Data Link Layer

Question 1

Purpose of Data Link Layer

Answer 1

• Responsible for communications between end-device NICs
• Prepares network data for the physical network
• Allows upper layer protocols to access the physical layer media
• Accepts data, usually layer 3 packets and encapsulates into layer 2 frames
• Performs error detection and rejects corrupt frames

Question 2

What are the two sublayers of the Data Link Layer

Answer 2

Logical Link Layer (LLC)

Media Access Control (MAC)

Question 3

What is the LLC

Answer 3

Communicates between the networking software at the upper layers and the device hardware at the lower layers. Places information in frame that identifies which network layer protocol is being used.

Question 4

What is the MAC

Answer 4

Responsible for data encapsulation and media access control. Provides data link layer addressing.

Question 5

What are the three common physical WAN topologies? Describe them?

Answer 5

Point-to-point - simplest and most common. Consists of a permanent link between two endpoints.
Hub and spoke - central site interconnects branch sites through point-to-point links
Mesh - high availability but requires every end system to be connected to each other

Question 6

What are the three modes that data transmitted over the network flow?

Answer 6

1. Simplex - data can only flow in one direction
2. Half-duplex - data flows in one direction at a time
3. Full-duplex - data flows in both directions at the same time

Question 7

When to use contention-based access?

Answer 7

Only when nodes are operating in half-duplex ie WLANS

Question 8

What types of contention-based access is there?

Answer 8

CSMA/CD - collision detection

CSMA/CA - collision avoidance

Question 9

What is collision detection? (CSMA/CD)

Answer 9

• Devices transmitting simultaneously will result in a signal collision on the shared media
• Devices detect the collision
• Devices wait a random period of time and retransmit data

Question 10

What is collision avoidance? (CSMA/CA)

Answer 10

• When transmitting, devices also include the time duration needed for the transmission
• Other devices on the shared medium receive the time duration information and know how long the medium will be unavailable.

Question 11

What is in a Data Link Frame Header?

Answer 11

Frame Start, Addressing, Type (protocol type), control (flow control services ie. QoS)

Question 12

What is in a Data Link Frame Trailer?

Answer 12

Error Detection and Frame Stop

Question 13

What are Layer 2 addresses used to connect?

Answer 13

only used to connect devices within the same shared media on the same IP network

Question 14

What is the order of address field?

Answer 14

L2 Header - Destination NIC, Source NIC

L3 IP Packet - Source IP, Destination IP