1. VoIP – Vs – PSTN
I am sure that all participants in this discussion thread appreciate that telecom networks have
been built over the years to serve a purpose or purposes.
1. Thus in the beginning they were built to facilitate speech communications and the PSTN
evolved to its present state through various phases of development. The PSTN network
of today consists of a subscriber access network from and to the Local Exchange. An
Access Network (AN) which connects the Local Exchanges in a city to the transport
network (TN) terminal at the city. The transport network terminal at the city is connected
to transport network terminals of other cities through mesh or tri-node topology, so that
in case of a TN link failure, alternate paths were available to the city. The transport
networks of the modern PSTN was originally built using PDH (Plesiochronous Digital
Hierarchy), and later replaced by SDH or SONET (US and Canada). SDH frames can
transport large volumes of voice and data in TDM, ATM, TCP/IP and Ethernet protocols.
2. Initially data communications were carried out through standard PSTN lines with dial-up
modems, and then through ISDN lines at higher data speeds than the dial-up modem.
Public data networks were also built using ISDN / PSTN lines access and p2p
connections between network nodes. These had a data speed limitation.
3. When higher data speeds were required by the subscribers of public data networks and
also for Internet access, TSPs round the world started building IP backbones with much
higher data speed capabilities. Typical topology and architecture of such networks is
shown in VPN1.pdf shown in http://www.slideshare.net/pankajmitra.
4. Enterprises/ Organisations could access these IP Backbones of their respective TSPs
(telephone service providers) using point-to-point (p2p) leased lines between their
Organisation LANs and the nearest POP of the TSP IP Backbone (IP Core) to achieve
the following purposes.
a. Have the facility of high speed Internet Access;
b. Have the facility of high speed data communications with other enterprises /
organisations through this TSP IP Backbone which served as the high speed
public data network of the TSP;
c. Set up VPNs between their own geographically separated locations.
d. These purposes were to have higher speed Internet access and higher speed
data communications with outsiders and insiders of the enterprise / organisation.
5. Most TSPs did not allow voice communications through their IP Backbones which were
built for high speed data communications and Internet access. They encouraged these
enterprise / organisations to carry on their speech communications needs through the
PSTN networks which were being run in parallel by the same TSPs. However, some of
them have now opened up VoIP over their IPBs after establishing proper methods of
billing and accounting.
6. Since it was not viable for individual subscribers to have p2p leased lines to get the high
speed data access to the IP backbones of TSPs, the latter deployed ADSL and ADSL2+
technology to provide individual telephone subscribers high speed data access
(broadband) over their existing telephone cable pair. This was done by placing DSLAMs
(digital subscriber line access multiplexers) at the Local Exchange Buildings of the TSP.
2. ADSL or ADSL2+ modems were placed at the subscriber premises to which were
connected the subscriber’s analogue telephone, and his/her computer / LAN switch /
router. The analogue telephone signal was fed through a filter into the ADSL modem
which carried combined signal to the MDF (main distribution frame) of the Local
Exchange. The filter at this end separated the voice signal and led it to the LEX (local
exchange) to facilitate metered PSTN telephony. The rest of the signal (data) was led
into the DSLAM which was in turn connected to the IP Backbone and led into the Tier 1
switch or the city and through this to the other services.
7. Thus up to the Broadband connectivity the TSPs encouraged separate voice and data
communication through the PSTN and the IP Backbone respectively, and this is the
present status. This however allowed one telephone call per broadband connection.
8. A VoIP implementation over broadband connection would facilitate multiple
simultaneous voice calls through single broadband connection. This is an advantage for
SOHOs more than for individual subscribers.
9. The document referred to above also explains why the security of internal databases are
vulnerable if they are connected through VPN networks, and recommends that to ensure
100% security of internal databases of an organisation their INTRANET WANs should
be built using p2p leased lines. Figure 2 in the document Telephony.pdf available in the
above mentioned URL shows how such p2p lines may be built over PSTN networks.
10. P2p leased lines required to ensure 100% security of internal databases of
enterprises / organisations cannot be set up over IPBs of TSPs.
