8/8/2019 expo_PSTN

1/51

8/8/2019 expo_PSTN

2/51

8/8/2019 expo_PSTN

3/51

8/8/2019 expo_PSTN

4/51

The first Telep hone

8/8/2019 expo_PSTN

5/51

Phy sical Cable between all p honesy P lace a p h ysical cable between every h ouse h old

requiring access to a telep h oney Such a setup is neit h er cost-effective nor feasibley For N p h ones require

N(N-1)/2 cablesy For 10 ph ones require

45 cables

8/8/2019 expo_PSTN

6/51

Introduction of Switc h Mec hanismy It could map any p h one to anot h er p h one

y Wit h th is device, t h e telep h one users needed only one

cable to t h e centralized switc h office, instead of seveny At first, a telep h one operator acted as t h e switc h

y Th is operator asked callers w h ere t h ey wanted to dialand t h en manually connected t h e two voice pat h s

y For N p h ones require N cablesy After 100 years or so, t h e h uman

switch was replaced by electronicswitch es

8/8/2019 expo_PSTN

7/51

U nderstanding PS TN Basicsy How voice is transmitted across a digital network?y W h y ph one numbers follow E.164 numbering format?y Basic circuit-switc h ing concepts?

8/8/2019 expo_PSTN

8/51

Analog and Digital Signalingy Sounds and Human speec h , are all in analog formy Until several decades ago, t h e telep h ony network was

based on an analog infrastructure as welly Analog communication is neit h er robust nor efficient

at recovering from line noisey Telep h one lines are very sensitive to inductance or

voltage produced by nearby electric circuits and linesy In t h e early telep h ony network, analog transmission

was passed t h roug h amplifiers to boost t h e signaly Analog communication is a mix of time and amplitude

8/8/2019 expo_PSTN

9/51

Analog Waveformy Th e amplitude varies wit h respect to time

8/8/2019 expo_PSTN

10/51

Line distortion wit h Analog Amplifiersy If you were far away from t h e end office switc h (wh ich

provides t h e ph ysical cable to your h ome), an amplifier

migh

t be required to boost th

e analog transmission(your voice).y Analog signals t h at receive line noise can distort t h e

analog waveform and cause garbled reception.y

Th is is more obvious to t h e listener if many amplifiersare located between your h ome and t h e end officeswitch .

8/8/2019 expo_PSTN

11/51

Line distortion wit h Analog Amplifiersy Figure s h ows th at an amplifier does not clean t h e

signal as it amplifies, but simply amplifies t h e

incoming signal, including distortion, if it existsy Th is process of going t h roug h several amplifiers wit h

one voice signal is called accumulated noise

8/8/2019 expo_PSTN

12/51

Digital Signals and Repeatersy In digital networks, line noise is less of an issue

because repeaters not only amplify t h e signal, but also

clean it to its original conditiony Th is is only possible wit h digital communication

because suc h communication is based on 1s and 0sy Th erefore w h en signals are repeated, a clean sound is

maintained wit h out distortion

8/8/2019 expo_PSTN

13/51

Digital Signals and Repeatersy W h en t h e benefits of t h is digital representation

became evident, t h e telep h ony network migrated to

pulse code modulation (P

CM

)

8/8/2019 expo_PSTN

14/51

Digital Voice Signaly P CM is th e most common met h od of encoding an

analog voice signal into a digital stream of 1s and 0sy

All sampling tec h niques use t h e Nyquist t h eorem, wh ich basically states t h at if you sample at twice t h eh igh est frequency on a voice line, you ac h ieve good-quality voice transmission

8/8/2019 expo_PSTN

15/51

Digital Voice Signaly Th e P CM process is as follows:

y Analog waveforms are put t h roug h a voice frequency

filter to ch

op out anyth

ing greater th

an 4000 Hzy Th ese frequencies are filtered to 4000 Hz to limit t h e

amount of crosstalk in t h e voice networky Using t h e Nyquist t h eorem, you need to sample at 8000

samples per second to ac h ieve good-quality voicetransmission

y Th e filtered analog signal is t h en sampled at a rate of 8000 times per second

