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1997-2005 R. LevinePage 1

Digital Switching

EETS8320

SMU/NTU

Lecture 9aSwitching Software

(print slides only, no notes pages)

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Overview Switching software is real-time event-driven:

The driving events are end-user actions such as dialing

digits, lifting or replacing handset, etc.

Circuit-switched voice telephone software mimicsthe human interface behavior of historical electro-mechanical switches Including incidental items like intentional post-dialing delay and non-

symmetrical treatment of origin/destination vis--vis disconnect (wirelineswitches)

Telephone switching software is often described ordesigned using finite state machine (FSM)formalism

Three isomorphic (equivalent) descriptions: Graphical linked points diagram

Graphical flow-chart-like (SDL= specification anddescription language)

Tabular row-column lists

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Historical Switching Original 1876 A.G.Bell installations were point-to-point hard

wired. Examples:

Office to warehouse of same firm (like a modernintercom circuit)

Palace to beach-house of the King of Hawaii

Manual cord-board switching introduced in Hartford, CT in

1880s. Teen-age boys pulled electric wires across the room and

temporarily connected them in response to verbal instructionsfrom subscribers

Later developments led to standard cord-board: a desk-likepanel with a retractable cord from each voice connection unit,

and a panel in front of the human operator with a socket foreach subscriber (and historically later, a socket for each trunkline to another switching center)

Parallel historical development of common battery power andsupervision technology also facilitated the cord switchboard

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Other 19th Century Improvements

Carbon Microphone (Edison and Berliner) Permitted loops of up to ~5 mi (8 km) due to greater

transmitted electrical audio power level

2-wire loop, instead of single wire using earth

conductivity for current return path Earth return was previous standard in telegraph systems, but

produced tremendous cross-talk for telephones

Loop greatly improved voice quality and reduced audio noise

Invented by J.J.Carty, later chief engineer of AT&T

Alternating current ringer (low maintenance) instead of

previous buzzer devices with vibrating electric contactssubject to sparking, corrosion and deterioration

Common (central office) battery for dc loop current usingtransformer to couple audio voice signal between twotelephones in a conversation

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Switchboard Plug

Same dimensions used today for 1/4 in(6.35 mm) diameter stereo headset plug

Tip (greenpositive

wire)Ring (red

negative

wire)

Sleeve (only in

electro-

mechanical

switches,no standard

outside-plant color)

Insulators

Tip

Tip

Ring

Sleeve

SleeveRing

Plug Assembly Graphic Symbol

Socket Assembly Graphic Symbol

Note: use of red

insulation for neg-

ative polarity is

unique to the

telephone industry.Other electrical

standards (power,

electronics, auto-

motive) use red

for positive.

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Historical Cord Circuit

_

+

Primitive Telephone set(dial, ringer, cradle switchnot shown). No directionalcoupler here as in latertechnology.

Primitive central office cord circuit. Positive battery terminal groundedto minimize electrolytic corrosion. Audio frequency voice signals coupledvia transformer. Does not show ringing power, sleeve wires, signal lamps and buzzer,operator exclusion switches, etc.

telephone set andsubscriber loop

Common battery feedand voice coupling

Note use

of same

CO battery

(with audio

bypass capacitor)

for all loops.

Earphone

Microphone

Other telephoneset not shown.Operator headsetalso in parallel withvoice wires tempo-rarily, not shown.

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Supervision Methods In traditional telephone jargon, supervision describes only the

aspects of signaling which relate to busy/idle status Dialed digit information was historically distinct (called signaling)

In modern cellular/PCS software both things are often described by theword supervision

therefore, be careful about jargon!

Historical method to get attention of the operator was a smallhand-cranked AC generator or magneto at subscriber end Produced about 90 V ac, at 20 Hz frequency.

Still standard ringing waveform for North America today

Then the common-battery circuit was introduced Subscriber switch-hook closed a current loop and operated a light

and/or buzzer near that subscribers socket on the switchboard panel Operator lifted a retractable cord from the desk-top, connecting her*

headset to the subscriber via a voice-frequency transformer

Operator then asked Number Please?* Boys were replaced by more polite ladies in 1890s; operator corps was exclusively female until 1960s.

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Call Connection Operator plugged other end of cord circuit into calld subscriber

socket (the second syllable of calld is artificially stressed in telephone

jargon to emphasize the spoken distinction with call) Outer part of socket and sleeve (called C wire in European jargon) of

plug carried a voltage when that line was busy. (No C wire in modernelectronic switches.)

Voltage (if present) on sleeve produced an audible click inoperator earphone, indicating busy line

If calld line is idle, cord circuit is plugged in, connecting voice circuit of bothtelephones

and connecting temporarily the operator as well

Operator presses momentary contact switch to apply 20 Hz, 90 V acringing to the calld loop

When calld person answers, operator presses a latching switch todisconnect operators headphone from the cord circuit

When either participant hangs up, dc loop current from common centraloffice battery stops, indirectly operating a distinct buzzer and light on thecord board via a relay.

Operator then tears down the connection by pulling both retractablecord plugs from the calld and calling part circuit sockets. Cords fall backinto desk surface due to weights under the desk.

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Cord Switchboard Capacity The number of simultaneous conversations is limited to the number of

cord circuits installed in a cord switchboard Each cord circuit is similar to a storage address (byte) in an electronic

switch vis--vis capacity

The BHCA (call processing) capacity is limited by the attention andoperational speed available from the human operator

Both were improved by providing more operator positions (cord circuits)

Each subscriber loop appeared at multiple sockets, each one withinreach of an individual operator position

Thus a historical need for busy status signal (sleeve or C wire)

Early example of switch concentration

Operator-handled calls were controlled by humanintelligence

Computer controlled (stored program controlled - SPC)switches merely strive to put back into automatic servicemany of the clever things human operators did historically(example, ring back to originator when initially busydestination finally becomes available)

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Some Human Operator Features

Call by name (no telephone number required) Response to: Please call the Smith home.

