Digital Telecommunications Technology - EETS8320 Fall 2006

1
Digital Telecommunications Technology -
EETS8320Fall 2006

• Lecture 1
• Overview and Introduction
• (Slides with notes.)

2
Introduction EETS8320

• Subject Area Digital coding and multiplexing of
  telecommunications transmissions (formerly in
  course EETS8302)
• Digital telecommunications switching (formerly
  in course EETS8304)
• Descriptive and semi-technical treatment
• About 70 of our students do not have an
  engineering or science undergraduate degree,
  although many work in the telecom industry.
• Each student can write a term paper at a
  technical level appropriate to their own
  background and knowledge

3
Course Administrative Matters

• 13 weeks of class each 3 hours (consecutive),
  with slides and notes
• Each student takes a multiple-choice midterm quiz
  (1hour) and writes a term paper (approx 20 pages
  or 5000 words) on a pre-approved topic.The letter
  grade on the term paper is substantially your
  final grade.
• If your midterm quiz numerical grade is above
  average your final letter grade is increased by
  one step on SMUs grade scale Example, B ? A-
• If your midterm is below average, no deduction is
  made. Your paper grade is then your course grade.
• Notes explain details.

4
Course Objectives

• One objective to give students sufficient
  understanding of the technology to make
  intelligent decisions in the present and future
• This course is focused on science and technology,
  because understanding technology is important.
• Adequate understanding of both technology and
  business are very important in the
  telecommunications industry.
• Knowledge of business-economics alone is not
  sufficient!
• Knowledge of technology alone, with ignorance of
  economics is also not sufficient!
• The Iridium system, ISDN and In-flight telephones
  are un-successful telecom products often cited as
  examples of economic or technological bad
  judgement.
• Human interface (ease of use) is also a factor in
  some cases.

5
Problem Products Iridium

• Iridium, a world-wide direct satellite telecom
  system of 1990s
• Technologically impressive, but
• Priced higher than most potential customers would
  pay
• Handset 3000 (price later significantly reduced)
• Service 3/min or more (price later slightly
  reduced)
• Designers and implementers were aware of possible
  low sales risk due to high prices.
• Unexpected low cost terrestrial competition in
  populated areas harmed Iridum .

6
Iridium More Recent History

• High-budget Customer Base was never large enough
• For example, oil exploration crews in Siberia?
  Very few of these!
• Native farmers in Kazakhstan? They could not
  afford Iridium.
• Callers from an ocean liner? Sounds promising
• Existing Inmarsat satellite telephone calls are
  10/minute!
• But e-mail to/from most ocean liners is free!!
• If you build it, will they come? Apparently, No!
• Customer enrolment was only a tiny fraction of
  Iridium managements estimates. Several top
  Iridium executives resigned.
• Iridium Corp. filed for Chapter 11 bankruptcy
  protection in August, 1999. This allows continued
  operation while a plan is made to hopefully
  reorganize and eventually pay creditors
  (bondholders, etc.). Shareholders are not
  protected in Chapter 11. Reorganized as Iridium
  Satellite LLC in Dec. 2000
• US Government subscribers are almost the only
  present users, while Iridium operates in
  reorganization.
• Development of several competitive LEO satellite
  systems (e.g. Globalstar) stopped. Licenses were
  cancelled or returned to the FCC.

7
Problem Products ISDN

• Fully digital end-to-end telecommunication via 64
  kbit/s channels derived from pre-existing digital
  telecom channels
• Was viewed as the unquestioned future direction
  of PSTN voice-data service in the 1980s.
• Bone of contention among major telephone switch
  manufacturers.
• Ultimately in limited use but not in consumer
  demand due to high cost
• Unanticipated availability of low-cost 53 kbit/s
  V.90 modems in 1990s diverted many potential
  customers

8
A Page from Economic Theory
100
100
D1
Motor Fuel.
In-flight Phone Service.
Quantity bought or sold
Quantity bought or sold
.W
.
Quantity minutes bought or sold
50
50
.
S2
30
Z
30
S1
D2
15
0.50
1.00
0.50
1.00
.25
Y
Unit price/min ()
X
Unit price (/gal)
Unit price ()
A
C
B

• Supply-demand curves graphically describe how
  buyers and sellers find an equilibrium price per
  unit and quantity sold. See notes.

9
Problem Products In-Flight Telephone Service

• Very costly to install due to severe aircraft
  radio interference standards.
• Originally allowed only aircraft-originated
  calls. Aircraft destination calls are supported
  in some systems in a somewhat inconvenient way..
• None were ever profitable. Many competitive
  systems addressed improved convenience, video
  games, etc., but not low price.
• Test market studies show that sales improve
  dramatically below 20 cents/min, but existing
  systems can't meet that price.
• Verizon Airfone plans, after 21 years, to
  discontinue service by the end of 2006. In-flight
  phone service will end on over 1,000 aircraft
  operated by United, Delta, Continental, US
  Airways, Air Canada and Cathay Pacific.
• Proposed systems using customers' existing cell
  phones inside the aircraft are still in
  preliminary development.
• Use of VoIP via Internet links (priced at
  10/h)) provided in some flights is another
  alternative
• Historical note First (analog) Airphone system
  developed by Jack Goeken, also famed as founder
  of MCI.

