Programmable Telephone Gateway

The gateway consists of an RTP gateway and a call processor.

Call Processor

The call processing language (CPL) is a Tcl-based language for processing incoming phone, ISDN and SIP calls.

For packet telephony and voice mail, RTSP requests should be supported directly (rtsp setup, rtsp play, rtsp pause, rtsp teardown).

Events

The following events can occur. Some events generate local variables that are visible only inside the event handler. Variables marked with (*) are only set for SIP calls. The device is either phone or. inet (for Internet). [Use callid instead for several concurrent SIP calls?]

name of event	description of event	SIP?	phone?	local variables	implementation note
- (initial state)	phone rings or SIP invitation arrives	x	x	$callid (phone: locally generated), $from (SIP From: header; phone: if caller id available), $to (SIP To: header; phone: ISDN extension) $media (list from SDP description) $subject (*), $priority (*), $organization (*)
busy	outgoing call found busy callee	x	x	retry-after
ringing	Outgoing call is ringing at the other side.	x	x
declined	Our outgoing call was answered, but the callee doesn't want to talk. to use	x	-	$reason (*)
answered [device]	Our outgoing call was accepted. The variables describe where audio is to be sent.	x	x	$host (*) $port (*)
disconnected [device]	Caller hung up.	x	x		SIP: received BYE or pmm BYE.
dtmf digit	The given DTMF digit was dialed. If no digit is given, any digit will do.	x	x	$digit
timeout ms	Timeout after the given number of milliseconds.	x	x

Commands

Implementation

The CPL program is parsed into the "entry" commands (the ones outside on) and a number of event handlers, one for each on command. Thus, each state is a list of Tcl "programs". The parsing may be done via Tcl.

The C program maintains a global state variable that is set by the goto command.

A number of generic file/socket event handlers always exist, independent of the state. Currently, these are:

• the serial port reader routine;
• the SIP port reader;
• the pmm reader.

When entering a state, the C program installs the event handlers requested and then executes (via eval) the entry statements. The exact mechanism of installing the event handlers depend on the event. Example implementation: for the DTMF handler, a pointer to the respective digit-specific Tcl handler is copied to a global Tcl array, which is accessed by the generic DTMF event handler. When a digit is recognized, the DTMF handler checks the array and does an eval on the entry, if found.

RTP gateway

This daemon program takes audio input from the workstation audio interface and generates RTP packets to one or more addresses. Similarly, it takes incoming audio packets and plays them to an audio interface. This program is NeVoT, but without a Tk user interface.

NeVoT is remote-controlled through a multicast interface (pmm).

NeVoT is also capable of recording audio to a file and playing back audio from a file.

The gateway needs to be enhanced to be able to detect DTMF ("touch tones") and indicate reception of DTMF back through pmm.

CPL syntax

States have the following generic syntax:

state S {arg1 arg2} {
  Tcl
  on E { Tcl }
  on E { Tcl }
}

Here, S is the name of the state, E the name of an event and Tcl can be any Tcl code.

Example

# number dialed
set number ""

# Initial state as phone rings

state initial { } {
  # pick up phone through command
  # ignored if we have already picked up
  answer

  # play outgoing message
  play -loop "welcome.au"

  on disconnected {
    play -stop
  }

  # If user presses "1", this message is interrupted
  on dtmf 1 {
    play -stop
    goto sales
  }

  # If user presses "2", go to recording
  on dtmf 0 {
    play -stop
    goto record_message
  }
}

# This is the sales menu; can be interrupted any time
state sales {
  play "sales.au"
  on dtmf 0 {
    goto initial
  }  
  on dtmf 2 {
    goto record_message
  }
}

# Record message until user hangs up or presses key
state record_message {
  on disconnected phone {
    goto 
  }
  on dtmf * {
  }
}

# Connect user to Internet by having him "type out" names via the keypad
state dial {
  on dtmf # {
    goto connect_internet $number
  }
  on dtmf * {
    set number "$number$digit"
  }
}

# Try connecting to SIP address.
state connect_internet {number} {
  dial sip://${number}@gateway.cs.columbia.edu

  on ringing {
    goto ringing $number
  }
  on timeout 30 {
    goto connect_internet $number
  }
}

state ringing {number} {
  on busy {
    speak "The number you have dialed, $number, is busy."
    speak "Please try again later."
    disconnect
    exit
  }
  on decline {
    speak "The number you have dialed, $number, refuses your call."
    disconnect
    exit
  }
  on answered {
    connected
    connect $host $port
    goto icall $host $port
  }
}

# Gatewayed phone call is in progress. User can press # to be connected
# to a different number.
state icall {host port} {
  on disconnected {
    # disconnect the other side
    disconnect $host $port
    exit
  }
  on dtmf # {
    goto initial
  }
}