CS294-3: Distributed Service Architectures in Converged Networks

1. CS294-3: Distributed Service Architectures in Converged Networks Randy H. Katz Computer Science Division Electrical Engineering and Computer Science Department University of California Berkeley, CA 94720-1776

2. Outline • Historical Background • Structure of the PSTN • Intelligent Network/Services • SS7 Structure • Common Themes and Observations

3. The Telegraph Learns to Speak Alexander Graham Bell • 1876: Demonstrates the telephone at US Centenary Exhibition in Philadelphia • Bell offers to sell patents to Western Union for $100,000--they refuse. Bell Telephone Company founded 9 July 1877 • 1878: Western Union enters into competition with rival system designed by Thomas Edison and Elisha Gray. Bell sues and wins.

4. Bell’s Early Telephones

5. Building the Network Almon Brown Strowger (1839 - 1902) • 1889: Invents the “girl-less, cuss-less” telephone system, also known as the mechanical switching system

6. “Ma Bell” • Bell’s patents expire in 1890s; over 6000 independent operators spring up • 1910: Bell System controls 50% of local telephone market • 1913: AT&T & U. S. government reach Kingsbury Agreement: AT&T becomes regulated monopoly while promising "universal" telephone service; Controls “toll” services in U.S. • Long distance interconnection withheld as a competitive weapon • 1950: Bell System controls 84% of the local telephone access market

7. Bell Telephone Equipment

8. Public Switched Telephone Network (PSTN) • Time Division Multiplexed Network • Single application: uncompressed voice (i.e., POTS—Plain Old Telephone Service) • 99.9994% uptime • Low latency and jitter • Highly scalable call routing through SS7 infrastructure • Value-added voice services: e.g., voice messaging, caller ID

9. Structure of the PSTN • Separation of data (switching) and control (signaling) planes • Hierarchical switches • Local Loop/Central Office/End Office • Intermediate/Regional Switches • Trunks (Long Distance) • Control signaling evolution • “Old” PSTN (1960s): analog signaling, encoded in voice band • Dual-tone multifrequency (DTMF) between CPE and EO;I.e., touch tone dialing • Single frequency (SF) between COs • “New” PSTN (1980s): digital signaling/separate control network (SS7) • SF still exists though made obsolete by SS7

10. Old vs. New Signaling Call “I’m busy!” Voice Trunk Call Set-up “I’m busy!” message Control Voice Trunk Free

11. Kinds of Signaling Messages • “I’m forwarding to you a call placed from 212-555-1234 to 718-555-5678. Look for it on trunk 067.” • “Someone just dialed 800-555-1212. Where do I route the call?” • “The called subscriber for the call on trunk 11 is busy. Release the call and play a busy tone.” • The route to XXX is congested. Please don’t send any messages to XXX unless they are of priority 2 or higher. • I’m taking trunk 143 out of service for maintenance.

12. Advantages of Out-of-Band Signaling for PSTN • It allows transport of more data at higher speeds, due to digital encoding rather than audio/analog encoding • Signaling takes place any time during call, not only at beginning • Allows signaling to network elements to which there are no direct trunk connections

13. PSTN Hierarchy • Before Deregulation • Class 5: End Office/Local Office • Interconnection among EOs only within a geographic service area • Class 4: Tandem Office • Also known as a toll office • Higher bandwidth trunking among service areas • Class 3: Primary Center • Part of toll (long distance) network • Class 2: Sectional Center • Class 1: Regional Center • Not a strict hierarchy in that Class 5 switch can be connected to switch other than a Class 5 or 4 switch

14. CO CO CO PSTN Hierarchy Local Area: 510 LATA Analog “local loop” Twisted Pair 8778 CO Digital 642 CO CO Trunk IXC LEC 64 Kbps links, time division multiplex/demultiplex Voice band: 3.4 KHz between 350 Hz and 3750 Hz 8000 samples/second x 8 bits (mLaw)

