ISDN

Integrated Services Digital Network

What is ISDN ?
1. End-to-end digital connectivity 2. Enhanced subscriber signaling
Original idea in the 1980s

3. 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 interaction is possible PSTN terminal

Evolution of the PSTN / ISDN

Step 1: all-analogue network (before 1960)

Evolution of the PSTN / ISDN

Step 2: digital transmission in the core network (1960 - 1980)

PDH transmission systems (2 - 140 Mbit/s)

Evolution of the PSTN / ISDN

Step 3: digital switching of 64 kbit/s channels (1970 - 1990)

Time switching technology

Evolution of the PSTN / ISDN

Step 4: common channel signalling in the core network (1980 - 2000)

SS7

Evolution of the PSTN / ISDN

Step 5: PDH systems are being replaced by SDH (1990 ...)

SDH transmission systems (155, 620 Mb/s)

Evolution of the PSTN / ISDN

Step 6: digital access lines (ISDN, ADSL) installed (1990 ...) End-to-end digital user data End-to-end digital signalling

Success of a new concept depends on:

Public network => standardization important (there may be different equipment vendors, operators ) => open interfaces Critical mass of services, subscribers, and terminal equipment is needed before concept can be made comercially attractive (chicken and egg problem)

Problem-free evolution & concept integration Does the user need this new concept?

Digital access: several alternatives

ISDN Bit rate (kb/s) Connection setup time Popularity 2 x 64 fast little modem max. 50 slow great ADSL much larger fast increasing

However, large impact on signalling protocols

PSTN vs. ISDN user access

PSTN Basic Rate Access ISDN Primary Rate Access ISDN

300 3400 Hz analogue transmission band poor-performance subscriber signaling 2 x 64 kbit/s digital channels (B channels) 16 kbit/s channel for signaling (D channel) 30 x 64 kbit/s digital channels (B channels) 64 kbit/s channel for signaling (D channel) concatenation of B channels possible

Telecommunication services
Basic telecommunication services
Bearer services provide the capability of transmitting signals between network access points. Higher-level functionality of user terminals is not specified. Teleservices provide the full communication capability by means of network functions, terminals, dedicated network elements, etc.

Supplementary services
A supplementary service modifies or supplements a basic telecommunication service. It cannot be offered to a customer as a stand-alone service.

Services examples
Some typical teleservices Telephony (normal, high quality) Telefax (Group 3, Group 4) Video-telephony Some typical bearer services Speech (transparency not guaranteed) 64 kbit/s unrestricted 3.1 kHz audio (non-ISDN interworking) Some typical supplementary services CLIP / CLIR Call forwarding / waiting / hold Charging supplementary services

Basic rate access user interface

Terminal Adaptor

S/T

Network Termination

Line Interface Circuit

R
Non-ISDN terminal

Bi-directional 192 kbit/s

160 kbit/s echo canceling or Exchange time compression

ISDN terminal

Subscriber (premises) network

Exchange

Primary rate access user interface

U

PBX
PBX equipment manufacturer specific solutions

Line Termination
Standard 2 Mb/s TDM connection (PDH or SDH) Exchange

64 kb/s D channel in one PCM time slot

ISDN Signalling Protocols

OSI reference model

7. Application 6. Presentation
A (protocol residing in a) lower layer provides certain services to a (protocol in a) higher layer Protocol Data Units (PDU) Headers

5. Session
4. Transport 3. Network 2. Data Link 1. Physical

Tx end

Rx end

Relation between network connection elements and OSI model

7. Application 6. Presentation
Some examples: Gateway (GW), Interworking function (IWF)

5. Session
4. Transport 3. Network 2. Data Link 1. Physical

Router, Switching function Bridge, Relaying function SDH cross-connect

Tasks of OSI layers:

7. Application 6. Presentation
User application Compression & coding Dialogue control

5. Session
4. Transport 3. Network 2. Data Link 1. Physical

End-to-end flow & error control

Switching & routing Link-layer flow & error control Multiplexing & transport of bits

