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.
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.
The broad
activities involved in the outside plant planning are: 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. All or some of the following will be
required for preparation of a detailed scheme: 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. 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. Aim would be to reduce back-feeding and the
distribution network cost 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.4.0 Economic Planning Periods
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.
All up to date records
should be obtained from the drawing office and in turn the planner should advise
the drawing office of any discrepancies found during the survey. Maintenance
should be asked for details of any plant which has been giving undue maintenance
difficulties so that where possible these can be cleared up in the design of the
new scheme. Similarly, external works, should be consulted about any
construction difficulties likely to be encountered. In addition information on
road works either by Municipalities or other services which may affect the
locality should be sought.5.1 Plant provision policy
5.2 Planning for new areas
5.3 Planning relief to existing
network
Methods of distribution and provision of plant for new
development and new estates are fairly straightforward. However, developing
and/or providing relief to existing networks creates extra difficulties as a
"live" network already exists and proposed improvements or augmentation
require to be properly integrated with least disturbance to the existing
network.
6.0 Planning Main Cable
Network
7.0 Duct Planning
The system of laying cables in pipes laid underground with provision of manholes/ joint boxes at specified distances so that, without resorting to repeated digging, these cables may be operated for rectification of faults or for joining with other cables. Additional cables may be laid subsequently on the same route in near future. Ducts are planned for long term.
Ducted cable system eliminates all the disadvantages of directly buried cables and gives many more advantages. It acts as a primary protective cover for the cable and minimizes the sheath punctures. The number of interruptions and breakdowns of services are reduced considerably. The life of cable is increased due to reduced handling and better protection. Unnecessary digging of roads for frequent burial of cables is avoided, consequently there is saving to the company. Unarmoured cables are laid in the duct resulting in 25 to 30% saving in the cost of cables. Installation of cables becomes easy and quick and expansion programmes can be carried out more economically and quickly. Jointing operations can be done more conveniently. The joints, PTVs, loading coils, PCM repeaters can be conveniently located in the jointing chambers and are accessible for maintenance easily. Interference with other utility services like water works, electricity authority etc. is also minimized.
While planning ducts appropriate clearance should be given from other installations like gas or water mains, sewers and subways, electric lamp standards, traffic signal posts and other similar plants. Special care should be taken to maintain appropriate clearances from high voltage cables. If a desired separation is not possible a slab of concrete can be made between the two.
On completion of the duct between any two jointing chambers, or sites thereof, a cylindrical brush and an iron test mandrel shall be passed once through each way to test the duct and remove any foreign matter which may have entered. The size of the test mandrel and brush should be as specified for the particular duct. This will ensure that a minimum of acceptable cabling space is available in each pipe.
It may be useful to pressure test all the completed duct sections. Compressed dry air is filled and a desired pressure is created(say 275 millibars). If pressure is retained for sufficient time then there is no leak. In case of leakage the area around the leak shall be thoroughly cleaned with methylated spirits and the leak should be plugged with PVC cement. Retesting should be done after the cement has dried up. It is all the more important to test the sections of ducts leading into exchange buildings. This is to avoid entry of poisonous gases into the cable chambers.
Jointing chambers are constructed on the duct routes to provide access to ducts and also provide space for housing ancillary plant items like regenerators. Jointing chambers can also be provided where severe change in direction is needed in a cable route. This could be at a road junction. Jointing chambers may also be need when a change in formation of duct is desired. These could be constructed in the form of joint-boxes or manholes. They could be made either on the footway or carriageway. They can be made of reinforced concrete or unreinforced concrete. All manholes are designed as carriageway structures and are used in footway and carriageway situations. All manholes will be of reinforced concrete construction. The jointboxes are constructed for carriageway or footway and can be unreinforced or reinforced. If in a situation the standard manhole does not serve the purpose then the dimensions are changed to suit the requirements.
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.
When a pipe has two or more cable(including the one proposed) then two situations may arise. Assuming that there would be three cables in all and the diameters of these are d, d1 and d2 then:
Case 1: d is the largest cable and sum of diameters of the smaller cables(d1 and d2)
is less than half the diameter of the largest cable then the space taken is only d mm. It is presumed that the smaller cables, d1 and d2, fit into the spaces between the large cable and the walls of the pipe as shown in the diagram
Fig 1 One large and several smaller cables
Case 2: If sum of the diameters of smaller cables (d1 and d2) is equal to or more than half the diameter of the largest cable (d) then the space required for these cables would be 0.7 x (d1 + d2 +d3)
Fig 2 Cables of similar sizes
With regard to subscribers’ loops, 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 relates to 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 equipment. This loop resistance consists of line resistance, transmitter resistance and resistance of the relay in the exchange. The administrations policy may therefore fix a safe limit from exchange to the DP for the purpose of design of the network.
The transmission loss affects 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.
The planner must strives for "the most economical provisioning 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.
