to the (network layer) routing software.3. The routing software chooses the appropriate outgoing line and passes the packet back down

to the data link layer software.

4. The data link layer software transmits it over the selected outgoing line.

Many of the principles at the data link layer, such as error control and flow control,are also found in transport and other layers and protocols as well.


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Framing

What is framing ?The process of breaking the bit stream offered by the

physical layer into discrete segments (frames).

Why framing ?

a) For computing the checksum for each frame.

b) For acknowledging the lost frames.

How to do framing ?


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A character stream

(a) Without errors. (b) With one error.

What happens if the count is garbled by an error ?


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Flag bytes

(a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

Disadvantages: closely tied to 8-bit characters and the ASCII character code.


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Bit stuffing

(a) The original data.

(b) The data as they appear on the line.

(c)The data as they are stored in receivers memory after destuffing.


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Physical layer coding violations

The IEEE 802 standard LAN uses the following (Differential

Manchester) encoding scheme: a 1 bit is encoded as HL,

and a 0 bit as LH.

The invalid encodings - HH and LL - can be used for framing.

Character count can be combined with others for extra safety.


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Error control and flow controlError control: the goal (of a reliable connection-oriented service):

to ensure each frame is ultimately passed to the network layer

at the destination exactly once, no more and no less.

The measures:1. Acknowledgements from the receiver to the sender.

2. Timers for preventing senders from hanging forever.

3. Sequence numbers for dealing with duplicated frames.

Flow control: What to do with a sender that systematically wantsto transmit frames faster than the receiver can accept them ?

Some kind of feedback mechanism is needed so that the sender canbe made aware of whether or not the receiver is able to keepup.

This is frequently integrated with error handling for convenience.


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Error-Correcting Codes

Basic idea: Including redundant information along with each block of data sent

to enable the receiver to deduce what the transmitted character must have been.

An unit containing m bits of data and rbits of redundancy, or check bits, is

referred to as an m + rbits codeword.

The number of bit positions in which two codewords differ is called the

Hamming distance. Two d apart codewords require dsingle-bit errors toconvert one into the other.

The minimum Hamming distance between any two codewords from a list of

codewords is the distance of the complete list.

To detect derrors, a distance of d + 1 is needed because with such a code there

is no way that dsingle-bit errors can change a valid codeword into another

valid codeword.

To correct derrors, a distance of 2d + 1 is needed because that way the legal

codewords are so far apart that even with dchanges, the original codeword is

still closer than any other codeword, so it can be uniquely determined.


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Hamming code

At the sending side:

1. The bits of the codeword are numbered consecutively, starting

with bit 1 at the left end.

2. The bits that are powers of 2 (1, 2, 4, 8, 16, etc.) are check bits.

The rest (3, 5, 6, 7, etc.) are filled up with the data bits.

3. Check bit forces the parity of the following bits to be even (or

odd): the check bit itself, and

all data bits whose position number has an element of2**i

when it is expended into a sum of powers of 2. E.g., for

check bit 1, the following data bits are checked: 3 = 1 + 2,5 = 1 + 4, 7 = 1 + 2 + 4, ...

One data bit is checked by all and just those check bits

occurring in its expansion.


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Hamming code

At the receiver side:1. A counter is initialized into zero.

2. Each check bit k(= 1, 2, 4, ...) and its checked data bits are

examined to see if the parity is correct.

3. If not, kis added into the counter.

4. If the counter is zero at the end, the codeword is accepted asvalid. Otherwise, it contains the number of the incorrect bit.

E.g., if the parities were wrong when examining check bits 1, 2, 4,

then the counter-value = 7 = 1 + 2 + 4 , we can conclude thatdata bit 7 was inverted since bit 7 is the only one checked by

bits 1, 2, and 4.


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Use of a Hamming code to correct burst errorsThe following figure shows some 7-bit ASCII characters encoded as 11-bit

codewords using a Hamming code. The data bits are found in bit positions 3, 5,

6, 7, 9, 10, and 11.

To correct burst errors by means of Hamming codes :1. A sequence of codewords are arranged as a matrix, one codeword per row.

2. The data is transmitted one column ( bits) at a time, starting with the leftmost column.

3. The matrix is reconstructed at the receiving side.

If a burst error of length occurs, at most 1 bit in each codeword will be effected, which

can be corrected by the Hamming code.
http://www.cit.gu.edu.au/teaching/3142CIT/slides/ch3/node3.html


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Error-detecting codes Error-correcting codes are suitable for simplex channels where

no retransmission can be requested. Error-detection plus retransmission is often preferred because

it is more efficient.

For comparison, consider a channel with error rate of 10**(-6)

per bit. Let block size be 1000 bits. To correct a single error (by Hamming code), 10 check bits per block

are needed. To transmit 1000 blocks, 10,000 check bits (overhead) are

required.

To detect a single error, a single parity bit per block will suffice. To

transmit 1000 blocks, only one extra block (due to the error rate of

10**(-6)per bit) will have to be retransmitted, giving the overheadof only 2001 (= 1000 + 1001) bits.

Under what conditions is error-correction more efficient ?


