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Title: networking notes

5. Introduction to Networking
What is a Network?
  • A network consists of 2 or more computers
    connected together, and they can communicate and
    share resources (e.g. information)

Why Networking?
  • Sharing information i.e. data communication
  • Do you prefer these?
  • Or this?

  • Sharing hardware or software
  • E.g. print document
  • Centralize administration and support
  • E.g. Internet-based, so everyone can access the
    same administrative or support application from
    their PCs

How many kinds of Networks?
  • Depending on ones perspective, we can classify
    networks in different ways
  • Based on transmission media Wired (UTP, coaxial
    cables, fiber-optic cables) and Wireless
  • Based on network size LAN and WAN (and MAN)
  • Based on management method Peer-to-peer and
  • Based on topology (connectivity) Bus, Star, Ring

Transmission Media
  • Two main categories
  • Guided ? wires, cables
  • Unguided ? wireless transmission, e.g. radio,
    microwave, infrared, sound, sonar
  • We will concentrate on guided media here
  • Twisted-Pair cables
  • Unshielded Twisted-Pair (UTP) cables
  • Shielded Twisted-Pair (STP) cables
  • Coaxial cables
  • Fiber-optic cables

Twisted-Pair Cables
  • If the pair of wires are not twisted,
    electromagnetic noises from, e.g., motors, will
    affect the closer wire more than the further one,
    thereby causing errors

Copper - Twisted Pair
  • Dialup over telephone line.
  • DSL (Digital Subscriber Line)
  • High-speed (256 Kbps 55 Mbps), Full-duplex.
  • Asymmetric Digital Subscriber Line (ADSL) and
    High-bit-rate Digital Subscriber Line (HDSL)
  • CAT5
  • Ethernet cable standard defined by the Electronic
    Industries Association and Telecommunications
    Industry Association (EIA/TIA).
  • Speeds up to 100 Mbps.
  • Connector
  • RJ-45 - Standard connectors used for unshielded
    twisted-pair cable.

  • Unshielded Twisted-Pair (UTP)
  • Typically wrapped inside a plastic cover (for
    mechanical protection)
  • A sample UTP cable with 5 unshielded twisted
    pairs of wires

  • Shielded Twisted-Pair (STP)
  • STP cables are similar to UTP cables, except
    there is a metal foil or braided-metal-mesh cover
    that encases each pair of insulated wires

  • Categories of UTP Cables
  • EIA classifies UTP cables according to the
  • Category 1 ? the lowest quality, only good for
    voice, mainly found in very old buildings, not
    recommended now
  • Category 2 ? good for voice and low data rates
    (up to 4Mbps for low-speed token ring networks)
  • Category 3 ? at least 3 twists per foot, for up
    to 10 Mbps (common in phone networks in
    residential buildings)
  • Category 4 ? up to 16 Mbps (mainly for token
  • Category 5 (or 5e) ? up to 100 Mbps (common for
    networks targeted for high-speed data
  • Category 6 ? more twists than Cat 5, up to 1 Gbps

Common Network Media
  • Electrical (copper)
  • Coaxial Cable
  • Single copper conductor in the center surrounded
    by a plastic layer for insulation and a braided
    metal outer shield.
  • Twisted pair
  • Four pairs of wires twisted to certain
  • Available in shielded and unshielded versions.
  • Fiber-optic - A cable, consisting of a center
    glass core surrounded by layers of plastic, that
    transmits data using light rather than
  • Atmosphere/Wireless Uses Electromagnetic waves.
    whose frequency range is above that of
    microwaves, but below that of the visible
  • Choose Media based on
  • Wiring configurations
  • Distance and location limitations
  • Speed
  • Reliability
  • Security
  • Budget

  • Coaxial Cables
  • In general, coaxial cables, or coax, carry
    signals of higher freq (100KHz500MHz) than UTP
  • Outer metallic wrapping serves both as a shield
    against noise and as the second conductor that
    completes the circuit

  • Infrared light is transmitted through fiber and
    confined due to total internal reflection.
  • Fibers can be made out of either plastic or
  • Used for high speed backbones and pipes over long
  • Comparatively expensive.

