Interworking Internet Telephony and Wireless Telecommunications Networks

1. Interworking Internet Telephony and Wireless Telecommunications Networks Jonathan Lennox Kazutaka Murakami Mehmet Karaul Thomas F. La Porta Presented By: Matt Vidal lennox kmurakami karaul tlp @bell-labs.com @bell-labs.com @bell-labs.com @bell-labs.com Bell Labs, Lucent Technologies July 1,2003

2. Introduction • Wireless Mobile Telephony and Internet Telephony are both growing at a rapid pace • Mobile: Global System for Mobile communication (GSM) • Internet Telephony: Session Initiation Protocol (SIP) • What is the best way for calls to be delivered and routed between the two networks? • Both SIP and GSM are designed to interface with Public Switched Telephone Network (PSTN) • PSTN Interface is a bad idea as it can be very inefficient in the mobile realm

3. Introduction (2) • Triangular Routing in mobile networks due to PSTN • Causes highly inefficient routing of both signaling and media packets Triangular Routing in Mobile Networks

4. Background: GSM Mobility and Call Delivery • MSC: Mobile Switching Center • Serving MSC routes all mobile calls within its serving area • Gateway MSC directs calls from PSTN into mobile access network • VLR: Visitor Location Register • Stores information about devices in its serving area • HLR: Home Location Register • Maintains subscriber information and location • GSM device has International Mobile Subscriber Identity (IMSI) • HLR gets mobile’s location using GSM Mobile Application Part (MAP) Protocol

5. Background: GSM Mobility and Call Delivery (2) • GSM call placed from Mobile Station ISDN Number (MSISDN) – Sent to gateway MSC (2 Phases) • 1. Gateway MSC obtains temporary Mobile Station Routing Number (MSRN) by contacting subscriber’s HLR • 2. The call is routed to the serving MSC using ISDN User Part Protocol (ISUP) • Intra-MSC Handover (Between base stations within one MSC’s control) – media traffic is simply redirected • Inter-MSC Handover (To a different serving MSC) – old serving MSC is required as an anchor, extending the call’s media circuit across the PSTN

6. Background: GSM Mobility and Call Delivery (3) Elements of a GSM Network

7. Background: SIP Mobility and Call Delivery • Simpler than GSM • All devices communicate by IP • All signaling occurs with SIP • GSM / SIP Mapping GSMSIP HLR Registrar Gateway MSC Home Proxy Server Serving MSC End System (for REGISTER) MSISDN User Address (in INVITE) IMSI User Address (in REGISTER) MSRN Device Address

8. Background: SIP Mobility and Call Delivery (2) • End Systems Contact Registrars Directly (Not 2 Phases) • New SIP device sends SIP REGISTER to its Registrar • SIP Address is in the form of User@Domain • New call sends SIP INVITE to SIP Address • Call is established and media flows directly between the endpoints • Handles handovers by reconfiguring the IP Address it is sending to without having to relay from home address as in mobile IP.

9. Background: SIP Mobility and Call Delivery (3) Elements of a SIP Network

10. Architecture: SIP/GSM Interworking • Three primary issues to consider when interworking Internet Telephony and Wireless Systems 1. How Calls May Be Placed From SIP to GSM 2. How Calls May Be Placed From GSM to SIP 3. How In-Call Mobility (Handovers) Is Handled • Issues two and three are easily dealt with • The first issue is the most complicated and will be investigated thoroughly

11. SIP/GSM Interworking: Calls From GSM To SIP • Calls from GSM to SIP are functionally similar to calls originating from PSTN to SIP • Difference is that they are addressed in telephone form rather than User@Domain form • Solution is to perform a lookup (Enum) based on the telephone number • Maybe use custom dialing plan prefixes • Translate number into User@Domain form • Proceed as usual PSTN-to-SIP

12. SIP/GSM Interworking: In-Call Handover • Two Categories 1. Intra-MSC 2. Inter-MSC • Intra-MSC needs no special treatment for SIP/GSM Interworking • Could optimize by using different IP Addresses to correspond to different base stations under serving MSC’s control • Inter-MSC affects SIP/GSM Interworking • Remains for future study

13. SIP/GSM Interworking: Mobile-Terminated Calls • Most Complex Aspect of SIP/GSM Interworking • Need direct media stream between caller and serving MSC • To do so, SIP signaling needs to reach Serving MSC for: • IP Address • Port Assignment Conventions • Media Characteristics • Three proposals for how SIP devices can determine the MSC at which the GSM device is registered • Each has various degrees of complexity, signaling traffic and call setup delay