8/8/2019 expo_PSTN

16/51

Digital Voice Signaly After t h e waveform is sampled, it is converted into a

discrete digital formy Th

is sample is represented by a code th

at indicates th

eamplitude of t h e waveform at t h e instant t h e sample wastaken

y Th e telep h ony form of P CM uses eig h t bits for t h e codeand a logarit h m compression met h od t h at assigns morebits to lower-amplitude signals

y If you multiply t h e eigh t-bit words by 8000 times persecond, you get 64000 bits per second (bps)

y Th e basis for t h e telep h one infrastructure is 64000 bps(or 64 kbps)

8/8/2019 expo_PSTN

17/51

Digital Voice Signaly T wo basic variations of 64 kbps P CM are commonly

used:y

-law, t h e standard used in Nort h America; andy a-law, t h e standard used in Europe

y Th e met h ods are similar as bot h use logarit h miccompression to ac h ieve from 12 to 13 bits of linear P CM

quality in only eig h t-bit words, but t h ey differ inrelatively minor detailsy E.g. Th e -law met h od h as a sligh t advantage over t h e a-

law met h od in terms of low-level signal-to-noise ratioperformance

8/8/2019 expo_PSTN

18/51

Local Loops, Trunks, and Inter-switc h Communication

y L ocal L oops is t h e telep h one infrastructure (cabling)th at starts wit h a simple pair of copper wires running

to yourh

omey Th e local loop p h ysically connects your h ometelep h one to t h e central office switc h (also known as aClass 5 switch or end office switc h )

y

Th e communication pat h between t h e central officeswitch and your h ome is known as t h e ph one line, andit normally runs over t h e local loop

8/8/2019 expo_PSTN

19/51

Local Loops, Trunks, and Inter-switc h Communication

y Th e communication pat h between several centraloffice switc h es is known as a trunk

y

Just as it is not cost-effective to place a p h ysical wirebetween your h ouse and every ot h er h ouse you want tocall, it is also not cost-effective to place a p h ysical wirebetween every central office switc h

y

A mes h ed telep h one network is not as scalable as one with a h ierarc h y of switch es

8/8/2019 expo_PSTN

20/51

20

1

10

9 8

7

32

4 5

6

1 2 3

1 2 3

1 2 3

65 66 67

228 229 230

1298 1299 1300

1 2 3 4 519,000

200 million telephones

19,000 endoffices

1300 tolloffices

230 primaryoffices

67 sectionaloffices

10 regionaloffices(full mesh)

Source: Computer Networks, Andrew S. Tanenbaum

Class 5

Class 4

Class 3

Class 2

Class 1

8/8/2019 expo_PSTN

21/51

Local Loops, Trunks, and Inter-switc h Communication

y Switch es are currently deployed in h ierarc h iesy End office switc h es (or central office switc h es)

interconnect t h roug h trunks to tandem switc h es (alsoreferred to as Toll office switc h es or Class 4 switc h es)y High er-layer tandem switc h es connect local tandem

switch es

8/8/2019 expo_PSTN

22/51

Local Loops, Trunks, and Inter-switc h Communication

y Central office switc h es often directly connect to eac h oth er

y

W h ere t h e direct connections occur between centraloffice switc h es depends to a great extent on callpatterns

y If enoug h traffic occurs between two central office

switch es, a dedicated circuit is placed between t h e twoswitch es to offload t h ose calls from t h e local tandemswitch es

y Some portions of t h e P ST N use as many as five levels

of switch

ingh

ierarch y

8/8/2019 expo_PSTN

23/51

PSTN Signalingy Generally, two types of signaling met h ods run over

various transmission mediay

User-to-network signalingy Th is is h ow an end user communicates wit h th e P ST N

y Network-to-network signalingy Th is is generally h ow th e switc h es in t h e P ST N

intercommunicate

8/8/2019 expo_PSTN

24/51

U ser-to- Network Signalingy W h en using twisted copper pair as t h e transport, a

user connects to t h e P ST N th roug h analog, Integrated

Services Digital Network (ISDN), or th

rough

aT

1carriery Th e most common signaling met h od for user-to-

network analog communication is Dual Tone M ulti-

Frequency (DTM

F)y DTM F is known as in-band signaling because t h etones are carried t h roug h th e voice pat h