Optical Fiber
  • Fiber-Optic Cables
  • Light travels at 3?108 ms-1 in free space and is
    the fastest possible speed in the Universe
  • Light slows down in denser media, e.g. glass
  • Refraction occurs at interface, with light
    bending away from the normal when it enters a
    less dense medium
  • Beyond the critical angle ? total internal

  • An optical fiber consists of a core (denser
    material) and a cladding (less dense material)
  • Simplest one is a multimode step-index optical
  • Multimode multiple paths, whereas step-index
    refractive index follows a step-function profile
    (i.e. an abrupt change of refractive index
    between the core and the cladding)
  • Light bounces back and forth along the core
  • Common light sources LEDs and lasers

  • Advantages and Disadvantages
  • Noise resistance ? external light is blocked by
    outer jacket
  • Less signal attenuation ? a signal can run for
    miles without regeneration (currently, the lowest
    measured loss is about 4 or 0.16dB per km)
  • Higher bandwidth ? currently, limits on data
    rates come from the signal generation/reception
    technology, not the fiber itself
  • Cost ? Optical fibers are expensive
  • Installation/maintenance ? any crack in the core
    will degrade the signal, and all connections must
    be perfectly aligned

  • 10BaseT
  • Ethernet specification for unshielded twisted
    pair cable (category 3, 4, or 5), transmits
    signals at 10 Mbps (megabits per second) with a
    distance limit of 100 meters per segment.
  • 10BaseF
  • Ethernet specification for fiber optic cable,
    transmits signals at 10 Mbps (megabits per
    second) with a distance limit of 2000 meters per
  • 100BaseT
  • Ethernet specification for unshielded twisted
    pair cabling that is used to transmit data at 100
    Mbps (megabits per second) with a distance limit
    of 100 meters per segment.
  • 1000BaseTX
  • Ethernet specification for unshielded twisted
    pair cabling that is used to transmit data at 1
    Gbps (gigabits per second) with a distance
    limitation of 220 meters per segment.

(No Transcript)
  • Local Area Network (LAN)
  • Small network, short distance
  • A room, a floor, a building
  • Limited by no. of computers and distance covered
  • Usually one kind of technology throughout the LAN
  • Serve a department within an organization
  • Examples
  • Network inside the Student Computer Room
  • Network inside CF502
  • Network inside your home

  • Definition LAN
  • "local area network"
  • Is a group of computers and associated devices
    that share a common communications line or
    wireless link and typically share the resources
    of a single processor or server within a small
    geographic area (for example, within an office
  • Usually, the server has applications and data
    storage that are shared in common by multiple
    computer users.
  • A local area network may serve as few as two or
    three users (for example, in a home network) or
    many as thousands of users.

  • Definition Wireless LAN
  • A local area network that transmits over the air
    typically in an unlicensed frequency such as the
    2.4GHz band.
  • A wireless LAN does not require lining up devices
    for line of sight transmission.
  • Wireless access points (base stations) are
    connected to an Ethernet hub or server and
    transmit a radio frequency over an area of
    several hundred to a thousand feet which can
    penetrate walls and other non-metal barriers.
  • Roaming users can be handed off from one access
    point to another like a cellular phone system.
  • Laptops use wireless network cards that plug into
    an existing PCMCIA slot or that are self
    contained on PC cards, while stand-alone desktops
    and servers use plug-in cards (ISA, PCI, etc.).

  • Characteristics LAN
  • Topology
  • The geometric arrangement of devices on the
    network or the shape of a local-area network
    (LAN) or other communications system.
  • Protocols
  • The rules and encoding specifications for sending
    data. The protocol defines the format and meaning
    of the data that is exchanged. The protocols also
    determine whether the network uses a peer-to-peer
    or client/server architecture.
  • Media
  • Devices can be connected by twisted-pair wire,
    coaxial cables, or fiber optic cables. Some
    networks do without connecting media altogether,
    communicating instead via radio waves.