14. Proposal 1: Modified Registration • Enhance MSC’s Registration Behavior • Serving MSC registers with Subscriber’s HLR and with “Home SIP Registrar” • MSC Needs to find SIP Registrar • This can be done with an Enum lookup using GSM device’s MSISDN number from registration • Initial Registration (Dual Registration) • 8-10 GSM MAP messages, 6 DNS messages, 4 SIP messages • Call Setup (Standard SIP) • 1 SIP message, 4 DNS messages

15. Proposal 1: Modified Registration (2) Modified Registration Procedure

16. Proposal 1: Modified Registration (3) • Advantages • Minimal Infrastructure Modifications • Requires updates to the serving MSC • Requires the addition of an Enum type of Database • Call Setup is SIP, one phase lookup • Disadvantages • With two databases, inconsistency could be a problem • GSM HLR is persistent • SIP Registrations require refreshing • Dual registration requires extra signaling overhead

17. Proposal 2: Modified Call Setup • Adapt GSM Call Setup • GSM Registration untouched • GSM call setup behaves like SIP • SIP Proxy Server determines MSISDN based on SIP Address • Queries HLR for MSRN • Enum Lookup for serving MSC SIP Address • SIP INVITE message sent to serving MSC • Initial Registration (Standard GSM) • 8-10 GSM MAP messages • Call Setup • 4 GSM MAP messages, 6 DNS messages, 1 SIP message

18. Proposal 2: Modified Call Setup (2) Modified Call Setup Procedure

19. Proposal 2: Modified Call Setup (3) • Advantages • GSM database remains the same • No possibility of out-of-sync database like in Modified Call Setup • Low registration overhead • Same as GSM • Disadvantages • Three Phase Call Setup! • GSM MAP query for the MSRN • Enum lookup for SIP device address • Call initiation • SIP and HLR need to be in communication

20. Proposal 3: Modified HLR • GSM HLR in Charge • Standard GSM registration • HLR determines the SIP device address at the serving MSC • HLR performs a Enum lookup to gather the serving MSC’s SIP domain (taking advantage of SIP address format) • HLR returns SIP Address (mapped from GSM registration, MSISDN@hostname.of.serving.MSC) to the SIP Proxy Server • Initial Registration • 8-10 GSM MAP messages, 2 DNS messages • Call Setup • 4 DNS messages, 1 SIP message

21. Proposal 3: Modified HLR (2) Modified HLR, Registration Procedure

22. Proposal 3: Modified HLR (3) Modified HLR, Call Setup Procedure

23. Proposal 3: Modified HLR (4) • Advantages • Very low signaling overhead • Nearly matches best registration overhead • Has the best call setup overhead • Low Call Setup Latency • Disadvantages • Significant modification of GSM HLR • SIP Proxy Server and HLR must be co-located, or have an interface protocol designed

24. Analysis • Two important measures for SIP and GSM Proposals: • Signaling Load • Setup Delay (to be investigated at a later time) • To compare total signaling load, signaling messages were weighted and compared • Modified HLR has lowest signaling load (20-30% less) • The other two depend on traffic parameters • Modified Call Setup best with low incoming call rate and low call/mobility ratio (higher setup time) • Modified Registration best with high incoming call rate and high call/mobility ratio (higher registration time)

25. Analysis (2) Weighted Signaling Load of all Three Proposals

26. Implementation • Implemented Modified Call Setup • Enhanced Mobile Call Processing (EMCP) of Bell Labs Router for Integrated Mobile Access (RIMA) • RIMA composed of Base Station Controller (BSC), PSTN Media Gateway (PSTN-GW), RTP Media Gateway (RTP-GW) and EMCP network • BSC: Circuit Voice Formatted to/from Packet Voice • PSTN-GW: RTP/IP Packet Voice Translated to/from Circuit Voice (RIMA <> PSTN) • RTP-GW: RIMA Media Connections to Internet • EMCP: Implements RIMA’s MSC and VLR Functionality (IP Telephony Signaling Gateway)

27. Implementation (2) RIMA-based Network Implementation

28. Implementation (3) EMCP Processing Engine

29. Implementation (4) EMCP Network Architecture

30. Conclusions • Proposed Three Schemes to Directly Interconnect SIP Internet Telephony and GSM Mobile Telephony Systems • Modified Registration • Modified Call Setup • Modified HLR • Eliminates Triangular Routing Created by PSTN • More Efficient Media Packet Routing • Eliminates Unnecessary Transcodings Due to PSTN • Better Voice Quality

31. Conclusions (2) • Modified Registration • Very Efficient When High Incoming Call Rate And High Call/Mobility Ratio (Higher Registration Overhead) • Modified Call Setup • Very Efficient When Low Incoming Call Rate And Low Call/Mobility Ratio (Higher Call Setup Overhead) • Modified HLR • Very Low Signaling Burden (20-30% Less Than Modified Registration / Modified Call Setup) • Significant GSM Equipment Modification • Development is in progress for a Modified HLR scheme

32. Questions?