8/8/2019 expo_PSTN

25/51

DTMF Keypady Th e DTM F keypad is laid out in a 4x4 matrix, wit h

each row representing a low frequency and eac h

column representing ah

igh

frequency y A single key press suc h as 0 will send a sinusoidal toneof th e two frequencies: 941 and 1336 h ertz (Hz)

y Th e two tones are t h e reason for calling it dual-tone

multi-frequency y Th ese tones are t h en decoded by t h e switc h to

determine w h ich key was pressed

8/8/2019 expo_PSTN

26/51

Signaling in ISDN y ISDN uses anot h er met h od of signaling known as out-

of-band Signalingy

Wit h th is met h od, t h e signaling is transported on ach annel separate from t h e voicey Th e ch annel on w h ich th e voice, data, fax is carried is

called a bearer (or B c h annel) and is 64kbpsy Th e ch annel on w h ich th e signal is carried is called a

data or a control c h annel (D c h annel)

8/8/2019 expo_PSTN

27/51

Signaling in BR I & PRIy Basic Rate Interface (BRI) service is t h e entry level and

offers two 64-kbps B c h annels and one 16-kbps D

ch

annel (2B+D)y It is intended to meet t h e needs of most individual usersand small offices

y P rimary Rate Interface ( P RI) service is a more scalable

form of th

e BRI servicey A P RI offers twenty t h ree 64-kbps B c h annels and one64-kbps D c h annel (23B+D)

y P RI is intended for larger enterprises wit h h igh er voice,

data, or fax traffic

8/8/2019 expo_PSTN

28/51

Network-to- Network Signalingy Network-to-network communication is normally

carried across t h e following transmission media:y T

1/E1 carried over twisted pairy T 1 is a 1.544-M bps link used in America and Japany E1 is a 2.048-M bps link normally used in Europey T 3/E3, T 4 carried over coaxial cabley T 3 is 44.736 M bps = 28 T 1s or 672 64-kbps connectionsy E3 is 34.368 M bps = 16 E1s or 512 64-kbps connectionsy T 4 is 274.176M bps = 168 T 1 circuitsy T 3, T 4 carried over a microwave linky SONET carried over fiber as OC-3, OC-12, and OC-48

8/8/2019 expo_PSTN

29/51

PSTN Services and Applications

P ST N Services

Custom CallingFeatures

(End office Switc h es)

Call Waiting

Call Forwarding

Th ree way Calling

Class Features(SS7 Signaling)

Display (ANI/C L I)

CL I Blocking

CallBlocking/Screening

Auto Callback

8/8/2019 expo_PSTN

30/51

PSTN Services and Applicationsy L ocal Exch ange Carriers ( L ECs) h ave been increasing

th e features t h ey offer to create a h igh er revenue

stream per consumery Numerous services are now available, for example, wh ich were not available just a few years ago

y Th ese services come in two common flavors:y

Custom calling features andy CL ASS features

8/8/2019 expo_PSTN

31/51

PSTN Services and Applicationsy Standard telep h ony service is commonly referred to as

plain old telep h one service ( P OT S).y

Th is service provides a dial tone to rotary and touc h -tone telep h ones, as well as access to national andinternational carriers

y Custom calling features rely upon t h e end office

switch

, not th

e entireP

ST

N, to carry information fromcircuit-switc h to circuit-switc hy CL ASS features, h owever, require SS7 connectivity to

carry t h ese features from end to end in t h e P ST N

8/8/2019 expo_PSTN

32/51

Custom Calling Featuresy Th e providers enable and control custom calling

features from wit h in end office switc h es directly.y

Signaling System 7 (SS7) messaging and serviceenablers are not required to operate t h ese features.y Th e following list includes a few of t h e popular custom

calling features commonly found in t h e P ST N today:y

Call waiting: Notifies customers w h o already placed acall th at t h ey are receiving an incoming cally Call forwarding: Enables a subscriber to forward

incoming calls to a different destinationy Th

ree-way calling: Enables conference calling

8/8/2019 expo_PSTN

33/51

CLASS Featuresy CL ASS (Custom L ocal Area Signaling Service )is a

popular suite of features available to subscribers.y

CL ASS features provide subscribers wit h a powerfuland convenient tool to control incoming and outgoingcalls.

y Wit h CL ASS, users interact wit h th e switc h software

from th

eir own teleph

one sets and give instructions on wh ich services t h ey want.y SS7 messages and functions are t h en invoked and sent

with in t h e network to perform t h e requested

operations.

