Chapter 5 LOCAL AREA NETWORK CONCEPTS AND ARCHITECTURES

1
Chapter 5LOCAL AREA NETWORK CONCEPTS AND
ARCHITECTURES
2
Objectives

• Introduce LAN
• Study OSI model
• Look at LAN Media
• Investigate LAN Architecture and Components
• Study standard LAN Architectures

3
What is a Local Area Network?

• LAN is a combination of hardware software tech.
  that allows computers to share a variety of
  resources e.g. printers, storage devices, Data,
  Applications, etc.
• It allows messages to be sent between attached
  computers ? Enable users to work together
  electronically Collaborative computing

4
What is a Local Area Network?

• Generally, LANs are confined to an area no larger
  than a single building or a small group of
  buildings
• It can be extended by connecting to other similar
  or dissimilar LANs, to remote users, or to
  mainframes computers LAN Connectivity or
  Internetworking
• Can be connected to other LANs of trading
  partners Enterprise Networking
• The computers themselves are not part of the LAN
  !!!

5
Categorizing LAN ArchitectureOSI Model

• Consists of 7 layers that loosely group the
  functional requirements for communication between
  two computing devices.
• Each layer relies on lower layers to perform more
  elementary functions and to offer total
  transparency to the intricacies of those
  functions. At the same time, each layer provides
  the same transparent service to upper layers.

6
OSI Model

1. Physical Layer responsible for the
   establishment, maintenance, termination of
   physical connection between communicating devices
   Point-to-Point data link.
2. Data-Link Layer responsible for the providing
   protocols that deliver reliability to upper
   layers for Point-to-Point connections established
   by physical layer protocols. To allow the OSI
   model to closely adhere to the protocol
   structure, operation of a LAN, Data-Link layer
   was splitted into two sublayers.

7
Data-Link Sublayers

• Media Access Control (MAC) interfaces with the
  physical layer is represented by protocols that
  define how the shared LAN media is to be accessed
  by the many connected computers.
• Logical Link Control (LLC) interfaces to the
  network layer.
• The advantage of splitting the Data-Link layer
  of having a single common LLC protocol is that it
  offers transparency to the upper layers while
  allowing the MAC sublayers protocols to vary
  independently.

8
OSI Model contd

1. Network Layer responsible for the establishment,
   maintenance, termination of end-to-end network
   links. Network layer protocols are required when
   computers that arent physically connected to the
   same LAN must communicate.
2. Transport Layer responsible for providing
   reliability for the end-to-end network layer
   connections. It provide end-to-end recovery
   flow control. It also, provide mechanisms for
   sequentially organizing network layer packets
   into a coherent message.

9
OSI Model contd

1. Session Layer responsible for establishing,
   maintaining, terminating sessions between user
   application programs.
2. Presentation Layer provide an interface between
   user applications various presentation-related
   services required by those applications. An
   example is data encryption/decryption protocols.
3. Application Layer it includes utilities that
   support end-user application programs but it does
   not include end-user application programs.

10
Figure 5-4 OSI Model - A Conceptual View
11
Encapsulation/De-encapsulation

• Encapsulation in this process, each successive
  layer of the OSI model adds a header according to
  the syntax of the protocol that occupies that
  layer.
• De-encapsulation in this process, each
  successive layer of the OSI model removes headers
  /or trailers processes the data that was
  passed to it from the corresponding layer
  protocol on the source client.
• These two processes describe how the various
  protocol layers interact with each other to
  enable an end-to-end communications session.