10
Customer Perception of Fairness is Important

• Some system proposals did not succeed due to
  negative customer perception of fairness
• Two types of limited play video disks were test
  marketed circa 1998 as no return methods for
  video rentals. Both rejected by customers.
• System software for wireless air time charges
  paid by land-line originator were developed, due
  to industry pressure circa 2000, but 100 of
  participants in US marketing tests would not
  choose this billing method.

11
Concerns about 3G Wireless

• Some telecom industry observers fear that 3G, and
  other advanced wireless data technologies, will
  suffer fates like those of Iridium and ISDN.
• First Generation (1G) wireless was analog
  cellular technology used from1981 to mid 1990s.
  Very few still in use.
• Second Generation (2G) wireless utilizes digital
  speech coding, used from early 1990s to the
  present. Technologies include GSM, CDMA, North
  American TDMA, iDEN (NexTel) and others. Range of
  available bit rate per user is about 6 kbit/s to
  20 kbit/s
• Two and a Half (2.5G or 2-1/2G) designs are
  packet data systems, achieving available bit rate
  per user up to about 60 kbit/s to 140 kbit/s.
  Used for Internet access and packet voice (VoIP).
• Third Generation (3G) utilizes various types of
  CDMA (UMTS, CDMA2000). Provides bit rates up to 2
  Mbit/s. Major applications are viewing high
  quality visual entertainment (HDTV images) or
  possibly transferring massive files via Internet.
• Fourth Generation (4G) utilizes OFDM to achieve
  16 Mbit/s or higher bit rates. Applications
  similar to 3G but even faster.

12
2.5 G

• These 3G sceptics believe that the major growth
  in cellular industry will come from lower cost
  voice service and less glamorous data services
  like e-mail.
• They therefore designed a packet data technology
  upgrade based on GSM, called GPRS (and a higher
  data rate version named EDGE), called 2.5G, or 2
  and a half G.
• The cost of installing GPRS or EDGE in an
  existing GSM base system is relatively small. (In
  contrast, all 3G systems require costly new or
  additional base radio replacements.)
• GPRS upgrades are already in place in some
  countries in Europe, N.America, Asia, Australia,
  etc.
• Use of GPRS (later EDGE) in USA (by ATT and
  Cingular) to replace IS-136 TDMA is under way
  today. Voice Stream (T-Mobile) has a GSM starting
  point technology and thus a less costly upgrade
  to GPRS and EDGE. Merger rumors between
  GSM-technology firms are continually rife.
• GPRS provides up to 171 kbit/s per subscriber,
  EDGE up to 384 kbit/s.

13
Other Aspects

• Previous slides did not analyze or compare
• Radio bandwidth required for higher bit rates.
• Sensitivity of each technology to noise and
  interference limits the number of simultaneous
  conversations in each cell and thus the system
  capacity
• Cost and complexity of each technology
• Power consumption in active and standby modes,
  affecting battery life.
• An accumulation of negative aspects like these
  can severely degrade the theoretical performance
  of real systems

14
Customer Preference Issues

• In some cases, potential customers won't buy
  because they perceive cost or terms of sale
  inherently unattractive. Examples
• Today North American wireless subscribers view
  air time costs over about US 0.20/min as
  excessive
• North American telephone users reject caller
  pays for wireless destination calls (although
  this is accepted in many other countries).
• In a non-telecom case, customers reject a self
  destructing or pay per view video disc.

15
Sometimes Non-Technical Problems Dominate

• Morse code telegraph was not practical for
  most end users because of the special skill
  required to send with a key and receive by
  listening to di-dahs
• Electro-mechanical Teletypewriter machines only
  require the ability to read and type (keyboard)
  but they were costly, bulky and noisy.
• Telephone station sets were always relatively
  small, quiet when idle, and require only the
  ability to speak and hear understand the language
  of the other person.
• Special teletypewriters are available for deaf or
  hard of hearing telephone users.

16
How Does Customer Perceive Acceptable Price vs.
Performance?

• In some cases, the technical performance is
  adequate, but end users perceive the price as
  excessive and wont buy the product.
• This non-technical aspect of product development
  is supposed to be addressed by customer surveys,
  focus groups, etc., but sometimes they predict
  incorrectly.
• Telecom items perceived as overpriced
• Iridium originally charged 3000 for a handset
  and 3 per minute air time
• Scheduled airline in-flight telephones. Those
  systems still charged at least 2 or more per
  minute,due to high operating costs.
• Most end users (apparently) wont pay over about
  0.10 to 0.20/minute for air time

17
Best to Understand Technology Yourself

• Make well-founded decisions yourself
• Less dependence on the opinions of others
• Your instructor earns most of his income from
  being a technology expert consultant, but would
  still rather have his clients understand the
  technology themselves!
• Separate the wheat from the chaff when
  exaggerated product claims are made
• Make realistic and profitable product and service
  plans
• Do customers exist for this product or service?
• Are they willing to pay a compensatory price for
  the product at projected costs?
• Why is the product competitively advantageous?
  What are the competitive products or services?

