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Networking > Datalink Layer Flow and Error Control > Flashcards

Datalink Layer Flow and Error Control Flashcards

1
Q

What is the problem with an error detection and correction approach?

A

Adds too much overhead.

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2
Q

What alternative is there to detection and correction? What is this called.

A

Send frames, find out if there is error, resend if not.

ARQ - Automatic Repeat reQuest Schemes

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3
Q

How are errors detected in ARQ?

A

Using CRC.

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4
Q

What is done in ARQ is CRC is (a) ok (b) not ok.

A

(a) Send an ACK to transmitter
(b) Send a NAK to transmitter

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5
Q

What happens if we miss a frame in ARQ?

A

Timer kicks in.

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6
Q

How long does it take to receive an ACK?

A

RTT

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7
Q

What is a? What is the formula for a?

A

The size of the link

a = RTT / transmission time

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8
Q

What is flow control?

A

Specifies how much data the Sender can transmit before receiving permission to continue from Receiver.

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9
Q

What is error control?

A

Allows the receiver to tell the sender about frames damaged or lost during transmission, and coordinates the re-transmission of those frames by the Sender.

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10
Q

T or F. After ARQ in datalink layer frames are passed to Network layer in order.

A

True.

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11
Q

Explain stop and wait flow control.

A

Sender waits for ACK back before sending next data.

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12
Q

Explain sliding window flow control.

A

Sender can transmit several frames continuously before needing an ACK. Each ACK’s number is the number of the next frame expected at the receiver and states that all previous were received correctly.

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13
Q

What is the length of the window in sliding window flow control?

A

Number of frames that can be transmitted without receiving an ACK.

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14
Q

Explain Go-Back-N ARQ.

A

N outstanding frames at most on link. When data is lost, we discard data above its sequence number and re-transmit from the first loss. When ACK is lost we have to transmit all data until last ACK and then we discard copies.

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15
Q

T or F. In Go-Back-N scheme. ARQ only accepts frames in the correct order.

A

True.

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16
Q

What rule must be applied in a Go-Back-N scheme to avoid sequence number starvation?

A

a < N

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17
Q

When is selective repeat better than go-back-N?

A

When a is large and there is likely to be an outstanding frame in error.

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18
Q

How is selective repeat worse?

A

More complicated
Frames out of order.

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19
Q

What happens in Go-Back-N as r goes toward 0?

A

Efficiency goes toward 1.

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20
Q

T or F. Selective repeat is more efficient than Go-Back-N.

A

True.

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21
Q

T or F. Under best conditions, Go-Back-N and Selective repeat are twice as efficient as Stop and Wait.

A

True.

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22
Q

What does HDLC stand for?

A

High Level Data Link Control Protocol

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23
Q

Explain HDLC.

A

3 types of frames.
I-Frame (User data from upper layer)
S-Frame (Supervisory frame transport control. E.g. ACK)
U-Frame (Unnumbered frame reserved for system management)

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24
Q

T or F. In back-to-back frame transmissions, the end flag of one frame can be used as the start flag of the next frame.

A

True.

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25
Q

How big is window size in HDLC?

A

2^N(S) - 1
or
2^N(R) - 1

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26
Q

What does FCS stand for?

A

Frame Check Sequence

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27
Q

What is N(S)? What is N(R)?

A

Sequence number of frame sent.
Sequence number of next frame expected.

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28
Q

What does (a) an I frame (b) an S frame and (c) a U frame contain?

A

(a) N(s) and N(R)
(b) CRC and N(R)
(c) Two CRC’s

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29
Q

What do the following stand for?
LAN
LLC
IEEE
MAC
DSAP
SSAP
CSMA/CD
MMAT

A

Local Area Network
Logical Link Control
Institute of Electrical and Electronics Engineers
Media Access Control
Destination Service Access Point
Source Service Access Point
Carrier Sense Multiple Access with Collision Detection
Maximum Media Access Time

30
Q

What layer is LAN, LLC and MAC found in?

A

Datalink layer.

31
Q

Distinguish between LLC and MAC?

A

LLC: common to all broadcast LANS. Has no physical addresses.
MAC: specific to the type of LAN being used. Provides addresses.

32
Q

What is a multi-access system?

A

When 2 or more nodes can transmit at the same time in one channel.

33
Q

What happens when 2 or more nodes transmit at same time?

A

All transmissions collide and are destroyed.

34
Q

Explain three contention resolution strategies for such multi-access systems.

A
  1. Divide the channel into independent sub-channels. E.g. TDM or FDM
  2. Collision resolution E.g. Ethernet
  3. Reservations E.g. Circuit switching
35
Q

Explain CSMA and CD.

