% >From jvialen@silver-surfer.hut.fi  Tue Apr 16 10:18:48 1991
% Return-Path: <jvialen@silver-surfer.hut.fi>
% Received: from silver-surfer by ngs.fi (5.61++/SMI-4.0)
% 	id AA02938; Tue, 16 Apr 91 10:18:48 +0300
% From: <jvialen@silver-surfer.hut.fi>
% Received: by silver-surfer.hut.fi (5.61++/6.8/S-TeKoLa)
% 	id AA00463; Tue, 16 Apr 91 10:20:24 +0300
% Message-Id: <9104160720.AA00463@silver-surfer.hut.fi>
% Subject: Computer_English_Latex
% To: Pekka.Nikander@ngs.fi
% Date: Tue, 16 Apr 91 10:20:22 EET DST
% X-Mailer: ELM [version 2.3 PL0]

%
%
%

\chapter{Telecommunications and computer networks}
\author{Jukka Vialen}

\section{Introduction}

This article is meant to give a short overview of the telecommunication 
and computer networks and also discuss where the communications is 
going in the near future. The main part of this text is kept at a very 
general level and details are collected in the nineth section --- there is a 
small list of concepts and abbreviations that one will very probably find 
in any article concerning the area.

\section{The need for network}
The need for a network arises quite naturally. Conceivably one could 
inter\-connect a small number of users directly. But consider the problem 
of directly connecting hundreds, thousands, or millions of users who 
desire to communicate with one another. Making direct connections, 
even if it were possible, would not make much sense because many of 
them might be used only infrequently.

\section{What is a network?}
A network consists essentially of network switches, or nodes, 
interconnected by transmission links. These links can be wire, cable, 
radio, satellite, or fibre optics facilities. The network systems range 
from small networks interconnecting data terminals and computers 
within a single building or campus-like complex, to large geographically 
distributed networks covering entire countries or, in some cases, 
spanning the globe. Some networks are privately operated; others are 
public --- available for a fee to all who want to use them.

Some networks use packet-switched technology, in which blocks of 
data called packets are transmitted from a source to a destination. 
Source and destination can be user terminals, computers, printers, or 
any other types of data-communicating and/or data-handling devices. In 
this technology packets from multiple users share the same distribution 
and transmission facilities. Each packet of data includes the addresses 
where the packet is going and where it is coming from.

Other networks are of the circuit-switched type, most common 
portions of the ubiquitous telephone networks to which we are all 
accustomed. In these networks, which generally transmit voice or data, 
a private transmission path is established between any pair or group of 
users attempting to communicate and is held as long as transmission is 
required. In digital transmission the 'private transmission path' is 
usually a private time slot where the data or digitized speech is 
transferred.

\section{About the physical media}
As mentioned the medium where information can  be transferred in the 
net\-works can be of several different types. This section discusses their 
use in today's and tomorrow's communication.

In the area of international telecommunications the upward trend in the 
capacity of major international links will probably continue, but the race 
between satellites and submarine cables awakes much more attention. 
The use of fibre-optic submarine cables has restarted this race. 

Optical fibres (which were first introduced in the early 1970s) use 
ultra\-short waves to transmit information using the wavelengths of light. 
In a natural environment, like air, this type of wave is attenuated too 
fast to be used, but it can be guided along fibre glass threads, i.e. 
optical fibres. In telecommunications, optical fibres are today a very 
important medium in the internal traffic of large cities. More and more 
they are also replacing the national links as a basic medium for speech 
(and data) transmission.

Many high speed LAN systems are also based on optical fibres (i.e. 
\linebreak[4] FDDI), though the majority of the commercial LANs use coaxial 
cables as transfer medium. The newest trend in this area is the 
"wireless LANs" --- use of infrared light or radio waves of the microwave 
region for transmission.

\section{Analog and digital networks}
The bulk of the telephone plant throughout the world is analog. It has 
been designed to handle voice and, despite the huge growth of data 
transmission, voice transmission is still the most common mode of 
communication world-wide. The transmission of data in telephone 
networks requires the conversion of data signals to voice-type analog 
signals using devices called modems. This limits the rate of 
transmission of data to at most 14.4 kbps (9.6 kbps transmission is 
more common as an upper limit).

