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Lead-In Cable:-
This cable is installed by the carrier (ie: Telstra / Optus) and brings the service into the building. The particular size of the cable is negotiated between the carrier, developer, landlord at the planning stages. The number of pairs should be sufficiently enough for the type of services expected within the building.
Number of Extensions:-
In this diagram (above) you must consider the future expansion of businesses inside and plan accordingly. For example the initial number of extensions required maybe 30 to allow for existing 30 staff members. You must also allow for future expansion of staff levels (roughly 10 new people over the next year). With IT a main topic, the installer must also allow for extra modems through the CSS.
Type of extensions:-
There are two types of extensions available on PABXs and Key Systems. These are either 2 wire or 4 wire and you must select the correct type of cabling with installing.
Number of Carrier Services:-
You then must determine from the customer or equipment supplier how many carrier services are to be installed. Typically, the ratio of the carrier to extensions is shown below
| Number of Extensions |
Ratio of Extensions to carrier services |
| 0-10 |
2:1 |
| 11-20 |
3:1 |
| 21-50 |
4:1 |
| Above 50 |
8:1 |
Note:- that these are indicative only and need to be estimated more accurately based on the traffic in and out. That is, the number of outgoing calls and incoming calls estimated per hour. This calculation should be done for what is known as the peak hour.
Type of Carrier Services:
By type we are referring, PSTN lines, ISDN lines, Tie lines etc.
Example on calculating the size of the system link cable:
| Service |
Type |
Quantity |
Pairs |
| Telephone users (initial staff numbers) |
2 wire |
20 |
20 |
| Telephone users (expansion in staff) |
2 wire |
10 |
10 |
| Telephone users (initial staff numbers) |
4 wire |
6 |
12 |
| Telephone users (expansion in staff) |
4 wire |
2 |
4 |
| Facsimile Machines |
2 wire |
2 |
2 |
| Modems |
2 wire |
1 |
1 |
| Night bells |
2 wire |
2 |
2 |
| Operators Console |
2 wire |
2 |
2 |
| PSTN Lines |
2 wire |
8 |
8 |
| Tie Lines |
4 wire |
2 |
4 |
| Minimum Cable Size |
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65 |
Logically for the table above, you would install 70 Pair (50 & 20 pair cables) the 65 required plus some extra spares, as expansion has already been allowed for in the intial calculation.
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For the brand Commander and their type of systems here is a way to estimate extension lines...
(Example) Commander Systems 1236
(12) = Trunk lines ( (36) x 2) = 72 Extension Lines - Totalling = 84 so, you would round this up to 100 pair system link cable.
(Example) Commander Systems 1224
(12) = Trunk lines ( (24) x 2) = 72 Extension Lines - Totalling = 60 pair therefore you would use cabling for 60 pairs:)
| This page was last modified on 22 January 2010 and maybe out of date with regards to newer practices or tariffs at the time of reading this article |
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