Where should services reside in Internet Telephony Systems? Xiaotao Wu, Henning Schulzrinne {xiaotaow, hgs}@cs.columbia.edu Department of Computer Science, Columbia University, New York

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

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

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

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

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

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

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

Service architecture Programming language model

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”

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

Handle Call Waiting in DFC LI LI Setup Upack Upack Setup Setup Router CW Setup Setup Switch CW Upack Setup LI B

Handle Call Waiting in DFC LI LI C Router CW LI B

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

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

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 Voicemail (*) = with information provided by end system  

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.