15. PSTN Hierarchy • Post AT&T Breakup (1980s) IXC: Interexchange Carrier LATA: Local Access Transport Area Intra-LATA connections Inter-LATA connections POP AT Allocate a facility (circuit) per switch per call Tandem Office Class 4 Switch TO TO End Office Central Office Class 5 Switch EO EO EO EO EO EO Local Loop CPE: Customer Premises Equipment ILEC: incumbent CLEC: competitive “Reciprocal Termination Revenue”

16. PSTN Hierarchy • Deregulation Complexities • RBOCs traditional service areas divided into LATAs (originally 146, now 300+) • Local operating company must use long distance provider to interconnect LATAs • Long distance carriers gain access to local networks through Point-of-Presence (POP) center • Every LATA must have a POP • LATA further divided into local market and toll market • Intra-LATA toll calls also open for competition

17. PSTN Hierarchy • Synchronous Network • Maintaining clock synchronization PRS Primary Rate Source Stratum 1 Stratum 2 Stratum 3 Stratum 4 PRS PRS Toll Office End Office PBX Channel Banks Sync Region Sync Region Sync Region Sync Region

18. Digital Signal Destination DS0 DS1 DS1C DS2 DS3 DS3 Bandwidth 64 kbps 1.544 mbps 3.152 mbps 6.312 mbps 44.736 mbps 274.176 mbps Channels (DS0s) 1 channel 24 channels 48 channels 96 channels 672 channels 4032 channels Carrier Designation None T-1 T-1c T-2 T-3 T-4 PSTN Hierarchy • Digital Signaling Hierarchy • SS-7 uses DS0s for signaling links • Channel banks: divide DS1 into 24 DS0s • Copper facilities replaced by fiber: • SONET, speeds to 9.6 gbps and beyond • OC-1: 51.84 mbps, OC-3: 155.52 mbps, OC-12: 622.08 mbps • OC-48: 2488.32 mbps, OC-96: 4976.64 mbps, OC-192: 9953.28 mbps

19. Intelligent Network • Motivation • More rapid evolution of the network • Easier to configure • Tailored “services” for subscribers • Services on demand: voice + communication + audio + video • But the most common services are “redirection”: e.g., 800 numbers, cellular network support, call forwarding, etc., but also services like call waiting, caller id, voice mail, etc. • Implemented by GUI to switching networking: Service Creation Environment (SCE) • SS-7 is IN’s backbone • Network that links together the Service Switching Points • Old PSTN: command signaling embedded in audio channel • New PSTN: interoffice signaling between switches via orthogonal network

20. Intelligent Network • Basic Call Sequencing • Features/services invoked on state transitions Disconnect Quiescent Put down phone Pick up phone Talking Off hook Send dial tone Dial Collect dial string Remote user Picks up phone Ringing Dialing Setup Setup complete Waiting for Setup Complete Busy detection Three way calling

21. Intelligent Network • Call Sequencing • Call set-up and tear-down handled by conventional SS-7 protocols • Dialed digits determine how to connect call • Request for call handling instructions sent to SCP • DB provides handling instructions • E.g., to map 800 number to local number under rules • Routing tables determine which trunks to use to connect call • SS7 message sent to adjacent exchange requesting circuit connection on specific trunk • Target exchange grants permission to connect to trunk by sending ACK to originating exchange • Uses Transaction Capabilities Applications Part (TCAP) Protocol

22. Kinds of Signals • Message Orientation • Bell ringing • Off hook • Voice transport/full duplex set-up • Bill recording • On-hook/call termination

23. Example Services • Find Me Service • Follow Me Service • Computer Security Service • Call Pickup Service • Store Locator Service • Call Routing Service • Multilocation Extension Dialing • Name Delivery • Outgoing Call Restriction

24. Example Services • Find Me Service • Call forwarding to another location • Call screening capabilities • Follow Me Service • Call forwarding based on time schedule • Computer Security Service • In-coming call blocking (e.g., to modem line) • Based on access code or originating phone number • Essentially Caller ID checking against SCP DB • Call Pickup Service • Unanswered call generates page on pager • Dial special code from any phone to connect to caller

