VoIP and SS7 Chapter 7
Basic functional parts of the PSTN PSTN Switching in exchanges Transmission (PDH, SDH) Databases in the network (HLR) Subscriber signalling (analog or ISDN=DSS1) Network-internal signalling (SS7)
PDH and SDH transmission bit rates PDH (Plesiochronous Digital Hierarchy) Japan USA Europe J1 1.5 Mbit/s T1 1.5 Mbit/s E1 2 Mbit/s J2 6 T2 6 E2 8 J3 32 T3 45 E3 34 J4 98 T4 140 E4 140 SONET (North Am.) SDH STS-1 51.84 Mbit/s STS-3 155.52 STM-1 STS-12 622.08 STM-4 STS-48 2.488 Gbit/s STM-16
Structure of E1 frame (2.048 Mbit/s) 0 1 2 16 31 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: received bit stream ... where does a new frame begin? ... ... Time slot 16 usually contains SS7 signalling information.
Subscriber signalling PSTN Switching in exchanges Transmission (PDH, SDH) Databases in the network (HLR) Subscriber signalling (analog or ISDN=DSS1) Network-internal signalling (SS7)
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. 1 2 3 4
Analog subscriber signalling in action User A LE A LE B User B SS7 signalling (ISUP) LE = local exchange Off-hook Dial tone B number Ringing signal Ringback tone (or busy tone) Off-hook (user B answers) Connection established
ISDN subscriber signalling in action User A LE A LE B User B SS7 signalling (ISUP) Off-hook DSS1 signalling messages Setup B number Setup Call proc Ringing Alert Tones generated in terminal Alert Off-hook (user B answers) Conn Conn Connection established
PSTN vs. ISDN user access 300 … 3400 Hz analog transmission band “Poor-performance” subscriber signaling PSTN 2 x 64 kbit/s digital channels (B channels) 16 kbit/s channel for signaling (D channel) => Digital Subscriber Signalling system nr. 1 (DSS1) Basic 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. Primary Rate Access ISDN
End-to-end digital signalling User interface PSTN Network User interface Q.931 Q.931 ISUP ISUP Q.931 Q.931 SS7 DSS1 MTP 3 MTP 3 DSS1 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
Introduction • Channel Associated Signaling • Still widely deployed today • Considered as old telephony • Common Channel Signaling • Separation of signaling and call paths • Signaling System 7 (SS7) • To enable a wide range of services to be provided to the end-user • Caller ID, toll-free calling, call screening, number portability, etc. • SS7 is the foundation for Intelligent Network (IN) services.
Channel-associated signalling (CAS) CAS means in-band signalling over the same physical channels as the circuit-switched user traffic (e.g. voice). Signalling is possible Exchange Exchange Exchange Signalling is not possible before previous circuit-switched link is established Circuit switched connection CAS has two serious draw-backs: Setting up a circuit switched connection is very slow. Signalling to/from databasesis not feasible in practice(setting up a circuit switched connection to the database and then releasing it would be extremely inconvenient).
Common channel signalling (CCS) In practice, CCS = SS7. Signalling is possible anywhere anytime Exchange Exchange Database The packet-switched signalling network is totally separated from the circuit-switched connections. Consequently: Signalling to/from databases is possible anytime. End-to-end signallingis possible before call setup and also during the conversation phase of a call. There is one drawback: It is difficult to check if the circuit-switched connections are really working (= continuity check).
SS7 Signaling ISUP Messages INAP/TCAP Messages The Telephone Network [1/2] Service Control Point Service Data Point + Signal Transfer Point Control Layer Intelligent Peripheral Transport Layer Class 4 Tandem Switch Class 5 End Office Switch Circuit Switched Network
The Telephone Network [2/2] • 5 Basic Components in Intelligent Networks • SSP/Service Switching Point • switching, service invocation • STP/Service Transfer Point • signal routing • SCP/Service Control Point • service logic execution • SDP/Service Data Point • subscriber data storage, access • IP/Intelligent Peripheral • resources such as customized voice announcement, voice recognition, DTMF digit collection SCP SDP TCAP messages IP STP STP SSP SSP ISUP messages Voice
Signalling example Tokyo Oulu Exch User A (calling user) User B (called user) Exch Exch London 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.
