chapter 6 - link layer

Question 1

What is a link-layer frame?

Answer 1

data unit used to send information between two directly connected devices

Question 2

What is the main job of the link layer?

Answer 2

Transfer datagrams between physically nodes over a link

Question 3

Where is the link layer implemented?

Answer 3

In the NIC (network interface card), as hardware

Question 4

What are the main services of the link layer?

Answer 4

Framing reliable delivery, flow control, error detection/correction

Question 5

What is a MAC address?

Answer 5

physical link-layer address unique to each NIC (not the same as IP)

Question 6

What is the difference between full-duplex and half-duplex?

Answer 6

Full: device can send and receive at once
half: only one at a time

half: walkie-talkie

Question 7

What is a checksum?

Answer 7

A value used to detect errors in transmitted data

Question 8

What does a parity bit detect?

Answer 8

Single-bit errors

Question 9

What is CRC (Cyclic Redundancy Check)?

Answer 9

strong error-detection method using polynomial division

Question 10

What is the purpose of a MAC protocol?

Answer 10

who gets to send data on a shared communication channel and when

who = device

Question 11

What are the 3 categories of MAC protocols?

Answer 11

Channel partitioning, random access, taking turns

Question 12

What is Time Division Multiple Access (TDMA)?

Answer 12

each node gets a time slot and the queue goes in a circle

hører til channel partitioning

Question 13

What is Frequency Division Multiple Access (FDMA)?

Answer 13

each node gets a frequency band and can send at the same time

sends link layer frames

Question 14

What is slotted ALOHA

Answer 14

Time is divided into equal time slots, and devices can only send at the start of a slot

when collisions -> retransmit at random time

random access

Question 15

slotted aloha - settings

Answer 15

all frames same size
time divided into equal size slots

Question 16

pure aloha

Answer 16

nodes transmit whenever - higher collision risk

Question 17

What is Carrier Sense Multiple Acces (CSMA)?

Answer 17

listen before sending

if sensing busy - defer transmission
if sensing free - transmit entire frame

Question 18

What is CSMA with Collision Detection (CSMA/CD)?

Answer 18

stop sending if collision detected -> wait and send at a random time later

random access

Question 19

pros cons - slotted aloha

Answer 19

pro - decentralized
cons - synchronization

Question 20

What improves CSMA/CD efficiency?

Answer 20

Short propagation delay (T_prop) and long transmission time (T_trans)

Question 21

ethernet csma/cd algorithm

Answer 21

• ethernet creates frame
• if idle -> retransmit
• if another transmission detected while sending -> abort

Question 22

How does a parity bit detect errors?

Answer 22

Adds 1 bit to ensure even (or odd) number of 1s; receiver checks if parity matches

Question 23

How does two-dimensional parity work?

Answer 23

Adds parity bits to rows and columns in a bit grid; detects and corrects single-bit errors

Question 24

How does CRC detect errors?

Answer 24

Divides data (polynomial) -> appends remainder so final result is divisible
receiver checks divisibility

Question 25

How does polling work?

Answer 25

A master device asks each device in turn -> do you have data to send -> if yes (send), if no (move on to next) | taking turn protocol

Question 26

difference between MAC and IP addresses?

Answer 26

IP = logical address (used for routing, can change with location) MAC = physical address (fixed per NIC, used for local delivery

Question 27

How does Address Resolution Protocol (ARP) work?

Answer 27

Translates IP address to MAC address if MAC unknown (arp request) -> caches result

Question 28

What is Ethernet?

Answer 28

defines how devices on a local network communicate using cables

Question 29

how does Ethernet work?

Answer 29

Data is sent in ethernet frames -> each frame -> source mac, destination mac, crc (error)

Question 30

What does an Ethernet switch do? - What if the switch doesn’t know the destination MAC?

Answer 30

forwards Ethernet frames only to the correct device using MAC addresses floods the frame to all ports except the incoming one

Question 31

How does a switch learn forwarding info?

Answer 31

When frame arrives on port, the switch looks at the source MAC address maps it in the table mac -> port

Question 32

What does a switch do when receiving a frame?

Answer 32

Looks up destination MAC in its table If known, forwards on correct interface If unknown, floods to all ports except incoming one

Question 33

Difference between a switch and a router?

Answer 33

Switch: link-layer device, uses MAC addresses, learns forwarding table Router: network-layer device, uses IP addresses, computes routes