©1999-2004 r. levine rev 1.7;page 1 eets8304 digital switching: introductory overview...

©1999-2004 R. LevineRev 1.7;

EETS8304 Digital Switching: Introductory Overview

EETS8304/TC715-N

SMU/NTU

Introduction and Overview(print slides only, no notes pages)

©1999-2004 R. LevineRev 1.7;

Review of Basic Electronics• Objective of the course is understanding of

underlying technology.• About 2/3 of students in come from non-

engineering backgrounds, primarily:– Computer Science, Mathematics– Business Administration or other non-technology

undergraduate degrees• Two forestry majors in the last 10 years!

• Many important and difficult executive decisions in the telecommunications industry require knowledge of technology– Many recent decisions led to later reversals, and

left technologists puzzled regarding motives.

©1999-2004 R. LevineRev 1.7;

Historical Embarrassing Retreats• Several multi-million $ telecom mergers or acquisitions canceled

or reversed (mostly early 1990s) after experience or further study proved unfavorable:– Mantras chanted before reality struck...

• “technological synergy…”• “new paradigm emerging…”• “good managers can manage anything; they don’t need to know the

technological details…”

– Explanations after the difficult reversals• No valuable synergy [of, for example, cable TV and telephone operations in

1991] discovered after careful study– Even without synergy, there is still cable in place and cable does provide a way to

offer competitive telephone or data services

• One supplier with both data processing computers and telecom equipment/service apparently not attractive to customers

– Supplier competing with customers viewed negatively by these customers

• CEO hired from another industry apparently not willing to “learn the ropes” and soon left…

• Note that some times there actually are synergies-- it is important to distinguish truth from illusion!

©1999-2004 R. LevineRev 1.7;

What to do?• Non-technical People Must Learn Sufficient

Technology to do their jobs properly• Typical problem:

– A certain technology is promising but currently very expensive. Example: ADSL for high bit-rate data via telephone wires

– How do customers judge the price/performance of this product versus available alternatives? Cable TV judged adequate at lower cost for entertainment. Internet and data applications are promising, but satellite and LMDS radio are potential competitors

– Will the cost drop in future? Is the cost forecast technologically reasonable and quantitatively accurate?• Semiconductor costs will likely drop due to large scale

integration. Cost of printed wiring boards, transformers,, and software development may not drop...

©1999-2004 R. LevineRev 1.7;

Digital SwitchingDigital telephone switches entered the public switched telephone

network (PSTN) in early 1970s– Followed digital transmission (T-1) success, and some

pioneering digital PBX switches– Distinct from electronic but not digital switches (like 1ESS)

that use computer control but analog electromechanical switching

– First PSTN application in transit/tandem switches (Lucent - then Western Electric – Model 4ESS switch)

– Then digital end office (“Class 5”) switches: Nortel DMS-10, DMS-100, Lucent 5ESS, etc. in late 1970s, early 80s

• Technological change permitted Nortel to open the US PSTN switching market, almost a previously de facto closed monopoly

– Digital switches had significant advantages in flexibility, smaller size, greater reliability (and incorporated automatic testing), lower power consumption

©1999-2004 R. LevineRev 1.7;

Success of Digital SwitchingDue to a combination of:• Theoretically flexible concepts such as stored program

control (SPC)– Intended to provide open-ended future development of new

capabilities, but in practice the complexity of software development and shortage of skilled programmers has limited this somewhat

• Available semiconductor technology, prior introduction of digital multiplexed telephone transmission

• Product design well focused on market needs– Digital switches are much smaller in size, power consumption than

electro-mechanical predecessors- typically 8:1 floor area ratio– Include important capabilities such as built-in test. Although not

inherently “digital,” these capabilities are valuable and came at the right time, and compensated for rising labor costs

• Culture of the telephone industry historically accepts and encourages “automatic” (e.g., no “operator assisted”) technology. Contrast with clothing manufacture, hairdressing (frisure), restaurants, etc.

©1999-2004 R. LevineRev 1.7;

Review of Semiconductor Devices• Semiconductor technology allows practical

electronics applications not feasible 40 years ago, due to:

• Low power consumption– Older vacuum tube technology required continuous

filament power

• High reliability– Older vacuum tube technology required frequent tube

replacement/maintenance

• High Component Density– Millions of transistors on a single integrated circuit chip

permit desktop computers, complicated cellular or PCS handsets, not feasible with earlier devices

©1999-2004 R. LevineRev 1.7;

Digital Logic Devices• Electronic devices and components can be

classified into two categories– Linear: resistors, capacitors, inductors, transformers,

transmission wires and cables– Non-linear: diodes, transistors of various types, etc.

