Structure of the PSTN

Structure of the PSTN

Transport or transmission (PDH, SDH)Switching (see previous lecture)Subscriber signalling (analog or digital)Network-internal signalling (SS7)Intelligent Network (IN) concept

Basic components also for circuit-switched core of mobile networks (PLMN)

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Basic functional parts of the PSTN

PSTNSwitching in exchanges

Subscriber signalling (analog or ISDN=DSS1) Network-

internal signalling (SS7)

Transmission (PDH, SDH)

Databases in the network (HLR)

PSTN Circuit-switched technology

Based on 64 kbit/s channels (TDM time slots)Time Division Multiplexing (TDM)Connection-oriented operation (setup & release connection => call)Charging is based on time duration of connectionOptimized for delay-sensitive services (speech)

No fixed channel concept (bit rate is not constant)Statistical multiplexing (greater flexibility)Connectionless operation (independent routing of packets) as defaultMore flexible charging solutionsQoS solutions required for delay-sensitive services

Circuit-switched network Packet-switched network

IP network as alternative to PSTN

Switching in exchanges





PSTN

IP network

Voice traffic can naturally also be carried over Packet-switched (IP) networks.This topic is covered in a future lecture.

Quality-of-Service (QoS) support needed!

Transmission: PDH or SDH systems






64 kbit/s channel (or TDM time slot)

This is the basic transport unit in both PDH and SDH transport systems. Note that switching in exchanges in the PSTN is also based on 64 kbit/s TDM time slots.

When used for voice transport, a 64 kbit/s channel contains PCM (Pulse Code Modulation) speech, generated according to ITU-T specification G.711.

time

Analog speech signal (300…3400 Hz)Sampling produces 8000 samples/s

Each sample is encoded into an 8-bit PCM code word

(e.g. 01100101)=> 8000 x 8 bit/s

PDH and SDH transmission bit rates

PDH (Plesiochronous Digital Hierarchy)

SONET (North Am.) SDHSTS-1 51.84 Mbit/sSTS-3 155.52 STM-1STS-12 622.08 STM-4STS-48 2.488 Gbit/s STM-16

Japan USA EuropeJ1 1.5 Mbit/s T1 1.5 Mbit/s E1 2 Mbit/s J2 6 T2 6 E2 8J3 32 T3 45 E3 34J4 98 T4 140 E4 140

Structure of E1 frame (2.048 Mbit/s)

32 TDM time slots (with 8 bits each / frame)

Time slots 1-31 carry digital signals (usually PCM speech) with a bitrate of 64 kbit/s.

Time slot 0 is used for frame synchronization:

0 1 2 3116

... ...received bit stream ... where does a new frame begin?

Time slot 16 usually contains SS7 signalling information.

Structure of STM-1 frame in SDH

SOH

SOH

AU pointer indicates where the virtual container starts in the payload field

3

5

9 261 bytes

1

STM-1 payload (contains the actual information)

STM = Synchronous transport moduleSOH = Section overheadAU = Administrative unit

Higher-order STM-4 signal is generated using synchronous byte interleaving:

byte from first STM-1 signalbyte from second STM-1 signalbyte from third STM-1 signalbyte from fourth STM-1 signal

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Bitrate of STM-1 signal

SOH

SOH

3

5

9 261 bytes

1

STM-1 payload Basic idea: bytes from a 64 kbit/s channel are carried in successive STM-1 frames (exactly one byte per frame).

STM-1 frame contains 9 x 270 bytes=> bitrate of STM-1 signal:

9 x 270 x 64 kbit/s = 155.52 Mbit/s

Mapping into STM-1 frames

SOH

SOH

VC-4 (Virtual container) POH

AU-4 pointer points to first byte of VC

1 260 bytes

Virtual container “floats” within the payload of STM-1

frames

9

POH = Path overhead

Filling of STM-1 payload in practice

P

In reality, the STM-1 payload is filled like this:

Beginning of next virtual container

Beginning of virtual container

P

Path overhead bytes

STM-1 frame N

STM-1 frame N+1

SDH pointer adjustment (1)

SOH

SOH

VC-4 (Virtual container)

When VC-4 clock rate is larger than STM-1 clock rate=> pointer value is shifted forward three bytes

Three “empty” bytes are

inserted here

oldnewPointer

value updated

SDH pointer adjustment (2)

SOH STM-1 payload

VC-4 (Virtual container)

When VC-4 clock rate is smaller than STM-1 clock rate => pointer value is shifted back three bytes

Three VC bytes are stored here

AU-4 pointer

oldnewPointer

value updated

Payload mapping

STM-1 can carry 63 E1 signals.SDH systems nowadays also

carry ATM and IP traffic.

