Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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) Basic functional parts of the PSTN PSTN Switching in exchanges Subscriber signalling (analog or ISDN=DSS1) Transmission (PDH, SDH) Networkinternal signalling (SS7) Databases in the network (HLR) PSTN Circuit-switched technology Circuit-switched network Packet-switched network Based on 64 kbit/s channels (TDM time slots) No fixed channel concept (bit rate is not constant) Time Division Multiplexing (TDM) Statistical multiplexing (greater flexibility) Connection-oriented operation (setup & release connection => call) Connectionless operation (independent routing of packets) as default Charging is based on time duration of connection More flexible charging solutions Optimized for delaysensitive services (speech) QoS solutions required for delay-sensitive services IP network as alternative to PSTN Voice traffic can naturally also be carried over Packetswitched (IP) networks. This topic is covered in a future lecture. Switching in exchanges Subscriber signalling (analog or ISDN=DSS1) PSTN Networkinternal signalling (SS7) IP network Quality-of-Service (QoS) support needed! Transmission (PDH, SDH) Databases in the network (HLR) Transmission: PDH or SDH systems PSTN Switching in exchanges Subscriber signalling (analog or ISDN=DSS1) Transmission (PDH, SDH) Networkinternal signalling (SS7) Databases in the network (HLR) 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. 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) time => 8000 x 8 bit/s PDH and SDH transmission bit rates PDH (Plesiochronous Digital Hierarchy) Japan J1 J2 J3 J4 1.5 Mbit/s 6 32 98 USA T1 T2 T3 T4 1.5 Mbit/s 6 45 140 SONET (North Am.) STS-1 STS-3 STS-12 STS-48 51.84 Mbit/s 155.52 622.08 2.488 Gbit/s Europe E1 E2 E3 E4 2 Mbit/s 8 34 140 SDH STM-1 STM-4 STM-16 Structure of E1 frame (2.048 Mbit/s) 012 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. Structure of STM-1 frame in SDH 9 3 SOH 1 5 SOH 261 bytes STM-1 payload (contains the actual information) AU pointer indicates where the virtual container starts in the payload field STM = Synchronous transport module SOH = Section overhead AU = Administrative unit Higher-order STM-4 signal is generated using synchronous byte interleaving: byte from first STM-1 signal … byte from second STM-1 signal byte from third STM-1 signal … byte from fourth STM-1 signal Bitrate of STM-1 signal 9 3 SOH 261 bytes STM-1 payload 1 5 SOH 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 AU-4 pointer points to first byte of VC SOH Virtual container “floats” within the payload of STM-1 frames P O H VC-4 (Virtual container) 9 POH = Path overhead 1 260 bytes Filling of STM-1 payload in practice In reality, the STM-1 payload is filled like this: P Beginning of virtual container STM-1 frame N P STM-1 frame N+1 Path overhead bytes Beginning of next virtual container SDH pointer adjustment (1) When VC-4 clock rate is larger than STM-1 clock rate => pointer value is shifted forward three bytes SOH Pointer value updated SOH Three “empty” bytes are inserted here old new VC-4 (Virtual container) SDH pointer adjustment (2) When VC-4 clock rate is smaller than STM-1 clock rate => pointer value is shifted back three bytes SOH STM-1 payload Pointer value updated VC-4 (Virtual container) AU-4 pointer Three VC bytes are stored here old new 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) Subscriber signalling PSTN Switching in exchanges Subscriber signalling (analog or ISDN=DSS1) Transmission (PDH, SDH) Networkinternal signalling (SS7) Databases in the network (HLR) Analog subscriber signalling 1 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. 2 The dialled called party (user B) number is sent to the local exchange in form of Dual Tone Multi-Frequency (DTMF) signal bursts. 3 Alerting (ringing) means that the local exchange sends a strong sinusoid to the terminal of user B. 4 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. Analog subscriber signalling in action User A LE A Off-hook Dial tone B number Ringback tone (or busy tone) Connection established LE B SS7 signalling (ISUP) User B LE = local exchange Ringing signal Off-hook (user B answers) ISDN subscriber signalling in action User A LE A Off-hook B number Setup Call proc Tones generated in terminal LE B SS7 signalling (ISUP) User B DSS1 signalling messages Setup Alert Ringing Conn Off-hook (user B answers) Alert Conn Connection established What does ISDN originally mean? 1. End-to-end digital connectivity 2. Enhanced subscriber signaling Idea originated in the 1980’s 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 PSTN vs. ISDN user access PSTN 300 … 3400 Hz analog transmission band Basic Rate