Wired Physical Layer

1
Wired Physical Layer

• EMC 165 Computer and Communication Networks
• Lecture 8
• Feb 10, 2004

2
Outline of todays lecture

• How Modems Work?
• How ISDN Works?
• How DSL Works?
• How Cable Modems Work?
• How Fiber Optics Work?

3
How Modems Work?

• Origin of Modems
• Modem contraction of the words
  modulator-demodulator.
• A modem Is typically used to send digital data
  over a phone line
• Sending modem modulates the data into a signal
  that is compatible with the phone line.
• Receiving modem demodulates the signal back into
  digital data.
• Wireless modems convert digital data into radio
  signals and back.

4
How Modems Work? - contd
5
How Modems Work? contd

• In this configuration, a dumb terminal at an
  off-site office can dial-in to a large central
  computer
• Modem speeds went through a series of
  improvements over the years
• 300 bps 1960s through 1983
• 1200 bps 1984-1985
• 2400 bps
• 9600 bps first appeared in late 1990 and early
  1991
• 19.2Kbps
• 28.8 Kbps
• 33.6 Kbps
• 56 Kbps became standard in 1998
• ADSL theorectical max of up to 8 Mbps

6
300bps Modems

• A 300 bps modem is a device that uses Frequency
  Shift Keying (FSK) to transmit digital
  information over a telephone line.
• In FSK, a different tone (frequency) is used for
  the different bits.
• The originating modem transmits 1070-Hz tone for
  a 0 and a 1270-Hz tone for a 1.
• The answering modem transmits a 2025-Hz tone for
  a 0 and a 2225-Hz tone for a 1.
• Because they use different tones, they can use
  the line simultaneously. This is known as
  full-duplex operation.
• When the letter a is typed, the ASCII code for
  this letter is 97 decimal or 01100001 binary.
• A device inside the terminal called a Universal
  Asynchronous Receiver/Transmitter (UART) converts
  the byte into its bits and sends them out one at
  a time through a serial port called RS-232 port.
• The terminals modem is connected to the RS-232
  port, so it receives the bits one at a time and
  its job is to send them over the phone line

7
Faster Modems

• To create faster modems, Phase-Shift Keying
  (PSK) and Quadrature Amplitude Modulation (QAM)
  are used.
• What is PSK? Shifting the phase of the wave.
• Higher speed modems incorporate a concept of
  gradual degradation, meaning they can test the
  phone line and fall back to slower speeds if the
  line cannot handle the modems fastest speed.

8
What is QAM?

• Simply a combination of amplitude modulation and
  PSK
• Assume use 2 amplitudes and 4 phase shift
• Bit value Amplitude Phase
• 000 1 None
• 001 2 None
• 010 1 ¼
• 011 2 ¼
• etc

9
ISDN

• ISDN stands for Integrated Services Digital
  Network
• Using the same copper phone lines that modems
  use, ISDN delivers a 5-fold speed improvement
  (compared to 28.8 Kbps modem) (up to 128 Kbps).
• ISDN can combine voice and data services over the
  same wires.

10
ISDN Basics

• ISDN provides a raw data rate of 144 Kbps over a
  single twisted pair.
• This 144 Kbps channel is divided into 2 64-Kbps
  channels (refer to as Bearer channels) and one 16
  Kbps channel (refers to as Data channel).
• Each B channel can carry a separate telephone
  call and has its own telephone number called a
  Directory Number (DN).
• One can combine the 2 B-channels together to form
  a single 128 Kbps data channel through a process
  called bonding.

11
How ISDN does It?

• If ISDN can squeeze 144Kbps out of my phone line,
  why cant modem do the same thing?
• A given pair of wires connecting 2 parties for
  communication can carry electrical signals in two
  forms analog or digital.
• An analog signal changes gradually through an
  infinite number of values, while a digital signal
  changes instantly (in theory) between just two
  values.
• An analog signals infinite number of variations
  makes it impossible to reproduce exactly. An
  analog signal will go only so far in copper wire
  to go further the signal must be regenerated
  electronically with a device called a repeater.
  The repeater converts the weak input signal to a
  stronger signal, unavoidably distorting it in the
  process. Each regeneration degrades the signal a
  bit more.
• Digital signals, on the other hand, are easy to
  regenerate precisely because there are only 2
  possible states for the signal.

