©1997-2005 r. levinepage 1 digital telecommunications technology eets8320 smu lecture 11 fall 2005...

©1997-2005 R. Levine

Digital Telecommunications Technology

EETS8320

SMU

Lecture 11

Fall 2005

Switching Features and Reliability(print slides only, no notes pages)

©1997-2005 R. Levine

Overview• Switching software vertical features

– Pictorial state machine descriptions of some vertical features

– More about state machine description

• Other parts of a telephone switch– Trunk signaling– Tone generators and receivers– Conference bridges– Internal modem functions, caller ID

• High-reliability design for telephone switches– Use of internal redundancy– Use of redundant processors for in-service software

upgrades

©1997-2005 R. Levine

Pictorial State Diagram• Recall that there are several ways to represent a telephone switch

as a FSM– The Pictorial point-line diagram is good for visualization– Two other forms: table or flow-chart like

• Pictorial form shown in previous lecture can be extended to illustrate other vertical features– POTS (Plain Old Telephone Service) is the starting point– Additional states will illustrate two widely available vertical services

• Call Waiting: 1 kHz “beep” at 10 sec intervals heard by destination person in existing conversation indicates an incoming call; that person must “flash” to swap active and held line

• 3-way conference: flash by instigator puts pre-existing conversation line on hold, gets “fresh” dial tone. Dial again, then flash to 3-way conference.

Note that “normal” hang up event is omitted from many states in succeeding page diagrams, to simplify diagrams. Where appropriate, it leads to Idle state.

©1997-2005 R. Levine

POTS Example

Idle

callèd

Ringing

replace handset

other caller

abandons

Dial tone

Distant lineanswers

Event: timer expires

Announce-ment: “Please hang up andtry again…”

replace handset

~~ ~~

Inert Howler

Distant line rings or busy.

Lift handset

Dial firstdigit

“Digilator”

~~

Complete validdigits

Collect digits

orConversation w/ black ‘phone [redrawn next page] invalid

digits.

©1997-2005 R. Levine

Call Waiting

Conversation w/ black

White OriginateWhite Answer

Red calls White

Red abandonscall attempt

Flash

~~

~~

10

~~

~~ Blackhangs up

Conversationw/ Red

“Call Waiting”*

*Called “call disturbing” by telecom consultant David Baum, because of the “beep” heard at 10 s intervals.

White hangs up (B hold) [continued next page]

Flash

Redhangs up

Flash~~

~~

~~

~~

Black on holdRed on hold

White hangs up (R hold)

Flash: Goto secondfollowing page

“White” hangs up (R hold)

©1997-2005 R. Levine

“White” Attempts Hang-up with One on Hold

Redhangs up

Black hears ringing toneRed hears ringing tone

White hangs up (R hold) White hangs up (B hold)

White Ringing White Ringing

These transition events come from previous page. Also from following page for green set, not shown.

To idle statetwo pagesprevious

To idle statetwo pagesprevious

Blackhangs up

White answers: go toConversation with Red,previous page

White answers: go toConversation with Black,previous page

©1997-2005 R. Levine

3-way Calling

Black on hold

~~

~~

Black on hold

Dial firstdigit

“Digilator”

Collect digits

or~~

Dial tone [note 2]

Black on hold

Validdigits

Greenanswers

~~

~~

Conference

Flash

~~

~~ Blackhangsup

Black hangs up

Green hangsup: go toconversation with Black, twopages back.

Green hangs up: go to conversation with Black, two pages back.

Note 1: Digit timer ex-piration applies duringDigilator operations. Not shown due to space limitations. Normal hang up returns to Idle State in all states on this page, but not shown. Note 2: So-called “stuttering” or “bouncing” dial tone in some implementations.

Flash causesthis state from p.4

White hangs up: Go to previous page and treat as shown there.

White hangs up: Goto previous page

White hangs up: see note

©1997-2005 R. Levine

Special Notes• The conversation state is treated the same here regardless of

answer or originate previous events– In most real “wireline” PSTN switches, the two states are distinct

• immediate release when originator hangs up vs. • delayed release (~10 sec.) when answering line hangs up.

– consolidating the two states makes our diagram simpler, and some PBX and most cellular/PCS systems actually do not distinguish disconnect time regarding originator vs. answer

• Hang up event not drawn from most states to simplify diagram– Normal hang-up leads to Idle state, where applicable– Hang up from 3-way conference disconnects other two lines in PSTN,

but can leave them connected in many PBX designs.