• Linear devices have output (current, voltage, etc.) directly proportional to input signal (when operating within a useful voltage or current range)– Audio amplifiers (high fidelity)– Radio amplifiers (cellular and PCS systems)

• Non-linear devices have regions where output is not proportional to input

• Digital electronics mostly exploits non-linear behavior

©1999-2004 R. LevineRev 1.7;

Non-linear Example• Electronic amplifier, constructed using

transistors (interior details later in semester)• Graphic symbol (often simplified by omission of

red colored lines at the bottom, the “common ground”)

Vin Vout

+ +

- -

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Input-Output• Represented approximately via a graph of input voltage vs.

output voltage– This ignores certain details concerning time delay of signal

inside amplifier, “noise,” etc.

Vin(volts)

Vout

1 2-2 -1 0.5

15

10

5

-15

-10

-5{{

Approximatelylinear outputrange

Approximately linear input range

Limiting regions(these are called saturationor cutoff)

Limiting regions(these are called saturationor cutoff)

©1999-2004 R. LevineRev 1.7;

Input Waveform• Typical of speech waveforms• Amplified (produces an output signal which is essentially

the same wave form scaled up in voltage) when the voltage is in the linear input range

• Waveform is not reproduced accurately if a larger input voltage range is used, exceeding the linear input range

time(milliseconds)

voltagevolts)

1

-1

0.5

-0.51 2 3 4 5 6

©1999-2004 R. LevineRev 1.7;

Output Example• Notice “flattening” of peaks

time(milliseconds)

voltagevolts)

1

-1

0.5

-0.51 2 3 4 5 6

©1999-2004 R. LevineRev 1.7;

Digital Coding

• For standard digital public telephone network purposes, the analog waveform of previous page is:– Sampled (voltage is measured) 8000 times per

second (125 µsec intervals)– Each voltage sample is digitally encoded as an

8-bit binary number– Each bit is transmitted sequentially as high or

low voltage pulse (symbolically 1 or 0)– Details available in EETS8302 notes, etc.

©1999-2004 R. LevineRev 1.7;

Boolean Algebra• In 1938, Claude Shannon (1916-2001) wrote a Master’s degree

thesis describing how to use the logical algebra, developed in the 19th century by George Boole, to systematically design electromechanical relay circuits

• Boole was a colleague of Charles L. Dodgson* at Cambridge University, where they studied mathematical logic and indulged in various word and puzzle games

• Shannon’s method automatically produced a workable design for any logical system which can be described by a list of states for each input. No “inspiration” or creative genius is needed.

– Example: An elevator has two direction-of-travel (DOT) states: up and down. If it is traveling up and a user presses a button for a floor above the present floor, it will stop at that floor. If the user presses a button for a floor below that floor, it will go up first to the highest previously requested floor number, and then reverse its DOT state to down, and then stop at all the floors, stored earlier, which could not be served when it was traveling up.

*Dodgson is better known under his pen name Lewis Carroll, as the author of Alice in Wonderland, etc.

©1999-2004 R. LevineRev 1.7;

Building Blocks

• We will show the use of three basic Boolean logical building blocks (devices, components, “gates”)

• Logical Inclusive OR• Logical AND• Logical inversion or NOT

– Other methods are also in use, starting with other basic building blocks. Our presentation simplifies design choices even though “real” engineers use more sophisticated methods as well.

©1999-2004 R. LevineRev 1.7;

• Word description: Output C is ON if either input A or B or both is/are ON.

– ON in this example is a HIGH output voltage (typically 5 volts)

– OFF is a LOW voltage (typically 0 to 0.2 volts)

Logical Inclusive OR

A

BC

1

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Logical AND• Word description: C is ON when both A

and B are ON simultaneously.

&A

BC

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Logical inversion or NOT

• Word description: C is ON when A is OFF, C is OFF when A is ON.

A C

©1999-2004 R. LevineRev 1.7;

Particular Applications

• The digital logic designs produced via Shannon’s Boolean algebra method perform the designated task, but may not be optimum with regard to various criteria:– Minimum component count– Minimum time delay of the signal– Minimum electric power consumption

• There are alternative design methods, and still some place for human creativity

• We will show several applications with simple (but not necessarily optimum) logical designs

©1999-2004 R. LevineRev 1.7;

Some Digital Applications

1.Symbolic arithmetic using binary numbers, to ADD, SUBTRACT, etc.

