The Electrical Interface 13
THE ELECTRICAL INTERFACE
2.1 INTRODUCTION
To transmit binary data over a transmission line, the binary digits making up each element to be transmitted must be converted into physical electrical signals. For example, a binary 1 may be transmitted by applying a voltage signal (or level) of amplitude +V volts to the sending end of a transmission line and a binary 0 by applying -V volts. On receipt of such signals, the receiving device would then interpret the +V volts as a binary 1 and the -V volts as a binary 0. In practice, however, the transmitted electrical signals can become attenuated (smaller) and distorted (misshapen) due to the imperfect nature of the physical transmission medium. This has the effect that in the limit the receiver is unable to discriminate between the binary 1 and 0 signals. Some of the effects of attenuation and distortion are shown in Figure 2. 1. The degree of each effect is strongly influenced by such factors as:
· the type of transmission medium;
· the bit rate of the data being transmitted; and
· the distance between the two communicating devices.
The various sources of impairment will be expanded upon later in this chapter.
As the effects caused by attenuation and distortion can be quantified for different types of transmission media and physical separations, there are now sets of well-defined, internationally agreed standards laid down for the electrical interface between a DTE and a piece of data communication-terminating equipment. These standards include not only a definition of the electrical signal levels to be used but also the use and meaning of any additional control signals and conventions that are used at the physical level. The two bodies that formulate standards for interconnecting pieces of equipment are the Consultative Committee of the International Telegraph and Telephone (CCITT) in Europe and the Electrical Industries Association (EIA) in the United States. Although the standards defined by both bodies use slightly different terminology, the basic signals and their meaning are the same.
This chapter is divided into three sections: the first describes some of the alternative transmission media that are currently in widespread use; the second, the different forms of electrical signals in use; and the third, some of the additional aspects of a number of the more common physical layer standards.
2.2 TRANSMISSION MEDIA
The transmission of an electrical signal between two pieces of equipment requires the use of a transmission medium which normally takes the form of a transmission line. In most cases, this consists of a pair of conductors or wires; however, transmission is sometimes achieved by passing a beam of light through a piece of glass fibre or an electromagnetic wave through free space. The type of transmission medium used is important, since it determines the maximum rate, in terms of binary digits (bits) per second or bps, that data can be transmitted. Some of the more common types of transmission media are discussed in the following sections.
2.2.1 Two-wire open lines
A two-wire open line is the simplest type of transmission medium. In such a line, each wire is insulated from the other and both are open to free space. This type of line is perfectly adequate for connecting two pieces of equipment that have a short physical separation (less than, say, 50 m) and a modest bit rate (less than, say, 19.2 kbps). The signal, which is typically a voltage or current level relative to some ground reference, is applied to one wire while the ground reference is applied to the other.
Although a two-wire open line may be used to connect two devices (DTEs) together directly, it is used mainly for connecting a DTE to a local piece of data communication-terminating equipment (DCE) - a modem, for example. As will be seen, such connections usually utilize multiple lines, the most common arrangement using a separate insulated wire for each signal and a single wire for the common ground reference. The complete set of wires is then either enclosed in a single protected cable called a multicore cable or moulded into the form of a flat ribbon cable as shown in Figure 2.2(a).
With this type of line, care is needed to avoid cross coupling of electrical signals between adjacent wires in the same cable. This is known as crosstalk and is caused by capacitive coupling between the two wires. In addition, the open structure of this type of line makes it susceptible to the pick-up of spurious noise signals from other electrical signal sources caused by electromagnetic radiation. The main problem with interference signals of this type is that they may be picked up in just one wire - the signal wire, for example - and not the ground wire. As a result, an additional difference signal can be created between the two wires and, since the receiver normally operates using the difference signal between the two wires, this can give rise to an erroneous interpretation of the combined (signal plus noise) received signal. These factors all contribute to the limited lengths of line and bit rates that can be used reliably.
2.2.2 Twisted pair lines
Much better immunity to spurious noise signals, referred to as noise immunity, can be obtained by employing a pair of wires that are twisted together. This is then known as a twisted pair line. The resulting close proximity of both the signal and ground reference wires means that any interference caused by extraneous signal sources is picked up by both wires and hence its effect on the difference signal is reduced. Furthermore, if multiple twisted pairs are enclosed within the same cable, the twisting of each pair within the cable further reduces interference effects caused by crosstalk. A schematic of a twisted pair line is shown in Figure 2.2(b).
Twisted pair lines are suitable, with appropriate line driver and receiver circuits that exploit the potential advantages gained by using such a geometry, for bit rates in the order of 1 Mbps over short distances (less than 100 m) and lower bit rates for longer distances. With some twisted pair cables, an additional protective screen or shield is used to reduce further the effects of extraneous interference signals. This is then referred to as a shielded twisted pair.
2.2.3 Coaxial cable
The main limiting factor of a twisted pair line is caused by a phenomenon known as the skin effect: as the bit rate (and hence frequency) of the transmitted signal increases, the current flowing in the wires tends to flow only on the outside surface of the wire, thus using less of the available cross-section. This has the effect of increasing the electrical resistance of the wires for higher frequency signals which in turn causes more attenuation of the transmitted signal. In addition, at higher frequencies, an increasing amount of signal power is lost due to radiation effects. Hence, for those applications that demand a bit rate higher than 1 Mbps, it is normal to use another type of transmission media. One type of transmission line that minimizes both of these effects is the coaxial cable.
