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Access Network Planning and Technologies

1

Introduction to telecommunication networks

1.0 Prologue

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 i.e. or the network connecting the subscribers and the exchange or the access network that has been traditionally simple copper based, point to point, passive network is now becoming increasingly complex. It therefore 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:

Make interconnection of end users possible
Facilitate exchange of information in a form desired and suitable for their terminals
Send and receive signals to/from the end users to facilitate the establishment, maintenance and dismantling of connections
Provide additional services such as wake-up calls, billing information etc.

It is very important for network planners to pay attention to the technical evolution of telecommunication systems. This would to enable proven new technologies to provide high quality telephone service and meet new demands of telecommunication services. Owing to prospective development of these technologies and increasing demand for new services other than telephony, telecommunication networks are changing from partly analog to fully digital.

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 fast.

1.1 Telecommunications network and layered model

Subscribers of a local area(municipal area or calls within which area chared at the same 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 multiexchange area when the number of subscribers are large and one exchange cannot effectively and economically serve all the subscriber. In this case 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.

SINGLE EXCHANGE AREA MULTI-EXCHANGE AREA

In a multiexchange area, however, the subscribers connected to different local exchanges can only communicate if the exchanges themselves are linked!. These links between 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.

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 exchanges in the local area. An example of such a network is shown below.

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.

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.

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

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 transit exchange as it collects and redistributes traffic.

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 links or submarine links or satellite.

The links among national switches, among international switches and between national and international switches are called trunks.

The telecommunications network can be described succicintly by a layered model consisting of the following layers:

The Switching and Services layer consists of all the switching nodes, local as well as transit. It also consists of any other equipment and like computers and software used to provide services to the customers.
The Transport Layer represents the links among the nodes and provides the medium and systems to carry the information from one node to the other. These are junctions and trunks. Junctions are links between the local switches and local and national switches. Trunks are the links between the national switches, the national and international switches and between the international switches i.e. the long distance network. The long distance or trunk network is composed of multiplexed channels of varying capacity connecting the National Switches and the International Switches. The trend has been to move from point-to-point links using Plesiochronous Digital Hierarchy(PDH) towards advanced networks with built in controllability based on Synchronous Digital Hierarchy(SDH) technique. The two most important trends in the long distance networks are digitization and introduction of fibre-optic technologies. These developments have reduced the transmission cost per channel-kilometer and improved the quality.
The Access Layer represents the access network that links the customers to the local switch.

1.2 Network Architectures

The two basic patterns of interconnecting switching centres are mesh and star formations. Actual networks are a result of combination of these two 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.

Digitalization and modernisation of networks

1.3 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.

1.4 Service development trends

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..

1.5 Bandwidth requirements

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.

1.6 Network requirements

Availability of maximum possible bandwidth
Delay in deployment of the network and provision of services should be minimum delay
Quality of transport to match the services
Reliability/availability
Interface
Ease of maintenance, low fault rate
Access network connectivity at high data rates

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. 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.