1. Integrated Services Digital Network
ISDN
Integrated Services Digital Network

2. interaction is possible
What is ISDN ?
1. End-to-end digital connectivity
2. Enhanced subscriber signaling
3. A wide variety of new services (due to 1 and 2)
4. Standardized access interfaces and terminals
Original idea in the 1980’s
ISDN is not a “new” network separated from the PSTN. Interworking with “normal” PSTN equipment is very important.
interaction is possible
ISDN terminal
PSTN terminal

3. Evolution of the PSTN / ISDN
Step 1: all-analogue network (before 1960)

4. Evolution of the PSTN / ISDN
Step 2: digital transmission in the core network ( )
PDH transmission systems ( Mbit/s)

5. Evolution of the PSTN / ISDN
Step 3: digital switching of 64 kbit/s channels ( )
Time switching technology

6. Evolution of the PSTN / ISDN
Step 4: common channel signalling in the core network ( )
SS7

7. Evolution of the PSTN / ISDN
Step 5: PDH systems are being replaced by SDH ( )
SDH transmission systems (155, 620 Mb/s)

8. Evolution of the PSTN / ISDN
Step 6: digital access lines (ISDN, ADSL) installed ( )
End-to-end digital user data
End-to-end digital signalling

9. 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?

10. Digital access: several alternatives
ISDN
modem
ADSL
Bit rate (kb/s) x 64 max much larger
Connection fast slow fast
setup time
Popularity little great increasing
However, large impact on signalling protocols

11. PSTN vs. ISDN user access
300 … 3400 Hz analogue transmission band
“poor-performance” subscriber signaling
PSTN
Basic Rate Access ISDN
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
Primary Rate Access ISDN

12. 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.

13. 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

14. Basic rate access – user interface
Exchange
Terminal Adaptor
S/T
Network Termination U
Line
Interface Circuit
Bi-directional 192 kbit/s
160 kbit/s echo canceling or
time compression R
Non-ISDN terminal
ISDN terminal
Exchange
Subscriber (premises) network

15. Primary rate access – user interface
PBX U
Line
Termination
PBX
equipment
manufacturer
specific
solutions
Standard 2 Mb/s TDM connection (PDH or SDH)
Exchange
64 kb/s D channel in one PCM time slot

16. ISDN Signalling Protocols

17. OSI reference model 7. Application 6. Presentation 5. Session
A (protocol residing in a) lower layer provides certain services to a (protocol in a) higher layer
6. Presentation
5. Session
Protocol Data Units (PDU)
4. Transport
3. Network
Headers
Rx end
Tx end
2. Data Link
1. Physical

18. Relation between network connection elements and OSI model
7. Application
Some examples:
6. Presentation
Gateway (GW), Interworking function (IWF)
5. Session
4. Transport
3. Network
Router, Switching function
2. Data Link
Bridge, Relaying function
1. Physical
SDH cross-connect

19. Tasks of OSI layers: 7. Application 6. Presentation 5. Session
User application
6. Presentation
Compression & coding
5. Session
Dialogue control
4. Transport
End-to-end flow & error control
3. Network
Switching & routing
2. Data Link
Link-layer flow & error control
1. Physical
Multiplexing & transport of bits

20. Typical protocol interaction
App
App : :
Switch, router
Relay, bridge
Tran
Tran
Net
Net
Net
Link
Link
Link
Link
Link
Phy
Phy
Phy
Phy
Phy
Phy
End
node
End
node
Intermediate nodes

21. Signalling protocols for end-to-end connection
User interface
PSTN Network
User interface
SS7
Q.931
Q.931
ISUP
ISUP
Q.931
Q.931
DSS1
DSS1
MTP 3
MTP 3
Q.921
Q.921
MTP 2
MTP 2
Q.921
Q.921
I.430
I.430
MTP 1
MTP 1
I.430
I.430
contains the signalling messages for call control

22. Layered DSS1 signaling structure
DSS1 = Digital Subscriber Signalling system no.1
Layer 1:
Bit sequence structure, framing & multiplexing
Layer 2:
Link control (HDLC-type protocol called LAPD)
Layer 3:
Signaling messages (application layer)
I.430
Q.921
Q.931