A

CSMA: A node wishing to transmit must first listen to the channel. When the channel is determined to be idle, our node can transmit.

CD: When a collision occurs all nodes involved stop transmitting immediately. Each node then computes a randomly-sized time interval, waits for that amount of time, and begins the transmission attempt again.

36
Q

What is the problem with CSMA/CD?

A

A node could be waiting forever.

37
Q

What does ethernet do?

A

Sends data from nodes at random times to try and avoid collisions.

38
Q

What is the worst case collision detection time?

A

2 * propagation delay.

39
Q

Why do we pad frames for Ethernet?

A

If frame takes at least 2 Prop to transmit, there cannot be any errors, so no need for ACK’s.

40
Q

What is a preamble?

A

56 bits of alternating 1s and 0s

41
Q

Explain token ring.

A

“ring” is a set of point to point links which is unidirectional.
“token” is special frame passed from one node to the next.

Each link can only transmit when it has the token and will release it when finished using it.

42
Q

T or F. In wired ethernet it can be assumed that when a frame is transmitted it is correctly received.

A

True.

43
Q

What problem exists with wireless networks?

A

Unclear boundaries. Coverage areas overlapping can cause interference.

44
Q

What is the most succesful LAN?

A

IEEE 802.11 WIFI

45
Q

What do the following stand for?
SFD
ISM
BSS
SSID
STA
AP
RF
DCF
PCF
HCF
QoS
WLAN

A

Start Field Delimiter
Industrial, Scientific and Medical
Basic Service Set
Service Set Identifier
Station
Access Point
Radio Frequency
Distributed Coordination Function
Point Coordination Function
Hybrid Coordination Function
Quality of Service
Wireless Local Area Network

46
Q

Regardless of whether it is an I-frame, S-frame or U-frame which byte has control information.

A

The 3rd.

47
Q

What do (a) I-frame (b) S-frame (c) U-frame control byte start with.

A

(a) 0
(b) 10
(c) 11

48
Q

Where is PF in control byte?

A

Always 4th bit (from right)

49
Q

T or F. When handing over in a mobile network, to make it seemless we have to make a connection before we break the prior connection.

A

True.

50
Q

T or F. The Generator for a CRC-8 could be represented by 10110010

A

False.

51
Q

T or F. If we increase the bit rate on a link, the size of the link, a, will decrease.

A

False.

52
Q

T or F. If the Hamming distance between code words is 7, we can correct 4 bit errors.

A

False.

53
Q

T or F. When we bit stuff a sequence of 1’s and 0’s we can ensure that the flags (01111110) are unique.

A

True.

54
Q

T or F. If we double the length of the link, the size of the link a will increase.

A

True.

55
Q

T or F. When we know in advance the size of a block of data to be bit stuffed, we then can work out how many extra bits we will get.

A

False.

56
Q

T or F. A generator for a CRC-32 can be represented as X^32 + X^26 + 1.

A

True.

57
Q

T or F. Selective repeat is not always more efficient than Go-Back-N.

A

False.

58
Q

T or F. When the propagation delay in Stop and Wait increases, the efficiency also increases.

A

False.

59
Q

T or F. When I increase the frame size I increase the probability of frame error.

A

True.

60
Q

T or F. The parameter a is given as the number of frames that could be sent back to back until we get a response from the receiver.

A

True.

61
Q

T or F. The efficiency of Go-Back-N is better than Stop and Wait.

A

True.

62
Q

T or F. In Stop and Wait there is only one frame outstanding on the link so we do not have to number the frames.

A

False.

63
Q

T or F. If the value of a is ten, then the maximum efficiency of Stop and Wait will be 10%.

A

True.

64
Q

T or F. When I increase the bit rate in Stop and Wait the efficiency increases.

A

False.

65
Q

T or F. There are no out of order frames in a Go-Back-N scheme.

A

True.

66
Q

T or F. There are four types of frames in HDLC.

A

False.

67
Q

T or F. If the control field starts with 10 this is a S-frame.

A

True.

68
Q

T or F. The N(S) and N(R) in the control field of an I-frame are related to each other.

A

False.

69
Q

T or F. There are four types of supervisory frames in HDLC.

A

True.

70
Q

T or F. Ethernet does not use ACK or NAK to try and make sure frame transmissions are successful.

A

True.

71
Q

T or F. You are able to send any frame size in Ethernet.

A

False.

72
Q

T or F. By sensing the medium before transmission we can ensure that there are no collisions in Ethernet.

A

False.

Networking (5 decks)

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