CS 294 -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 1
Outline • • • Historical Background Structure of the PSTN Intelligent Network/Services SS 7 Structure Common Themes and Observations 2
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. 3
Bell’s Early Telephones 4
Building the Network Almon Brown Strowger (1839 - 1902) – 1889: Invents the “girl-less, cuss-less” telephone system, also known as the mechanical switching system 5
“Ma Bell” • Bell’s patents expire in 1890 s; 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 6
Bell Telephone Equipment 7
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 SS 7 infrastructure • Value-added voice services: e. g. , voice messaging, caller ID 8
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 (1960 s): 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 (1980 s): digital signaling/separate control network (SS 7) » SF still exists though made obsolete by SS 7 9
Old vs. New Signaling Call “I’m busy!” Voice Trunk Call Set-up “I’m busy!” message Control Voice Trunk Free 10
Kinds of Signaling Messages • “I’m forwarding to you a call placed from 212555 -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. 11
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 12
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 13
PSTN Hierarchy Local Area: 510 LATA 8778 Analog “local loop” Twisted Pair Digital CO CO 642 CO CO LEC Trunk IXC 64 Kbps links, time division multiplex/demultiplex Voice band: 3. 4 KHz between 350 Hz and 3750 Hz 8000 samples/second x 8 bits (m. Law) 14
PSTN Hierarchy • Post AT&T Breakup (1980 s) IXC: Interexchange Carrier LATA: Local Access Transport Area POP Tandem Office Class 4 Switch Intra-LATA connections Inter-LATA connections TO EO EO Allocate a facility (circuit) per switch per call AT TO EO Local Loop CPE: Customer Premises Equipment EO EO EO End Office Central Office Class 5 Switch ILEC: incumbent CLEC: competitive “Reciprocal Termination Revenue” 15
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 16
PSTN Hierarchy • Synchronous Network – Maintaining clock synchronization PRS Stratum 1 Primary Rate Source PRS Stratum 2 Toll Office Stratum 3 End Office Stratum 4 PBX Channel Banks Sync Region 17
PSTN Hierarchy Digital Signal Destination DS 0 DS 1 C DS 2 DS 3 Bandwidth 64 kbps 1. 544 mbps 3. 152 mbps 6. 312 mbps 44. 736 mbps 274. 176 mbps • Digital Signaling Hierarchy Channels (DS 0 s) 1 channel 24 channels 48 channels 96 channels 672 channels 4032 channels Carrier Designation None T-1 c T-2 T-3 T-4 – SS-7 uses DS 0 s for signaling links – Channel banks: divide DS 1 into 24 DS 0 s – 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 18
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 19
Intelligent Network • Basic Call Sequencing – Features/services invoked on state transitions Disconnect Put down phone Quiescent Talking Pick up phone Off hook Remote user Picks up phone Ringing Dialing Setup complete Setup Waiting for Setup Complete Send dial tone Dial Collect dial string Busy detection Three way calling 20
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 » SS 7 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 21
Kinds of Signals • Message Orientation – – – Bell ringing Off hook Voice transport/full duplex set-up Bill recording On-hook/call termination 22
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 23
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 24
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 25
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 26
Intelligent Network SSP SSP • SSP: Service Switching Point SSP – Computers where service logic resides SCP STP • STP: Signal Transfer Point – Packet routers carrying the message-based signaling protocol • SCP: Service Control SMS Point SMS STP SSP – Service databases STP SSP • SMS: Service Management System SSP – User interface to service DBs & descriptions 27
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 28
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 SS 7 signal units • Database access – Queries through SS 7 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 29
Signal Transfer Points (STP) • Provide the routing paths for SS 7 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 30
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 31
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 32
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 SS 7 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 33
SCP Databases • Call Management Services DB (CMSDB) – Call processing » Routing instructions for special service numbers (800, 877, 976, 900, …) » Billing information: billing address, 3 rd 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 34
SCP Databases • Line Information DB (LIDB) – Subscriber information: calling card service, 3 rd 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 35
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 36
Routing in SS 7 • 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 37
Routing Redundancy Combined Link STP SSP Alternative “Extended” Connect Link STP STP SSP STP Same destination Alternative signaling points SSP Alternative Direct Connect Link SSP 38
Link Types • Access Links (A-links) – Direct link between SSP--STP or SCP—STP – At least two A links SSP STP STP SSP 39
Link Types • Bridge Links (B-links) – Connect mated STP pairs to other mated STP pairs SSP STP STP SSP 40
Link Types • Cross Links (C-links) – STP to mated STP – Used primarily to deal with congestion situations, enhance reliability SSP STP STP SSP 41
STP Link Types • Diagonal Links (D-links) – Connect mated STP pairs at different hierarchical levels STP SSP STP STP SSP 42
Link Types STP • Extended Links (E-links) – Connect SSP to remote STP pairs to achieve diversity in routes SSP STP STP – Backup paths for greater reliability STP SSP 43
