THE OPTICAL FIBRE DISTRIBUTED CONCENTRATOR (OFDC)

ABSTRACT

A.B.A. Zeh,

Divisional Manager, SDA, Box 7055, Pretoria 0001.

South Africa

The following paper looks at several products that are used for rural communications with emphasis on one particular product, the OFDC. The first part of the paper looks at the problem of providing cost effective rural communications and touches on various 'possible' solutions. The OFDC solation is then discussed in more detail. Emphasis is placed on the available features and services as well as the cost effectiveness of the solution,

INTRODUCTION

Rural communications is a problem that faces most telecommunications administra- tions in the developing world. The major problem in providing a service to the sparely populated areas is the cost of the service. There are many different environments that have to be catered f o r and to date there is no single soiution to the problem. Different solutions are more amiable to different environments. In the following section some of these solutions will be briefly discussed.

POSSIBLE SOLUTIONS

The solutions that are most often proposed for rural communications are:

Multi party line

This service heis the advantage that only one physical carrier is needed tu provide service to a group of, normally 10, users. The disadvantage, however, is that if one user is busy none of the other parties can use the system. In the case of an automated party line (APL) the method of originatincj or receiving calls is improved by the provision of user direct dialling and selective user ringing. The basic disadvantage of one call at a time still exists.

The multi party line system is used in remote areas for the provision of low density traffic communications . Point to Point/Multi-Point Radio

In, this situation the basic carrier of information is a radio system. Many sophisticated systems are available, e . g . RUTEL , IRT1500/2000. The advantaige of the system i s the relative ease of being a b l e ta provide! service to the subscriber, shce all that is required is the provision of a radio transmitter in the environment. In the case of multi-point radio a certain amount of cabling still needs to be provided from the receiver point to the individual telephones. A problem with the system is that line of sight is requireld and the cost is often relatively high. Also, in the case of environments that need high traffic rates the

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transmission bandwidth is limited.

This system is used in those areas that are sparsely populated and the terrein allows a wide coverage area.

Cellular Radio

This solution is very similar to the previous one. The difference is that in this system the possibility is given to roam from one base station coverage area to another. The special case of fixed point cellular network, whereby a fixed base station covers a large area with a number of mobile users that do not have roaming facilities is very suitable where it is difficult to provide infrastructure, e . g . in Game reserves etc.. The disadvantage is that this service is expensive to provide especially if the country does not already have an extensive cellular network.

Satellite Network

In those areas where coverage of a very sparsely populated area is required or where the user, in a very remote area, does not have a fixed base the concept of mobile satellite communications has become a possibility. The cost is very high but the reliability is good.

Alternative

It is not proposed in this paper to compare the various systems to each other and to establish which may be the best. As can be seen from above, various systems have applications in different environments and there is no system that will cover all different environments, traffic and feature requirements or cost models. In the rest of the paper a different system is presented

that allows one to treat the user as a normal user, i.e. as if the user was connected to a digital main exchange directly, and that has certain cost advantages. This system is known as the Optic Fibre Distributed Concentrator (OFDC). This system is very suited ta medium density areas and comes in to its own when spare capacity is available on an existing fibre optic route.

OPTIC FIBRE DISTRIBUTED CONCEN?RZhTOR(OFDG)

System Overview

The OFDC system in principle is an extens'ion of the Siemens EWSD public exchange system. The OFDC connects directly to the digital line unit ( D L U ) or concentrator. This concentrator can be located either remotely or locally to the exchange. To allow the system to be independent of the EWSD system it can also be connected to a Small Digital Exchange ( S D E ) . The SDE is a stand alone autonomous exchange that caters for about 930 subscribers. The various system components of the OFDC systems are shown in figure 1.