23. 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

24. Q.931 Call-related messages
Call establishment messages: ALERTING
CALL PROCEEDING
CONNECT
CONNECT ACKNOWLEDGE
PROGRESS
SETUP
SETUP ACKNOWLEDGE
Call clearing messages:
DISCONNECT
RELEASE
RELEASE COMPLETE
Similar functions as ISUP in SS7

25. Typical content of ISDN Set-up message
Called party (user B) number & numbering plan
Calling party (user A) number (+ CLIP/CLIR)
Bearer capability (64 kbit/s unrestricted, speech, 3.1 kHz audio, packet mode B-channel, packet mode D-channel)
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
Show to B?

26. Structure of Q.931 message (Release)
Message type: RELEASE Significance: Local Direction: Both
Info Element Direction Type Length
Protocol Both M 1
discriminator
Call reference Both M 2-
Message type Both M 1
Cause Both O
Display n  u O
Signal n  u O
Cause description may require many bytes

27. Setup of a PSTN call off-hook SS7 ISUP dial tone B number ringing tone
User A
Exchange A
Exchange B
User B
off-hook
SS7 ISUP
dial tone
B number
ringing tone
ringing tone
user B answers
connection ok

28. Setup of an ISDN call using Q.931
User A
Exchange A
Exchange B
User B
Setup
off-hook
Setup
Call proceed
SS7 ISUP
Alert
“ring”
Alert
Connect
user B answers
Connect
connection ok

29. Intelligent Network concept
SS7
Common Channel
Signalling System Nr. 7 IN
Intelligent Network concept

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

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

32. 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.

33. CAS vs. CCS Tokyo Oulu Exch User A (calling user) Exch Exch
User B (called user)
Exch
Database
1) Accessing database
Espoo
2) Signalling before call setup
3) Signalling during conversation phase
(user-to-user => digital access technology required)

34. Signalling points (SP) in SS7
Every SP is identified by a unique signalling point code
Signalling Transfer Point (only related to SS7 network)
STP
STP
Signalling Point (in a database, such as HLR in GSM) SP
MAP INAP CAP
Application protocols used in SS7
STP
Exchange SP
Signalling Point (signalling termination in an exchange)
ISUP

35. Intelligent Network (IN) Concept
Intelligence => access to various databases
Operator implements service logic (IN Service)
STP
SCP
Service Control Point
(a network element containing the service logic, is often also called database or register)
MAP INAP CAP
Exchange
SSP
Service Switching Point
(enables service triggering in an exchange)
ISUP

36. Typical call-related IN procedure (1)3.
SCP 2. 4.
SSP 1. 5.
Exchange
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)

37. Typical call-related IN procedure (2)3.
SCP 2. 4.
SSP 1. 5.
Exchange
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)

38. Typical call-related IN procedure (3)3.
SCP 2. 4.
SSP 1. 5.
Exchange
Exchange
4. SCP provides information
Example: Number translation in SCP
SSP sends 800 number ( )
SCP translates into ”real” number which
can be used for routing the call
( )
translation may be based on several variables

39. 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)

40. Protocol layers (”levels”) of SS7
Application protocols
TUP
ISUP
MAP
CAP
INAP
TCAP
SCCP
routing
MTP level 3
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

41. 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 ...)

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

43. MTP level 2 frame formats
Level 3 signalling message
MSU (Message Signal Unit) F CK
SIF
SIO LI
Control F
Network:
National
International
User part:
TUP
ISUP
SCCP
Network
management
LSSU (Link Status Signal Unit) F CK SF LI
Control F
FISU (Fill-In Signal Unit) F CK LI
Control F

44. 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

45. 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
SLC
OPC
DPC
MTP SCCP message:
SLS – 4 bit signalling link selection
SLS
OPC
DPC
MTP TUP message:
CIC – 12 bit circuit ID code
CIC
OPC
DPC

46. Structure of SS7 ISUP message
Level 3 signalling message in SIF (Signalling Information Field)
Routing label
MTP ISUP message:
SLS – 4 bit
CIC – 12 bit
Max octets
CIC
SLS
OPC
DPC
ITU-T structure
ANSI => different
OpP
MaVP
MaFP
MTC
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)

