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This section deals with the planning issues involved in the access networks. The accent is more on the traditional network with appropriate mentions of the new technologies. Planning issues of new technologies in the access network would however be taken up in another section If you wish to go back to the introduction page click on the link here
A large part of investment made in telecommunications is made in the subscriber or access part of the network. This investment, in many cases, may exceed fifty percent of the total capital investment made in a local exchange area. With such large investments at stake and increasing subscriber expectations, how does one continue meeting subscriber requirement in an effective manner? In order to continue meeting customers requirements in an effective manner it becomes imperative to properly plan networks based on a sound forecast using a scientific approach.
2.0 Flexible Networks
We saw earlier that the copper based access network may have one or two flexibility points a.k.a the cross connection points(CCP). The first one from the exchange is called the primary CCP or cabinet and the second one, if provided, is called the secondary CCP or pillar. The network therefore gets divided into two or three parts viz., primary, secondary and distribution. In many networks only cabinet exists and therefore we have only two parts of the plant: primary and distribution.
Any pair in the cable from a DP to a cabinet can be connected to any pair of the cable between the cabinets and the exchange. In this way, all smaller pair groups from the DPs (distribution cables) can be combined to form larger pair group to the exchange (primary cable). Distribution areas are smaller(tens of subscribers) and the demand can fluctuate from the forecast by a large percentage. Considering the whole cabinet area, however, the fluctuations largely balance out and considerably smaller number of spare pairs in the primary cable section is necessary.
Some of the advantages of having flexible networks are: sections of the network can be developed independently, making unexpected situations easier to handle. Flexibility obtained by the use of cabinets permits a high cable fill of nearly 90% for the primary cables. Cable fault localization is facilitated by the possibility of disconnection and testing at the cabinet.
Some of the disadvantages of this system are: cabinets are unsuitable for highly humid areas with accompanying risk of low insulation. Frequent handling of a cabinet's jumper field and consequent liability of faults can not be totally eliminated.
Advantages, however, far outweigh the disadvantages.
3.0 General Steps for
outside plant planning
The broad
activities involved in the outside plant planning are:4.0 Economic Planning Periods
When cable is laid it is necessary to provide some spare capacity to meet the future requirements. The number of years for which provision is made in advance is called the planning period. If a cable is provided for a long planning period, the initial cost per pair becomes less but considerable cable capacity remains idle for a long time period and such provision becomes uneconomical. On the other hand, if the cable is provided for too short a planning period, the cost per pair becomes very high and the cable provision again becomes uneconomical. There is an optimum period of planning for provision of cables. The economical planning periods for the primary network is short to medium term while that for distribution network is long term. Generally for bigger systems where the demand/rate of growth is high, it is not practicable to have long planning periods in view of uncertainty in forecast, the high capital cost involved, large scale introduction of fibre in the network and technological advances in the access network. It is therefore becoming increasingly common to plan the primary network for 3 to 5 years and distribution network for a 5-10 year forecast.
5.0 Planning Distribution Cable
Network
The necessity of a distribution scheme results from the need
to
While it may not always be possible to do so, it should be a planning objective to ensure that schemes are planned such that plant is provided in advance so that service can be given on demand. In many cases, information on new development is acquired well in advance and as such the preparation of a scheme to meet the demand should pose few problems.
Information required
All or some of the following will be required for preparation of a detailed scheme:
5.1 Plant provision policy
The policy when designing relief to an existing distribution area is the overlay principle. The existing network should be disturbed as little as possible except for requests to uplift existing plant which is or likely to cause maintenance or construction problems. Pairs' changeover should be avoided unless it can be established that it would be economic or would be preferable engineering method. Sufficient pairs should always be provided to enable exclusive service to be given to all customers. As with the main cable network, continuous monitoring of the network will indicate which parts need relief. Individual relief requirements will form a series of schemes that will be executed when 'triggered' by demand or when resources are available.
