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How do you deliver TDM over IP?
TDM (time-division multiplexing) circuits carry multiple voice or data channels in a timed stream. To transport one over an IP network, equipment at each edge adapts the circuit into packets and reconstructs it at the other end. This is commonly called circuit emulation or pseudowire (PW) transport.
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“TDM over IP” describes a family of approaches rather than one universal protocol. The endpoints must use a compatible PW type and agree on relevant settings, while the intervening packet network must be engineered for the service’s delay, delay variation, loss, and timing needs. The standards do not establish a universal latency threshold or guarantee that an arbitrary IP network will meet a particular circuit’s requirements.
What is the difference between SAToP and CESoPSN?
| Approach | How it handles TDM | When its design may fit | Important consideration |
|---|---|---|---|
| SAToP | Structure-agnostic: carries the TDM bitstream without interpreting its framing. RFC 4553 specifies T1, E1, T3, and E3 bitstreams. | When the endpoints do not need to interpret framing, individual channels, or signaling. | The complete stream is transported, and packet loss can affect it. Delay, delay variation, and jitter-buffer delay contribute to end-to-end delay. RFC 4553 |
| TDMoIP | Structure-aware: can recognize TDM structure and expose multiplexed channels and signaling. | When channel visibility or access to signaling can support functions such as per-channel loss concealment or bandwidth conservation. | These are possible advantages, not a guarantee of application quality; network conditions and implementation still matter. RFC 5087 |
| CESoPSN | A structure-aware circuit-emulation approach. RFC 5287 defines pseudowire types including basic mode and a TDM-with-CAS type. | When the required service mode, including channel-associated signaling (CAS) where applicable, is supported at both endpoints. | Confirm the precise PW type and setup parameters at both ends. Cisco documents SAToP and CESoPSN for T1/E1 CEM on specified ASR 900 configurations. RFC 5287 |
Choose transparent carriage or structure-aware transport
SAToP treats the input as a bitstream rather than interpreting its TDM structure. RFC 4553 says SAToP provides sequencing and synchronization functions, including detection of lost or misordered packets and compensation. That does not mean packet loss has no service impact: replacement data may keep an occasional loss from shutting down the customer-edge interface, but it cannot prevent every consequence, such as errored blocks.
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#1 Best Overall
- FOUR ANALOG PHONE LINES OVER YOUR EXISTING NETWORK - Carries 4 real POTS lines (RJ11) across an ordinary Ethernet LAN, so handsets, fax machines, alarm diallers and elevator phones keep working after the copper is gone. Dial tone, ring voltage and caller ID pass through exactly as on a physical pair. Fax transmission supported as well.
- SUPPLIED AS A MATCHED FXO/FXS PAIR - TWO UNITS - The FXO unit terminates the CO or PABX side; the FXS unit terminates the CPE side (phone sets, fax). Both units are included and are required - the system does not function single-ended.
- TDM OVER IP ON A PRIVATE LAN - NOT VoIP, NOT SIP - No SIP registration, provider account or subscription. Both units must reside on the same IP subnet. Do NOT use an internet/WAN connection as the uplink; the transport is not designed to traverse a routed public link.
- FX PORT ACCEPTS A 155M (100FX) SFP - MODULES SOLD SEPARATELY - The FX uplink takes a 155 Mbps Fast Ethernet SFP for transport over dark or active fiber. Two matching transceivers are required, one per unit, and are NOT included; the cage ships empty. Transceiver parameters must match at both ends. All Ethernet/IP and direct RJ-45 modes operate without any module fitted.
- WEB-MANAGED, PRE-CONFIGURED AT THE FACTORY - Units ship with matched IP addresses already set. Subnet, frame length and timing parameters are changed through the built-in web manager. No console cable, terminal software or driver installation.
Structure-aware methods can make information about framing, channels, or signaling available to the implementation. RFC 5087 describes opportunities for per-channel loss concealment and bandwidth conservation in TDMoIP. Whether those capabilities are useful depends on the circuit and equipment. They do not remove the need to engineer delay, loss, and timing.
Can you carry E1 or T1 over an IP network?
