06 Datalink Layer-QA

Question 1

What is the purpose of the datalink layer?

Answer 1

Purpose is to prepare network data for the physical network.

Question 2

What is the datalink layer ultimately responsible for?

Answer 2

NIC-to-NIC communications within the same network

Question 3

What does the datalink layer do (6 things)?

Answer 3

1. Enables upper layers to access the media
2. Accepts data and encapsulates it into L2 frames
3. Controls how data is placed/received on the media
4. Exchanges frames between endpoints on the network media
5. Receives encapsulated data, directing it to upper-layer protocol
6. Performs error detection, rejection of bad frames

Question 4

How would not having the datalink layer impact network communications?

Answer 4

Network layer protocols would have to make provisions for every type of media that could exist along the path

Question 5

What are the two major functions of the LLC?

Answer 5

1. Communicates between network software at upper layers and device hardware at lower layers
2. Places information in the frame identifying the L3 protocol used within the frame; adds L2 control information

Question 6

What are the major functions of the MAC sublayer?

Answer 6

1. controls the NIC hardware
2. provides encapsulation. incl. delimiting, addressing, error detection
3. provides media access control (required for half-duplex)

Question 7

What access control methods are available for shared media?

Answer 7

1. contention-based access
2. controlled access

Question 8

What is the main flaw with contention-based access control methods?

Answer 8

all devices operate in half-duplex—only one device can transmit at a time

Question 9

Controlled-access networks can also be described as …

Answer 9

deterministic

Question 10

How do controlled-access networks function?

Answer 10

each node has its own time to use the medium

Question 11

Why are controlled-access networks considered inefficient?

Answer 11

each device must wait its turn to access the medium

Question 12

Why do modern Ethernet networks not require an access method?

Answer 12

Modern Ethernet networks operate in full-duplex mode.

Question 13

Describe CSMA/CD

Answer 13

1. Nodes operate in half-duplex
2. If two devices send at the same time, both nodes notice the collision
   - jamming signal
   - random backoff timer
3. If one device wants to send, it will look for a carrier signal on the wire.
4. If the device does not detect a carrier signal, it assumes the network is available.

Question 14

Describe CSMA/CA

Answer 14

1. Nodes operate in half-duplex
2. devices attempt to avoid collisions by waiting before transmitting
3. Each transmission includes the time required to complete the transmission.
4. other nodes “hear” this information, thus know how long each xmit should take

Question 15

List common datalink layer protocols

Answer 15

Ethernet
802.11 wireless
PPP
HDLC
Frame relay