A section would correspond to one DP area and tenancies should be grouped into DP areas such that a standard size cable will meet the 10 year pair requirement calculated in accordance with the pair provision norms. All the sections may not require DPs to begin with and one DP can serve more than one section. As the area develops, more DPs will be opened and subscribers suitably transferred. DP cables may be grouped into larger cables in the course of proceeding back towards the cabinet. Normally all DP pairs are connected to the cross-connection point but the number of through pairs from residential DPs which have flexibility, (e.g. overhead radials), may be reduced to the forecast number of tenancies where this will result in smaller cables towards the cabinet. The overall aim is to achieve a clean layout. Joints with an unequal number of pairs in and out should be avoided wherever possible.
Where additional pairs have to be provided to meet an increased demand in existing areas, problems are more difficult. We need to answer questions like "How many extra pairs to provide?" Or "Which other DPs should be relieved at the same?" or type and size of DPs to be opened, location of DPs, transfer of DPs from one cabinet to another, diversion of pairs, etc. Many of these do not have such straightforward answers. It is possible, however, to predict the future exhaustion of DPs from an inspection of the spare pairs when considered in relation to the line plant forecast. Absolute accuracy is not so important as slight over-provision can not only be tolerated but also acts as a safety margin.
This calls for good engineering planning. The extreme "cure" for exhausting existing networks would be to completely replace them with new plant and recover the old. This radical solution is unlikely to be adopted except perhaps in areas where the existing network has deteriorated to the extent that there is a very high maintenance cost liability or it will not support new services. Normally such a course of action would be a totally uneconomical
The simplest and most economic way of providing service is by means of overhead distribution and this method should be used wherever it is acceptable and appropriate. However, in some residential areas, the quality of housing and nature of the locality may dictate, apart from Town Planning agreement that an underground distribution system is required. The method of providing service cables and distribution cables to the tenancies concerned is known as under ground radial distribution. The service cables, are laid from the house or villa to the footway and then along the footway to a common point where they are jointed to the distribution cable in a sleeve joint. Radial systems give a good service reliability, are easy to maintain and have an accessible test point inherent in the design.
Multi-story residential and business blocks should be served by underground cable terminated within the building on an internal DP.
6.0 Planning Main Cable
Network
The need for a cable scheme is indicated by the anticipated shortage of cable pairs between the exchange MDF and one or more cabinets or direct DPs. When planning relief to the exhausted area, consideration should also be given to relieving any other cabinets on the common cable route where existing plant is insufficient to cater for the planning period which, as mentioned earlier, is 3 to 5 years for the primary cable.
Each relief scheme should be based on a line plant forecast of growth for the area concerned. Confirmation that a forecast is up to date should be obtained before it is used in the design of a scheme.
For this purpose all existing and forecast connections should be considered with an addition for miscellaneous included to give the total forecast connections. The additional pair requirement is then calculated by subtracting the existing pairs from the forecast connections at the end of the third year from completion of the scheme or for the 5 year planning period.
This process is repeated for all subsequent relief schemes so that all unforeseen changes in the forecast growth may be taken into account. Thus each successive relief will be considered as the initial installment of a new relief sequence.
The objective is a clean layout with all conductors connected through at joints, giving full utilization of the plant provided and a simply executed job. Cable should be provided in the longest possible lengths consistent with other considerations. Where joints are unavoidable their positions should be selected having regard to economics and the work location. In some cases economies can be achieved by having parallel cables over a section of route so as to avoid costly enlargement of existing jointing chambers to accommodate extra large joints.
As in the case of planning distribution cables, maintenance staff should be consulted to ascertain whether any plant is giving particular problems so that, if possible, these can be cleared up on the proposed scheme. Schemes must be planned economically but planners should always bear in mind the finished layout and future maintenance. Joints should be accessible and duct-ways not obstructed. Joints in any event are a maintenance liability and their numbers should be kept as low as possible.
New cabinet may be required if the existing cabinet in any area does not have enough primary termination space or distribution termination space as dictated by the planning requirements. In such a situation the existing cabinet area is bifurcated and suitable transfer of the existing DPs is planned.
The location of the cabinet will be at a place where a number of distribution cables of this area meet. Generally the location of the cabinet will be at the beginning of the cabinet area towards the exchange side to avoid any back feeding of the distribution cable. In this way, the number of cabinets for the exchange area under study as well as their locations are finalised.
