Lesson 3—Networking BASICS1 Networking BASICS Network Design Unit 2 Lesson 3

1Lesson 3—Networking BASICS

Networking BASICS

Network Design

Unit 2

Lesson 3

Lesson 3—Networking BASICS 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.


Objectives (continued)

• Define channel access method.

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

methods.


Topologies

• Physical layout of network devices

• Four types: mesh, bus, ring, and star


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.


Mesh Topology


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.


Bus Topology


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.


Ring Topology


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.


Star Topology


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.


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.


Channel Access Methods

• These are rules of cooperation to eliminate collisions.

• There are four types of channel access methods.


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.


Polling• Each computer is asked in

sequence whether it needs to transmit.

• Polling provides equal access to all devices.


Token Passing

• A special token packet is passed around the network.

• A station must secure a token before sending a packet.

• It eliminates collisions.


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.


Demand Priority


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.


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.


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.


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.


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.


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.


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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Lesson 3—Networking BASICS1 Networking BASICS Network Design Unit 2 Lesson 3