Computer Communications System Noise Computer Science Essay

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This report intends to discuss different types of noise which can cause impairments within a data transmission system. It will briefly cover some of the most common noise types in telecommunications then move on to discuss noise specific to a guided media standard '802.3ab'. Wireless media will then be discussed, with the most common methodologies used for modulating RF signals. The report will be concluded with a self-reflection stating how I feel this report has benefitted me and where I could have applied myself better to gain a better end product.

Introduction

Within busy telecommunications systems noise is a massive issue which had the ability to impair signals and cause for whole data packets to be lost. Noise by definition is the unwanted addition to a signal, as it has unintentionally become part of the signal it causes affects such slowing down data rate, causing distortion and affecting the general signal as a whole. This noise could either be fundamental noise (noise which is caused by some random phenomenon and cannot be cancelled out) or interference which is caused by other signals interfering with the source signal; there are many techniques which can be used to combat this type of noise.

Chapter 1-Types of Noise

1.1 Guided Media

1.1.1 Thermal Noise

Thermal noise was first detected in 1962 by John B. Johnson and later theorised by Harry Nyquist. It is for this reason that thermal noise can now be known as other names such as Johnson noise, Nyquist noise and Johnson-Nyquist noise. [1]

Thermal noise is a by-product of thermal agitation of electrons and can be found present on all forms of transmissions and media. Thermal noise is dependent on the temperature, thus the hotter the temperature the more thermal noise. It will always be present as achieving the conditions required keeping the electrons stable enough is simply not feasible at this moment in time.

There are various methods of modulation which can be applied to reduce thermal noise all of which are centred on lowering the frequency at which the signal is transmitted.

1.1.2 Intermodulation (IMD)

Intermodulation noise is when 2 signals share the same transmission medium at different frequencies. "The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original frequencies" [2]

Intermodulation is caused by nonlinearities of a system, thus causing for the output to differentiate from the signal which was the original input.

This type of noise can be experienced when you drive near by an airport and have the car radio on, suddenly you pick up interference from the signal tower on your car radio. This is likely to happen when one signaller is operating on 5Hz and the other 10Hz, intermodulation would cause the both to be combined and overlay onto the 15Hz frequency which happens to be what you are tuned into in your car.

1.1.3 Crosstalk

Crosstalk is the result of noise leaking from one channel/circuit of a transmission medium onto another. The most common effect of crosstalk is when you are on the phone and pick up a neighbours conversation.

When we are talking of analogue we can combat crosstalk by either twisting the pairs or converting the signal into a digital square wave as this type of signal is less susceptible to this type of noise.

1.1.4 Impulse noise

Impulse noise is as the name would suggest is impulsive, they consist of short on/off pulses. These impulses are more damaging to a digital signal that what they are to an analogue. The effect on the analogue signal is a cause of crackling or a loss of a couple of pixels, however with digital it is more damaging and can cause whole blocks of data being lost at once.

These impulses are caused by a sort of electromagnetic instability which could be anything from a physical impairment in the cable to a lightning storm, anything which causes an electromagnetic field which will interfere with the signal.

Due to the unpredictable nature of impulse noise it is impossible to remove it completely. However modulation techniques such as Coded OFDM can be implemented to limit the amount of effect the impulses have.

1.2 Unguided Media

1.2.1 Intersymbol interference

Intersymbol interference is a distortion which is caused by multipath propagation; a signal is transmitted to the receiver through many different paths, with the eventual symbols arriving out of sequence and overlapping each other. There are many factors which can cause this phenomenon to occur, such as; Reflection, a signal being bounced off of a structure. Refraction, signal is distorted through vegetation. The atmosphere, through ionosphere reflection. It is due to these paths being of different lengths that cause the signal to arrive at the receiver at varying times. These differing paths can potentially distort the amplitude or the phase of the signal.

Techniques used to combat multipath Intersymbol include interference error correcting codes and adaptive equalisation.

Chapter 2- Guided Media

2.1 802.3ab

802.3, otherwise known as 1000BASE-T is a gigabit Ethernet standard which provides data rates of up to 1000Mbps. Full-duplex transmission throughput is achieved by using 4 pairs of Category 5e cabling, sending 250 Mbps over each pair. The transmitted symbol rate is that of 100Base-TX (125 M Baud/s), this clock frequency allows for physical implementations of 100/1000 Mbps. 1000BASE-T is the first Ethernet standard to use all 8 pins of the RJ45 connector.

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Figure 1) [3]

The physical layer of 1000BASE-T interface with the above Medium Access Control (data link) layer via the GMII (Gigabit Media Independent Interface) which has the clock signal, receive and transmit lines. The physical layer is made up of the PCS (Physical Coding Sublayer) and the PMA (Physical Medium Attachment) blocks.

A code redundancy of higher than 100% is allowed for when coding 250Mbps in 125Mbaud with 5 symbols and also gives guaranteed good immunity against noise. As this is a complex coding scheme 1000BASE-T compensates for such by utilising FEC (Forward Error Correction) as a four dimensional, 8-state Encoder.

