Telephony

By: Boris Rousseau

The history of mankind has been greatly influenced by his successes and failures in communicating his perception of the world around him to others. Marshall McLuhan, the Canadian communications expert, argued that the technology of communication greatly affects our understanding of the message to be communicated. "The medium is the massage" [1]

In the developed world, telephony is the dominant communications technology. Over 20% of the world's population has access to a telephone [2]. Indeed an advanced and reliable telephony network is considered necessary for a nation to consider itself developed. Like most everyday technologies, telephony can seem a like magic. Strictly speaking, telephony involves reproducing sounds a distance. Typically the sounds are those produced by the human voice and are transmitted as electrical impulses over wires in a similar manner to that proposed by Alexander Graham Bell in 1876. Other transmission technologies such as wireless or radio transmission and optical fiber are also widely used but rarely provide the direct connection between the home and the network. This direct connection is often known as the access network or 'last mile'.

The network itself is made up of a collection of telephony switches or exchanges that are used to route voice data from one telephone to another. Exchanges are the practical alternative to connecting every telephone to every other telephone using a wire or group of wires for each connection. A phone call is a virtual connection or circuit between two telephones that is maintained by the network for the duration of the call and provides a consistent and reliable flow of voice data between phones.

In Ireland the vast majority of home telephones are analogue. This refers to the data transmission method used. Voice data is represented by electrical impulses that are analogues of the voice that produced them. The strength of the electrical signal varies linearly with the loudness of the human voice

that produced it. This is an acceptable method of transmission over short distances. Unfortunately the world is an extremely hostile place for telephone wires and analogue telephony transmissions. There are many factors that can produce small degradations in the signal, cumulatively producing inaudible and unclear reproductions at the other end of the 'phone line'. To preserve the signal's integrity over long distances it becomes necessary to represent the voice as digital data. This happens before transmission in the case of digital transmission systems like Integrated Services Digital Network (ISDN) and between or at telephone exchanges for a normal 'analogue call'. This protects data's integrity two ways. The level of the signal is sampled at regular intervals and then approximated or quantised by a fixed number within a certain tolerance of the signal's strength or amplitude. The samples are then transmitted as binary data. Simply put, binary or base-2 data represents numbers using a sequence of 1s and 0s. It is much easier to recover the state of a signal where there are only two possible states at any one time then it is if the state is an analogue of something infinitely variable, like loudness of the human voice. Counter-intuitively it is also possible to transmit binary voice data faster. The information is conveyed in a more elaborate form but is harder to corrupt and can therefore be transmitted faster and using a wider range of transmission technologies e.g. Ultra High Frequency (UHF) radio or optical fiber.

I've only provided a brief introduction into the complexity of the communications devices that we take for granted. Mobile phones and the wireless network will be explored in more detail in later columns.

[1] The Medium is the Massage, Marshall McLuhan et al., 1967
[2] State of the World Forum, State of the World Index, 2000