18
Now Telecom Technology

• Having said enough for now about the reasons and
  motivations for telecom products, we turn to the
  technology of telecommunication.
• There are two ways to convey information
• Send a physical object. Historically, the
  customary object is a letter (e.g. on papyrus,
  parchment and later on paper).
• Send some energy in the form of an
  electromagnetic wave. In ancient times, light was
  involved in viewing signals or semaphore signals
  at a distance. Privacy and data rate improvements
  had to wait for the discovery and a minimal
  understanding of electricity.

19
Historical Overview Telegraph

• Invented by Samuel F.B.Morse (an artist, not a
  scientist) greatly assisted by Alfred Vail .
• Inter-city telegraph demonstrated by Morse in
  1837.
• Several less practical European telegraph systems
  preceded Morse
• For example, Morse (and others) thought that
  electrical signals travelled instantaneously
  from telegraph key to the sounder (receiver),
  since the complete theory of electromagnetic
  waves was not formulated until 1860-90 by J.C.
  Maxwell, O. Heaviside, et al.
• Coincidentally, a relative of Theodore Vail,
  president of ATT about 60 years later

20
Telegraph Main Features

• Current flow around a circuit including a
  battery, telegraph key (on-off switch), a single
  wire (typically iron, later copper) with the
  earth as a return path.
• Worked adequately up to about 30 miles, depending
  on earth conductivity.
• About 1849 the repeater allowed longer links by
  chaining 30 mi sections via an electro-mechanical
  relay (switching contacts operated by an
  electromagnet).

21
Telecom Overview Telephone

• The telephone was invented in 1876 by Alexander
  G. Bell (a speech teacher, not a scientist). Born
  in Scotland, Bell immigrated to Canada and then
  the USA.
• The telephone had the significant advantage that
  no special skill (such as learning Morse code)
  was required to use it!
• Ease or convenience of use is often a deciding
  factor in the success of one technology over
  another.
• Bells microphone (called transmitter) produced
  electric current proportional to instantaneous
  air pressure. Earphone (receiver) reversed the
  process, converting the electrical waveform back
  into acoustic (sound) form.

22
Some Business History

• Bell was financed by his wealthy industrialist
  father-in-law, Gardiner G. Hubbard, a man with a
  history of business and legal contention with the
  (then) large Western Union Telegraph Company
• Bells original objective was to send several
  independent telegraph signals over the same
  circuit
• Today we would describe his plan as frequency
  division multiplexing (FDM) of amplitude
  modulated Morse code.
• He discovered by accident that his equipment
  could transmit speech
• He added a new claim to his already filed patent
  covering this
• When the telephone became commercially important,
  major patent litigation followed, ultimately
  decided by the US Supreme Court

23
Business Conflicts

• Bell and Hubbard offered the patent to Western
  Union (WU) at first for 100,000
• This was an immense sum in 1876, when a large
  house cost less than 1000.
• WU turned them down, due to dislike of Hubbard
• A famous negative evaluation letter (probably not
  authentic) is available on this course web site.
• The letter also is a prime example of
• Not Invented Here (NIH) attitude, ignoring good
  outside ideas
• Lack of proper appreciation of the advantages of
  the invention
• Inability to accurately foresee that improvements
  are possible to overcome the initial
  disadvantages of the invention
• The Internet web site http//eh.net/ehresources/h
  owmuch/dollarq.php that contains a history of US
  dollar inflation, indicates that 100,000 in 1876
  had the purchasing power of 1,705,922.48 in the
  year 2005.

24
Early Competitive Moves

• WU, after recognizing the fast growth of the
  telephone, quickly decided to get back into
  competition
• They hired the best available inventor, Thomas A.
  Edison, to invent a significantly improved
  microphone circa 1878
• Edison studied the telephone, found its most
  important weakness, and came up with a solution
  based on Bells liquid transmitter. Bells
  liquid transmitter was a variable resistance
  microphone used in his first working voice
  transmission, but it was impractical because it
  used an acid-water solution as the variable
  resistance material. Edison substituted a sealed
  capsule of powdered carbon as the pressure
  sensitive variable resistance element. This
  carbon microphone invention was also later
  improved by German-American Emil Berliner as well.

