Data Transmission Across Media

Transmitting data across media implies sending bits through the transmission medium. Physically, the data is sent as electric signals, radio waves or as light signals. Let’s now discuss the use of electric current to transfer digital information. For this, the bits are encoded and sent as characters.

Transmission Modes

The direction in which data can be transmitted between any two linked devices is of three types—(1) Simplex, (2) Half-duplex, and (3) Full-duplex, or duplex. Simplex transmission is uni- directional data transmission. Of the two linked devices, only one of them can send data and the other one can only receive data. Half–duplex transmission is bi-directional data transmission, but the linked devices cannot send and receive at the same time. When one device is sending data the other can only receive. Full-duplex transmission is bi-directional, and the linked devices can send and receive data simultaneously. The linked devices can send data and at the same time receive data. Figure  shows the different kinds of transmission modes used for interaction.

Satellite transmission

Transmission modes

Transmission Speed

When the signals are transmitted between two computers, two factors need to be considered— (1) Bandwidth, and (2) Distance.

· Bandwidth is the amount of data that can be transferred through the underlying hardware i.e. the communication medium, in a fixed amount of time. Bandwidth is measured in cycles per second (cps) or Hertz (Hz). The bandwidth of the transmission medium determines the data transfer rate.

· Throughput is the amount of data that is actually transmitted between the two computers. Throughput is specified in bits per second (bps). The throughput capability of the communication medium is also called bandwidth. The bandwidth of the communication medium is the upper bound on the throughput, because data cannot be sent at a rate more than the throughput of the communication medium.

· Higher throughput is achieved by using a large part of the electromagnetic spectrum (large bandwidth). Technology that uses large part of the electromagnetic spectrum to achieve higher throughput is known as broadband technology. The technology that uses small part of the electromagnetic spectrum is known as baseband technology.

· Throughput is affected by the distance between the connected computers or devices. Even if a transmission medium is designed for a specific bandwidth, the throughput is affected by the distance of communication.

· The bandwidth of transmission medium is limited by the distance over which the medium needs to transmit the signal. The bandwidth decreases with the increase in the distance between the connected devices. When a signal has to travel long distance, the signal strength decreases; the signal strength is utilized to overcome the resistance offered by the connecting medium (cable or wire). The gradual deterioration of signal strength across long distances is called attenuation.

· Moreover, with increasing distance the external disturbance increases, which causes the signal to deteriorate and results in less amount of data to be transferred. The degradation of signal due to internal or external disturbances is called distortion.

· The bandwidth and distance of the transmission medium is selected so that it offers minimum attenuation and minimum distortion.

· The cat-5 UTP cable has a throughput of 100 Mbps over a distance of 100m. The 10 base2 coaxial cable has a throughput up to 10Mbps over a distance of 185 m. The 10 base 5 coaxial cable has a throughput up to 10Mbps over a distance of 500 m.