STM-1

More about SDH…

SDH pocket guide (there is a link to this material on the course home page)

www.iec.org/online/tutorials/sdh

Section 4.4.1 in ”Understanding Telecommunications 1” by Ericsson Telecom, Telia and Studentlitteratur 1998 (the corresponding online course is sometimes available at www.ericsson.com)

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Subscriber signalling






Analog subscriber signalling

The calling party (user A) tells the local exchange to set up (disconnect) a call by generating a short (open) circuit in the terminal => off-hook (on-hook) operation.

The dialled called party (user B) number is sent to the local exchange in form of Dual Tone Multi-Frequency (DTMF) signal bursts.

Alerting (ringing) means that the local exchange sends a strong sinusoid to the terminal of user B.

In-channel information in form of audio signals (dial tone, ringback tone, busy tone) is sent from local exchange to user. User can send DTMF information to network.

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Analog subscriber signalling in action

LE AUser A User B

Ringing signal

Off-hook (user B

answers)

Off-hook SS7 signalling

(ISUP)Dial toneB number

Ringback tone (or

busy tone)

LE B

Connection established

LE = local exchange

ISDN subscriber signalling in action

LE AUser A User B

Ringing

Off-hook (user B

answers)

Off-hook SS7 signalling

(ISUP)B number

Tones generated in terminal

LE B

Setup

Call proc Setup

Alert

ConnAlert

Conn

DSS1 signalling messages

Connection established

What does ISDN originally mean?

1. End-to-end digital connectivity2. Enhanced subscriber signaling3. A wide variety of new services (due to 1 and 2)4. Standardized access interfaces and terminals

ISDN is not a “new” network separated from the PSTN. Interworking with “normal” PSTN equipment is very important.

ISDN terminal

PSTN terminal

interaction is possible

Idea originated in the 1980’s

PSTN vs. ISDN user access

300 … 3400 Hz analog transmission band“Poor-performance” subscriber signaling

2 x 64 kbit/s digital channels (B channels)16 kbit/s channel for signaling (D channel) => Digital Subscriber Signalling system nr. 1 (DSS1)

PSTN

Basic Rate

Access ISDN

Primary Rate

Access ISDN

30 x 64 kbit/s digital channels (B channels)64 kbit/s channel for signaling (D channel)Mainly used for connecting private branch exchanges (PBX) to the PSTN.

End-to-end digital signalling

ISUPQ.931

Q.921I.430

Q.931

Q.921I.430

MTP 3MTP 2MTP 1

Q.931

Q.921I.430

Q.931

Q.921I.430

ISUPMTP 3MTP 2MTP 1

contains the signalling messages for call control

User interface User interfacePSTN Network

DSS1

SS7

DSS1

Signalling System nr. 7 (SS7)






History of inter-exchange signalling

SS6 = CCIS (common channel interoffice signaling) was deployed in North America as an interim solution, but not in Europe. CCIS is not the same thing as SS7.

Starting from 1980 (mainly in Europe), CAS was being replaced by SS7. The use of stored program control (SPC) exchanges made this possible. Like CCIS, signalling messages are transmitted over separate signalling channels. Unlike CCIS, SS7 technology is not monolithic, but based on protocol stacks.

Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, the signalling is carried in-band along with the user traffic.

CAS

CCIS

SS7

Channel-associated signalling (CAS)CAS means in-band signalling over the same physical channels as the circuit-switched user traffic (e.g. voice).

Signalling to/from databases is not feasible in practice (setting up a circuit switched connection to the database and then releasing it would be extremely inconvenient).

