Telecommunications today is perhaps the fastest evolving field of study. It is continuously offering new challenges and opportunities to telecommunications network planners. The subscriber part of the telecommunications network or the network connecting the subscribers to the central office or the access network that has been traditionally simple twisted copper pair based, point to point, passive network is now becoming increasingly complex. In the present scenario it becomes imperative for the access network planner to be familiar with both traditional and new technologies, structures and methods as their plans would have a profound long term impact on how the network shapes up and meets the desired objectives..
The basic idea of telecommunication is the exchange of information. The information may include voice, text, data, image and video. A telecommunications network is therefore a system which can provide these services to a number of end users. From the end users' perspective, the network has some main tasks:
Demand and traffic patterns will change faster in the future than they do today. To cope with this, one important property a network should have is flexibility. Flexibility in simple term implies being able to provide bandwidth on demand. If bandwidth can be provided on demand then the network becomes capable of deploying and supporting a vide variety of services and with greater ease and speed.
Subscribers of a local area(municipal area
or calls within which area charged at a uniform rate) are connected to their
respective telephone exchange called local-exchange or local
switch or terminal exchange. The local area could be a
single exchange local area in which case all the subscribers are terminated on
the same switch or a multi-exchange area when the number of subscribers are
large and one exchange cannot effectively and economically serve all the
subscriber. In the case of multiexchange area each local exchange has its own
area called exchange area and the envelope of all exchange areas
would be the local area.
Calls among subscriber of the same exchange can be switched through without the need of any other kind of links except the pairs linking subscribers to this exchange.
In a multi-exchange area, however, the subscribers
connected to different local exchanges can only communicate if the exchanges
themselves are linked!. These links between the local exchanges are called junctions.
Whereas each
subscriber normally has one dedicated pair up to the exchange, the junctions are
dimensioned based on the traffic between exchanges and the grade-of-service
required. Variations on this classical theme are coming and we'll see them as we
proceed. Here
the diagram depicting the junction network also shows a new element viz. a
local transit exchange(TR). A transit would normally be used in bigger sized
network to ease traffic routing and cost-optimizing the junction
network. In this example the local area of the city is geographically divided
into two by a physical obstruction i.e. the river and the transit would make
it easier and less expensive to interconnect the local exchanges on both the
sides to each other as also the local exchanges on the other side to the
national switch. It
may be noted in the diagram that all the interexchange circuits are junctions.
The term trunk-junction has been used just to differentiate between the two
types of circuits shown. The national switch may be located in the same building
as one or more local exchanges but is still not a part of the local network All the international calls are routed through international
gateways to which the national exchanges would be connected.
International gateways of different countries would be linked through
terrestrial, submarine or satellite links.
The links among national switches, among international
switches and between national and international switches are called trunks.
A multi-exchange local area may have another type of exchange called transit or
local transit. A transit, unlike a local exchange, does not have subscribers
connected to it and therefore does not act as a source or sink for traffic in
the network. It only collects and redirects the traffic among the local
exchanges in the local area. An example of such a network is shown below.
The term local network is used denote the
network in a local area: This would therefore refer to the access network, the
junction network, the local switches and local transits(tandems), if installed.
What other links would be required if the subscribers of two
different local areas need to communicate? As we linked all the local exchanges
of one local area to each other, we could also directly link all the local
exchanges of one local area to all the exchanges of other local areas in the
country. This, though technically feasible, would be economically a disaster.
Telecommunications network therefore have another type of exchange called national
switch or trunk automatic exchange. All the local
exchanges of one local area are connected to atleast one such switch. All the
national switches of a country are then connected to each other based on the switching
plan. A national switch is also a type of transit exchange as
it collects and redistributes traffic.
The telecommunications network can be described succicintly
by a layered model consisting of the following layers:
Three basic patterns of interconnecting switching centres are mesh, star and ring formations. Actual networks are a result of combination of these basic types.