Access ISDN 2 x 64 kbit/s digital channels (B channels) Primary Rate Access ISDN 30 x 64 kbit/s digital channels (B channels) “Poor-performance” subscriber signaling 16 kbit/s channel for signaling (D channel) => Digital Subscriber Signalling system nr. 1 (DSS1) 64 kbit/s channel for signaling (D channel) Mainly used for connecting private branch exchanges (PBX) to the PSTN. End-to-end digital signalling User interface Q.931 Q.931 DSS1 PSTN Network ISUP SS7 ISUP MTP 3 MTP 3 User interface Q.931 Q.931 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 Signalling System nr. 7 (SS7) PSTN Switching in exchanges Subscriber signalling (analog or ISDN=DSS1) Transmission (PDH, SDH) Networkinternal signalling (SS7) Databases in the network (HLR) History of inter-exchange signalling CAS Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, the signalling is carried inband along with the user traffic. CCIS 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. 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. 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 Circuit switched connection Signalling is not possible before previous circuitswitched link is established CAS has two serious draw-backs: • Setting up a circuit switched connection is very slow. • 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). 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 signalling is 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). Signalling example Tokyo User A (calling user) Oulu Exch Exch Exch London User B (called 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. Protocol layers (“levels”) of SS7 ISDN User Part (ISUP) MTP user SS7 application protocol for managing circuitswitched connections Application protocols (e.g. Mobile Application Part, MAP) Transaction Capabilities Application Part (TCAP) Signalling Connection Control Part (SCCP) MTP level 3 (routing in the signalling network) MTP MTP level 2 (link-layer protocol) MTP level 1 (64 kbit/s PCM time slot) SS7 protocols vs. OSI model SS7 protocol stack MAP OSI protocol layer model … TCAP Application Presentation ISUP Session Transport SCCP MTP level 3 Network MTP level 2 Data link MTP level 1 Physical OSI protocol layer model Application layer Presentation layer Session layer User application (in this case, the actual signalling messages) Data compression & coding Dialogue control Transport layer End-to-end flow & error control Network layer Switching & routing through the communications network Data link layer Link-layer flow & error control Physical layer Multiplexing & transport of bits, time slots in PDH or SDH systems Message Trasfer Part (MTP) functions MTP level 1 (signalling data link level): Digital transmission channel (64 kbit/s TDM time slot) MTP level 2 (signalling link level): Frame-based protocol for flow control, error control (using Automatic Repeat reQuest, ARQ), and signalling network supervision and maintenance functions. MTP level 3 (signalling network level): 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 2 frame formats Level 3 user information MSU (Message Signal Unit) F CK SIF SIO LSSU (Link Status Signal Unit) F CK SF LI Control FISU (Fill-In Signal Unit) F LSB CK LI Control F MSB F LI Control Network: • National • International User part: • ISUP • SCCP • Signalling network management F MTP level 2 frames MSU (Message Signal Unit): • Contains actual SS7 signalling messages • The 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) 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 at receiving end • The received frame is FISU if LI = 0 Signalling points (SP) in SS7 Network elements (relevant from signalling point of view) contain signalling points identified by unique signalling point codes. Signalling Transfer Points only relay signalling messages STP STP SP Signalling Point (in a database, such as HLR in mobile network) MAP STP SP ISUP Exchange Signalling Point (signalling termination in an exchange) Signalling point code (SPC) SS7 signalling messages contain MTP level 3 routing information in the form of a routing label: MSB International (and most national) signalling networks (ITU-T): LSB SIO octet 14-bit Destination Point Code (DPC) 14-bit Originating Point Code (OPC) 4-bit Signalling Link Selection (SLS) DPC DPC OPC OPC OPC SLS North American national signalling network (ANSI): Signalling message payload 24-bit DPC and OPC, 5-bit SLS code Format for international SPC: Zone 3 bits Area/Network 8 bits SP 3 bits For examples, see: www.numberingplans.com 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. F CK SIF SIO LI Control F 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) Message type (one octet) Mandatory fixed part Mandatory variable part Optional