Typical protocol interaction

App : Tran Net Link Phy Phy App : Tran Net Link Phy Phy Link Phy Phy

Switch, router Net Link Link

Relay, bridge

End node

Intermediate nodes

End node

Signalling protocols for end-to-end connection

User interface PSTN Network User interface

Q.931
DSS1

Q.931 Q.921 I.430

ISUP SS7 ISUP MTP 3 MTP 2 MTP 1 MTP 3 MTP 2 MTP 1

Q.931
DSS1

Q.931 Q.921 I.430

Q.921 I.430

Q.921 I.430

contains the signalling messages for call control

Layered DSS1 signaling structure

DSS1 = Digital Subscriber Signalling system no.1

I.430 Layer 1: Bit sequence structure, framing & multiplexing Q.921 Layer 2: Link control (HDLC-type protocol called LAPD)

Layer 3: Q.931 Signaling messages (application layer)

LAPD (Q.921) is used for

Establishing data link connections identified by the Data Link Connection Identifier (DLCI = SAPI + TEI) Frame delimiting, alignment and transparency, allowing recognition of frames transmitted over the D-channel Flow control: (a) to maintain the sequential order of frames across a data link connection, (b) temporarily stopping transmission Error Control: detection of errors on a data link connection, recovery from errors, and notification to the management entity of unrecoverable errors

Q.931 Call-related messages

Call establishment messages: ALERTING CALL PROCEEDING Similar CONNECT functions as CONNECT ACKNOWLEDGE ISUP in SS7 PROGRESS SETUP SETUP ACKNOWLEDGE Call clearing messages: DISCONNECT RELEASE RELEASE COMPLETE

Typical content of ISDN Set-up message

Called party (user B) number & numbering plan Calling party (user A) number (+ CLIP/CLIR)
Show to B?

Bearer capability (64 kbit/s unrestricted, speech, 3.1 kHz audio, packet mode B-channel, packet mode Dchannel) Channel identification (B1, B2, or D channel request) Low-layer compatibility (type of bit rate adaptation, type of modem ) High-layer compatibility (teleservice-related issues)

Keypad facility

Structure of Q.931 message (Release)

Message type: RELEASE Significance: Local Direction: Both

Info Element

Protocol discriminator Call reference Message type Cause Display Signal

Direction
Both

Type
M

Length
1

Both Both Both nu nu

M M O O O

21 2-32
2-3

Cause description may require many bytes

Setup of a PSTN call

User A Exchange A Exchange B User B

off-hook dial tone B number ringing tone

SS7 ISUP ringing tone user B answers

connection ok

Setup of an ISDN call using Q.931

User A Exchange A Exchange B User B

offhook

Setup
Call proceed Setup

SS7 ISUP

Alert

ring user B answers

Alert Connect Connect

connection ok

SS7
Common Channel Signalling System Nr. 7

IN
Intelligent Network concept

History of inter-exchange signalling

CAS Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, signalling always occurs in-band (i.e. over voice channels). SS6 = CCIS (common channel interoffice signaling) was widely deployed in North America, but not in Europe (=> concentrating on SS7 instead). 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 based on protocol stacks.

CCIS

SS7

Channel-associated signalling (CAS)

CAS means in-band signalling over voice channels.
signalling possible Exchange signalling not possible (yet) Exchange Exchange

circuit switched connection

CAS has two serious draw-backs: 1) Setting up a circuit switched connection is very slow. 2) Signalling to/from databases is not possible (setting up a circuit switched connection to the database would be extremely inconvenient).

Common channel signalling (CCS)

In practice, CCS = SS7 (except maybe North America). In Finnish: CCS = yhteiskanavamerkinanto (YKM)
signalling possible anywhere anytime Exchange Exchange Database

The packet-switched signalling network is separated from circuit switched connections. Consequently: 1) Signalling to/from databases is possible anytime. 2) End-to-end signalling is possible before call setup and also during the conversation phase of a call.

