Lecture 4

Question 1

What are the two different DATA link protocols?

Answer 1

Ethernet (IEEE802.3)

Wireless LAN (e.g., IEEE802.11)

Question 2

What does the data link layer do?

Answer 2

It transfers the frame over a link (wired or wireless)

While the network layer handles the end-to-end (host to-host) datagram transfer

It is coupled by the technology between adjacent nodes

Question 3

What is a simplex transmission mode?

Answer 3

Transmission in one direction only

Question 4

What is a half-duplex transmission mode?

Answer 4

Transmission in both directions, but not concurrently

Question 5

What is a a full-duplex transmission mode?

Answer 5

Transmission in both directions concurrently

Question 6

What are nodes in the Data Link Layer?

Answer 6

Host & routers

Question 7

What are Links in the Data Link Layer?

Answer 7

They connect adjacent node

Question 8

Describe the sending node

Answer 8

It encapsulates datagram in a frame

The frame is transmitted via the link

Question 9

Describe the receiving node

Answer 9

The receiving node
o It receives the frame
o It de-encapsulates the datagram

Question 10

What is a broadcast in terms of a link

Answer 10

Broadcast
o Multiple receivers
o Anyone can send a frame
o All the others node receive the frame

Question 11

Describe network adapters

Answer 11

• Data Link is implemented in a adapter
• Ethernet, NICs

Sender Node:
o Datagram Encapsulation in a frame
o CRC, flow control, etc.
Receiving Node:
o Error Check, flow control, etc.
o De-encapsulates the datagram

Question 12

What does MAC stand for?

Answer 12

Media Access Control

Question 13

Why are MAC address used?

Answer 13

o Determine source and destination within the Data Link

o Reliable Transfer of frames between sending-receiving node

Question 14

What is Flow control in terms of M.A.C

Answer 14

o Control of the data flow between sender and receiver

o Prevents underflow at the sender and overflow at the receiver

Question 15

What is congestion control in terms of M.A.C

Answer 15

There are possible errors introduced at the physical layer
o Congestion control defines the management of the error
o In case of errors detected by the receiver
o Retransmission/Frame Drop
o Undetected errors: The datagram is sent to Layer 3

Question 16

What is the Multiple Access protocol?

Answer 16

o Distributed algorithm defining how the nodes share the channel
o The algorithm must decide the node that will have access to the channel and the time that the channel is reserved

Question 17

Describe MAC protocols

Answer 17

o Channel separation into smaller “parts” (time slots, frequency, code): TDMA, FDMA, CDMA
o Random Access
o Recovery from collisions

Question 18

Describe multiple access

Answer 18

The nodes are connected to a common communication channel

Question 19

What is access control?

Answer 19

Access control: It limits the performance degradation due to conflicts and empty periods

Question 20

What are some examples of random access MAC protocols:

Answer 20

Examples:
o Slotted ALOHA
o ALOHA
o CSMA / CD, CSMA / CA

Question 21

What does the Random access MAC protocol do ?

Answer 21

Two or more nodes transmit concurrently=>collision

Random access MAC protocol specifies:
o How to detect collisions
o How to recover from collisions