2.Store and retrieve binary data in addressable memory a numbered storage location for each item of datastorage organized into “bytes” or “octets” (8-bit groups)

3.Simple example of a multi-purpose arithmetic logic unit (ALU)performs different operations (ADD, logical AND, etc) on two

inputs as controlled by a number code (operation code)

4. Combine these items to make a simple programmable computer (conceptual description)– Aside from ALU, requires a sequence controller– Memory used to store:

• Data to be processed, data results• Codes representing the program steps or operations

©1999-2004 R. LevineRev 1.7;

Input/Output (I/O)• Both computers and digital switches use similar I/O devices• Early computers moved all data in and out of memory via the ALU and

central processor unit (CPU)

• Later computers (1960s onward) incorporated separate direct memory access (DMA) hardware to handle I/O

• CPU sets up a starting address and a block size (or stop address) in DMA, then DMA autonomously accesses memory byte by byte until the entire block is input or output

CPU

DMAMemory

control signals

I/Owires

start

stop

Eleven bytes of data in the memory are specifically represented by smallrectangles

©1999-2004 R. LevineRev 1.7;

Telephone Circuit Switch• Very similar to computer DMA but:

– In computer, DMA device accesses memory bytes at sequential address

– In circuit switch, one DMA device stores bytes from digital input in sequential addresses, other DMA extracts bytes in a different non-sequential order as controlled by control signals

DMA no.2Memory

control signalsT-1 orE-1digitalmulti-plexlinks

start

stop

DMA no.1

control signals

©1999-2004 R. LevineRev 1.7;

Time and Space Switching• A result of the sequential and non-sequential data I/O operations

is a re-arrangement of the various bytes in their time order• This structure is known as a digital time switch• A similar structure with 3 or more I/O ports can be used to route

incoming bytes to one of the two output ports. This is called “space switching” as well as time switching, since the switch can chose different parts of space (different ports) to send the output

• Electromechanical analog switching is always space switching because the only switching operation is the choice of different output ports. No memory implies no time switching.

• Certain small digital or sampled-data analog switches perform only time switching in some parts of their structure

– Switches which connect to individual analog telephone subscriber lines must ultimately perform space switching to select the proper telephone line

©1999-2004 R. LevineRev 1.7;

Historical Strowger Step-by-step Switch• Almon B. Strowger, a mortician in Kansas City, KS, invented the

first practical automatic dialing system, installed in LaPorte, Indiana, ca. 1895

– Famous story: fearing that the human operator was always directing calls for a mortician to his competitor, he invented an automatic user-controlled telephone switch

– First version used extra wires and push buttons at each telephone – Rotary dial with impulsive current on the voice wire pair was a later

development

• Strowger’s manufacturing firm later took the name Automatic Electric, later absorbed by GTE, later moved to Phoenix, Arizona (now AG Communication Systems,* affiliated with Lucent)

– “Stepper” progressive control switches were manufactured world wide for many decades

– Electromechanical common-control switches, initially designed by other competitor vendors, such as “panel” and “crossbar” types succeeded steppers in the 1930 - 1960 decades

* An interesting working exhibit of a Strowger step-by-step system is in the lobby of their building in Phoenix.

©1999-2004 R. LevineRev 1.7;

Schematic Stepper Diagram• Many details omitted here

Vertical Motion due tovertical lifting electro-magnet,not shown.

Rotary Motion due torotary electro-magnetmechanism, not shown.

Rank 1

Rank 9

Rank 0

1

23

4 5 6 7 89

0

Tip, Ring, Sleevewires from Rank 8,column 7.

Electromagnets andsprings activate the motionsof the wiper arm in responseto dial impulses.

Rank 8

©1999-2004 R. LevineRev 1.7;

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

switch “inlet” and 100 “outlets” (tip,ring wire pair with “sleve” wire)– 10 contact pairs (Also a third “sleeve” wire in addition) arranged in a horizontal arc,

selected by rotating the wiper switch arm. – 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” single motion switch acts as input concentrator (reverse of selector action)

– Wiper arm contacts act as the single outlet– Line finder single-motion rotary stepper wired to 10 subscriber lines,

selects the line that goes off-hook• Stepper starts stepping from line to line when any of the 10 lines go off hook,

then stops when correct “off-hook” line is “found”• If an originating call engages the connection on one line finder, a second or

third line finder will handle the next originating call from that group of 10 lines• 10 parallel line finders are needed to allow non-blocking origination

– analogous to operator responding to buzzer and light– Multiple line finders wired to same 10 telephone sets analogous to multiple

operator stations with each having access to the same subscriber sockets.

