Data Communications & NetworkingLecture-10

Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

Lecture overview

Transmission modeParallel and serial transmissionSynchronous and asynchronous

transmission

Transmission mode

A given transmission on a communications channel between two machines can occur in several different ways.

The transmission is characterized by: the direction of the exchanges. the transmission mode: the number of bits sent

simultaneously. synchronization between the transmitter and receiver.

The direction of the exchanges

There are 3 different transmission modes characterized according to the direction of the exchanges.

Simplex Half Duplex Full Duplex

Simplex Mode

In simplex mode the communication is uni-directional as on a one-way street.

Only one of the two stations on a link can transmit, the other can only receive.

Examples: Keyboard, Monitor, Mouse

Half Duplex

In half-duplex mode, each station can both transmit and receive but not at the same time.

When one device is sending, the other can only receive. The half-duplex mode is like a one-lane road with two-directional traffic. The entire capacity of the communication channel is taken over by

whichever of the two devices is transmitting at the time. e.g. walkie-talkie

Full duplex In full-duplex mode both stations can transmit and receive

simultaneously. The full duplex mode is like a two-way street with traffic flowing in

both directions at the same time. In full duplex mode signals going in either direction share the capacity

of the link. This sharing can occur in two ways either the link must contain two

physically separate transmission paths, one for sending and the other for receiving or the capacity of the channel is divided between signals traveling in opposite directions.

Data Transmission

The number of bits sent simultaneouslyWhen we talk about transmission of data from one

device to another the important issue is wiring.When we consider wiring the important thing is

how a data stream is sent. Parallel Transmission Serial Transmission

Parallel Transmission

Binary data, consisting of 1’s and 0’s,may be organized in to groups of n bits each.

Computers produce and consume data in groups of bits.

By grouping we can send data n bits at one time.Each bit has its own wire.N bits of one group can be transmitted with each

clock tick from one device to another.

Parallel Transmission

Serial TransmissionIn serial transmission one bit follows another, so we

need only one communication channel instead of n to send n bits.

The advantage of serial over parallel transmission is that with only one communication channel.

Serial transmission reduces the cost of transmission over parallel roughly by a factor of n.

Communication within devices is parallel, conversion devices are required at the interface between the sender(parallel-to-serial) and receiver (serial-to-parallel).

Serial Transmission

Serial Transmission

Serial transmission can occur in one of two ways.

SynchronousAsynchronous

Asynchronous Transmission

Asynchronous is so named because the timing of a signal is unimportant.

Information is received and translated by agreed upon patterns.

The receiving device can retrieve the information without regard to the rhythm in which it is sent.

Patterns: Patterns are based on grouping the bit stream in to bytes .

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Asynchronous Transmission continued…

Each group usually 8 bits is sent along the link as a unit.

The sending system handles each group independently, relaying it to the link whenever the link is ready, without regarding to a timer.

Without synchronization, the receiver cannot use timing to predict when the next group will arrive???

Answer: Use Start & Stop bits

Start & Stop bitsStart bit To alert the receiver to the arrival of a new group, therefore an

extra bit is added to the beginning of each byte. This bit is usually a 0,is called the start bit.

Stop bit To let the receiver know that the byte is finished, one or more

additional bits are appended to the end of the byte. These bits usually 1’s are called stop bits.

In addition, the transmission of each byte may be followed by a gap of varying duration.

This gap can be represented either by an idle channel or by a stream of additional stop bits.

Asynchronous Transmission

Why called asynchronous???The synchronization is at the byte level.The sender and receiver do not have to be

synchronized all the time. When the receiver detects a start bit,it sets a timer and begins counting bits as they come in. After n bits,the receiver looks for a stop bit. As soon as it detects the stop bit,it waits until it detects the next start bit.

Synchronous Transmission

In synchronous transmission, the bit stream is combined in to longer frames which may contain multiple bytes.

Each byte is introduced on to the transmission link without a gap between it and the next one.

It is left to the receiver to separate the bit stream in to bytes for decoding purposes.

Timing is important in synchronous transmission

Synchronous Transmission

Advantages of synchronous transmission

The advantage of synchronous transmission is speed.

Byte synchronization is accomplished in the data link layer.

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