11. The availability high bandwidth on the ADSL 2+ (8MB+ at 3 Km from the exchange)
prompted some TSPs to provide additional services using their IP Backbones (IPB) and
they wanted to and did introduce TV Broadcast and Video on Demand (VOD) using IP
transport over their respective IPBs from Head-end to the DSLAMs for distribution to the
ADSL2+ subscriber. To view the video images the subscriber had to install an additional
set-top-box which would enable them to see the broadcast or on-demand video content
on their TV screens. This service would run concurrently with high speed data / Internet
access and metered telephony through the PSTN and was popularly called “Triple Play”.
12. However, the folly of IP transport for real-time communications like video soon surfaced
and showed up in the form of
a. Packet loss resulting in fall in QoS
b. High Zap times – time between change of channels
c. Higher maintenance costs
which the subscribers faced, reducing the popularity of the triple play service of the TSP
over the competing cable TV network and the DTH satellite based TV services offered
by various service providers.
13. These problems can be easily resolved by having flexible channelized TDM transport
from the Head-end of the TV Broadcast and the Regional Content Farms for VOD, to the
DSLAMs in the LEX building. This step would replace the IP transport of Video signals
from the broadcast Head-end of the TSP and various VOD content Farms to the
DSLAMs at the LEX buildings, by the flexible channelized TDM transport and
3. a. Ensure 100% QoS, zero packet loss
b. Reduced Zap time
c. Lower cost of maintenance
14. The above points in 12 and 13 explain the advantage TDM transport has over IP
transport for real time communications like Video / Broadcast TV.
15. This then is the backdrop of the present telephony network which forms the basis of our
present discussion on VoIP-Vs.-PSTN.
16. For public video conferencing multiple ISDN (a subset of PSTN) links is still the best
option and facilitates real-time quality video calls / conferencing.
17. The networks PSTN and IPB have been basically set up to ensure good toll quality
telephony over the PSTN network and high speed data networking and Internet access
through the IPB.
18. I see from the sketches sent to me by Christine against earlier posts, that theirs is a
company similar to what we call the ISPs (Internet Service Providers) in this country.
Such companies set up their own IP backbones (IPBs) using p2p leased lines from
TSPs to connect their POPs. These networks were formed prior to the formation of the
TSPs’ own IPBs, and served as the high speed Internet access for their subscribers. In
addition to providing Internet access, they also offered to set up VPNs for individual
enterprises / organisations. In countries where VoIP was permitted by the TSPs, the
ISPs also offered VoIP through the VPNs of enterprises / organisations, and also to the
general public through their IPB. A substantial portion of the discussions that have
happened in this discussion thread have veered around this activity of ISPs.
19. The users of these services have been alluding to statements like “...most people are
unaware that their telephony is being carried over IP Backhauls”.
20. It would be interesting to know what percentage of the total telephony traffic is being
carried through such services in countries in which these services are prevalent. In India
till recently VoIP traffic was negligible, and more than 95% of the total telephony traffic
was been carried through CS voice in the PSTN and the CMTS mobile services. VoIP is
now being offered by some TSPs over their IPBs, in India. However, the costs have
been kept more or less at par with the PSTN call rates.
21. I see from some statistics that the total installed base of VoIP telephony has now
reached 100 million against a total telephone subscriber base of around 4 to 5 billion
(about 59% to 74% of the total world population of 6.8 billion).This works out to about
2.5% to 2% of the total subscriber base. This is after VoIP has been around for many
years now. I do hope this gives followers and subscribers to this discussion thread
a perspective of the actual status of VoIP, and folly of the claims that “PSTN days
are numbered and that this will be completely replaced by VoIP”
22. Some references have also been made to the mobile networks where from 4G onwards
there is a move afoot to have PS voice in place of CS voice which is available in
systems from 2G, 2.5G, 3G, 3.5 / 3.9G. Kindly see the document Mobile.pdf in the URL
http://www.slideshare.net/pankajmitra. It may be seen from this document that prior to
4. 4G, all voice is on CS and all data is on PS supported by the PSTN and IPB or the TSP /
MSP (mobile phone service provider).