12
Simple ISDN hookup
UTP Unshielded Twisted Pair
13
ISDN Basics

• The B channels carry customer voice or data
  signals.
• The D channel carries signals between your ISDN
  equipment and the phone companys central office.
  
• The 2 bearer plus one data channel is called the
  Basic Rate Interface (BRI) in telco lingo.
• One can buy 23 B channels with a single 64 Kbps D
  channel. This service is called the Primary Rate
  Interface (PRI).
• A single 4-wire cable carries the 2BD channels
  into another box called the Terminal Adapter
  (TA). Unlike the Network Terminator (NT1), which
  provides only a single function (creating the
  2BD channels), the TA can do many things.
• The TA can connect any of the terminal equipment
  (TE) computers, fax machines, LANs or telephone
  sets to one or both of the B channels.
• External ISDN reference points labeled R, S/T,
  and U. Each interface point requires an
  electrically different device connection and
  cabling. The U reference point is the incoming
  unshielded twisted pair. The S/T reference point
  is a four-wire UTP cable.

14
ISDN Basics

• A typical TA for data-only applications might
  simply emulate a pair of ordinary modems,
  transmitting standard modem setup and dialing
  commands into ISDN call-setup commands.
• One connects a computer to this kind of TA with a
  normal RS232 cable and uses the usual modem or
  fax software to set the speed to 64 Kbps.
• The TA provides automatic rate adaptation to
  match whatever data rate the computer supports
  with ISDNs 64 Kbps channel.
• Advantage of ISDN data is sent digitally so
  higher reliability when compared to the analog
  modems which suffer from all kinds of maladies
  ranging from intermittent line noise to speed
  mismatches and protocol conflicts.

15
Beyond ISDN Basics

• One can set up ISDN to simulate the features of
  an office PBX, using advanced TAs or direct
  computer-integrated ISDN hardware.
• ISDN offers flexible options for mixing voice and
  data
• Up to 8 devices can share access to the channels
  using a feature of ISDN called passive bus.
  Passive bus uses a 2nd kind of network
  terminator, called NT2 to let up to 8 separate
  TAs share a single 2BD circuit.
• TAs that support passive bus have a port labeled
  S/T to indicate that you are making the
  connection at the S/T ISDN reference points.
• ISDN also allows you to construct sophisticated
  integrated voice/data applications e.g. call
  appearances. POTS allow you to put a call on hold
  while taking a second call. ISDN expands that
  capability to up to 15 separate calls.

16
Multiple ISDN circuit appearances

• For incoming ISDN calls, the telcos Central
  Office sends a call setup message to the TA via
  the D channel, indicating that a call is
  available to be picked up.
• The TA answers the call and assign it to an
  available B channel. If both B channels are used,
  it can free a channel by placing an active call
  on hold and making the new call active. These
  calls can be either data or voice, in any
  combination. Thus a single TA can handle as many
  as 15 simultaneous calls in progress, with any 2
  of those calls active.

17
Using analog modems in ISDN environment

• This kind of TA accepts an ordinary voice or
  modem audio signal through a standard RJ11
  modular jack and digitizes it for transport
  across the ISDN interface. It interprets the
  touch-tone dialing signals put out by the
  telephone set or modem and generates the required
  ISDN call setup signals.
• If the number you call is not an ISDN POP, the
  telco equipment at the remote end automatically
  trnaslates the digitized audio back to analog
  audio, where the destination modem hears what
  its always heard before ISDN came along.

18
Cost of ISDN

• Cheapest service (PacBell) - 30/month for local
  access plus msg-unit charges of 4 cts for 1st
  minute and 1ct for each additional minute.
• Long-distance digital charges 2-3 times higher
  than voice long-distance calls.
• NT1 costs between 100-200
• ISDN TAs cost range from 300 to 1500.

19
Alternatives to ISDN

• Copper-wire digital services such as T1
  (1.544Mbps)
• Frame Relay services (56 Kbps to 1.544 Mbps)
• Asynchronous Transfer Mode (ATM) (25Mbps to 100
  Mbps).
• Alternative is to replace copper wire with fiber
  optic cabling
• The last mile also called the local loop is telco
  talk for the twisted wire pair between the CO and
  the subscriber.
• Each telephone user requires a dedicated pair of
  copper wires. The length is more than 1 mile but
  fewer than 20 miles and averages over 5 miles in
  metropolitan areas.
• Faster digital services require digital repeaters
  at least once per mile.
• But normal copper pairs dont have such
  repeaters.
• The copper wires have been there for 50 years.
  The cost of replacing existing copper with fiber
  would be 250 billion.

