computer networks chapter 11 – data link control and protocols

Computer Networks

Chapter 11 – Data Link Control and Protocols

Spring 2006 Computer Networks 2

Flow Control

Flow control – controling the sender not to overflow the receiver Buffers – temporary memory where the receiver

stores data sent by the sender before it process them

Buffers can be filled in with data when sender is running on the fast (or lightly loaded

machine) and the reciever is running on a slow (or highly loaded) machine

In case no buffers are available, data can be lost. Therefore protocols need to control the sender not to

overflow the reciever Protocols for flow control can exist at the data-

link layer or the upper layers


Error Control

Errors can appear due to: Impairments of transmission media Loss of data because of limitted buffer

space Other unpredictable circumastances

It is necessary that the receiver has mechanisms to discover the occurence of errors When the error is detected, the receiver

can ask the sender to retransmit data.


How to Ask for Retransmission?

Two possibilities When the frame is recieved without errors, the

receiver can send an acknowledgement (ACK) When there are errors in the frame, the receiver can

send a negative acknowledgement (NACK)

ACKs are used in data communication It is easier to detect lost frames when ACKs are used Any time error is detected the data are retransmitted The process is called ARQ (Automated Repeated

reQuest)


How long to wait for ACK?

When a frame is sent the sender starts a timer The timer is used to detect frame loss or

acknowledgement loss When the timer expires, the sender knows that it

needs to send the frame again

The sender keeps a copy of the frame in its sending buffers

For frame identification sequence number are used Sequence numbers are carried as a part of the

header


Protocols for Flow and Error Control

Stop-and-Wait A single frame is sent at a time Inefficient with respect to the use of the

available bandwidth

Sliding window Several frames are sent at a time and after

each ACK is received a new frame is sent Effitiency is increased, but complexity is

increased, too


Stop-and-Wait ARQ

The sending device keeps a copy of the last frame until it receives an acknowledgement for that frame Keeping this copy allows the sender to

retransmit lost or damaged frames

For identification purposes, both data frames and ACKs are numbered alternatively 0 and 1. A data frame is acknowledged by an ACK1

frame, indicating that the receiver has has got the frame 0 and is expecting the frame 1


Stop-and-wait ARQ (no lost frames or acknowledgements)

Sender Receiver

.

.

.

Tim

e

Data 0

ACK 1

Data 1

ACK 0

Data 0


Stop-and-wait ARQ (a frame is lost or damaged)

Sender

Timeout

ReceiverT

ime

Data 0

ACK 1

Data 0

.

.

.


Stop-and-wait ARQ (ACK is lost or damaged)

ReceiverT

ime

Data 0

ACK 1

Data 0

Timeout

.

.

.

Sender

ACK 1


Performace of Stop-and-Wait Protocol

Good for relatively short links for which the ratio between the transmission and propagation time is very small This includes short links or longer links with low data

rates The limitation is a consequense of the rule to wait for

an acknowledgement before sending another frame

Require one-bit storage for the sequence numbers

Inadequate for longer links and higher data rates


Improving the Stop-and-wait Protocol

Stop-and-wait is inefficient if one channel is used for frames and the other for acknowledgements.

Piggybacking ACK is attached to the data frame in the oposit

direction, thus using both channels for frames and for ACKs .

Sliding windows A set of frames are sent without the

acknowledgement. The number of frames of this set can be used for the flow control purposes, too.


Sliding Window

...

Tim

e

...

ReceiverSender

The number of frames sent without acknowledgement is called the window size.

If the window size is w = 2n, than, the number of sequence numbers for the frames is in the range from 0 to 2n-1.

n-bit field is required in the frame to store the sequence number


Sliding Window (Cont.)

Problems arise when some of the frames are discarded. Two strategies are developed to deal with this problem Go back n strategy

The reciever simply discards all frames after the damaged frame without sending acknowledgement.

Selective repeat strategy The receiver keeps all the frames after the damaged

frame. When sender finaly notice that something is wrong it retransmits the bad frame.

