A related technology is circuit emulation, which enables transport of TDM traffic over cell-based (ATM) networks.
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.
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.
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).
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.
In such cases all structural overhead must be transparently transported along with the payload data, and the encapsulation method employed provides no mechanisms for its location or utilization.
Encapsulation signifies placing the adapted payload into packets of the format required by the underlying PSN technology.
A side benefit of this choice of payload types is simplified interworking with circuit emulation services carried over ATM networks.
For statically allocated, constant bit-rate (CBR) TDM links, TDMoIP employs ATM adaptation layer 1 (AAL1).
AAL1 operates by segmenting the continuous stream of TDM data into small 48-byte cells and inserting sequencing, timing, error recovery, and synchronization information into them.
This mechanism, defined in ITU-T standard I.363.2, was developed for carrying variable bit rate (VBR) services over ATM.
ITU-T G.114/G.131 states that one-way transmission times of up to 150 ms are universally acceptable, assuming adequate echo control is provided.
VoIP systems that try to be very bandwidth efficient may also add tens of milliseconds of algorithmic delay in the voice codec.
The problem is that the TDM source time reference is no longer available, and the precise rate at which the data are to be "clocked out" of the jitter buffer is unknown.
Since TDMoIP packets are so small, it is acceptable to simply insert a constant value in place of any lost speech samples.
Assuming that the input signal is zero-mean (i.e. contains no DC component), minimal distortion is attained when this constant is set to zero.