Time-division multiplexing (TDM) is a method of sharing one link among several signals by dividing its capacity into time slots. This entry focuses on synchronous, fixed-slot TDM, where each signal gets the same repeating slot: the circuit cycles through the slots many thousands of times a second, so each call or data stream gets a steady share of capacity whether or not it is using it. Statistical TDM, by contrast, hands out slots on demand to whichever signals have data to send. Fixed-slot TDM is the basis of the legacy telephone network and of services such as T1 lines, Primary Rate Interface (PRI) circuits and much of the carrier infrastructure behind analog lines. For buyers, “TDM” usually means old circuit-switched voice and data services that carriers are moving to IP and retiring, and the question is how to replace them.
At a glance
- In the fixed-slot TDM used by telephone networks, each call or data stream gets its own time slot on a shared circuit, with capacity reserved whether it is used or not.
- A North American T1 carries 24 slots of 64 kbps each; a European E1 carries 32.
- T1, PRI, much of the public switched telephone network (PSTN) and some older transport services are built on TDM.
- Carriers are moving these services to IP-based networks and retiring TDM, on timelines that vary by carrier and location.
- Replacement usually means SIP trunking or cloud phone systems for voice, and IP or cellular-based services for single analog lines.
What problem it solves
Running a separate pair of wires for every call between telephone exchanges was expensive. Fixed-slot TDM let carriers digitize calls and interleave many of them on one circuit, which made long-distance and trunk capacity far cheaper and gave each call predictable quality, because its slot was reserved for the whole call.
The trade-off is efficiency. In fixed-slot TDM, a reserved slot sits idle during silence and when no call is using it, while packet-based networks share capacity dynamically. As data traffic overtook voice, carriers moved to IP networks and voice followed as voice over IP (VoIP). The legacy TDM networks that remain are costly to maintain, which is why many carriers are retiring them.
How it works
Sampling and slots. In telephony, each voice call is sampled 8,000 times a second, and each sample is encoded as 8 bits, giving a 64 kbps channel, often called a DS0. A TDM circuit assembles frames that contain one sample from each channel in turn and sends those frames continuously.
T1 and E1. A T1 line carries 24 DS0 channels; an E1 carries 32, with one or two used for framing and signaling depending on configuration. Higher-capacity TDM circuits, such as DS3 and the SONET and SDH hierarchies, bundle many of these together for carrier transport.
Circuit switching. When a call is placed on a TDM network, the switches reserve a slot along the whole path for the duration of the call. That is what gives TDM calls their consistent quality and what makes them inefficient for bursty data.
Voice services built on it. A PRI, a form of ISDN, uses a T1 or E1 with one channel set aside for signaling. Traditional POTS lines are analog to the customer, but the carrier commonly converts them to TDM in its network.
The transition to IP. Carriers are replacing TDM switches and copper with IP networks and fiber. Customers on TDM services are typically offered IP alternatives, such as SIP trunking, and may see price increases on legacy circuits in the meantime. In the US, retirements are subject to regulatory processes that differ by service and state.
For planning the move off TDM voice circuits, see our PRI replacement solution page.
When it matters for buyers
- A carrier retirement or price increase notice. Legacy TDM circuits are a common target; plan the replacement before the deadline drives it.
- Inherited sites and acquisitions. T1s, PRIs and analog lines are often undocumented, and some carry alarms, elevator phones or point-of-sale traffic.
- Replacing a PBX. Moving the PBX and its TDM trunks to the cloud together is usually simpler than replacing each separately.
- Auditing telecom spend. Unused TDM circuits are a frequent source of waste.
- Life-safety lines. Fire alarm, elevator and emergency phone lines need replacements that meet the device’s and the local authority’s requirements.
Questions to ask vendors
- Which of our services are TDM, and what are your retirement plans and dates for each?
- What IP or cellular replacement do you propose for each, and how does pricing compare?
- Will legacy circuit prices change before retirement?
- Can our phone numbers be ported to the replacement service, and what will cutover look like?
- How will alarms, elevators, fax and other analog devices be supported after the change?
- If we keep an older PBX, which gateway do you support to connect it to SIP?
How it differs from VoIP
TDM voice circuits and VoIP carry voice in different ways. A TDM voice circuit reserves a fixed slot on a circuit for each call, so quality is consistent but unused capacity is wasted. VoIP breaks voice into packets that share an IP network with other traffic, which is more efficient and flexible but depends on the network having enough capacity and suitable quality-of-service handling. Most new business phone services are VoIP; TDM remains mainly as legacy circuits awaiting replacement.