智能电表集中抄表系统入户子系统 第4页
Most of the design activity carried out to create a scheme for a greenfield network is done between the primary cross connection point (PCP) (made available at the entry point of the scheme) and each customer (the D-side network). This activity takes place across and within four layers. The site developer’s initial geography and actions define the civil works layer, e.g. the position of customers and the layout of site trenching. The duct layer is contained within the civil works layer and consists of the duct pipes and intermediate boxes that serve to connect sections of ducting together. Enclosed within the duct layer is the cable layer. In this layer, groups of bearer pairs are aggregated into cable sheaths. Within the cable sheaths are the bearers themselves which deliver services to the connected customers. It begins with the site plans of the proposed site being received from a developer (typically on an A0 sheet of paper). Upon this plan, the network planner would lay out a duct network, and submit it for consideration to the developer. If the proposal is acceptable, a cable network can then be produced. Additionally, a jointing schedule is prepared to specify how each working pair on every cable is to be jointed back from each customer to the exchange. The duct network is comprised of a spine network and a lead-in network. The spine network consists of 90 mm diameter duct laid between the entry point to the site — usually a PCP — and access points which will subsequently contain the flexibility or distribution points (DPs) of the scheme. The lead-in network consists of 49 mm diameter duct and is used to establish connections between the DP housings and the site customers. Between the PCP and each DP, the main spine duct comprises sections of ducting which run between access points on the network. These access points are usually boxes which can either be pre-fabricated or built on site by the developer and they enable junctions to be created on the duct network. Junctions serve two major functions in the duct network. Firstly, they allow cables to be introduced into any part of the duct network. Secondly, they permit the introduction of points of flexibility or jointing into cables that pass through or are terminated at a box. Between the DP housings and each customer, the lead-in network consists of 49 mm diameter duct sections and optional branch elements (duct tees). Tees obviate the need to run individual sections of duct between each customer and the serving DP. Instead, cable can be introduced by rodding from each customer through an intervening duct tee up to the box which houses the serving DP. Figure 3 illustrates this process. There are, however, constraints on the use of duct tees which will be discussed further in section 6.2. In both the spine and lead-in networks, changes of duct link direction are achieved by using a variety of duct bends.  
Fig 2 Duct tees in the lead-in network.
Once a duct network has been agreed, a cable network can be introduced. The cable network also consists of two parts — the spine network which is located between the PCP and each DP, and the lead-in network which is located between each DP and customer. In reality, because cables are manufactured in discrete sizes, with 100 pairs being the largest deployed size in the D-side network, a number of cable networks may exist, with each network serving up to a maximum of 100 pairs. Beginning from the customer edges of the duct network, pairs are allocated based on the predicted level of demand per customer. At this point, a decision about the number of customers that will be served (and therefore the total number of copper pairs) and the position of the serving DP needs to be made. DPs are usually sited at the most central box in the cluster of customers. This position tends to minimise the run of cables between the DP and each customer. Once all the DP pair counts and positions are determined, the design of the spine network cable can begin (Fig 4). The goal in the design of the spine network is to provide the smallest number of cables to provide service to groups of DPs. In order to do this, the DPs are grouped in such a way that the aggregate number of pairs for each set of DPs does not exceed 100. Each cable that provides service to a DP group has its outer sheath exposed at the box which contains a DP, and then only the required number of pairs are exposed for jointing at the DP. Thus the cable spine network forms a logical bus. At the bearer level, of course, the configuration is a tree.
 Fig 3 Spine and lead-in topologies.
Upon completion of the cable network, a jointing schedule needs to be prepared. This document defines how connectivity at the bearer level is achieved for a given network. In order to minimise the risk of damage, the pairs on the outermost part of a cable are allocated to the most upstream DPs or customers, with the assignment of the innermost pairs being to the more downstream DPs or customers. Figure 5 is a simplified illustration of this process.

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