  • Wide Area Network (WAN)
  • A network that uses long-range telecommunication
    links to connect 2 or more LANs/computers housed
    in different places far apart.
  • Towns, states, countries
  • Examples
  • Network of our Campus
  • Internet

Your home
Student Computer Centre
  • Example WAN technologies
  • ISDN Integrated Service Digital Network
  • Basic rate 192 Kbps Primary rate 1.544Mbps
  • T-Carriers ? basically digital phone lines
  • T1 1.544Mbps T3 28?T1
  • Frame relay
  • Each link offers 1.544Mbps or even higher
  • ATM Asynchronous Transfer Mode
  • Support B-ISDN 155Mbps or 622Mbps or higher
  • SONET Synchronous Optical Network
  • Basic rate OC1 51.84Mbps
  • Support OC12 and up to OC192 (9953.28Mbps) or
    even higher in the future

  • Example of WAN Broadband Cable Network
  • Cable TV services have been extensively developed
    in most modern cities
  • Cable TV companies try to make use of their
    coaxial cable installed (that are supposed to
    carry TV signals) to deliver broadband data
  • Many cable network wiring has been replaced with
    hybrid fiber-coax (HFC) ? i.e. use of fiber-optic
    cable to connect to the subscribers buildings,
    and then the original coaxial cable to connect to
    each household

The connection is shared by a number of
subscribers, hence may raise performance and
security problems
Fiber-optic cable
Cable Drop
Cable company
Coaxial Cable
  • Cable is an asymmetrical technology
  • Downstream max 36 Mbps
  • Upstream max 10 Mbps
  • May be reduced to 3 10 Mbps downstream and 2
    Mbps upstream, depending on no. of subscribers
  • Need a special cable modem

Ethernet link to PC
Teryon Cable Modem
Coaxial link from cable TV socket
Wireless Media
  • Wireless LAN or WLAN
  • Wireless local area network that uses radio
    waves as its carrier
  • Wi-Fi ("Wireless Fidelity)
  • A set of standards for WLANs based on IEEE 802.11
  • Wi-Max
  • Emerging technology that can cover ranges up to
    10 miles or more
  • Satellite/Microwave
  • High speed media used for longer distances and
    remote locations

Sample LAN ImplementationHome Configuration
Sample LAN ImplementationBusiness Configuration
Sample LAN ImplementationBusiness Configuration
Topology ? 3 basic types
  • How so many computers are connected together?
  • Bus Topology Ring Topology
  • Star Topology

  • Topology types
  • bus topology All devices are connected to a
    central cable, called the bus or backbone. Bus
    networks are relatively inexpensive and easy to
    install for small networks. Ethernet systems use
    a bus topology.
  • star topology All devices are connected to a
    central hub. Star networks are relatively easy to
    install and manage, but bottlenecks can occur
    because all data must pass through the hub. This
    is not much of a problem anymore with the
    widespread deployment of switches.
  • ring topology All devices are connected to one
    another in the shape of a closed loop, so that
    each device is connected directly to two other
    devices, one on either side of it. Ring
    topologies are relatively expensive and difficult
    to install, but they offer high bandwidth and can
    span large distances.

  • Bus Topology
  • Simple and low-cost
  • A single cable called a trunk (backbone, segment)
  • Only one computer can send messages at a time
  • Passive topology - computer only listen for, not
    regenerate data
  • Star Topology
  • Each computer has a cable connected to a single
  • More cabling, hence higher cost
  • All signals transmission through the hub if
    down, entire network down
  • Depending on the intelligence of hub, two or more
    computers may send message at the same time

How to construct a network with Bus / Star
Bus Topology
Coaxial cable
Star Topology
BNC T-Connector
Network Card
  • Ring Topology
  • Every computer serves as
  • a repeater to boost signals
  • Typical way to send data
  • Token passing
  • only the computer who
  • gets the token can send
  • data
  • Disadvantages
  • Difficult to add computers
  • More expensive
  • If one computer fails, whole network fails

