Link Layer I

Question 1

Link Layer: Key Responsibilities

Answer 1

Framing
Breaks the network layer datagram into frames.

Adds header and trailer (e.g., MAC addresses, error detection info).

Each frame is transmitted over a single link.

Question 2

Link Access

Answer 2

Controls access to the shared link (especially in wireless and broadcast networks like Ethernet).

Uses MAC protocols (e.g., CSMA/CD for Ethernet, CSMA/CA for WiFi).

Question 3

Error Detection and Correction

Answer 3

Adds error checking bits (e.g., CRC in Ethernet).

Detects and sometimes corrects bit errors introduced by noise, signal distortion, etc.

Question 4

Reliable Delivery (optional)

Answer 4

Some link-layer protocols provide acknowledgments and retransmissions.

E.g., 802.11 (WiFi) provides reliable delivery.

Ethernet does NOT – relies on higher layers.

Question 5

MAC (Media Access Control) Addressing

Answer 5

Each device has a unique MAC address (48-bit).

Used to identify sender and receiver at the link level.

Independent of IP addressing.

Question 6

Flow Control (sometimes)

Answer 6

Prevents fast sender from overwhelming the slow receiver.

More common at higher layers, but can exist at link layer in some technologies

Question 7

Multiplexing and Demultiplexing

Answer 7

Handles multiple network layer protocols (e.g., IP, ARP) over the same link.

Uses EtherType field in Ethernet frame.

Question 8

Channel partitioning MAC protocols: TDMA

Answer 8

TDMA: time division multiple access
▪ access to channel in “rounds”
▪ each station gets fixed length slot (length = packet transmission
time) in each round
▪ unused slots go idle

Question 9

Channel partitioning MAC protocols: FDMA

Answer 9

FDMA: frequency division multiple access
▪ channel spectrum divided into frequency bands
▪ each station assigned fixed frequency band
▪ unused transmission time in frequency bands go idle