Public Switched
Telephone Network (PSTN II/II)

Topics today in PSTN

A: Switching types

Connectionless/ connection oriented

Packet/circuit

B: PSNT exchanges and interfaces

interface Q.512

using access and trunk networks

signaling

network management

internetworking (telecommunications between networks)

Exchange control functions

Maintenance functions

supervision of subscriber lines and trunk circuits

Operational functions

administrative data as

subscriber database

routing database

statistical data as

from where and whom subscribers call

holding times for different equipment types

utilization of IN services

User services

Exchange user services (examples)

Absent-subscriber services as the answering machine

Call booking: connection at the desired time

Person-to-person call: ensures that call goes to a right person

Serial call: setting up several calls

Telephone conferencing: several persons participate to call in real-time (compare: tele-conferencing)

Directory inquiries: also speech recognition, recorded messages

(many of these nowadays available in terminals)

The space-switch (used as a cross-switch and concentrator)

The time-switch

One of the time slots of any full-duplex lines is connected to some other line (at a time)

Thus two switches / time slot connect a line

For 100 full-duplex lines at 19.6 kbps a 1.92 Mbps bus is thus required for no blocking

If no fixed assignment of input lines to time slot but on demand allocation -> blocking switch that reduces number of switches and switch clock frequency. For instance 200 lines of 19.6 kbps with bus of 1.92 Mbps
-> about half of the devices can connect at any time, eg concentration is 2:1

The time-space-time (TST) switch

Works in local exchange and subscriber stage

Performs PCM concentration, usually 10:1 … 3:1

Connects subscribers also to information tones
and test equipment

Time switch contains one bus for incoming and outgoing calls (full-duplex)

PSTN ISDN exchange
interfaces (Q.512)

Exchange interfaces and tasks, V1

Purpose of exchange is to organizes connection between exchange terminators!

V1: Access to basic ISDN (This is user’s ISDN-u interface that can be used to connect small PBX also)

Basic ISDN V1-functions:

2 B + D (2x64 kbps + 16 kbps) channeling structure

timing and frame synchronization

activate and deactivate terminator

operation and maintenance

feeding power supply

ISDN basic access parameters defined in G.961

Exchange interfaces and tasks, V2-V4

V2: Interface serves typically concentrators

2048 kbit/s eg

30 B + D

Electrical standard G.704

V3: Resembles V2 but intended for interface other exchanges (PABX)

Electrical standard G.703

30 B + D at 2048 kb/s

also 23 B +D at 1544 kb/s (I.431)

V4:Interface to private networks (not ITU-T specified), for instance DSLAM (ADSL-interface)

Exchange interfaces and tasks, V5

Between access network and exchange

2048 kbit/s

Specifies basic interfaces for

Analog access

ISDN-access

Electrical interface G.703

Channel control and signaling

V5 supports interface rates 2048 kbit/s … 8448 kbit/s

Connecting the local loop:
Line interface circuit (LIC)

Line interface circuit components

Over-voltage protection

Test equipment to connect to monitor the line condition faults

Voltage feed

ringing

telephone current supply

Detection of

hook stage, pulse generated, or dual-tone receiver

The hybrid junction (2 wire - 4 wire interface)

An A/D converter (uses PCM techniques at 64 kbps)

The hybrid circuit

4-wire connection is used between exchanges and 2-wire connections from exchange to subscribers

The hybrid-circuit

The hybrid circuit summarized

The hybrid circuit transforms two-wire connection into 4-wire connection.

If the hybrid is unbalanced echo will result

Hybrid is balanced when no own voice is leaked into own loudspeaker

Hybrid unbalance can result from line impedance changes due to weather conditions

Unbalance results echo

Echo cancellation circuits are harmful in data connections

Nowadays realized by operational amplifier based circuitry that automatically monitors line impedance changes

Network echo suppressor (NES)

R: transmission gate, A: attenuator, L: logic circuit

When the signal is present on the receiving line the transmitting line is cut-off

A kind of semi-duplex approach to solve the echo problem

Network echo canceller (NEC)

Signal echo is extracted and subtracted from the received signal

More effective than echo suppressor. Often NEC and NES are however both used

PSTN operation and maintenance (Q.513)

Different alarm classes

Vital functions and circuits
(as SS7 and group switch)
use secured paths and
backups
Procedures provided for:

troubleshooting

fault diagnostics

hardware faults can
be isolated

Supervision is realized also
by connecting maintenance
units to the network

Important switches have
extensive backup equipment

Modern PSTN hierarchy

PSTN Hierarchy cont.