12
Figure 5-5 OSI Model - An Architectural View
13
LAN Media

• Not Twisted Pair
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
• Coaxial Cable (Coax)
• Fiber Optic

14
Not Twisted Pair

• Phone wire
• RYBG
• Flat Gray Modular Wiring
• 4, 6 and 8 wires

15
Unshielded Twisted Pair

• No Shielding
• EIA Cat (1 5)
• AWG
• Attenuation Loss of signal volume and power over
  a long distance
• NeXT a strong signal overpowering a weaker
  signal on an adjacent pair

16
Shielded Twisted Pair

• Shielding is metallic foil or copper braid
• Shielded from EMI and RFI

17
Coaxial Cable (coax)

• Reliable High speed data transmission over
  relatively long distance
• Used in Ethernet and comes in different thickness

18
Figure 5-5 Coax Cable Cross-Section
19
Fiber Optic

• Untappable and Immune to EMI and RFI
• Glass Vs. Plastic
• Multimode Step Index 200Mbps lt 1Km
• Multimode Graded Index 3Gbps several Kms
• Single mode light rays are more focused only one
  wavelength can pass at a time. (most expensive)

20
Figure 5-6 Fiber Optic Cable Cross Section
21
How is a LAN Implemented ?

1. Appropriate networking hardware software must
   be added to every computer or shared peripheral
   device that is to communicate via the LAN.
2. Some type of network media must physically
   connect the various networked computers and
   peripheral devices to converse with each other.

22
The LAN Architecture Model

• All network architecture are made up of the same
  logical components.
• To accurately describe a given network
  architecture, one needs to know the following
• Access methodology.
• Logical topology.
• Physical topology

23
Access Methodology

• Since many users is to send requests onto the
  shared LAN media at the same time, there must be
  some way to control access by multiple users to
  that media. These media-sharing methods are named
  Access methodologies.
• Sharing the media is an important concept in
  LANs, which are sometimes called media-sharing
  LANs.
• There is two access controlling methods
• 1- CSMA/CD 2-Token Passing

24
CSMA/CD

• Its based on the philosophy Lets just let
  everyone onto the media whenever they want if
  two users access the media at the same second,
  well work it out somehow.
• Carrier sense multiple access with collision
  detection
• Carrier sense the PC wishing to put data onto
  the shared media listens to the network to see if
  any other users are on line by trying to sense
  a neutral electrical signal known as the carrier.
• If no transmission is detected, multiple access
  allows anyone onto the media.

25
CSMA/CD

• If two user PCs should access the same media in
  the same time, a collision occurs collision
  detection lets the user PCs to know that their
  data wasnt delivered controls retransmission
  is such a way to avoid collisions.
• Another factor of collisions is propagation
  delaying, which is the time it takes to a signal
  from a source PC to reach a destination PC.
• Because of this delay, its possible for a
  workstation to sense if there is no signal on the
  shared media, when in fact another distant
  workstation has transmitted a signal that hasnt
  yet reached the carrier sensing PC.

26
Token Passing

• Dont you dare access the media until its your
  turn. You must first ask permission, only if I
  give you the magic token may you put your data on
  the shared media.
• It ensures that each PC user has 100 of the
  network channel available for data requests
  transfers by insisting that no PC accesses the
  network without processing a specific packet of
  data (Token).
• The token is first generated by a specified PC
  known as active monitor and passed among PCs
  until one PC would like to access the network.

27
Token Passing

• The requesting PC seizes the token, changes the
  token status from free to busy, puts its data
  frame onto the network, doesnt release the
  token until its assured that its data was
  delivered.
• Successful data delivery is confirmed by the
  destination workstation setting frame status
  flags to indicate a successful receipt of the
  frame.
• Upon receipt of the original frame with frame
  status flag set to destination address
  recognized, frame copied successfully the
  sending PC rests the token status from busy to
  free release it.
• The token is passed along the next PC.

28
Figure 5-6 Token-Passing Access Methodology
29
Logical Topology

• After the data message ha reached the
  shared-media LAN, the next step is to determine
  how that message will be passed from workstation
  to workstation until the message reaches its
  intended destination.
• This passing technology is known as Logical
  Topology.
• There are two known logical topologies
• 1- Sequential 2- Broadcast

30
Sequential Topology

• Also known as ring logical topology.
• The data is passed from one PC (or node) to
  another.
• Each node examines the destination address of the
  data packet to determine if this packet is meant
  for it
• If the data was not meant to be delivered at this
  node, the data packet is passed along to another
  node in the logical ring.

