Can We Get There From Here?

The impact of the evolving PSTN on the user's perception of QoS

Dave MustillBT ExactIP/ATM Performance & QoS Standards

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

What do we mean by delay in the PSTN?

Sound goes into the mouthpiece of one phone and comes out ofthe earpiece of the other.....

THE PSTNSound in

Sound out

What do we mean by delay in the PSTN?

Sound goes into the mouthpiece of one phone and comes out ofthe earpiece of the other.....BUT NOT STRAIGHT AWAY.

THE PSTNSound in

Sound out

What do we mean by delay in the PSTN?

Sound goes into the mouthpiece of one phone and comes out ofthe earpiece of the other.....BUT NOT STRAIGHT AWAY.

IT CAN TAKE TENS OF MILLISECONDS.

THE PSTNSound in

Sound out

What do we mean by delay in the PSTN?

One-way delays over 25ms can result in echo being perceivedunless it is controlled or cancelled.One-way delays over ~100ms start to affect interactivity.

THE PSTNSound in

Sound out

Electrical echo

Acoustic echo

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.

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

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

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

Where are we now?

UK PSTNcall

Where are we now?

UK PSTNcall

trans-global call ormobile to landline

call

Where are we now?

UK PSTNcall

trans-global call ormobile to landline

callmobile to

mobile call

Where are we now?

UK PSTNcall

trans-global call ormobile to landline

callmobile to

mobile call

international call with satellite hop

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....

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....

We want to be about here....

UK PSTNcall

trans-global call ormobile to landline

callmobile to

mobile call

international call with satellite hop

We want to be about here....

UK PSTNcall

trans-global call ormobile to landline

callmobile to

mobile call

international call with satellite hop

But we want here tooffer a wider range of services and

be easier and cheaper to run.

Where are we headed?Let’s add IP technology to the PSTN...

Sound still goes into the mouthpiece of one phone and comes out ofthe earpiece of the other.....it’s packetised, buffered andde-packetised in the IP bit of the network and this can addup to 30 or 40ms of delay. But the PSTN is not this simple...

THE PSTNSound in

Sound out

THE IP BIT

Number Portability can produce some long speech paths in the UK

NGNPDonor

GNPDonor

OriginatingNetwork

NGNP Recipient& Service Logic

GNPRecipient

Transit Network

This call path passes through nine network hops (plus two accesslines). Even if each hop adds ~10ms delay we are still on theG.114 plateau so customers should be satisfied with the call quality.

What if one of the operators puts IP technology in its network?

NGNPDonor

GNPDonor

OriginatingNetwork

NGNP Recipient& Service Logic

GNPRecipient

Transit Network

We’ve added up to another 30ms and we are starting to move outtowards the G.114 slippery slope to dissatisfaction.

IP to TDM orTDM to IPconversion

What if the transit operator also puts IP technology in its network?

NGNPDonor

GNPDonor

OriginatingNetwork

NGNP Recipient& Service Logic

GNPRecipient

Transit Network

Each time the path enters the transit network it can experience anadded delay of 30 to 40ms. Even when entering the other IP-basednetwork we have to convert to TDM for interconnect – this could meanan end-to-end delay of 150 to 250ms.

IP to TDM orTDM to IPconversion

Where does this put us on the curve?

UK PSTNcall

complex UK call withsome IP elements

Where does this put us on the curve?

UK PSTNcall

complex UK call withsome IP elements

add a mobile to oneend of the above

Calls like this hardly ever happen...

NGNPDonor

GNPDonor

OriginatingNetwork

NGNP Recipient& Service Logic

GNPRecipient

Transit Network

But calls like this do...

NGNPDonor

GNPDonor

OriginatingNetwork

NGNP Recipient& Service Logic

GNPRecipient

Transit Network

Even if some parts of the call are owned by the same operator andwe can avoid some of the multiple IP packetisations andde-packetisations we could still be adding three or four lots of30 to 40ms delay if all operators migrate to IP with TDM interconnect.

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!

How do we get to where we want to be?

How do we get to where we want to be?• Standards• Standards• Standards• Standards• Standards• Standards• Standards• Standards

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

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

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.

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?

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.

Thanks for your attention.

Dave MustillBT ExactIP/ATM Performance & QoS Standards