Chapter 2: The Physical Layer

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Chapter 2 The Physical Layer
2
Learning Objectives

• The Theoretical Basis for Data Communication
• Transmission Media
• Wireless Transmission
• The Telephone System
• Narrowband ISDN
• Broadband ISDN and ATM

continued
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Learning Objectives

• Explain ISDN communications for data networks
• Describe ATM communications
• Cellular Radio
• Communication Satellites

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The Theoretical Basis for Data Communication

• Fourier Analysis
• Bandwidth-Limited Signal
• The Maximum Data Rate of a Channel

5
Transmission Media

• Magnetic Media
• Twisted Pair
• Baseband Coaxial Cable
• Broadband Coaxial Cable
• Fiber Optics
• Transmission of Light Through Fiber
• Fiber Cables
• Fiber Optic Networks

continued
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Transmission Media

• Comparison of Fiber Optics and Copper Wire

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Transmission Media

• Thick coaxial cable
• Thin coaxial cable
• Shielded twisted-pair cable
• Unshielded twisted-pair cable
• Fiber-optic cable
• Wireless technologies

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Considerations for Media Selection

• Data transfer speed
• Use in specific network topologies
• Distance requirements
• Cable and cable component costs
• Additional network equipment required
• Ease of installation
• Immunity to outside interference
• Upgrade options

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Coaxial Cable (Coax)

• Components
• Copper core surrounded by insulation
• Insulation surrounded by another conducting
  metal, such as braided wire
• All covered by an outer insulating material
• Varieties
• Thick
• Thin

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Thick Coaxial Cable
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Thin Coaxial Cable

• Resembles television cable
• Attached to a BNC connector, which is then
  connected to a T-connector
• Middle of the T is connected to the NIC in the
  computer or network device
• A T-connector with a terminator at one end

Terminator
BNC connector
BNC T-connector
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Coaxial Cable Definitions

• Impedance is the total amount of opposition to
  the flow of current. In coaxial cable, a 50-ohm
  impedance influences how fast a packet can travel
  through the conductive material under optimal
  conditions.

continued
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Coaxial Cable Definitions

• In baseband transmission, the entire channel
  capacity of the medium is used by one data
  signal. Thus, only one node transmits at a time.
• Broadband transmission employs several
  transmission channels on a single communications
  medium. More than one node can transmit at a time.

continued
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Coaxial Cable Definitions

• Bandwidth is the capacity of a channel to
  transmit data.

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Twisted Pair Cable

• Components
• Pairs of insulated copper wires twisted together
  (for reduction of EMI and RFI)
• Covered with an outer insulating jacket
• Varieties
• Shielded twisted-pair (STP)
• Unshielded twisted-pair (UTP)

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Shielded Twisted-Pair Cable

• Pairs of insulated wires are twisted together,
  surrounded by a shielding material for added EMI
  and RFI protection, all inside a protective
  jacket.

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Unshielded Twisted-Pair Cable

• Most frequently used network cabling
• Low cost
• Relatively easy installation
• Has no shielding material between the pairs of
  insulated wires twisted together and the cables
  outside jacket

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Unshielded Twisted-Pair Cable
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Most Commonly Used Twisted-Pair Cable Categories
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To Attach UTP Cable to an RJ-45 Connector

• Connecting twisted-pair cable to an RJ-45
  connector

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Fiber Optic Cable

• Components
• One or more glass or plastic fiber cores inside a
  protective cladding material
• Covered by a PVC outer jacket

Optical fiber (core)
Glass cladding
Protective outer sheath (jacket)
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Fiber Optic Cable Definitions

• A fat pipe is fiber-optic cable used on a network
  backbone for high-speed communications.
• Attenuation is the amount of signal that is lost
  as the signal travels through the communications
  medium from its source (transmitting node) to the
  receiving node, measured in decibels.

continued
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Fiber Optic Cable Definitions

• The power budget is the difference between the
  transmitted power and the receiver sensitivity,
  measured in decibels. It is the minimum
  transmitter power and receiver sensitivity needed
  for a signal to be sent and received intact.

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Alternatives for Enterprise and WAN Requirements

• Fast Ethernet
• FDDI
• ISDN
• Frame relay

• Cell relay
• ATM
• SMDS
• SONET

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Fast Ethernet

• Fast Ethernet is Ethernet communica-tions at
  speeds up to 100 Mbps and is defined under the
  IEEE 802.3u standard. It can send and receive
  packets at the same time.

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FDDI

• Fiber Distributed Data Interface (FDDI) is a
  fiber-optic data transport method capable of a
  100 Mbps transfer rate using a ring topology.