Wake up calls (at pre-determined time)

Re-connect calls accidentally disconnected*

Notify busy line of incoming call waiting Set up 3-way (or more) conference call

Connect call to alternate line when subscriber isaway from home (call forwarding)

Note that modern feature-rich PBX, small business key systems, and some

PSTN switches now do these things via computer control

Several experts have calculated that there are not enoughpeople on earth to support the todays (2001) level of publictelephone traffic using operator cord board switching!

*The GSM cellular system can optionally be configured to do this.

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Strowger Step-by-step Switch Almon B. Strowger, a mortician (undertaker) in Kansas City

KS, invented the first practical automatic dialing system Famous story: fearing that the human operator was directing calls for a

mortician to his competitor, he invented an automatic user-controlledswitch

First version (installed in LaPorte, IN, circa 1895) used extra wires andpush buttons on each subscriber set

Rotary dial with impulsive current on the voice wire pair was a laterdevelopment

Strowgers manufacturing firm, Automatic Electric, movedto suburban Chicago, IL, later absorbed by GTE, latermoved to Phoenix AZ, now AG Communication Systems(partly owned by Lucent)

Stepper progressive control switches were manufactured world widefor many decades

Electromechanical common-control switches developed by othermanufacturers, such as panel and crossbar types partiallysucceeded steppers in the 1930 - 1960 decades

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Schematic Stepper Diagram

Many details omitted here

Vertical Motion

Rotary Motion

Rank 1

Rank 0

1

2

34 5 6 7 8

9

0

Tip, Ring, Sleeve

wires from Rank 8,

column 7.

Electromagnets and

springs activate the motions

of the wiper arm in response

to dial impulses.

Ten places on

each circular

rank where

a 3-contact

assembly is

located -- not

illustrated in

detail.

Rank 9

Axle

Arm

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Stepper Switching Strowger switches evolved into an assembly with a movable wiper

switch inlet and 100 outlets (wire pairs with sleeve wire) 10 contact pairs arranged in a horizontal arc, selected by rotating the

wiper switch arm. (Also a third sleeve wire in addition) 10 such horizontal arc sub-assemblies stacked and selected via vertical

motion of the axle (actually the first motion is vertical)

Single-motion (rotation only) switch assemblies were also used

Line Finder switch (mostly single motion) acts as inputconcentrator (inverse of selector action)

Wiper arm contacts act as the single outlet Line finder single-motion stepper typically wired to 10 subscriber lines,selects a line when that line goes off-hook

Stepper starts stepping from line to line when any of the 10 lines gooff hook, then stops when correct off-hook line is found

analogous to operator responding to buzzer and light

Multiple line finders wired in parallel to the same 10 telephone sets

analogous to multiple operator stations with each having access to thesame subscriber sockets.

Number of simultaneous originating conversations limited to thenumber of line finder switches connected to those lines. Ten linefinders wired to ten subscribers is non-blocking with regard to linefinders. (Overall system may still block at later stages)

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Selector Switches Line finder outlet goes through a transformer cord circuit

Connected to dial-tone generator until the first dialed digit.

Then the circuit is switched through a chain of two-motion selector stepperswitches, with a motion for each digit. Each burst of impulses (dialed digit)produces a rotary or vertical motion constituting the next stage of the wiperarm selection process

Dial pulses from rotary dial (typically 10 impulses per second, each oneapproximately 60 millisec current OFF and 40 ms current ON) are passedaroundthe cord circuit by special electro-mechanical relays

A relay employs magnetically operated switch contacts, so that current ON/OFF statusin the contacts mimics the current ON/OFF status in the wire coil causing themagnetic action.

Special slow release relays hold the line finder so the 60 ms OFF intervals donot cause a disconnection

After turning and releasing the telephone set rotary dial from anangle labeled with a specific number, the returning rotation of the

dial to its normal position produces 1 to 10 current impulses Simultaneously, an Off-normal switch contact in the telephone set

temporarily short-circuits earphone so clicking is not heard Following a stage of selection motion, a slow release relay is automatically connected

into that line to prevent further disturbance of that particular selection due to thesucceeding bursts of dialing impulses

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Incidental Information

Rotary dial label 0 represents 10 impulses everywhere in the world(except Sweden, where the dial is labeled 0, 1, 29)

However, touch-tone dials in Sweden use the same digit labels for DTMFtones as the world standard.

Impulsive signaling must be converted at international boundaries toSwedish telephone system. But symbolic signaling (binary digit codes usedin CCS7, etc.) is the same everywhere.

Alphabetic dial labels (2=ABC, 3=DEF, etc.) were introduced in NewYork City in ~1923 when subscribers complained about long 5 digit

directory numbers. Alphabetic dial labels were introduced in US, Canada, UK, France,

Scandinavia and USSR (three cities only) but not all the same: Examples: Q on French dial, Russian (Cyrillic A B... G ...F) letters in Moscow,

Leningrad, Odessa,

Considered an obstacle to direct international dialing, alphabetic exchangenames were purged from telephone directories in 1960s by internationalagreement.

The anti-digit dialing league and other grass roots groups in the USopposed all-digit directories in the 1960s.

Letter labels still appear on the dial in most of these named countries.Business users highly value so-called Anagram numbers such as 1-800-FLOWERS, or 1-800-NORSTAR, 1-800-AMERICAn, etc.

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Significant Properties of Stepper Switches To add more traffic capacity, install more line finders and more

selector switches This increases parallel path (traffic) capacity through the switch, sincemultiple last stage selectors lead to the same destination lines.

Only one last stage selector can connect at a given time. The sleeve wire isalso connected to each corresponding position on the selectors and isused to divert the call to a busy signal generator if the sleeve voltage is ONfor that destination line and a call is attempted while destination line is

busy. A non-blocking Strowger step switch assembly would require 100 last

stage selector switches connected to 100 destination telephone lines, andsimilar replication of parallel paths all the way to the originating lines (linefinders, earlier stepper stages, etc.).

This automatically increases the call processing capacity (BHCA)

of the switch as well Each selector is both a traffic path and a part of the digit processing hardware When there is a traffic path available to the destination, there is also the

hardware to respond to the succeeding dialed digits.