Central Office Terminal(C0T)

The OFDC interfaces to the concentrator via a 2 card solution. The one card is the Optic Fibre Line Termination Equipment (OLTE) and the other the Central Off ice Terminal (COT). The COT takes care of the processing needed and the multiplexing of various data streams into the optic fibre frame format. Duplicated COT'S are provided for redundancy purposes. The OLTE provides the electrical to light conversion interface. The fibre is based on a 1300nm monomode fibre. The

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F n t s r f n r e r an aiso G r j r T e a batceq backups ?'he distance multimode fibre if so required. between RT's is 1.6 k r i . if a

standard PIN-driver i s used. Optic Fibr_e Format_ This can 1,s increased i ,O 3 0 krm

if a LASER driver is used, but this has an impact on the cost

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o r i m a t i > w

8 to 64 subcribers ," per remote termin

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L o c a l O M T ( o p t i o n a l )

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Figure 1: OFDC Systeni overview

T h e optic fibre frame format is based or! a 18,432 Mbps transmission rate. T h e frame consists of a superframe with f o u r subframes, each of 4,608 Mbps, Two subframes are used for 120 channels of speech and associated control data. T h e other two-f rames provide a possible 120 channels for leased lirie data transfer. These c a n n e l s can be used for point to p o i n t tributary data were each user can be provided with n x 64 kbps w i t h a maximum 02 2,048 Mbps data traffic.

Remote Terminals (RT)

The interface to the optic fibre r i n g is called remote terminal ( R T ) . The system can cater €or up to 6 3 remote terminals. Each HT can have one of three sizes, 8, 32 or 64 subscribers. The various R T ' s can be either mains or solar powered and are provided with

of the drivers. The Subscriber L i n e interface modules (SL&'s 1 used to connect the users to the R T ' s i s identical to those that would have been used at the concentrator, if service had been provided direct f r o m the DJ.,U and thus this does not r e q u i r e additional interfacing cost. In those areas where t h e provision of an optic fibre loop is a problem, the cable between RT's can be replaced with ai microwave l i n k . T h i s link is based at this time on a

protocol. standard G.703 Investigations a r e taking place to have a specialized interface developed for this microwave interface to allow the cheapest connectivity.

Maintenance I_- Interface

The COT is connected to either a personnel computer or a modem. This connection is used for maintenance, to set up the

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initial system and for configuration of the optic fibre cable and remote subscribers. Both the EWSD and the SDE have built in test robots, that is used from the maintenance of the subscriber loop. The various RT’s can , optionally, be provided with these test units. The concentrator is normally equipement with a three module test unit, however, to save cost and space only two of the three modules are catered for in the RT. The test units in the RT will react transparently to test commands from the EWSD mother exchange in the case of the EWSD system. Those tests that cannot be catered for due to the reduction in the number of modules will get acknowledged in such a way that the EWSD maintenance personnel will not get unexplainable data. The tests that cannot be catered for concern the transmission parameters and are not considered to be that important because of the digital nature of the optic fibre loop to the RT. The EWSD or SDE does not cater for the additional tests required because of the Optic Fibre link and this will still need to be done through the communication link either on the local PC or remotely on a modem.

Features

As ststed before, a major advantage of the OFDC system is that the users on the system have the same features as if they were connected to the digital exchange directly, i.e. sophisticated features like call forwarding, do not disturb etc.. ISDN facilities can also be provided. The traffic handling capacities are the same as for the concentrator, i.e. 0,l Erl/line €or low density and 0,2 Erl/line for high density concentrators. The fibre provided can be configured in a redundant loop or star configuration. In the redundant loop configuration it will be

possible to have automatic switch over to allow as little interruption to service as possible when a fibre break occurs.

In conclusion the proposed OFDC system will provide the user with the same service and facilities as if he were connected to an exchange directly. To provide this service, however, is still not cheap. The major cost is the optic fibre cable. However, there where spare fibre is already available the cost is significatly less. Also once the fibre has been installed it becomes very cheap to provide service to additional subscribers. It must be stressed again that the system will not be used in all situations, but we are confident that this system can be used effectively and cheaply in many applications where at this time it is very difficult to provide service.

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