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Transcoding is the process of conversion between the switched and packet environments. However, Asynchronous Transcoding is to be avoided. According to Intel, “The term “asynchronous transcoding” refers to a situation when, for example, one endpoint is talking G.711 to another endpoint talking G.723 (two different encodings).”
Transmission lines are typically 1 msec per 100 miles of cable. RTT for geo-synchronous satellite links are 250 to 270 msec and multiple hops can yield even longer delays. Serial port interface delay can be from 1 msecs for a 128KBPS packet on a 2MBPS line to 130 msecs for a 1024K packet on a 64Kbps line. VoIP gateway node can be 50 to 100 msec and decompression delay is typically 10 msecs or less. In a VoIP [Voice over Internet Protocol] network, the End, Short or Echo Tail is defined as the RTT [Round Trip Time] from the gateway to the hybrid (2-wire-4-wire-wire converter) and back to the gateway.
The time duration of the echo tail is referred as the Tail Length. However, a hybrid circuit does not create a brick wall echo. A brick wall echo refers to one where the response of a far end signal would be an echoed signal. The EC [Echo Canceller] needs only to compensate for the circuit-switched (TDM [Time Division Multiplexing]) segment of the call. A Line Echo Canceller is used with Short Tail problems.
A Network Echo Canceller is used with Long Tail problems. The speed of electricity (light) is 300,000,000 meters per second or 186,000 miles per second. For example, the distance between Boston and San Diego is 3,000 miles. Without any delay, RTT [Round Trip Time] for voice or data transmission would take 32 msecs (microseconds). Here are some of the types of delays found in various network tests. Actual delay results may be considerably higher. CODEC [COmpression-DECompression] (computer processing) of different voice compression protocols such as G.723.1: 40 msec, G.729: 10-20 msec, G.728: 2-5 msec. Router inter-process hand-offs are approximately 10 msec at each end router (hop).
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[tags]voip,ec,echo canceller,echo tail,long tail problem[/tags]