A stepper switch assembly automatically has enough callprocessing capability if it has adequate traffic path capacity

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Stepper Properties

Stepper switches are extremely reliable overall Because of parallel path capability through a large stepper switch, the failure

rate of these switches (when properly maintained) is very good Failures affecting only one useramount to only about 1 hour cumulative in 20 years

Failure of the entireswitch is only 1 or 2 minutes in 20 years, and when this occursit is mostly due to power supply or other aspects of the system

Steppers can be adapted to many improvements Touch-tone dialing (by means of a tone-to-pulse converter)

Computer control has been adapted to steppers to make advanced features available (such as

call waiting, 3-way conference, etc.) Unfortunately, basic reliability, power consumption and size not improved!

Inter-switch signaling between stepper switches requires electricaltransmission of dialing impulses conversion between modern digital signaling (common channel 7) and

impulse switching is feasible, but slow acting

European version of SS7 signaling allows transmission of one dialed

digit at a time, but North American (ANSI) version does not send dialednumber onward until the last digit is dialed.

several earlier electronic but non-digital switching systems still usedelectromechanical switching (small relays) and analog transmission(example: No. 1 ESS), but digital computer central control or stored programcontrol

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Undesirable Stepper Properties

High maintenance Gross Motion or Large Motion wiping contacts

Require lubrication, cleaning, adjustment, etc.

Susceptible to corrosion from sparking, air pollution (such asSO2 in the air, etc.)

Slow mechanical operation Even when tone-to-pulse converters support Touch-tone

dialing

Slow signaling Cant take full advantage of CCS7 and other electronic

signaling systems

Big and bulky Digital switches use ~1/50th the floor space of steppers; ~1/10th the

floor space of crossbar switches.

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Some Other Historical Electro-Mechanical Switches

Panel A huge mechanical monster switch using continuously running electric motors

and electrically operated clutches to move wipers vertically and horizontally on arectangular wall panel of contacts. A high maintenance problem.

Crossbar An assembly of rocking contacts attached to vertical and horizontal rotating

actuator axles. Because of relatively small motion and compact size, this was the

heir apparent to the stepper switch in both North America and Europe untilelectronic switching appeared.

X-Y A horizontally platform with rows and columns of contacts with wipers actuated

by magnetic coils. Gross motion problems, but more compact than Strowgerdesign.

Rotary

Similar to X-Y switch, but platforms had contacts arranged in semi-circles ofincreasing radius. More compact than Stepper, but same gross motion problems.

Multi-relay Rocking contact motion, but still rather complex and difficult to maintain.

The last 3 were mainly used by independent telcos in North America. All here exceptCrossbar and Multi-relay were gross motion switches.

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Common Control Many of these electro-mechanical designs had separate relay

assemblies to count (decode) the dial impulses, completelyseparate from the switching portion of the system. These so-calledcommon control portions were analogous to the computercontrol in a digital switch.

Once the desired destination directory number was decoded, it wastranslated by special purpose wired logic devices

One method for this was to use magnetic core memory of a specialwired type (not addressable RAM like modern computer memory)

The equipment numbers resulting from the translation were used toselect a path through the switching part of the system.

The result of the translation was a code designating the properbay, shelf, and switch outlet wire for the internal destination calls,or the proper outgoing trunk group for outgoing (other switch)

calls. The first non-busy channel in a trunk group was selected byan appropriate special outgoing trunk switch.

These systems first demonstrated the need for provisioningseparately both sufficient call processing capacity (BHCA) and alsosufficient switching capacity (Erlangs)

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Electronic Switches ESS No. 1: Electronic but not Digital!

Computer control/stored program control (SPC)

Analog Relay switching, using sealed contact reed switches

Most of the design problems for high reliability wereaddressed in this design. Duplicated processors, etc.

ESS No. 4: Fully Digital but Trunks Only

When designed (1960s-70s) the cost of A/D conversion(CODECs) on each subscriber line was seen as prohibitive

Depended on T-1 channel banks at distant ends of trunkgroups for A/D (analog/digital) conversion

4 ESS is a transit or tandem switch, not a central office or end office

4 ESS has only T-1 links at its ports (no telephone sets except for a

few test telephones) Above are all Lucent (then AT&T) products. Competitors had similar

designs shortly after or almost contemporaneously. In that era WesternElectric (manufacturing division of AT&T) only sold products to the BellSystem operating companies, and was required to license all its patents asone result of an earlier anti-trust settlement.

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Subscriber Interface to Electronics Was Difficult

Integrated circuits made the digital central office (end office switch)economically feasible The most elegant hardware design requires a dedicated analog-digital

converter at each subscriber loop. A long time economic problem forend office switches.

Subscriber loop circuitry usually on a removable printed wiring

card Replaceable in case of failure or damage from lightning, etc.

Handles some so-called BORSCHT* functions Battery Feed

Over voltage (from lightning and accidental power line contact)

Ringing

Supervision Codec (A/D inter-conversion, also low pass audio filtering)

Hybrid (directional coupler, 2-wire to 4-wire inter-conversion)

Testing (a capability of the switch, not of the telephone set)

* This term credited to John Iwerson of AT&T Bell Labs ca. 1960s

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Modern Digital Switch Subscriber Loop

Block Diagram

Hybrid andmatchingnetwork

Hybrid andmatchingnetwork

_

+

Telephone set (dial,ringer, cradle switchcircuits for loop lengthlevel compensationnot shown)

Wireloop,up to~8 km

Central office switch equipment. Actual switching is not shown, but isoff to the right of this page. Audio frequency voice signals coupledvia transformer. Ringing power, loop current detection (supervision)not shown.