Local (example, within a city)

Subscriber connections

Switching within the local exchange

Switching to other exchanges

Transit (county level, say between Tampere and Helsinki)

Switching traffic between different geographical areas within one country

International

Gateway-type traffic between

different countries

DWDM (Dense Wavelength Division Multiplexing) routes

Rates can follow SONET or SDH standard

Subscriber signaling for local calls

Inter-exchange signaling

Channel associated signaling (CAS) as No.5, R1, R2

analog and digital connections

Modern ISDN exchanges apply SS7(digital), that is a common channel signaling method (CSS) that is discussed later in its own lecture

CAS is divided into line and register signaling:

Line signaling:

line state between the trunk-links as

answer, clear-forward*, clear-back

Register signaling:

routing information as

B-number, A-category, B-status

Inter exchange signaling (cont.)

Three categories of information is transmitted:

setup, supervision clearing

service related information as

forwarding, callback, charging

status change information

transmission network congestion

neighborhood exchange congestion

Example of inter-exchange signaling

Inter-exchange signaling (cont.)

A case study: DX 200 Exchange

Various control units apply common busses to control the exchange

A case study: DX 200 Exchange

SSU: Subscriber Signaling Unit: controls access network

CCSU:Common Channel Signaling Unit (SS7).

CCMU: Common Channel Signaling Management Unit: (as MTP, SCCP)

PAU: Primary Rate Access Unit: controls basic (64 kbit/s) system interfaces

LSU: Line Signaling Unit: takes care of signaling between transit exchanges and access networks

MFSU: MULti-frequency Service Unit: Takes care of signaling when multiple frequency signals are used

A case study: DX 200 Exchange (cont.)

BCDU:Basic Data Communication Unit: Serves various data services to OMU as access to X.25 and LANs

M: Marker Unit: Controls concentrators / space switches

CM: Central Memory: Contains user database, charging, signalling, routing and exchange ensemble.

STU: Statistical Unit: Collects statistical information on traffic and charging.

CHU:Charging Unit: Maintains charging database obtained from signalling units.

OMU:Operation and Maintenance Unit: Allows personnel access to exchange memory, perform tests an traffic measurements.

Slide 39



	A: Switching types
		Connectionless/ connection oriented
		Packet/circuit
	B: PSNT exchanges and interfaces
		interface Q.512
		using access and trunk networks
		signaling
		network management
		internetworking (telecommunications between networks)


Maintenance functions
	supervision of subscriber lines and trunk circuits
Operational functions
	administrative data as
		subscriber database
		routing database
	statistical data as
		from where and whom subscribers call
		holding times for different equipment types
		utilization of IN services
User services



		Absent-subscriber services as the answering machine
		Call booking: connection at the desired time
		Person-to-person call: ensures that call goes to a right person
		Serial call: setting up several calls
		Telephone conferencing: several persons participate to call in real-time (compare: tele-conferencing)
		Directory inquiries: also speech recognition, recorded messages (many of these nowadays available in terminals)


	One of the time slots of any full-duplex lines is connected to some other line (at a time)
	Thus two switches / time slot connect a line
	For 100 full-duplex lines at 19.6 kbps a 1.92 Mbps bus is thus required for no blocking
	If no fixed assignment of input lines to time slot but on demand allocation -> blocking switch that reduces number of switches and switch clock frequency. For instance 200 lines of 19.6 kbps with bus of 1.92 Mbps -> about half of the devices can connect at any time, eg concentration is 2:1


	Works in local exchange and subscriber stage
	Performs PCM concentration, usually 10:1 … 3:1
	Connects subscribers also to information tones and test equipment
	Time switch contains one bus for incoming and outgoing calls (full-duplex)


