1. Where should services reside in Internet Telephony Systems?
Xiaotao Wu, Henning Schulzrinne
{xiaotaow,
Department of Computer Science,
Columbia University, New York

2. Outline Most services can be in end system Where should service reside
PSTN vs. Internet Telephony
Call waiting
Where should service reside
End system vs. Network server
Service architecture
Programming language model
DFC
Service examples for different models and feature interaction between end system and network server

3. PSTN vs. Internet Telephony
end system
PSTN
Number of lines
or pending calls
is virtually unlimited
Generally, end system in Internet Telephony logically have multiple lines. It can deal with unlimited incoming phone calls.
PCs can be considered as the end-user devices and can perform many service functionalities.
Unlimited lines, no feature interaction, good or not? Keep all the things in human’s mind?
More intelligence, PCs
can be considered to be
end-user devices
Single line,
12 buttons and
hook flash to signal

4. PSTN vs. Internet Telephony
Signaling & Media
Signaling & Media
With a few exceptions, in Internet telephony, end systems are the only entities where signaling and media flows converge. Thus, any service that requires interaction with user media is likely to be easier to implement in the end systems.
More seperately
Internet
Telephone:
Signaling
Signaling
Media

5. Call waiting Wait 2 minutes Line 2 ringing Press line 2 A 180 Ringing
INVITE
Talk on line 1
180 Ringing
INVITE,
SDP’s c=0
200 OK
182 Wait 2 minutes
Traditional call waiting services do not make sense in an Internet environment, where the number of ``lines'' or pending calls is virtually unlimited. It’s not necessary to use traditional ‘hook flash’ to switch calls.
Though, logically, the number of lines is unlimited, the user is limited to the ability to handle multiple conversations. And the device is also limited to provide multiple media output. So, generally, the user can not handle two calls at the same time, the user need to put on party on hold.
The busy handling service requires human interaction, it’s more convenient to handle it in the end system.
The end system can easily provide more information, like ‘Hold for 2 minutes’ B C

6. Call waiting A
200 OK
Talk on line 2
Hold on line 1 C B

7. End system vs. Network server
Permanent IP address
Always on
(User can have unique address
and can always be reached)
Ample computational capacity
High bandwidth
(Conference)
Indirect user interaction
Usually only deals with signaling
(Based on predefined mechanisms,
or indirect user interaction, like
through web page)
End system
Temporary IP address
Powered off so often
(User’s address always changed
and can not be reached sometime)
Limited computational capacity
Low bandwidth
(One to one or small size conf.)
Direct user interaction
Signal and media converge
(easier to deal with human
interaction, easier to deal with
interaction with media)
More latency for user interaction

8. End system vs. Network server
Information hiding
Logical call distribution
Gateway
End system
Busy handling
Call transfer
Distinctive ringing

9. Service architecture Programming language model

10. Service architecture DFC
DFC and SIP proxy model are all try to distribute services. The main difference is, in DFC model, router is introduced to set the order of the feature interaction (the feature box can interact with router to set its own order). In SIP proxy model, the order of feature execute is “hop-by-hop”

11. Call forwarding on busy
c.cgi A
New INVITE
Run c.cgi
c.cgi handle busy
302
INVITE
INVITE
486 busy
302 Ok
Talk on line 1
INVITE
200 Ok C
If the service logic keeps track of the end system, it can based on rule to do the call forwarding. E.g.
The user can set the rule on the proxy, like 'Don't disturb rule'. When the user get some important call (like talking with my boss), he can just enable the rule on the proxy (or the rule can automatically enabled based on the caller information). This rule can also be set in the end system. In the end system, it can either handle the rule the same as the proxy, or simply set the maximum call can be handled as 1, if the maximum callnum can be configured.
If the rule is set on proxy, when the user moves, the same rule can be applied to every end system he uses.
If we use the ‘Don’t disturb rule’, the service logic can redirect the call.
For the first INVITE, pingtel phone use name alias for the INVITE, the name alias can be translated to the valid username by the service logic. B D

12. Handle Call Waiting in DFCLI LI
Setup
Upack
Upack
Setup
Setup
Router CW
Setup
Setup
Switch CW
Upack
Setup LI B

13. Handle Call Waiting in DFCLI LI C
Router CW LI B

14. Call forwarding on busy in end system
INVITE C A
302
200
INVITE
For call forwarding on busy in end system, it’s quite simple. Just send 302 to the caller.
Talk on line 1 D B

15. Feature interaction between end system & network server
c.cgi
302
(Don’t disturb rule)
(I can accept
call on line 2)
Run c.cgi
INVITE
302 Ok A
Talk on line 1 C
INVITE
200 Ok
Mention CPL, bring two parts together
Centralized service v.s. service in end system. B D

16. Service location examples
End system
Network (proxy)
Network with Media (UA)
Distinctive ringing
Yes
Can assist
Visual call id
Call waiting No
Yes(*)
CF busy
CF no answer
CF no device
Location hiding
Transfer
Conference bridge
Gateway to PSTN
Firewall control
Voic
(*) = with information provided by end system

17. Conclusion
Powerful end systems offer benefits such as flexibility and personalized services
End system implementation are good for user interaction
DFC and SIP proxy implementations make it possible to distribute services
The interaction between end system services and network services is still an open issue.