Among foreseeable developments, the introduction of the integrated 
services digital network (ISDN), which affects the very structure of the 
networks, is already well on the way. Implementation of the ISDN will 
mean the replacement of networks which now exist separately, such as 
telex, data transmission, images and voice etc, by a single fully 
digitized network to which all sorts of terminals can be connected 
indiscriminately : the telephone of course, but also facsimile machines, 
telex, various kinds of computers, videophones and so on.

This does not mean the physical creation of a new network but rather it 
means replacing the old analog telephone network by a new fully 
digitized general purpose network.

\section{Cellular networks}
The early 80s brought cellular networks, which are characterized by full 
mobility without service interruptions, and fully automated call 
procedures. First generation cellular networks --- the Scandinavian NMT, 
the Bell-orig\-inat\-ed AMPS and its British derivative TACS, together 
with smaller systems --- are all intended for speech communication. 
Phones for these systems are called analog mobile phones mainly 
because speech transmission in these is analog.

The next generation of cellular mobile telephone's (CMT) like the
Pan-European GSM-system aim at expanding network capacity and further
stan\-dard\-iza\-tion of systems by using digital speech transmission and
TDMA tech\-niques. They also make more value-added services like short
message services, facsimile and other data and speech applications
possible in a mobile environment. Further advantages of digital
systems are improved speech quality and , at least theoretically, the
possibility of further capacity increases by reducing bit rates in
speech coding.

\section{Communications in the 1990s}
(The word "communications" here means both voice and data 
transmission through networks.)

The infrastructure of the public communications networks in the future can 
probably be divided into three parts : the general purpose network 
(ISDN), radio networks and broadband networks (B-ISDN). The locally 
used networks (LANs) inside companies and offices and campus-like 
complexes will not lose their meaning, of course.

The biggest change in using communications will happen in the area of 
data transmission (this evolution is already well on its way). Every 
office, working hall, factory or car will have its own data terminal. It is 
not only the internal data-transmission in companies that will grow but 
also the communication between companies will grow rapidly.

The data-transmission will soon be an integrated part of the business 
operations of a successful company. A company can distribute its 
activities around the world but the internal communication can work in 
real time because of the modern telecommunication techniques.

The rapidly increasing use of optical fibres offers new possibilities also 
in the area of mass communications (TV, radio). The capacity of the 
optical fibres is so great that TV-broadcasting can easily be done 
through networks designed for communications purposes. This opens 
new possibilities for the distribution of TV-programmes.

In homes the use of optical fibres will be insignificant in the 1990s, but 
there is no doubt that in new buildings the future use of optical fibres 
will be taken into account.

Wireless (radio) transmission is probably the most rapidly growing area 
of communications today and in the near future. The main reason for 
this is the growing use of mobile phones.

\section{Some basic concepts and abbreviations}
{\it
\begin{description}
\item[AMPS -] Advanced Mobile Phone System

\item[ANSI -] American National Standards Institute

\item[ARPAnet -] A network developed for research purposes by the Department 
of Defence in the early 1970s (USA).

\item[ATM -] Asynchronous Transfer Mode. A switching system that will be used 
in the broadband networks of the future.

\item[B-ISDN -] Broadband ISDN. Capacity exceeded up to 2 Mbit/sec by using 
30 B-channels.

\item[CityNet -] a product offered by HPY (The local telephone company in 
Helsinki). Based on optical cables.

\item[CRC -] Cyclic Redundancy Check. A method for ensuring that data has not 
been corrupted during transmission.

\item[CSMA/CD -] Carrier Sense Multiple Access with Collision Detection. The 
stations monitor the channel and start transmission when the channel is 
free. If two or more stations begin their transmission at the same time 
a collision occurs. Powerful in small quantities of traffic.

\item[DataNet -] a 'virtual-network' service. A customer's LANs can be 
connected to the DataNet network.

\item[Datel -] a network for business-customers from TELE.

\item[Datex -] a circuit-connection based network from TELE. Widely used in 
the Nordic countries.

\item[DQDB -] Distributed Queue Dual Bus. IEEE standard number 802.6. A 
standard for a MAN based on the use of fiber.