20
How DSL Works?

• DSL is a high-speed connection that uses the same
  wires as a regular telephone line.
• The wires themselves have the potential to handle
  frequencies up to several million Hertz but for
  voice communications, we limit them to 3.4 KHz
• By limiting the frequencies carried over the
  lines, the telephone system can pack lots of
  wires into a very small space without worrying
  about interference between lines.
• Modern equipment that sends digital rather than
  analog data can safely use much more of the
  telephone lines capacity. DSL does just that

21
ADSL

• ADSL stands for asymmetric digital subscriber
  line.
• Asymmetric data sent in one direction is faster
  than in the other direction.
• An ADSL modem has a dedicated copper wire running
  between it and phone companys nearest
  multiplexer (MUX) or central office.
• This dedicated copper wire can carry far more
  data than the 3Kbz signal needed for our phones
  voice channel.
• With a dedicated copper wire between the phone
  company and the home, the capacity is something
  like 1Mbps between the home and the phone company
  (referred to as upstream) and 8 Mbps between the
  phone company and the home (referred to as
  downstream) under ideal conditions.

22
ADSL

• Most homes and small business users are connected
  to an ADSL.
• ADSL divides up the available frequencies in a
  line on the assumption that most internet users
  look at or download much more information than
  they send, or upload.
• Under this assumption, if the connection speed
  from the Internet to the user is 3-4 times faster
  than the connection from the user back to the
  Internet, then the user will see the most
  benefit.
• Other types of DSL include
• Very high bit-rate DSL (VDSL)
• Symmetric DSL
• Rate-adaptive DSL a variation of ADSL where the
  modem adjusts the speed of connection depending
  on the length and quality of the line.

23
ADSL - contd

• ADSL is distance-sensitive.
• As the connection length increases, the signal
  quality decreases and the connection speed goes
  down.
• The limit for ADSL services is 5460 m (18,000
  ft).
• ADSL technology can provide a max of 8 Mbps
  downstream at a distance of 6000 ft and upstream
  speeds of up to 640 Kbps.
• In practice, the best speeds widely offered today
  are 1.5 Mbps downstream and 64-640 Kbps upstreams.

24
ADSL contd

• The approach an ADSL modem takes is as follows
• The phone lines bandwidth between 24KHz and
  1,100KHz is divided into 4KHz bands and a virtual
  modem is assigned to each band. Each of these 249
  virtual modems tests its band and does the best
  it can with the slice of bandwidth it is
  allocated. The aggregate of the 249 virtual
  modems is the total speed of the pipe.

25
ADSL - contd

• There are 2 competing standards for ADSL, namely
  discrete multitone (DMT) and carrierless
  ampltitude/phase (CAP) system.
• CAP operates by dividing the signals on the
  telephone line into distinct bands voice
  conversations are carried in the 0-4KHz band.
  Upstream channel is carried in a band between
  25-160 KHz and downstream begins at 240KHz and
  goes up to a point that varies depending on a
  number of conditions (line length, line noise, no
  of users in a particular phone switch) but has a
  max of about 1.5MHz.
• DMT divides the data into 247 separate channels,
  each 4KHz wide. You get an equivalent of 247
  modems connected to your computer at once. Each
  channel is monitored and if the quality is too
  impaired, the signal is shifted to another
  channel. This system constantly shifts signals
  between different channels.
• DMT is more complex to implement than CAP but
  gives it more flexibility on lines of differing
  quality.

26
DSL Equipment

• ADSL uses 2 pieces of equipment, one on the
  customer end and one at the Internet service
  provider, telephone company or other DSL service
  provider.
• At the customer end, we have the DSL transceiver.
  At the service provider end, we have the DSL
  access multiplexer (DSLAM) to receive customer
  connections.

27
DSL Equipment

• DSL Transceiver also called ATU-R. It is the
  point where data from the computer is connected
  to the DSL line
• DSL DSLAM takes connections from many customers
  and aggregates them onto a single, high-capacity
  connection to the Internet.
• DSLAMs are generally flexible to support
  multiple types of DSL.
• DSLAM may provide additional functions including
  dynamic IP address assignment.
• ADSL provides a dedicated connection from each
  user back to the DSLAM but cable modem (which we
  discuss next) users generally share a network
  look that runs through a neighborhood. Thus, ADSL
  users wont see performance decrease as new users
  are added.