Amplifier

and A/Dconverter

Amplifierand D/Aconverter

Transmitsignal

Receivesignal

CO part

telephone set andsubscriber loop

Common battery feedand voice coupling

Tip

Ring

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Subscriber Line Interface

Card/Chip (SLIC) Due to large volume of use, integrated

circuits are available which perform mostof these BORSCHT functions

ICs designed for line card in switch andchips for use in a low-cost telephone setare both available

Spoken acronym SLIC /slI

k/ sounds likeanother acronym, Subscriber LineConcentrator (SLC). Ask for fully spelledout version if context is not clear!

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Some BORSCHT Explanations Battery Feed via split winding on audio coupling

transformer

Over-voltage (lighting, power line crossing)protection primarily based on arc-over at sparkgaps installed where outdoor wiring enters

subscriber premises and CO building. Enclosed gas spark gaps provide uniform electrical breakdown at~300 volts between wires or wire-to-ground

Hermetic enclosure prevents variations due to air pressure andhumidity, a problem in older lightning arrestor devices

Non-linear series resistance devices limit high current surges due tolightning or accidental cross with power voltage wiring

Light bulbs or heat coils

Positive Temperature Coefficient (PTC) resistors using conductivepolymer plastics

Noton subscriber loop circuit card, except for PTCresistor.

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More BORSCHT Explanations Ringing voltage from ringing generator via

electro-mechanical relay contacts on tip & ring

Supervision (dc loop current sensing) via variousmethods: Sensing relay coil in series with subscriber loop. Loop current

actuates separate relay contacts. Inductance of relay coil affects frequency response somewhat, but

can be bypassed for audio frequencies via a capacitor

Non-linear magnetic material (saturable magnetic core) with loopcurrent coil and sensing coil

Loop current changes magnetization point behavior

Sensing coils small signal inductance decreases when loopcurrent is on.

Smaller, less costly, less effect on voice signal.

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Still More BORSCHT

CODEC (COder/DECoder) in switch (except for ISDN sets):

Low pass filter the audio on analog side of A/D conversion

Active RC filter, switched capacitor filter, or CCD (charge controldiode array) transversal filter are different analog technologies

Purpose is to attenuate audio power above about 3.5 kHz

Then measure voltage, 8000 samples/second for coding

Encode each voltage sample as a compressed digitally coded 8 bitsign-magnitude binary code

Mu-law approximately logarithmic compression rule used in NorthAmerica and Japan

A-law log-linear compression rule used in other national PSTNsystems

Digital/Analog conversion in opposite direction as well

Hybrid or directional coupler is analog device using multiple windings ontransformers, together with a matching network composed of resistors

and capacitors Separates incoming and outgoing electrical waveforms on 2-wire

subscriber loop into separate unidirectional signals with good but notperfect accuracy

All digital transmission operations in a digital switch comprise twoopposite-flowing unidirectional signal paths

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Automatic Test in Digital Switches Electro-mechanical relay installed in the line card can switch

subscriber loop temporarily to an auxiliary test device Main test operation is to measure (tiny) microamperes of

leakage current between wires and from each wire to ground High leakage current indicates imminent failure due to moisture in cable,

damaged insulation, etc.

Testing is usually done circa 2 AM when traffic is minimal

Even so, if subscriber lifts handset or a call comes in, test issuspended until loop is again idle

Suspicious test results are automatically reported to repair craftspersons

Tremendous reduction in staff is feasible when their repairwork can be scheduled, rather than waiting for an emergency,

unexpected customer complaint, or loop failure! Most repairs are thus done before customer notices noisy line

problems!

Consequently, most unexpected failures today are due tohuman error or accident, rather than slow cable deterioration

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Digital Switch Advantages

Automatic test reduces staff costs significantly

Predominant cost saving in many cases!!

Feature-rich, increases income of public

telephone company by selling optional verticalfeatures (e.g. Call waiting, conference, etc.)

Inter-works with digital trunks (T-1 etc.) withoutuse of channel banks

Smaller size allows more CO capacity growth insame building

Less electric power consumption, reducesoperating costs somewhat

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Internal

SwitchingMatrix or

Network

Block Diagram Control Processor and Switching Matrix are duplicated for reliability

Line and trunk interface cards are not duplicated because failure rate on outsideplan wiring is greater than loop or trunk electronics.

Internal

Switching

Matrix or

Network

Line/Loop

Interface

Trunk

Interface

Control

Processor

(CPU, RAM

etc.)

Line/Loop

Interface

Line/Loop

Interface

Trunk

Interface

Trunk

Interface

More

not shown

More

not shown

Multiple station loopsMultiple T-1 trunk groups

optional

remote

lineconcen-

trator

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Translation Tables

Each end office switch has at least 3 translation tables in itscontrol processor 1. Internal line appearance number (ILAN) translated to directory

number (DN)

Identifies billing number for originated calls, and for calling line ID

ILAN is a proprietary number indicating the rack, shelf and circuitcard number of a line

2. Inverse table of above: DN to ILAN Used to route incoming call to proper destination line

3. Translates from NPA/NXX (or just NXX) into the proper outgoingtrunk group to reach that destination.

Two inverse tables are used for fast look-up Like using both a Spanish-English and a separate English-Spanish

dictionary for human language translation Tiny switches (example: 16 lines) use just one table and perform

exhaustive search for the inverse translation function

Additions, removals, and changes in DNs are made by entries inthese tables, not by rewiring the external subscriber loops.

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Other Switch Configurations A switch can be configured with only trunk interfaces (no line

interfaces). Applications include everything excepttraditional endcentral office use: Tandemor transitswitch use in local or long-distance network

A cellular or Personal Communication System (PCS) radio system switch

Connections to base radio stations are via trunks (e.g., T-1)

Historically, a switchcan be configured with only line interfaces(no trunks) for use as an intercom or PAX inside a building.Seldom installed today since a standard PBX with both inside andoutside connections is less costly than two track systems.

A line module can be located remote from the switch locationwhen a distant cluster of subscribers needs service.

Connects to main switch via T-1 links thru a trunk interface

Subscriber Line Concentrator (SLC-96) is an example of this.

PBX also performs this function, but has different signaling and is typicallysubscriber owned. PBX and SLC systems are not covered here In detail.