25. Example Services • Store Locator Service • Advertise one number, map to local number based on caller phone number • Caller number prefix matched against SCP DB of store locations • Call Routing Service • Reroute calls during congestion times or outside of business hours • Routing instructions in SCP DB • Multilocation Extension Dialing • Abbreviated extension numbers to reach personnel regardless of location and without need for a PBX

26. Example Services • Name Delivery • Display caller name based on in-coming caller number • Based on line subscriber database • More sophisticated version of Caller ID (Automatic Number Identification-ANI) • Outgoing Call Restriction • Block calls to specific numbers and area codes, including 900/976 services

27. SSP: Service Switching Point Computers where service logic resides STP: Signal Transfer Point Packet routers carrying the message-based signaling protocol SCP: Service Control Point Service databases SMS: Service Management System User interface to service DBs & descriptions SSP SSP SSP SSP SCP SCP STP STP SMS SMS STP STP SSP SSP SSP SSP Intelligent Network

28. SS-7 Signaling Points • Service Switching Points (SSP) • Computers • Signal Transfer Points (STP) • Packet Routers • Usually deployed in mated pairs and interconnected • Service Control Points (SCP) • Databases, usually deployed in pairs (not direct connected) • Redundancy and Diversity • Signaling points deployed in pairs • 56 kbps facilities (links) between signaling points deployed in pairs

29. Service Switching Points • Communicates signals to and from the switches of the voice network • Command primitives and packets/signal units • Translates voice switch signals into SS7 signal units • Database access • Queries through SS7 network to centrally located computers providing the function of SCPs • E.g., 800 number lookup • Majority of traffic is circuit-related messages for call set-up and tear-down

30. Signal Transfer Points (STP) • Provide the routing paths for SS7 signaling messages • Usually embedded in voice switches as integrated functionality or as an attached computer; rarely found as a standalone box • Three kinds of STPs • National Signal Transfer Point • Router for a national network (ANSI);Does not support international standards (ITU-TS) • International Signal Transfer Point • Part of international network; Interconnects different national networks • Gateway Signal Transfer Point • Interworks between national and international protocols, between two different national protocols, or between PSTN protocols and the cellular network • Implements screening mechanisms/packet filtering

31. Signal Transfer Points (STP) • Two basic operations provided by SSP • Call Connections • Use dial string to route connection message to terminating end office • Database Queries • Message is routed to an SCP • SSP provides lookup request to STP keyed on dialed digits (aka global title digits) • Global Title Translation: STP uses internal translation tables to map this to database address • Address consists of a Subsystem Number and a Point Code of the interface to the DB

32. Signal Transfer Points (STP) • Network Measurements • Counts on types of messages entering and leaving the network • Network event logging (e.g., link outage duration, local processor outage duration) • “Peg Counts” • Count of originating and terminating messages • Aggregated and sent to Regional Accounting Office (RAO) • Usage invoice generated and sent to customers • Owners: Regional Bell Operating Companies through Bellcore (is this still true?) • Customers: Long Distance Telcos and Independent Telcos

33. Service Control Points (SCP) • Interface to Telco databases • Subscriber service definition • Routing for special service numbers (800, 900) • Credit card validation/fraud protection • Subscriber service creation • SCP is really a computer intermediary between SS7 and the database • Identified by subsystem number, basis for SSP routing • Standard databases: • Call Management Services • Line Information • Business Services • Home Location Register • Visitor Location Register

34. SCP Databases • Call Management Services DB (CMSDB) • Call processing • Routing instructions for special service numbers (800, 877, 976, 900, …) • Billing information: billing address, 3rd party billing • Network management • Instructions for rerouting around congestion • Call sampling/traffic studies • Report generation for kinds of calls being made • Used in provisioning studies

35. SCP Databases • Line Information DB (LIDB) • Subscriber information: calling card service, 3rd party billing instructions, originating line number screening • Calling card validation, e.g., PIN storage • Originating line number screening: call forwarding, speed dialing, etc. • Subscriber features are network-specific: not all features implemented by all operators, or implemented in the exact same fashion • Business Services Database (BSDB) • Call processing instructions • Network management procedures • Specific aspects of private network • Time of day internal rerouting of numbers