28
How Cable Modems Work?

• Cable Modem Basics
• Inside the Cable Modem
• Tuner
• Demodulator
• Modulator
• MAC
• Microprocessor
• Cable Modem Termination System

29
Cable Modem Basics

• Each television signal is given a 6 MHz channel
  on the cable. The coaxial cable used to carry
  cable TV can carry hundreds of megahertz of
  signals.
• When a cable company offers internet access over
  the cable, the cable modem system puts downstream
  data into a 6MHz channel.
• On the cable, the data looks just like a TV
  channel. So, internet downstream data takes up
  the same amount of cable space as any single
  channel of programming.
• Upstream data requires even less of the cables
  bandwidth, just 2MHz, since the assumption is
  that most people download far more information
  than they upload.
• Putting both upstream and downstream data on the
  cable TV system requires two types of equipment,
  a cable modem on the customer end and a cable
  modem termination system (CMTS) at the cable
  providers end.

30
Inside the Cable Modem.

• All cable modems contain certain key components
• A tuner
• A demodulator
• A modulator
• A media access control (MAC) device
• A microprocessor

31
Inside the Cable Modem

• Tuner
• connects to the cable outlet.
• Sometimes, with the addition of a splitter to
  separate the Internet data channel from normal
  CATV programming.
• In some cases, tuner will contain a diplexer
  which allows the tuner to make use of one set of
  frequencies for downstream traffic and another
  set for upstream data.
• In some cases, the cable modem tuner is used for
  downstream data and a dial-up telephone modem is
  used for upstream traffic.
• The tuner passes the received signal to the
  demodulator.

32
Inside the Cable Modem

• Demodulator
• A QAM demodulator takes a radio-frequency signal
  that has had information encoded in it by varying
  both the amplitude and phase of the wave, and
  turns it into a simple signal that can be
  processed by the Analog/Digital (A/D) converter.
• The A/D converter takes the signal and turns it
  into a series of digital 1s and 0s.
• An error correction module then checks the
  received information against a known standard so
  that problems in transmission can be found and
  fixed.
• Network frames may be in MPEG format, so an MPEG
  synchronizer may be used to make sure the data
  groups stay in line and in order.

33
Inside the Cable Modem

• Modulator
• If cable system is used for upstream, a modulator
  is used to convert the digital computer network
  data into radio-frequency signals for
  transmission. This component is called a burst
  modulator, because of the irregular nature of
  most traffic between a user and the Internet.
• Consists of 3 parts
• A section to insert information used for error
  correction on the receiving end.
• A QAM modulator.
• A digital to analog (D/A) converter.
• MAC
• Sits between the upstream and downstream portions
  of the cable modem, and acts as the interface
  between the hardware and software portions of the
  various network protocols.
• Some of the MAC functions will be assigned to a
  central processing unit (CPU).
• Microprocessor
• Depends on whether the cable modem is designed to
  be part of a larger computer system or to provide
  internet access with no additional computer
  support. In systems where the cable modem is the
  sole unit required for internet access, the
  microprocessor picks up MAC slack and much more.
  Motorolas PowerPC processor is one of the common
  choices for system designers.

34
CMTS

• CMTS takes the traffic coming in from a group of
  customers on a single channel and routes it to an
  internet service provider (ISP) for connection to
  the Internet
• At the head-end, the cable providers will have
  servers for accounting and logging, Dynamic Host
  Configuration Protocol (DHCP) for assigning and
  administering IP addresses of all the cable
  systems users, and control servers for a
  protocol called CableLabs Certified Cable Modems
  (or DOCSIS), the major standard used by US cable
  systems in providing internet access to users.

35
CMTS contd

• The downstream data is sent to all users just
  like in an Ethernet network.
• Individual network connection decides whether a
  particular block of data is intended for it or
  not.
• On the upstream side, information is sent from
  the user to the CMTS. Other users dont see that
  data at all.
• The narrower upstream bandwidth is divided into
  slices of time, measured in milliseconds, in
  which users can transmit one burst at a time to
  the Internet.
• A CMTS will enable as many as 1,000 users to
  connect to the Internet through a single 6MHz
  channel. Each channel is capable of 30-40 Mbps of
  total throughput.