25
Business Strategies

• The improvement in audio loudness (permitting
  longer telephone wires and thus more wire
  coverage area per central office) gave the carbon
  microphone a strong economic competitive
  advantage.
• But WU could not operate a telephone system
  without infringing the basic Bell patent, either.
• Negotiations were stalled, until the Bell company
  suggested something which would be illegal under
  present anti-trust law
• WU agreed in 1879 to stay out of the telephone
  business for 20 years in return for 1 of the
  income from the telephone.
• The telephone industry grew so fast that Bell was
  soon able to buy most of WU shares. WU became a
  subsidiary of Bell from circa 1900 until divested
  in a famous 1914 anti-trust case.

26
Early 20th Century

• American Telephone Telegraph (the renamed Bell
  Telephone company), was headed for many years by
  Theodore Vail, coincidentally a nephew of Alfred
  Vail, Morses collaborator.
• Vail vigorously bought out other telephone
  operating companies in most major cities, leaving
  only rural areas to the independents (formed
  after the Bell patents expired). This acquisition
  stopped in 1914.
• ATT purchased Western Electric Co. (electric
  equipment manufacturer originally so named to
  save the cost of repainting the entire sign in a
  former Western Union repair shop), vertically
  integrating manufacturing and telephone
  operations
• ATT established its Long Lines division,
  providing long distance connection between all
  North American and foreign cities.
• In 1914 ATTs negotiator made the Kingsbury
  commitment to not buy out any more independent
  telephone companies, thus settling a federal
  antitrust lawsuit.

27
Some Technological Transmission Advances

• Single wire with earth return was replaced in
  1890s by a subscriber loop of current carrying
  copper wire.
• An innovation by J.J.Carty, who became head of
  ATT RD and ultimately established Bell
  Telephone Laboratories.
• Some ATT accomplishments during the first half
  of the 20th century
• DeForests Audion triode vacuum tube amplifier
  was improved and adapted for analog voice
  frequency amplification, leading to coast to
  coast long distance telephone connections.
• Gilbert S. Vernam invented the Vernam Cipher
  cryptography method for teletypewriters during WW
  1
• The quality and noise of analog telephone
  connections were improved in 1920s by H.S.Blacks
  invention of negative feedback at Bell
  Laboratories.
• Frequency Division Multiplexing (FDM) using
  single side band (SSB) modulation was developed
  by John R.Carson at Bell Laboratories. Basis of
  Analog telephone multiplexing.
• Microwave co-ax cable was developed by Lloyd
  Espenscheid of Bell Labs. Used today for T-3 and
  other signals.

28
More Business Developments

• The Anti-trust Division of the US Justice
  Department investigated ATT in 1914, 1937, 1948,
  1965, 1972. Each investigation led to consensual
  settlements which further restricted the scope of
  ATTs business.
• 1914 Kingsbury Commitment stopped acquisition
  of independent telephone companies and divested
  WU from ATT
• 1937 ATT divested non-telephone businesses
  (appliances, motion picture sound systems, etc.)
  and offshore manufacturing.
• ITT (originally International Telephone and
  Telegraph Corp.) was founded by brothers
  Sosthenes and Hernand Behn. They were sugar
  brokers in Porto Rico who first bought the Puerto
  Rico telephone company. Then they founded Cia.
  Telefonica Espana in 1923. In 1937, using J.P.
  Morgan funds, they bought all  off-shore Western
  Electric factories. They later founded other
  telephone companies in Latin America. ITT sold
  its telephone manufacturing businesses in 1990s
  to Alcatel, and now owns hotels, insurance
  companies and some non-telephone manufacturing
  firms.
• 1948 ATT agreed to license all patents to
  competitors
• 1969 ATT agreed to allow connection of
  customer-owned equipment (result of FCC and court
  CarterPhone decision) rather than renting. ATT
  had previously always rented equipment to the
  subscriber, a method learned from the United Shoe
  Machinery company in the early Boston years.
• 1984 ATT divested local telcos (RBOCs) but
  retained long distance and manufacturing.
  (Manufacturing later separated under the Lucent
  name.)

29
Other Business Events

• ATT, until 1984 divestiture, received 1 of
  gross income of all RBOCs
• Also was part owner of Bell Canada and Northern
  Electric, its manufacturing subsidiary, until
  1970s. This became Nortel Networks, no longer
  owned by ATT.
• Extensive cross-licensing of patents with other
  major telephone equipment manufacturers in other
  countries as well.
• Example Crossbar telephone switch was developed
  under cross-licensing agreement with L.M.Ericsson
  of Sweden
• ATT acquired NCR (formerly National Cash
  Register) in 1989, then spun it off as part of
  the 1996 separation into three businesses. Lucent
  (with Bell Laboratories) is only a manufacturer
  and recently merged with Alcatel. ATT is today
  an operating company in long distance. Its
  cellular/PCS activity is a separate corporation,
  now merged with Cingular Wireless.
• Both ATT and Lucent have started several
  spin-offs also