CAS vs. CCS

Tokyo Exch Exch Oulu

User A (calling user)

Exch

Exch

User B (called user) Database

1) Accessing database 2) Signalling before call setup

Espoo

3) Signalling during conversation phase (user-to-user => digital access technology required)

Signalling points (SP) in SS7

Every SP is identified by a unique signalling point code
STP
Signalling Transfer Point (only related to SS7 network)

STP
MAP INAP CAP

SP

Signalling Point (in a database, such as HLR in GSM) Application protocols used in SS7

STP

SP
ISUP

Signalling Point (signalling termination in an exchange)

Exchange

Intelligent Network (IN) Concept

Intelligence => access to various databases
Operator implements service logic (IN Service)

STP
MAP INAP CAP

SCP

Service Control Point (a network element containing the service logic, is often also called database or register)

SSP
ISUP

Service Switching Point (enables service triggering in an exchange)

Exchange

Typical call-related IN procedure (1)

3. 4. 5. Exchange SCP

1.

2. SSP
Exchange

1. Call routing proceeds up to Exchange 2. Trigger activated in Basic Call State Model at SSP 3. SSP requests information from SCP (database) 4. SCP provides information 5. Call routing continues (routing to next exchange)

Typical call-related IN procedure (2)

3. 4. 5. Exchange SCP

1.

2. SSP
Exchange

2. Trigger activated in Basic Call State Model at SSP Typical triggers: Called number (or part of number) Access code or ID information Time (hour, day) or location (mobile system) Calling number (or part of number)

Typical call-related IN procedure (3)

3. SCP

1.

2. SSP
Exchange

4.
5. Exchange

4. SCP provides information Example: Number translation in SCP SSP sends 800 number (0800 1234) SCP translates into real number which can be used for routing the call (+358 9 4512343) translation may be based on several variables

IN service examples
Traditional IN services: Freephone / customised charging schemes Virtual Privat Network (VPN) Number portability Televoting

IN in mobile networks: - Mobility management (HLR, VLR = databases) - Security management (Authentication ...) - CAMEL IN in mobile networks (Customised Applications for Mobile networks Enhanced Logic)

Protocol layers (levels) of SS7

Application protocols TUP ISUP

MAP SCCP
MTP level 3

CAP TCAP routing

INAP

MTP level 2 (link-layer protocol) MTP level 1 (64 kbit/s PCM time slot) MTP - Message Transfer Part SCCP - Signalling Connection Control Part UP - User Part AP - Application Part

Application protocols in SS7

TUP (Telephone User Part) is being replaced by ISUP ISUP (ISDN User Part) for all signalling related to setting up, maintaining, and releasing circuit switched connections MAP (Mobile User Part) for transactions between exchanges (MSC, GMSC) and databases (HLR, EIR, AuC) in mobile networks INAP (Intelligent Network Application Part) for IN applications in fixed networks CAP (CAMEL Application Part) for extended IN functionality in mobile networks (where MAP is not sufficient ...)

MTP functions
MTP level 1 (signalling data link level): Physical transmission (e.g. 64 kbit/s PCM time slot) MTP level 2 (signalling link level): HDLC-type frame-based protocol for flow control, error control (using ARQ), and signalling network supervision and maintenance functions. MTP level 3 (signalling network level): Routing in the signalling network (using OPC, DPC) between SPs with level 4 users (see SIO at level 2).

MTP level 2 frame formats

MSU (Message Signal Unit)
Level 3 signalling message

CK

SIF

SIO

LI

Control

LSSU (Link Status Signal Unit)

CK

SF

LI

Control

Network: National International User part: TUP ISUP SCCP Network management

FISU (Fill-In Signal Unit)

CK

LI

Control

MTP level 2 frames

MSU (Message Signal Unit): Contains signalling messages (User Part SIO) The received frame is MSU if LI > 2 (number of octets) LSSU (Link Status Signal Unit): Contains signalling messages for link supervision The received frame is LSSU if LI = 1 or 2 FISU (Fill-In Signal Unit): Can be used to monitor quality of signalling link The received frame is FISU if LI = 0

Routing information in SS7 message

Level 3 signalling message in SIF (Signalling Information Field)

Routing label
MTP management message: SLC 4 bit signalling link code
MTP SCCP message: SLS 4 bit signalling link selection MTP TUP message: CIC 12 bit circuit ID code