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State Machine Standards, user documentation, or software design documents can be

written in natural human language, but this often leads tomisunderstandings and differing implementations

Readers disagree on meaning of natural language, regarding sequenceof steps, etc.

Actual operational test of compliance to standards requires testing viainter-working against pre-existing implementations

Often, first-to-market implementations actually supersede the writtenstandard when discrepancies occur

A better form of description is needed for: Software algorithm design

Description and documentation of existing systems

for testing or design of compatible equipment

for user training

Finite state machine (FSM) formalism (also called Discrete StateMachine -- DSM) serves this purpose

SDL-Specification and Description Language, ITU-T standardsZ.100 and SDL 2000, formalize a graphic flow-chart-like symbolismfor this purpose.

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Finite State Machine (FSM) FSM formalism can describe a computer or a telephone

switching system quite well

A FSM has a number of distinct states

States are distinguished from each other by the unique

binary value of: At leastone bit somewhere in the CPU, RAM or mass

storage (disk, etc.) is distinct (1 vs. 0 value) from thecorresponding bit value in another state

In an electro-mechanical telecom switch, at least onerelay/switch contact is distinct (ON vs. OFF)

A FSM is driven from state to state by events An event is often an external cause such as a customer dialing

a digit, lifting or replacing a handset, etc.

The expiration of a timer/counter is also an event

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Some SimplificationsStrictly speaking, each combination of busy vs. idle telephone lines in a switchis a different state of that overall state machine

Because of the similarity of operation of all telephone lines, we can simplify thedescription by describing the telephone switch in terms of just the 2 (or 3, etc. linesfor a conference call) involved in the conversation

The distinctions due to different optional vertical features (call waiting, etc.) can behandled by means of a general FSM description which handles everypossiblefeature, with clearly defined options to allow or deny each specific feature

dependent on a data table entry defining the class of service (COS) for that line*.We consider the states of one telephone and the aspects of the switch whichrelate to it, and also the events at the calld telephone as well.

The general historical approach to FSM design is to describe what historicalelectro-mechanical switches do, and then program the digital switch to presentthe same behavior to the customer

When new features are designed, feature conflicts sometimes arise. These

include discovery of ambiguous operations, etc. Feature conflicts are usually resolved by re-design of the

feature at the human interface level.*Some local service providers (e.g. SouthWestern Bell -- SBC) now allow allsubscribers touse most previously optional services, charging on a per use basis until a maximummonthly fee is accumulated.

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FSM Description Formats

Logically equivalent (isomorphic) descriptions can be madein several forms1. Graphic point or picture for each state, with directed lines or

arrows representing event-caused transitions between states

Useful for human visualization, particularly withcute-sy pictures.

2. Table with column for each state, row for each event (or thereverse), and entries describing the target state and relatedinformation.

Often very large if all events are treated explicitly, and oftenhas many null (not possible) entries. Good for certain table-driven computer software systems.

Usually notinstructive for human visualization3. Flow-chart like description such as SDL

Convenient starting point for software development

One-to-one correspondence to formal software language isunder study (e.g. ITT CHILL language occasionally used)

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Simplifying Conventions Certainevents invariably lead to the same result,

regardless of current state (whenever logicallyconsistent) Example: hang up of the handset leads to

disconnection (this line idle) state

To avoid pictorial clutter, this is omitted but implied ingraphic point-line diagram. Shown only where essentialfor understanding.

Multiple states can be symbolically combined intoone covering state to clarify the explanation

aspects All the internal details must still be explicitly defined for

a working description (perhaps separate diagram)

Example: dialed digit collection (digilator)

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Pictorial FSM Example

Illustrates only 3 states

Idle state

Event: called by

another caller.

Ringing state

Conversation (answered) state

Event: lift handset

Event: replace handset

(note: some designs haveintermediate state with

10 sec. timeout)

Event: other caller

abandons call attempt,

ring-no-answer.

Babble, babble

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Originate-Answer Distinctions

Most PSTN wireline switches actually handle disconnectdifferently for an originator vs. an answerer. Such switchesdisconnect: immediately when originator disconnects

after (typically) 10 second timer expires, when answering persondisconnects.

Distinction is software controlled based on a bit set in RAM Mimics a historical property of some electro-mechanical

switches Allows calld person to hang up and then quickly run from one

extension set to another on same line without disconnection

Many PBX and Cellular/PCS switches do not distinguishoriginator vs. destination (except for billing!!) We do not distinguish in our diagram for simplicity

If distinguished, two separate conversation states would be requiredin the diagram

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Example Extended

More states but still not complete

Idle

calld

Ringing

Conversation

replace handset

other caller

abandons

Lift handset Dial tone

Dial first

digit

Distant lineanswers

Digilator

~~

Valid

digits

Event: timer expires

or invalid digits.

Collect digits

Announcement:

