This tutorial explains Asynchronous Transcoding and SIP [Session Initiation Protocol] available at TECHtionary.com.
Transcoding is the process of conversion between circuit-switched (PSTN-Public Switched Telephone Network) and packet-switched networks such as Frame Relay, Internet, and ATM [Asynchronous Transfer Mode]. 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 or transmitter codings).”
Echo can also occur as a result of Asynchronous Transcoding. This TECHtionary.com tutorial also described in detail Echo or rather EC [Echo Cancellation]. Echo is the reflection of the original back to the sender. There are many causes for echo but it can occur in many types of network including SIP [Session Initiation Protocol]. SIP is a signaling protocol for Internet conferencing, telephony, presence, events notification (emergency calling) and instant messaging. Hybrid Echo is an impedance mismatch within the 2-wire to 4-wire (T-1) conversion called the Hybrid. Impedance is a measure (expressed in Ohms) of the opposition (resistance) to the flow of electricity and changes value as the frequency of the electricity changes. Capacitive reactance causes impedance to rise as the signal frequency decreases, whereas the inductive reactance causes impedance to increase as the frequency increases which means the higher the impedance, the lower the current.
A typical POTS [Plain Old Telephone Service] analog line is 600 Ohms. In other words, electrical echo is generated when the incoming signal voltage from the destination speaker is reflected back toward the speaker as a slightly altered and delayed replication due to impedance mismatch in the hybrid. The presence of echo occurs whenever the signal delay exceeds ten milliseconds. The speaker can hear echo as reflected voice when the delay exceeds as little as 16 msec. It is manifested to the far end as a distorted/duplicated or altered replica copy of the original speaker. In such a case, a gateway in the network would be performing the transcoding from G.711 to G.723. Transcoding is asynchronous in the sense that the two different coders (G.711 and G.723) have different packet sizes (in milliseconds [msecs]). Worst case scenario could be that one endpoint might have 15 msecs packets, the other 25 msecs packets. The buffering required could introduce a significant latency into the voice path. Transcoding between the switched and packet environments requires the following steps. Some of the relevant components are: the TIC-Telecommunications Interface Card (often a T-1-Transmission Level one interface); the signal processing or transcoding card – often called a DSP [Digital Signal Processing] farm (chipsets); and the packet NIC [Network Interface Card].
The TIC processes the incoming voice using the appropriate communications stacks and signaling sequences into the relevant multiplex standard – H.324 for PSTN [Public Switched Telephone Network] (analog); H.320 for ISDN [Integrated Services Digital Network] or others. The multiplexed data is then forwarded to the transcoder. Transcoding between standards takes place on the DSP card. The DSP engine takes multiplexed data from the telecoms processor card, demultiplexes it and translates this data into the packet network format such as IP [Internet Protocol]. The transcoded data is then remultiplexed into the packet network standard data structure (such as ethernet) before being sent to the NIC. In the packet NIC engine, the newly multiplexed data is processed by the appropriate network stack – such as ATM [Asynchronous Transfer Mode] or Ethernet, for example – before communication into the network.
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 is an echo 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.
See in-depth tutorials on this topic and more at TECHtionary.com.