Initially, the residential PSTN traffic that is expected to originate in each node of the network must be calculated. To achieve this there are a number of steps that the planner must follow.
3.4.1.1 Adjustment of Traffic Generation and Penetration Factor
After having decided the cities that are initially going to host the nodes of the network in the area of interest, their population figures must be adjusted to take into account the remaining population of the area of interest. The planner decides the nodes in which the PSTN traffic, generated by the rural population outside the cities where the nodes will be hosted, is going to appear.
If there is information (from the operator or other sources) about the existing, or future access network that is linked to each node of the area of interest then we can easily add the population related to each access network to the population of the city which hosts this node. For the case where no information exists, the network planner has two options depending on the time and recourses available.
If enough time is available then the statistics of the population and the geography of the area of interest should be carefully examined. The population figures of the smallest, in terms of geographic granularity, sub-areas (e.g. parishes) within the area of interest must be found. The population figure of each sub-area should be added in the population of the nearest city which will host a node of the transport network. If the distance of a sub-area from two or more nodes seems to be equal then its population figures can be divided accordingly between these nodes.
If not enough time is available for the network planner to follow the above procedure (and this is usually the case), a uniform distribution of the rural population is assumed. The adjusted population in the cities where the nodes of the network are going to be deployed is easily calculated. If P is the population of each city. Sa is the
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sum of the population of the whole area and Sc is the sum of the population of the
cities of the area then the adjusted population P' in each node is:
p ' = ~ p
Sc
After having calculated the above, the number of residential phones must be calculated. This is derived by using a constant produced annually by telecommunication business researchers called the penetration factor. This varies from country to country and sometimes varies between areas of the same country (e.g. US). For the UK the penetration factor is currently 0.45. The penetration factor, multiplied by the population of an area, gives the number of phones in this area.
In the special case where the penetration factor is significantly different between the rural and the urban areas of the region where the network is going to be deployed, the adjustment of population must be modified. The number of phones using the penetration factor in each area must first be calculated, and then an adjustment of phones from the whole region to the selected nodes must take place. In this case the number of phones in the whole region is summed and assuming that the phones are uniformly distributed in the rural areas, the adjusted number of phones in the nodes of the network is calculated, following the same procedure as the one used for adjusting the population.
3.4.1.2 Market Share
As the next step, the network planner must know the expected market share for the specific operator. These could be based on the expectation of the operator on entering the market. The market share has to be considered, for the worst case scenario where other networks are going to compete for the same market. The percentage of market
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share expected (or guaranteed), multiplied by the number of phones in each node shows the number of phones in each node that are going to generate traffic for the network being designed.
3.4.1.3 Calculation of Residential Traffic in Erlangs
As a next step the Erlang factor must be given or assumed, in order to calculate the originating traffic in Erlangs [124]. This factor derives from the average Busy Hour
Call Attempts (BHCA) since it is the busy hour that generates the peak demand that
has to be satisfied [124]. The average duration of a call is another important metric which is also termed as the average holding time.
The physical meaning of the traffic measured in Erlangs according to ITU-T, is the average number of simultaneously busy circuits (average number of calls in progress) [124-126]. The number of Erlangs represents the traffic intensity. It is a dimensionless quantity named after the Danish A. Erlang, who first developed the principles of traffic theory [127]. Another view of traffic in Erlangs is that it represents the proportion of time that a phone is busy during a specified period of time (usually during the busy hour) [124]. Using this last definition, the Erlang factor can be measured as follows:
Let C be the average BHCA of a specific type of traffic (i.e. residential, business etc.), and let D be the average holding time during the busy hour. Having 3,600 seconds every hour, we can easily calculate the Erlang factor E for this specific type of traffic:
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If for example there are 2 busy hour call attempts generated from a phone, and the average duration of these calls was 120 seconds, then the Erlang factor describing this phone (or the number of Erlangs generated by this phone) is 0.066.
In North America traffic is sometimes expressed in hundreds of call seconds per hour (CCS). Since one hour contains 3600 seconds, 1 Erlang = 36 CCS.
The Erlang factor for residential traffic can vary according to the type of people living in each area. There are studies defining different uses of phones between different social groups. It is also possible that the operator might propose a different Erlang factor according to its expectations, beliefs or requirements. If no information for the Erlang factor is given to the network planner, then 0.06 is commonly assumed for residential traffic.
3.4.1.4 Creation of Different Residential PSTN Traffic Tables
Following the steps described above a table listing the originating traffic in Erlangs in the nodes of the currently being designed network can be generated. Not all of this traffic is destined to nodes of the network though. By assuming that other networks are operating in the area of interest a fraction of this traffic is going to terminate at a node of another network. In addition, a fraction of the traffic will be destined to termination points outside the overall network of the operator (far national or international connections). Therefore, before starting to quantify the traffic distribution in the network the amount of traffic that is actually going to circulate within the network needs to be calculated. The amount of the other two types of traffic also needs to be found.
The amount of traffic destined for other networks in the same area of interest can simply be calculated by using the market share again. By multiplying the originating residential PSTN traffic in each node with the market share factor, the amount of traffic that is going to originate and terminate within the network can be found. The
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remaining originating residential PSTN traffic is destined to other networks in the area of interest.
The percentage of traffic that is destined to networks outside the area of interest varies according to the circumstances. If no data is available, a very general case where around 10% of all traffic originating in a node is destined towards a very far national or an international end point can be considered. This figure is obtained by considering the existing average fraction of originating traffic with international destinations. Following these assumptions, the network planner ends up with three tables:
1. A table showing the traffic originating from each node going to another network in the area of interest.
2. A table showing the traffic originating from each node going to a far away destination point outside the area of interest.
3. A table showing the traffic originating from each node that is going to be distributed among the nodes of the same network.
Often the gateway(s) for all traffic leaving the network lies on a single node. In this case tables one and two can merge to a single table containing all the traffic that exits the network. Since this is not always the case though, the concept of the three different tables retained covers all possible cases.