Please hang

up and

try again

replace handset

~~~~

Inert Howler

Distant line rings or busy.

or

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Other Representations

Not shown explicitly in these notes are two other FSMrepresentations of a telephone switch:

Tabular representation could have (for example) a columnfor each state, and a row for each event

Entry for Idle column and Lift Handset row is Go toDial Tone state

Entry for Dial Tone column and Lift Handset row isnot applicable

Entry for Dial Tone column and Dial first digit row is:Start timeout for inter-digit max time, go to next Digilatorinternal state

SDL description comprises a distinct flow-chart-like diagramfor the computer processing steps which must follow eachevent

Proto-representation of the software in the interrupthandling routine invoked by that event.

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Time-Space Diagrams In some cases the sequence of events and messages betweendifferent parts of a telecom system is displayed via a time-space

diagram. Time (not to correct scale) usually increases in downward direction

Various horizontal positions (not to scale) represent different physicaldevices (subscriber set, end office switch, transit switch, etc.) in thesystem

Only one (usually representing a successful case) event sequence isdisplayed.

End Office

SwitchDistant End

Office Switch

Transit

Switch

Destination

Station SetOriginating

Station Set

IncreasingTime

Off Hook

Dial Tone

Dial Digits

Recognize non-local number andextend to destination switch

Ring

Answer

Connect Voice Channel

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Variations on a Theme Use of diagonal lines (as in previous page example)

emphasizes transmission time delay aspect Often called a zig-zag diagram

Horizontal lines may also be used

Then often called a ladderdiagram. Still exhibits sequence of

events but de-emphasizes transmission delay

Another name: Ping PongDiagram Similar diagrams in other subject areas illustrate sequential

events at different locations

e.g., Feynman diagrams in quantum physics

Time-Space diagrams cannot clearly illustrate all exceptional

cases in one figure e.g. destination busy, or ring-no-answer, timer in software

expires, etc. etc.

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Distant Line Ring or Busy

When the destination telephone is in anotherswitch and the trunk signaling is one of the moreprimitive types (notcommon channel No.7), theoriginationswitch cannot distinguish distantringing from busy The human caller must listen to the call

progress tones to distinguish busy/ringing

When the calld telephone line is in the same

switch or CCS7 signaling is used, this switch candistinguish, and two states should be drawn.

Two distinct states are notillustrated in thesenotes.

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Digilator Digit Collection A contraction of the words digit and percolator, the Digilator

state is really a collection of many internal states and events.

Digit collection strategy can be described as a tree-structureddata collection decision process

Any pause before or between digits which exceeds maximum time

causes a state transition to a recorded announcement Internal timer (typically 6 to 20 seconds) is started after each digit but the last,and reset again when the next digit is sensed

Expiration of the timer causes a time out interrupt

Valid digit strings are described by both Numbering plan: assignment of specific number groups to local, long-

distance, and service lines

Dialing plan: assignment of specific digits (usually prefixes) forpurposes not described in dialing plan:

Example: dial 9 prefix to get outside line in a PBX

Example: dial initial 1 (North America) or 0 (many Europeannations) for non-local or special service calls (1-411, etc..)

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Digilator Information In North American Numbering Plan (NANP)

Initial digits 2,3, 9 (not 1,0 ; traditionally represented by N) arevalid for local directory number

Subsequent digits can by any of 1,2,3,9,0 (called X). First 3 digits formexchange code NXX, remaining digits XXXX. Special treatment for 3-digitservice codes NXX=911, 311, and in some areas 611, 411 etc.

Digit Collection will be stopped after 7 digits (local call) are collected

Recently, mandatory 10 digit local dialing in numerous cities like Dallas. Then further testing or connection required to determine if this number is in

service, corresponds to a valid NXX, etc. Data for this decision may not beavailable in the originating switch.

The complicated cases are initial zero (0) and initial one (1)

Initial 1 mayimply: service call: 1-411 for directory assistance, 1-611 for repair (initial 1 not used

universally) Non-local (inter-exchange carrier) butin e.g. Los Angeles, just non-local

first digit of 11XX, a rotary dial substitute for special feature prefix/code suchas *69 (1169) for call back most recent caller

first digit of 1010XXX prefix for selected inter-exchange carrier (e.g., 1010222for MCI, 1010288 for ATT, etc.)

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NANP Initial Zero Initial zero (0) has several subsidiary choices, depending

upon succeeding input:

Followed by no other digits (typically 6 second timeout)connects to local operator/attendant (0|)*

Followed by second 0 and timeout (00|), connects to

preferred inter-exchange carriers operator/attendant Followed by 11 indicates international call (011-) prefix

010- is international operator assisted prefix

Followed by any valid foreign directory number (countrycode, area code, local number) indicates an operator

assisted call (so-called zero-plus call) Example, person-to-person or English-language-only

international call, etc.* The non-standard symbol | is used here to represent a timeout

with no succeeding digits.

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NANP Areas US, Canada, and certain coastal* and Caribbean islands are under the NANP

Each area has a 3 digit area code of form NXX, sometimes represented by the 3letters NPA. In some documents, the 10 digits are represented ABC DEF GHIJ.

Historically the B digit was restricted to 1 or 0 due to historical use of NNXfor local office codes. No longer done (now NXX)

Size of an area is dependent on total directory numbers in use in that area.High population density areas have required many area code splits oroverlays in recent years and will again in the future

Caller pays special surcharges is the source of questionable billing in

some Caribbean nations. Bewareof area code 809 and others... Certain pseudo-area codes are used to cover the entire NANP:

800, 888 and 877 for calld-line-pays long distance numbers.

Actualtarget number is determined by a translation table data base usingthe succeeding 7 digits

Call forwarding to existing line can be altered based on originatorsdirectory number, time/date or other factors

Your call to Sears Roebuck or Dominos Pizza is routed to thegeographically nearest store (central office) to your point of origin

When calling some large firms, your call to the same number may gothe the east coast customer service department in the morning, andthe west coast department in the afternoon and evening.

*e.g. St.Pierre and Miquelon, French possessions off Canadas Atlantic coast. AlsoBermuda, etc.

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ITU Numbering Plan The world is divided into 9 zones, each with a

specific initial digit used in national prefix: 1 North America (US, Canada and some islands)

2 Africa

3 Europe (part) 4 Europe (other part)

5 Central and South America

6 Australia and pacific (part)

7 Former USSR

8 China, Japan and Pacific (other part)

9 India and Middle East

smaller nations have 3-digit code, larger nations have 2- or1-digit codes.

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How is Connection Routed? A connection is directed from an originating line or trunk to