• Here, flash is described as a event distinct from hang up– Alternative description is possible– The elementary physical events are loop current ON or OFF– The time relationship between these distinguishes hang-up from flash– Within 2 seconds, they can be distinguished– Flash is viewed as a distinct event in many call processing software

designs

©1997-2005 R. Levine

What is a Cradle-switch Flash?• Some proprietary PBX sets use a pushbutton to send a

message instead of using cradle switch.• “Flash” in PSTN is an interruption in loop current:

– duration T seconds; where 1<T<2 seconds– caused by a brief press of the cradle switch (switch hook). Note

distinctions:• A short (~60 ms) interruption is a rotary dial pulse• A longer interruption is a disconnect request

• When loop current ceases, start a 1 second and 2 second timer as first action in an interrupt service routine:

– If current returns ON before 1 sec timer expires, then ignore the interruption and return with both timers cleared.

– If current returns ON after 1 second but before 2 seconds, treat as a flash event. Return with both timers cleared.

– If 2 second timer expires before current returns ON, treat as hang up. Return with both timers cleared.

©1997-2005 R. Levine

Flash in SDL FormAlso a simple illustration of SDL

START: loopcurrent OFF

Note: This routine is an interruptservice routine initiated by the loopcurrent OFF event.

Begin 1 second timer and2 second timer

Wait state

Current ON 1 sec timer

2 sec timerRETURN.No event.

Clear both timers.

Clear 2 sec timer

Current ON

Graphic symbols:

Send a message or signal.

Receive a message or signal.

GOTO: Flash handling.

GOTO:Hang up handling

wait

©1997-2005 R. Levine

Additional Points• Interrupts in most control computers have the ability to be armed

(active awaiting an event) or disarmed (inactive, will ignore designated external event)

– This has not been brought out explicitly in previous explanations– Thus, some types of events or some designated devices may be

intentionally temporarily ignored.

• At “system programming” level, the software must ordinarily arm or disarm specific interrupts as required

– For example, during the first few operations in an interrupt service routine (such as saving the return address) other interrupts are temporarily ignored. This avoids a disruption of the return setup process.

• Most computers also have designed-in priority levels for different types of interrupts and traps

– Memory or CPU error has higher priority than an external interrupt– Thus if two events occur simultaneously, the interrupt service routine

written for the more important one will execute first, while temporarily ignoring the lower priority interrupt, which will eventually execute also.

©1997-2005 R. Levine

Types of FSM Description

• We have seen examples of pictorial point-line FSM and SDL-type description.

• A tabular description is also used in some documentation

• The objectives of these forms are:– Human understanding of the system

• For design• For evaluation and analysis (e.g., is there a feature

conflict?)– Rapid and accurate production of software

• SDL or tabular forms are sometimes better for this than pictorial diagrams

©1997-2005 R. Levine

Partial Tabular Description of POTS State

Event

Idle State Dial ToneState

DigilatorCompoundState

Distant LineRing or BusyState

Ringing State ConversationState

LiftHandset

Go to DialTone State

NotApplicable

N/A N/A Go toConversation

N/A

Hang Up N/A Go to IdleState

Go to IdleState

Go to IdleState

N/A Go to IdleState

Dial InitialDigit

N/A Go ToDigilatorState**

N/A No event N/A No event*

DialSuccessiveDigits

N/A N/A Progressthru sub-states

No event N/A No event

Dial LastDigit

N/A N/A Go toDistant ---Busy State

No event N/A No event

* Note: End to end signaling (to a remote control answering machine) does not involve the telephone switch or network, since tone receivers are removed after the Digilator compound state** Note 2: Also set inter-digit timer. Notice that states invoked by inter-digit timer are not listed here for simplicity.

©1997-2005 R. Levine

FSM Description Comments• Tabular descriptions have low “information density”

– Many entries are “Not Applicable” (meaning you can’t do that in this state) or “No event” (you can do it, but the system does not recognize it as an event in this state, and consequently there is no state change)

• But a table allows a very complete description of the state transition process with nothing omitted for the sake of “pretty pictures”

– Some efforts have been reported to compile software directly from a table description or SDL description (in list form)

• Also some graphic display formats are supported in software development systems (notably ObjecTime™)

– The ITT software pseudo-language CHILL (CCITT High Level Language) was designed to be used for telecom switching software development, but it is not widely used in North America

– Many system level routines are written in assembly language to optimize CPU “speed” and program size

– Many non-system routines are written in widely known languages such as “C++”(used by Lucent) or Protel (a Nortel Networks proprietary language, similar to Pascal)

– Since many projects are “programmer limited,” learning a new programming language is an undesirable step in a project!

