Telecoms glossary • T
TDM (Time-Division Multiplexing)
How the old phone network carried calls. Mostly a word you now hear while somebody describes equipment you are about to replace.
What is TDM?in VoIP and business telephony
1. Time-division multiplexing carries several calls over one physical line by giving each call its own repeating time slot. Each slot is 64 kbit/s and stays reserved for the whole call whether anyone is speaking or not. It is the transmission method behind ISDN, E1 circuits and the traditional telephone network.
2. VoIP replaced it with packet switching, where voice shares a line with everything else and consumes capacity only when there is something to send.
How TDM carries calls
A single pair of copper cannot carry thirty conversations at once, so the network takes turns instead. Time is chopped into frames, each frame is divided into slots, and each call is given the same numbered slot in every frame.
ITU-T Recommendation G.704 defines the frame structures used at the 1544 and 2048 kbit/s hierarchical levels, which are the T1 and E1 lines the industry actually deployed. An E1 frame carries 32 slots of 64 kbit/s each, giving 2.048 Mbit/s. Slot 0 carries framing and synchronisation, slot 16 carries signalling, and the remaining 30 carry voice, which is why an E1 primary rate circuit is described as 30 channels.
One E1 frame: every call owns a slot, speaking or not
The reserved slot is the whole design. Once a call is set up it has a guaranteed, fixed allocation until it ends, so there is no congestion, no jitter and no packet loss by construction. There is also no way to use that capacity for anything else while the caller is listening rather than speaking.
TDM against packet switching
The comparison is not simply old against new. TDM traded efficiency for certainty, and modern voice networks trade the other way and then spend effort buying the certainty back with QoS.
| TDM circuit | Packet (VoIP) | |
|---|---|---|
| Capacity model | Reserved per call | Shared, used on demand |
| Silence on a call | Still occupies the slot | Sends little or nothing |
| Quality when healthy | Fixed and predictable | Depends on the network under it |
| Adding channels | New circuit, site visit | A change on the account |
| Features | Whatever the PABX has | Platform features, updated centrally |
| Working from elsewhere | Not without diversion charges | An extension registers anywhere |
The row that closes the argument commercially is channels. A TDM circuit is bought in blocks of 30 whether you need 8 or 28, while SIP trunking is bought per concurrent channel.
Where you still meet TDM in South Africa
Three things in a South African comms room are TDM, and all three are on somebody’s replacement list.
- ISDN circuits
- A primary rate interface presented as an E1 with 30 channels, or a basic rate line with two. If a quote mentions PRI or BRI, that is TDM.
- Legacy PABX hardware
- An on-premise switch with digital line cards. It can often be kept alive on an IP service through a VoIP gateway that converts between the two worlds, which buys time without buying a new system.
- Analogue extensions
- Not TDM themselves, but usually hanging off equipment that is. Lift phones, gate intercoms and fax machines are the ones people forget until the migration weekend.
The direction of travel is not in doubt, and our guide to moving away from Telkom copper services covers the migration sequence. What matters commercially is that TDM equipment is a depreciating asset with a shrinking pool of people who can maintain it, so the question is when you replace it rather than whether.
Working out what you actually have
Before accepting a migration quote, establish what is physically installed. Look for a card labelled PRI or E1 in the PABX, a network terminating unit on the wall, or a bill that itemises channels rather than lines. A supplier who has not asked which of those you have is quoting from a template.
The useful follow-up question is what your peak concurrent call count is, because that number, not your extension count, decides how many channels the replacement service needs. Offices routinely discover they have been paying for 30 channels to carry six calls.
About this entry
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