36
How does Fiber Optics Work?

• What are Fiber Optics?
• How does an Optical Fiber Transmit Light?
• A Fiber-Optic Relay System
• Advantages of Fiber Optics

37
What are Fiber Optics?

• Fiber Optics are long, thin strands of very pure
  glass about the diameter of a human hair. They
  are arranged in bundles called optical cables and
  used to transmit light signals over long
  distances.
• A single optical fiber has the following parts
• Core the glass center of the fiber where the
  light travels
• Cladding the outer optical material surrounding
  the core that reflects the light back into the
  core
• Buffer coating the plastic coating that
  protects the fiber from damage and moisture

38
What are Fiber Optics? - contd

• There are two types of fibers single-mode and
  multi-mode fibers
• Single mode fibers have small cores (about
  3.5x10-4 inches or 9 microns in diameter) and
  transmit infrared laser light (wavelength 1300
  to 1550 nanometers)
• Multi-mode fibers have larger cores (about
  2.5x10-3 inches or 62.5 microns in diameter) and
  transmit infrared light (wavelength850 and 1300
  nm) from light-emitting diodes (LEDs).

39
How Does an Optical Fiber Transmit Light?

• The light in a fiber-optic cable travels through
  the core by constantly bouncing from the
  cladding, a principle called total internal
  reflection.
• Because the cladding does not absorb light, the
  light wave can travel great distances.
• Some of the light signal degrades within the
  fiber due to impurities in the glass. The extent
  of degradation depends on the purity of the glass
  and the wavelength of the transmitted light e.g.
  850 nm light degrades 60-70 percent/Km while 1550
  nm light degrades 50 percent/Km. But some premium
  optical fibers show much less signal degradation
  less than 10 at 1550 nm.

40
What is Total Internal Reflection?

• When light passes from one medium with one index
  of refraction (m1 ) to another medium with a
  lower medium of refraction (m2), it bends or
  refracts away from an imaginary line
  perpendicular to the surface (normal line). As
  the angle of the beam through m1 becomes greater
  with respect to the normal, the refracted light
  through m2 bends further away from the line.
• At one particular angle (critical angle), the
  refracted light will not go into m2, but instead
  will travel along the surface between two media.
  If the beam through m1 is greater than the
  critical angle, then the refracted beam will be
  reflected entirely back into m1 (total internal
  reflection, even though m2 may be transparent.

41
A Fiber-Optic Relay System

• Fiber-optic relay systems consist of the
  following
• Transmitter produces and encodes the light
  signals
• Optical fiber conducts the light signals over a
  distance
• Optical regenerator may be necessary to boost
  the light signal
• Optical receiver receives and decodes the light
  signals.
• Transmitter
• Directs the optical fiber device to turn the
  light on or off in the right sequence
• Has a lens to focus the light into the fiber.
  Lasers have more power than LEDs but vary more
  with changes in temperature and are more
  expensive.
• The most common wavelengths of light signals are
  850nm, 1300 nm, and 1550 nm.

42
Fiber-Optic Relay System - contd

• Optical Regenerator
• Some signal loss occurs especially over long
  distances (more than 0.5 mile) such as undersea
  cables.
• Optical regenerator consists of optical fibers
  with a special coating (doping). The doped
  portion is pumped with a laser. When the degraded
  signal comes into the doped coating, the energy
  from the laser allows the doped molecules to
  become lasers themselves. The doped molecules
  then emit a new, stronger light signal with the
  same characteristics as the incoming weak light
  signal. It is like a laser amplifier.
• Optical Receiver
• Takes incoming digital light signals, decodes
  them and sends the electrical signal to the other
  users computer. The receiver uses a photocell or
  photodiode to detect the light.

43
Advantages of Fiber Optics

• Less expensive several miles of optical cable
  can be made cheaper than equivalent lengths of
  copper wire.
• Thinner Optical fibers can be drawn to smaller
  diameters than copper wire
• Less signal degradation the loss of signal in
  optical fiber is less than in copper wire
• Low power because signals in optical fibers
  degrade less, lower-power transmitters can be
  used.
• Digital signals optical fibers are ideally
  suited for carrying digital information, which is
  especially useful in computer networks.
• Non-flammable no electricity is passed through
  optical fibers, there is no fire hazard.
• Lightweight an optical cable weights less than
  a comparable copper wire cable.
• Flexible because they can transmit and receive
  light, fiber optics are used in many flexible
  digital cameras.

44
References

• www.HowStuffWorks.com
• How Modems Work
• How ISDN Works?
• How Cable Modems Work?
• How Fiber Optics Work?