Public Switched
Telephone Network (PSTN)

Topics in PSTN

	Introduction
		review of early exchanges
		PSTN Standards
	User services & terminals
	Modern exchange technology
		interface standards
		access and trunk networks
		signaling
		network management
		internetworking (telecommunications between networks)

Introduction

	PSTN switching is based on circuit switching by duplex* connections
	Temporary bidirectional connections
	Originally for speech (voice) only at 300-3400 Hz
	Earlier two subscribers connected by purely physical connection (physical switch contacts)
	Nowadays by time slots ~ ISDN is integrated to PSDN
	PCM is the TDMA standard for the digital transmission
	PCM time slots consist of 8 bit samples
	For voice digital exchange sets up 64 kbit/s connections
	Data connections by (1) modems, (2) ISDN interface (3) leased lines via X.25 / Frame relay, or (4) ADSL

History

	1876 A. G. Bell telephone patent
	1878 The first exchange constructed in La Porte, the US
		could connect any two of the 21 subscribers
		manual switching (!)
	1891 first automatic exchange: Strowger Switch by Almon B. Strowger: an undertaker in Kansas City
	A 100 line Strowger switch:
		each user has its own selector
		no concentrators
		expensive

An early analog PBX: 100 subscriber exchange (Subscriber controlled call set-up)

An early exchange, call setup

	One of the 100 subscribers lifts his handset -> Call finder is activated to search the line.
	After the line is located other relays connect the dial-tone generator.
	The subscriber selects two digits.
	The first digit selects the subscriber group by using the group selector.
	The second digit selects the line selector.
	Selection is done by sending pulses that move the selectors stepwise.
	When connection is established a ringing tone is sent.
	Note that only 10 subscribers of 100 can call at the same time to different numbers! (why?) (concentration is 1:10)

PSTN exchange development

	Register-controlled setup (1920 -)
		B-subscriber number receiver by a register
		register controls all the remaining call setup stages
	Distributed control
		Markers indicate idle switches
		Thus markers control path routing
	Stored program control, 1960s and 1970s (SPC)
		New services
			supervision (operation & maintenance O&M)
			integrated charging
			gathering statistics
		IN services
		Easier updating and maintenance

Categorizing switching

Some features in PSTN of ´60

	Coil loading was used to enhance higher frequency range
	Frequency division multiplexing (FDM) with single sideband (SSB) modulation was used in trunk networks

Some features of PSTN of ´60 (cont.)

	Network intelligence and value-added services
		not supported as such
		operators were anyhow intelligent :)
			value added services by tracking what happens in the area!
	Inter-exchange signaling
		call setup took about 15 seconds
		channel-associated signaling (CAS: No.5, R1,R2*)
		about 10% of trunk line capacity was taken by signaling
	Operation and maintenance
		using local info-bases and local workforce
		network maintenance was based on on-field check-ups

Present-day PSTN ‘terminals¹’

	Fixed-line phones (analog, ISDN)
	Cordless phones (PBX-RF interface: DECT2)
	Fax
	Pay phones
	Private Branch Exchange (PBX)
	Gateways to Public Land Mobile Networks (PLMN):
		GSM
		wireless local area networks (WLAN)
	Local loop data extensions
		modems
		ADSL technology
		(leased lines)

Present-day PSTN services

	Basic service
		Bearer service (local loop access): analog (/ISDN)
	Value-added services (telephonist-originated) services as
			directory inquiry (118)
			weather, stock exchange, ticket reservation ...
	Supplementary services (Intelligent Terminal (IN) implementation)
		distributed supplementary as ‘call forwarding unconditional’, ‘call waiting’, ‘queuing’ ...
		centralized supplementary services (IN) use specialized routing & charging as VPN, credit card calls, free phone (receiver pays), universal access number (connected automatically to the nearest office), ...

PSTN today summarized

	Gets still more subscribers!
	ISDN very popular in switches (in Finland all-digital exchanges)
	ISDN getting popular also for local loop access
	Versatile access part
	Conventional local loop technology develops fast
	Remote controlled O&M
	IN services fully-developed - Intelligence moves to terminals
	Fiber-optical DWDM links connect exchanges
	Common channel signaling (SS7)
	SDH-based (Synchronous Digital Hierarchy) trunk-networking

Analog local loop interface

Basic telephone terminal

	A basic phone can be made by using just four units
		The bell
		The hook switch
		The keypad
		The speech circuit
	Modern keypads use dual-tone dialing
	The speech circuit adapts voice levels and isolates mic and speaker

Dual-tone dialing

	Dual-tone dialing is used in subscriber loop to transmit the selected B-subscriber number
	Earlier pulse selection was applied (very rare nowadays)

Basic modules of a modem

	Diagnostic unit
		Checks faults and controls the modem
	Interface and line units
		Adapt the modem and terminal
	Modem performs A/D and D/A conversion and select rate such that transmission quality criteria (error rate) can be meet

What is specified in a modem recommendation?

Some modem specifications

	ITU-T specifies several modem standards as
		V.26 (11/88) - 2400 bits per second modem for use on 4-wire leased lines
		V.27 (11/88) - 4800 bits per second modem for use on leased lines
		V.27ter (11/88) - 4800/2400 bits per second modem for use in the general switched telephone
		V.29 (11/88) - 9600 bits per second modem for use on point-to-point 4-wire leased lines
		V.90 (09/98) - 56 000 bit/s downstream and up to 33 600 bit/s upstream modem for use in the general switched telephone

Slide 20

Fax communications over PSTN

	Faxes follow standard PSTN modem communications recommendations or IEEE recommendations, as V.17 (02/91) (- Wire modem for facsimile applications with rates up to 14 400 bit/s)
	Faxes are divided into groups:
		Group 1 (´68): Analog scanning, 2400 bits/s
		Group 2 (´76): Analog scanning, 4800 bits/s
		Group 3 (´80): Digital scanning, 14400 bits/s
		Group 4 (´84): Digital scanning, 64 kbit/s (ISDN)
	Example of modules in group 3 transmitting fax:

PSTN in ITU-T standards (www.itu.org)

	Series D Recommendations - General tariff principles
	Series E Recommendations - Overall network operation, telephone service, service operation and human factors
	Series G Recommendations - Transmission systems and media, digital systems and networks
	Series I Recommendations - Integrated services digital network (ISDN)
	Series M Recommendations - Network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile, and leased circuits

More PSTN standards...

	Series O Recommendations - Specifications of measuring equipment
	Series P Recommendations - Telephone transmission quality, telephone installations, local line networks
	Series Q Recommendations - Switching and signaling (Signaling Systems no:4,5,6, and 7, Register Signaling no: R1, R2, IN - Service)
	Series V Recommendations - Data communication over the telephone lines

Example: Q-recommendations:
Switching and signalling*

Slide 25

If you don’t find it from Recommendations something is lost:  How to use pay phones?

	Recommendation E.134 (03/93) - Human factors aspects of public terminals: Generic operating procedures

Switching:
Transfer modes & connections