In digital data transmission where we have more than one bits to send from sender to receiver. Our primary when we are considering the wiring is the data stream.

Do we send 1 bit at a time; or do we group bits into larger groups and, if so, how? The transmission of binary data across a link can be accomplished in either parallel or serial mode.

In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

1. Parallel Transmission

Binary data, consisting of 1s and 0s, will be organized into groups of n bits each. Computers produce and consume data in groups of bits. By grouping, we can send data n bits at a time instead of 1. This is called parallel transmission. The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission.

Shortcoming of Isochronous parallel transmission it requires n communication lines just to transmit the data stream. Hence it is expensive, parallel transmission is usually limited to short distances.

2. Serial Transmission

In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices.

The advantage of serial over parallel transmission is that with only one communication channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of n.

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

Serial transmission occurs in one of three ways: asynchronous, synchronous, and isochronous.

  1. Asynchronous
  2. Synchronous, and
  3. Isochronous.

2.1 Synchronous Transmission

In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits. 

2.2 Isochronous Transmission

A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The isochronous transmission guarantees that the data arrive at a fixed rate.

In real-time audio and video, in which uneven delays between frames are not acceptable, synchronous transmission fails.

For example, TV images are broadcast at the rate of 30 images per second; they must be viewed at the same rate. If each image is sent by using one or more frames, there should be no delays between frames.

2.3 Asynchronous Transmission

In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1) at the end of each byte. There may be a gap between each byte.