36. SCP Databases • Home Location Register (HLR) • Billing information, allowed services • Current location of the cell phone • Uniquely identified by Mobile ID number (MIN) • Used just like POTS number • Visitor Location Register (VLR) • Track roaming subscribers • Verify validity of MIN via HLR • Operations Support Systems (OSS) • Remote maintenance centers monitor networks • Allows remote reconfiguration/repair • SMS: standard interfaces for commands for DB administration and monitoring/measurement

37. Routing in SS7 • Link sets • All links with same adjacent STP are called a link set • Switching equipment does round robin scheduling to insure equal usage • Routes • Sequence of links to reach a destination switching point • Set of routes called a route set • Members of route set allow alternative routing • Router tables indicate next hop link set to follow to reach destination

38. STP STP STP STP STP STP Routing Redundancy Alternative “Extended” Connect Link Combined Link SSP SSP SSP SSP SSP Same destination Alternative signaling points Alternative Direct Connect Link

39. STP STP STP STP Link Types • Access Links (A-links) • Direct link between SSP--STP or SCP—STP • At least two A links SSP SSP SSP SSP

40. STP STP STP STP Link Types • Bridge Links (B-links) • Connect mated STP pairs to other mated STP pairs SSP SSP SSP SSP

41. STP STP STP STP Link Types • Cross Links (C-links) • STP to mated STP • Used primarily to deal with congestion situations, enhance reliability SSP SSP SSP SSP

42. STP STP STP STP STP STP Link Types • Diagonal Links (D-links) • Connect mated STP pairs at different hierarchical levels SSP SSP SSP SSP

43. STP STP STP STP STP STP Link Types • Extended Links (E-links) • Connect SSP to remote STP pairs to achieve diversity in routes • Backup pathsfor greaterreliability SSP SSP SSP SSP

44. STP STP STP STP STP STP Link Types • Fully Associative Links (F-links) • Direct interconnection among SSP in the presence of large amounts of traffics SSP SSP SSP SSP

45. BTS BSC BTS BSC BTS BSC BTS BSC STP Cellular Network and SS-7 VLR HLR EIR BSS AC MSC SS7 PSTN MSC BTS: Base Transceiver StationBSC: Base Station ControllerBSS: Base Station SubsystemMSC: Mobile Switching Center HLR: Home Location RegisterVLR: Visitor Location RegisterEIR: Equipment RegisterAC: Authentication Center

46. Basic Call Setup X W • Switch A analyzes the dialed digits and determines that it needs to send call to switch B • A selects idle trunk, formulates initial address message (UAM) addressed to B; identifies initiating switch, destination switch, trunk selected, calling and called numbers, misc other information • A picks A-link AW to xmit message to B Signaling links A B Subscriber line Subscriber line Voice Trunk

47. Basic Call Setup X W • W receives msg, determines dest, sends it to B along BW link • B receives msg, B determines it serves called number which is idle • B forms address complete message (ACM), containing recipient switch (A), sending switch (B), selected trunk • B sends ACM msg to X, sends ring tone to A over trunk, rings called subscriber’s phone Signaling links A B Subscriber line Subscriber line Voice Trunk

48. Basic Call Setup X W • X receives msg from B, forwards it to A • A connects calling line to indicated trunk, caller hears ringing tone • Called subscriber picks up phone, B formulates answer message (ANM), indicating intended recipient switch (A), sending switch (B), selected trunk Signaling links A B Subscriber line Subscriber line Voice Trunk

49. Basic Call Setup X W • B same link as ACM to send ANM message (BX). Trunk now connects calling/called lines • X forwards ANM to A via AX • A insures calling subscribed is connected to correct trunk in both directions • If calling subscriber hangs up first, A generates Release (REL) msg, addressed to B, via AW Signaling links A B Subscriber line Subscriber line Voice Trunk

50. Basic Call Setup X W • W forwards REL to B via WB • B receives REL, disconnects trunk from called subscriber, return trunk idle, generate release complete (RLC) msg, send it back via BX • X receives RLC, forwards to A via AX • A receives the RLC, idles the indicated trunk Signaling links A B Subscriber line Subscriber line Voice Trunk