23. The primary focus of the progressive generations of mobiles is to facilitate higher speeds
of data and Internet access, through the IPB of the MSP. Voice is not the focus. Hence
by opting for PS voice in 4G makes very little sense, and it is best left alone as CS voice
as in 3G. 3.5 / 3.9G. This will not affect the data speed enhancement in 4G.
24. LTE is the protocol which transports the combined voice and data signal across the radio
spectrum using OFDM (Orthogonal Frequency Division Multiplexing) in place of TDMA /
FDMA (a combination of frequency division multiple access and time division multiple
access) for GSM and CDMA (code division multiple access) for cdmaOne (IS95) and
25. These transport protocols do not affect the voice and data integration which uses a
multiplexer type of device when CS voice and PS data are used up to 3.9G, and a
packet aggregator in the all IP network proposed from 4G onwards.
26. Retaining CS voice in the 4G system onwards will facilitate the MSPs using
existing telephony infrastructure and they have only to increase the capacity of
their IPB for the additional data speeds.
27. By moving over to PS voice from 4G onwards, will warrant the MSPs installing IP trunks
in all their BSCs and the MSC to communicate with the 2G, 2.5G, 3G, and 3.5 / 3.9G
subscribers who are on CS voice. This population is around 90 to 95% of the total
mobile phone population. In addition to manage the PS voice through the IPB, the MSPs
will have to install the Soft switches at each TSP / MSP exchange building (BSCs and
MSC) and also the Media Control Gateway. VoIP Billing mechanism will be separate
from the present CS voice telephone billing mechanism in the MSP’s existing network.
The maintenance costs of these VoIP networks will be an additional cost over the
existing maintenance cost of the MSP network. These are additional costs which the
MSP will have to incur by moving into an all IP network as proposed in the 4G initiative.
28. By retaining voice on CS as in 3G / 3.5 / 3.9G, the MSPs will be saved these
additional expenditures, and there will be no major benefits to be derived by the
subscriber or the MSP by moving into PS voice as proposed in 4G.
29. Now let me dwell on the specific points raised by Johan and David. I will answer these
seriatim starting with Johan’s post.
a. The primary issues that require the continuance of TDM networks are
i. The need to have p2p leased lines for setting up INTRANET WANS to
ensure 100% security of internal databases.
ii. The need to have p2p leased lines to connect the IPB POPs of the
various ISPs. If p2p leased lines are not available the ISPs will have to lay
their own Optical Fibre Cable (OFC) backbone or hire dark fibre from the
TSPs’ existing infrastructure to set up their own IPBs and connect their
POPs through DWDM networks running over the OFC backbone.
iii. The availability of ISDN network for public video conferencing using
multiple ISDN connections. ISDN is a circuit switched network and a sub-
set of the PSTN network.
5. iv. Toll quality speech for business communications
v. It is not going the same way as telex...
b. I am glad you agree with that bandwidth required for VoIP calls are higher.
However, your comparison of the DOS-Windows transformation is not correct,
where the benefits in the changeover were immense. That is not the case with
change to VoIP.
c. Points 12, 13, and 14 show that the so called simplicity of convergence
through an all IP environment is a myth, and all is not honky dory with this
approach. We therefore advocate the mixed network approach – CS (for
voice/fax/video) and PS (for data and other IP services) – for convergence.
It is much more cost effective and less bandwidth hungry.
d. I would be very interested to know how many and which countries have done
away with their TDM switch infrastructure, since you mention “...a whole load of
e. I see you have not commented on the higher management costs of VoIP
networks and presume you agree with me on this point.
f. I do hope the dissertations above will explain to you how convergence can be
carried out through a mixture of TDM and IPB networks of TSPs, very cost
g. The coexistence will continue as long as equipment is available to expand and
maintain the existing TDM infrastructure. As long as steady business is coming in
some of the more successful manufacturers of such equipment will continue to
make such equipment available, as the less successful ones will leave the field in
pursuit of newer opportunities. Considering the fact that around 97.5 to 98% of
the total telephony is run using CS voice, your “time being” is going to be a very
30. Going on to David’s post I would comment as under, but only after thanking David for the
very informative URLs.
