Transcript Review of Basic Electronics - Lyle School of Engineering

EETS8304 Digital Switching:
Introductory Overview
EETS8304/TC715-N
SMU/NTU
Introduction and Overview
(print slides only, no notes pages)
Rev 1.7;Page 1
©1999-2004 R. Levine
Review of Basic Electronics
• Objective of the course is understanding of
underlying technology.
• About 2/3 of students in come from nonengineering 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.
Rev 1.7;Page 2
©1999-2004 R. Levine
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!
Rev 1.7;Page 3
©1999-2004 R. Levine
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...
Rev 1.7;Page 4
©1999-2004 R. Levine
Digital Switching
Digital 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 DMS10, 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
Rev 1.7;Page 5
©1999-2004 R. Levine
Success of Digital Switching
Due 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.
Rev 1.7;Page 6
©1999-2004 R. Levine
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
Rev 1.7;Page 7
©1999-2004 R. Levine
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
Rev 1.7;Page 8
©1999-2004 R. Levine
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
-
Rev 1.7;Page 9
©1999-2004 R. Levine
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.
Vout
15
Limiting regions
(these are called saturation
or cutoff)
10
-2
Limiting regions
(these are called saturation
or cutoff)
Rev 1.7;Page 10
Vin
-1
-5
0.5
1
-10
{
{
Approximately
linear output
range
5
-15
Approximately linear input range
©1999-2004 R. Levine
2
(volts)
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
voltage
volts)
1
0.5
1
2
3
4
5
6
-0.5
-1
Rev 1.7;Page 11
©1999-2004 R. Levine
time
(milliseconds)
Output Example
• Notice “flattening” of peaks
voltage
volts)
1
0.5
1
2
3
4
5
6
-0.5
-1
Rev 1.7;Page 12
©1999-2004 R. Levine
time
(milliseconds)
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.
Rev 1.7;Page 13
©1999-2004 R. Levine
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.
Rev 1.7;Page 14
©1999-2004 R. Levine
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.
Rev 1.7;Page 15
©1999-2004 R. Levine
Logical Inclusive OR
• Word description: Output C is ON if
either input A or B or both is/are ON.
A
1
C
B
– ON in this example is a HIGH output voltage (typically 5
volts)
– OFF is a LOW voltage (typically 0 to 0.2 volts)
Rev 1.7;Page 16
©1999-2004 R. Levine
Logical AND
• Word description: C is ON when both A
and B are ON simultaneously.
A
&
C
B
Rev 1.7;Page 17
©1999-2004 R. Levine
Logical inversion or NOT
• Word description: C is ON when A is
OFF, C is OFF when A is ON.
A
Rev 1.7;Page 18
C
©1999-2004 R. Levine
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
Rev 1.7;Page 19
©1999-2004 R. Levine
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 data
storage 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
Rev 1.7;Page 20
©1999-2004 R. Levine
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
control signals
CPU
Eleven bytes
of data in the
memory are
specifically
represented by
small
rectangles
Memory
Rev 1.7;Page 21
start
stop
©1999-2004 R. Levine
DMA
I/O
wires
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
control signals
DMA no.1
Memory
control signals
start
stop
Rev 1.7;Page 22
©1999-2004 R. Levine
DMA no.2
T-1 or
E-1
digital
multiplex
links
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
Rev 1.7;Page 23
©1999-2004 R. Levine
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 usercontrolled 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.
Rev 1.7;Page 24
©1999-2004 R. Levine
Schematic Stepper Diagram
• Many details omitted here
3
Rank 0
Rank 9
4
5
6
7
2
Tip, Ring, Sleeve
wires from Rank 8,
column 7.
8
9
0
1
Electromagnets and
springs activate the motions
of the wiper arm in response
to dial impulses.
Rank 8
Rotary Motion due to
rotary electro-magnet
mechanism, not shown.
Vertical Motion due to
vertical lifting electro-magnet,
not shown.
Rank 1
Rev 1.7;Page 25
©1999-2004 R. Levine
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.
Rev 1.7;Page 26
©1999-2004 R. Levine
Other Electro-Mechanical Switches -1
• From about 1920 to 1950 many other electromechanical 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.
Rev 1.7;Page 27
©1999-2004 R. Levine
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
Rev 1.7;Page 28
©1999-2004 R. Levine
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
Rev 1.7;Page 29
©1999-2004 R. Levine
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)
Rev 1.7;Page 30
©1999-2004 R. Levine
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.
Rev 1.7;Page 31
©1999-2004 R. Levine
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.
Rev 1.7;Page 32
©1999-2004 R. Levine
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)
Rev 1.7;Page 33
©1999-2004 R. Levine
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)
Rev 1.7;Page 34
©1999-2004 R. Levine
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
Rev 1.7;Page 35
©1999-2004 R. Levine
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
Rev 1.7;Page 36
©1999-2004 R. Levine
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”
Rev 1.7;Page 37
©1999-2004 R. Levine