a destination line or trunk outlet by setting the correctnumbers in the connection memory associated with theswitching matrix. The choice of destination or outlet is based on an internal

translation table which uses a part of the directory number asinput (e.g. NPA or NXX) and physical equipment identificationnumber (trunk number or ILAN) as output.

The contents of this translation table may be changed fromtime to time (or another table substituted) to accommodate:

Malfunction or traffic saturation of certain links (useof alternate path or route)

Known or expected better choices for economic ortraffic reasons Time of day changes in traffic in various time zones

Utilize different price strategies on leased or outgoingtrunks

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Historical North American* Signaling Methods

DC (direct currrent) or baseband dial-pulse signaling 1900-1940s

SF (single frequency) impulsive tone signaling for supervision and dialing.Needed for FDM multiplexing.

MF (multi-frequency) dialing digit representations.

The above 3 methods are each historically called in band even when analog

frequency band is not involved (for example, in T-1).

When digital multiplexing (eg, T-1) arrived, MF dialing signals and robbed bit

supervision signaling was used. Many telephone networks formerly or presently use(d) in-band tone signals to

represent dialed digits (DTMF touch tone, or another set of audible tones

called multi-frequency MF)

DTMF uses two simultaneous tones taken from a set of 8

MF uses two simultaneous tones taken from a different set of 5

Common channel signaling (SS7 ISUP) is dominant today Earlier versions were SS6 and SS7 TUP.

*R2 and other signaling methods used outside of North America are not covered in thislecture.

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DC Pulsing Signals Before dial telephones, for inter-office (inter-switch) signaling (usually in

same city) the human operator connected through a trunk to anotherhuman operator at destination switch. Requests for a destination numberwere verbal. When subscribers ended the call, buzzers at each cord boardsignaled the operators to manually tear down (unplug) each link.

Earliest step switch dial signaling methods accessed specific inter-officetrunks in response to dialing the first (typically) 3 digits. The remainingdialed digits were out-pulsed in the same form as locally dialed signals

(that is, brief 40 ms interruptions in the loop current) on the the samewires as the eventual voice channel, and operated the last stages of stepswitches in the destination switch. A long interruption in the loopcurrent ultimately caused automatic disconnect of all the step switchesinvolved in the call, just as for a local (same switch) call.

This system was difficult to evolve further: Required many permanently installed, directly-connected but not always

frequently used inter-office trunks. Long distance voice transmission with electronic (vacuum tube) amplifiers

began about 1914. (Before that, very costly thick low-resistance trunk lines wereinstalled between some cities.) Amplifiers also required conversion from 2-wireswitching (used in each end switch) to 4-wire switching in the trunk link.

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Single Frequency (SF) Signaling In the 1920s and 1930s, analog frequency division

multiplexing (FDM) was introduced into the long distancetelephone network. Typically 12 conversations weremultiplexed on the same 4 trunk wires, by means ofamplitude modulation of each conversation (in eachdirection) onto a distinct carrier frequency.

Similar to having 12 radio broadcasting stations, with 12 pre-tuned radio receivers. Except the radio frequency signals wereall carried by wires and notover the air.

DC or baseband pulsing was not feasible with FDM.

In each conversation signal in each direction, a singlefrequency (2600 Hz) tone was transmitted in the voice

channel to indicate the absenceof subscriber loop current. Steady SF tone indicated that the channel was idle, or that the

subscriber at the relevant end had hung up.

Out-pulsing of the SF tone at the beginning of a call setupconveyed dialing digits as from a rotary dial.

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Multi-Frequency (MF) Digit Signals

SF for dialing was slow, and was also susceptible to theftof service fraud from SF tones generated at the originatingsubscriber end by hackers.*

MF dialing encoded each digit, and also a start (KP) andstop signal, each by means of a pair of tones taken from a

menu of five distinct audio frequencies. MF dialed digit signals were initially used with SF supervision. This required a more sophisticated multi-tone receiver to

decode the tones and produce electric current pulsescompatible with the switching equipment.

New types of switch equipment (example: crossbar) were

introduced in the 1930s that did not need sequential digitpulsing and thus connected the call faster when MF was used.

MF signaling can out-pulse 10 digits in one second. Basebandand SF out-pulsing could take up to 10 seconds for this.

*Large scale hacker fraud did not become a widespread problem until the 1960s

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MF with Digital Multiplexing

When T-1 transmission was introduced (1961), MFwas still used to convey dialed digits.

Supervision was signaled via robbed bit signaling

(described in EETS8302 lecture notes and in Bellamy

recommended reference) The same advantages and disadvantages still

applied to MF encoding of the destination number

Plus a new requirement:

Not flexible for evolving new services. Next slide showsand example where other information is needed.

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Automatic Routing of Subscriber Dialed Calls Automatic routing of long distance calls via several intermediate switches

(first introduced in late 1950s) called for more sophisticated signaling Knowing only the destination telephone number was not sufficient

A problem called ring around or looping can occur when an automatic call

routing system tries several suburban intermediate switches surrounding adestination city, and eventually revisits the same intermediate switches

The call signaling must identify each call so that the previous intermediateswitches on its attempted route are not visited twice

Better to signal to originator to try again later and abandon the call

Unlimited ring around will eventually gobble up allcircumferential trunkchannels, making the situation worse than it already is.

from originating switch

Destination switch

Blue represents trunksUsed in call setup attempt.

Red represents trunk groupsWith all trunks already busy.

Attempted connectionIs heading again forSwitch A..

A

B

C

D

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Common Channel Signaling The first common channel signaling system

(1970s, CCITT No.6 or common channelinteroffice signaling CCIS) used fixed lengthfixed structure messages via a 2400 bit/secondmodem in a pre-designated channel.

Signals in the commonchannel controlled callsetup and disconnect and some routing featuresin any voice channel. Prevented hacker fraud

Still not versatile for future evolution

Replaced starting in 1980s with common channelsystem number 7. Higher bit rate, faster call setup

Versatile for future evolution

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Signaling System 7 Almost a world wide standard

Some variations in versions and features in differentnational telephone networks

High bit rate. Two implementations: Uses one voice channel, carrying 56 kb/s or 64 kb/s

Alternative implementation uses all of a T-1 bit rate(1536 kb/s) except the synchronizing bit.

Flexible Message formats provide for future addition of new

parameters

Reliable Typical SS7 network has extensive duplication of data