31
Broadcast Topology

• A data message is sent simultaneously to all
  nodes on the network.
• Each node decides individually if the data
  message was directed toward it. If not, the
  message is ignored.
• No need to pass the message to a neighboring node.

32
Physical Topology

• The clients servers must be physically
  connected to each other according to some
  configuration be linked by the shared media of
  choice.
• The physical layout configuration can have a
  significant impact on LAN performance
  reliability.
• There are three physical topologies
• 1- Bus 2- Ring 3-Star

33
Bus Topology

• A linear arrangement with terminators on either
  end devices connected to the Bus via
  connectors /or transceivers.
• A break or loose connection anywhere along the
  entire bus will bring the whole network down.

34
Ring Topology

• Each PC connected via a ring topology is actually
  an active part of the ring, passing data packets
  in a sequential pattern around the ring.
• If one of the PCs dies or a network adapter card
  malfunctions, the sequence is broken, the token
  is lost, the network is down !

35
Star Topology

• It avoids the drawbacks of both Bus Ring
  topologies by employing some type of central
  management device. This central device may called
  a Hub, a wiring center, a concentrator, a MAU
  (multistation access unit), a repeater, or a
  switching hub.
• By isolating each PC or node on its own leg or
  segment of the network, any node failure only
  affects that leg.
• If this central device goes down, the whole
  network goes down too.

36
Figure 5-7 LAN Physical Topology Choices
37
NETWORK ARCHITECTURES

• Classic Architectures
• Ethernet
• Token Ring
• FDDI
• High Speed Architectures
• Family of Fast Ethernet
• 100BaseT
• 100VG-AnyLAN
• Gigabit Ethernet (1000BaseT)
• 10 Gigabit Ethernet
• HSTR (High Speed Token Ring)
• Fibre Channel
• iSCSI
• LAN-Based ATM
• Home Network Architectures
• HPNA.
• Bluetooth and PAN

38
Ethernet

• Origins
• Invented by Robert Metcalfe (founder of 3Com
  CO.).
• Although Ethernet IEEE 802.3 are different
  standards.
• Ethernet is used to refer to IEEE 802.3
  compliant network.
• Functionality
• Access methodology CSMA/CD.
• Logical topology broadcast.
• Physical topology traditionally, bus
  currently, star.

39
Ethernet
Figure 5-8 Ethernet and IEEE 802.3 Standards
40
Ethernet

• Media related Ethernet standards

41
Token Ring

• Origin
• Olaf Soderbulm in 1969.
• IBM standardized it as IEEE 802.5.
• Functionality
• Access methodology token passing.
• Logical topology sequential.
• Physical topology before, ring now, star.

42
Token Ring

• Standards
• IEEE 802.5 no speed specification.
• Operate at speed of 4 16 Mbps.
• 24-bit data packet
• The starting delimiter field alert the token
  ring card installed in workstation that a frame
  is approaching.
• receive access control field.
• Workstation distinguish btw tokens MAC sub
  layer frames.
• If token bit 0 then frame represents free
  token.
• If token bit 1 then frame represents busy
  token.
• Routing info used with source routing bridges
  that link multiple token ring LANs (LAN-to-LAN).
• Sequential logical topology message passing
  form neighbor to neighbor.
• Token ring architecture logical ring,
  physical star.

43
Active Monitor

• Removes Dead frames
• Replace lost or damaged token
• Responsible for master clock
• Makes sure there is only one active monitor
• Provide buffer for token in small networks

44
FDDI

• Origins
• Fiber Distributed Date Interface.
• 100 Mbps network architecture.
• Specified 1984 by ANSI(X3T9.5).
• No IEEE standard.
• supports IEEE802.2 protocol. It is most popular.
• Functionality
• Access methodologyModified token passing.
• Logical topologySequential.
• Physical topologyDual counter-rotating rings.