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Cyclic Redundancy Code (CRC) method

A k-bit frame is regarded as the coefficient list for a

polynomial with kterms, ranging fromx**(k-1) tox**0. Polynomial arithmetic is done modulo 2. Both addition and

subtraction are identical to EXCLUSIVE OR:

10011011 01010101

+ 11001010 - 10101111

--------------- ---------------

01010001 11111010

The basic idea of the CRC method:1. The sender appends a checksum to the end of the frame in such a way that the

polynomial represented by the checksummed frame is divisible by agenerator polynomial G(x).

2. When the receiver gets the frame, it tries dividing it by the same G(x). If there

is a remainder, there must have been an error and a retransmission will be

requested.

How to compute the chechsum for a given frame ?
http://www.cit.gu.edu.au/teaching/3142CIT/slides/ch3/node3.htmlhttp://www.cit.gu.edu.au/teaching/3142CIT/slides/ch3/node3.html


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Calculation of the CRC checksumIn any division problem, if you diminish the dividend

by the remainder, what is left over is divisible by

the divisor.

Detectable errors by CRC:1. All single errors are detectable ifG(x) contains

two or more terms.

2. All double errors are detectable ifG(x) does not

dividex**k + 1 for any kup to the maximum

frame length.3. All odd number of errors ifG(x) contains a factor

ofx + 1 .

4. All burst errors of length r(the degree ofG(x) )

or the number of check bits.

5. The probability of a burst error of lengthr + 1

being accepted as valid is (0.5)** (r

1).6. The probability of a bad frame, garbled by a long

burst or multiple shorter bursts, getting through

unnoticed is (0.5)**r.

Simple hardware has been built to compute and

verify the CRC checksum.


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Elementary Data Link Protocols

Assumptions:

1. The physical layer, data link layer, and network layer are independentprocesses.

2. The network layer contacts the data link layer by generating properevents.

There exist suitable library procedures for the data link layer to delivery a

packet (To_network_layer), and to fetch a packet (from_network_layer).

3. The data link layer provides a reliable, connection-oriented service to the

network layer.

4. The physical layer computes/checks checksums in outgoing/incomingframes, and contacts the data link layer by generating properevents.

There exist suitable library procedures for the data link layer to send a frame

(to_physical_layer), and to fetch a frame (from_physical_layer).


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Protocol Definitions

Continued

Some definitions needed in the protocols to follow.

These are located in the file protocol.h.


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Protocol

Definitions

(ctd.)

Some definitions

needed in the

protocols to follow.

These are located in

the file protocol.h.


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Unrestricted

SimplexProtocol


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Simplex

Stop-and-

WaitProtocol


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A Simplex Protocol for a Noisy Channel

Continued

A scheme using timer and

acknowledgement:1. The sender starts a timer after

sending a frame.

2. The receiver sends back an

acknowledgement only when

the incoming frame were

correctly received.

3. The sender will retransmit the

frame if the timer expired

before receiving an

acknowledgement.

What is the fatal flaw in the

above scheme ?

Loss of an acknowledgement

duplicated frame.

How to distinguish a frame

being seen for the first

time from a

retransmission ?

Use a sequence number in the

header of each frame.


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A Simplex Protocol for a Noisy Channel (ctd.)

A positive acknowledgement with retransmission protocol.


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Sliding Window Protocols

How to achieve full-duplex data transmission ?

1. Two separate simplex data channels -- waste of bandwidth.

2. One circuit for data in both directions and using the field in the frame

header to distinguish a data from an acknowledgement.

Piggybacking: temporarily delay outgoing acknow-ledgements so that they

can get a free ride on the next outgoing data frame. Save bandwidth,number of interrupts and buffers.

How long should the receiver wait for an outgoing frame onto which to

piggyback the acknowledgement ?

An ad hoc scheme: waiting a fixed number of msec (timer).


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Basics ofsliding window protocols A sequence number is assciated with each transmitted frame. Sequence

numbers range from 0 up to some maximum (2**n - 1) circularly.

A window is a list of consecutive sequence numbers. The size of awindow is determined by two pointers: the lower edge and the higher

edge. The size can be fixed or dynamically extended and shrunk.

The sender maintains a sending window with consecutive sequence

numbers corresponding to frames sent but has yet not acknowledged.

Whenever a new packet arrives from the network layer, it is given the next highestsequence number, and the upper edge of the window is advanced by one.

When an acknowledgement comes in, the lower edge is advanced by one.

The size of the sender's window is a dynamical one.

The receiver maintains a receiving window with consecutive sequence

numbers corresponding to frames it is permitted to accept.

When a frame whose sequence number is equal to the lower edge of the

window is received, it is passed to the network layer, an acknowledgement is

generated, and the window is rotated by one.

The receiver's window always remains at its initial size.

A lidi i d f i 1 ith 3 bit


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A sliding window of size 1, with a 3-bit

sequence number

(a) Initially.

(b) After the first frame has been sent.

(c) After the first frame has been received.

(d) After the first acknowledgement has been received.


