A hookflash indication is a brief on-hook condition that occurs during a call. It is not long enough in duration to be interpreted as a signal to disconnect the call. Create a hookflash indication by quickly depressing and then releasing the hook on your telephone.
PBXs and telephone switches are frequently programmed to intercept hookflash indications and use them as a way to allow a user to invoke supplemental services. For example, your local service provider may allow you to enter a hookflash as a means of switching between calls if you subscribe to a call waiting service.
In the traditional telephone network, a hookflash results in a voltage change on the telephone line. Because there is no equivalent of this voltage change in an IP network, the ITU H.245 standard defines a message representing a hookflash. To send a hookflash indication using this message, an H.323 endpoint sends an H.245 user input indication message containing a "signal" structure with a value of "!". This value represents a hookflash indication.
Cisco H.323 Version 2 software includes limited support for relaying hookflash indications using the H.245 protocol. H.245 user input indication messages containing hookflash indications that are received on the IP call leg are forwarded to the plain old telephone service (POTS) call leg if the POTS interface is Foreign Exchange Office (FXO). If the interface is not FXO, any H.245 hookflash indication that is received is ignored. This support allows IP telephony applications to send hookflash indications to a PBX through the Cisco gateway and thereby invoke the IOS supplementary services of the PBX if the PBX supports access to those features using hookflash.
The gateway does not originate H.245 hookflash indications in this release. For example, it does not forward hookflash indications from foreign-exchange-station (FXS) interfaces to the IP network over H.245.
The acceptable duration of a hookflash indication varies by equipment vendor and by country. Although one PBX may consider a 250-ms on-hook condition to be a hookflash, another PBX may consider this condition to be a disconnect. Therefore, the timing hookflash-out command allows the administrator to define the duration of a hookflash signal generated on an FXO interface.
The figure below illustrates an FXS hookflash being translated to an H.245 user input.
||Translating an FXS Hookflash to an H.245 User Input
In Cisco H.323 Version 2 software, an FXS hookflash relay is generated only if the following two conditions are met:
The other endpoint supports the reception of an H.245 hookflash and advertise this using the "Receive User Input Capability" message during H.245 capabilities exchange.
The call is established with either the h245-alphanumeric or h245-signal variant of the dtmf-relaycommand.
This implies that the VoIP dial peer is configured for dtmf-relay h245-alphanumeric or dtmf-relay h245-signal, but not cisco-rtp.
Enter the timing hookflash-input command on FXS interfaces to specify the maximum length of a hookflash indication. If the hookflash lasts longer than the specified limit, then the FXS interface processes the indication as an onhook.
To configure hookflash relay on a gateway, use the following commands beginning in global configuration mode.