If you are a telecommunications service provider and you have set up you first switch. What is the minimum requirement between the user equipment and your switch. Well you dont need a lot. You just need to link the customer premises equipment(CPE) to you witch with a pair of copper conductors.

Fig 1. Simple interconnection of subscribers to exchange

Though the above would work, in practice it gives some problems to the service provider. To say some would be an understatement. It gives probles in all the areas viz. Planning, Construction and Maintenance!

The planner of such a network does not have much choice. In fact he does not have much of role in such a network. Since each pair is laid on demand he cannot provide for future. There is no flexibility in the network. If another customer next door to the existing customer asks for a connection then another pair has to be laid right from the exchange to the customer.

If we lay a slightly bigger cables, say 10 or 20 pair cables upto the vicinity of a neighbourhood, then any nearby subscriber can be given service much faster by allocating on of the spare pairs. The multi-pair cable is teminated on a distribution point(DP)

Fig 2. Introducing distribution points

This makes life easier but only a little bit. Any of the 20 pairs going to DP1 or 10 pairs going to DP2 can be given to any of the customers the designated areas of these DPs. Laying and managing one 20 or 10 pair cable from the exchange to the DP may be a bit easier than dealing with individual pairs in these sections. This is definitely an improvement over the situation that we had before. However, consider a case where if all the 10 pairs of the cable going to DP2 get exhausted and there are more subscriber in this area while on the other hand the 20 pair cable going to DP1 still has half the pairs unused! This situation is not uncommon where pairs are available but not at the places where they are required. How can we make use of the extra pairs of DP2 area for customers in DP1 area?

The way we grouped several subscribers into DPs we could group a number of DPs and serve them with a common cable. We need to set up another point, a cross connection point, upto which we would bring a still larger cable than we brought to the DP and serve a number of DPs from that point. Let's see how it will look.

Now there is one larger cable going to the the crossconnection point(CCP). This CCP provides the capability of connecting any of the pairs of this cable to any of the DPs, in its area, thus offering great flexibility in pair management For this reason a CCP is also called a flexibility point. Introduction of a CCPcould be called a revolutionary change in the access network. A common name for this cross connection point is cabinet. An urban network would have a number of cabinets each serving a number of DPs. The cabinet area may have hunreds of customers while the DP areas would have tens of customers.

On a pessimistic note one could argue that now we have big cables going from the exchange upto the cabinets. We could for example have a 1200 pair cable going to one cabinet and say a 800 pair going to the other. Now if we have exhausted all the pairs of one of them then would it be possible to use pairs from the other for the customers in the exhausted area? Again a case of pairs being available in the network but not at the right place!

To make a larger part of the cable common to many more subscribers we could add one more cross connection point. Is'nt this going too far? How many cross connection points can we possibly have between the exchange and the DP? Theoretically as many as the designer wants but going by the international norms upto two such cross-connection points are used. If we did this the network would look like this:

The above structure has all the rudiments of the real-life access network. The first CCP from the exchange is called the Primary Cross-connection Point(PCP) or the cabinet and the cable linking the PCP to the Exchange is known as the Primary cable. The network of all the cabinets and the primary cables is referred to as primary network. The second CCP is known as the Secondary Cross-connection Point(SCP) or the pillar and the cables linking pillars to the cabinet are known as secondary cable. The cables linking DPs to the SCP are known as Distribution cables and together the distribution cables, DPs and other related infrastructure form the distribution network.

So what are the advantages of having cabinets (and possibly pillars) in the network instead of having direct pairs from the exchange to the customers? We surely have not incurred the extra cost of installing cabinets and pillars for fun! Let's enumerate some of these..

1. These flexibility points divide the network between the exchange and the DPs into sections as seen above. This gives the flexibility of planning and constructing different parts of the access network at different times and by different planners.
2. Any pair of the primary cable can be "jumpered" to any pair of any of the distribution cables in the same cabinet thus making a large part of the cable common to a larger number of subscribers. In two CCP networks the same flexibility is available on both the flexibility points
3. Planning of the network now involves providing pairs flexibly for a number of years. Larger cables ensure availability of enough spare pairs during the planning period to provide service on demand.
4. Construction of the plant is eased because we are laying large cables in place of laying one pair at a time. This saves on construction labour cost and time.
5. Maintenance becomes easier as cabinets and pillars provide good test points for testing different parts of the plant. Once the fault has been localized, only the affected part(Primary, secondary or distribution) cable pair can be replaced.
6. Cost of the network is optimized as the pairs are more efficiently utilized. Effort and labour cost for construction and maintenance are reduced. Cable cost comes down as per pair cost in the larger cable is less than cost of single pair cables or cost per pair in small sized cables.
7. Time taken to construct and maintain the flexible network is much less than rigid networks with no flexibility points.
   

   More on structure and components of access network...
   

   Or you could study one of these..

   1. Introduction to telecommunication networks
   3. Forecasting
   4. Planning access networks
   5. New technologies in the access network
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