A switching network connects any two telephones through a shared exchange instead of a dedicated wire between every pair, so 100 telephones need 100 lines rather than 4,950 pairs. The exchange can be a person with patch cords, a chain of stepping mechanisms driven by dialled pulses, or a grid of crosspoints closed by a controller. Every design trades cost, capacity and who or what can see the call being set up.
Part of Telecommunications
How a telephone network joins any caller to any other through an exchange, and what the main kinds of exchange trade away.
Direct Wiring Explodes Quadratically, A Switch Grows Linearly
If every telephone had its own pair to every other, N telephones would need N × (N − 1) / 2 pairs:
- 10 telephones: 10 × 9 / 2 = 45 pairs
- 100 telephones: 100 × 99 / 2 = 4,950 pairs
- 1,000 telephones: 1,000 × 999 / 2 = 499,500 pairs
With an exchange each telephone has one line to the centre, so the same three networks need 10, 100 and 1,000 lines. The exchange does the pairing at the moment of the call, and the pair of wire that carried the call is released afterwards. That is why switching exists, as Switching Systems puts it.
Three Families Of Exchange Do The Same Job Differently
Manual. A person watches for a signal, asks who is wanted and pushes a cord into the called line’s jack. See Manual Exchange and Manual Switchboard. It needs no machinery beyond the board and handles whatever number of lines the operator can watch. A board of 25 cord pairs carries 25 calls at once.
Step-by-step. The pulses of each dialled digit directly step a selector up and round. Two stages with two-digit numbers reach 10 × 10 = 100 lines, three stages 1,000. See Automatic Switching and Step-by-Step Switch.
Crossbar. A grid in which closing one crosspoint joins one row to one column, so a 10 × 10 matrix can hold 10 calls at once, with a shared controller finding the path. See Crossbar Switch.
Later exchanges replaced the mechanics with electronics and then with digital switching of coded voice, but the problem each one solves is the one above.
Blocking Means An Exchange Can Say No
No exchange has as many paths as lines, because most lines are idle most of the time. When every path is busy the next call is blocked. Traffic is measured in erlangs: 100 subscribers each talking 10 minutes in the hour generate 100 × 10 / 60 = 16.7 erlangs. Manual Switchboard gives about 1.5% blocking for 25 cord pairs at that load. Recomputing with the standard Erlang B recursion gives about 1.4%, the same order. A designer chooses how many paths to buy for an acceptable blocking rate rather than for every line at once.
What You Need
- A count of lines you must serve now, and a guess at growth
- A busy-hour estimate of minutes each subscriber talks, for the erlang arithmetic above
- A person for a manual board, or the machining and relay skills for a step-by-step switch
- A numbering plan, written down, before the first call is dialled
Common Mistakes
- Wiring a mesh because it is simple. It is simple at 5 telephones and impossible at 50.
- Sizing paths for every line. That wastes most of the board; size for the busy hour.
- Treating the exchange as only equipment. Whoever stands or maintains the centre sees or can see who is calling whom, which is the subject of the next two articles.
Safety
Ringing and battery at the exchange
An exchange concentrates the ringing generator (60 to 90 V AC from a magneto) and a central battery of commonly 48 V. A lead-acid bank can also release hydrogen while charging, so ventilate the battery room and keep sparks away from it. Use a fused feed to every board.
What’s Next
- Learn next: Manual Exchange, the first exchange and its social consequences
- Combine with: Network Architecture for joining several exchanges