– Developers realistically are constrained to use programming languages which are known to the programming staff

©1997-2005 R. Levine

What Else is in a Telecom Switch?• A telephone switch needs a number of related internal devices to work as

designed

– Tone generators and receivers, recorded announcements• Used to generate and recognize audible tones

– Call progress tones:• Dial tones, busy signals, etc.

– Trunk “in-band” signaling such as MF

– Trunk interface signaling conversion• Particularly for robbed-bit trunk signaling vs. internal continuous

channel busy/idle status indication.

– Conference bridges– Internal modem functions

• Used for caller ID in PSTN– Caller digits (and directory name) transmitted on

subscriber loop between first two rings• Used for “modem pool” function in some PBX or cellular

systems having internal digital data transmission

©1997-2005 R. Levine

Audible Tones: Mostly for People• When A.B. Strowger invented the automatic dial telephone

mechanism, he also eventually had to invent dial tone*• Audible call progress tones are standardized in North America

– Telcordia ** documents describe the “Precise Tone Plan”– Most tones are combination of two frequencies, so a limited number of frequency

generators can produce all desired combinations

• Ringing tone is 350 Hz combined with 440 Hz, for example– Cadence (on-off interval timing) and loudness are also standardized

• 2 sec ON, 4 sec OFF for Precise Tone Plan ringing cadence

• Call Progress tones are very different in many other countries– Consequently it is difficult to reliably determine call status by machine

(requires tone filters, digital signal processors, etc.)– Even in North America, monitoring call status by machine using audible

call progress tones is questionable (primarily due to non-standard loudness)

• This was a more significant practical problem in the early days of competitive long distance service, but not as important today for carriers who now have full signaling access (ENFIA-D).

*Some competitors (for example, AT&T) claim the invention of dial tone.**Formerly Bellcore before 4/1/99.

©1997-2005 R. Levine

In-band Signaling• Audible tones are now little used for machine-to-machine

signaling in trunk call processing– Phased out because of slow signaling and susceptibility to fraudulent

manipulation by “Phone Phreaks” (also called “hackers,” “crackers”)– Mostly replaced by common channel No. 7 digital signaling in PSTN

• Multi-frequency (MF) and Dual Tone Multi-frequency (DTMF or Touch Tone) are most widely used in-band methods today:

– Mostly on private networks, leased lines– MF consists of various pairs of 2 frequencies taken from a set of 5– DTMF consists of 2 frequencies taken from a different set of 8– DTMF is standardized world wide

• Also extensively used for end-to-end signaling between various customer equipment

– Example: remote control of answering machine.

©1997-2005 R. Levine

Audio Generators and Receivers• Some very small office (key) telephone systems use a dedicated

DTMF tone receiver on each line– One tone receiver per line is a costly design strategy for a large system

with present hardware technology

• In most switches, shared tone generators and receivers are provisioned on special printed wiring cards and mounted in a digital switch

– Temporary connections are established internally (via the internal switching matrix) between the originator’s voice channel and the tone receiver device during the dialing interval (“Digilator compound state”) only,

• This connection is separate and distinct from the talking connection established later in the call

• Since tone receiver is removed after necessary digits are dialed, advertisers can advise callers to dial memorable (but ignored) extra digits. Example: dial “1 800 AME RICA n” to reach American Airlines. The final “n” is ignored.

Incidentally, telephone numbers that spell a word or company name using letters on the North American dial are sometimes called “anagram” numbers.

©1997-2005 R. Levine

DSP in Telecom

• Today most tone generators use digital signal processors (DSPs) to generate a digital representation of the desired audio waveform

– the audio waveform is represented by a sequence of 8-bit pulse code modulation (PCM) values corresponding to samples taken at intervals of 8000 samples/second (same method used for digital voice coding)

– Versatile DSPs can generate any repetitive waveform desired• New specifications or foreign applications can be accommodated

by installing or modifying contents of a “firmware” chip• DSPs or wired logic devices can also transcode voice waveforms

which are encoded at very low bit rates using other coding algorithms

– Applications include recorded announcements “Please hang up and try your call again…”, etc.

