Tuesday, February 19, 2008

Bit Rate, Baud Rate & Attenuation

Bit Rate, Baud Rate & Attenuation

Bit Rate

Number of bits transmitted per second or bps rate. The amount of data that a computer network can transmit is called bandwidth of network and is usually measured in kilo bits per second (Kbps) or mega bits per second (Mbps). A bit the smallest unit of information what computer can process can have one of two values, either 0 or 1. A kilo bit in one thousand bit while a mega bit is one million bit the speed at which information can be transmitted across the internet depends on the lowest information transporting capacity along the route and the number of people using that route at any given time. The bit rate is the speed of communication.

Baud Rate

The signaling rate of a line is measured in bauds. It is the switching speed or number of the transition (Voltage or frequency changes) that are made per second. The speed at which data is transmitted referred to as baud. Baud is commonly identified as the number of bits per second that can be transmitted over a communication line.



Attenuation

The loss of signal’s energy due to the resistance of the medium is called attenuation.

Thursday, February 7, 2008

Data Communication & Data Transmission Modes

Data Communication

In data communication, four basic terms are used i.e.

Data

Signal

Signaling

Transmission

Data refers to information that conveys or contains something of meaning to source/recipient. The information in data may be raw data or contain interpreted result from raw data.

Signal is the data encoding in the form of electric or electromagnetic waves to produce digital or analog signal.

Signaling is the process by which computer interacts with transmission media & sends the signal down the media.

Transmission is the communication of data achieved by propagation or processing of signals.

Data Transmission Modes

There are three ways as modes for transmitting data from one point to another. These are:-

Simplex

Half-Duplex

Full-Duplex

Simplex Mode of Transmission

In Simplex transmission, communication takes place only in one direction. Devices connected to such a circuit are either send-only or receive only device.


Fig-1.1 -- Simplex Mode of Data Transmission


Half-Duplex Mode of Transmission

A Half-Duplex system can transmit data in both directions, but only one direction at a time. This requires two wires. This is a common type of communication because one person can speak at a time. In this case line must be turned around each time the direction is reversed. This involves a switching circuit.


Fig-1.2 -- Half-Duplex Mode of Data Transmission


Full-Duplex Mode of Transmission

A Full-Duplex system allows information to flow simultaneously in both the direction on the transmission path. Use of Full-Duplex line improves efficiency because the line turn around time is removed. It requires four wires.


Fig-1.3 -- Full-Duplex Mode of Data Transmission



Friday, February 1, 2008

Switching Techniques

Switching Techniques

Circuit Switching

When you or your computer places a call (Telephone), the switching equipment within the telephone system search for a physical copper path all the way from your telephone to receiver’s telephone, this technique is called as circuit switching.

Once the call has been setup, a dedicated path between both ends exists & will continue to exist until the call is finished.

An important feature of circuit switching is the need to set-up an end-to-end path before any data can be sent. The time between the end of dialing and start of ringing can easily be 10 seconds and may increase on long-distance or international calls. During this time interval, the telephone system is hunting for a copper path.

As a consequence of copper path between the calling parties, once the set up has been completed, the only delay for data is he propagation time for electromagnetic signal.

Once the call has been put through, you never get busy signals; you might get busy signals before the connection has been established due to lack of switching.

Message Switching

A Message is a logical unit of information and can be of any length. In this method, if a station or a switching office wishes to send a message to another, it first attaches the destination address to message. When the sender has a block of data to be sent, it is stored in the first switching office and then forwarded later. This method is known as store-and-forward. Each block is received entirely, checked for errors and then retransmitted.

In message switching, no physical connection is required between the source and destination.

But one disadvantage is that the message length is unlimited i.e. each switching node must have sufficient storage to buffer message and another one is that a message is delay at each node because of time required to receive the message plus a queuing delay waiting for a chance to retransmit message to next node.

Packet Switching

This method works similar to message switching, however it overcomes the disadvantage of message switching because in this method routing is done on packet basis and not on message basis.

Here, a message is split up into packets of fixed size. Besides the block of data to be sent, a packet has a header that contains source and destination addresses, control information, acknowledgement and error checking bits. With message switching, there was no limit on block size, which means that routers must have disks to buffer long blocks. It also means that a single block may tie up a line for minutes restricting switches for the other traffic. To get around these problems, packet switching was invented.

Packet switching makes sure that no user reserves the transmission line for very long time.

Thus computer networks usually use packet switching, occasionally circuit switching, but never message switching.

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