©1999-2004 R. LevineRev 1.7;

Other Electro-Mechanical Switches -1• From about 1920 to 1950 many other electro-

mechanical switches were designed• Stromberg-Carlson XY Switch: Gross motion switches

involved two-dimensions of motions over a plane surface with 10x10, or 100 lines, like an “unrolled” Strowger switch. Plane surfaces were stacked more tightly to use less building (central office) space.

• The AT&T “Panel” switch used two electrically operated clutches (similar to the electric clutch in an automobile air conditioner) and a continuously rotating electric motor, to move a contact arm in a 10x10 plane. An ill-fated device due to heavy maintenance needs.

©1999-2004 R. LevineRev 1.7;

Other Electro-Mechanical Switches -2• Crossbar “fine motion” switch. Made by AT&T and

Ericsson under cross-licenses 1930s-1960s. Contacts are supported on armatures that rocked or rotated through a small angle to make contact with one of two lines. Path through several stages of such rocker switches allowed connection of caller and called lines. Notable because the dialed digits were counted by separate relay circuits and a “common control” relay structure (predecessor of computer control) set up the connection path in the switch.

• All-relay switches were used for small installations like a Private Branch Exchange (PBX)– A relay comprises one or more switch contacts that can be

electromechanically opened or closed by the magnetic force of a current-carrying coil of wire

©1999-2004 R. LevineRev 1.7;

Human Interface of Telephone Switch-1• Each new generation of telephone switching (electronic,

digital, cellular) was designed so that it had the same human interface for dialing, ringing, answering, etc. (as much as possible– no “dial tone” for cellular!)

• The earliest telephones (ca. 1877) required some type of loud alerting device to call the destination person from across the room to the telephone set for a conversation. – Various improvements ultimately led to the use of relatively high

voltage alternating current “ringer” for wired telephone systems, and a similar loud alerting sound for cellular radio telephones, etc.

• Hands-free telephone technology makes automatic answer feasible, but this is only used inside a business among participants who implicitly agree to be disturbed by such a call at any time– A “do not disturb” option button is typically provided

©1999-2004 R. LevineRev 1.7;

Human Interface of Telephone Switch-2• This two step process (non-voice alerting followed by voluntary

answer and conversation) fits well with the user’s concept of desiring control over answering and use of the telephone.– The social concept of suddenly speaking to a person not acquainted

with the caller was a new and somewhat uncomfortable concept in the 1880s

– Thomas Edison is frequently credited with popularizing the word “Hello” when originating or answering a telephone call

• Some cultures use other terms such as:– “Jones here” Identifying the answering person– “Pronto!” (I am ready) in Italian– “¡Digame!” (speak to me, tell me) in Spanish

• Two extensions of this answering process have evolved:– More private: pre-answer visual “caller ID” – Less private: Automatic answer for “Push to talk” radio, used between

pre-consenting subscribers (members of a military unit, or members of a work crew such as a dispatcher and worker)

©1999-2004 R. LevineRev 1.7;

Caller ID Properties• It has proven to be one of the most popular and lucrative optional

telephone services in the last decade.– Originating caller’s number and optionally directory name are

transmitted to destination telephone via a modem tone signal between the first two ringing bursts. Detected and displayed by means of modem receiver and alphanumeric display

– Income from Caller ID has justified the almost-complete upgrade of the North American PSTN to SS7 signaling (discussed later in the course)

• ISUP version of SS7 signaling transmits the originator’s telephone number to the destination switch. There are multiple uses for Caller ID data.

– An existing data base (Line Information Data Base – LIDB) was already available to find the directory listing name from the originator's number

• Typically only available when originator is in the same RBOC operating company area as the destination

• Social controversy: When Caller ID was introduced in late 1980s, many subscribers felt entitled to block display of their originating number without cost. This option is therefore available on a permanent or per-call basis.

©1999-2004 R. LevineRev 1.7;

Push To Talk - PTT

• Military walkie-talkies, vehicle dispatcher systems, and other early radio systems shared only one channel in a “half duplex” manner: voice transmitted in only one direction at a time. Transmit manually controlled by a Push-to-Talk button.

• NexTel, using Motorola iDEN technology, allowed both traditional telephone service (dial, ringing, answer, with full duplex conversation) and also PTT. PTT has immediate half duplex connection to designated individual or group destination.