30
Some Major Telecom Vendors with Dallas-Ft.Worth
Presence

• Alcatel (France) acquired most ITT manufacturing
  operations and Rockwell (Collins) telecom
  products, and Digital Switch Corp. (DSC), and
  integrated them with its existing products in the
  1980-90s.
• Ericsson (Sweden), another long term telecom
  manufacturer worldwide, has operations here.
• Fujitsu, NEC (Nippon Electric Co.) are two
  separate independent Japanese telecom
  manufacturers with Dallas area operations
• Motorola, primarily in Fort Worth (cellular and
  paging equipment)
• Nokia (Finland), strong in cellular/PCS handsets
  but also makes cellular infrastructure and
  landline telecom switchgear
• Nortel Networks (formerly Northern Telecom) is a
  descendant of Northern Electric of Canada.
• Siemens (Germany) a long term telecom and general
  electrical equipment maker, now reducing its
  presence in telecom.
• This area is sometimes called Telecom Corridor
  or Switch Alley

31
Some Telephone Operating Companies

• Originally 7 (now 4) Regional RBOCs, with
  consolidation of SWBell-PacTel-SNET and
  NYNEX-GTE-Bell Atlantic (now Verizon), etc.
• GTE, arising from mid-century consolidation of
  many independent local telcos, merged with Bell
  Atlantic and Primeco wireless to form Verizon
  (rhymes with horizon) in 2000
• Scattered remaining independents in some smaller
  cities (e.g. Rochester NY, etc.)
• Numerous Inter-Exchange Carriers (IXCs) the
  largest 3 being ATT, MCI and Sprint.
• The government operates Post, Telephone and
  Telegraph (PTT) administrations in many other
  countries but privatization is spreading
  rapidly

32
Digital Telecom Revolution

• The T-1 digital multiplexing system, introduced
  by Bell Labs in 1961, ultimately led to an almost
  complete conversion of the North American public
  switched telephone network (PSTN) to digital
  transmission and (later) digital switching
• T-1 was a rare and uniquely successful product
  because it is
• Immediately equal or lower in cost than the prior
  analog FDM multiplexer. Cost improved more later
  with product evolution as well.
• Carefully designed to be backwards compatible
  with all switching and prior art transmission
  equipment at connection interfaces
• Better signal quality than FDM multiplex
• More capacity (24 voice channels on the same
  wires that previously carried only 12 channels)
• Also written T1. Since T-1 is a trade name, DS-1
  is an approximately equivalent term used in
  standards documents, etc.

33
Is Digital Always Better?

• The error introduced by conversion from analog
  to digital representation can be controlled and
  limited in advance by the designer of the A/D
  converter. Called quantizing error.
• Digital representation of information does not
  suffer from cumulative noise errors.
  Transmission over a longer distance only causes
  time delay, not distortion.
• This is the result of a system design in which
  the two binary digital voltage levels (typically
  0 and 5 volts) differ by much more than the
  typical noise voltage level (typically 0.001
  volts).
• But digital representation typically uses more
  (radio) bandwidth than analog representations.
• This problem can be reduced by use of data
  compression coding in some cases.
• When the channel is extremely noisy, (e.g., a
  cellular radio link) error protection coding must
  be used, and this requires part of the total
  channel bit rate to be devoted to bits for this
  purpose. Cellular radio systems typically devote
  half the physical bit rate capacity to error
  protection.

34
T-1 Benefited From Prior Technology

• PSTN voice signals were historically already
  low-pass audio-frequency filtered to attenuate
  audio power above approx. 3.5 kHz audio frequency
• Necessary for FDM multiplexing and well-verified
  to support intelligible conversation
• Permits accurate digital waveform coding at 8000
  samples/second
• T-1 design was an early application for
  transistors
• Repeaters are installed at 6000 ft. intervals in
  outdoor or difficult-access locations and must
  operate reliably and consume little power
• Vacuum tube devices would not be practical
• T-1 uses PCM (pulse code modulation) waveform
  coding with logarithmic companding
• 8-bit binary coding of each waveform sample, with
  non-uniform voltage steps, produces uniform
  signal to noise ratio over a wide range of audio
  loudness
• 8 bit/sample 8000 sample/sec 64,000
  bit/second 64 kb/s

35
Incorporation of Call-Processing Signals

• Two methods for signaling are in general North
  American use
• 1. Robbed bit signaling uses the least
  significant bit of the PCM in every 6th frame to
  convey supervision (channel busy/idle) status.
  Five of every six consecutive waveform samples
  are not affected.
• Systems for 12 and 24 multi-frame synchronizing
  patterns are used to ensure that the signaling
  equipment uses the proper bit
• Robbed bit signaling leaves 56 kb/s (7 bits of
  every sample) for the subscriber, even if not
  multi-frame synchronized
• 2. Common channel signaling uses a reserved
  digital channel (either 64 or 1536 kb/s in North
  America) to convey messages in packet data form
  between switching systems regarding the call
  processing on numerous other channels
• Common channel signaling system Number 7 is
  todays world-wide standard, with some national
  variants There are many abbreviations (SS7,
  CCS7, etc.) In some cases, different
  abbreviations imply different national variants
  of Common Channel Number 7.