– Also voice message service (trade names like “call notes” etc.), using with mass memory peripherals such as hard disk to store the voice

• Used to generate and recognize audible tones– a DSP can be used to “filter” waveforms, that is to determine the

amount of power in one portion of the frequency spectrum– Call progress tones:

• Dial tones, busy signals, etc.– Trunk “in-band” signaling: MF, DTMF, etc.

©1997-2005 R. Levine

Tone Receiver Function• Receives MF or DTMF tone combinations, and produces a digital

representation of the symbolic value for use by the CPU– Facilitated by use of VERY LOUD DTMF or MF tones (1 or 2 milliwatt DTMF

levels, vis-à-vis sub-milliwatt speech levels). A muting resistor is connected in parallel with the telephone earphone when you press a DTMF button, so the audio is not so loud to the originator.

– DTMF tones must meet minimum duration standards

– Internal coding of dialed digits typically uses a 4-bit code (binary coded decimal -BCD)

• Compatible with SS7 signaling

– BCD digit value is passed to the CPU along with an interrupt in some designs

• Decadic (rotary dial) impulses can be detected and counted by means of• Same hardware on line card already used for supervision detects current

ON, OFF

• Typically a microprocessor handling control of several line cards also verifies timing of impulses and counts the impulses, producing a BCD coding of the dialed digit

– Then passes dialed digit information to CPU internally

• Tone receiver is not needed for rotary dial pulses

©1997-2005 R. Levine

Trunk interface Signaling Conversion• Digital trunk signaling in many cases uses a combination of

methods– This is typical of most PBX installations. Also used in some private

networks, leased lines• Tone “in-band” signaling for dialed digits (via DTMF)• A, B bit signaling for “supervisory” signaling (busy, idle)

– Older (electro-mechanical) equipment used A,B bit replica of rotary dial loop current pulsing (at 10 or 20 pulses/second)

• A and B bit supervision requires trunk interface to synchronize with the 12 or 24 frame sequence established by the binary pattern of successive framing bits

– The least significant bit of the PCM waveform sample data is examined at one correct frame out of each 6 consecutive frames

– Status of this bit indicates channel is busy or idle– Sample of busy/idle status occurs once every 6 or every 12 or every 24 frames

(750 or 1500 or 3000 milliseconds) depending on framing and signaling– Trunk interface hardware is normally designed to present a software control

interface to the internal switch control processor simply indicating busy or idle, and all details of A,B bit synchronization are handled by dedicated hardware.

©1997-2005 R. Levine

Common Channel No. 7 (SS7) Signaling• North American PSTN mostly uses common channel No. 7 (CCS7, SS7) signaling

today– One 64 kb/s channel is dedicated to packet data communication for call

processing of many voice channels (or a 1536 kb/s T-1 link in some cases)– Usually 24th channel on the first installed T-1 link is used for SS7

• Certain high message traffic situations use up to 1.536 Mb/s channels (entire T-1 carrier capacity except the synch bit)

– Packet messages in this channel are separated by means of HDLC* “flag” bit patterns (01111110). Message content is “bit-stuffed” with added binary zeros after each 5 consecutive binary 1s to ensure that the flag pattern never occurs inside the message. Theses “stuff” bits are removed at the end of each link.

– Error checking and automatic re-transmission is accomplished by message transfer part (MTP) software (Level 2 in OSI model). Each packet ends with a checksum error detection code.

– Content of CCS7 messages are passed to/from the control CPU internally– Many defined SS7 message types: used to request a call, report busy/idle

status of telephone lines at each end, and to disconnect. – Message is linked to the correct voice channel by a channel reference number.*HDLC= high data-rate link controlSome authors use distinct abbreviations (C7, SS7, CCS7) to distinguish North American ANSI version of

signaling from ITU/European version. Others use one abbreviation for all versions.

©1997-2005 R. Levine

Advanced Intelligent Network (AIN)

• Packet data switches, some combined with voice switches, some separate, transport the SS7 messages around the telephone network.

– Digital channels used for these messages are either derived from a channel in a digital voice traffic link such as T-1, or sometimes use physically separate T-1 or other links.

