What are typical TDM over IP solutions? TDM over IP (TDMoIP) is the simulating equipment of time division multiplexing over a packet switched network (PSN). TDM is based on T1, E1, T3 or E3 signals. TDMoIP is an integration of E1 into new IP networks. It is a type of pseudo wire (PW). However, other traffic types that can be carried over pseudo wires are ATM, frame relay etc. But, TDMoIP is a real time bit stream having unique characteristics.
WORKING OF TDMoIP:
The working of the TDMoIP is discussed in this part.
- The synchronous bit streams such as T1 (an American context digital signal) / E1 (a European context digital signal) or T3/E3 are chopped into packets.
- IP headers are added to transmit packets.
- These packets are forwarded to destination over IP network.
- At destination, the original bit stream is constructed by removing IP headers, concatenating the packets.
- This is TDMoIP.
TDM is used to carry arbitrary bit streams at the rate of G.702. The standardized method of carrying these bit streams in larger units which contains the same number of bits, called frames. There are 8000 frames per second; T1 frame consists of 193 bits and E1 frame consists of 256 bits.
It is divided into unframed and framed TDM. In unframed TDM, bits are used for payload whereas, in framed TDM required bits for synchronization and various other functions. This is used to multiplex multiple voice channels each consisting of 8 bit samples per second. It is known as ‘channelized TDM’.
The features of the TDMoIP are listed here.
- It is easy to setup and maintenance, since TDMoIP is user friendly web server.
- TDM over IP supports point to point and point to multipoint applications.
- It acts as a low processing delay for E1 channels.
- Supports cascade concatenate for more than four E1 ports.
- TDMoIP resists packet loss.
It enables a smooth migration over next generation transport. TDMoIP has significant savings on infrastructure by minimizing E1/T1 leased lines. It has low latency to meet TDM services. It maintains integrity of framed or unframed E1/T1. This is simpler and less expensive and also provides higher quality voice with much lower latency. It is transparent to protocols and signaling. The benefits are comparatively more.
Figure.1: 8E1 multiplexed over IP (TDM over IP) using AD-net’s AN-TDM-IP-8E1 muxes
ISSUES OF TDMoIP:
There are two main issues in the TDMoIP and whole TDM over IP solutions available in market. Let us discuss the two issues here and what the issues cause them here.
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 IP this synchronization is absent.
Another issue of TDMoIP is 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. All PSNs suffer to some degree from packet loss, and this must be compensated when delivering TDM over a PSN.
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