Lecture-6

Transmission mode

One of the primary concerns when transmitting data from one device to another is the way the data is communicated between the two users. A given transmission on a communications channel between two machines can occur in several different ways. The transmission is characterized by:

  • According to the direction of the exchanges: Simplex, Half Duplex, Full Duplex
  • According to number of bits sent simultaneously: Parallel and Serial Transmission
  • synchronization between the transmitter and receiver: Synchronous and Asynchronous Transmission
  • According to direction of data Exchange:

Simplex : In this mode data is sent in one direction only the other end/device can only listen the transmission but cannot respond back. Examples of simplex mode of transmission are broadcasting, telecasting etc.

Half Duplex :In this modeboth device able to communicate to each other, but one device at a time. While one device is transmitting the other device listen only and vice versa. Example of this type of mode is walkie-talkie

Full Duplex: This mode provides simultaneous bidirectional communication between the end devices. Examples are Mobile phone communication, Telephonic conversation.

Type

  1. Parallel Vs Serial transmission:

In parallel communication there are as many physical wires as the number of bits in a data word.

Advantage: Transmission Speed

Disadvantage: Cost

Serial Communication:

In this type only a single wire is used for carrying the transmission. The transmission is done bit by bit.

Advantage: Used for long distance over a single wire

Disadvantage: Speed

  1. Asynchronous vs Synchronous Transmission:

Asynchronous Communication: Data is transmitted over a single wire. The two communicating device can transmit at any time. The transmission as per agreed upon pattern. Some of the characteristics are:

  • No clock is used as timing is not important
  • Redundant bits for recognizing the start and stop of transmission word additional bits are used. These bits are called start and stop bits.
  • Inter byte gap is used making the transmission slow
  • No synchronization at byte level but at bit level some sort of synchronization is desired. This is in the form of a counter that counts the number of bits received and to recognize the next bit as stop bit.
  • This system of communication is simple, cheap and effective.
  • Example of short duration communication is keyboard to computer communication.

Synchronous Communication:

In this type a large chunk of data is transmitted without any inter byte gaps, start and stop bits. These chunks of information are called as frames. Multiple bytes are packed together as a stream of 0’s and 1’s. This type of communication requires proper synchronization for separating the bytes. Some of the characteristics are:

  • Timing is important between transmitter and receiver.
  • Multiple bytes are packed together in a frame
  • It is the responsibility of the receiver to count the bits and separate the frame into bytes.
  • There is no extra bits like start, stop and inter byte gaps(or idle time)
  • Synchronous transmission is faster than asynchronous transmission
  • Most suitable for data transmission between two computers
  • Though there is no gap between the bytes but there may be some gap between the frames.

Questions:

  1. Differentiate between synchronous and asynchronous communication
  2. What do you understand by the term transmission mode? How it is characterised?