– Dedicated packet switches (Signal Transfer Points -STPs) can route and “switch” packets.

• Data bases (service control points – SCPs) can translate destination/ routing numbers to provide new services. Significant early application was “translation” of dialed “800-” numbers (now also 888, 877, etc.)

– Early 800 service required “foreign exchange” connection of special telephone lines to one of a few specially installed 800-service switches.

– Modern 800 service translates the 800 number into a pre-existing “ordinary” directory number, and then routing the call to that customer line.

– The 7-digit (800 NXX-XXXX) translation permits service from any competitive inter-exchange carrier and can be modified based on time, date, or originator’s directory number

• Thus the caller is connected to the nearest Domino’s pizza retail store, or when calling a large nationwide firm, the caller is connected to the east coast office in the morning, and the west coast office in the afternoon

©1997-2005 R. Levine

Local Number Portability -LNP• AIN service facilitates changing your telephone wiring to a competitive local exchange

carriers (CLECs) to offer service from a new switch while retaining your “old” (pre-existing) directory number.

– A large central data base (in a Service Control Point - SCP) contains translation tables which map your “ported” number to a new “recipient”switch NXX.

– When a call is dialed, the origination switch1 sends a data base query SS7 message to this data base to find the new NXX, and your call is routed to this recipient switch. The message used for call setup contains this “ported” directory number.

– The recipient switch has internally pre-established a translation table to translate from the “ported” DN to a line appearance internal code. That line rings, (presumably answers) and conversation ensues.

– A similar result could have been achieved by “call forwarding,” but that would require more directory numbers (bad for the number exhaustion problem) and is the responsibility of the “old” or donor switch (a potential competitive “conflict of interest”)

– These new SCP data bases (duplicated for reliability, but one in each “RBOC” area) mostly have high capacity high data rate links from other STPs and end office clusters. Their operation and data fill is performed by a separate national contractor NeuStar (formerly Lockheed-Martin) which is contractually competitively neutral in the telecom business.

Note 1: In some cases, a later switch in the routing path, called the “N-1” switch, interrogates the data base and consequently re-routes the connection.

©1997-2005 R. Levine

In-band Signaling• Tone receiver or generator for DTMF or MF is needed only for brief

interval at the beginning of a call– The same tone generators and receivers used for subscriber lines can be

temporarily connected to the trunk channels via the internal switching matrix when needed

– PBX typically uses Direct Inward Dialing (DID) • a quantity of trunks is provisioned based on expected incoming/outgoing

traffic• PSTN “spills forward” 3 (or 2 or 4 in some installations) DTMF digits at

beginning of call, taken from last digits of callèd number• PBX completes connection to correct extension via internal switching

matrix– PBX also typically provides audible ringing or busy tones to the PSTN

• At present, audible call progress tones (ringing, busy, etc.) are passed in the assigned voice channel even when SS7 is in use

– Theoretically, when all the switches can work quickly, the assignment of a voice channel could be postponed in a SS7 system until the callèd line answers. Tones could be generated locally only at the originator’s switch before connection.

– This would allow much more “useful” traffic in existing circuit-switched telephone systems, since many (perhaps up to 30%) of all calls are busy or ring-no-answer.

©1997-2005 R. Levine

Conference Bridge• To support a 3-way (or multi-way) conference call, we cannot just “add” the digital

PCM signals from all the participants– First, the binary value of the µ-law (or A-law outside North American and Japan) PCM code is

intentionally “compressed” at the high end of the voltage scale• This is a feature of µ-law coding to use the minimum number of coding bits by allowing

more quantizing error at high instantaneous voltage levels, where it is less perceptible– Second, the PCM code is not in the normal “2’s complement” form for negative values,

required for modern CPU arithmetic.

• Without getting into the circuit level detail, the major functions of a bridge comprise:• A conference bridge converts the 8-bit PCM values into 12 (to 16) bit uniform binary

2’s complement code values, suitable for CPU arithmetic in a DSP or dedicated ALU– For those speakers whose voice is included in the sum, their own voice is subtracted out from

the signal going back to their particular earphone (to reduce “side tone”), but not subtracted out from other participants’ signals

– Appropriate signals are added and sum converted back to µ-law PCM

• Furthermore, in a multi-line conference bridge, the two loudest simultaneous speakers are identified based on a 10 to 20 millisecond sliding time window

– Audio from other sources is suppressed and not added into the total waveform– This prevents adding incoherent background noise and produces a “clean” audio result

• The 3 (or more) participants in the conference call are each connected to a distinct port of the multi-port conference bridge

– Treated as 3 (or more) separate internal connections– If one of the 3 hangs up, the connection is immediately re-directed through the internal

switching matrix as an ordinary connection.