The two strategies are trade-offs between bandwidth and data-link buffer space.


Go Back n Strategy

Sending window of size n, receiving window of size 1. The sender has to buffer n frames, bandwidth is

wasted.


Selective Repeat Strategy

Only retransmit the frames that are in error Both sending and receiving window are of

size n


Visualisation of Data-link Protocols

You can check the animation of stop-and-wait and sliding window protocol using the java applet presented on the following page

http://www.cs.bham.ac.uk/%7Egkt/Teaching/SEM335/dlsim/Simulation.html


Example Data-link Protocols

Character oriented protocols Kermit, X modem Simple transfer protocols used extensively

for PC to PC communication

Bit oriented protocols HDLC -

Data-link protocols in the Internet (SLIP and PPP)

ATM and Data-link protocols in ATM


SLIP (Serial Line IP)

Predecessor to PPP (not an Internet standard)

Very simple, but has some serious problems Supports only IP as a network layer protocol Does not do any error detection or

correction (left to the upper protocols) Each side must know the other’s IP address Does not provide any form of authentication


PPP (Point-to-Point Protocol)

Internet Standard (RFC1661) Handles error detection Supports multiple protocols in the

network layer (IP, IPX, OSI CLNP, XNS) Allows IP addresses to be nagotiated at

connection time Used for dial-up and for leased lines


Framing Data Link Layer packets are called

frames. Frames are always transmitted between

directly connected ends (there is a physical connection).


Synchronization

A sequence of binary digit, transmitted at some regular rate over a link, has to be recovered at the reciever

Synchronization Clocking or timing (the instance at which the

reciever will be looking for individual bits) is an important issue

Different places where synchronization is needed Bit synchronization (start of bit) Character or byte synchronization (start of byte or

character) Frame synchronization (start of frame)


Two Approaches Exist

Asynchronous transmission The reciever has a free-running clock (ussually

fixed multiple of the sender clock). Short bit sequences are emitted to allow the

reciever to know when to start looking for bits. The receiver clock assumes to maintain correct

timing over short spans of few bits. Synchronous transmission

Clock information has to be embeded within the bit stream, so that the reciever can either extract the clock information, or use the embeded information to fine tune its clock.

The clock signal can be sent via separate channel (additional bandwidth required).


Asynchronous Transmission

If the line is in the idle state (some electrical state) and one 7 or 8 bit group need to be sent, these bits are preceded by a start bit and followed by one to two stop bits.

The receiver continously samples at rate nominally N times the transmit clock rate.

It is looking for the first transition from ”high” to ”low” after its previous idle or ”stop” bits.

After this, it sets the next sampling instant to be half of the transmitter clock period (this should be near the center of the start bit).

It then samples at multiples of N receiver clock pulses until ending stop bits return the line to logic high


Example of Asynchronous Transmission


Synchronous Transmission

Assynchronous transmittion is ineficient (extra overhead bits needed) and cannot be used at high transmittion rates.

In synchronous transmission we tailor the way in which binary data is encoded onto the electrical signal to obtain a signal with properties such that the receiver can receive the clock information.


Frame Synchronization

Character count Specify number of characters in the header Not convinient because of possibility to get out of

synchronization when there is a transmission error Starting and ending characters

STX: start-of-text character signal at start of frame, ETX: end-of-text character signal at start of frame

Starting and ending flags Each frame starts and ends with a special bit pattern called

a flag byte Using special coding sheme

If Manchester encoding sheme is used the high-high and low-low combinations can be used for this purpose


Character and Bit Stuffing

STX and ETX can be data DLE-STX signal for start of frame, DLE-ETX

signal for end of frame DLE can be data, thus some solution is necessary

Character stuffing – insert a DLE character for any DLE character in data

Bit stuffing – analogous to character stuffing If the flag byte = 01111110, than after five 1’s a

0 is inserted in the data string Stuffed 0s are removed at the receiver

computer networks chapter 11 – data link control and protocols

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