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CS294-3: Distributed Service Architectures in Converged

Networks

Randy H. KatzComputer Science Division

Electrical Engineering and Computer Science Department

University of CaliforniaBerkeley, CA 94720-1776

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Outline

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

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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.

The Telegraph Learns to Speak

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Bell’s Early Telephones

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Building the NetworkAlmon Brown Strowger (1839 - 1902)

– 1889: Invents the “girl-less, cuss-less” telephone system, also known as the mechanical switching system

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“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

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Bell Telephone Equipment

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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

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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

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Old vs. New Signaling

Call

“I’m busy!”VoiceTrunk

Call Set-up

“I’m busy!” message Control

VoiceTrunkFree

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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.

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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

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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

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PSTN Hierarchy

CO

CO

CO

CO

CO

CO

Local Area: 510LATA

642

8778

LECTrunkIXC

64 Kbps links, time division multiplex/demultiplexVoice band: 3.4 KHz between 350 Hz and 3750 Hz8000 samples/second x 8 bits (Law)

Analog “local loop”Twisted Pair

Digital

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PSTN Hierarchy

• Post AT&T Breakup (1980s)

POP

IXC: Interexchange CarrierLATA: Local Access Transport Area

EO EO EO EO EO EO

TO TO

ATTandem OfficeClass 4 Switch

End OfficeCentral OfficeClass 5 Switch

Local LoopCPE: Customer Premises Equipment

Allocate a facility(circuit) per switch

per call

ILEC: incumbentCLEC: competitive“Reciprocal Termination Revenue”

Intra-LATA connectionsInter-LATA connections

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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

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PSTN Hierarchy

• Synchronous Network– Maintaining clock synchronization

PRS

PRS PRS

Toll Office

End Office

PBXChannel Banks

Primary Rate Source

Stratum 1

Stratum 2

Stratum 3

Stratum 4Sync Region Sync Region Sync Region Sync Region

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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

Digital SignalDestination

DS0DS1

DS1CDS2DS3DS3

Bandwidth

64 kbps1.544 mbps3.152 mbps6.312 mbps44.736 mbps274.176 mbps

Channels(DS0s)1 channel

24 channels48 channels96 channels672 channels4032 channels

CarrierDesignation

NoneT-1T-1cT-2T-3T-4

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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

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Intelligent Network

• Basic Call Sequencing– Features/services invoked on state transitions

Quiescent

Off hook

Dialing

Waiting forSetup Complete

Ringing

Talking

Pick up phone

Send dial toneDial Collect dial string

SetupSetup complete

Remote userPicks up phone

DisconnectPut down phone

Busy detectionThree way calling

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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

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Kinds of Signals

• Message Orientation– Bell ringing– Off hook– Voice transport/full duplex set-up– Bill recording– On-hook/call termination

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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

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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

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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

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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

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Intelligent Network• 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

SCPSCP

SMSSMS

SSP SSP SSP SSP

SSP SSP SSP SSP

STP STP

STP STP

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Routing Redundancy

SSP

STP

STP

CombinedLink

Same destinationAlternative signaling points

STP

STP

STP

STP

SSP

SSP

SSP

SSPAlternative Direct

Connect Link

Alternative “Extended”Connect Link

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Link Types

• Access Links (A-links)– Direct link between SSP--STP or SCP—STP– At least two A links

STP

STP

STP

STP

SSP

SSP

SSP

SSP

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Link Types

• Bridge Links (B-links)– Connect mated STP pairs to other mated STP pairs

STP

STP

STP

STP

SSP

SSP

SSP

SSP

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Link Types• Cross Links (C-links)

– STP to mated STP– Used primarily to deal with congestion situations,

enhance reliability

STP

STP

STP

STP

SSP

SSP

SSP

SSP

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Link Types

• Diagonal Links (D-links)– Connect mated STP pairs at

different hierarchical levels

STP

STP

STP

STP

SSP

SSP

SSP

SSP

STP

STP

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Link Types• Extended Links

(E-links)– Connect SSP to

remote STP pairs to achieve diversity in routes

– Backup pathsfor greaterreliability

STP

STP

STP

STP

SSP

SSP

SSP

SSP

STP

STP

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Link Types• Fully

Associative Links (F-links)

– Direct interconnection among SSP in the presence of large amounts of traffics

STP

STP

STP

STP

SSP

SSP

SSP

SSP

STP

STP

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Cellular Network and SS-7

BTS BSCMSC

HLR VLR EIR

AC

BTS BSC

MSCBTS BSC

BTS BSC

STPSS7

PSTN

BTS: Base Transceiver StationBSC: Base Station ControllerBSS: Base Station SubsystemMSC: Mobile Switching Center

HLR: Home Location RegisterVLR: Visitor Location RegisterEIR: Equipment RegisterAC: Authentication Center

BSS

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Basic Call Setup

• 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

A B

W X

Subscriberline Voice Trunk

Signalinglinks

Subscriberline

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Basic Call Setup

• 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

A B

W X

Subscriberline Voice Trunk

Signalinglinks

Subscriberline

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Basic Call Setup

• 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

A B

W X

Subscriberline Voice Trunk

Signalinglinks

Subscriberline

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Basic Call Setup

• 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

A B

W X

Subscriberline Voice Trunk

Signalinglinks

Subscriberline

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Basic Call Setup

• 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

A B

W X

Subscriberline Voice Trunk

Signalinglinks

Subscriberline

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Database Query Example

• 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

A

W

X

L

M

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Database Query Example

• 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

A

W

X

L

M

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SS7 Protocol Stack

PhysicalData LinkNetwork

SCCP

ASP

TCAP

TUP

ISUP

BISUP

MTPNSP Message

TransportProtocol

SignalingConnectionControl Part

TransactionCapabilities

Application Part TelephoneUser Part

ISDNUser Part

OMAP: Operations, Maintenance, Admin PartMAP: Mobile Application Part

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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

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SS7 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

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SS7 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 x 8 bits)– 0 not used, 255 reserved “for future use”– Networks: RBOCs, major independent telcos, IXCs

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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

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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

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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!

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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