Yes. SAToP explicitly covers T1 and E1, as well as T3 and E3. Vendor equipment also documents T1/E1 circuit-emulation interfaces. A suitable gateway or router adapts the circuit at each edge; the packet network carries the resulting pseudowire between them.
Rank #2
- Support for 2 SIP account profiles, caller ID for various countries/regions.Failover SIP server feature in case main SIP server goes down
- Support for T.38 fax, flexible dialing plans, security protection, and comprehensive voice codecs
- Two RJ-45 ports 10/100 (switched or routed)
- 4 and 8 FXS port media gateways
- Supports PSTN/PBX analog telephone sets or analog trunks
Before selecting equipment, establish whether the service is framed or unframed, whether it uses the full circuit or fractional timeslots, and whether the endpoints must preserve or expose signaling. “T1/E1 supported” alone does not prove support for the required interface module, framing, channel mapping, signaling mode, or software release.
What to compare before choosing a method
- Circuit type and channel needs: Identify T1, E1, T3, or E3, and whether the service uses fractional channels or the complete stream.
- Framing and signaling: Decide whether endpoints need to inspect framing, expose channels, or use signaling such as CAS.
- Delay budget: Account for packetization delay, packet-network edge-to-edge delay, and jitter-buffer delay. Estimate delay and variation before setup, as RFC 4553 recommends.
- Delay variation and loss: Assess the packet network under expected operating conditions, including congestion and the service’s tolerance for packet loss. Do not assume a generic IP service meets the timing needs.
- Clock recovery: Confirm how the endpoint pair will provide and recover timing, and that the chosen modes are supported by the equipment.
- Interoperability: Verify that both endpoints support the same PW type and compatible setup parameters.
- Network engineering: Check quality-of-service treatment, congestion, maximum transmission unit (MTU), and resilience for the intended path.
- Operations and lifecycle: Check device support, software and interface-module compatibility, availability, alarms, and the team’s ability to operate the solution.
These are design checks, not a substitute for service-specific thresholds. Standards describe protocol behavior; they do not establish one acceptable delay or loss target for every application. Set targets against the actual circuit and service requirements.
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RFC 5287 defines TDM pseudowire types and requires both endpoints to agree on the same PW type. Depending on the mode and selected behavior, setup parameters include TDM payload bytes and bit rate. Check the complete configuration as a matched pair rather than relying on a product’s general TDM-over-IP label.
- Identify the service: Record the circuit type, framing, full-rate or fractional-channel use, and signaling requirements.
- Match the PW type: Confirm that both devices support the same mode—such as SAToP or the required CESoPSN type—and any needed signaling behavior.
- Match settings: Compare timeslot mapping, payload and packetization settings, bit rate, timing mode, and other parameters required for the selected mode.
- Check the path: Validate MTU, QoS, congestion handling, and resilience across the entire packet route, not just the device interfaces.
- Validate operations: Confirm management and alarm visibility, then test the end-to-end service against its timing and error requirements before production use.
RFC 5287: TDM Pseudowire Setup.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Documented equipment examples and lifecycle checks
RAD Megaplex-1
RAD describes the Megaplex-1 as a multiservice access node for transporting analog and TDM traffic from legacy circuit-switched equipment over packet-switched networks. Its product page lists E1/T1 services and standard pseudowire technology. Verify the port modules required for the particular circuit and confirm current availability with the vendor. RAD Megaplex-1 product information.
Cisco ASR 900 CEM
Cisco’s IOS XE 17 configuration guide documents T1/E1 circuit-emulation interfaces and SAToP and CESoPSN pseudowire types for the covered ASR 900 configurations. This is platform- and software-specific documentation, not a claim that every ASR 900 model or release supports the same features. Confirm the exact router, interface module, IOS XE release, and network mode for the deployment. Cisco T1/E1 CEM Interface Module Configuration Guide.
Cisco TDM Gateways
Cisco identifies its TDM Gateways family as no longer sold. Its lifecycle page lists end of sale as January 20, 2025, and end of support as January 31, 2030. Those dates apply to that family; they should not be confused with CEM functionality documented for other Cisco platforms. Cisco TDM Gateways support and lifecycle information.
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