TDMoIP - Delay

Delay

The telephony network severely constrains end-to-end delays. 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. These constraints are not problematic for TDM networks, where the major component of the end-to-end delay is electrical propagation time ("light speed delay"). By contrast, IP-based systems typically add various forms of delay, one of which is based on the time it takes to form packets (packetization delay), which is proportional to the packet size divided by the data rate. Packet sizes cannot be made too small or the packet header overhead will become overwhelming. The other form of delay introduced by IP systems is the playout delay, which needs to be added at the recipient to buffer packet delay variation and ensure a smooth playout. VoIP systems that try to be very bandwidth efficient may also add tens of milliseconds of algorithmic delay in the voice codec. Historically, bad implementations have added additional, operating-system induced delays, which together with the other delays in practice sometimes approach 100 ms even before taking propagation delays into account.

In contrast, TDMoIP maps TDM octets directly into the payload with no voice compression algorithms and no resultant algorithmic delay. The packetization latency added by TDMoIP depends on the number of cells per packet but is typically in the single millisecond range due to the higher data rate of a complete multiplex as compared to a single VoIP flow. Playout delay considerations do not differ materially between TDMoIP and VoIP, however, so both work best on paths with controlled packet delay variation (strong overprovisioning or "QoS").