In a coaxial cable, the signal and ground reference wires take the form of a solid centre conductor running concentrically (coaxially) inside a solid (or braided) outer circular conductor as shown in Figure 2.2(c). The space between the two conductors should ideally be filled with air, but in practice it is normally filled with a dielectric insulating material with either a solid or honeycomb structure.
Due to its geometry, the centre conductor is effectively shielded from external interference signals and also only minimal losses occur due to electromagnetic radiation and the skin effect. Coaxial cable can be used with a number of different signal types, but typically 10 or even 20 Mbps over several hundred metres is perfectly feasible. Also, as will be expanded upon later, coaxial cable is applicable to both point-to-point and multipoint topologies.
2.2.4 Optical fibre
Although the geometry of the coaxial cable significantly reduces the various limiting effects, the maximum signal frequency, and hence the information rate, that can be transmitted using a solid (normally copper) conductor, although very high, is limited; this is also the case for twisted pair lines. Optical Fibre Cable differs from these types of transmission media in that it carries the transmitted information in the form of a fluctuating beam of light in a glass fibre, rather than an electrical signal in a piece of wire. Light waves have a much wider bandwidth than electrical waves and hence optical fibre cable can be used for transmitting very high bit rates, in the order of hundreds of megabits per second. Furthermore, the use of a light beam makes optical fibre cable immune to the effects caused by spurious electromagnetic interference signals and crosstalk effects. Optical fibre cable, therefore, is also extremely useful for the transmission of lower bit rate signals through extremely noisy electrical environments - in steel plants, for example, which employ much high-voltage and current-switching equipment. It is also being used increasingly in environments that demand a high level of security, since it is difficult to physically tap an optical fibre cable.
An optical fibre cable consists of just a single glass fibre, for each signal to be transmitted, contained within a protective cover, which also shields the fibre from any external light sources. A schematic diagram of such a cable is shown in Figure 2.2(d). The light signal is generated by a special optical transmitter unit, which performs the conversion from normal electrical signals as used in a DTE. Similarly, at the other end of the line, a special optical receiver module is used to perform the reverse function. Typically, the transmitter uses a light-emitting diode (LED) to perform the conversion operation and the receiver a light-sensitive photodiode or phototransistor. As the fibre is coated with a reflective film, the majority of the light produced by the LED remains inside the fibre and hence the attenuation effect is low. In general, optical fibre cable systems are more expensive than coaxial cable and, because of their construction, they are mechanically weaker, which makes them more difficult to install. It is also more difficult to join (or split) fibre cable due to the high coupling losses that occur, and hence they are only considered when either very high bit rates are required or enhanced levels of noise immunity are needed.
2.2.5 Microwaves
All the transmission media mentioned so far have used a physical line to carry the transmitted information. However, data can also be transmitted using electromagnetic (radio) waves through free space. One example of such a media is satellites: a collimated microwave beam, on to which the data are modulated, is transmitted to the satellite from the ground and this is then received and retransmitted (relayed) to the predetermined destination(s). A typical satellite channel has an extremely high bandwidth and can provide many hundreds of high bit rate data links using a technique known as multiplexing. This will be described in more detail later but, essentially, the total available capacity of the channel is divided into a number of subchannels, each of which can support a high bit rate link.
Satellites used for communication purposes are normally geostationary, which means that the satellite orbits the earth in synchronism once every 24 hours and hence appears stationary from the ground. The orbit of the satellite is chosen so that it provides a line-of-sight communication path to both the transmitting station(s) and the receiving station(s). The degree of the collimation of the microwave beam retransmitted by the satellite can be either coarse, so that the signal may be picked up over a wide geographical area, or finely focused, so that it may only be picked up over a limited area. With the latter, the signal power is higher and hence smaller diameter receivers, such as antennas or dishes, can be used. Satellites are in widespread use as a data transmission medium and the applications range from interconnecting different national computer communication networks to providing high bit rate interconnecting paths to link communication networks located in different parts of the same country. A schematic of a typical satellite system is shown in Figure 2.2(e).
Microwave links are also widely used to provide communication links when it is impractical or too expensive to install physical transmission media; for example, across a river or perhaps a busy motorway or highway. Such links are referred to as terrestrial microwave links. As the collimated microwave beam travels through the earth's atmosphere with this type of application, it can be disturbed by such things as man-made structures and adverse weather conditions. With a satellite link, on the other hand, the beam travels most of its path through free space and hence it is less affected by such effects. Nevertheless, line-of-sight microwave communication through the earth's atmosphere can be used reliably over distances up to 50 km.
2.3 SIGNAL TYPES
When two pieces of communicating equipment (DTEs) are situated relatively close to one another and only modest bit rates are used, the data can be transmitted by using just two-wire open lines and simple interface circuits, which change the signal levels used within the equipment to a suitable level for use on the interconnecting cable. However, as the physical separation between the two pieces of equipment and the bit rate increase, more sophisticated circuits and techniques must be employed. Moreover, if the two pieces of equipment are situated in, say, different parts of the country (or world) and there are no public data communication facilities available, then the only cost-effective approach is to use lines provided by the various PTT authorities for telephone purposes. When using this type of communication medium, currently, it is necessary to convert the electrical signals output by the source DTE into a form analogous to the signals used to convey spoken messages. Similarly, on receipt of these signals, it is necessary to convert them back into a form suitable for use by the destination DTE. The equipment used to perform these functions is known as a modem. Some of the different signal types used by modems and also other forms of transmission lines are discussed in the following sections.