36
Further Digital Multiplexing

• Higher level digital multiplexing systems were
  developed with better economy for high traffic
  corridors
• T-1 (DS-1) so called North American (and Japan)
  Primary Rate digital multiplexing. 24 channels at
  1.544 Mb/s
• T-1C a double capacity system (48 channels) now
  rarely used. Not mentioned in international
  standards. 3.152 Mb/s
• T-2 (DS-2) a quadruple capacity system (96
  channels). Called M12 or Secondary Rate. Combines
  4 DS-1 tributaries. Seldom installed today. 6.312
  Mb/s
• T-3 (DS-3) Combines 7 DS-2 tributaries. M13
  multiplexers produce this Tertiary level rate
  from 28 T-1 tributaries. 44.736 Mb/s. Uses
  co-axial cable or microwaves.
• Different and mostly incompatible T-4 higher
  level digital multiplexers using co-axial cable
  or microwaves were developed in different
  countries (US, Canada, Japan) but were relatively
  little used since only a few routes have enough
  traffic to make this economically feasible.
• European digital multiplexers of similar
  characteristics are widely used in other
  countries.

37
Higher Level Multiplexer Trends

• DS-1, DS-2, DS-3 multiplexers are designed to
  accommodate small time-varying inaccuracy in the
  bit rate of the incoming tributaries
  (plesiochronous multiplexing)
• An undesirably large portion of the total bit
  rate (bit overhead) is needed to handle this,
  and the necessary process for de-multiplexing a
  single DS-1 or single 64 kb/s voice channel
  (DS-0) is complicated and costly
• These difficulties, and the increased use of
  fiber optic transmission and more accurate
  digital bit stream synchronization, has led to
  development of new and fundamentally improved
  multiplexing designs

38
EM Wave Transmission Media

• Radio transmission. Non-guided via open space
• Inferior channel characteristics due to fading,
  interference, etc.
• But portability makes cellular service valuable,
  and absence of intermediate equipment between
  microwave towers gives lower cost.
• Guided electromagnetic waves
• Via twisted pair wires, co-axial cable. Typically
  using repeaters to compensate for signal loss
• Via optical fiber, in the infra-red optical
  frequency range. Electro-optical or all-optical
  signal amplifiers are used to compensate for
  losses.

39
SONET and SDH

• A multiplexing format normally used on optical
  fiber, but lowest bit rate members of the family
  can be transmitted via co-axial cable or
  microwave radio.
• SONET (Synchronous Optical Network) in North
  America
• SDH (Synchronous Digital Hierarchy) elsewhere
• In a refreshing departure from previous
  international incompatibility, these standards
  are virtually identical. SONET includes a lowest
  bit rate version at 51.84 Mb/s which is not used
  in SDH, but higher rates such as 155.52 Mb/s etc.
  are common to both standards and are compatible
  when similarly configured.

40
Digital Transmission and Switching

• The rapid growth of digital multiplexing
  transmission systems (almost 100 of the North
  American network today) led to a parallel
  development of digital local and long distance
  switches. These switches are more compact, use
  less power, and are more reliable than their
  electro-mechanical predecessors, and mostly
  contain automatic self-test equipment to permit
  efficient use of fewer repair personnel
• Digital Switching is now included in the present
  course EETS8320.

41
Digital Switch Basics

• End office digital switches typically support
  traditional analog telephone sets, and in some
  cases ISDN or proprietary digital telephone sets.
  The analog voice waveform voltage is periodically
  measured (sampled) and each voltage is
  converted via analog/digital converters and
  digitally coded into a bit stream.
• From trunk connections, separate channels of
  digital information are separated from the bit
  streams.
• Digital channel data is stored temporarily
  (typically for 125 microseconds) in a local
  memory in the switch.
• Desired outgoing channel bit streams are
  multiplexed together to connect to other
  switches.
• Microprocessor internal to the switch controls
  routing of connections.

42
Switch Types

• End-office switch both trunks and telephone sets
• Formerly called Class 5
• Trunk-trunk switch (no telephone sets)
• Used to complete long distance connections
  between end switches
• Used (with radio base stations) for cellular
  radio systems
• Formerly designated as Class1 to Class 4 based on
  details of application in the network.
• Private Branch Exchange (PBX) switches, used
  primarily for business users to establish both
  external and internal calls
• Intercom switch. Connects telephone sets or
  hands-free stations for internal calls only.
  Rare today.

43
Switch Features

• All digital switches are microprocessor
  controlled and have many features. Some examples
• Call waiting (signal during a conversation that
  another caller is attempting to reach you, and
  ability to answer that caller)
• Incoming call forwarding to another number when
  desired
• 3-way conference calling via a conference bridge
• PBXs in particular have a large repertoire of
  sophisticated features.