©1997-2005 R. Levine

Internal Modem Functions• In recent years, many digital switches incorporate modem functionality

– The versatile DSP can also produce and receive any modem signal– Many existing tone generators/receivers can be “updated” to modems as well

by provisioning an interchangeable printed wiring card with new and more versatile circuitry.

• Used for vertical feature: caller ID in analog PSTN– The Caller ID signal is transmitted by the switch on an analog telephone line

between the first two ringing bursts via a standard modem tone signal– The information source for the calling number ID is via the SS7 ISUP IAM

message. Optional caller name information comes from the “Line Information Data Base” (LIDB) maintained by local carriers.

• Used for “modem pool” function in some PBX or cellular systems having internal digital data transmission

– Some PBXs have a proprietary internal digital data link encoding format for proprietary digital extensions

– Recent digital cellular systems such as GSM, IS-95 and IS-136 support digital data terminals associated with the mobile unit by using a special digital transmission code (incorporating error protection coding) over the radio link

• To connect to users of standard modems in the PSTN, the switch must provide a data conversion capability including a modem functionality on the PSTN side

©1997-2005 R. Levine

Reliability• Electro-mechanical telephone switches have established a

high level of reliability– Partly due to inherent internal redundancy of control and

signal paths in progressive control switches– Similar levels of reliability are achieved in common control

electro-mechanical designs by appropriate use of redundant modules

• Customers and subscribers expect and deserve high reliability communication– Often literally a matter of life or death– Until recently, there was no backup in the form of competitive

local telephone service, so the burden of high reliability fell completely on one service provider.

• Digital switches designed for PSTN use meet similar failure rate targets as their electro-mechanical predecessors.– Achieved by appropriate use of redundant modules– Also extremely thorough testing and rapid repair from a pool

of spare parts kept on hand.

©1997-2005 R. Levine

How is Reliability Measured?• “Probability of failure” is a vague term, since it does not take

into account the timing or duration of the failure– Every device will likely fail at some time if used long enough

• A more meaningful description is failure rate– Ratio of “down” time to total time based on long term testing of

many samples– We can estimate the failure rate (given certain reasonable

assumptions) of an assembly of devices from knowledge of the failure rate of individual modules• For example, estimate overall failure rate from knowledge of the

failure rate of each resistor, integrated circuit or connector on a printed wiring board,

– “Down time” also depends on rapidity of replacement (time to repair) as well as how often the part fails to work. Depends on time to dis-assemble, locate replacement part, etc.

– “Up time” is also called “availability” in certain contexts

©1997-2005 R. Levine

Definitions

• Find mean (average) of these quantities:• MTBF = Sum of all TBFk/number of samples• MTTR = Sum of all TTRk/number of samples• then: Failure rate = MTTR/ (MTBF+MTTR)

= Fraction of time in “down” (non working) condition

• MTTR depends on “speed” of discovery and repair of a fault, as well as intrinsic module physical susceptibility to failure

Working status

“Up”

“Down”

Time To Repairk (TTRk) Time Between Failuresk (TBFk) Time To Repairk+1 (TTRk+1)

time

©1997-2005 R. Levine

Failure Rate of Combined Modules

• When all modules must simultaneously work (“series” connections) for the entire assembly to work:Combined Failure Rate= sum of individual failure rates [- fraction

of time when multiple failures overlap]When individual failure rates are very small, we can numerically

ignore the double-counting correction term in brackets [ ].

• When multiple redundant modules perform the same task in parallel, and the result is correct so long as just one parallel module can function correctly,Combined Failure Rate: product of individual failure rates [+

fraction of time when only n-1 of the n modules fail but 1 keeps working]• Since worst case failure rates are significant, we make an

error on the conservative side by ignoring the term in [ ]

©1997-2005 R. Levine

“Series” Modules

• Assuming that overall failure rate is the sum of the two individual failure rates does include a double counting of the shaded time interval. When the individual failure rates are low enough so that the probability of simultaneous failure is small, the error from this approximation is slight.