Exchange ExchangeExchange

Circuit switched connection

Signalling is possible

Signalling is not possible before previous circuit-

switched link is established

CAS has two serious draw-backs:Setting up a circuit switched connection is very slow.•

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Common channel signalling (CCS)In practice, CCS = SS7.

Exchange Exchange

Signalling is possible anywhere anytime

Database

End-to-end signalling is possible before call setup and also during the conversation phase of a call.

The packet-switched signalling network is totally separated from the circuit-switched connections. Consequently:

Signalling to/from databases is possible anytime.• •

There is one drawback: It is difficult to check if the circuit-switched connections are really working (= continuity check).

Signalling example

ExchExchUser A

(calling user)

Database

A typical scenario: User A calls mobile user B. The call is routed to a specific gateway exchange (GMSC) that must contact a database (HLR) to find out under which exchange (MSC) the mobile user is located. The call is then routed to this exchange.

OuluTokyo

London

User B (called user)

Exch

ISDN User Part

(ISUP)

Protocol layers (“levels”) of SS7

MTP level 3 (routing in the signalling network) MTP level 2 (link-layer protocol)

MTP level 1 (64 kbit/s PCM time slot)

Signalling Connection Control Part (SCCP)

Transaction Capabilities Application Part (TCAP)

Application protocols (e.g. Mobile Application Part, MAP)

MTP

MTP user

SS7 application protocol for managing

circuit-switched connections

MAP

ISUPTCAP

SCCP

SS7 protocols vs. OSI model

MTP level 3MTP level 2MTP level 1

…Application

PresentationSession

Transport

Network

Data linkPhysical

SS7 protocol stack OSI protocol layer model

OSI protocol layer model

Multiplexing & transport of bits, time slots in PDH or SDH systems

Switching & routing through the communications network Link-layer flow & error control

End-to-end flow & error control

User application (in this case, the actual signalling messages) Data compression & coding

Dialogue control

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data link layer

Physical layer

Message Trasfer Part (MTP) functions

MTP level 1 (signalling data link level): Digital transmission channel (64 kbit/s TDM time slot)

Frame-based protocol for flow control, error control (using Automatic Repeat reQuest, ARQ), and signalling network supervision and maintenance functions.

Routing in the signalling network between signalling points (using signalling point codes). MTP level 3 ”users” are ISUP and SCCP (other ”users” such as TUP or DUP are not widely used any more).

MTP level 3 (signalling network level):

MTP level 2 (signalling link level):

MTP level 2 frame formats

F CK SIF SIO LI Control F

F CK SF LI Control F

F CK LI Control F

MSU (Message Signal Unit)

LSSU (Link Status Signal Unit)

FISU (Fill-In Signal Unit)

Level 3 user information

Network: • National• International

User part: • ISUP• SCCP• Signalling network management MSBLSB

MTP level 2 frames

MSU (Message Signal Unit):Contains actual SS7 signalling messagesThe received frame is MSU if LI > 2 (LI = number of octets)

LSSU (Link Status Signal Unit):Contains signalling messages for MTP level 2 (signalling link) supervisionThe received frame is LSSU if LI = 1 or 2

FISU (Fill-In Signal Unit):Can be used to monitor quality of signalling link at receiving endThe received frame is FISU if LI = 0

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Signalling points (SP) in SS7

Network elements (relevant from signalling point of view) contain signalling points identified by unique signalling point codes.

Exchange

STP SP

SPSTP

Signalling Point (in a database, such as HLR in mobile network)

Signalling Transfer Points only relay signalling messages

Signalling Point (signalling termination in an exchange)

STP

MAP

ISUP

Signalling point code (SPC)SS7 signalling messages contain MTP level 3 routing information in the form of a routing label:

SIO octetDPC

DPC

LSBMSB

OPCOPC

OPC SLSSignalling message

payload

International (and most national) signalling networks (ITU-T):14-bit Destination Point Code (DPC)14-bit Originating Point Code (OPC)4-bit Signalling Link Selection (SLS)

North American national signalling network (ANSI):24-bit DPC and OPC, 5-bit SLS code

Format for international SPC: Zone Area/Network SP3 bits 3 bits8 bits

For examples, see: www.numberingplans.com

Same SPCs can be reused at different network levels

SPC = 277

SPC = 277

International

National

SPC = 277 means different signalling points (network elements) at different network levels.