44
Digital Speech Coding

• A technical race has continued for the last
  quarter century between speech coding technology
  and transmission technology
• Lower bit rate speech coders are typically more
  complex devices, but they allow carrying more
  conversations in a transmission medium with a
  fixed total bit rate
• Innovations such as fiber optic transmission and
  integrated circuits have reduced the cost of high
  transmission bit rates
• The public telephone industry almost changed over
  to 32 kb/s ADPCM speech coding in the early
  1980s, but the lower cost of fiber stopped this
  plan
• Radio systems such as cellular and PCS appear to
  be the main present use for lower bit rate speech
  coders.
• Adaptive Differential PCM

45
Other Speech Coding

• Digital speech coding methods generally fall into
  one of two categories
• 1. Waveform coding. Examples include
• PCM (Mu-law and A-law pulse code modulation used
  in DS-1 and E-1)
• ADPCM (adaptive differential PCM - typically 32
  kb/s)
• Delta Modulation (DM) and CVSD (continuously
  variable-slope DM)
• 2. Audio Power Spectrum Coding. Examples include
• Sub-band coding
• RELP (regular - or residual - pulse excited
  linear predictive coding)
• CELP (code excited linear predictive)
• VSELP, ACELP (vector sum ELP, Algebraic code ELP)

46
Some Speech Coder Bit-rates Typical Applications

• Lower bit rate coders are generally less
  satisfactory than higher bit rates.
• PCS Personal Communications Service, a cellular
  radio system usually with digital speech coding

47
Non-voice Bearer Services

• Due to their near-ubiquitous presence, readily
  available investment capital, and the franchise
  held by many telephone operating companies to
  install wire, cable or fiber, many other services
  are also under development and use in the
  telephone system and related systems
• Images telefax, video, other images
• Data Internet access, data bases, and related
  information
• Digital coding of any originally analog
  information (such as video) is seen as the
  optimum method for combined transmission
• but verify that the entire system is really
  advantageous!!

48
Telephone Data Modems

• Digital data can be transmitted via telephone
  voice channels using an audio frequency carrier
  signal which is modulated to convey binary
  information by changing its
• Amplitude (instantaneous voltage or power level).
  This method is used alone only for Morse Code
• Phase (relative time delay of oscillatory
  waveform peaks and valleys vis-à-vis a standard
  clock signal)
• Frequency (the quantity of cycles per second the
  musical pitch)
• Recent modem (modulator-demodulator) designs
  mostly use QAM (quadrature amplitude modulation)
  a combination of amplitude and phase modulation
• V.90 or V.92 In one direction, various voltage
  amplitude levels are each used to represent a
  specific 7-bit binary data value.
• ADSL A special type of multi-carrier QAM modem
  is used via telephone subscriber wires to carry
  high bit-rate digital Internet signals in a
  frequency band above the usual voice frequencies.
• Modem is an invented word made of the first
  syllables taken from the two words Modulator and
  DEModulator.

49
Modem Properties

• Data modems today also include automatic
  equalizers to compensate for individual voice
  channel characteristics that would otherwise
  cause undesired waveform changes.
• Data rates of up to 9.6, 14.4, 28.8 and 33.6 kb/s
  are feasible using classic adaptive QAM modem
  technology
• Higher bit rates up to 56 kb/s use direct PCM
  encoding at one end
• Fully digital connection at transmitting end.
  Analog connection at receiving end. Signal
  voltage can be measured with sufficient accuracy
  at receiving end to infer the PCM code value
  used.
• Full 64 kb/s throughput requires a specifically
  installed digital line such as ISDN or DDS.
• V.90 and V.92 modems today are legally limited
  to 53 kb/s. The highest voltage levels of PCM are
  prohibited to avoid crosstalk with other wire
  pairs in the same cables.

50
Fully Digital Telephone Services

• ISDN (integrated services digital network) and
  proprietary digital services (DDS, etc.)
• Special digital signals used on the subscriber
  loop
• Permits end-to-end 64 or 56 kb/s digital service
• For voice, analog-digital conversion is performed
  in the ISDN telephone set rather than in the
  central office switch
• Unfortunately ISDN is very costly, but has had a
  recent small surge in utilization due to Internet
  access applications. Some critics view ISDN as an
  early example of the Iridium syndrome
• Emergence of 56 kb/s V.90/92 modems has severely
  reduced the use of ISDN

51
Packet Data Systems

• In several types of data networks, data is
  transmitted in packet format
• A small block of consecutive data bits from each
  particular source has a header pre-pended. The
  header contains, among other things, a code
  number indicating the destination. This is used
  to control routing.
• Typically an error-detecting code is appended to
  the end of the packet.
• In some systems, all packets are the same size
  (length) in others each packet is of different
  size, typically based on source data rate.
• Packets from different sources are transmitted
  via the same channel, one after another
• Most systems use a special flag bit pattern,
  01111110, as a separator between packets.
• The internal packet bit stream is pre-modified
  (bit stuffing) to exclude any false occurrences
  of theflag pattern.
• At the receiving end, the bit stuffing process is
  undone