45
FDDI

• Built-in reliability Longer distance
• Support 100Mbps of bandwidth.
• High degree of reliability security.
• Reliability comes from fiber EMI RFI design
  of physical topology of FDDI.
• EMI (Electromagnetic Interference).
• RFI (Radio Frequency Interference).
• FDDI physical topology compromised of two
  separate rings in which data moves simultaneously
  in opposite directions.
• 1st ring Primary data ring.
• 2nd ring Secondary or backup data ring used in
  failure of primary ring or an attached
  workstation.

46
FDDI
Figure 5-13 FDDI Network Architecture and
Technology
47
FDDI

• Both rings attached to a single hub or a
  concentrator.
• Distance FDDI LAN cover 500 nodes at 2km
  apart.
• If repeaters used every 2km media can stretch
  up to 200km.
• Interoperate with IEEE 802.3 10-Mbps Ethernet.
• Interoperation needs FDDI-to-Ethernet bridge.
• Bridge can connect many Ethernets.
• PCs, and mainframes etc. must be equipped with
  either FDDI NIC or external FDDI controllers if
  they wish to access the FDDI LAN.

48
FDDI

• To cut down costs benefit of 100 Mbps
  bandwidth managers only connect one of the 2 FDDI
  fiber rings.
• This is known as SAS (Single Attachment
  Stations).
• Else if both fiber rings connected it is called
  DAS (Dual Attachment Stations).
• The heart of the FDDI LAN is the FDDI
  concentrator or hub.
• The design of the hubs is modular with backbone
  connections to both FDDI rings.
• The dual counter rotating rings network
  architecture of FDDI has a self-healing
  capabilities.

49
FDDI
Figure 5-14 FDDIs Self-Healing Ability
50
FDDI

• Standards
• Two ways of modification to the token passing
  access methodology.
• FDDI removes the token from the ring transmit
  a full data frame. If the transmition is complete
  it releases a new token. Collision is avoided as
  only one station can have the free token at a
  time, and a station cannot put a data message
  onto the Network without a token.
• A Station can send more than one message per
  token.

51
FDDI

• FDDI could be run under copper wires, shielded
  or unshielded twisted pair (UTP) CDDI copper
  distributed data interface. It still support
  100Mbps but limit distance to 100m/segment. ANSI
  standard for CDDI is TP-PMD (Twisted Pair
  Physical Media dependant).

Figure 5-15 FDDI Token and Data Frame Layouts
52
FDDI

• Applications
• Network architecture trends.
• Campus backbone
• attach multiple devices to FDDI rings (dual ring
  of trees).
• A server may be attached to more than one FDDI
  concentrator to provide redundant
• connections and avoid fault tolerance (dual
  homing).
• High bandwidth work groups
• used when connecting less than 20 PCs for high
  bandwidth communications.
• E.g. Multimedia workstations, engineering
  workstations, CAD/CAM workstations.
• As a power user require GUIs like windows.
• High bandwidth sub workgroup connections
• Only 2 or 3 servers

53
FDDI
Figure 5-16 Alternative Applications of the FDDI
Network Architecture
54
HIGH-SPEED NETWORK ARCHTECTURES

• 100BaseT
• Represents a family of fast Ethernet standards
  offering 100 Mbps performance and adhering to the
  CSMA/CD access methodology.
• The three media-specific physical layer standards
  of 100BaseT are
• 1- 100BaseTX the most common of the three the
  one for which the most technology is available.
  It specifies 100-Mbps performance over two pair
  of category 5 UTP (Unshielded twisted pair) or
  two pair of type 1 STP (Shielded twisted pair).