– Much faster connection than dial, ring, answer. Intentionally lower speech coder quality.

– Emulates earlier analog or other PTT system that typical niche user is familiar with

– Designed for certain niche markets such as ambulance, taxicab, repair crew, etc.

• Popularity of NexTel PTT for its niche market has led both CDMA (Sprint and Verizon) and also GSM technologies to include a PTT option for those users who desire it.

• As in hands-free wired telephones, PTT automatic answer is socially acceptable only within a pre-designated group of subscribers who implicitly agree to accept such calls.

Note: PTT is also an abbreviation for Post, Telephone and Telegraph administration in some governments.

©1999-2004 R. LevineRev 1.7;

Digital and Electronic Switching• Most large telecommunication switches built since the 1960s are

electronically controlled (stored program control: SPC) by means of a dedicated control computer

– Examples: 1ESS, ESS-101, GTX, SP-1, DanRay

• Some of these perform(ed) switching via electromechanical crossbar switching, sealed reed relay contacts, or sampled-data analog waveforms.

• Digital switches use a time switch or space-time switch to direct digital bits to and from the proper ports in the proper time order

– Examples: DMS-10, DMS-100, ROLM PBX, 4ESS, 5ESS

• SPC switches can have many new features added by “merely” upgrading and modifying the call processing control software

– About 80% of the technical staff at the many telecom firms in the Dallas-Ft.Worth area primarily design and develop switching software

– Shortage of skilled programmers is the limiting factor in most system development projects today

• Some features require new or special hardware as well (example: conference bridge for multi-party conference calls)

©1999-2004 R. LevineRev 1.7;

Software for Switching• This course gives only an introduction to

switching software• SMU offers separate courses devoted entirely to switching

software development (EETS8305) and to PCS/cellular

– Switching software is controlled by real-time events (callers dialing digits, etc.) and must respond quickly

– Telephone switching software is characterized by many subscribers who can, in principle, do the same generic things (establish connections) but with different specific ports and time-slot channels

• Multiprogramming and multiprocessing software structures are useful here, with data structures which are dynamically constructed to serve all currently active subscribers

– Reliability requirements are very high, particularly in the public switched telephone network (PSTN)

©1999-2004 R. LevineRev 1.7;

Some Network Switching Features• Digital switching systems make extensive use of translation via

data tables contained in memory– Subscriber telephone directory number is related to a particular port

via a data table. Port is defined via an internal number comprising the number of the particular rack of equipment, the particular shelf, and the particular plug-in printed wiring card on that shelf.

– When subscribers move or relocate to different lines or ports (on same switch), a change can be made in the corresponding table, rather than re-arrange wiring at the central office building.

– In long distance networks, the dialed number may often be translated into a completely different destination telephone number via a translation table in a data base

• Many 800 and 888 toll-free numbers are translated based on the calling central office code, so that a caller who dials the “800” number of the US Postal Service, Sears Roebuck, or Domino’s Pizza will actually be connected to the nearest “retail” store or location

• Calls may be routed to different offices of a firm in different time zones at different hours of the day, to serve callers over a longer work day than could be accomplished with one office location

©1999-2004 R. LevineRev 1.7;

Digital Switching in Cellular and PCS• Digital switching is used with all present cellular and PCS

systems– Analog switches were used with prototype (late 1970s) analog

FM cellular systems, but the next generation and all since are digital, primarily for the same general economics-based reasons as other telecom applications

– Newest generation of PCS uses digitally coded speech over the radio link, so internal digital switching is valuable technologically as well.

• PCS requires continually changing the identity relationship between the subscriber’s handset and the radio channel – Roaming service requires location of the subscriber

possibly anywhere in the world– Handoff/handover (transfer of a call from one base

station to another) may occur during a conversation

©1999-2004 R. LevineRev 1.7;

2G, 2+1/2G and 3G• Cellular radio technologies using digitally coded

speech (called 2nd generation -- 2G) were introduced in early 1990s– Examples: IS-136 (also called TDMA), GSM (also named

PCS-1900 in North America), IS-95 (CDMA)

• Packet-data technologies at moderately high (typically up to 384 kb/s) data rates, called 2+1/2G, now being introduced, based on GSM or IS-95 CDMA technology

• Packet data using very high bit rates (approx. 2 Mbit/s or more) mostly with CDMA radio technology, called 3G.– 2+1/2G (also written 2.5G) was designed last as a lower

cost, easier migration technology, by “3G doubters”