F CK SIF SIO LI Control F

The Service Information Octet (SIO) indicates whether the DPC and OPC are international or national signalling point codes.

ISDN User Part (ISUP)ISUP is a signalling application protocol that is used for establishing and releasing circuit-switched connections (calls).

Only for signalling between exchanges (ISUP can never be used between an exchange and a stand-alone database)

Not only for ISDN (=> ISUP is generally used in the PSTN)

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Structure of ISUP message:SIO (one octet)

Routing label (four octets)CIC (two octets)

Message type (one octet) Mandatory fixed part

Mandatory variable partOptional part

Must always be included in ISUP messageE.g., IAM message

E.g., contains called (user B) number in IAM message

ISUP signalling messagesBasic ISUP signalling messages:

Call setup:

IAM (Initial address message)ACM (Address complete message)ANM (Answer message)

From LE A to LE B

From LE B to LE A

Call release:

REL (Release message)RLC (Release complete message)

Direction depends on releasing party (user A or user B)

Difference between SLS and CIC

The four-bit signalling link selection (SLS) field in the routing label defines the signalling link which is used for transfer of the signalling information.

The 16-bit circuit identification code (CIC) contained in the ISUP message defines the TDM time slot or circuit with which the ISUP message is associated.

Exchange

STP

ExchangeCircuit

Signalling link

Signalling using IAM message

Exchange ExchangeExchange

SPC = 82

Circuit 14

SPC = 22 SPC = 60Circuit 20

STPSL 4

SL 7

STP

Outgoing message:OPC = 82 CIC = 14DPC = 22 SLS = 4

Processing in (transit) exchange(s):Received IAM message contains B-number. Exchange performs number analysis (not part of ISUP) and selects new DPC (60) and CIC (20).

Setup of a call using ISUP

LE A LE BTransit exchange User A User B

Setup IAMIAM

Setup

Alert

Connect

ACM

ANM

ACM

ANM

Alert

ConnectCharging of call starts now

Number analysisDSS1 signalling assumed

Call setup: Signalling sequence 1

User A User B

Off hook

Dial tone

B number

Local exchange detects setup request and returns dial tone

Local exchange: analyzes B number determines that call should be routed via transit exchange (TE)

LE A LE BTE

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User A User BLE A LE BTE

Initial address message (IAM)

ISUP message IAM is sent to transit exchange (TE).TE analyzes B number and determines that call should be routed to local exchange of user B (LE B).IAM message is sent to LE B.There now exists a circuit-switched path (the path is “cut through”) between user A and LE B.



Address complete message (ACM)

Ringing signalRingback tone

Ringing signal is sent to user B (=> user B is alerted).Ringback tone (or busy tone) is sent to user A.(Ringback/busy tone is generated locally at LE A or is sent from LE B through circuit switched path.)

or



Answer message (ANM)User B answers

User B answers, connection is cut through at LE B.Charging of the call starts when ISUP message ANM is received at LE A (the normal case).The 64 kbit/s bi-directional circuit switched connection is now established.

Charging starts now

Conversation over this “pipe”

E.164 numbering scheme

00

0

358 9

9

1234567

1234567

1234567

International number

National number

User numberPrefixCountry codeArea code

3589

In each exchange, the B number is analyzed at call setup (after the IAM message containing the number has been received) and a routing program (not part of ISUP) selects the next exchange to which the call is routed.

or mobile network code, e.g. 40

E.164 number structure

00 358 9 1234567

Prefix

For examples, see: www.numberingplans.com

Country code (1-3 digits)

National destination code (1-3 digits)

Max. 15 digits

Subscriber number

Area code, e.g. 9Mobile network code, e.g. 40

MSISDN number

Signalling sequence for call release


On hook Release message (REL)

Release complete message (RLC)

The circuits between exchanges are released one by one.(The generation of “hanging circuits” should be avoided, since these are blocked from further use.)