55
100BaseT

• 2- 100BaseT4 Physical layer standard for
  100-Mbps transmission over four pairs of Category
  3,4,or 5 UTP.
• 3- 100BaseFX Physical layer standard for
  100-Mbps transmission over fiber optic cable.
• Network Architecture it use the same IEEE802.3
  MAC sublayer frame layout yet transmit it at 10
  times faster than 10BaseT. There must be a
  trade-off that comes in the maximum network
  diameter
• 10BaseTs maximum network diameter is 2500 m with
  up to 4 repeaters between any two nodes.
  gtgt

56
100BaseT

• 100BaseTs maximum network diameter is 210 m with
  up to only 2 repeaters between end nodes.
• Technology
• Most of the 100BaseT NICs are called 10/100 NICs
  which means that they are able to support either
  10BaseT or 100BaseT but not simultaneously.
• 10BaseT 100BaseT networks can only interoperate
  with the help of internetworking devices such as
  10/100 bridges routers.
• Some Ethernet switches can support 100BaseT
  connection can auto-sense, or distinguish
  between 10BaseT 100BaseT traffic.

57
Figure 5-17 100BaseT Network Architecture
Implementation
58
HSTR

• High Speed Token Ring
• 100Mbps Token Ring
• No IEEE Standard
• Supports IEEE 802.1q which allows Ethernet frames
  to be encapsulated within Token Ring frames
• Important for organizations that must support
  both network architectures

59
Gigabit Ethernet

• From the family of Fast Ethernet, IEEE 802.3z
  standard.
• Known as (1000Base-X)
• 1000BaseSX Multimode Fiber Optic, horizontal
  floorplanning
• 1000BaseLX Singlemode Fiber Optic, vertical
  backbone
• 1000BaseCX Copper Wire (Dead)
• 1000BaseTX 4 pairs Cat 5 UTP, max. 100m.
• - The final standard retains Ethernets CSMA/CD
  access methodology.
• - Ethernet frame size did not change.

60
Gigabit Ethernet

• Gigabit Ethernet combined Speed with Maximum
  Transmission distance by using single mode fibers
  that can run up to 5Km, This reflects on its
  applications
• Resolving Server bandwidth constraints
• Removing bottlenecks from backbone.
• Beyond Gigabit Ethernet 10 Gigabit Ethernet

61
Fiber Channel

• Alternative Gigabyte Ethernet NT Architecture.
• ANSI standard X3T9.3.
• Speed of 133 to 1.062 Gbps.
• Uses optical fiber and copper cables.
• Used to connect high-performance storage devices
  and RAID (Redundant Arrays of Independence/Inexpen
  sive Disks) subsystems to computers.
• Fiber channel switches and Network Interface
  Cards are also available.

62
LAN-Based ATM

• ATM (Asynchronous Transfer Mode).
• It is a switched NT technology.
• Speed range from 25Mbps to several Gbps.
• NICs are available for servers work stations.
• For ATM based to communicate with non-ATM based
  computers a process known as LAN emulation must
  be implemented.
• For applications to take advantage of ATMs
  speed and features they must be ATM aware.
• ATM has been implemented in animation and
  stock-trading industries.

63
Home Networks

• HPNA (Home Phone Line Networking Alliance).
• Runs Ethernet over the RGYB Telephone line by
  using available bandwidth (10Mbps).
• Wireless like BlueTooth.
• CSMA\CA, more overhead, only 1.6Mbps, support up
  to 16 nodes.
• Operate in the range of 2.4GHz.
• They jump frequencies to avoid conflict and
  interference.
• Known also as Spread Spectrum Technologies

64
Figure 5-22 HPNA Implementation
65
Guided Transmission Media

• Transmission capacity depends on distance and
  type of network (point-to-point or multipoint)
• Twisted Pair
• Coaxial cable
• Optical fiber

66
Twisted Pair

• Least expensive and most widely used
• Two insulated copper wires arranged in regular
  spiral pattern
• Number of pairs bundled together in a cable
• Twisting decreases crosstalk interference between
  adjacent pairs in cables
• Using different twist length for neighboring pairs

