Thursday, November 21, 2013
Factors to consider when selecting a transmission medium (TRANSMISSION IMPAIRMENTS)
There are many transmission media available and each media type has certain characteristics, so it’s necessary to aware about all the benefits and shortcomings of each one.
• Cost & Ease of installation
• Delay Distortion
• Capacity (Bandwidth and throughput)
• Transmission delay
• Propagation delay
• Thermal/White Noise
• Intermodulation Noise
• Impulse Noise
• Cost & Ease of installation : Costing is an important factors , when we select a media. Because absolute cost and ease of installation data are difficult to provide without referring to specific implementations, one can make relative judgments by comparing each medium to the others.
• Attenuation : Attenuation refers to the tendency of electromagnetic waves to weaken or become distorted during transmission. It is loss of energy as the signals propagates outwards. Attenuation increases with distance, as a wave passes through a medium, some of its energy is absorbed or scattered by the medium’s physical properties. The loss is measured is decibels per kilometer (db/km).attenuation is higher at higher frequencies.
• Delay Distortion: one property of signal propagation is that the speed of travel of frequency is highest at the centre of bandwidth and lowest at both the ends. Delay Distortion is caused by the fact that the signals of varying frequencies travel at different speeds along the medium.
• Capacity: the capacity of a transmission medium is often stated as bandwidth. Bandwidth is the range of cycle frequencies, measured in hertz (Hz) or cycle per second, that a transmission medium can physically accommodate.
Throughput is a measurement of the amount of data transmitted within a specified time period, usually measured in bits per second (bps).
• Transmission delay: it is the delay, which is present due to link capacity
• Propagation delay: Propagation delay is the time between the last bit transmitted at the head node of the link and the last bit received at the tail node.
• Interference: interference occurs when undesirable electromagnetic waves affect the signal. Interference can be caused by many factors, including
• Electromagnetic Interference (EMI)
• Radio wave interference (RFI)
• Noise : Noise is unwanted energy from sources other than transmitter. Thermal noise is caused by random motion of the electrons in a wire and is unavoidable. Intermodulation Interference Occurs whenever signals of different frequencies share the same medium. Cross Talk is caused by inductive coupling between two wires. Impulse noise caused by spikes on the power line or other causes.