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Timed Token Method
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Types of Packets Sent by FDDI

• Synchronous communications are continuous bursts
  of data controlled by a clock signal that starts
  each burst.
• Asynchronous communications occur in discrete
  units in which a start bit at the front signals
  the start of a unit and a stop bit at the back
  signals the end of the unit.

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FDDI Packet Format
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FDDI Error Detection

• Long periods of no activity
• The token is presumed to be lost.
• Long periods in which the token is not present
• A node is assumed to be transmitting continuously.

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FDDI Communications Media

• Single-mode fiber-optic cable
• Allows one bundle of light to enter the fiber
• Used for network backbones in which data must
  travel over long distances
• Multimode fiber-optic cable
• Allows many bundles of light to enter at a given
  time
• Used for desktop workgroup applications that
  involve shorter transmission distances

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FDDI Dual Ring

• Data transmission redundancy makes FDDI networks
  highly reliable.
• Redundancy is accomplished by using two network
  rings.

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FDDI Wrapping in an Error Condition
Path blocked due to a network problem
Network path doubles back to become a single
ring, still reaching all nodes.
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Classes of Nodes that Connect to FDDI

• A Class A node, or dual-attached node, on an FDDI
  ring is a network device, such as a hub, that is
  connected to both rings in the FDDI architecture.
• A Class B node, or single-attached node, in FDDI
  is a workstation, server, or host computer that
  connects to the primary ring only and through
  that connects to a Class A node (hub).

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X.25

• X.25 is a packet-switching protocol for
  connecting remote networks at speeds up to 64
  Kbps.

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Switching Techniques

• Circuit switching is a network communication
  technique that uses a dedicated channel to
  transmit information between two modes.
• Message switching sends data from point to point,
  with each intermediate node storing the data,
  waiting for a free transmission channel, and
  forwarding the data to the next point until the
  destination is reached.

continued
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Switching Techniques

• Packet switching establishes a logical channel
  between two transmitting nodes but uses several
  different paths of transmission to continually
  find the best routes to the destination.

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X.25 Transmission Modes

• A switched virtual circuit is a communication
  channel that is established for only as long as
  the communication session lasts.
• In a permanent virtual circuit the communica-tion
  channel stays connected at all times.
• An X.25 datagram does not use a particular
  communication channel. Data arrives at different
  times because each datagram may follow a
  different route to the destination.

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X.25 Packet Switching

• Data terminal equipment (DTE) consists of
  terminals, workstations, servers, and host
  computers that operate on a packet-switching
  network.
• Data communications equipment (DCE) consists of
  network devices that perform packet switching.

continued
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X.25 Packet Switching

• A packet assembler/disassembler (PAD) is a device
  that converts data from a format used by a DCE to
  one that can be placed on an X.25 communications
  network, and it translates data received in an
  X.25 communications format to a format that can
  be read by a DCE.

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X.25 Network
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X.25 Communication Layers
Relationship of X.25 communication layers and the
OSI model
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ISDN

• Integrated Services Digital Network (ISDN) is a
  standard for delivering data services over
  telephone lines, with a current practical limit
  of 64 Kbps and a theoretical limit of 622 Mbps.

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Advantages of ISDN

• Provides voice, data, and video services over one
  network
• Has a layered protocol structure compatible with
  OSI
• Offers communications channels in multiples of 64
  Kbps, 384 Kbps, and 1,536 Kbps
• Has switched and nonswitched connection services
• Has broadband ISDN capabilities of 155-622 Mbps

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I.200 Services for Networking

• Bearer services
• Circuit-mode option (see next slide)
• Packet mode option
• Include virtual call and permanent virtual call
  circuits that are modeled after X.25 switched and
  permanent virtual circuits
• Teleservices
• Provide for 3.1-KHz speech communications
• Supplementary services
• Available for voice communications

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ISDN Circuit Modes
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Digital Communications Techniques

• ISDN supports two interfaces
• The basic rate interface (BRI) consists of three
  channels. Two are 64-Kbps channels for data,
  voice, and graphics transmissions. The third is a
  16-Kbps channel used for communication signaling.
• The primary rate interface (PRI) consists of
  switched communications in multiples of 1,536
  Kbps.

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Digital Communications Definitions

• Broadband ISDN (B-ISDN) is being developed to
  provide an initial ISDN data transfer rate of 155
  Mbps. The theoretical limit is 622 Mbps.
• Multiplexing is a form of circuit switching in
  which several physical channels are connected to
  a switch called a multiplexer. Multiple computers
  are also connected to the switch.

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T-Carrier

• A T-carrier is a dedicated telephone line for
  data communications.

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T-Carrier Servicesand Data Rates
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ISDN and OSI Layered Communications
Relationship of ISDN layered communications to
the OSI model
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Frame Relay

• Frame relay is a communications protocol that
  relies on packet switching and virtual circuit
  technology to transmit data packets.
• It does not incorporate extensive error checking,
  assuming that intermediate nodes will perform
  that task.