a. There are many reasons which drive change of systems, not all of which are
technical or for the benefit of the end-user.
i. The push from the manufacturers who have the new products to sell. In
the case of VoIP it is the manufacturers of Gateways, Soft Switches,
CPE, iPhones, etc.
ii. Once they have succeeded in pushing their idea through to and
influencing the decision makers in the TSPs, the latter go about finding
the justification for carrying out the change. The document shown in the
URL sent by you is such an exercise.
iii. Any expenditure carried out by the operators benefits the vendors and
some of the architects of the change.
iv. I am surprised that there are so many hullabaloos in extending broadband
connectivity to 8 to 10% of the American households who do not have this
facility particularly in the remote areas. They are already within the reach
of mobile services, and these can be extended more easily than wire-line
services, without all the associated additional investments.
b. This is called keeping up with the Joneses. Approximately 97.5 to 98% of the
total world population of mobile phones run on CS voice, and are primarily used
for voice communication and SMSs. Only 10 to 15% (at least in India) are using
the IP based services on mobile phones. This situation is not going to be wished
away. How many people will benefit from the 4G services is a matter of debate,
particularly since the 3G services are only just being launched and WBB
6. (wireless broadband) to follow. The 4G spectrum is yet to be auctioned in India. I
have already mentioned above and in earlier posts that keeping voice on CS
voice as in 3G/3.5G will not affect other aspects of 4G implementation – higher
upload and download speeds of data.
c. As analysts and Telecom Professionals I do not think it is right for us to
just see and know what is happening, but to see the relevance of the
various initiatives, and try to guide and influence these to meet the
d. The total worldwide count of 100 million VoIP subscribers taking Skype, Vonage,
all into account is a very small percentage of the total world telephone
population. It works out to only 14.66% of India’s telephone population of 682
million almost all of which is on circuit switching. In terms of the total world
telephone population this would be 2.5 to 2% of 4 to 5 billion telephones (wire-
line and mobile) worldwide which is 59 to 74% of the total world population of 6.8
billion people. Considering the time that VoIP has been around, this is not really
of any significance. If it was really all that great then with the free calls provided
by companies like Skype, the use of VoIP would have sky rocketed. The real
reason why this has not happened is that a vast majority of telephone
users both wire-line and wireless (mobile) are not very data savvy. Voice
and at best SMS is their primary mode of communication. I do hope this
gives the correct perspective to your question 3.
e. Regarding increase of wireless / mobile phones over wire-line phones, the
reason is the convenience of usage and the personal nature of the phone. In
India the growth was very tardy as long as incoming calls were being charged
for. As soon as this was removed, the mobile phone usage catapulted and so did
the tele-density which is now a little over 50%, i.e. one phone for every two
persons. Almost all the mobile and all the wire-line phones in India are on circuit
switched voice. We do not envisage any major shift to PS voice in either service.
f. To answer the simple question posed in point 5 of your post, the answer is that
PSTN (POTS) will continue to be in existence, as even the mobile phone system
which is growing at a rapid pace is on circuit switching and is supported by the
PSTN / TDM system.
g. I agree with you that the PSTN / TDM systems will last as long as equipment
required to support them are available. There is no real reason for manufacturers
to stop making them available as long as the business is profitable.
h. However, I cannot agree with you that the TDM switches “write off” period has
arrived yet. This will still take a long time. And with the increased use of p2p
leased lines for building INTRANET WANs for 100% security of internal
databases, and the increased usage of ISDN for public domain video
conferencing, the continuance of TDM/PSTN is very necessary. These functions
cannot be carried out over IP Backbones of TSPs.
31. I do hope the foregoing comments and the various documents referred to and all
available in the URL http://www.slideshare.net/pankajmitra. will give followers and
subscribers to this discussion thread a realistic perspective of the actual state of VoIP,
and a clearer understanding of the on-going debate on VoIP – Vs – PSTN.