Data Link Layer

Question 1

What is the data link layer responsible for?

Answer 1

Controlling the hardware and for error detection

Question 2

What is MAC?

Answer 2

Media Access Control, which controls when a device is allowed to transmit. It solves the main problem in many networks, which is that only a single device is allowed to transmit at a time.

Question 3

Explain the two approaches to MAC

Answer 3

Controlled Access - One device is allowed to send information at a time, and there is an authority assigning permissions to send, or the permission gets passed from device to device.

Contention based access - Access provided at a first come first serve basis. This requires a mechanism to detect when a frame collision has happened, and order a re-transmission of those frames.

Question 4

CSMA/SC method

Answer 4

Carrier Sense (CM) - A device listens and only transmits when no other device is doing do

Multiple Access(MA) - Multiple devices share the same medium

Collision Detection(CD) - Mechanism to detect and order re-transmission in case of a collision

Question 5

What was Ethernet based on?

Answer 5

A single coaxial cable that devices tapped into physically

Question 6

What is a shared bus topology?

Answer 6

When all devices share the same bus (original Ethernet)

Question 7

What is a consequence of a shared bus?

Answer 7

All devices receive all the messages sent an the network.

Question 8

What is a MAC address?

Answer 8

A unique address of 6 bytes to each device connected to a network that can differentiate the messages sent to the devices,

Question 9

What are the disadvantages of shared-bus Ethernet?

Answer 9

• Half-duplex ( only one device)
• Network broadcasts ALL messages
• Heavy reliance of collision detection limits size of network

Question 10

What is a star topology?

Answer 10

Central hub component with limited number of sockets, so each device connects with an individual cable.

Question 11

What is the caveat of shared topology?

Answer 11

It made the network behave as if all the computers were connected with one cable. the physical topology was a star, whilst the logical topology was a bus.

Question 12

How do switches work on the data link layer?

Answer 12

The switch receives a message from a connected device and doesn’t know where the receiver is, but it knows it isn’t from the sender port. Then is broadcasts the message to all ports, with an ethernet frame that contains the source MAC address.

Question 13

What is a forwarding table?

Answer 13

The area where the source ports’ information are stored after a switch broadcasts a message.

Question 14

What does WLAN use to communicate?

Answer 14

A strictly regulated spectrum of radio waves.

Question 15

What do 2.4GHz and 5GHz refer to?

Answer 15

The two main bands of frequency which WLAN devices use

Question 16

What is attenuation?

Answer 16

The effect of a higher frequency becoming weaker much faster over longer distances that lower frequencies.

Question 17

How many channels are available in the 2.4 GHz band?

Answer 17

13 channels approx. 22 MHz wide.

Question 18

What is the simplest setup for a wireless network?

Answer 18

ad-hoc/independent Basic Service Set

Question 19

Explain an ad-hoc network

Answer 19

Behaves like an og shared Ethernet (no central hub and random frames sent to network with MAC address). But many use a central AP.

Question 20

Explain an Infrastructure BSS.

Answer 20

Access point of connected to rest of network with a cable-based Ethernet. All communication goes through the AP (Access point)

Question 21

What is an ESS?

Answer 21

Multiple APs work together connected by the same cable-based network, designed to overlap to transmit the same identifier (ex. eduroam)