Question: Explain how data is transmitted through the communication channel? Mention some of the standards ……

Data transmission :

Data transmission means transmission of digital or analog data to one or more network or communication devices, over a communication medium.

In practice, three different types of data transmission channels are used :

• TYPES OF CHANNELS

1. A simplex channel can only be used for data transfer in only one direction. The sender generally has a ‘‘modulator’’ to translate binary data into analog signals, and the receiver has a ‘‘demodulator’’ for the reverse translation.
   For example, a radio station usually sends signals to the audience but never receives signals from them.
2. A duplex, or full-duplex, channel can transfer information in both directions simultaneously. Each station has both a ‘‘modulator’’ and a ‘‘demodulator’’ combined into a single instrument called a ‘‘modem.’’
   Eg : Telephone as a bast example, we can speak and be heard by each other at the same time because there are two communication paths between them.
3. A half-duplex channel, finally, can transfer data in both directions, but not simultaneously. The stations have to be switched from sending to receiving or back.
   Eg :
   Walkie-talkie is a typical half duplex device. It has a “push-to-talk” button which can be used to turn on the transmitter but turn off the receiver. It means that you can transmit you message but can’t receive from your partner.

• Serial and Parallel

1. Serial Transmission: In this type of transmission, one data bit is transmitted at a time in series. This is commonly used for long distances transmission because it requires less signal processing and less chances of error.
2. Parallel Transmission: In this type of transmission, several data bits are transmitted at a time in parallel. This is normally used for short distances.

• ASYNCHRONOUS AND SYNCHRONOUS

1. On a serial line both the sender and the receiver have a separate clock that sets the transmission rate. The sender uses its clock to drive the line (i.e., to transmit the bits), and the receiver uses its clock to scan it. In asynchronous transmissions the two clocks need not be in perfect synchrony when no data are transmitted. Data is transmitted in chunks of, for instance, 7 or 8 bits, preceded by a special start symbol and followed by a stop symbol. The receiver uses the start symbol to synchronize its clock with the sender.
2. It is sufficient if the two clocks can stay in synchrony for only the 7 or 8 bits that make up a data word. The length of the data word is sometimes called the ‘‘synchronization gap,’’ the period of time that the two clocks must stay in synchrony. The stop symbol is usually either 1.5 or 2 bits long, to allow the receiver to process the data and catch up with the sender and to restore synchrony at the next start symbol. The period of time that passes between the stop symbol and the next start symbol, however, need not be an integral number of bit times.

IEC 60870-5-101:

This is an international standard. This protocol is mostly used in energy sector. It follows EPA architecture (Enhanced Performance Architecture). It defines only the physical link and application layers of the OSI model.

Features

·         This supports both unbalanced & balanced modes of data transfer.

·         In this, data is classified into different information objects and each information object is provided with a specific address.

·         It provides facility to classify the data into high priority (class-1) and low priority (class-2) and transfer the same data using separate mechanisms.

·         It provides facility for time synchronization

IEC 60870-5-101 standard is a Companion standard further extended by IEC 60870-5-2 which defines the link layer transmission mode.

In Data Link layer, below are the features of this protocol:
1. Asymmetric transmission (unbalanced mode): In this mode, Party line or point to point transmission takes.
2. Symmetric transmission (balanced mode): In this mode, only point to point transmission is possible.