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A One-Bit Sliding Window Protocol

Continued

1. The maximum window size is 1.

2. Stop-and-wait.


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A One-Bit Sliding Window Protocol (ctd.)


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A One-Bit Sliding Window Protocol (2)

Two scenarios for protocol 4. (a) Normal case. (b) Abnormal

case. The notation is (seq, ack, packet number). An asterisk

indicates where a network layer accepts a packet.

A P l U i G B k N


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A Protocol Using Go Back N

A tacit assumption in previous protocols: the time in the following period isnegligible:

1. The transmission time for a frame to arrive at the receiver.2. The processing time for the receiver to handle the incoming frame.

3. The transmission time for an acknowledgement to come back at the sender.

Consequently, the sender is blocked during the above period, which can be verylarge in channels with long round-trip propagation delay.

How to solve this problem ?

The pipelining solution: Set the maximum sender's window size (the number ofoutstanding frames) so large that the sender is allowed to transmit multipleframes until the acknowledgement for the first frame comes back. Thereafter,acknowledgements will arrive one after another, so the sender always gets

permission to transmit.

What to do if a frame in the middle of a long stream is damaged or lost ?

Pi li i d


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Pipelining and error recovery

(a) The go back n strategy:1. The receiver discards all

subsequent frames, sending noacknowledgements. The

receiver's window size is 1.

2. After time out, the sender

retransmits all unacknowledged

frames in order, starting with the

damaged or lost one.

(b) The selective repeat strategy:

1. The receiver buffers correct

subsequent frames, but sending

no acknowledgements (or Nak).

The receiver's window size is

larger than 1.2. After time out (or receiving

Nak), the sender retransmits the

damaged or lost frame.
http://www.cit.gu.edu.au/teaching/3142CIT/slides/ch3/node5.html


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Sliding

WindowProtocol

Using Go

Back N

Continued


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Sliding Window Protocol Using Go Back N

Continued


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Continued


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A Slidi Wi d P t l U i S l ti R t


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A Sliding Window Protocol Using Selective Repeat

Continued

A Slidi Wi d P t l U i S l ti R t (2)


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A Sliding Window Protocol Using Selective Repeat (2)


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Continued

A Slidi Wi d P t l U i S l ti R t (4)


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(a) Initial situation with a window size seven.

(b) After seven frames sent and received, but not acknowledged.

(c) Initial situation with a window size of four.

(d) After four frames sent and received, but not acknowledged.


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Example Data Link Protocols

HDLCHigh-Level Data Link Control

The Data Link Layer in the Internet

Hi h L l D t Li k C t l


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High-Level Data Link Control

The frame is delimited with flag sequence 01111110.

Address can be used to identify a terminal on multidrop lines or to distinguishcommands from responses for point-to-point lines.

Controlis used for sequence numbers, acknowledgements, and others.

Data can be arbitrarily long.

Checksum uses CRC-CCITT.

The nice thing about standards is that you have so many to choose from.

At the data link layer, we have IBM SDLC (Synchronous Data Link Control),

ANSI ADCCP (Advanced Data Communication Control Procedure), ISOHDLC (High-level Data Link Control), CCITT LAP (Link Access Procedure),

etc. If you do not like any of them, you can just wait for next year's model.

All are bit-oriented and using bit stuffing.

All use the same frame structure:


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High-Level Data Link Control (2)

Three kinds of frames distinguished by the control field:

(a) An information frame.

(b) A supervisory frame.

(c) An unnumbered frame.



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A home personal computer acting as an internet host.

There are two situations in which point-to-point communication occurs in Internet:

Router-router leased line connection between Internet routers.

Host-router dial-up line connection between home computers and the Internetservice providers' routers, as shown in the following Figure.

PPP Point to Point Protocol


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PPPPoint to Point Protocol

PPP provides three features:

1. A framing method that unambiguously delineates the end ofone frame and the start of the next one. The frame format

also handles error detection.

2. A link control protocol for bringing lines up, testing them,

negotiating options, and bringing them down again

gracefully when they are no longer needed. This protocol is

called LCP (Link Control Protocol).

3. A way to negotiate network-layer options in a way that is

independent of the network layer protocol to be used. The

method chosen is to have a different NCP (NetworkControl Protocol) for each network layer supported.

A typical scenario of PPP


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A typical scenario of PPPA typical scenario of a home user calling up an Internet service provider to

make a home PC a temporary Internet host:

1. Physical connection setup phase: The PC calls the provider's router via a modem.

The router's modem answers the phone and establishes a physical connection.

2. Data link layer options negotiation phase: The PC sends the router a series of LCP packets in the payload field of one or

more PPP frames. These packets and their responses select the PPP parameters

to be used.3. Network layer options negotiation phase:

A series of NCP packets are sent to configure the network layer and to assign an

IP address for the PC (if the PC wants to run a TCP/IP protocol stack).

4. Data communication phase: The PC sends and receives IP packets over the established connection.

5. Connection release phase: When the PC is finished, NCP is used to tear down the network layer

connection and free up the IP address.

The LCP is used to shut down the data line layer connection.

The computer tells the modem to hang up the phone, releasing the physical

connection.


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Chapter3 Data Link Layer