52
Why Packets?

• Many types of digital information sources are
  bursty in time
• Brief burstsof high bit rate data are separated
  by some time intervals during which no data bits
  are generated
• Data coding methods which remove redundant
  information from raw speech or video typically
  produce bursty data
• A number of different packet transmissions can be
  multiplexed on a shared channel in a high bit
  rate medium (co-ax, fiber, etc.) more efficiently
  than using a separate channel for each source,
  provided that all data sources do not continually
  produce data bursts simultaneously

53
ATM (Asynchronous Transfer Mode)

• ATM Payload data is transmitted in fixed size
  packets (called here cells) of 48 bytes (384
  bits) with a 5 byte identification header (53
  bytes total)
• ATM signals can be transmitted e.g. via the
  payload of SONET/SDH at 50 Mb/s or more gross
  bit rate
• Due to its small packet size, ATM has little
  signal delay, and is theoretically superior to
  other packet formats for digitally coded voice.
• ATM is an interesting alternative to LAN/WAN
  technologies such as Ethernet, although presently
  far more costly

54
Telefax (Facsimile,FAX)

• Groups 1 and 2 FAX are obsolescent.
• Group 3 FAX is in worldwide use. A page image is
  typically transmitted in less than a minute at
  9.6 kb/s binary data rate via internal modem over
  a voice grade PSTN channel.
• Group 3 FAX uses binary data compression coding
  of black/white pixel (or pel) picture elements
  (dots)
• Line difference coding takes advantage of
  vertical lines in the image
• Run-length coding takes advantage of large
  contiguous areas of white or black, and of long
  run zero line differences produced by line
  difference coding.
• Huffman coding takes advantage of repeated
  appearance of certain binary bit patterns in the
  FAX bit stream

55
Other Data Compression Methods

• Lossless data compression methods exploit
  redundant data bit patterns when present, and
  accurately regenerate original data when decoded
• Plain language text has well-known frequently
  occurring characters (E T A O I N etc.) and
  infrequently occurring characters (J Z Q etc.), a
  fact that is exploited by Morse code and Huffman
  coding
• LZW (Lempel-Ziv-Welch) coding dynamically
  adjusts transmission codes to use short binary
  patterns for frequent symbols and longer binary
  patterns for infrequent symbols. LZW is one type
  of algebraic coding.
• Huffman coding is a non-dynamic formal lossless
  data compression method similar to LZW

56
Lossy Coding

• The word lossy implies data compression with
  imperfect reconstruction of the original
  information
• Used when human perception can be fooled, for
• Video and still pictures with continuous
  brightness range (gray or color)
• Audio spectrum (speech) coding. Does not
  reproduce the exact sound waveform
• Lossy image coding typically approximates the
  spatial brightness pattern using a family of
  orthogonal functions
• Discrete Cosine Transform is popular for images,
  video
• Frame difference and motion extrapolation methods
  are used with video as well
• Video, which requires over 40 Mb/s with simple
  waveform coding, can be lossy-encoded at 64 to
  128 kb/s (via Px64 code) with acceptable (not
  high) quality and coding delays

57
Error Protection Coding

• Use of additional bits with the payload data
  can be used to
• Correct a limited quantity of bit errors
• Detect (but not correct) larger quantity of bit
  errors
• Error detection codes are often used in
  conjunction with an Automatic request to
  Retransmit (ARQ) strategy to retransmit pieces of
  the data (typically packets) unless they are soon
  acknowledged as received OK, via a message sent
  back to transmitter from the receiving end
  equipment.
• Widely used with error-prone radio channels and
  delay-tolerant signals such as for wireless call
  processing messages
• Also used in T-1 extended super frame version,
  and SONET/SDH multiplexing systems, to
  continuously monitor transmission accuracy.

58
Encryption

• Important when the transmission is physically
  open to interception by unauthorized persons
• Particularly for wireless, radio, microwave, etc.
• Encryption methods can also be used to
  authenticate messages
• Only a person who knows the correct secret key
  can properly encrypt or decrypt a message
• The most widely used physical level encryption
  method is the Vernam cipher
• A secret encryption cipher bit stream is
  added to the bits before transmission, then the
  same cipher bit stream is subtracted out at the
  receiver
• The practical complications in this process
  relate to generating, distributing and
  synchronizing the cipher bit streams at both the
  transmit and receive ends.
• Not normal arithmetic addition and subtraction.
  Rather the XOR, or ring sum or modulo-2 logical
  operation

59
End of Lecture 1

• The rest of the sessions involve a more detailed
  description of the technical topics just listed.