TDM over IP

In computer networking and telecommunications, TDM over IP (TDMoIP) is the emulation of time-division multiplexing (TDM) over a packet-switched network (PSN). TDM refers to a T1, E1, T3 or E3 signal, while the PSN is based either on IP or MPLS or on raw Ethernet. A related technology is circuit emulation, which enables transport of TDM traffic over cell-based (ATM) networks.

TDMoIP is a type of pseudowire (PW). However, unlike other traffic types that can be carried over pseudowires (e.g. ATM, Frame Relay and Ethernet), TDM is a real-time bit stream, leading to TDMoIP having unique characteristics. In addition, conventional TDM networks have numerous special features, in particular those required in order to carry voice-grade telephony channels. These features imply signaling systems that support a wide range of telephony features, a rich standardization literature and well-developed Operations and Management (OAM) mechanisms. All of these factors must be taken into account when emulating TDM over PSNs.

One critical issue in implementing TDM PWs is clock recovery. In native TDM networks the physical layer carries highly accurate timing information along with the TDM data, but when emulating TDM over PSNs this synchronization is absent. TDM timing standards can be exacting and conformance with these may require innovative mechanisms to adaptively reproduce the TDM timing.

Another issue that must be addressed is TDMoIP packet loss concealment (PLC). Since TDM data is delivered at a constant rate over a dedicated channel, the native service may have bit errors but data is never lost in transit. All PSNs suffer to some degree from packet loss and this must be compensated when delivering TDM over a PSN.

In December 2007 TDMoIP was approved as an IETF RFC 5087 authored by Dr. Yaakov Stein, Ronen Shashua, Ron Insler, and Motti Anavi of RAD Data Communications.

Communications service providers and enterprise customers are interested in deployment of voice and leased line services over efficient Ethernet, IP and MPLS infrastructures. While Voice over IP (VoIP) is maturing, its deployment requires an investment in new network infrastructure and customer premises equipment (CPE). TDMoIP presents a migration path, whereby modern packet switched networks can be used for transport, while the end-user equipment need not be immediately replaced.

TDMoIP was first developed in 1998 by RAD Data Communications (see U.S. patent number 6,731,649) and first deployed in Sweden in 1999 by Utfors (later acquired by Telenor). Utfors employed the first generation TDMoIP product (known as IPmux-4) to provide bundled services including TDM private lines, TDM leased lines and a variety of IP and Ethernet services. In 2001, the IETF set up the PWE3 working group, which was chartered to develop an architecture for edge-to-edge pseudowires, and to produce specifications for various services, including TDM. Other standardization forums, including the ITU and the MPLS - Frame Relay Alliance, are also active in producing standards and implementation agreements for pseudowires.

Although TDM can be used to carry arbitrary bit streams at the rates defined in G.702, there are standardized methods of carrying bit streams in larger units each containing the same number of bits, called frames. TDM framing locks the frame rate to the sampling frequency of voice traffic, so that there are always 8000 frames per second; a T1 frame consists of 193 bits and an E1 frame of 256 bits.