SLC

OPC

DPC

SLS

OPC

DPC

CIC

OPC

DPC

Structure of SS7 ISUP message

Level 3 signalling message in SIF (Signalling Information Field) MTP ISUP message: SLS 4 bit CIC 12 bit

Routing label CIC SLS OPC DPC

Max 256 + 1 octets

OpP

MaVP

MaFP

MTC

ITU-T structure ANSI => different

MTC: Message Type Code (name of ISUP message) MaFP: Mandatory Fixed Part (no LI, no parameter names required) MaVP: Mandatory Variable Part (LI, no parameter names required) OpP: Optional Part (LI and parameter names required)

Difference between SLS and CIC

SLS defines the signalling link which is used for transfer of signalling information. CIC defines the circuit (used for a certain circuit switched connection) with which the ISUP message is associated.

signalling link
STP SSP Exchange circuit SSP Exchange

Role of DPC and OPC in SS7

DPC Destination Point Code (14 bit 16384 SPs) Termination point of application transaction Key information for routing within SS7 network DPC is inserted by the originating MTP user. OPC Originating Point Code (14 bit) Originating point of application transaction The network indicator in the SIO octet determines whether the DPC or OPC is an international, national, or network dependent SP identifier.
F CK SIF SIO LI Control F

Same signalling point codes can be reused at different network levels

International
SPC = 277

National
Network specific
SPC = 277

SPC = 277

SPC = 277 means different SPs at different network levels

Functions at signalling network level

MTP user

Message distribution

Message discrimination

Signalling link

Message routing
ISUP SCCP

Signalling message handling

MTP level 2

Signalling network management

ISUP (Integrated Services User Part)

Essential for circuit-switching related signalling Not only ISDN (can be generally used in PSTN) Features:

Establishment / release of circuit switched connections (basic call control) using link-by-link signalling
End-to-end signalling between two exchanges (for this purpose SCCP + ISUP is used) see Bhatnagar, p.77 General (non-user-related) circuit management

Example: link-by-link routing

Using MTP-level routing table, STP routes message to DPC = 22 STP STP SL 2 SL 7 SPC = 82 Circuit 14 Exchange Exchange SPC = 22 Circuit 20 SPC = 60 SPC = 18 Outgoing MTP MSU: OPC = 22 CIC = 20 DPC = 60 SLS = 2

SL 4

SPC = 15

Exchange

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

Processing in (transit) exchange(s): Received message is sent to user (ISUP) that gives B-number to exchange. Exchange performs number analysis and selects new DPC (60) and CIC (20)

MTP + ISUP in SS7

The routing capability of MTP is rather limited (routing tables are entirely based on signalling point codes). Exchanges perform the routing through the network(s) during the establishment of circuit switched connections on an exchange-to-exchange basis, using the dialed digits and routing tables.
+358 9 4512343 exchange ID

Country code

National region

Subscriber number

Example: link-by-link signalling

Using MTP-level routing table, STP routes message to DPC = 22 STP STP SPC = 18 Outgoing MTP MSU: OPC = 22 CIC = 20 DPC = 60 SLS = 2

Otherwise like link-by-link SL 2 SPC = 15 routing, SL 4

SL 7 only difference is here SPC = 82 Circuit 14 Exchange

SPC = 22

Circuit 20

SPC = 60

Exchange

Exchange

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

Processing in (transit) exchange(s): Using routing table and incoming routing label, exchange inserts DPC (60) and CIC (20) into outgoing routing label (no number analysis )

Setup of a call using ISUP

User A Exchange A Transit exchange Exchange B User B

Setup

IAM

IAM

Setup Alert

Q.931 Alert

Link-by-link routing (number analysis)

ACM

ACM

Connect

ANM

ANM

Connect

Charging of call starts now

Link-by-link signalling (no number analysis)

Some basic ISUP messages

user A user B

IAM Initial Address Message ACM Address Complete Message ANM Answer Message REL Release Message RLC Release Complete

Signalling sequence 1 (call setup)

User A Off hook Dial tone B number LE A TE LE B User B

Local exchange detects setup request and returns dial tone Local exchange: analyzes B number determines that call should be routed via transit exchange (TE)

Signalling sequence 2 (call setup)

User A LE A TE LE B User B

Initial address message (IAM) ISUP message IAM is sent to transit exchange.