	Purpose of exchange is to organizes connection between exchange terminators!
	V1: Access to basic ISDN (This is user’s ISDN-u interface that can be used to connect small PBX also)
	Basic ISDN V1-functions:
		2 B + D (2x64 kbps + 16 kbps) channeling structure
		timing and frame synchronization
		activate and deactivate terminator
		operation and maintenance
		feeding power supply
		ISDN basic access parameters defined in G.961


	V2: Interface serves typically concentrators
		2048 kbit/s eg
		30 B + D
		Electrical standard G.704
	V3: Resembles V2 but intended for interface other exchanges (PABX)
		Electrical standard G.703
		30 B + D at 2048 kb/s
		also 23 B +D at 1544 kb/s (I.431)
	V4:Interface to private networks (not ITU-T specified), for instance DSLAM (ADSL-interface)


	Between access network and exchange
	2048 kbit/s
	Specifies basic interfaces for
		Analog access
		ISDN-access
	Electrical interface G.703
	Channel control and signaling
	V5 supports interface rates 2048 kbit/s … 8448 kbit/s


	Over-voltage protection
	Test equipment to connect to monitor the line condition faults
	Voltage feed
		ringing
		telephone current supply
	Detection of
		hook stage, pulse generated, or dual-tone receiver
	The hybrid junction (2 wire - 4 wire interface)
	An A/D converter (uses PCM techniques at 64 kbps)


	4-wire connection is used between exchanges and 2-wire connections from exchange to subscribers


	The hybrid circuit transforms two-wire connection into 4-wire connection.
	If the hybrid is unbalanced echo will result
		Hybrid is balanced when no own voice is leaked into own loudspeaker
	Hybrid unbalance can result from line impedance changes due to weather conditions
	Unbalance results echo
	Echo cancellation circuits are harmful in data connections
	Nowadays realized by operational amplifier based circuitry that automatically monitors line impedance changes


	R: transmission gate, A: attenuator, L: logic circuit
	When the signal is present on the receiving line the transmitting line is cut-off








	A kind of semi-duplex approach to solve the echo problem


	Signal echo is extracted and subtracted from the received signal
	More effective than echo suppressor. Often NEC and NES are however both used


Different alarm classes
Vital functions and circuits (as SS7 and group switch) use secured paths and backups Procedures provided for:
	troubleshooting
	fault diagnostics
		hardware faults can be isolated
Supervision is realized also by connecting maintenance units to the network
Important switches have extensive backup equipment


Local (example, within a city)
	Subscriber connections
	Switching within the local exchange
	Switching to other exchanges
Transit (county level, say between Tampere and Helsinki)
	Switching traffic between different geographical areas within one country
International
	Gateway-type traffic between
		different countries
		DWDM (Dense Wavelength Division Multiplexing) routes
Rates can follow SONET or SDH standard


Channel associated signaling (CAS) as No.5, R1, R2
	analog and digital connections
Modern ISDN exchanges apply SS7(digital), that is a common channel signaling method (CSS) that is discussed later in its own lecture
CAS is divided into line and register signaling:
	Line signaling:
		line state between the trunk-links as
			answer, clear-forward*, clear-back
	Register signaling:
		routing information as
			B-number, A-category, B-status


Three categories of information is transmitted:
	setup, supervision clearing
	service related information as
		forwarding, callback, charging
	status change information
		transmission network congestion
		neighborhood exchange congestion


	Various control units apply common busses to control the exchange


	SSU: Subscriber Signaling Unit: controls access network
	CCSU:Common Channel Signaling Unit (SS7).
	CCMU: Common Channel Signaling Management Unit: (as MTP, SCCP)
	PAU: Primary Rate Access Unit: controls basic (64 kbit/s) system interfaces
	LSU: Line Signaling Unit: takes care of signaling between transit exchanges and access networks
	MFSU: MULti-frequency Service Unit: Takes care of signaling when multiple frequency signals are used


	BCDU:Basic Data Communication Unit: Serves various data services to OMU as access to X.25 and LANs
	M: Marker Unit: Controls concentrators / space switches
	CM: Central Memory: Contains user database, charging, signalling, routing and exchange ensemble.
	STU: Statistical Unit: Collects statistical information on traffic and charging.
	CHU:Charging Unit: Maintains charging database obtained from signalling units.
	OMU:Operation and Maintenance Unit: Allows personnel access to exchange memory, perform tests an traffic measurements.