.A B

Double counting interval

A alone

B alone

A with B

Failure intervals on the bottom time line are approximately the sum of the failure intervals on the two individual module time lines above. The intervals of overall failure represent the inclusive OR combination of individual module failures.

©1997-2005 R. Levine

“Parallel” Modules

• The input selector must be made with very high internal reliability to have very much lower failure rate than the overall target failure rate, and there must be an accurate way to determine which module is OK. This method used in telephone switching.

Redundant Modules

Input selector, controlled by diagnostic error detecting equipment (not shown)

Simultaneous failure

A alone

B alone

A with B

AB

Failure interval on the bottom time line is indicated by the overlap of simultaneous failure intervals in all the individual modules. This is approximately equal to the product of all the failure rates. The interval of overall system failure is the logical AND combination of the two individual module failures.

©1997-2005 R. Levine

Designed-in Duplications in Telephone Systems• Power supply is redundant

– In PSTN, commercial electric power (110 or 220/208 V ac) is rectified into dc and used to charge storage batteries

• Lead-acid secondary cells are similar to automobile batteries, but use extremely pure lead, resulting in very long total battery life

– Backup generator with fuel is normally installed (or available on a truck) in case of long-term commercial power failure

– Such batteries also power all electronic equipment in the digital switching and transmission plant, some emergency lighting, etc.

– Different groups of 48 V batteries provide redundant power to different portions of the equipment

– Two battery feeds are brought to each module (shelf) of equipment. • Diode circuits are used to select the battery which is at “highest” voltage. • Thus a failed battery group is automatically backed up

• Hardware design uses mechanical interlocks, to prevent:– turnoff of both power sources simultaneously– Extracting both duplicated modules simultaneously

• Despite (or perhaps because of) this, human error is one of the major present causes of failures

– And occasionally sabotage!– Several telecom outages in 1960s through 1980s (fires, widespread SS7

message congestion, etc.) were found to be due to human operational errors.

©1997-2005 R. Levine

Which Modules are Paralleled?• Internal modules such as the control computer, switching

matrix, tone generators and receivers, etc.• Not port modules leading to outside plant, such as

– Line cards for subscriber loops– Trunk cards for T-1 links, etc.

• These port devices are essentially in a “series” connection with outside plant wiring– Overhead or aerial wire on telephone poles– Buried cable– Buried fiber optics, etc.

• which all suffer to some extent from failures beyond the control of the telephone operation organization. These failure rates are comparable to failure rates of an individual line or trunk card.

– Some modern fiber installations use intentional redundant paths to increase outside plant reliability. Then higher port device reliability is justified.

©1997-2005 R. Levine

Pointless Redundancy

• The failure rate for these outside plant transmission wires or fibers is known to be comparable to (or worse than) a non-duplicated printed wiring card. Causes are

– Cable corrosion, moisture, cuts from careless excavation activity– Vehicle accidents affecting poles, cable, etc.– Human error in wiring installation and repairs– Flood, fire, other natural disasters

• Use of duplicated redundant line or trunk cards would increase the cost of the system without decreasing the failure rate

– For highly reliable service, get 2 distinct outside lines!• When the very lowest failure rate is needed, get diverse

geographical placement (routing) of these 2 lines!

Proposed redundantport (line, trunk)module.

Outside plant module(wire, cable) withhigh failure rate.

©1997-2005 R. Levine

High Reliability with Medium Reliability Parts

• Based on individual component failure rate data, a moderately complex printed wiring board with about 100 components is expected to have a failure rate approximately ~2•10-6; a few minutes in 3 to 5 years.

• By using 2 such boards in a parallel process design, the overall failure rate is approximately the square of the above figure, giving 4 •10-12; or about 1 minute in 475,600 years.

• Digital switches have reliability similar to electro-mechanical switches, although the theoretical reliability is much better. Many failures are due to:– Software

• Extensive efforts are underway to improve the reliability of software as well

– Human errors in hardware and software use• Duplicated control computers require special strategies.