8/8/2019 expo_PSTN

34/51

CLASS Featuresy A few of t h e CL ASS features are mentioned in t h e

following list:y

Display: Displays th

e calling party's directory number, or Automatic Number Identification (ANI)y Call blocking: Blocks specific incoming numbers so t h at

callers are greeted wit h a message saying t h e call is notaccepted (a.k.a Call Screening)

y Calling line ID blocking: Blocks t h e outgoing directory number from being s h own on someone else's display

y Automatic callback: Enables you to put a h old on t h elast number dialed if a busy to call back w h en free (a.k.a Automatic Recall)

8/8/2019 expo_PSTN

35/51

Voice Maily P ST N-based voice mail enables S P s to offer an

alternative to answering mac h ines.y

Th is is attractive because subscribers do not need topurc h ase or operate any additional equipment.y Anot h er benefit of network-based voice mail is t h at

voice messaging is still available even if t h e called

party's line is busy.y Network-based voice mail also enables t h e subscriber

to retrieve t h eir voice mail from remote locations.

8/8/2019 expo_PSTN

36/51

Voice Maily Th e two main voice mail services available to

residential and small-business users are:y

Voice messaging: Enables subscribers to store and play recorded greetings and receive, review, and distributemessages from outside users

y Fax messaging: Enables subscribers to receive faxes and view th em at a later time

8/8/2019 expo_PSTN

37/51

PSTN Numbering Plansy In some places in t h e United States, it is necessary to

dial 1+10 digits for even a local cally Th

is will become more and more prevalent as moredevices require telep h one numbersy Th e need to dial 1+10 digits for a local number is

normally due to an overlay y

An overlay can result in next-door neig h bors h avingdifferent area codes. An overlay is w h en a region wit h anexisting area code h as anot h er area code "overlayed."Th is offers t h e existing customers t h e benefits of noth aving to switc h area codes, but forces everyone in t h atregion to dial 10 digits to call anyw h ere.

8/8/2019 expo_PSTN

38/51

PSTN Numbering Plansy Essentially, two numbering plans are used wit h th e

P ST N:y Th

e North

American NumberingP

lan (NANP

) andy Th e International Telecommunication Union

Telecommunication Standardization Sector (I T U-T ;formerly CCI TT ) International Numbering P lan

8/8/2019 expo_PSTN

39/51

NANPy NANP is an 11-digit dialing plan t h at contains t h ree

parts (referred to as N P A-NXX-XXXX ):y Th

e NumberingP

lan Area (NP A, a.k.a. Area code),

y Central Office (CO) Code (NXX), andy Station Number.

y NANP uses NXX, wh ere N is a value between 2 to 9

and X is a value between 0 to 9y NANP is also referred to as 1+10y h ttp://www.nanpa.com

8/8/2019 expo_PSTN

40/51

ITU -T International Numbering Plany IT U-T Recommendation E.164 specifies t h at:

y A Country Code (CC),y

National Destination Code (NDC), andy Subscriber Number (SN) be used to route a call to a

specific subscribery Th e CC consists of one, two, or t h ree digits. Th e first

digit(s) defines world numbering zones. A list of allth e defined CCs is found in I T U-T RecommendationE.164 Annex A

y NDC and SN vary in lengt h based on t h e needs of t h e

country. Neith

er oneh

as more th

an 15 digits

http://www.itu.int/ITU-T/inr/

8/8/2019 expo_PSTN

41/51

Drawbacks of t he PS TNy P ST N does a good job to switc h voice calls, but t h ere

are many business w h ereby voice is an application ontop of a data network for several reasons:

y Data h as overtaken voice as t h e primary traffic on many networks built for voice

y Th e P ST N cannot create and deploy features quickly enoug h (Equipment Vendors provide applications forth eir P ST N)

y Data/Voice/Video (D/V/V) cannot converge on t h eP ST N as currently built

y Th e arch itecture built for voice is not flexible enoug h tocarry data

8/8/2019 expo_PSTN

42/51

Packet Telep hon y Network Driversy Th e integration of D/V/V is more t h an just a c h ange in

infrastructurey

D/V/V integration also enables new features to bedeveloped more quickly and opens up applicationdevelopment to t h ousands of Independent Software

