EETS8304 Digital Switching: Introductory Overview

1
EETS8304 Digital Switching Introductory Overview

• EETS8304/TC715-N
• SMU/NTU
• Introduction and Overview
• (print slides only, no notes pages)

2
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.

3
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 dont
  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!

4
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...

5
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 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

6
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 81 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.

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

8
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

9
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
-
-
10
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

5
Approximately linear output range
Vin (volts)
1
2
-2
-1
0.5
-5

-10
-15
Limiting regions (these are called saturation or
cutoff)
Approximately linear input range
11
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
time (milliseconds)
1
2
3
4
5
6
-0.5
-1
12
Output Example

• Notice flattening of peaks

voltage volts)
1
0.5
time (milliseconds)
1
2
3
4
5
6
-0.5
-1
13
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.

14
Boolean Algebra

• In 1938, Claude Shannon (1916-2001) wrote a
  Masters 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
• Shannons 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.

15
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.

16
Logical Inclusive OR

• 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)

?1
A
C
B
17
Logical AND

• Word description C is ON when both A and B are
  ON simultaneously.

A
C
B
18
Logical inversion or NOT

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

A
C
19
Particular Applications

• The digital logic designs produced via Shannons
  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

20
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

21
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
I/O wires
Memory
DMA
start
stop
22
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

T-1 or E-1 digital multi- plex links
control signals
control signals
Memory
DMA no.1
DMA no.2
start
stop
23
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

24
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
• Strowgers 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.

25
Schematic Stepper Diagram

• Many details omitted here

Tip, Ring, Sleeve wires from Rank 8, column 7.
6
5
4
7
Electromagnets and springs activate the
motions of the wiper arm in response to dial
impulses.
8
3
9
Rank 0
2
0
Rank 9
1
Rank 8
Rotary Motion due to rotary electro-magnet mechani
sm, not shown.
Vertical Motion due to vertical lifting
electro-magnet, not shown.
Rank 1
26
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.

27
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 ATT 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.

28
Other Electro-Mechanical Switches -2

• Crossbar fine motion switch. Made by ATT 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

29
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

30
Human Interface of Telephone Switch-2

• This two step process (non-voice alerting
  followed by voluntary answer and conversation)
  fits well with the users 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)

31
Caller ID Properties

• It has proven to be one of the most popular and
  lucrative optional telephone services in the last
  decade.
• Originating callers 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
  originators 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.

32
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.

33
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)

34
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)

35
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 Dominos 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

36
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 subscribers 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

37
2G, 21/2G 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
  21/2G, 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.
• 21/2G (also written 2.5G) was designed last as a
  lower cost, easier migration technology, by 3G
  doubters