Data Communications and Networking > The Link Layer > Flashcards

The Link Layer Flashcards

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1
Q

What is the main role of the link layer?

A

“Move frames between adjacent nodes over a single link”

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2
Q

What is a frame?

A

“A link-layer packet containing header

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3
Q

What is encapsulation at the link layer?

A

“Wrapping the network-layer packet inside a frame”

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4
Q

What are common link-layer services?

A

“Framing
Link Access
Reliable Delivery
Error Detection and correction
Flow Control
Half-duplex and full-duplex”

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5
Q

What are three types of link types?

A

“Point-to-point, broadcast links, Multipoint”

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6
Q

What does ARP do?

A

“Maps IP addresses to MAC addresses within a LAN”

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7
Q

What is a MAC address?

A

“48-bit globally unique identifier assigned to network interfaces”

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8
Q

What addressing difference exists between MAC and IP?

A

“MAC is flat and permanent; IP is hierarchical and assigned”

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9
Q
A
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10
Q

What is error detection?

A

“Techniques to check whether bits were corrupted during transmission”

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11
Q

What is error correction?

A

“Ability to reconstruct original data without retransmission”

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12
Q

What is a codeword?

A

“Data bits plus redundant bits used for error detection/correction”

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13
Q

What is the Hamming distance between two codewords?

A

“The number of bit positions in which they differ”

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14
Q

What is the minimum Hamming distance of a code?

A

“The smallest Hamming distance between any two valid codewords”

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15
Q

What is the error detection capability of a code with distance d?

A

“Can detect d−1 bit errors”

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16
Q

What is the correction capability of a code with distance d?

A

“Can correct ⌊(d−1)/2⌋ bit errors”

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17
Q

What Hamming distance is required to correct 1-bit errors?

A

“Distance 3”

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18
Q

What Hamming distance is required to detect 2-bit errors?

A

“Distance 3”

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19
Q

What Hamming distance is required to detect 3-bit errors?

A

“Distance 4”

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20
Q
A
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21
Q

What is even parity?

A

“Number of 1s in codeword made even by parity bit”

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22
Q

What can parity detect?

A

“Only odd numbers of bit errors”

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23
Q

What is a checksum used for?

A

“Detecting corruption by summing data words and comparing inverted sum”

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24
Q

What arithmetic does checksum use?

A

“One’s complement addition”

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25
26
What is CRC used for?
"Reliable burst error detection using polynomial division"
27
What does CRC append to the message?
"Remainder of dividing data bits by generator polynomial"
28
What is the divisor in CRC?
"The generator polynomial represented as a bitstring"
29
What does the receiver check when verifying CRC?
"Whether received bits divided by generator polynomial give remainder 0"
30
What error can CRC NOT detect?
"Errors that produce a remainder identical to the correct one (rare for good polynomials)"
31
What polynomial degree determines about CRC?
"Number of CRC bits appended (degree = r → r CRC bits)"
32
What type of errors are CRCs especially good at detecting?
"Burst errors"
33
What is the CRC algorithm step 1?
"Append r zero bits to data (r = degree of generator polynomial)"
34
What is the CRC algorithm step 2?
"Divide augmented data by generator polynomial using XOR subtraction"
35
What is the CRC algorithm step 3?
"Replace zeros with the remainder bits"
36
What is the CRC check at the receiver?
"Divide received bits by generator polynomial; remainder must be 0"
37
38
What is a multiple access channel?
"A link shared by multiple active nodes"
39
What is channel partitioning?
"Divide channel into time/frequency slots"
40
What is TDMA?
"Time Division Multiple Access—nodes take turns using fixed time slots"
41
What is FDMA?
"Frequency Division Multiple Access—nodes use separate frequency bands"
42
What is the main drawback of TDMA/FDMA?
"Idle slots when nodes have no data; low efficiency with bursty traffic"
43
44
What is random access?
"Nodes transmit whenever they want and recover from collisions"
45
What is the pure ALOHA vulnerability period?
"2 × frame time"
46
What is the maximum throughput of pure ALOHA?
"18%"
47
What is slotted ALOHA?
"Time divided into slots; transmissions begin only at slot boundaries"
48
What is the max throughput of slotted ALOHA?
"37%"
49
Why is slotted ALOHA better than pure ALOHA?
"Halves vulnerability period to one frame time"
50
51
What is CSMA?
"Carrier Sense Multiple Access—listening before transmitting"
52
What is the advantage of CSMA?
"Reduces collisions by detecting busy channel"
53
What is propagation delay's role in CSMA collisions?
"Collisions can still occur because propagation is not instantaneous"
54
What is CSMA/CD?
"CSMA with Collision Detection—used in classic Ethernet"
55
What does a node do upon detecting a collision in CSMA/CD?
"Stops transmitting immediately and sends jamming signal"
56
What is binary exponential backoff?
"After collision, each station waits for a random backoff time chosen from a range that doubles with every new collision before trying to retransmit."
57
Why does CSMA/CD not work on wireless networks?
"Cannot detect collisions while transmitting (hidden node problem)"
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59
What is controlled access?
"Protocols that avoid collisions by giving permission to send"
60
What are examples of controlled access?
"Token passing
61
What is token passing?
"A token circulates; only holder may transmit"
62
What is polling?
"Master node invites slaves one at a time to transmit"
63
What is the drawback of controlled access?
"Single point of failure (token holder or master)"
64
65
What is the link-layer CRC field?
"Error detection field in Ethernet frames using CRC-32"
66
What part of an Ethernet frame is protected by CRC?
"Header and payload (not the preamble)"
67
What does the Ethernet preamble do?
"Allows clock synchronisation before real data arrives"
68
What is the Ethernet MAC protocol?
"CSMA/CD (for wired half-duplex legacy Ethernet)"
69
70
What is a switch?
"A link-layer device forwarding frames based on MAC addresses"
71
What table does a switch maintain?
"Switching/forwarding table mapping MAC → port"
72
What is switch learning?
"Automatically populating forwarding table by inspecting source addresses"
73
What is flooding in switches?
"Sending frame on all ports when destination MAC unknown"
74
What feature do switches use to avoid loops?
"Spanning Tree Protocol (STP)"
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