47. 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
Exchange
SSP
SSP
Exchange
circuit

48. Role of DPC and OPC in SS7
DPC – Destination Point Code (14 bit  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

49. Same signalling point codes can be reused at different network levels
International
SPC = 277
National
SPC = 277
Network specific
SPC = 277
SPC = 277 means different SPs at different network levels

50. Functions at signalling network level
MTP
user
ISUP SCCP
Signalling link
MTP level 2
Message
distribution
Message
discrimination
Message
routing
Signalling message handling
Signalling network management

51. 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)
General (non-user-related) circuit management
see Bhatnagar, p.77

52. Example: link-by-link routing
Using MTP-level routing table, STP routes message to DPC = 22
STP
STP
Outgoing MTP MSU:
OPC = 22 CIC = 20
DPC = 60 SLS = 2
SL 2
SPC = 15
SPC = 18
SL 4
SL 7
Exchange
SPC = 82
Exchange
SPC = 22
Exchange
SPC = 60
Circuit 20
Circuit 14
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)

53. 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.
exchange ID
Country code National region Subscriber number

54. Example: link-by-link signalling
Using MTP-level routing table, STP routes message to DPC = 22
Otherwise like link-by-link routing,
only difference is here
STP
STP
Outgoing MTP MSU:
OPC = 22 CIC = 20
DPC = 60 SLS = 2
SL 2
SPC = 15
SPC = 18
SL 4
SL 7
Exchange
SPC = 82
Exchange
SPC = 22
Exchange
SPC = 60
Circuit 20
Circuit 14
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 … )

55. Setup of a call using ISUP
User A
Exchange A
Transit exchange
Exchange B
User B
Setup
IAM
IAM
Setup
Q.931
Link-by-link routing (number analysis)
Alert
ACM
ACM
Alert
Connect
ANM
ANM
Connect
Link-by-link signalling (no number analysis)
Charging of call starts now

56. 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

57. Signalling sequence 1 (call setup)
User A
LE A TE
LE B
User B
Off hook
Local exchange detects setup request and returns dial tone
Dial tone
B number
Local exchange:
analyzes B number
determines that call should be routed via transit exchange (TE)

58. 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).

59. Signalling sequence 3 (call setup)
User A
LE A TE
LE B
User B
Address complete
message (ACM)
Ringing signal
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)

60. Signalling sequence 4 (call setup)
Conversation over this “pipe”
User A
LE A TE
LE B
User B
Answer message (ANM)
User B answers
Charging starts now
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.

61. E.164 numbering scheme 00 358 9 1234567 International number 9 12345679
National number
User number
Prefix
Country code
358
Area code 9
or mobile network code 40
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.

62. Signalling sequence (call release)
Conversation over this “pipe”
Signalling sequence (call release)
User A
LE A TE
LE B
User B
On hook
Release (REL)
Charging stops
Release complete (RLC)
The connection links are released one by one.
(“Hanging links” are blocked from further use)

63. SCCP (Signalling Connection Control Part)
Essential for non-circuit-switching related signalling
Features:
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 3 functionality
OSI Layer 4 functionality

64. SS7 connection setup using SCCP
Signalling connection, not circuit switched connection (= call)
User
(AP)
User applications
User
(AP)
User
(AP)
User
(AP)
SCCP
SCCP
GT translation
SCCP
SSN analysis
MTP
MTP
MTP
SSP STP SCP

65. 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
STP
SCP
Global title (GT) examples:
Translation in STP
0800 number => SCP
IMSI => HLR
GT => DPC + SSN

66. 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
SCP
SCP
SSP
STP
SSP
SCP
SSP

67. Example: SCCP connection with GTT
No SCCP functionality
STP
STP
SCCP functionality
STP
SPC = 32
SCCP
SCCP
MSC/VLR located in Espoo
HLR located in Oslo
SPC = 82
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

68. 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

69. 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
MAP
TCAP
MAP
SCCP
ISUP
BSSAP
BSSAP
TCAP
MTP
SCCP
SCCP
SCCP
HLR
MTP
MTP
A interface
BSC
MSC / VLR
to GMSC

70. 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:
(the course book)