A telephone network is built in layers: each subscriber has its own line to a local exchange, exchanges are joined by shared trunk circuits sized to the traffic between them, and a numbering plan and a directory tell the system where each number lives. Joining two 100-line exchanges takes a handful of trunks, not the 19,900 wire pairs that wiring 200 phones to each other would need. Decide the layers, the numbers and who keeps the directory before laying wire.
Part of Telecommunications
The design of a multi-exchange telephone network: subscriber lines, trunks, numbering, power and administration, with the arithmetic for sizing the links.
The Architecture Is Its Layers And The Rules That Join Them
A single exchange is a star: every subscriber line runs to the centre. The next step is not a bigger star but several, connected by trunks.
- Subscriber loop. The pair from a telephone to its exchange. It is limited to about 1,500 Ω of loop resistance, which is about 8 km of 0.5 mm wire (Long-Distance).
- Local exchange. Switches calls among its own subscribers, either by a person (Manual Exchange) or by machinery (Automatic Switching).
- Trunks. Shared circuits between exchanges, each carrying one call at a time. They are the part that gets expensive on long routes and are where repeaters and carrier systems earn their keep.
- Numbering and directory. The plan that maps a number to an exchange and line, and the written list of who has which number.
- Power and signalling. Which batteries feed which loops, and how ringing and dialling cross from one exchange to the next.
- Test and repair. Who can test lines, with what authority, and what is logged.
Switching Systems offers a guide to scale: step-by-step suits about 10 to 200 subscribers and crossbar 100 to 10,000 (Switching Systems). Past one exchange, a second layer is justified by distance as much as by count.
Trunks Are Sized From Traffic, Not From The Number Of Subscribers
Mesh wiring grows as N × (N − 1) / 2, so 200 phones wired directly need 200 × 199 / 2 = 19,900 pairs. Two exchanges of 100 lines each use 200 subscriber lines plus a trunk group between them.
To size the trunks, measure how much of the traffic leaves the local exchange. The 16.7 erlangs generated by 100 subscribers each talking 10 minutes in the busy hour (100 × 10 / 60, per Manual Switchboard) is the total. Assume, for illustration, that 10% of it is outbound: 1.67 erlangs. The standard Erlang B recursion for 1.67 erlangs gives:
- 5 trunks: about 2.1% of calls blocked
- 6 trunks: about 0.57% blocked
So 6 trunks keep blocking below the 1% that Step-by-Step Switch suggests as engineering practice. The 10% is an assumption, not a measurement. Count real inter-exchange calls over a week, then size from the count.
A Numbering Plan Is A Piece Of Architecture
With two-digit numbers one exchange reaches 100 lines. Add a leading digit that names the exchange and the same two digits repeat at each: one exchange digit with up to 10 exchanges times 100 lines gives 10 × 100 = 1,000 numbers, as the 1,000-line three-stage step-by-step exchange in Step-by-Step Switch does. Write the plan down before the first number is issued, since renumbering means re-labelling every directory, jack and tag.
How The Layers Are Joined Matters As Much As How Many
Topology Design works through the choices (star, mesh, hierarchy) and Reliability Design the failures. Topology Comparison gives the questions to ask for any network: how many devices, what budget, what reliability, any special needs.
Architecture Also Decides Who Controls It
Every layer has a keeper: the directory, the trunk routes, the exchange itself. Put the keepers on paper with the oversight each needs. The principles are in Institutional Design, and the privacy issues at the exchange are in Manual Exchange and Automatic Switching.
What You Need
- A map of settlements, route lengths and where poles can go
- A count of lines per settlement, now and in a few years
- A numbering plan and a written directory with a named keeper
- A traffic estimate from real call counts, for trunks
- A power plan per exchange
Common Mistakes
- Wiring exchanges to every neighbour. Mesh trunks multiply; use a hub or a ring first.
- Sizing trunks by head count. Size by measured erlangs.
- Numbering as you go. Patching a numbering plan later breaks every directory.
- One keeper for everything. The person who holds the exchange, the directory and the log can see and control the whole network.
Safety
Exchanges and long routes
Every exchange has a battery of commonly 24 or 48 V and a ringing source of 60 to 90 V AC, and every long route is a lightning antenna (see Lightning Protection). Specify protectors and earths at every building and repeater, and a ventilated, fused battery room.
What’s Next
- Learn next: Topology Design, star, mesh and hierarchy
- Combine with: Reliability Design, keeping the network up when a link fails