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Integrated Services Digital Network

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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

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 1234567
9 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)

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