2.13 Transmission Problems
Most data transmission takes place by using conduction over copper wire. There are a number of problems associated with this that will cause problems with transmission.
- Attenuation to Crosstalk Ratio (ACR)
Attenuation is the loss or reduction in the amplitude (strength) of a signal as it passes through a medium. As a signal travels through a copper wire some of the signal will be absorbed by the medium (this is called heat dissipation). The effect of attenuation is one of the reasons why there are cable standards that specify the maximum length of cable run for particular types of cable, eg: 10BaseT maximum recommended run length is 100 metres.
Attenuation is measured in decibels (dB).
A general rule exists with regards to decibels and power:
Increase of 3 dB = Double the power
Decrease of 3 dB = Half the power
Increase of 10 dB = Ten times the power
Decrease of 10 dB = One-tenth the power
Note: You can think of attenuation as being the opposite of amplification.
Crosstalk occurs when there is more than one cable being used. The electromagnetic force of one cable may be sufficient to have an effect on another. Some cables are twisted to reduce the possibility of crosstalk.
There are two main types of crosstalk but both involve some form of unwanted noise between sender and receiver.
NEXT is near end crosstalk and happens at the sending end of the cable
FEXT is far end crosstalk and happens at the receiving end of the cable
In badly made twisted pair cables crosstalk will occur when the cable at the jack is untwisted or exposed out of the outer jacket.
Attenuation to Crosstalk Ratio (ACR)
This is the difference in decibels (dB) between the signal attenuation and near-end crosstalk (NEXT). The ACR is calculated to make sure that a signal transmission is stronger at the receiver than any interference due to crosstalk.
Attenuation and crosstalk must be minimised in order for acceptable signal transmission. Attenuation depends on the length and type of cable and as such cannot be changed. However, crosstalk can be minimised by making sure that the cables meet certain standards.
ACR is also referred to as headroom.
Noise is unwanted electrical signals that find their way onto the wire. If the noise is low then it will not interfere with the communication. If the noise is too high then it will begin to interfere with the signal. Noise can come from other cables in the circuit or from external sources. To overcome problems with noise shielded cable can be used.
Skew is a problem related to timing in parallel cables. If data is carried over multiple lines (parallel) it is possible that the data may arrive at different times at the receiver.
Note: Skew is not possible in serial communications.