Need For duct sharing
Having determined that it is feasible to share ducts between a cable and a telecommunications service, BAH has calculated what percentage of the horizontal network of the cable television requirements can be met by the existing duct network.
First BAH evaluated how many kilometres of duct space was needed for a cable service in Hong Kong. HKT has estimated that it would need to put in coaxial cable for television service on approximately 85% of all of the existing ducts by the end of the build period in 1994, for a total of 2890 kilometres of duct. BAH believes that this estimate is rather high, primarily because of the high degree of routing redundancy within the existing network. As an example, maps of ducts provided by HKT show that in many cases there are ducts along both sides of many roads. This amount of redundancy is not needed for a cable service. BAH has estimated that 2400 km of duct (which includes the amount of duct needed for the building lead-ins) would be needed if HKT were building the cable network using its existing duct system.
Spare capacity in existing duct network
BAH has also conducted engineering studies of the maps provided by HKT to determine the percentage of spare capacity which could be used by HKT for its proposed cable television network. The methodology for assessing the effective spare capacity within the duct network and our general findings are presented below.
As a first step in the analysis, BAH gathered data to understand the age of the duct network. HKT did not have records on the ages of the ducts, but was able to approximate a breakdown of the ages by examining the age of the local exchange and the number of ducts which serve that exchange. In the absence of specific records, BAH believes that this method of determining the age of the ducts is relatively sound.
The findings were that approximately 22% of the ducts were over 20 years old and 26% of the ducts were between 15 and 20 years old.
This finding is particularly important for two reasons: firstly, it should be expected that the older ducts will be closer to full capacity and as such not so easily suitable as a conduit for cable television; secondly, since the economic life of a duct is approximately 25 years, it is important to understand how many of the ducts will be expected to be replaced (and possibly upgraded) within the time frame of the installation of the cable television network.
On the first point, BAH examined the maps of the Mongkok area, which has ducts which are over 20 years old. From this BAH found that although there were several routes which are too congested for cable television there was sufficient capacity in the ducts to provide cable service assuming that the ducts are actually shared between a cable television and telephone service. It is not important to determine if there is spare capacity in all routes, but rather whether or not there is sufficient spare capacity in the routes needed for cable television service.
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