©1997-2005 R. Levine

Special Methods for Redundant Computers• One approach is “load sharing”

– Two or more computers perform diverse tasks– One computer alone can handle an acceptable minimum emergency

load (e.g., support a stated BHCA capacity, perhaps 2/3 of “normal”)– If the other computer(s) fail, the system will at least support a reduced

BHCA level until repairs are made– This is a lower cost approach than complete redundancy

• Combining both the methods of load sharing and fully duplicated individual computers is also feasible, although more costly than load sharing alone

• Other approach is duplicated “hot standby” or “immediate transfer” computer

– Even more than 2 computers are used in some critical applications, such as aircraft navigation control for the SR-21 “Blackbird” fighter airplane, for example. More than 2 computers is considered too costly for less critical applications

• 3 or more (an odd number of) units can use “majority logic” to determine with good accuracy when just one is faulty

©1997-2005 R. Levine

Computer Changeover• With 2 computers synchronized to the same digital clocking signal:

• Comparison of the binary signals on the CPU-Memory Data Bus of the two computers gives a continuous evaluation to ensure that the two computers agree at all times– The comparison module (a combination of XOR devices)

must itself be internally redundant and very reliable• To determine which computer has a fault:

– Disagreement at the data bus level is detected by the high-reliability comparison module

– This is designed to cause a trap signal, which initiates an interrupt service routine containing a rapid test of all the CPU, Memory and peripheral devices. The test program is so designed that it runs to completion in only a millisecond or so when all functions work correctly, but runs longer or possibly gets stuck in a loop without exit if there is a flaw in any of the intermediate calculations.

– The computer which finishes first and correctly is given control (its output is directed to the other devices in the system). The other device is flagged as faulty and its output is not routed to other devices. Alarms ring so that technicians will replace the faulty computer board.

©1997-2005 R. Levine

Minimize MTTRUpon detecting a failed printed wiring board:• Alarms are sounded • On-site technicians are given directions via control

console display to replace the particular suspected bad computer circuit boards– Of course, a full set of good spare circuit boards is kept

at hand at all times!

• The system continues to run using one computer until the replacements are made. Then a special test and start up process is manually initiated to return to use of duplicate control computers.– During the time interval with only one control computer,

the system is more susceptible to failure since there is no backup computer, of course. This interval should be only minutes in duration out of an interval of several months to years.

©1997-2005 R. Levine

Dual Computer Operations• The design of a telecom switch with dual computers is

intended to support– High reliability (as just explained)– Continuous availability

• Even when changing or upgrading software or data tables• When the computer is not fully available, the design

objective is – Continue conversations in progress

• Do not disconnect them– Briefly deny new attempted calls if necessary, with preference

to calls in progress! (long standing telephone industry tradition)

• Some networks (such as SS7) have redundant switches in two different geographic locations so only one will be affected by a local disaster.

©1997-2005 R. Levine

Software Structure• Call processing computer programs often have a

multiprogramming structure• Similar to a general purpose time-sharing computer, but

– Small variety of programs• Line call processing• Trunk call processing

– Many different users• Each user has a semi-permanent data area

– Translation between directory number and equipment number– Class of Service (COS) data: shows if each customer subscribes

to optional “vertical” features such as call waiting, etc.

• Also separate call-related temporary data area– Data relevant to a call currently in progress (dialed digits,

identification of trunk currently in use, etc.)– This data area is freed and used by others when call ends– Temporary data areas are often arranged in a push-down stack

structure

©1997-2005 R. Levine

Load New Software while Running• Software upgrades are usually designed to have backward

compatible data structures– Software upgrades are called “Generics” or “Releases” or “BCSs” by various switch

vendors. Upgrades issued several times each year.– New features obviously require new data elements in the temporary data stacks, but

the data structure is designed so that a new program upgrade can continue to process calls in progress using temporary data generated by the previous version of the software

• To install upgraded software while a switch is in service– Start when traffic is at a minimum (typically late at night: 2 A.M.)– Use installation software which puts call processing control entirely under

computer No. 1 of the two duplicated control computers. Computer No. 2 is now in a standby mode, not doing call processing

• Computer No. 1 continues to write duplicate temporary call related data in the data RAM of both computers

• The upgrade call processing software is loaded into computer 2. When fully loaded, control is transferred to computer 2, and computer 2 continues to handle all calls in progress and any new calls.

• Meanwhile the computer 1, now in standby, also has the upgraded software loaded. When the loading is complete, the two computers are returned to their normal parallel duplicate processing.

©1997-2005 r. levinepage 1 digital telecommunications technology eets8320 smu lecture 11 fall 2005...

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