1. Can We Get There From Here? The impact of the evolving PSTN on the user's perception of QoS Dave Mustill BT Exact IP/ATM Performance & QoS Standards

2. Can we get there from here? What do we mean by delay in the PSTN? Where are we now? What about the future? Where are we headed? How do we get to where we want to be? Conclusions

3. What do we mean by delay in the PSTN? Sound goes into the mouthpiece of one phone and comes out of the earpiece of the other..... THE PSTN Sound in Sound out

4. What do we mean by delay in the PSTN? Sound goes into the mouthpiece of one phone and comes out of the earpiece of the other.....BUT NOT STRAIGHT AWAY. THE PSTN Sound in Sound out

5. What do we mean by delay in the PSTN? Sound goes into the mouthpiece of one phone and comes out of the earpiece of the other.....BUT NOT STRAIGHT AWAY. IT CAN TAKE TENS OF MILLISECONDS. THE PSTN Sound in Sound out

6. What do we mean by delay in the PSTN? One-way delays over 25ms can result in echo being perceived unless it is controlled or cancelled. One-way delays over ~100ms start to affect interactivity. THE PSTN Sound in Sound out Electrical echo Acoustic echo

7. ITU-T Recommendation G.114 - One-Way Transmission Time Revised version recently agreed. Recommends that, for general network planning and regardless of the type of application, one- way delay (UNI-to-UNI) should not exceed 400ms. Notes that if end-to-end (i.e. "mouth to ear" in the case of speech) delays can be kept below 150ms most applications will experience essentially transparent interactivity.

8. The Effects of Absolute Delay (from ITU-T Recommendation G.114)

9. Where are we now? Delay objectives for the UK are given in the National Transmission Plan - NPDS7(94)4 For UK connections without echo cancellation: – the NTP-NTP one-way delay should be less than 15ms for at least 95% of calls – for the small proportion of calls that exceed the 15ms limit, an absolute limit of 25ms is recommended

10. Where are we now? For UK fixed line to fixed line, or fixed line to digital mobile with echo cancellation: – 95% of national connections should meet a 125ms one-way delay limit – no connections should exceed 150ms For UK digital mobile to digital mobile with echo cancellation: – 95% of national connections should meet a 215ms one-way delay limit – no connections should exceed 230ms

11. Where are we now? UK PSTN call

12. Where are we now? UK PSTN call trans-global call or mobile to landline call

13. Where are we now? UK PSTN call trans-global call or mobile to landline call mobile to mobile call

14. Where are we now? UK PSTN call trans-global call or mobile to landline call mobile to mobile call international call with satellite hop

15. What about the future? The PSTN is becoming expensive to maintain (in fact, it’s always been expensive!). The 64kbit/s time division multiplexed (TDM) architecture of the PSTN offers limited scope for service innovation. Voice, data, multimedia applications are becoming more sophisticated and more useful, and it would be great to offer them on a reliable, secure and high quality network (like the PSTN). Bring on the Internet Protocol....

16. What about QoS in our IP-based PSTN? User expectations of PSTN quality of service (QoS) are unlikely to become more relaxed. Evolution will not happen quickly enough for our brains to adapt to longer transmission delays. Our boys at the labs have have ruled out genetic engineering for the foreseeable future. So....

17. We want to be about here.... UK PSTN call trans-global call or mobile to landline call mobile to mobile call international call with satellite hop

18. We want to be about here.... UK PSTN call trans-global call or mobile to landline call mobile to mobile call international call with satellite hop But we want here to offer a wider range of services and be easier and cheaper to run.

19. Where are we headed? Let’s add IP technology to the PSTN... Sound still goes into the mouthpiece of one phone and comes out of the earpiece of the other.....it’s packetised, buffered and de-packetised in the IP bit of the network and this can add up to 30 or 40ms of delay. But the PSTN is not this simple... THE PSTN Sound in Sound out THE IP BIT

20. Number Portability can produce some long speech paths in the UK NGNP Donor GNP Donor Originating Network NGNP Recipient & Service Logic GNP Recipient Transit Network This call path passes through nine network hops (plus two access lines). Even if each hop adds ~10ms delay we are still on the G.114 plateau so customers should be satisfied with the call quality.

21. What if one of the operators puts IP technology in its network? NGNP Donor GNP Donor Originating Network NGNP Recipient & Service Logic GNP Recipient Transit Network We’ve added up to another 30ms and we are starting to move out towards the G.114 slippery slope to dissatisfaction. IP to TDM or TDM to IP conversion

22. What if the transit operator also puts IP technology in its network? NGNP Donor GNP Donor Originating Network NGNP Recipient & Service Logic GNP Recipient Transit Network Each time the path enters the transit network it can experience an added delay of 30 to 40ms. Even when entering the other IP-based network we have to convert to TDM for interconnect – this could mean an end-to-end delay of 150 to 250ms. IP to TDM or TDM to IP conversion

23. Where does this put us on the curve? UK PSTN call complex UK call with some IP elements

24. Where does this put us on the curve? UK PSTN call complex UK call with some IP elements add a mobile to one end of the above

25. Calls like this hardly ever happen... NGNP Donor GNP Donor Originating Network NGNP Recipient & Service Logic GNP Recipient Transit Network

26. But calls like this do... NGNP Donor GNP Donor Originating Network NGNP Recipient & Service Logic GNP Recipient Transit Network Even if some parts of the call are owned by the same operator and we can avoid some of the multiple IP packetisations and de-packetisations we could still be adding three or four lots of 30 to 40ms delay if all operators migrate to IP with TDM interconnect.

27. And it could get worse... The G.114 curve assumes we use the PSTN G.711 codec end-to-end, most other codecs move the curve towards more dissatisfaction. If poor echo control is implemented then the adverse effects of delay will become more noticeable. Varying packetisation sizes in different networks could add more delay. Let’s put VoIP in all our private networks!

28. How do we get to where we want to be?

29. Standards

30. Who is working on the necessary standards? International Telecommunications Union –ITU-T Study Groups 2, 11, 12, 13 & 16 European Telecommunications Standards Institute –work started under Project TIPHON, now in Technical Committee TISPAN UK End-to-End QoS Task Group

31. UK End-to-End QoS Task Group (E2E QoS TG) Reports to NICC’s Public Network Operators Interest Group (PNO-IG) Established in autumn 2000 to recommend changes to the NPDS to take account of the introduction of new technologies into the PSTN –Look at IP, ATM, other technologies –Only consider POTS phone (no IP phones, no MM terminals) –Users not to see any degradation of quality

32. Current status of the NPDS No changes have yet been agreed to the delay section of the NPDS. An Annex was approved in 2002 which gives guidelines on how to limit the QoS impact of the incorporation of new technologies the PSTN. This Annex recommends that each operator should aim to add no more than 12.5ms delay per “network hop” (including access lines). N.B. This document is not yet “in force” and may be revised further.

33. Some outstanding issues... For the E2E QoS TG –Speech sample size for VoIP –Echo cancellation guidance –Post Dial Delay (currently unnoticeable for most UK PSTN calls) Other issues –International compatibility –Can we manage with TDM interconnect? –Do we need to do anything about those weird routings?

34. Conclusions The PSTN offers high quality voice communication and we mustn’t mess that up. IP technology offers a lot of possibilities - but it must be implemented with care or there is a real danger that the QoS of the PSTN could dip in the near future. End-to-end delay must be carefully managed and echo must be properly controlled. There are tough engineering decisions to be made and standards offer us a way forward.

35. Thanks for your attention. Dave Mustill BT Exact IP/ATM Performance & QoS Standards