Data Networks, Networking basics

Transmission modes

Parallel/serial

In parallel transmission a number of bits are transmitted in parallel over separate channels (wires). This requires expensive cables and can only propogate over short distances before bits get out of step with each other. Used mainly for printer and high speed peripherals.

Serial transmission transmits one bit after another along a single channel - used for all networking applications.

Analogue/digital

In analogue transmission, bits are encoded as different frequency signals, allowing data to be sent over analogue voice circuits. Over long distances, amplifiers are needed to counter signal attenuation, which unfortunately amplify any noise present as well, leading to a lower bandwidth. Used in local-loop to exchange from consumer premises.

Digital transmission encodes bits as different signal levels. Repeaters can be used at intervals to clean-up and re-transmit the bit pattern with no degradation. Used in modern telephony between exchanges and increasingly made available to consumers through such facilities as ISDN, DSL etc.

Synchronous/asynchronous

In synchronous communication, a continues timing signal is sent (in band or out-of-band) with the message to ensure that the receiver keeps in step with the sender. Involves more expensive hardware and appropritae for constant, high volume transmissions.

Asynchronous communication is more appropriate for spasmodic, short bursts of data such as characters transmitted from a terminal keyboard. Individual characters can be sent at any time and sender and receiver must agree on the characteristics of the transmission (see RS232).

Duplex/simplex

Serial communication can be in one direction only (simplex), two directions, possibly simultaneously (duplex) or both directions, but only one at a time (half duplex).

Bits/baud

Data transmission rates are usually measured in bits per second (bps) but channels are also characterised by baud. The baud rate is the rate at which the signal can change on the channel (named after M.Baudot) - the corresponding bit rate is either the same (as in RS232) or it might be a modemple of the baud rate (e.g. 10Mb/s Manchester-encoded Ethernet requires 20MBaud and 36Kb/s modems use 9KBaud channels)

Signal forms

Signals can either be electric (voltages or currents) or electromagnetic (radio, microwave, infrared, visible light with increasing bandwidth) and all propogated at a speed somewhat less than the speed of light.

Local asynchronous communication - RS232/RS422

A standard which has been in extensive use for a long time for connecting terminals to modems in the PSTN local loop is RS232. Superceeded by an improved standard called RS422 - but still often referred to as RS232.

Transmitting bits on a serial line

Bits can be represented by voltages appearing across a pair of copper wires. E.g. a 1 might be represented by a negative voltage and a 0 by a positive one.

RS232 protocol

RS232 is a hardware protocol that specifies the following :-

Voltages - a 1 bit is -15 volts and a 0 bit is +15 volts
Idle line - is represented by -15 volts
Frame start - a 0 bit
Frame end - to separate frame from next one, either 1,1.5 or 2, 1 bits called stop bits.
Error check - an optional odd or even parity bit.
Connector - a 25 pin connector with pin 2 for transmit and pin 3 for receive.
Other pins - an earth (or ground) line, and control lines such as request-to-send (rts) and clear-to-send (cts) for optional hardware handshaking.

Framing errors and break key

The sender and receiver must agree on number of stop bits and type of parity check and also the baud rate (each has a clock which it uses to sample the line in the middle of each bit position). The signal is also sampled at other times and if the correspondence is not correct a framing error is generrated.

The keyboards break key puts the line into the 0 state whilst held down and artificially generates a framing error which can be detected by software.

Channel capacity, bandwidth and noise

The capacity of a channel is measured by its bandwidth expressed in cycles-per-second (cps or Hertz) and represents the fastest rate at which the channel (including the transmitter and receiver attached) can change the signal and reliably detect such changes.

Increased transmission distance produce attenuation and distortion of signals and external interference produces noise which further distorts the signal, limiting the receivers ability to interpret it.

Nyquist's theorem

Nyquist's work provided a theoretical maximum data rate D (in bps) for a channel based on its bandwidth B and the number of different signal levels K as follows :-


  D = 2 B log₂ K

Shannon's theorem

In practice, this theoretical maximum is not acheivable due to the presence of noise, and Shannon produced a more realistic estimate based on bandwidth B, and the signal-to-noise ratio S/N :-


  D = B log₂(1 + S/N)

The signal-to-noise ratio is normally measured in decibels which is defined as :-

                                ratio in dB = 10 log₁₀S/N

For example, an analogue dial-up line to the local exchange provides 4000 Hz bandwidth. If the signal to noise ratio is 30 dB, then the maximum theoretical data rate is approximately 40 kbps.

Copper - twisted pair

Categories of twisted-pair lines

A pair of parallel conductors used to transmit a signal will radiate an electric field and in so doing, lose energy. They would also pick up external electric or magnetic fields and be subject significant noise.

By twisting the wires in a spiral, both these effects are minimised. Such conductors are available in two forms - unshielded twisted pait (UTP) and shielded twisted pair (STP) which has a metallic shield wrapped around the twisted wires to reduce noise.

A number of standards exist which specify the characteristics of different grades of STP & UTP in terms of bandwidth and operating distance between repeaters.

Applications of twisted pair

Twisted-pair is used in the local loop to PSTN and has also been adopted as the most common form of ethernet (LAN) wiring.

Advantages & Disadvantages

The main advantages of twisted-pair lines is that they are cheap and easy to install or move and maintain.

They do have a number of significant disadvantages however :-

Bandwidth - effectively limited to 1 MHz.
Data-rate - ranges from 100 Mbps over 30 feet downto 2 Mbps over 3.5 miles.
Repeaters - because of this many repeaters are needed over long distances.
Noise - highly susceptible to external interference.
Security - very easy to eavesdrop on communications (no need for a physical connection).

Copper - coaxial cable

Characteristics of coax

Coaxial cable, in which one conductor is surrounded by insulation and then the second conductor in the form of a tube has much better electrical properties, with less external interference and attenuation due to radiation.

Various categories of coax exist having bandwidth ranging from 370 MHz up to 1000 MHz and hence providing significantly better data rates.

Applications

Coax is used extensively in cable networks and was at one time used for PSTN trunks (now replaced by fiber-optic cable).

Advantages & disadvantages

Coax has a significant number of advantages over twisted-pair :-

Broadband - sufficient bandwidth to support modemple channels through Signalling.
Error rates - due to better insulation and sheilding, experiences much lower error rates than twisted-pair.
Repeaters - as a result, repeaters or amplifiers (analogue) can be spaced out more.
Security - much more difficult to eavesdrop onto than twisted pair.

It is however much more expensive and more costly to install or move than simple twisted-pair.

Wireless transmission

Microwave communication

Microwave channels can potentially operate ata bandwidth of 100 GHz. But in practice, this is limited to 50 GHz due to atmospheric absorbtion in some of the higher frequencies. Licensing regulations also frequently impose limits on allowed bandwidth use.

Microwave signals are subject to attenuation through atmospheric conditions - particularly rain and as transmitter and receiver must have line of sight, the Earth's curvature restricts distance to about 90 miles usually.

Applications of microwave

Microwave links can provide a cheaper alternative to leased PSTN lines and are particularly good in MAN's where access is difficult for cabling.

Communication satellites

Satellites can be categorised into three groups :-

Low-Earth-orbit (LEO) - typically at an altitude of a few hundred miles, such satellites orbit faster than Earth rotation and from the ground are seen to move across the sky (orbit times being a few hours).
Middle-Earth-orbit (MEO) - these orbit at altitudes of 6000 to 9000 miles.
Geostationary orbit (GEO) - at an altitude of 22,300 miles above the equator, these orbit at the same speed as Earth rotation and from the ground appear to be stationary.

Geostationary satellites

Very attractive for network communications as the ground stations can be fixed. A very effective alternative to terrestrial channels for WAN's where the different centres all fall within the footprint of the satellite.

One significant problem is the round-trip delay of about 0.5 seconds which make voice dialogue difficult.

LEO's & MEO's

To obtain constant communication channels with LEO's and MEO's you need between 5 and 20 satellites to ensure that 1 is always visible over the horizon.

GPS (global positioning system) uses such a scheme whereby 4 satellites are always visible over the horizon and a communication network can be built in which satellites relay signals between each other in order to reach specific locations on the ground - rather like a cellular phone network where the transceivers are in motion rather than the cellphones.

Fibre-optic cable

Categories of fibre-optic

There are three basic types of fibre-optic cable :-

Multi-modal - this is 62.5 micron in diameter and suffers from dispersion of the light wave, reducing its bandwidth.
Graduated modem-modal - by varying the refractive index along the fibre radius, dispersion is reduced.
Mono-modal - with 8 micron diameter, little or no dispersion.

Transmitters can be either light-emitting diodes which produce an unfocused beam or lasers which produce coherent light for mono-modal cable.

Signalling methods

Signals are sent using amplitutde modulation (AM) of visible light and currently there are three frequencies defined for each of which the bandwidth is about 25 THz. Repeaters, which converted the optical signal to an electrical one and then regenerated the optical signal are being replaced by optical-amplifiers making long distances easy to support.

Wave Division Signalling (WDM)

Another name for frequency-division Signalling (FDM) makes possible the creation of as many as 15000, 25 THZ channels in a single fibre.

Applications of Fibre-optics

Now the established medium for PSTN trunks and adopted for local-area networks using FDDI.

Advantages and disadvantages

The advantages of fibre-optic cable are :-

Bandwidth - enormous bandwidth potential with WDM.
Interference - not susceptible to external electromagnetic interference.
Security - extremely difficult to eavesdrop and such action detectable as an optical leak.
Physical - very light and compact.

There are a number of disadvantages however :-

Installation - high installation cost.
Damage - vulnerable to physical damage.
Hardware - expensive receivers, transmitters and test equipment needed.

Summary of transmission media characteristics

In summary, the following table gives a rough idea of the overall characteristics of the above transmission media.

Media	Bandwidth	Error rate	Repeater interval	Security	Cost
Twisted-pair	1 MHz	Poor ~10^-5	Short ~1 mile	Poor	Low
Coax	1 GHz	Good ~10^-8	Short ~2 miles	Good	Moderate
Microwave	100 GHz	Good ~10^-9	Medium ~45 miles	Poor	Moderate
Satellite	100 GHz	Good ~10^-9	Long <=23000 miles	Poor	High
Fibre	75 THz	V Good ~10^-12	Long ~4000 miles	V Good	High