Peer-to-Peer Networks
  • Peer-to-peer network is also called workgroup
  • No hierarchy among computers ? all are equal
  • No administrator responsible for the network

  • Advantages of peer-to-peer networks
  • Low cost
  • Simple to configure
  • User has full accessibility of the computer
  • Disadvantages of peer-to-peer networks
  • May have duplication in resources
  • Difficult to uphold security policy
  • Difficult to handle uneven loading
  • Where peer-to-peer network is appropriate
  • 10 or less users
  • No specialized services required
  • Security is not an issue
  • Only limited growth in the foreseeable future

Clients and Servers
  • Network Clients (Workstation)
  • Computers that request network resources or
  • Network Servers
  • Computers that manage and provide network
    resources and services to clients
  • Usually have more processing power, memory and
    hard disk space than clients
  • Run Network Operating System that can manage not
    only data, but also users, groups, security, and
    applications on the network
  • Servers often have a more stringent requirement
    on its performance and reliability

  • Advantages of client/server networks
  • Facilitate resource sharing centrally
    administrate and control
  • Facilitate system backup and improve fault
  • Enhance security only administrator can have
    access to Server
  • Support more users difficult to achieve with
    peer-to-peer networks
  • Disadvantages of client/server networks
  • High cost for Servers
  • Need expert to configure the network
  • Introduce a single point of failure to the system

Network Hardware
  • Hub
  • An unintelligent network device that sends one
    signal to all of the stations connected to it.
  • All computers/devices are competing for attention
    because it takes the data that comes into a port
    and sends it out all the other ports in the hub.
  • Traditionally, hubs are used for star topology
    networks, but they are often used with other
    configurations to make it easy to add and remove
    computers without bringing down the network.
  • Resides on Layer 1 of the OSI model

OSI Model Layers
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical
  • Switch
  • Split large networks into small segments,
    decreasing the number of users sharing the same
    network resources and bandwidth.
  • Understands when two devices want to talk to each
    other, and gives them a switched connection
  • Helps prevent data collisions and reduces network
    congestion, increasing network performance.
  • Most home users get very little, if any,
    advantage from switches, even when sharing a
    broadband connection.
  • Resides on Layer 2 of the OSI model.

OSI Model Layers
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical
Network Hardware
  • Bridge
  • Connects two LANs and forwards or filters data
    packets between them.
  • Creates an extended network in which any two
    workstations on the linked LANs can share data.
  • Transparent to protocols and to higher level
    devices like routers.
  • Forward data depending on the Hardware (MAC)
    address, not the Network address (IP).
  • Resides on Layer 2 of the OSI model.

OSI Model Layers
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical
Network Hardware
  • Repeater
  • Used to boost the signal between two cable
    segments or wireless access points.
  • Can not connect different network architecture.
  • Does not simply amplify the signal, it
    regenerates the packets and retimes them.
  • Resides on Layer 1 of the OSI model.

OSI Model Layers
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical
Network Hardware
  • Router
  • A device that connects any number of LANs.
  • Uses standardized protocols to move packets
    efficiently to their destination.
  • More sophisticated than bridges, connecting
    networks of different types (for example, star
    and token ring)
  • Forwards data depending on the Network address
    (IP), not the Hardware (MAC) address.
  • Routers are the only one of these four devices
    that will allow you to share a single IP address
    among multiple network clients.
  • Resides on Layer 3 of the OSI model.

OSI Model Layers
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical
Network Hardware
  • Additional Network Hardware Devices
  • Network Interface Cards (NICs)
  • Puts the data into packets and transmits packet
    onto the network.
  • May be wired or wireless.
  • Gateway
  • Connects networks with different protocols like
    TCP/IP network and IPX/SPX networks.
  • Routers and Gateways often refer to the same
  • Proxy server
  • Isolates internal network computers from the
  • The user first access the proxy server and the
    proxy server accesses the internet and retrieves
    the requested web page or document. The user then
    gets a copy of that page from the proxy server.

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