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Switching and Virtual Circuits

• Frame relay uses multiple virtual circuits over a
  single cable medium.
• Permanent
• A continuously available path between two nodes
• Switched
• Based on the need to establish a transmission
  session

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Cell Relay

• Cell relay is a communications protocol that uses
  large fixed-length cells to transmit voice,
  video, and data.
• A cell is a large fixed-length data-carrying unit
  primarily consisting of a header with
  transmission control information and a large
  payload section that contains data.

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ATM

• Asynchronous Transfer Mode (ATM)
• Uses multiple channels and switching to send
  voice, video, and data transmissions on the same
  network
• Stresses efficient, quality of service (QOS),
  high-capacity data transport
• Provides an international communications standard

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Acceptance of ATM

• ATM handles data, voice, and video transmissions.
• Because there is flexibility in geographic
  distance, it can be used for LAN and WAN
  communications.
• It can accommodate high-speed communications.
• It can provide high-speed communications between
  Ethernet, token ring, Fast Ethernet, FDDI, and
  other kinds of networks.

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ATM Cell
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ATM and OSI Layered Communications
OSI network layer
ATM layer 3 Responsible for physical and logical
routes and for constructing voice, video, and
data into cell payload
OSI data link layer
ATM layer 2 Responsiible for cell construction
and point-to-point connectivity
OSI physical layer
ATM layer 1 Responsible for physical
connectivity via electrical transport interface
Relationship of ATM layered communications to the
OSI model
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Advantages of ATM Switching

• ATM switching enables data to be transmitted at
  access speeds appropriate to the type of data
  sent.
• It permits use of higher bandwidths.
• Each ATM connection has its own dedicated
  bandwidth.
• Connection processes are more clearly defined
  because they are handled by the switch from point
  to point.

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SMDS

• Switched megabit data service (SMDS), also called
  multimegabit data service, was developed by
  regional telephone companies to provide
  cell-based, high speed communications between
  MANs.

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SMDS and OSI Layered Communications
Relationship of SMDS layered communications to
the OSI model
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Applications of SMDS

• Providing a high-speed link for regional networks
• Transmitting large image files, such as medical
  X-rays
• Transmitting architectural drawings and other CAD
  graphics
• Providing fast access to library holdings and
  electronic catalogs

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SONET

• SONET (for synchronous optical network)
  fiber-optic technology allows for high-speed
  (more than 1 Gbps) data transmission.
• Networks based on SONET can deliver voice, data,
  and video.
• Advantage It is nonproprietary.

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Applications of SONET

• Providing very high-speed data connectivity
  between distant networks
• Video conferencing between distant sites
• Long-distance teaching
• High-quality sound and video reproduction
• High-speed transmission of complex graphics, such
  as topographical maps and images created through
  satellite photography

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Communications Media and Characteristics

• Uses single-mode fiber-optic cable andT-carrier
  communications (starting at T3)
• Operates at a base level of 51.84 Mbps

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SONET Transmission Rates
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SONET Network Topology and Failure Recovery

• SONET travels in a ring topology capable of
  providing three options for failure recovery
• Unidirectional path switching
• Automatic protection switching
• Bidirectional line switching (highest level of
  redundancy)

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SONET Bidirectional Line Switching
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SONET Layers and the OSI Model
Relationship of SONET layered communications to
the OSI model
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Wireless Transmission

• Radio Transmission
• Infrared and Millimeter Waves
• Microwave Transmission

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Radio Transmission
Radio wave communications between buildings
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Radio Transmission Definitions

• Hertz is the measurement of the frequency of the
  alternation of an electrical current or radio
  wave.
• Kilohertz (kHz) 1000 hertz
• Megahertz (MHz) 1 million hertz
• Terahertz (THz) 1 trillion hertz
• In a line-of-site transmission, the signal goes
  from point to point.

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Infrared Transmission

• Can be broadcast in a single direction or in all
  directions, using an LED to transmit and a
  photodiode to receive

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Microwave Transmission

• Terrestrial microwave systems
• Satellite microwave systems

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Terrestrial Microwave Systems

• Transmit the signal between two directional
  antennas shaped like dishes
• Have transmissions in the frequency range of 4-6
  GHz or 21-23 GHz
• Require operator to obtain an FCC license

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Satellite Microwave Systems

• Transmit the signal from transmitting antenna to
  a satellite in space, then to the receiving
  antenna
• Have transmissions in 11-14 GHz range
• Require that you launch a satellite or lease
  service from a company offering this technology

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Advantages of Wireless Communications
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Disadvantages of Wireless Communications
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Communications Media Costs and Considerations

• Installation costs
• Maintenance costs

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