Transit exchange 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. Within all exchanges, the path is cut through (circuit switched path between user A and LE B).

Signalling sequence 3 (call setup)

User A Ringing tone or Ringing signal is sent to user B (user B is alerted). Ringing tone is sent to user A. (Ringing tone is generated locally at LE A or is sent from LE B through circuit switched path) LE A TE LE B User B

Address complete message (ACM)

Ringing signal

Signalling sequence 4 (call setup)

User A Charging starts now LE A TE LE B User B

Answer message (ANM)

User B answers

Conversation over this pipe

User B answers, connection is cut through at LE B. Charging of the call starts when ANM message is received at LE A. Conversation can take place over the bi-directional circuit switched connection.

E.164 numbering scheme

00 358 0 Prefix Country code Area code 9 9 9 1234567 1234567 1234567 358 or mobile network code 40 International number National number User number

In each exchange, the B number is analyzed at call setup and a routing program (algorithm) selects the next exchange to which the call is routed.

Signalling sequence (call release)

User A LE A TE LE B User B

Conversation over this pipe On hook Charging stops Release (REL) Release complete (RLC)

The connection links are released one by one.

(Hanging links are blocked from further use)

SCCP (Signalling Connection Control Part)

Essential for non-circuit-switching related signalling

Features:

OSI Layer 3 functionality

Essential for end-to-end signalling & database access Global Title Translation (GTT) for enhanced routing SubSystem Number (SSN) analysis at destination

4 Transport Service Classes

OSI Layer 4 functionality

SS7 connection setup using SCCP

Signalling connection, not circuit switched connection (= call)

User (AP)

User applications
SCCP GT translation

User (AP)

User (AP)

User (AP)

SCCP

SCCP SSN analysis

MTP

MTP

MTP

SSP

STP

SCP

Global title translation (GTT)

Global title translation (GTT) is required when the originating exchange (SSP) knows a global title but does not know the DPC of the database (SCP). SSP Global title (GT) examples: 0800 number => SCP IMSI => HLR STP Translation in STP GT => DPC + SSN SCP

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 SSPs (exchanges).
SSP SSP SSP SCP STP SSP SCP SCP

SSP

Example: SCCP connection with GTT

No SCCP functionality STP SCCP functionality STP

STP
SCCP MSC/VLR located in Espoo SPC = 82 SPC = 32 SCCP HLR located in Oslo SPC = 99

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

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

Four classes of service in SCCP

Class 0: Basic connectionless class. Each information block (SCCP message) is transmitted from one SCCP user to another SCCP user independently. Class 1: Sequenced (MTP) connectionless class. All messages use the same SLS code. Class 2: Basic connection-oriented class. Virtual connections are set-up and released + using same SLS code + segmentation & reassembly (SAR)

Class 3: Flow-control connection-oriented class. VC control + same SLS codes + SAR + flow control

Signalling in GSM core network

ISUP for signalling between exchanges (MSC, GMSC) MAP for signalling to/from databases (VLR, HLR, AuC, EIR) MM / CM RR BSSMAP / DTAP BSSAP SCCP MTP BSC BSSAP SCCP MAP TCAP MAP TCAP SCCP MTP

ISUP

SCCP MTP HLR

A interface

MSC / VLR

to GMSC

Further information on SS7

Tutorials: Modarressi, Skoog: SS7: a tutorial, IEEE Comm. Magazine, July 1990 Jabbari: CCSS7 for ISDN and IN, Proc. IEEE, Feb. 1991 Books: Bhatnagar: Engineering networks for synchronization, CCS7, and ISDN, IEEE Press, 1997 Van Bosse: Signaling in telecommunication networks, Wiley, 1998 Web material: www.iec.org/online/tutorials/ss7 www.ericsson.com/about/telecom (the course book)