L4 The Data Link Layer

Question 1

Describe the data link layer

Answer 1

The datalink layer presents the physical layer as a reliable transmission service to the network layer.

• repackaging upper layer into smaller chunks.
• reassembling physical layer data into meaningful datagrams.
• performs error detection and correction.

Question 2

What are the two sublayers to the data link layer?

Answer 2

LLC (logical Link control)

MAC (media access control)

Question 3

Describe the MAC layer (Media Access Control)

Answer 3

Media Access Control (MAC) responsible controlling what and when the network device transmits.

Question 4

What are the two approaches to media access control (MAC)?

Answer 4

Contention

Controlled access

Question 5

Describe the Media Access Control (MAC) approach: Contention

Answer 5

Contention is when data is only transmitted when the medium is free.

Question 6

Media Access Control: What are collisions and How does contention deal with collisions?

Answer 6

A collision occurs when two devices transmit data at the same time. Contention aims to detect when a collision occurred. Solution: random back off by both parties and try again.
Large # users = serious network degradation

Question 7

Describe the Media Access Control (MAC) approach: controlled access

Answer 7

Central mechanism controls access to medium.

Question 8

Media Access Control (MAC) Controlled access: Describe roll call polling

Answer 8

Access controller makes clients take turns in round robin fashion: A, B, C, D; Arbitrary order: A, B, A, C, A, D. (Hub polling involving tokens is dead)

Question 9

What are some sources of error?

Answer 9

Line distortion
White noise
Impulse noise
Crosstalk
Echoes
Attenuation
Intermodulation noise

Question 10

Error: line distortion

Answer 10

different frequency components of the signal travel at different speeds through the medium

Question 11

Error: White noise

Answer 11

caused by the natural vibrations of electrons in matter. Sets limits on receiver sensitivity.

Question 12

Error: impulse noise

Answer 12

Primary source of errors in data communications. Caused by sudden, substantial change in electrical state of perfecting entity.

Question 13

Error: crosstalk

Answer 13

Occurs in electric media when the current flowing in one cable induces a current in a cable that is close to it. Guardbands may be too narrow.

Question 14

Error: Echoes

Answer 14

Signal propagates from the transmitter to far end of medium and then reflects from the far end back to the transmitter. Can occur with loosely connected cable, improper termination of cables and ports, and mismatched impedance (electrical resistance)

Question 15

Error: Attenuation

Answer 15

Signal becomes weaker the further troubles from the transmitter. It may be absorbed by matter such as water. Can be reduced by repeaters or amplifiers.

Question 16

Error: intermodulation noise

Answer 16

Occurs when the signals from two transmitters combine to produce a third signal.

Question 17

How are errors detected? What are the three error detection techniques?

Answer 17

By sending additional bits along with the data. Some common techniques include:

• parity checking
• checksum
• cyclic redundancy check

Question 18

Error detection: Describe and draw Parity checking.

Answer 18

Used with low-speed serial links.
Odd parity: # transmitted 1 bits odd
even parity: # transmitted 1 bits even
Only catches 50% of bit errors.