Module 7

Question 1

Describe how CSMA/CD works, in terms of CS, MA and CD.

Answer 1

CS (Carrier Sense) means
When a station has a frame to send, The station checks the link if there is any signal (i.e., transmission).
If there is a signal (i.e., electricity), then wait a bit, and go to 2.
Otherwise, send the frame.

CD (Collision Detection) means
CSMA +
The sender keeps listening to the link while it is transmitting a frame.
If there is a signal collision, then stop the transmission, wait a bit, and go to 1.
Otherwise, done.

Question 2

List two types of channelization.

Answer 2

FDMA
-The available bandwidth of the common channel is divided into bands that are separated by guard bands.

-The same concept as frequency division modulation (FDM)

TDMA

• The bandwidth is just one channel that is timeshared between different stations.
• The same concept as time division modulation (TDM)

Question 3

Describe the architecture of WLAN.

Answer 3

A BSS without an AP is called an ad hoc network. Two stations communicate directly.

A BSS with an AP is called an infrastructure network.
Any two stations communicate through the AP.

See Figure 15.6 for different types of WLAN.

ISM (Industrial, Scientific, and Medical) bands are used. See Table 15.4.

802. 11 – 2.4 GHz; 2 Mbps
803. 11a – 5.725 GHz; 54Mpbs
804. 11b – 2.4 GHz; 11 Mbps
805. 11g – 2.4 GHz; 54 Mbps

Question 4

Describe the architecture of Bluetooth.

Answer 4

Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A

Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously.

• Master/slave (maximum 8 stations)
• 2.4 GHz ISM band
• 1Mbps; 3Mbps in the 2nd generation TDMA

Question 5

Match major layers with hub, bridge, switch, router and gateway.

Answer 5

Physical = Hub
Data-link = Switch
Network = Router

Question 6

List advantages of using VLANs.

Answer 6

• Flexible logical reconfiguration
• Seperates broadcast domains
• Security
• Easily change/create IP logical networks
• Less networking devices at a central router

Question 7

Q13-1 Why is there no need for CSMA/CD on a full duplex ethernet LAN.

Answer 7

In a full-duplex Ethernet, each station is connected to the switch and the
media is divided into two channels for sending and receiving. No two stations
compete to access the channels; each channel is dedicated.

Question 8

Q13-9

Answer 8

A bridge can raise the bandwidth and separate collision domains.

Question 9

Q13-9 What are the advantages of dividing an Ethernet LAN with a bridge?

Answer 9

A bridge can raise the bandwidth and separate collision domains.

Question 10

Q13-8 What is the difference between multicast, unicast, and broadcast addresses?

Answer 10

A uni cast address refers to a single device on a network.
192.168.0.1 for example.

A multicast address defines multiple devices on a network.
224.x.x.x - 239.0.0.0

A broadcast address is sent to all devices on a network or LAN segment.

A broadcast address on a given network is usually the last address of the scope.

192.168.0.255 /24

Question 11

Q13-10 What is the relationship between a switch and a bridge?

Answer 11

A bridge is generally used to separate L2 collision domains. In the past two hubs would be connected via a bridge and this would be two seperate collision domains.

Question 12

Q17-2 What do we mean when we say a switch can filter traffic? Why is filtering important?

Answer 12

A switch has the intelligence to know which port a frame should be sent out of. This limits
broadcast traffic and collisions and removes the need to use CSMA/CD.

Question 13

Q17-6 How does a VLAN save a company time and money?

Answer 13

A VLAN allows configuration flexibility of a VLAN by logically defining LANs using software. It
allows a logical LAN to span multiple switches (between offices and or buildings) and to be
reconfigured without having to change the physical configuration.

Question 14

Q17-1 How is a repeater different from an amplifier?

Answer 14

An amplifier amplifies the signal, as well as noise that may come with the signal, whereas a repeater regenerates the signal, bit for bit, at the original
strength.

Question 15

Q17-7 How does a VLAN provide extra security for a network?

Answer 15

Members of a VLAN can send broadcast messages with the assurance that
users in other groups will not receive these messages.

Question 16

Q17-11 Which one has more overhead a switch or a router? Explain your answer.

Answer 16

A router has more overhead than a switch.

a. A router process the packet at three layers; a switch processes a frame at
only two layers.

b. A router needs to search a routing table for finding the output port based on
the best route to the final destination; A switch needs only to consult a filtering table based on the location of stations in a local network.

c. A routing table is normally longer than a filtering table; searching a routing
table needs more time than searching a filtering table.

d. A router changes the link-layer addresses; a switch does not.

Question 17

Q17-4 How is a hub related to a repeater?

Answer 17

A hub is similar to a repeater in that it just repeats what is received on one port out all the other ports.

Question 18

Q17-10 What do we mean when we see a switch can filter traffic? Why is filtering important.

Answer 18

A switch knows which device is on which switch port. It knows when it receives a frame which device it is destined for and on which LAN segment it resides. This avoids collisions, congestion and prevents devices from having to dispose of many undesired frames.

Question 19

Q17-2 What do we mean when we say a switch can filter traffic? Why is filtering important?

Answer 19

A switch has the intelligence to know which port a frame should be sent out of. This limits broadcast traffic and collisions and removes the need to use CSMA/CD.

Question 20

Q17-6 How does a VLAN save a company time and money?

Answer 20

A VLAN allows configuration flexibility of a VLAN by logically defining LANs using software. It allows a logical LAN to span multiple switches (between offices and or buildings) and to be reconfigured without having to change the physical configuration.

Question 21

Q17-12 Which one has more overhead a hub or a switch?

Answer 21

A switch has more overhead because it keeps track of all machines that are connected and determines which frames are required by each device. A hub on the other hand has a simpler job and only needs to send a frame out every port but the one on which it was received.