A full mesh is one in which each node is connected to all other nodes. How many one way circuits would be required to achieve this? Well, for an n-node network n*(n-1) one way circuit groups are required or half of that if bothway circuits are used.
In star configuration all calls between any two switching centres of the same level are routed through a transit exchange at a higher level.
Rings connect switches in a closed path. Traffic can be
carried both in the clockwise and anti-clockwise directions. Rings offer more
reliability as they offer survivability in case of single link failures.
Digitalization of a telecommunications network is to introduce digital transmission and switching components in the network. The reasons were initially mainly economic but there are also other reasons, such as improved and extended services to the subscribers, improved transmission quality, improved operation and maintenance facilities etc.
The best economy and performance is achieved if both transmission and switching are digital and integrated (no A/D conversion required). However, since the existing telephone sets are analog and costs of replacing them with digital sets are substantial, the subscriber lines will take longer to get digitalized. When the switches and the links between them are digital we have Integrated Digital Network (IDN), which is the first goal of the digitalization process.
With increasing demand of broad-band services the subscriber network will see an increase in the fibre content and digitalization. The final digitalization of the subscriber lines opens up the future possibilities of connecting a wide variety of digital equipment at the subscribers' premises, such as data terminals, telefax, teletex etc.. We then have an Integrated Services Digital Network (ISDN), where telephone services, data services, video services etc are switched by one integrated telecommunications network.
The service explosion in recent year has imposed new requirements on the networks. In order to facilitate the planning of a simple, flexible and cost effective network that can offer high quality services, it is important to specify these services as well as possible . It will be necessary to have a much more service centered view of the telecommunications network and planning of thc network actually ought to start with a definition of service scenarios. When possible and needed services are determined for the strategic future point of time, the corresponding technical requirements such as use of CCS7, IN, B-ISDN should be defined. Some of the services that are available currently are Call forwarding, Call distribution, Queuing, Call back, Hot line, Free phone, Terminal mobility, Information service, Telefax, Data communication, Videoconferencing. It is important to determine whether the existing infrastructure would be suitable for mobility--terminal and personal, Bandwidth on demand, Multimedia services, distributive services, interactive services etc..
Bandwidth emerges as an important requirement. References are made to "Narrowband", "Wideband" and "Broadband". Estimated bandwidth requirements for various services are given below-
Voice POTS 64kbps GSM mobile 13kbps
Text Telex 50bps Teletex 2.4kbps Electronic mail <64kbps
Data Normal 2.4 - 9.6kbps File transfer 64kbps - 8Mbps LAN <=100Mbps
Image Facsimile Gr 1-3 2.4kbps Facsimile Gr 4 2.4 - 64kbps CAD/CAM 64KBPS - 34Mbps
Image/Video Simple Videophone 64kbps Normal Videophone 2Mbps Videoconference (C) 64kbps - 2 Mbps TV(C) 34Mbps HDTC(C) 140Mbps HDTV >140Mbps Note: (C) Indicates Compressed.
The trend today is toward increased use of optical fibre communication systems based integrated services digital network. The infrastructure of the access network comprises ducts, cables and associated equipment and represents considerable capital investment for the network operator. Changing this infrastructure requires long term decisions. One solution to avoid development of new, initially costly, infrastructure is to use radio access. However, to serve a more demanding market, cable oriented solutions are required as their long run costs are lower and capacity higher. This means in practice optical fibre solution. By using sophisticated coding techniques, bit rates in access of 2 Mbps can be carried on copper, offering possibility that a number of future services will be delivered over the copper network. In the longer term only the optical fibre and to some extend radio will have the bandwidth capability that can deliver an access network which will meet all future service requirements. Unfortunately the cost of fibre systems is high but Passive Optical Networks(PONs) offer imminent possibility that fibre systems can be deployed economically not only to small and medium businesses but also to residential customers requiring broadband services.
You may be interested in one these...
2. Structure of the Access Network
3. Subscriber Demand and Traffic
Forecasting
4. Planning Access Networks
5. New Technologies in the Access Network