part Must always be included in ISUP message E.g., IAM message E.g., contains called (user B) number in IAM message ISUP signalling messages Basic ISUP signalling messages: Call setup: IAM (Initial address message) ACM (Address complete message) From LE A to LE B From LE B to LE A ANM (Answer message) 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. Signalling link STP Exchange Exchange Circuit Signalling using IAM message STP STP SL 4 SL 7 SPC = 82 SPC = 22 Circuit 14 Exchange Outgoing message: OPC = 82 CIC = 14 DPC = 22 SLS = 4 Exchange Circuit 20 SPC = 60 Exchange 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 User A Setup DSS1 signalling assumed Alert Connect LE A Transit exchange IAM LE B IAM User B Setup Number analysis ACM ANM Charging of call starts now ACM ANM Alert Connect Call setup: Signalling sequence 1 User A Off hook LE A TE LE B Dial tone Local exchange detects setup request and returns dial tone B number Local exchange: • analyzes B number • determines that call should be routed via transit exchange (TE) User B Call setup: Signalling sequence 2 User A LE A TE LE B User B 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. Call setup: Signalling sequence 3 User A LE A Ringback tone TE Address complete message (ACM) LE B User B Ringing signal or 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.) Call setup: Signalling sequence 4 User A Charging starts now LE A TE LE B Answer message (ANM) User B User B answers Conversation over this “pipe” 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. E.164 numbering scheme 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. 00 358 9 1234567 International number 0 9 1234567 National number 1234567 User number Prefix Country code 358 Area code or mobile network code, e.g. 40 9 E.164 number structure Max. 15 digits 00 358 Prefix Country code (1-3 digits) 9 1234567 Subscriber number National destination code (1-3 digits) Area code, e.g. 9 Mobile network code, e.g. 40 For examples, see: www.numberingplans.com MSISDN number Signalling sequence for call release User A LE A TE LE B User B Conversation over this “pipe” On hook Charging stops 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.) 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. 2. SCCP performs global title translation in the STP (0800 or IMSI number => DPC) and the SCCP message can now be routed to the database. 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 Exchange Database Database STP Exchange Exchange Exchange Exchange 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 MSC located in Espoo SPC = 82 Outgoing message: OPC = 82 DPC = 32 SCCP: IMSI global title SPC = 32 SCCP HLR located in Oslo SPC = 99 Processing in STP: Received message is given to SCCP for GTT. SCCP finds the DPC of the HLR: DPC = 99 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) To sum it up with an example… Part B, Section 3.3 in ”Understanding Telecommunications 2” PSTN Typical operation of a local exchange Subscriber signalling (analog or ISDN=DSS1) Transmission (PDH, SDH) Networkinternal signalling (SS7) Databases in the network (HLR) Basic local exchange (LE) architecture Modern trend: Switching and control functions are separated into different network elements (separation of user and control plane). Subscriber stage LIC Time switch LIC Tone Rx Tone generator Line interface circuit Switching system Group switch TDM links to other network elements ETC ETC Sign. Exchange terminal circuit • Switch control • E.164 number analysis • Charging • User databases • O&M functions SS7 Signalling equipment Control system Setup of a call (1) Phase 1. User A lifts handset and receives dial tone. Local exchange of user A 4. Tone Rx is connected 1. Off hook LIC LIC 5. Dial tone is sent (indicating “network is alive”) Time switch Tone Rx Tone generator Switching system ETC Group switch ETC 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 LIC 1. User A dials user B number Time switch Tone Rx 2. Number (DTMF signal) received Group switch ETC ETC 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 LIC Time switch LIC Tone Rx Group switch 1. Tone receiver is disconnected ETC E.g., CIC = 24 ETC Sign. 2. Outgoing circuit is reserved 3. Outgoing signalling message (ISUP IAM) contains user B number Control system IAM (contains information CIC = 24) Setup of a call (4) Phase 4. ACM received => ringback or busy tone generated. ANM received => charging starts. Local exchange of user A Switching system LIC Time switch LIC 2. Ringback or busy tone is locally generated 4. Call continues… Tone generator Group switch ETC ETC Sign. 1. ISUP ACM message indicates free or busy user B 3. Charging starts when ISUP ANM message is received Control system ACM, ANM