Vendors (ISVs)

8/8/2019 expo_PSTN

43/51

Circuit Switc h ing Versus Packet Switc h ingy Circuit-switc h ing model is breaking into a new model

by wh ich open standards exist between all t h ree layersy

A packet infrastructure will carry th

e actual voice(media)y Th e call-control layer will be separate from t h e media

layer, andy open A P Is (Application P rogramming Interfaces) will

enable new services to be created by ISVs

8/8/2019 expo_PSTN

44/51

Circuit Switc h ing Versus Packet Switc h ing

8/8/2019 expo_PSTN

45/51

Standards-Based Packet Infrastructure La yery Th e packet infrastructure replaces t h e circuit-

switch ing infrastructure in t h is new modely Th

is infrastructure is usually IP, alt

houg

hth

is modelalso works if A TM is th e underlying transporty IP is so attractive as t h e packet infrastructure because

of its ubiquitous nature and t h e fact t h at it is t h e de

facto application interfacey Th is means t h at software applications running over I P do not h ave to be known

y IP simply transports t h e data end to end, wit h no realinterest in t h e payload

8/8/2019 expo_PSTN

46/51

Open Call-Control La yery Call-control is t h e process of making a routing

decision about w h ere a call needs to go to establis h th ecall

y In t h e P ST N today, t h ese decisions are carried out by SS7 and are made by Service Control P oints (SC P s)

y In t h is new model of separating t h e bearers (R T P

streams) from th

e call-control layer and separating th

ecall-control layer from t h e services, it is necessary tomake sure t h at standards-based protocols are used

8/8/2019 expo_PSTN

47/51

Open Call-Control La yery M any different I P routing protocols exist and eac h is

specifically designed for a certain type of network e.g.y

Router InformationP

rotocol (RIP

),y Interior Gateway Routing P rotocol (IGR P ),y Enh anced Interior Gateway Routing P rotocol (EIGR P ),y Intermediary System to Intermediary System (IS-IS),y

Open S h ortest P ath First (OS P F), andy Border Gateway P rotocol (BG P )

8/8/2019 expo_PSTN

48/51

VoIP Call-Control Protocolsy Th e main VoI P call-control protocols are:

y SIP - being developed as a protocol t h at will enable enddevices (endpoints or gateways) to be more intelligent,

y H.323 - IT U-T recommendation wit h th e largestinstalled base,

y M GCP - developed in 1998 to reduce t h e cost of gateways wh ere call-control occurs in gateway controllers,

y H.248/ M EGACO IET F and I T U joint internationalstandard for media gateway control

y P eer to P eer ( P 2P ) VoI P IP Teleph ony M odel as used by Skype

8/8/2019 expo_PSTN

49/51

Open Service Application La yery Wit h out good applications, t h e network infrastructure

is of no usey

W h

en building a network th

ath

as open interfacesfrom t h e packet layer to t h e call-control layer and fromth e call-control layer to t h e application layer, vendorsno longer h ave to develop applications rat h er everyonecan develop applications

y L egacy applications suc h as call-centers for enterprisenetworks, and standard P ST N applications suc h as call

waiting and call forwarding, must be ported onto anew infrastructure wit h out t h e end user realizing t h atth e ch an e occurred

8/8/2019 expo_PSTN

50/51

Summar yy Voice in t h e P ST N is a fairly complex weave of different

tech nologies t h at h ave been evolving since 1876 It isabout to undergo a revolution.

y Th e tec h nology required to enable true multimediaconversations on a daily basis is readily available

y Th e telep h one/communications infrastructure is

moving to a new model and will soon be able to carry th ese multimedia conversationsy Th e remaining piece of t h e puzzle is t h e bandwidt h

necessary to complete t h ese multimedia conversations

8/8/2019 expo_PSTN

51/51

Q uestionsy

????