Transmission Media: Types, Interference, and Channel Capacity
In a data transmission system, the transmission medium is the physical path through which the sender and receiver communicate. We distinguish two types of media: guided and unguided. In both cases, transmission takes place through electromagnetic waves. Guided media direct the waves along a physical path; examples of these media are coaxial cable, fiber optic, and twisted pair. Unguided media transmit waves but do not direct them; examples include air and vacuum.
The nature of the medium, along with the signal being transmitted, determines the characteristics and quality of the transmission. For guided media, the medium itself primarily determines the limitations of the transmission: data transmission speed, bandwidth support, and spacing between repeaters. However, when using unguided media, the frequency spectrum of the signal produced by the antenna is more critical than the transmission medium itself.
Interference in Transmission
All electrical and electronic devices emit interference and/or are susceptible to problems affecting data transmission. Some of these problems are:
- Attenuation: Signal power decays with distance. Therefore, it is crucial to ensure that enough energy reaches the receiver circuitry. Additionally, the noise should be significantly lower than the original signal. To maintain signal energy, amplifiers or repeaters are used.
- Because attenuation varies with frequency, analog signals become distorted. Systems must be used to return the signal to its initial characteristics (using coils that change the electrical characteristics or by amplifying higher frequencies more).
- Delay Distortion: In guided media, the speed of propagation of a signal varies with frequency. Some frequencies arrive before others in the same signal, and therefore, the different frequency components of the signal reach the receiver at different times. To mitigate this problem, equalization techniques are used.
- Noise: Noise is any unwanted signal that is inserted between the transmitter and receiver. There are different types of noise:
- Thermal noise: Due to the thermal agitation of electrons within the conductor.
- Intermodulation noise: Occurs when different frequencies share the same transmission medium.
- Crosstalk: Occurs when there is coupling between the lines carrying signals.
- Impulsive noise: Consists of intermittent pulses of short duration and high amplitude that affect the signal.
- Channel Capacity: Channel capacity is the speed at which data can be transmitted in a data communication channel.
Key Factors in Data Transmission
- The data rate is the rate, expressed in bits per second, at which data can be transmitted.
- The bandwidth is the bandwidth of the transmitted signal, limited by the transmitter and the nature of the transmission medium (in hertz).
- The error rate is the rate at which errors occur.
For a given bandwidth, it is advisable to achieve the highest possible transmission speed, but not so high that the error rate becomes unacceptable. The biggest drawback to achieving this is noise.
For a given bandwidth W, the highest possible transmission speed is 2W. However, if it is allowed (with digital signals) to encode more than one bit in each cycle, it is possible to transmit a greater amount of information. Nyquist’s formulation tells us that by increasing the levels of differentiable stress in the signal, it is possible to increase the amount of information transmitted.