Asyn Sync Transmission

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ICT 2002. All rights reserved. No part of this material may be reproduced and sold.

Communication Modes

There are three communication modes.

Simplex Communication

Sender Receiver

Only One-way communication is possible.

Examples : TV, Radio


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Communication ModesHalf Duplex

Sender

Receiver

Two-way communication is possible, but

not simultaneously.

ExampleWalky Talky.

Receiver

Sender


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Communication ModesFull Duplex

Sender

Receiver

Two-way simultaneous communication is

possible.

ExampleTelephone System.

Receiver

Sender


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Serial Transmission

Data are transmitted in a bit by bit fashion.

1 0 0 1 1 1 0 1 0 0 0 0 0 1 0 1

Only one channel is needed for the link.

Used in almost all the data communication

applications.


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Parallel Transmission

1

0

0

11

1

0

1

0

0

0

00

1

0

1

All the bits in a

byte or a wordare transmitted

simultaneously

Because of the need of the multiple channels,

only used to connect devices in short range.


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Synchronization

Since the information is encoded into digital

signals at the transmitting side, for the receiver

to interpret the incoming information correctly,

the receiver must be able to identify the exact

boundaries of each bit, each byte, and each

block of information.

Enabling this capability in a transmission link

is called synchronization.


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Synchronization Ctd.

Synchronization in a transmission link isachieved in three levels.

Bit or Clock Synchronization

Byte or Character Synchronization Block or Frame Synchronization.

There are two approaches followed in common

practice. Asynchronous Transmission

Synchronous Transmission


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Asynchronous Transmission

The bit rate is known in advance by bothtransmitting and receiving ends.

The level of the link is maintained at high

when no bits are transmitted.

The number of data bits, use and the type of

the parity bit, and the number of stop bits are

also agreed upon in advance, and each

transmitted element (byte, character,etc.) is

encapsulated with a Start bit and one or more

Stop bits.


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Asynchronous Transmission Ctd.

Start of Transmission Time

Start Bit 8data bitsEven Parity Bit

Stop Bit(s)

1 1 0 0 0 1 0 1

Direction of Transmission


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1st

Byte 2nd

Byte

1 1 0 0 0 1 0 1 1 0 0 0 0 1 0 1



Start BitStart Bit

Stop Bit(s) Stop Bit(s)


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The bit rates known by the both ends

enable the Bit or Clock synchronization.

The use of start and Stop bits enables theByte or Character synchronization.

The communication control characters

such as STX, ETX are used to achievethe Block or Frame synchronization.


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When one Start bit, one Parity bit and

one Stop bit is used for each transmitted

element, for example a byte, the actual

throughput can be examined by the

efficiency.

A synchronous transmits is used forlow bit rate character based

communication.


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This is a method used to transmit data in amore efficient manner.

There are no gaps or extra bits placed inbetween the transmitted bytes.

A complete frame containing the dataelements to be transmitted is constructed andthe frame is transmitted as a single entity.

There are two approaches, namely

a) Character Oriented SynchronousTransmission

b) Bit Oriented Synchronous Transmission


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SYN SYN STX STX

Data and Error Detection Info.


Frame

SYNSynchronizing Character

STXStart of Text

ETXEnd of Text

Character Oriented Synchronous Transmission


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Synchronous Transmission Character Oriented Synchronous

Transmission with Character Stuffing

SYN SYN DLE STX DLE STXDLE DLE

Start of Text sequence End of Text sequence

DLE inserted by

the transmitter

This scheme is made data-transparent by

Character Stuffing.

Frame Content


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Bit Oriented Synchronous Transmission

01111110 1001110111111. ..110110 01111110

Opening Flag Closing Flag

Frame Content

Frame

Opening Flag is used to achieve Byte or

Character synchronization

Clock information is included within the

encoding method and hence the Bit or Clock

synchronization is achieved.


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Bit Oriented Synchronous Transmission withBit Stuffing

01111110 10011101111101..11011011 01111110

Zero bit inserted

The Zero Bits inserted by the transmitter

after every set of consecutive five binary 1sin the content, to avoid the date being mis

interpreted go the opening flag.


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Dales Delivery Services

There are two types of Services :

a) Connection-Oriented Services

b) Connectionless Services

In connection Oriented Services, a continues

path from the source to the destination is

established, then it is used for the information

transfer and finally it is released.


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Types of Services Ctd.

In Connectionless services, there is no continues

path established between the source and the

destination, instead the transmitted information

are encapsulated in packets which contain thefull addressing information of the destination.

The addressing information enables the

connecting devices in the network to forward

the packets on the correct routes so that the

packets can reach the destination.


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Types of Services Ctd.

Each service can be characterized by a Quality of

Service.

Some services are characterized as reliable as they

guarantee that the data are delivered correctly to

the destination. Such services provide some means

of acknowledgement which confirms the correctreception. Convection oriented services are

typically reliable.

The services which do not provide suchacknowledgements are known as unreliable (best

effort) services. Convectional services are typically

unreliable.


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Flow Control

Involved with controlling the flow of

information so that the receiver can accept all

the information without any overflow or loss

due to the fact that the sender is sending at ahigher speed.

The implementation of Flow Control requires

some kind of feedback mechanism, so the

sender can be made aware of whether or not

the receiver can keep up.


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Unrestricted Simplex Protocol

This is the simplest - But unrealistic !!!

It is based on the following assumptions.

Data are transmitted in one direction only.Both transmitting and receiving sides are

always ready.

Processing time can be ignored.The communication channel is totally error

free and does not cause any data losses.


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Unrestricted Simplex protocol

Is a

Frame

Coming?

Receive it

Yes

NoFrameReady to

send?

Send it

Yes

No


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Simple Stop-and Wait Protocol

Is a

Frame

ready?

Is the

Receiver

Ready?

No

YesNoYes

Send

the frame

At the Sender :-


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Simple Stop-and Wait Protocol

Is theprocessingof framecompleted?

No

YesNo

YesProcessthe frame

At the Receiver :-

Is thenext framearrived?

Send anacknowle-dgement


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Sliding Window Protocols

More frames are sent without waiting for

individual acknowledgements.

The number of frames sent at a time can be

changed dynamically.

Sender maintains a set of sequence numbers

corresponding to the frames it is permitted tosend. (Sending Window)


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Receiver also maintains a Receiving Windowcorresponding to the set of frames it is

permitted to accept.

The Senders Window and the ReceiversWindow need to have the same lower and upper

limits.

Slid window to protocol is more efficient thenthe stop & wait protocol

Slid window protocol is used in connation

oriented service typically.

Sliding Window Protocols Ctd.

Documents

Asyn Sync Transmission