TDM routes carry calls over circuit switched links (E1/T1 lines using SS7 signaling), while VoIP routes carry them as packets over IP using SIP. Most wholesale voice is now exchanged over SIP, but TDM still exists at some operator interconnects, so many calls travel part of the way on each.
The two technologies in short
| Aspect | TDM | VoIP (SIP) |
|---|---|---|
| Transport | Dedicated circuits, one fixed 64 kbit/s timeslot per call | IP packets over private links or the public internet |
| Signaling | SS7 (ISUP) or ISDN | SIP |
| Capacity unit | E1 (30 voice channels) or T1 (24 channels) | Channels and CPS set in software |
| Setup time | Weeks: physical circuits, cross connects, point codes | Days or hours: IP addresses and a test |
| Codec | G.711 by design | G.711, G.729 and others |
| Cost to interconnect | High: hardware, ports, circuits | Low: a session border controller and bandwidth |
Why the market moved to VoIP
A SIP interconnect needs no physical circuit between the two carriers, so it is cheap to open, cheap to scale and easy to move. A carrier can add a new supplier in a day, test it, and route traffic to it the same week. With TDM every new partner needs circuits, ports and SS7 setup. This flexibility is what makes modern least cost routing and fast route changes possible. See how a SIP interconnect works for the setup steps.
Where TDM still matters
- Some national operators still accept international traffic only on TDM or SS7 interconnects at their gateway.
- Legacy fixed networks that have not been migrated to IP.
- Some large carriers keep TDM links as a stable, high quality path for big volumes.
In those cases a carrier converts SIP to TDM at a media gateway close to the operator. You still buy a SIP route; the conversion happens further down the chain.
Quality differences
TDM gives a fixed timeslot per call, so there is no jitter or packet loss on the circuit itself. VoIP quality depends on the network: on a well managed path it matches TDM, on a congested internet path it suffers. Each conversion between TDM and IP, or between codecs, adds delay and can reduce quality, which is why fewer hops is usually better. See transcoding and MOS, jitter and packet loss.
Caller ID and signaling
Information carried in SIP headers has to be mapped into SS7 fields at the conversion point, and some of it can be lost. Caller ID usually survives, but extra data does not always. In the United States, STIR/SHAKEN call authentication was built for SIP and does not natively cross TDM segments, which is why regulators have pushed for IP interconnection and for non-IP authentication methods. See STIR/SHAKEN explained.
What to ask a carrier
- Is the route SIP end to end, or is there a TDM segment, and where?
- Which codecs are accepted on your side, and is transcoding done?
- Is CLI preserved across any conversion?
- Is the interconnect over the public internet or a private link?
Buying a route
For nearly all buyers, a SIP interconnect is the right choice. VoIPMatcher sources wholesale VoIP termination over SIP and manages the route with its carriers. Request a voice route with your destinations, codecs and expected volume.
Frequently asked questions
What is a TDM route in VoIP?
A TDM route is a voice route where at least part of the path runs over circuit switched links, such as E1 or T1 lines with SS7 signalling, instead of SIP over IP. Buyers usually still connect over SIP; the carrier converts the call to TDM at a media gateway close to the destination operator.
What is the difference between a TDM PBX and a VoIP PBX?
A TDM PBX connects to the phone network over physical lines such as ISDN PRI, E1 or T1 and switches calls on circuit channels. A VoIP PBX handles calls as SIP and RTP over IP and connects to carriers through SIP trunks. Most new systems are VoIP, and many older TDM PBXs connect through gateways.
How many channels does an E1 or T1 have?
An E1 carries 30 voice channels, using 32 timeslots of 64 kbit/s with two reserved for framing and signalling, and is used in Europe and most of the world. A T1 carries 24 channels and is used in North America and Japan. On SIP routes, capacity is set in software as channels and CPS.
Is TDM being phased out?
Gradually. Many operators are retiring legacy circuit switched networks and moving interconnection to IP, and in the US regulators have pushed IP interconnection partly so that STIR/SHAKEN signatures survive end to end. TDM still remains at some national gateways and in legacy fixed networks, so the transition is uneven by country.
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