Link Types STP • Fully Associative Links (F-links) – Direct interconnection among SSP in the presence of large amounts of traffics SSP STP STP SSP 44
Cellular Network and SS-7 HLR BSS BTS BSC MSC VLR EIR AC SS 7 STP PSTN MSC BTS: Base Transceiver Station BSC: Base Station Controller BSS: Base Station Subsystem MSC: Mobile Switching Center BSC HLR: Home Location Register VLR: Visitor Location Register EIR: Equipment Register AC: Authentication Center 45
Basic Call Setup X W Subscriber line A Voice Trunk Signaling links B Subscriber line • 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 46
Basic Call Setup X W Subscriber line A Voice Trunk Signaling links B Subscriber line • 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 47
Basic Call Setup X W Subscriber line A Voice Trunk Signaling links B Subscriber line • 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 48
Basic Call Setup X W Subscriber line A Voice Trunk Signaling links B Subscriber line • 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 49
Basic Call Setup X W Subscriber line A Voice Trunk Signaling links B Subscriber line • 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 50
Database Query Example W L A X M • Subscriber served by switch A wants to reserve rental car, calls 800 number, should be mapped to closest location • When finished dialing, A recognizes 800 number, needs assistance to handle • X receives query from A, selects database M to respond to query 51
Database Query Example W L A X M • X sends query to M over MX link; M receives query, extracts passed info, selects real tel # or net to route call • M forms response msg to process call, send to A via MW link • W receives msg, routes to A over AW link • A receives msg, determine call routing, picks trunk, generates IAM, does call set-up as before 52
SS 7 Protocol Stack OMAP: Operations, Maintenance, Admin Part MAP: Mobile Application Part Transaction Capabilities Application Part TCAP ASP Signaling Connection Control Part NSP T U P SCCP I S U P Telephone User Part B I S U P ISDN User Part Network Data Link Physical MTP Message Transport Protocol 53
Message Transfer Part • Routing – – – Incoming message not for local node Physical addresses are called point codes Called/dialing party address determines next hop physical address Routing tables as service provider maintained/network dependent Destination point code not necessary the final destination, but the penultimate address: allows rerouting at the last step • Message Distribution – Incoming message for local node – Routes message to internal user based on user part of message (telephone vs. ISDN higher level protocols) • Network Management – Link Management – Route Management – Traffic Management 54
SS 7 Protocols • Built on MTP • Communications to Telco Databases (SCPs) – Signaling Connection Control Part » Allows addressing of specific applications within a signaling point, i. e. , a specific database on an SCP » Apps identified by subsystem number, e. g. , 800 call processing, calling-card processing, AIN processing, custom local-area signaling service (CLASS) services (e. g. , repeat dialing, call return, etc. ) » Incremental routing via Global Title Translation (GTT)— basically query is sent to an intermediary STP with a GTT request, it will then determine how the message should be routed and perform the routing » This feature used to break down single database into multiple regional or network operator databases 55
SS 7 Protocols • Communications to Telco Databases (SCPs) – Application Service Part » Not widely used: Connection-oriented services – Transaction Capabilities Applications Part » Uses SCCP for transport » Query, retrieval, results transmitted as messages » Remote control of entities (switches) via TCAP messages • Addressing – – Point codes: uniquely identify a signaling point Network/Cluster/Member Number (8 bits x 8 bits) 0 not used, 255 reserved “for future use” Networks: RBOCs, major independent telcos, IXCs 56
Keshav’s Common Design Techniques • • • • Multiplexing: Trading time and space Pipelining and parallelism: trading computation for time Batching: trading response time for throughput Exploiting locality: trading space for time Optimizing the common case Hierarchy Binding and indirection Virtualization Randomization Soft State Exchanging state explicitly Hysteresis Separating data and control Extensibility 57
Common Themes • Pervasive use of redirection/number mapping • Call blocking & filtering • Exploitation of geographic proximity based on phone number prefixes • Enabled by DB lookups indexed by phone numbers 58
Observations • Complicated! Teasing out principles is difficult • Separation of data from control • Separation of control network into service logic, routing, and databases • Addresses/locality very strongly tied to telephone number semantics • Ownership of the signaling system not so clear! 59
Drivers for Convergence • PSTN is expensive, limits innovation from start-ups; complex and expensive system maintenance; requires large operations staffs to manage • Proprietary, limits integration of external apps like billing, customer service • No support for IP-based products, multimedia, content delivery • No model as a contributing element of a “system of systems”; trying to evolve to be the be-all network • On-line billing/reporting via Web desirable but difficult to integrate with legacy systems • Sprint study [ref? ]: data equipment 70% less expensive, data access lines 60 -80% cheaper than voice, maintenance of packet networks 50%, provisioning costs 72% less • CLECs looking for a lower cost way to enter local market • Dial-up circuit overload 60
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