Charging stops

Conversation over this “pipe”

Signalling Connection Control Part (SCCP)SCCP is required when signalling information is carried between exchanges and databases in the network.

An important task of SCCP is global title translation (GTT):

STP DatabaseExchangeSTP with GTT capability

Exchange knows the global title (e.g. 0800 number or IMSI number in a mobile network) but does not know the DPC of the database related to this global title.

1.

SCCP performs global title translation in the STP (0800 or IMSI number => DPC) and the SCCP message can now be routed to the database.

2.

Why GTT in STP network node?

Global title translation (GTT) is usually done in an STP.

Advantage: Advanced routing functionality (= GTT) needed only in a few STPs with large packet handling capacity, instead of many exchanges.

Exchange

STP

Database

Exchange

Exchange

ExchangeExchange

Database

Exchange

Example: SCCP usage in mobile call

SCCPSCCP

MSC located in Espoo HLR located in Oslo

STP

SPC = 82 SPC = 99

SPC = 32

SCCP/GTT functionality

Outgoing message:OPC = 82 DPC = 32SCCP: IMSI global title

Processing in STP:Received message is given to SCCP for GTT. SCCP finds the DPC of the HLR: DPC = 99

Mobile switching center (MSC) needs to contact the home location register (HLR) of a mobile user identified by his/her International Mobile Subscriber Identity (IMSI) number.

More about SS7…

Chapter 4 in ”Engineering Networks for Synchronization, CCS7, and ISDN” by P.K.Bhatnagar 1997 (this belongs to the distributed course material)

www.iec.org/online/tutorials/ss7

Part E in ”Understanding Telecommunications 2” by Ericsson Telecom, Telia and Studentlitteratur 1998 (the corresponding online course is sometimes available at www.ericsson.com)

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To sum it up with an example…

PSTNTypical operation of a local exchange





Part B, Section 3.3 in ”Understanding Telecommunications 2”

Basic local exchange (LE) architecture

Time switch

TDM links to other network elements

• Switch control

Switching system

• E.164 number analysis• Charging• User databases

LIC

LICToneRx

Group switch

Sign.

ETC

ETC

Exchange terminal circuit

Line interface circuit

SS7 Signalling equipment

Control system• O&M functions

Subscriber stage

Modern trend: Switching and control functions are separated into different network elements (separation of user and control plane).

Tone generator

Setup of a call (1)

Time switch

2. Check user database. For instance, is user A barred for outgoing calls?

Switching system

3. Reserve memory for user B number

LIC

LICToneRx

Group switch

Sign.

ETC

ETC

Control system

Phase 1. User A lifts handset and receives dial tone.

1. Off hook

Local exchange of user A

4. Tone Rx is connected

5. Dial tone is sent (indicating “network is alive”)

Tone generator

Time switch

3. Number analysis

Switching system

4. IN triggering actions? Should an external database (e.g. SCP, HLR) be contacted?

LIC

LICToneRx

Group switch

Sign.

ETC

ETC

Control system

Phase 2. Exchange receives and analyzes user B number.

2. Number (DTMF signal) received1. User A

dials user B number

Setup of a call (2)


Time switch

2. Outgoing circuit is reserved

Switching system

LIC

LICToneRx

Group switch

Sign.

ETC

ETC

Control system

3. Outgoing signalling message (ISUP IAM) contains user B number

Phase 3. Outgoing circuit is reserved. ISUP Initial address message (IAM) is sent to next exchange.

Setup of a call (3)

1. Tone receiver is disconnected


E.g., CIC = 24

IAM (contains information CIC = 24)

Time switch

1. ISUP ACM message indicates free or busy user B

Switching system

LIC

LIC Group switch

Sign.

ETC

ETC

Control system

3. Charging starts when ISUP ANM message is received

Phase 4. ACM received => ringback or busy tone generated. ANM received => charging starts.

Setup of a call (4)


ACM, ANMTone generator2. Ringback

or busy tone is locally generated

4. Call continues…

Documents

Structure of the PSTN