67
Twisted Pair
68
Twisted Pair - Applications

• Most common transmission medium for both analog
  digital signals
• Telephone network
• Between house and local exchange (subscriber
  loop)
• Within buildings
• Telephones connected to private branch exchange
  (PBX) for voice traffic
• Connections to digital switch or digital PBX
  (64kbps)
• For local area networks (LAN)
• 10Mbps or 100Mbps

69
Twisted Pair - Pros and Cons

• Cheap
• Easy to work with
• Low data rate
• Short range

70
Twisted Pair - Transmission Characteristics

• Analog
• Amplifiers every 5km to 6km
• Digital
• Use either analog or digital signals
• repeater every 2km or 3km
• Attenuation is strong function of frequency
• Susceptible to interference and noise
• Easy coupling with electromagnetic fields
• A wire run parallel to power line picks up 60-Hz
  energy
• Impulse noise easily intrudes into twisted pairs

71
Attenuation of Guided Media
72
Twisted Pair - Transmission Characteristics

• Measures to reduce impairments
• Shielding with metallic braids or sheathing
  reduces interference
• Twisting reduces low frequency interference
• Different twist length in adjacent pairs reduces
  crosstalk
• Limited distance
• Limited bandwidth
• For point-to-point analog signaling, 1MHz
• Limited data rate
• For long distance digital point-to-point
  signaling, 4 Mbps
• For very short distances, 100Mbps-1Gbps

73
Unshielded and Shielded TP

• Unshielded Twisted Pair (UTP)
• Ordinary telephone wire
• Cheapest
• Easiest to install
• Suffers from external EM interference
• Shielded Twisted Pair (STP)
• Metal braid or sheathing that reduces
  interference
• Better performance at higher data rates
• More expensive
• Harder to handle (thick, heavy)

74
Unshielded Twisted-Pair (UTP)

• Quality of UTP vary from telephone-grade wire to
  extremely high-speed cable
• Cable has four pairs of wires inside the jacket
• Each pair is twisted with a different number of
  twists per inch to help eliminate interference
• The tighter the twisting, the higher the
  supported transmission rate and the greater the
  cost per foot

75
UTP Categories

• Cat 3
• up to 16MHz
• Voice grade found in most offices
• Twist length of 7.5 cm to 10 cm
• Cat 4
• up to 20 MHz

76
UTP Category 5

• Up to 100MHz
• Commonly pre-installed in new office buildings
• Twist length 0.6 cm to 0.85 cm
• Current standard for data
• 100 meter maximum segment length
• 100 mb/s available
• GB/s over short distances
• Inexpensive
• Can be pulled in existing conduit

77
Unshielded Twisted Pair Connector

• The standard connector for unshielded twisted
  pair cabling is an RJ-45 connector.
• A plastic connector that looks like a large
  telephone-style connector
• RJ stands for Registered Jack connector follows
  a standard borrowed from telephone industry.
• Standard designates which wire goes with each pin
  inside the connector.

78
Guided Media Twisted pair (Category 5)
79
Cat 5 Network Cables
Category 5 Cable composed of 4 twisted pairs
Cat 5Cable RJ45 composed of 4 twisted pairs
Shielded Cat 5 Network Cable RJ45
80
Comparison of Shielded Unshielded Twisted Pair
Attenuation (dB per 100m)
Near-End Crosstalk (dB)
Frequency (MHZ) Cat. 3 UTP Cat. 5 UTP 150-ohm STP Cat. 3 UTP Cat. 5 UTP 150-ohm STP
1 2.6 2.0 1.1 41 62 58
4 5.6 4.1 2.2 32 53 58
16 13.1 8.2 4.4 23 44 50.4
25 10.4 6.2 41 47.5
100 22.0 12.3 32 38.5
300 21.4 31.3
81
Coaxial Cable

• Hollow outer cylindrical conductor surrounding a
  single inner conductor
• Inner conductor held by regularly spaced
  insulating rings or solid dielectric material
• Operates at higher frequencies than twisted pair

