APPLICATION NOTE P32mxi Advanced Channel Bank Voice Activity Detection (VAD) Answer Supervision/Disconnect Supervision by Susan L. Pirzchalski, CTO Answer Supervision A recurring problem for anyone trying to connect loop extension equipment (especially when installing Voice Over IP gateways) to existing POTS lines is the lack of Answer Supervision. POTS circuits are used because: a) It’s the only thing available from the PTT, or b) The cost of DID trunks is cost prohibitive for the application. By definition, a POTS line is a simple DC loop circuit designed to connect to a regular telephone. When you call someone you know they’ve answered because you hear them talking. When they hang up you will hear dead air for a while, or possibly the Central Office (CO) disconnect tone, which usually gives you the hint that it’s time to hang up the phone! As shown in the diagram below, the off-hook signal sent from the called phone is not propagated through the local CO to the calling phone. The only way the calling phone knows that the called phone has answered is when the calling phone stops hearing the audible ringing and hears the called party say “hello.” Pleiades Communications, Inc. Page 1 VAD Application Note 2002-01-17 Local PTT Central Office Calling Phone Called Phone off-hook dial # ringing audible ringing off-hook talking For loop extension applications (including Digital Loop Carriers, satellite systems and VoIP gateways), not having a definite signal that indicates when the call becomes “busy” or “idle” is problematic. The only signals that can be sent from the Central Office (CO) to the phone is ringing or not ringing. The only control that is sent back to the CO is on-hook or off-hook. There are no control signals from the CO to tell the phone that a called party has either answered the phone or hung up. This means that Answer Supervision can only be generated through two methods: a) By using a timing device to automatically send out an “answer” signal after a specified timeout, or b) By actually listening to the audio coming from the CO to determine if someone has answered the called phone, then sending the “answer” signal to the Gateway. The P32mxi Advanced Channel Bank can use either method to generate Answer Supervision. P32mxi Advanced Channel Bank Local PTT Central Office Compression Equipment (or VoIP Gateway) T1/E1 Circuit FXO E&M or R2 Digital Called Phone (POTS Circuit) Signaling off-hook off-hook dial # ringing audible ringing off-hook talking off-hook/"answer" (after voice detection or timer expiry) Pleiades Communications, Inc. Page 2 VAD Application Note 2002-01-17 In the diagram above, the Compression Equipment/VoIP Gateway terminates the compressed (or packetized) call and converts it back to normal PSTN audio on the T1/E1 circuit. The P32mxi breaks the T1/E1 into 24/30 analog channels (FXO interfaces) that can be plugged into normal line-side (POTS) circuits on the local Central Office. Inbound calls cause the P32mxi to take the POTS circuit off-hook; the Gateway then dials the phone number of a user on the PTT network. The Voice Activity Detection method of Answer Supervision in the P32mxi ignores the audible call progress tones provided by the local CO and waits until someone starts talking on the destination phone. Once voice or modem/fax tones are detected, “answer” signaling is sent towards the Gateway. The VAD method provides superior performance by eliminating false supervision signals and delivering accurate billing to the customer. It is an optional hardware/software package for the P32mxi. Disconnect Supervision The VAD package also offers Disconnect Supervision. This prevents POTS lines from getting “stuck” when the called phone hangs up: Local PTT Central Office Calling Phone Called Phone talking (call in progress) on-hook time passes... (10 seconds or so) disconnect tone As with Answer Supervision, there is no way for the CO to electrically signal the calling phone that the called phone is hung up (remember, the CO can only send ringing or not ringing to a POTS circuit). Without Disconnect Supervision, the call is not torn down until the calling party hangs up and idle signaling is propagated through the network to the channel bank. If the network does not clear properly, the channel gets stuck. The signaling for Disconnect Supervision is shown below: Pleiades Communications, Inc. Page 3 VAD Application Note 2002-01-17 P32mxi Advanced Channel Bank Local PTT Central Office Compression Equipment (or VoIP Gateway) T1/E1 Circuit FXO E&M or R2 Digital Called Phone (POTS Circuit) Signaling talking (call in progress) on-hook time passes... (10 seconds or so) disconnect tone on-hook/"idle" (after disconnect tone detected) on-hook on-hook When the P32mxi sends on-hook/”idle” to the Gateway, the Gateway will send on-hook back to the P32mxi. Now the call is torn down completely, eliminating any potential hung circuits. Disconnect Supervision has the added benefit that the channel from the gateway is cleared for the next call very quickly. This allows for a higher call density through the unit by making channels available to the pool much sooner than if they were waiting for the network signaling to clear. Pleiades Communications, Inc. Page 4 VAD Application Note 2002-01-17