packet switches and geographically diverse alternatetransmission channels, and includes continual built-inchannel testing and route around failed links/switches.

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Main SS7 Message Types Standard SS7 uses 4 Message types for ordinary call setup,

but other types exist and more can be added Initial Address Message (IAM) orig->dest

Contains dialed and originators numbers, and otherinformation

Address complete msg. (ACM) dest->orig

Indicates if destination Line is ringing or busy

Answer (ANS) dest->orig

Indicates that destination line has been answered.

SS7 is designed to allow postponing assignment of a voicechannel until this event. This feature uses voice channelcapacity much more effectively but will be implemented

only in the future when the world PSTN is 100% SS7. Release (RLS) and its acknowledgements- either direction

Causes disconnect process.

In addition to the standard parameters of each messagetype, new parameters can be added when required.

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SS7 Services Examples:

Calling Line Identification (Caller ID) Originator telephone number sent as a parameter in theIAM to the destination switch. Sent to destination linebetween first two rings via a modem tone, if notblocked. This was the economic killer app for SS7

Call back to last unanswered caller Call completion to busy subscriber Monitors busy line when denied caller requests, and

establishes connection with caller after it hangs up.

Redirect 800 calls and other pre-designated

calls to alternate numbers When you dial 1 800 DOM-INOS, directs call to the

nearest Dominos pizza retail store. Uses data baseindexed by first 6 digits of the originators number.

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Future Telephone Signaling Voice over Internet Protocol (VoIP) using speech

digitally coded at low bit rate into data packetshas caused great interest in use of the Internetand the traditional PSTN to set up calls. Twoprotocols are in use for these applications:

H.323, originally designed for multi-media andconference calling via Internet, is the firsthistorical method. Similar in many ways anddesigned to be backward compatible with SS7.Uses binary number parameters in messages.

Session Initiation Protocol (SIP) is simpler fornon-telephone experts to understand, and usesalphabetic (ASCII) characters and printablenumeric digit parameters.

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Advanced Intelligent Network (AIN)

Numerous advanced features utilize SS7 messages. Examples: Call completion to busy subscribers (CCBS). Originator dials *66 (auto

redial) upon reaching a busy line, then hangs up.

Destination switch automatically sends a special SS7 messagewhen desired but busy destination person hangs up.

Origination switch rings back the originating/requesting line. Iforigination person answers, origination switch sends a message to

destination switch causing destination line to ring. If destination person answers, conversation proceeds.

SCP data base can translate 800/888/877 dialed numbers intodestination numbers appropriate to the caller, the time of day, etc. Callers to Sears Roebuck, Dominos Pizza, actually reach the

geographically closest retail outlet, or the east coast customer ordercenter in the morning and the west coast center later in the day, so

order takers work shorter hours at each such location! Translation based on origination calling line ID and extensive data base

relating each NPA/NXX to the directory number (DN) of the nearestretail outlet, etc.

This marketing name is misleading, because the network does not actually re-dial the desireddestination repeatedly. It is dialed only once, after the originator answers the ring back.

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Local Number Portability The objective of LNP is to promote local service competition.

The FCC (and some other national telecom regulators such asOfTel in UK) have legally mandated portability of telephonedirectory numbers when a subscriber changes from an existing(donor) LEC switch to a new (recipient) competitive localexchange carrier (CLEC) telephone service provider switch.

In North America, several centralized duplicated SCP/STP data basesprovide a translation between the subscribers DN and a specialdummy number (called a location routing number-LRN) located onthe new recipient CLEC switch.

After this translation, the call is routed to the recipient destinationswitch using the LRNs NPA/NXX. The actualdialed number is carriedin a separate special information parameter in the IAM message.

When the call is routed to the recipient destination switch, thedestination switch connects the call to the internal subscriber linedesignated in a specialtranslation table which relates suchported DNs to the physical lines (ILANs) in the switch used forthat purpose.

Proposed System Beta

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Proposed System Beta

Your instructor proposed a software-based network technology* in 1998to allow allof one subscribers business or residence telephone lines to

each be dialed using the same7 (or 10) digit decimal telephone number. The subscriber may group all business lines (voice, fax, cell/PCS,

pager, data, etc.) under one number and all residential lines(teenagers line, home fax, etc.) under another number

The different lines in a group are distinguished from each other by

means of functional purpose (FP) codes which are mostly pre-set bythe user

FP codes for each line are stored in a suitable data base(perhaps the same data base is used for LNP!)

The internal network SS7 signal messages carry the FP codes inseparate parameters of existing messages

Multi-use lines have a preset normal FP (typically voice) which can betemporarily superceded on individual calls by a dialed prefix (e.g., a faxmachine automatically dials a *329 prefix, indicating that the desireddestination is a fax line during this call only).

*US patent 6,076,121 issued June 13, 2000, and foreign patents pending.

Beta Status

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Beta Status Aside from making it easier to remember all the different telephone numbers of

your correspondents, System Beta reduces the problem of telephone numberexhaustion by reducing the quantity of telephone numbersfor most subscribers tojust one or two, regardless of the quantity of lines in service! In some cases mostof the lines used by a subscriber will have internal non-decimal BCD

digits in their internalnetwork telephone numbers. This is not visible to the end user, but itstops the present growth in use of multiple decimal telephone numbers by subscribers.

Also permits blocking undesired callers by functional purpose (e.g., block allunsolicited sales calls) and other new optional features FCC now considering System Beta due to above benefit and uses for the disabled (e.g.

automatic routing of special 911 or teletypewriter for deaf calls)

Estimated complete one-timedevelopment and installation cost is approximately $7Billion to the telephone industry (mostly administrative and testing costs). At present the telephone industry spends about $1billion each year due to area code

changes, with no end in sight to this recurring cost. Possibly additional $7 Billion in 2005 to2020* to change to 4-digit area codes.

Cost of all area code changes and 10-digit dialing to the public is estimated to be muchmore ($50 Billion to $150 Billion).

System Beta was tabled by the T1S1.3 standards committee which defines NorthAmerican SS7 signaling message standards. Waiting for direction from a major carrier or FCC to take it off the table.

Could permit recombining previously split area codes, restoring 7-digit localcalling, in perhaps 2 to 8 years after installation.

*Industry predictions of date when 11 digit NANP will be needed varies with the source and the dateof estimate

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