Data Link Layer - CH 4 (EXAM 1)

Question 1

What is the DL layer?

Answer 1

moves messages from one device to another on a binary level

controls the way messages are sent on device

Question 2

What are the 3 major functions of the DL layer?

Answer 2

media access control
error control
message delineation (start and end)

Question 3

What is media access control?

Answer 3

controls which device transmits and when

Question 4

What are the 2 types of MAC?

Answer 4

contention - small networks (exponential)
controlled - large networks with high traffic (linear)

important on multipoint and half-duplex circuits

Question 5

What is contention access?

Answer 5

transmits when the circuit is available
no centralized limit

Ethernet LAN

collision occurs when devices transmit at the same time

devices must be polite

Question 6

What is controlled access?

Answer 6

wireless LAN

access request, polling (roll call and hub polling with tokens)

Question 7

What are the 2 types of error control?

Answer 7

corrupted and lost data

Question 8

What should networks be designed with?

Answer 8

error prevention
error detection
error correction

Question 9

What are some sources of network errors?

Answer 9

electrical signal disruption
missing bits
flipped bits
noise and distortion

Question 10

White noise

Answer 10

cause: electron movement
prevent: increase signal strength

Question 11

Impulse noise

Answer 11

cause: sudden increase in electricity
prevent: shield/move wires

Question 12

Cross-talk

Answer 12

cause: wires too close together
prevent: move the wires

Question 13

Echo

Answer 13

cause: poor connection
prevent: fix connection

Question 14

Attenuation

Answer 14

cause: gradual decrease in signal over distance
prevent: repeaters/amplifiers

Question 15

Intermodulation noise

Answer 15

cause: circuit signals combine
prevent: move wires

Question 16

How can you detect errors?

Answer 16

adding a check value to the message
through formula at source and destination to ensure a match

parity-check - based on the number of 1’s in the message
reduces 50% of errors

checksum - algorithm
detects 95% of errors

cyclic redundancy check - uses remainder as check value
more capacity, more data

Question 17

How can you correct errors?

Answer 17

retransmission (backward error correction) - common

forward error correction - device corrects messages without retransmission

stop and wait (acknowledge/timeout)

continuous

Question 18

What are the DL protocols?

Answer 18

async serial transmission

synchronous transmission - data sent in large blocks (frame)

SDLC (bit stuffing)

Ethernet (Ethernet II frame)

Point to point (PPP)

Question 19

Formula for transmission efficiency

Answer 19

# of information (bits) / 
# of info + overhead bits

Question 20

How can efficiency increase?

Answer 20

increase data bits

decrease overhead bits

Question 21

What is throughput?

Answer 21

total # of information bits received per second

after overhead bit count and correcting errors