Communications Environment Using SIP and Hybrid Internet-IN/PSTN

Fig. 1 shows an example of the communications deployed by two colleagues that collaborate across the Atlantic to develop a presentation. Henry works from home, while Alan can be reached at the office phone at work, at the home phone or cellular phone in the car. Alan can also be reached at his PC at work or at home.   When at home, Alan dials into the company's intranet for PC access.

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Fig. 1  Communications environment example for using SIP 

There is an audio/video/data conference server available that can be accessed both from within the intranet and also from the Internet by authenticated users.

Overview of Communications

Henry calls Allan's work phone via by Internet telephony to set up with SIP an a/v/data conference to jointly edit a presentation.

Alternatives for placing the call:

Use a phone with Internet telephony gateway - shown here below in detail, or
Use a PC phone with PINT server and VPN

Allan does not answer his work phone. Possible alternatives:

Allan leaves a message where he can be contacted, or
A CTI/PSTN forwarding service connects to Alan's home phone and then to Alan's cell phone, or
An IP contact manager returns Alan's cell phone number - shown below in detail.

Henry calls or is redirected to Allan's cell phone. Conversation via Alan's cell phone: "Let's have an a/v/data conference next morning, OK ?"

They set up a multiparty a/v/data conference

Using a T120 conference server, or
Using SDP and SIP - shown here below
The a/v/data meeting is conducted and the participants keep the file of the presentation.
Jeff can participate in the conference only by phone and will get the presentation faxed to him. The alternative meeting protocols for Jeff are:
H.323 meeting - diagram for audio call setup (Henry will provide a Fig. 2 later) - shows lots of messages
SDP-SIP meeting - diagram for audio call SIP setup (Henning will provide a Fig. 3)
A copy of the presentation will be faxed to Jeff in real time. We show her a fax call using ESMTP.
Step-by step messages and comments

Call Setup Step

Message/Code(pseudo-code)

Use phone and InternetPhone Company

 
1 - Dial InternetPhone Co.

1-800-XXX-XXXX
2 - Enter PIN

home number + 4 digits
3 - Dial number

+44 1473 + 6 digits
4 - Call is answered

 

Or, use PC and click to dial.

To: phone://+44 1473 + 6digits

As an alternative use SIP from the PC. The nearest SIP server can be registered in the client. It locates the remote IPtel gateway and the call is routed over the local PSTN. A similar sequence could be used to route the call over the enterprise virtual private network (VPN) using the IN and PINT. The E.164 address would be replaced by the VPN phone number and a VPN authorization code is required. This is a case of Hybrid IN and Internet use that will not be developed here any further, since it is more complex.

The SIP call sequence below has 3 messages (see Fig 3), vs. 19 for Q.931 signaling ( see Fig 2).
SIP server locates gateway

Static routing table or resource location
SIP call to gateway

 
Gateway confirms OK

 
Call answered by recorder

 

The message on Alan's recorder indicates Alan is not in the office. Henry could have saved the trouble by using first an Internet address and triggering the location server to find Alan. But OK to show as an the example...

Click to dial using location a server

INVITE alan@bt.co.uk
Redirect to cell phone

 
Call cell phone

 
Call answered by Alan

 

Alan and Henry have the conversation and agree to call an a/v/data conference call.

An a/v/data conference call is announced by e-mail using SDP. (Henning, please expand or change freely here. I feel humble when it comes telling you about SDP and SIP) Jeff is at the "LittleHotel" and has there only plain phone and fax communications via a fax gateway located at "littlehotel.com"..

Alan invites Henry and Jeff to the conference.

INVITE henry@mci.net

INVITE To: phone://1-703-715-XXXX

Call answered by Henry on his PC

 

Call answered by Jeff on his phone

 

a/v/data conference is proceeding. Alan and Jeff retain a copy of the presentation. During the conference, the presentation drafts are faxed to Jeff's hotel.

The Internet fax transport protocol ESMTP [4] is used to communicate from Alan's PC to a plain fax machine in Jeff's hotel. ESMTP is also used between the PC client and the fax gateway talking to the fax machine.

Fax to Jeff - immediate delivery

RCPT TO: 1-703-715-XXXX@littlehotel.com; SESSION

Fax confirmed

250;