\item[Ethernet -] IEEE 802.3. Theoretical speed 10 Mbit/s (in 1989). Uses 
main\-ly coaxial cable as a physical medium and a 'channel' as topology. 
Uses CSMA/CD. Most widely used LAN. 

\item[FDDI -] Fiber Distributed Data Interface. ANSI X3T9.5. A fiber-based 
high speed LAN and probably the standard for a 100-Mbit/s token-passing 
ring network in the near future. Maximum 500 stations can be attached to 
the network, with up to 200 kilometers total distance end to end. The 
standard also specifies that there can be no more than 2 kilometers 
between active attachments.

\item[FRAME -] 	a packet of information ready to be sent to the network. 
Includes both the data and address of the receiver. The basic 'idea' 
behind any kind of asynchronous transmission.

\item[GSM -]	Groupe Special Mobile. 

\item[IEEE -]	The Institute of Electrical and Electronics Engineers, Inc.

\item[Internet -]	Internet is a worldwide computer network. Its 
development was started by the DARPA ARPANET - an inspection office of the 
U.S. defense Department. It consists of several independent networks which 
are connected together. Most of these networks are located in the USA and 
Canada.

\item[ISDN -] Integrated Services Digital Network. The public 
telecommunication network of the future. It has so called 'B'-channels 
(capacity 64 kbit/s) and 'D' channels (capacity 16 kbit/s). The D-channel 
will be used mainly for signalling purposes whereas the B-channels will be 
used for the transfer of the actual information. ISDN is fully digital which 
means that you for example do not need a modem with your computer when 
transmitting data. Moreover it allows, for example, that you can be connected 
simultaneously to two or more places with your normal extension.

\item[ISO -] 		International Standard Organisation

\item[ITU -]		International Telecommunication Union

\item[LAN -] 	Local Area Network

\item[MAC -] 	Media Access Control. Actions related to the transmission 
path. For example CSMA/CD.

\item[MAN -] 	Metropolitan Area Network

\item[MUX -]	Multiplexer.

\item[NMT -]	Nordic Mobile Telephone

\item[OSI -] Open System Interconnection. A reference model of the ISO. Has 
seven 'layers' by which a network developer can describe how his/her 
network is implemented. The layers cover everything from the physical 
medium to applications.

\item[PCM -]	Pulse Code Modulation. A modulation method used in digital 
speech transmission.

\item[SMDS -] 	Switched Multimegabit Data Service.

\item[SNA -]	System Network Architecture. IBM's standard for a common 
architecture for communication within a company's diverse equipment.

\item[TACS -]	Total Access Communication System

\item[TCP/IP -] 	Transmission Control Protocol / Internet Protocol.

\item[Telex -] 	Based on the public telecommunications (telephone) network. 
Not very useful in data transmission because of its slowness but is very 
useful when sending short messages because of its wide spreadness.

\item[Token-passing -] in a ring-based network (FDDI) encircles a 'token' that
reaches one station at a time. One station at a time can seize the token 
and begin to transmit. The station appends the token to the end of its 
transmission. So there can be several messages going in the ring at the 
same time.

\item[Token Ring -]   IEEE standard 802.5. Known as IBM's product since 1985. 
Theoretical speed 4 Mbit/s. Uses ring topology and point-to-point 
con\-nect\-ions.

\item[WAN -]	Wide Area Network

\item[X.25 -]	a standard for a packet-switched network
\end{description}
}

\section*{Glossary}

\begin{tabular}{ll}
conceivably  		&		mahdollisesti		\\
to span the globe	&	ulottua koko maapallolle	\\
ubiquitous		&		kaikkialla oleva	\\
accustomed to sth	&		tottua jhk		\\
diverse			&		sekava, kirjava		\\
\end{tabular}

\section*{References}

{\parindent=0pt\parskip=3pt
Dimitri Bertsekas/Robert Gallager, {\it Data Networks}, Prentice Hall Inc, 1987

Juha Rapeli,{\it ASIC's for Cellular Mobile Telephones}, Nokia Mobile
Phones Ltd Oulu Re\-search Center, 1990

{\it Prosessori} magazines, Erikoislehdet Oy, 13/1990, 2/1991
}