82
Coaxial Cable
83
Coaxial Cable Layers
outer jacket (polyethylene)
shield(braided wire)
insulating material
copper or aluminum conductor
84

85
Optical Fiber
86
Optical Fiber

• Thin, flexible material to guide optical rays
• Cylindrical cross-section with three concentric
  links
• Core
• Innermost section of fiber
• One or more very thin (diameter 8-100 mm) strands
  or fibers.
• Cladding
• Surrounds each strand
• Plastic or glass coating with optical properties
  different from core

87
Optical Fiber

• Jacket
• Outermost layer, surrounding one or more
  claddings
• Made of plastic and other materials
• Protects from environmental elements like
  moisture, abrasions and crushing

88
Optical Fiber Single fiber

89
Optical Fiber Cable
90
Optical Fiber Structure
91
Optical Fiber - Benefits

• Greater capacity
• Data rates of hundreds of Gbps over tens of Kms
• Smaller size weight
• Significantly lower attenuation
• Electromagnetic isolation
• Not affected by external EM fields
• Not vulnerable to interference, impulse noise, or
  crosstalk
• No energy radiation little interference with
  other devices security from eavesdropping
• Greater repeater spacing
• 10s of km at least
• Lower cost and fewer error sources

92
Optical Fiber - Applications

• Long-haul trunks
• Increasingly common in telephone networks
• About 1500 km in length with high capacity
  (20,000-60,000 voice channels)
• Metropolitan trunks
• Average length of about 12 km with capacity of
  100,000 voice channels
• Mostly, repeaters not required
• Rural exchange trunks Lengths from 40 to 160 km
  with fewer than 5000 voice channels
• Subscriber loops Handles image, video, voice,
  data
• LANs 100Mbps to 1 Gbps, support hundreds of
  stations on campus

93
Transmission Characteristics

• Single-encoded beam of light transmitted by total
  internal reflection.
• Fiber has two basic types
• Single mode
• Multimode
• Graded index
• Step index.

94
Transmission Modes

• Single mode fiber
• the light is guided down the center of an
  extremely narrow core
• Multimode step-index fiber
• the reflective walls of the fiber move the light
  pulses to the receiver
• Multimode graded-index fiber
• acts to refract the light toward the center of
  the fiber by variations in the density

95
Transmission Modes
96
Step-index multimode

• Core made of one type of glass.
• Light traveling in fiber travels in straight
  lines, reflecting off the core/cladding interface
• Rays at shallow edges reflected and propagated
  along fiber
• Other rays absorbed by surrounding material
• Allows for multiple propagation paths with
  different path lengths and time to traverse fiber
• A pulse of light is dispersed while traveling
  through the fiber
• Limits rate at which data can be accurately
  received
• Best suited for transmission over very short
  distances

97
Graded Multimode Fiber

• Core is composed of many different layers of
  glass, with indices of refraction producing a
  parabola index profile
• A properly constructed index profile will
  compensate for the different path lengths of each
  mode
• Bandwidth capacity of graded fiber 100 times
  larger than step index fiber
• Normally uses inexpensive LED laser transmitter
  and receivers
• Maximum distance up to 2 km
• Most common type is 62.5/125mm
• Uses wavelengths of 850nm and 1300nm
• Often used for building backbones and short
  inter-building communications

98
Graded Multimode Fiber

• Higher refractive index at center makes rays
  close to axis advance slower than rays close to
  cladding
• Light in core curves helically reducing traveling
  distance (does not zigzag off cladding)
• Shorter path higher speed makes light at
  periphery as well as axis travel at same speed

99
Single-Mode Fiber

• Shrinks core size to a dimension about 6 times
  the wavelength of the fiber, causing all the
  light to travel in only one mode
• Modal dispersion disappears and bandwidth of the
  fiber increases by at least a factor of 100 over
  graded index fiber
• Can be used for distances of 30 km or when high
  data rates are required

