TELEPHONYAS IT EXISTS TODAY

DIFFERENCE BETWEEN DIFFERENCE BETWEEN

CIRCUIT & PACKET SWITCHED

NETWORKS

DIFFERENCE BETWEEN

DIGITISED & PACKETISED

VOICE

1. In Fig 1a. Ear transducer 1 audio signal to Ear 1ab. Mouth audio signal 2a to transducer in mouth piece 2c. Electrical signal from transducers 1 and 2 carried over 2 wire

twisted pair to either local exchange (in case of direct extensionlines) 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 ofthe EPABX or the DELs connected to the LEX..

f. Or they may be connected through the trunks of these exchangesto other Local Exchanges (LEX) in the telephone network.

g. The analogue (DEL) connections to the EPABX from the codex atthe LEX is terminated on the CO (Central Office) trunk cards (5) inthe EPABX through twisted pair copper connections to carry theanalogue signals between the LEX and the EPABX.

h. For digital circuits between the EPABX and the LEX, 2 twistedpair 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 theLEX, the digitised voice signals switched through the PCM TDMLEX, the digitised voice signals switched through the PCM TDMswitch in the EPABX is transmitted digitally without conversionwhich happens if analogue trunks are used.

j. The PCM TDM switches are designed for totally unblockedcommunication between all extensions. Thus if there are 100extensions in the EPABX, there will be 50 simultaneouscommunications between these extensions.

k. For unblocked communications through and beyond the EPABX adiversity of usage for external communications known as Erlangloading is used. This means that one trunk is provided for nextensions, where n could be 10 or 8 or 6 depending on how busythe exchange is. When this is done, every time the outside line isattempted to be seized by an extension, it gets a dial tone whichmeans it is free to dial. This is called unblocked communication.

l. In the LEX, there are usually large number of direct extensions, ofthe order of thousands (typically 5000 or 10000). Using the sameErlang loading principle to evacuate the traffic from a 10000 lineexchange in an unblocked fashion we would need around 1000trunks going out to the transport layer of the PSTN system. Thisis usually provided through Link Access Protocol (LAP) like V5.2which provides 16 E1 circuits over a 4W digital circuit between theAccess Network (AN) and the Local Exchange (LE or LEX). Toevacuate 10000 DELs in an unblocked fashion you would need2 – V5.2 .

m. From Fig 1 we see that from and after the localexchange the voice is totally digitised.

n. Fig 1a shows the schematic of a total telephonenetwork 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 intopology 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

connection.

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

channels.

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

formation.

12. Packet formation is required when there are no end–to-end channels for each communication, andvarious users are sharing a common network (nodedicated circuits or channels) . The packets aresmall and have a header containing the destinationdetails and a trailer containing the sending enddetails.

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 can14. 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

in nature.

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

bandwidth.

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

networks).networks).

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

WAN.

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

enterprise.

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,

MPLS, TCP

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 itsover the Internet is based on this protocol with its

attendant shortcomings.

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)

above.

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

the PSTN.

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

recorder).

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 customerspeeds 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

TSP.

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

Backbone).

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 stopslistener. 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.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 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 significantadvantage 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 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.

Fig 1

2

1

3

1a

2aAnalogue

P

C

M

T

D

M

P

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EPABX

5

4

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Digital BRI

Or PRI

Analogue

6

10

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1a

2a

9

3 8

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

links (OFC)

9. IP Trunk for interfacing with IP networks to communicate with

VoIP phones

10. Link to IP Network

EPABX10

9

Fig. 1aTotal Telephone System

2

1

31a

2a

Analogue

P

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M

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D

M

P

C

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EPABX

5

4

LEX

4

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Or PRI

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6

10

6

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TSP

TDM / SDHTSP

IP BACKBONE

INTERNET

2

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Analogue

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38

TDM / SDH

TRANSPORT

NETWORK

IP BACKBONE

LEXLEXLEXLEX

TAXTAX

FOM

HOM

PHOM

PHOM

TM

NTM

HMHM

TM

NTM

CITY BCITY A

INTER-CITY CARIER

Fig 2GENERAL SCHEMATIC OF

PUBLIC SWITCHTED TELEPHONE NETWORK (PSTN)

SHOWING TRANSPORT AND ACCESSNETWORK

ALSO LEASED LINE BUILD UP (IN RED)

MDF at LEX

Building

Primary Cables

Street Pillar Boxes

Building DBs

Secondary Cables

EPBAX

EPBAX

VDPS

Channel Splitter

Data Router

Point-to-pointLeased lines

EPBAX

EPBAX

VDPS

Data Router

Point-to-pointLeased lines

Channel Splitter

RPDN – ROUTED IP NETWORK

R3

R1

R2

R4

L

L

R – Router at each organisation location

L - LAN hub / Switch at each organisation location

Private Network

Fig. 3

Fig. 3a

L

L

L

L

Fig. 3a

TTSP IP BACKBONE

FOR SHARED NETWORKS &

VPN

R

3

R

4

R

2

R

1

Fig. 3a

L

L

5

20

P

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171

94

171

99 6

78

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22

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Fig 4 - 2.5G (GPRS) System

TDM

TDM

TDM

IP

IP

TDMA / FDMA

TDMA / FDMA

41

8

1

8

16

16 5

Digi CS

Digi CS

P78CDMA

Fig.5 - 2.5G cdmaOne (IS95)System

5 20

P

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CDMA

CDMA

1

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Fig 6