Some considerations in locating a cabinet are as follows.
a) The function of the cabinet is to provide flexibility. Unless there are 5 to 6 DPs in a block, a separate cabinet for the block may not be justified instead the DPs can be served from the cabinet in an adjoining block. In such cases, the entire block must be included in adjoining cabinet area.
b) The best point for location of cabinets is on the primary cable route after it enters the cabinet area.
c) The cabinet area should be so demarcated that no major road crossings are encountered while laying distribution cables to feed the DPs of the area.
d) Primary cables being ducted are safer as compared to the distribution cables which may be directly buried. The distribution cables are the weakest element in the system. Therefore, the distribution cable length from cabinet to the last DP should be the minimum possible keeping in mind the objective of reducing back feeding as well.
e) The cabinet should not be target for any vehicular traffic.
f) It should be away from the electrical cabinets.
g) It should be away from the kerb stone i.e. adjacent and parallel to the wall.
h) It should be so located that even on heavy rainy days the lowest connection strips do not get submerged. For this local enquiries may have to be made to find out maximum flooding level. If a cabinet has to be located in water logged areas then it should be on a suitable foundation.
Normally larger cabinet areas are more economic than smaller ones so the aim should be to plan for these rather than a greater number of smaller areas.
Full regard to safety aspects is essential. Economic considerations should not outweigh the safety of staff, public and plant. Where possible, work in dangerously situated jointing chambers should be avoided. This may mean giving thought to the construction of a new jointing chamber or providing extra joints or even seeking an alternative route.
The site of all proposed work should be surveyed and changes made to initial proposals if necessary. The planner should "think through" the works operations and if appropriate discuss any problems with works staff. If required, the planner should give guidance during the work and be prepared to modify the proposals to suit site conditions.
7.0 Duct Space Planning
After determining the cable required as per the planning policies it is necessary to ensure that duct space is available. The existing duct space is said to be ample when space is available for the proposed first and second installments and in this case the initial cable will be provided without further study. The duct space is limited if the space will accommodate a larger size cable than calculated but its provision would preclude any further relief cable being drawn in. Space is just sufficient if the space is available for the proposed initial cable and none remaining. Space is inadequate if it is insufficient space even for the initial cable.
If duct space is ample or just sufficient then the initial cable installment is provided. In case of limited duct space cable can be provided to fill duct space. In case of inadequate space, rearrangement to make space, providing smaller cable, using alternate route and constructing new ducts are the options to be considered. In case of no space new ducts need to be provided.
8.0 Transmission and Signalling
Limits
With regard to subscribers’ loop, mainly two factors affect the establishment of call and quality of speech. One factor is called as "Signalling Limit" and the second factor is called as "Transmission Loss limit". The former is affected by the direct current (dc) performance of the line while the latter is affected by the alternating current (ac) performance of the line.
The signalling limit impairs calling, dialing or ring trip functions etc. The d.c. loop condition is used to indicate origination of call by a subscriber and d.c. loop interruption pulses are used to indicate dialing in many cases. However for call originating condition, the d.c. loop resistance decides the signalling aspect. Electronic exchanges are capable of accepting loop resistance in the region of 2000 ohms. This limit differs with different exchange equipments. This loop resistance consists of line resistance, transmitter resistance and resistance of the relay in the exchange. The administrations policy may therefore fix an upper limit from exchange to the DP for the purpose of design of the network.
The transmission loss impairs the level of the speech. The end to end loss is important to determine the quality of speech from calling subscriber to called subscriber. The end to end section consists of various components such as calling and called subscriber loops, junction networks and long distance network. Different values of transmission losses have been allocated to different sections. The maximum transmission loss limit prescribed for subscriber loop is 8 dB at 1600 Hz. Examples of loss parameters for normally used cables are given below:
For Private automatic Branch Exchanges (PABXs) the set up involves exchange line upto the PABX and extension line from PABX to the subscriber. The PABX and its extensions consumes about 150 ohms and therefore the parameter used for the exchange line should be reduced by this amount. The dB loss for the PABX is about 1dB and planners should keep this in mind while planning the DP serving PABXs.
10.0Summary
The planner must strives for "the most economical provision of plant, to meet service requirements, consistent with sound engineering principles". We will see more aspects of planning of the access network when we read about new technologies.