100
Transmission Modes
101
Optical Fiber Light Sources

• Semiconductor devices that emit light when
  voltage applied
• Light Emitting Diode (LED)
• Cheaper
• Wider operating temp range
• Longer operational life
• Injection Laser Diode (ILD)
• More efficient
• Greater data rate
• Wavelength Division Multiplexing (WDM)
• Multiple beams of light at different frequencies
  transmitted simultaneously
• 100 beams operation at 10 Gbps, for a total of 1
  trillion bps

102
Fiber Optic Attenuation

• Attenuation of optical fiber is a result of two
  factors, absorption and scattering
• Absorption is caused by absorption of light and
  conversion to heat by molecules in the glass.
• absorption occurs at discrete wavelengths, and
  occurs most strongly around 1000 nm, 1400 nm and
  above1600 nm.
• Scattering occurs when light collides with
  individual atoms in the glass
• Light scattered at angles outside the numerical
  aperture of fiber will be absorbed into the
  cladding or transmitted back toward the source.

103
Fiber Optic Attenuation

• Scattering is a function of wavelength,
  proportional to inverse fourth power of
  wavelength of light
• doubling wavelength of light, reduces scattering
  losses 16 times
• For long distance transmission, use longest
  practical wavelength for minimal attenuation and
  maximum distance between repeaters
• Fiber optic systems transmit in the "windows"
  created between the absorption bands at 850 nm,
  1300 nm and 1550 nm
• Plastic fiber has a more limited wavelength band,
  that limits practical use to 660 nm LED sources

104
Fiber Optic Attenuation
105
Fiber Types and Typical Specifications
106
Fiber Optic Cables
Duplex Multimode 62.5/125 mm
Duplex Single-mode 9/125 mm
Fiber optic cable
107
Coaxial Cable

• Outer conductor covered with a jacket or shield
• Diameter from 1 to 2.5 cm
• Shielded concentric construction reduces
  interference crosstalk
• Can be used over longer distances supports more
  stations on a shard line that twisted pair

108
Coaxial Cable Applications

• Most versatile medium
• Television distribution
• Ariel to TV, Cable TV
• Can carry hundreds of TV channels for tens of
  kms.
• Long distance telephone transmission
• Can carry 10,000 voice channels simultaneously
• Being replaced by fiber optic
• Short distance computer systems links
• Local area networks

109
Coaxial Cable - Transmission Characteristics

• Used to transmit both analog digital signals
• Superior frequency characteristics compared to
  twisted pair (1KHz-1GHz)
• Less susceptible to interference crosstalk
• Constraints on performance are attenuation,
  thermal noise, intermodulation noise
• Analog
• Amplifiers every few km
• Closer spacing if higher frequency (up to 0.5
  GHz)
• Digital
• Repeater every 1km
• Closer spacing for higher data rates

110
LAN Media

• Unshielded Twisted Pair (UTP) is currently the
  most popular.
• There are different grades of UTP
• Category 1, 2, 3, 4, 5
• Shielded Twisted Pair (STP) is similar, except
• it has a foil shielding or copper braid to reduce
  EMI
• costs considerably more

111
LAN Media

• Coaxial cable
• features a solid metal core surrounded by a
  plastic insulator, then a foil shield braid,
  and finally a plastic or vinyl protective jacket
• cable-tv uses coaxial cable
• there are different grades of coax cable

112
LAN Media

• Fiber Optics
• glass core surrounded by glass cladding, and
  protected by a plastic or vinyl jacket
• very secure
• unaffected by EMI
• typically capable of 200 Mbps - 3 Gbps
• different grades are available
• multimode step index, multimode graded index,
  single mode

113
LAN Switching vs. Classical Segmentation

• Classical Segmentation
• works best when most traffic local
• max. throughput backbone speed
• Switching
• provides high degree of segmentation
• max. throughput switch speed