Transcript Networking BASICS - Phantrung's Blog

Networking BASICS
Network
Design
Unit 2
Lesson 3
Lesson 3—Networking BASICS
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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.
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Objectives (continued)
• Define channel access method.
• Give the advantages and disadvantages
of contention, polling, token passing,
and demand priority channel access
methods.
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Topologies
• Physical layout of network devices
• Four types: mesh, bus, ring, and
star
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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.
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Mesh Topology
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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.
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Bus Topology
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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.
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Ring Topology
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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
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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.
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Design Considerations
• The best topology involves
matching with the environment it is to be used in.
• The physical arrangement of
computers do not dictate the
necessary topology.
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Channel Access Methods
• These are rules of cooperation to
eliminate collisions.
• There are four types of channel access
methods.
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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.
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Token Passing
• A special token packet is
passed around the network.
• A station must secure a
token before sending a
packet.
• It eliminates collisions.
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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.
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Demand Priority
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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 responsible 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.
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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 altogether. 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.
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