SS7 Protocol Suite OSI Layers INAP MAP Application Presentation Session ISUP TCAP SCCP Transport Network MTP Level 3 MTP Level 2 Data Link MTP Level 1 Physical
MTP Levels 1 & 2 • Message Transfer Part • Level 1 • Handling the issues related to the signals on the physical links between one signaling node and another • Closely to layer 1 of the OSI stack • Level 2 • Dealing with the transfer of messages on a given link from one node to another • Providing error detection/correction and sequenced delivery of the SS7 messages • signalling network supervision and maintenance functions
MTP Level 3 • Signaling message handling • Providing message routing between signaling points in the SS7 network • May pass a number of intermediate nodes (STP, Signal Transfer Point) • MTP level 3 ”users” are ISUP and SCCP • Signaling network management • Rerouting traffic to other SS7 signaling links in the case of link failure, congestion or node failure • Load-sharing
Provides a number of services to the protocol layer above it • The transfer of messages • Indicating availability of resources • MTP-Transfer request, MTP-Transfer indication, MTP_Pause indication, MTP-Resume indication, and MTP-Status indication
ISUP • ISDN User Part • Used as the protocol for setting up and tearing down phone calls between switches • Initial Address Message (IAM) • To initiate a call between two switches • Answer Message (ANM) • To indicate that a call has been accepted by the called party • Release Message (REL) • To initiate call disconnection
A connection-oriented protocol • Related to the establishment of connections between users • The path of messages and the path of the bearer might be different
SCCP • Signaling Connection Control Part • Used as the transport layer for TCAP-based services • freephone (800/888), calling card, wireless roaming • Both connection-oriented and connectionless • Mostly connectionless signaling • Global title translation (GTT) capabilities • The destination signaling point and subsystem number is determined from the global title
TCAP, MAP and INAP • TCAP (Transaction Capabilities Applications Part) • Supporting the exchange of non-circuit related information between signaling points • Queries and responses sent between SSPs and SCPs are carried in TCAP messages • Provides services to • INAP (IN Application Part) • MAP (Mobile Application Part)
SS7 Network Architecture • Figure 7-4 depicts a typical SS7 network arrangement. • This configuration serves several purposes. • No direct signaling links • A fully meshed signaling network is not required. • The quad arrangement ensures great robustness.
Signaling Point (SP) • Each node in an SS7 network is an SP. • The signaling address of the SP is known as a signaling point code (SPC). • Linkset • Group of signaling links directly connecting two SPCs • For capability and security reasons • Service Switching Point (SSP)
Signal Transfer Point (STP) • To transfer messages from one SPC to another
Service Control Point (SCP) • A network entity that contains additional logic and that can be used to offer advanced services • The switch sends a message to the SCP asking for instructions. • The SCP, based upon data and service logic that is available, will tell the switch which actions need to be taken. • An good example – toll-free 800 number
An example • A subscriber dials a toll-free 800 number • The SSP knows that it needs to query the SCP • The SCP contains the translation information • The SCP responds to the SSP with a routable number • The SSP routes the call • Connectionless signaling • The application use the services of TCAP, which in turn uses the services of SCCP
Message Signal Units (MSUs) • The messages sent in the SS7 network • Backward Sequence Number • BSN Indicator Bit • Forward Sequence Number • Length Indicator
Message Signal Units (MSUs) • The messages sent in the SS7 network • The format of an MSU • SIO – Service Information Octet • Indicate the upper-level protocol (e.g., SCCP or ISUP) • A sub-service field indicating the signaling numbering plan • SIF – Signaling Information Field • The actual user information • The ANSI version and the ITU-T version • The routing label • The Destination Point Code (DPC) • The Originating Point Code (OPC)
Signaling Link Selection (SLS) • The particular signaling link to be used
SS7 addressing • The ANSI version, 24 bits • Member, cluster, network codes • An operator has a network code • The ITU-T version, 14 bits • International Signaling Gateway • Use sub-service field • National, Nation Spare, International, International Spare • An international gateway has one national point code and one international code
