Interworking Internet Telephony and Wireless

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Barnaby Cain
6 years ago
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1 Interworking Internet Telephony and Wireless Telecommunications Networks Bell Laboratories & Columbia University Kazutaka Murakami, Mehmet Karaul, Thomas F. La Porta Bell Laboratories Tuesday, April 3, 2001 Work performed in the Networking Techniques Research Department, Bell Labs Research, Lucent Technologies April 3,

2 Interworking SIP and Mobile Telephony: Outline Motivation Interworking Architecture and Proposals Evaluation Analysis Conclusion April 3,

3 Motivation Two rapidly-growing components of telephony: Internet Telephony (SIP) Mobile Telephony In particular, 2nd-generation digital systems: GSM (Europe) IS-41 (USA) Third-generation mobile telephony systems (3GPP) will be IP networks with telephony provided by voice-over-ip This is still a ways away We need able to interwork Internet telephony with second-generation systems as well The obvious way to connect them would be to use PSTN in the middle Directly interworking them can be much more efficient April 3,

4 Why is direct interworking more efficient? Eliminate triangular ( trombone ) routing Local Switch Caller Destination Serving Switch Intermediate Switch Eliminate media transcoding Many SIP terminals can use GSM Ease transition in future evolution 3GPP IP-based switching centers April 3,

5 Interworking Architecture 2G air interfaces (GSM, CDMA/TDMA) are unchanged Fixed parts of mobile network (inter-msc) use SIP (and RTP) HLR VLR SIP SIP Proxy SIP serving MSC 2G April 3,

6 Interworking: easy parts Calls from 2G to SIP Subscriber s HLR knows whether calls should be placed using SIP Could use custom dialing plan prefixes, e.g. Enum lookups of phone numbers Same as PSTN-to-SIP In-call mobility Re-INVITE for intra-msc mobility Possibly REFER, possibly anchoring for inter-msc mobility April 3,

7 Interworking: Registration and calls from SIP to 2G We want to have calls from SIP to 2G devices be SIP/RTP to the serving MSC So, the user s home SIP proxy server needs to be able to find the serving MSC Three techiques possible: Modified Registration Modified Call Setup Modified HLR Will illustrate 2G with GSM April 3,

8 SIP-to-GSM Technique 1: Modified Registration VLR 3 2 UPDATE_LOCATION INSERT_SUBSCRIBER_DATA (MSISDN) HLR 1 GSM Location Update Request serving MSC 6 REGISTER SIP Proxy & Registrar IMSI 5 SIP Registrar 4 Address MSISDN Enum Distributed Database (MSISDN - Registrar) Serving MSC locates subscriber s home registrar based on MSISDN Parallel registrations GSM and SIP SIP call setup procedures are standard April 3,

9 SIP-to-GSM Technique 2: Modified Call Setup VLR 3 PROVIDE_ROAMING_NUMBER 4 (IMSI) Response (MSRN) Response 5 (MSRN) HLR 2 SEND_ROUTING_ INFO (MSISDN) GSM INVITE INVITE serving SETUP (to serving MSC SIP SIP Proxy (to user address) MSC device address) 6 MSRN 7 Enum Distributed Database (MSRN - Device) Serving MSC SIP Device Address GSM/IS-41 registration is standard SIP proxy server initiates GSM MSRN lookup procedure Locates SIP address of smsc based on MSRN April 3,

10 SIP-to-GSM Technique 3: Modified HLR GSM HLR co-located with SIP registration database Can map GSM registrations to SIP locations Proxy server queries registration database April 3,

11 Enum Distributed Database (MSC (E.164) - MSC (SIP)) 3 4 Serving MSC E.164 Address Serving MSC SIP Domain VLR 2 UPDATE_LOCATION HLR 1 GSM Location Update Request IMSI serving MSC SIP Proxy Modified HLR Registration April 3,

12 VLR HLR 3 GSM 2 SETUP serving MSC INVITE (to MSISDN@serving MSC) SIP Proxy 1 INVITE (to user address) Modified HLR Call Setup April 3,

13 SIP-to-GSM Techniques: Evaluation (1) Modified Registration Least modification to existing infrastructure SIP proxy and registrar, GSM HLR and gmsc unchanged Call setup is fast Two separate databases can get out of sync High overhead with low mobility rates Modified Call Setup No database inconsistencies Low registration overhead Triple-phase call setup: high signalling load, latency HLR, SIP Proxy must be able to communicate April 3,

14 SIP-to-GSM Techniques: Evaluation (2) Modified HLR Relatively low signalling requirements for registration; quite low requirements for call setup Call setup latency similarly low Invasive modifications of HLRs SIP proxy and HLR must be co-located, or have communication channel April 3,

15 SIP-to-GSM Techniques: analysis We analyzed message weights for each of the three approaches Depending on characteristics, modified registration or modified call setup has lower signalling load Modified HLR is always lowest April 3,

16 Tot. packet weight Modified Registration Modified Call Setup Modified HLR Mod. Reg = Mod. C.S Call / Mobility Ratio Incoming Call Rate call / hour Weighted signalling load of the three proposals April 3,

17 Implementation Implemented modified call setup scheme atop Bell Labs RIMA experimental distributed MSC/VLR. Didn t (at the time) have a HLR or gmsc to play with; had to fake it by having SIP proxy contact VLR Further development of the modified HLR scheme is in progress April 3,

18 Conclusions Efficient interworking of SIP and GSM is possible Modified HLR is the best technique for registration and call setup, though it requires the most development work April 3,

Three kinds of number portability

Number Portability Three kinds of number portability Location portability: a subscriber may move from one location to another location without changing his or her telephone number Service portability: