AS IT EXISTS TODAY
CIRCUIT & PACKET SWITCHED
DIGITISED & PACKETISED
1. In Fig 1
a. Ear transducer 1 audio signal to Ear 1a
b. Mouth audio signal 2a to transducer in mouth piece 2
c. Electrical signal from transducers 1 and 2 carried over 2 wire
twisted pair to either local exchange (in case of direct extension
lines) or to EPABX (in case of extension).
d. These analogue signals are carried to the Codecs 6.
e. These codecs digitise the analogue signals and these are
switched in the PCM TDM switches to connect to extensions of
the EPABX or the DELs connected to the LEX..
f. Or they may be connected through the trunks of these exchanges
to other Local Exchanges (LEX) in the telephone network.
g. The analogue (DEL) connections to the EPABX from the codex at
the LEX is terminated on the CO (Central Office) trunk cards (5) in
the EPABX through twisted pair copper connections to carry the
analogue signals between the LEX and the EPABX.
h. For digital circuits between the EPABX and the LEX, 2 twisted
pair copper wires are used - one for Tx (transmit) and one for Rx
(receipt). These could be through the basic rate interface (BRI)
which have 2 B (bearer) channels and one signalling channels D.
Or they may be connected through primary rate interface (PRI)
which have 30 B channels and 2 D channels.
i. When digital links are being used between the EPABX and the
LEX, the digitised voice signals switched through the PCM TDM
switch in the EPABX is transmitted digitally without conversion
which happens if analogue trunks are used.
j. The PCM TDM switches are designed for totally unblocked
communication between all extensions. Thus if there are 100
extensions in the EPABX, there will be 50 simultaneous
communications between these extensions.
k. For unblocked communications through and beyond the EPABX a
diversity of usage for external communications known as Erlang
loading is used. This means that one trunk is provided for n
extensions, where n could be 10 or 8 or 6 depending on how busy
the exchange is. When this is done, every time the outside line is
attempted to be seized by an extension, it gets a dial tone which
means it is free to dial. This is called unblocked communication.
l. In the LEX, there are usually large number of direct extensions, of
the order of thousands (typically 5000 or 10000). Using the same
Erlang loading principle to evacuate the traffic from a 10000 line
exchange in an unblocked fashion we would need around 1000
trunks going out to the transport layer of the PSTN system. This
is usually provided through Link Access Protocol (LAP) like V5.2
which provides 16 E1 circuits over a 4W digital circuit between the
Access Network (AN) and the Local Exchange (LE or LEX). To
evacuate 10000 DELs in an unblocked fashion you would need
2 – V5.2 .
m. From Fig 1 we see that from and after the local
exchange the voice is totally digitised.
n. Fig 1a shows the schematic of a total telephone
network as it exits today
2. Fig 2 shows how the Local Exchange LEX is connected
using V5.2 LAPs, and PHOM (public higher order MUX) in
the LEX and TAX buildings through the TAX and the
transport layer to other cities.
3. This schematic drawing also shows how point-to-point (p2p)
leased lines are built up over PSTN and how it denies
physical access from the PSTN.
4. Such p2p leased lines connect all locations of a MLO (multi-
locational organisation) into a real private data network as
shown in Fig 3a, or each location of the MLO to the nearest
POP (point-of-presence) of the TSP IP Backbone to form a
VPN as shown in Fig. 3b. A typical TSP IP Backbone
topology is shown in Fig1 and its network architecture in
Fig2 of VPN.ppt shown in URL
http://www.slideshare.net/pankajmitra . This presentation
also explains the security vulnerability of VPN networks.
5. In the circuit switched PSTN network the digitised voice is
sent through TDM channels each 64 kbps, one per circuit,
achieved through time slicing or time division multiplexing.
6. The BRI link provides 2 B and 1D channel or one ISDN
7. The PRI link provides 30B + 2D channels or one E1 channel.
8. The V5.2 LAP interface provides 16 E1 circuits.
9. The HOM combines these to larger number of 64 kbps
10. In circuit switched networks for each voice or fax or data
communication an end-to-end virtual circuit is set up each of
64 kbps . The different types of communications can be
carried out simultaneously one on each channel.
11. Since an end -to-end communication channel is
available for each communication taking place, the
digitised voice, fax, or data is sent along these digital
channels as they are without any need for packet
12. Packet formation is required when there are no end
–to-end channels for each communication, and
various users are sharing a common network (no
dedicated circuits or channels) . The packets are
small and have a header containing the destination
details and a trailer containing the sending end
13.Therefore, over a single point-to-point (p2p) link several
and different communications can travel together in the
form of packets with header and trailer addresses, sent
from different sources and to different destinations.
14. In the above example the flow of communications can
be simultaneously in both directions.
15.The concept of packetisation was developed to send data
over shared data networks to make better utilisation of
link and network bandwidth and avoid idling which would
happen in the case of dedicated point-to-point links for
data only networks, since data communications is bursty
16.In the PSTN circuit switched network the WAN bandwidth
utilisation is ensured using the Erlang loading and user
diversity. Thus in dial-up data circuits the problem of
unutilised data communications is non – existent as the
circuit will be called up only when the data
communications are required. However, even in the
connected state, the very nature of computer
communication entails under-utilisation of the connected
17.The problem of under-utilisation could occur in pure
data p2p networks if this was used in the circuit
switched or TDM mode. The problem gets substantially
alleviated if packetisation is resorted to as then several
sources could be in communication with several
destinations at the same time over the same link.
18. In the circuit switched network the voice routes through
the EPABX and / or the LEX.
19. The fundamental difference between the PSTN circuit
switched system and the packet switched data
networks is that the communications (voice or data)
do not route through the LEX.
20.The voice packets along with the data packets are sent
to the router in the customer premises and get
connected to other organisation locations either through
direct p2p leased circuits or through a service
providers IP backbone (as in the case of VPN
21. The voice packetisation takes place at the user end in
either of the following ways.
a. Using IP PBX where analogue voice through the
copper cables get digitised through codecs and
switched in the EPABX. The output streams
moving out of the EPBAX get packetised in the IP
trunk cards and these are then fed into the router
for onward transmission through the IP WAN.
b. Using standard EPABXs whose analogue trunks
are fed into the voice packetising cards in the
routers . The voice packets are then sent by the
router along with data packets through the IP
c. Using IP phones connected to the LAN which
caries the voice packets created in the IP phones
along with the data packets from the different
computers in the LAN through the router to and
across the WAN.
d. The IP WANs could be private using p2p links
between enterprise locations or shared using TSP
IP backbones (VPN) or Internet.
e. The above options apply to users within an
f. For individual subscribers the only way he can use
VoIP is through the Internet Connection he / she
has through dial-up or broadband connection.
g. Majority of the individual telephone subscribers
today are on PSTN with DEL (direct extension lines)
22. There re two basic types of packet switched networks
a. Connectionless - IP, Ethernet, UDP, RTP
b. Connection oriented – X.25, Frame Relay, ATM,
23. TCP / IP protocol suit is the prevalent standard data
communications in packet switched networks today.
They are not very effective for VoIP, although all voice
over the Internet is based on this protocol with its
24. When individual enterprises are setting up their VoIP
over their own private networks, they may use UDP or
RTP / SIP.
25. Similarly if TSPs wish to they can for their VPN
subscribers use UDP or RTP/SIP for VoIP and TCP / IP
for the data communications. The MPLS connection
oriented network can have multi-protocol suits
encapsulated in one label.
26. Broadband access networks over existing telephone
cables of the PSTN networks of various TSPs are
arranged through DSLAMs fed from the IP Backbones
of the TSP and an ADSL modem placed at the customer
premises (CPE). The Internet Feed to the DSLAM is
through TCP/IP from the NIXI (the national internet
exchange interface) in the city where the DSLAM is
situated. See figure 2 of VPN.ppt mentioned in (17)
27. The metered telephony through the Broadband access
network over the same copper cable is separated using
voice filters and run as analogue signals into the Local
Exchange of the PSTN. Thus the metered telephony over
the broadband connection is still circuit switched through
28. These broadband connections may be extended for
IPTV using a set-top box associated with the ADSL
modems in the customer premises and use IGMP
(internet group management protocol) for live TV, and
RTSP (real time streaming protocol) for VOD (Video on
Demand) and NPVR (network based personal video
29. The total Internet and IPTV signals are carried over the
ADSL2+ which facilitates a maximum download speed of
24 mbps and an upload speed of 1.4 mbps. These
speeds deteriorate with distance of the customer
premises from the DSLAM located in the LEX (local
exchange) building. At the average distance adequate
bandwidth is available for downloading up to two video
signals (Broadcast TV, VOD, gaming) compressed in
MPEG4 (H.264) besides facilitating Internet connection
up to 2 mbps. The entire DSLAM feed for Internet, and
IPTV signals are received from the IP backbone of the
30. The metered telephony through voice filters are fed
from the PSTN network.
31. VoIP if used and permitted by the TSP over their
Broadband connections (many of them do not allow this)
has to route through the IP Backbone of the TSP /
Internet (if it is meant for destinations beyond the TSP IP
32. Fig 4 shows the key elements of a GSM network
structure for mobile telephone. Fig 5 shows a typical
layout of the CDMA one or the IS-95 network. The first
generation (1G) GSM network used analogue network
access from the mobile phone to the BTS. To make use
of the frequency spectrum available for maximum number
of cell phones covered by the BTS (Towers) and the
BSC, FDMA (frequency division multiple access) was
used. The 2G GSM and the IS-95 uses digital network
access from the mobile phone. To make the best use of
the frequency spectrum available 2G GSM uses TDMA
(time division multiple access), and the IS-95 network
uses CDMA (code division multiple access).
33. For comparison of these access methods, imagine a
cocktail party, where couples are talking to each other in
a single room. The room represents the available
bandwidth. In GSM, a speaker takes turns talking to a
listener. The speaker talks for a short time and then stops
to let another pair talk. There is never more than a pair of
speakers talking in the room, no one has to worry about
two conversations mixing. In CDMA, any pair of speakers
can talk at any time; however each uses a different
language. Each listener can only understand the
language of their partner. As more and more couples talk,
the background noise (representing the noise floor) gets
louder, but because of the difference in languages,
conversations do not mix. In FDMA, each speaker pair
speaks in a different pitch audible only to conversing pair.
34. From the BSC (base switching centre) to the MSC (main
switching centre), the communication is TDM for both the
GSM and the IS-95 systems. The MSC is connected to
the PSTN network through TDM circuits and to the PSTN
access network also through TDM to the TAX (trunk
automatic exchange) in the city and through the transport
layer through TDM / SDH to the TAX of other cities.
35. For your ready reference, GSM has a market share of
80 to 85% and the IS-95 a share of 10 to 15% globally.
36. The above dissertation shows that the mobile
telephony is entirely circuit switched up to 2.5 G.
37. For 3G mobile also the telephony is circuit switched.
The only additional feature is that the voice and data
communication will be simultaneous and data
communications will be over a larger bandwidth to
support TV and video transmission.
38. Thus as long as the basic communications systems
need circuit switching, we will have to continue with
the PSTN network.
39. In India we have 500+million mobile subscribers and
40+ million fixed line subscribers. The entire
540+million telephone users are on circuit switching.
All these use digitised voice through the various
switches and access and transport networks. But
they do not use packetised voice.
40. 4G mobile has been designed with packetised voice,
and hence VoIP. When this is implemented by any
TSP they will need to relay their core network from
their central switching station to the base switching
stations to an IP backbone, and these will be
connected to the IP Backbone of the TSP networks. It
is like setting up a parallel IP network to the existing
circuit switched core network.
41. A lot will depend on how the market takes to 4G.
Firstly 4G handsets are going to be very expensive
and certainly not affordable to a large majority of the
present mobile phone subscribers – at least certainly
not in India. Secondly there the issues of additional
bandwidth and QoS for any VoIP telephony. Further
the facilities that can be provided by 3.5G which is an
extension of 3G with the same type of infrastructure
already provides what 4G is promising.
42. We should always view technology on its relevance
and not as a stand alone novelty and newness.
43. To move from circuit switching to packet switching it is
necessary to build TSP IP Backbones to the same extent
of coverage as their present PSTN networks – a
mammoth task, to say the least.
44. The interesting fact to remember is that TSP IP
backbones are laid out over point-to-point TDM or SDH
circuits connecting all the POPs (points-of-presence) of
the IP network through full mesh or tri-node topology (for
re-routing traffic against temporary link failures). TDM or
SDH are essentially synchronous communications
networks and circuit switched.
45. Thus we need TDM or SDH circuits to form IP
backbones of TSPs for running all IP communications.
46. Packetisation of data and the asynchronous IP
communications were made to take care of better
bandwidth utilisation by sharing the bandwidth for
multiple communicators, to overcome the bursty nature
of data communications. It was not designed for or
intended for real time communications like voice, fax,
and video. Such real time communications are best
carried out over synchronous communications networks
or circuit switched networks.
47. The whole concept of VoIP was born with the desire for
unified communication. Some technologists and
researchers felt this could be done by sending voice
packets along with data packets in queue. Since voice is
a real time communications VoIP came up with problems
which are being attempted to be resolved by constant
research and development in this field.
48. It is important to note that there are other alternatives
for unifying communications in a more bandwidth
efficient manner – combining circuit and packet
switching over the same network by allocating
bandwidth for each. The former may be used for all
real time communications like voice and fax, and the
latter for data and all IP services.
49. This alternative principle has been used in 3G/3.5G
mobile telephony, and very successfully.
50. In my view the VoIP initiative for 4G is a retrograde
step and should be seriously reviewed and the line
taken in 3G/3.5G be accommodated in the 4G / LTE
programme, particularly since no significant
advantage is gained by moving into VoIP in place
of circuit switched voice, and there are many more
problems to be overcome.
51. Step in (50) will reduce the implementation cost for 4G
by the MSPs (Mobile phone service providers) as then
they will not need to IP Trunks at all BSCs and MSC to
connect to their IP core network for communicating
with their present 2G, 2.5G, 3G, and 3.5G subscribers
with the onset of 4G.
52. Finally in Fig. 6 (Inset Fig.1), I have given a Venn
diagram which gives the inter-relationship between the
different communications options available to
individual and enterprise users.
1a 1 Analogue 4
2a 2 P 6
1a 1 T 8
4 P3 4
C D 6
Analogue M M
D Digital BRI
M 5 Or PRI 10
1. Transducer in ear piece of telephone instrument
a. Audio signal from ear transducer to ear
2. Transducer in mouth piece of telephone handset
a. Audio signal from mouth to mouth transducer
3. CO trunk card in EPABX
4. Codecs which convert analogue signals into digital signals for
use in the TDM / PCM switches.
5. Digital trunk BRI / PRI
6. V5.2 LAP interfaces
7. Higher Order Multiplexers (HOM)
8. Link to HOM at trunk automatic exchange (TAX) building.
These could be cable PCM, digital micro wave, or Fibre Optic
9. IP Trunk for interfacing with IP networks to communicate with
10. Link to IP Network
Total Telephone System
3 Analogue 4 P
2a C 7
P 3 Analogue
2a 2 D 6
D Digital BRI
M 5 Or PRI
IP BACKBONE TDM / SDH
3 Analogue 4 P
2a 2 C 6
1 Analogue 8
3 4C 3
D Digital BRI
M 5 Or PRI
GENERAL SCHEMATIC OF
PUBLIC SWITCHTED TELEPHONE NETWORK (PSTN)
SHOWING TRANSPORT AND ACCESSNETWORK
ALSO LEASED LINE BUILD UP (IN RED)
CITY A CITY B
HM INTER-CITY CARIER HM
LEX LEX FOM LEX LEX
MDF at LEX
Street Pillar Boxes
E E E E
P P P P
B B B B
A A A A
X X X X
Leased lines Leased lines
Channel Splitter Channel Splitter
Data Router V Data Router V
RPDN – ROUTED IP NETWORK
R – Router at each organisation location
L - LAN hub / Switch at each organisation location
L R TTSP IP BACKBONE R L
3 FOR SHARED NETWORKS & 1
TDMA / FDMA Digi CS
8 7 P
1 1 TDM TDM
16 5 9 4 6 S
8 17 9 20
TDMA / FDMA
5 9 6 1 1
16 4 8 17 21
Fig 4 - 2.5G (GPRS) System
8 7 P
2 6 1 1 S
5 17 20
7 8 9 T
8 17 1
Fig.5 - 2.5G cdmaOne (IS95)System