A 4-Dimensional level 5 pulse amplitude modulation (4D-PAM5) is used in Gigabit Ethernet, this unlike in 10/100 Mbps Ethernet allows for all 4 pairs of the UTP to be utilised for transmitting and receiving simultaneously. In this system we have 5 symbol levels; +2, +1, 0, -1, -2 which are mapped to the voltage levels +1, +0.5, 0, -0.5, -1. [4]

Due to the amount of data on the cable, 1000BASE-T is very susceptible to crosstalk. 802.3ab consists of both NEXT (Near End Crosstalk) and FEXT (Far End Crosstalk).

F

figure 2) [5]

NEXT is noise which is caused by unwanted coupling of the signal, this is induced to a neighbouring receiver by a transmitter. The addition of NEXT cancellers gives an extra level of immunity towards noise over the twisting of the cable, this is new to Gigabit Ethernet. The value used to express this is dB, unlike when we speak of attenuation high dB values are desired. The signal which is detected on the disturbed pair should in comparison with the signal injected into the disturbing pair be as small as possible.

FEXT is caused at the transmission end of the pair and is unwanted coupling among 2 or more of the transmitting pairs. FEXT is less than NEXT when operating at frequencies higher than 4MHz, this due to the attenuation of the signal. This is due to FEXT being the coupling between pairs which the signal uses to propagate and is attenuated over 100 meters

Hybrid devices enable full-duplex transmission on single pairs. This is achieved by filtering the transmit signal out at the receiver. Hybrid networks which have a good trans-hybrid loss dictate that they reduce the amount that the transmitter signal is coupled into the receiver, however it still cannot remove all of the transmit signal. Cancellers must be added to each pair of wires to remove any echo signal.

The last type of noise which can affect this standard is; background noises (Ambient) such as lightning storms, overhead power lines, interference from wireless symbols and alien crosstalk. The random nature of ambient noise means that it cannot be cancelled and thus directly detracts from the SNR margin for the system.

The noise which is a cause of internal to the system factors can go through cancelation techniques however does not eradicate the interference on the signal completely. Impairments which are a by-product of fundamental noise cannot go through cancellation techniques and in turn directly affect BER of the system.

Chapter 3 - Wireless media

3.1 Wi-Fi

Wi-Fi standards define a subset of standards which operate within the unlicensed radio frequencies between 2.4GHz and 5GHz range, due to this being an unlicensed frequency range we tend to get devices such as Bluetooth, microwaves and cordless phones as well as many others. These signals along with the signals being created from your own wireless network leave your intelligence signal highly susceptible to interference.

The bandwidth at this range is split up into a total of 13 different channels for 2.4GHZ band, with each channel operating at a bandwidth of 22MHz and sub dividing the 5GHz band into 19 channels Europe and 29 for America Wireless devices can be configured to automatically detect the least congested channel and in doing so will switch over to that channel, as well as this it is possible for you to manually select your channel using an application to manually view which channel has the least amount of traffic on it.

3.2 802.11a

802.11a-1999 is one of the sub standards which operate at 5GHz band of this unlicensed bandwidth and utilises OFDM (Orthogonal Frequency-division multiplexing) which uses 52 individual subcarriers to provide a transmission data rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. The 802.11a standard states that the data rates; 6, 12 and 24 Mbps are mandatory. The system uses four of the subcarriers as pilot subcarriers, a reference for disregarding of either frequency or phase shifts within the signal during a transmission.

A pseudo random binary sequence is via the pilot channels for the prevention of spectral lines [6]. The remaining 48 of the subcarriers are used for the purpose of providing separate parallel pathways which are used for sending information in a parallel format. This results in the frequency of the 802.11a standard sub frequency being 0.3125 MHz.

Each of the sub carriers may be either modulated as; Binary PSK, Quadrature PSK, 16-QAM or 64-QAM.

After all the signals have been generated onto their own frequency they are then combined together and in turn give us the key OFDM signal. The subcarriers overlap one another, thus ensuring no intermodulation and allowing for the intelligence signal to be received unimpaired.

Conclusion

We have looked over the different types of noise which can cause impairments within a data transmission system. We discussed the Guided media standard, 802.3ab and we could see from analysis that it was susceptible to both ambient noise and inner system interference. We could also see from this that although we can use cancelling techniques for impairments such as NEXT and echo it doesn't fully eradicate the noise, however with the random nature of ambient noise we can't use techniques such as cancellers.

Reflection

Summary

This report intends to discuss different types of noise which can cause impairments within a data transmission system. It will briefly cover some of the most common noise types in telecommunications then move on to discuss noise specific to a guided media standard '802.3ab'. Wireless media will then be discussed, with the most common methodologies used for modulating RF signals. The report will be concluded with a self-reflection stating how I feel this report has benefitted me and where I could have applied myself better to gain a better end product.

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