Friday, March 13, 2009

TDM Or IP Transmission .... Which Is More Efficient?

Both modes of voice transmission has its own advantages and disadvantages . For example TDM transmission works on circuit switched mode. If you take a telephone call on a ordinary phone a 64 Kbps is allocated for the entire length of your conversation which makes the quality of voice extremely good .... but we would not be able to make use of the bandwidth allocated for that circuit even if there is lot of idle time during the conversation. Whereas, in case of IP transmission as packets are switched, as long as you are having control over the bandwidth, usage can be optimised giving optimal quality based on our specs. But the key term here is control, if you do not have control over the bandwidth allocation and ratio of concurrent voice channels transmission, then voice quality can suffer if the traffic exceeds the limitation. This is again factored based on the compression equipment in place.

Any traffic on a TDM circuit can be considered travelling over a clear channel relating to the ordinary telephone call example mentioned earlier.

Other differences are TDM is a secure channel, but IP bandwidth if it is on Public Internet can't be termed fully "secure". However with the advent of MPLS technology, IP transmission has gained more predominance as packets getting switched has enabled security. Apart from this QoS is possible on MPLS networks and thereby applying CoS on the bandwidth allocate can prioritize voice and data traffic according your specs. (Your MPLS may have a T1 or DS3 bandwidth backbone)

All these points can significantly skew the results in favour of IP transmission over TDM. In a business case scenario, for accomodating multiple voice channels with optimum quality and cost, IP transmission as on date would win hands down. However, this can vary on a case to case basis depending on various other factors which are to be considered.

Now .... if you consider TDM over IP that's a different ballpark.

TDM over IP is often touted by people as a very "next gen" capability. However, it very often doesn't make much sense. It's actually not very bandwidth efficient at all. It's certainly less bandwidth efficient than TDM circuits over, say, SDH. The reason is regardless of the carrier technology, a TDM circuit like and E1 requires the full bandwidth of the circuit to be reserved across the network. There is no way of compressing it or statistically multiplexing it. The big efficiency benefit of IP networks is due to the statistical multiplexing capability for native IP apps. This benefit is destroyed when dealing with TDM transport.

The only real benefit of TDM over IP is that is *may* allow you to use an existing IP network. This may give you some efficiency that is specific to your application, location, or network setup. there may also be some efficiency in consolidating the management into a single network. There may also be commercial reasons why running over IP is better for you. However, these will be specific to your application. As a general rule, TDM over IP is less efficient than the common alternatives.

Consider that an IP network has to run over a transport network. Often this will be SDH (Sonet). Traditionally TDM circuits would run directly on the SDH network:TDM circuit over SDH. If you run them over the IP layer, then you will have TDM over circuit emulation over IP over SDH. The circuit emulation/IP is an additional layer, and additional layers add overhead.

However, this all depends on whether you have lots of TDM circuits or just a one or two. If you have lots then it may make sense to add the capability to the SDH network. If it's only one or two, then it may be better (cheaper/easier) to add this to the IP layer.

As I said, TDM over IP over SDH is nominally less efficient (will use more bandwidth) than TDM over SDH. However, specific local circumstances may skew this significantly away from the nominal.

For help walking through the options available to you for the most cost effective solution ... at ZERO cost to you .... request support through Business VoIP Solution

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