Networking BASICS

1
Networking BASICS

• Network Design
• Unit 2
• Lesson 3

2
Objectives

• List and describe the characteristics of the
  mesh, bus, ring, star, and hybrid topologies.
• Tell what items should be considered when
  selecting a network topology.

3
Objectives (continued)

• Define channel access method.
• Give the advantages and disadvantages of
  contention, polling, token passing, and demand
  priority channel access methods.

4
Topologies

• Physical layout of network devices
• Four types mesh, bus, ring, and star

5
Mesh Topology

• It is also called a point-to-point topology.
• Each device is connected directly to all other
  network devices.
• It provides fault tolerance.
• It is only found in wide area networks.

6
Mesh Topology
7
Bus Topology

• It is a multipoint topology.
• Each device shares the connection.
• The bus has one starting and one ending point.
• Packets stop at each device on the network.
• Only one device at a time can send.

8
Bus Topology
9
Ring Topology

• It is a circle with no ends.
• Packets are sent from one device to the next.
• It does not slow down as more devices are added.

10
Ring Topology
11
Star Topology

• All devices are connected to a central device
  (hub).
• The hub receives and forwards packets.
• It is the easiest topology to troubleshoot and
  manage.
• It has a single point of failure.

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Star Topology
13
Hybrid Topologies

• They are variations of two or more topologies.
• Star bus used to connect multiple hubs in a
  star topology with a bus.
• Star ring wired like star, but functions like a
  ring.

14
Design Considerations

• The best topology involves matching with the
  environ-ment it is to be used in.
• The physical arrangement of computers do not
  dictate the necessary topology.

15
Channel Access Methods

• These are rules of cooperation to eliminate
  collisions.
• There are four types of channel access methods.

16
Contention

• Each device contends or fights for the ability to
  send a packet.
• Carrier Sense Multiple Access/Collision Detection
  (CSMA/CD) devices must listen before
  transmitting.
• A collision causes two devices to wait for random
  intervals.
• Carrier Sense Multiple Access/Collision Avoidance
  (CSMA/CA) all devices wait for random intervals.

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Polling

• Each computer is asked in sequence whether it
  needs to transmit.
• Polling provides equal access to all devices.

18
Token Passing

• A special token packet is passed around the
  network.
• A station must secure a token before sending a
  packet.
• It eliminates collisions.

19
Demand Priority

• The central device sends the demand signal and
  waits for an acknowledgment signal.
• It allows for certain devices to be given higher
  priority.
• It reduces network traffic.

20
Demand Priority
21
Summary

• Topology refers to the physical layout of the
  network devices and cabling and how all the
  components communicate with each other. Computer
  network topology refers to how the computer
  network is arranged. There are four basic types
  of computer network topologies mesh, bus, ring,
  and star.
• A mesh topology is also called a point-to-point
  topology. Each device is connected directly to
  all other devices on the network. A mesh topology
  is sometimes used in a wide area network (WAN)
  setting to ensure that all the sites continue to
  transmit in the event of a cable failure or
  another similar problem. Mesh topologies are
  rarely used in a LAN.

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Summary (continued)

• LANs typically use a multipoint topology in which
  each computer on the network has just one
  connection. This connection is attached to a
  single cable that is shared by all other devices
  on the network. The most common type of
  multipoint topology is a bus topology. A computer
  network bus topology is characterized by one
  starting point and one ending point. A bus
  network topology works well for smaller networks
  and is inexpensive to install. However, it can
  become slow when more devices are added to the
  network.
• A star topology describes a network in which all
  the devices are connected to a central device
  known as a hub, which is respon-sible for
  receiving and forwarding packets to other devices
  on the network. Because everything is centralized
  in a star topology, it is the easiest topology to
  manage and troubleshoot.

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Summary (continued)

• A multipoint ring topology is the opposite of a
  bus. A bus has two end points, but a ring is a
  circle with no ends. All the devices on the
  network are connected to the ring. Unlike a bus
  topology, in which the network devices do not
  play an active role in the network, each device
  has additional network functions in a ring
  topology. A ring topology works well for a
  computer network with many devices attached to
  it.
• Not all topologies are strictly mesh, bus, star,
  or ring. Some variations of these topologies,
  known as hybrid topologies, are also used. These
  variations are actually combinations of two or
  more topologies.

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Summary (continued)

• A star bus topology is typically used to connect
  multiple hubs in star topologies with a bus
  topology and provides additional fault tolerance.
  A star ring topology is wired like a star, but
  functions like a ring. The packets in a star ring
  do not go directly to the next device on the
  ring, but instead go to a central hub.
• For multipoint topologies to function properly,
  there must be rules of cooperation to ensure that
  transmissions arrive intact. These rules are
  called channel access methods.
• One type of channel access method is polling.
  Each computer on the network is asked, or polled,
  in sequence whether it wants to transmit. If it
  does, it sends its packet while everyone else
  waits if it has nothing to send, the next
  computer is polled.

25
Summary (continued)

• Another type of channel access method is
  contention, which means that each device
  contends, or fights, for the ability to send a
  message. There are two types of contention
  methods. The first type of contention is called
  Carrier Sense Multiple Access/Collision Detection
  (CSMA/CD). CSMA/CD says that, before one computer
  starts to send a message, it should listen to
  determine whether another device is currently
  sending a packet. If a collision happens, all
  computers on the network are told to hold their
  transmissions. The two computers that caused the
  collision each wait a different amount of time
  before listening again to the medium and
  restarting the process.

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Summary (continued)

• Another contention variation attempts to avoid
  collisions alto-gether. This is known as Carrier
  Sense Multiple Access/Collision Avoidance
  (CSMA/CA). CSMA/CA handles the situation
  differently. Instead of making just the two
  stations responsible for the collision wait a
  random amount of time before attempting to
  resend, CMSA/CA has all stations wait a random
  amount of time after the medium is clear.
• The channel access method of token passing
  involves a special packet called a token that is
  passed from one computer to the next on the
  network. A computer can send a message only if it
  has the token. If a computer does not need to
  send a packet, it passes the token on to the next
  computer. If a computer needs to send a packet,
  it waits for the token and then attaches the
  packet to it.

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Summary (continued)

• The channel access method of demand priority is
  similar to raising a hand and waiting to be
  called on. If a computer wants to send a message,
  it first transmits a demand signal to a central
  device. Once the central device receives the
  demand signal, it sends an acknowledgment signal
  back to the computer the computer can then begin
  sending its message.