F CK SIF SIO LI Control F Same SPCs can be reused at different network levels International SPC = 277 SPC = 277 National SPC = 277 means different signalling points (network elements) at different network levels. 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) Structure of ISUP message: SIO (one octet) Routing label (four octets) CIC (two octets) Must always be included in ISUP message Message type (one octet) Mandatory fixed part E.g., IAM message Mandatory variable part E.g., contains called (user B) number in IAM message Optional part
The ISDN User Part (ISUP) • ISUP • The most-used SS7 application • The establishment and release of telephone calls • IAM • Called number, calling number, transmission requirement, type of caller, … • ACM • The call is through-connected to the destination • A one-way-audio path is opened for ring-back tone • Optional • If not returned, no ring-back tone at all
CPG, Call Progress • Optional; provide information to the calling switch • ANM, Answer Message • Open the transmission path in both directions • Instigate charging for the call • REL, Release • RLC, Release Complete • CIC, circuit identification Code • Indicates the specific trunk between two switches • OPC, DPC, and CIC
Difference between SLS and CIC The four-bit signalling link selection (SLS) field in the routing labeldefines the signalling link which is used for transfer of the signalling information. The 16-bit circuit identification code (CIC) contained in the ISUP messagedefines the TDM time slot or circuit with which the ISUP message is associated. Signalling link STP Exchange Exchange Circuit
ISUP Call Establishment and Release • A given circuit between two switches is identified by OPC, DPC and CIC.
Signalling using IAM message STP STP SL 4 SL 7 Exchange SPC = 82 Exchange SPC = 22 Exchange SPC = 60 Circuit 20 Circuit 14 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). Outgoing message: OPC = 82 CIC = 14 DPC = 22 SLS = 4
Setup of a call using ISUP User A LE A Transit exchange LE B User B Setup IAM IAM Setup DSS1 signalling assumed Number analysis Alert ACM ACM Alert Connect ANM ANM Connect Charging of call starts now
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 with GTT capability Exchange STP Database 1. 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. 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.
Example: SCCP usage in mobile call 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. SCCP/GTT functionality STP SCCP SPC = 32 SCCP MSC located in Espoo HLR located in Oslo SPC = 99 SPC = 82 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
To sum it up with an example… Part B, Section 3.3 in ”Understanding Telecommunications 2” PSTN Typical operation of a local exchange Transmission (PDH, SDH) Databases in the network (HLR) Subscriber signalling (analog or ISDN=DSS1) Network-internal signalling (SS7)
Basic local exchange (LE) architecture Modern trend: Switching and control functions are separated into different network elements (separation of user and control plane). Switching system Subscriber stage TDM links to other network elements LIC Time switch ETC Group switch Tone Rx ETC LIC Tone generator Sign. Exchange terminal circuit Line interface circuit • Switch control • E.164 number analysis SS7 Signalling equipment • Charging • User databases • O&M functions Control system
Setup of a call (1) Phase 1. User A lifts handset and receives dial tone. Local exchange of user A Switching system 4. Tone Rx is connected 1. Off hook LIC Time switch ETC Group switch Tone Rx ETC LIC 5. Dial tone is sent (indicating “network is alive”) Tone generator Sign. 2. Check user database. For instance, is user A barred for outgoing calls? 3. Reserve memory for user B number Control system
Setup of a call (2) Phase 2. Exchange receives and analyzes user B number. Local exchange of user A Switching system LIC Time switch ETC Group switch Tone Rx ETC LIC 2. Number (DTMF signal) received 1. User A dials user B number Sign. 3. Number analysis 4. IN triggering actions? Should an external database (e.g. SCP, HLR) be contacted? Control system
Setup of a call (3) Phase 3. Outgoing circuit is reserved. ISUP Initial address message (IAM) is sent to next exchange. Local exchange of user A Switching system E.g., CIC = 24 LIC Time switch ETC Group switch Tone Rx ETC LIC IAM (contains information CIC = 24) 1. Tone receiver is disconnected Sign. 2. Outgoing circuit is reserved 3. Outgoing signalling message (ISUP IAM) contains user B number Control system