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Communication Networks > Link Layer > Flashcards

Link Layer Flashcards

1
Q

What are the 4 link layer services?

A
  1. framing
  2. synchronisation
  3. flow control
  4. errors in transmission
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2
Q

How does framing work in the link layer?

A
  • encapsulates datagram into frame
  • adds header and trailer
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3
Q

What are MAC addresses and what are they used for?

A
  • used if shared medium
  • used in frame headers to identify source and destination
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4
Q

How does the link layer use reliable delivery between adjacent nodes?

A
  • seldom used on low bit-error link
  • half/full duplex
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5
Q

Do wireless links have high or low error rates?

A

high

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6
Q

How are link layer functions implemented?

A
  • in an adaptor (network interference chip NIC)
  • on a chip
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7
Q

What are the two types of links?

A
  • point-to-point
  • broadcast
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8
Q

What are the 3 classes of MAC protocols?

A
  1. Channel Partitioning
  2. Random Access
  3. Round robin
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9
Q

How does channel partitioning work?

A

allocate to node exclusively

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10
Q

How does random access work?

A

contention mechanisms

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11
Q

How does round robin work?

A

Nodes take turns

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12
Q

How does point-to-point data link control work?

A
  • 1 sender
  • 1 receiver
  • 1 link
  • no MAC
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13
Q

What are the 2 most popular DLC protocols?

A
  1. PPP: point-to-point protocol
  2. HDLC: high level data link control
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14
Q

How is a PPP data frame structured, what does each part do and how many bytes for each section?

A

8 - Flag: delimiter
8 - Address: nothing
8 - Control: nothing
8/16 - Protocol: upper layer protocol to which
frame delivered
Variable: Info: upper layer data being carried
16/64 - Check: cycle redundancy check for error
detection
8 - Flag

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15
Q

What are the 3 performance considerations for the link layer?

A
  1. Delay between ready to transmit and completion of successful transmission
  2. Throughput - total rate of data being transmitted between stations
  3. Utilisation of the medium - fraction of total capacity being used
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16
Q

What is propagation delay?

A

time taken for signal to propagate from one node to the next

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17
Q

What is transmission delay

A

time taken for transmission to send out block of data

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18
Q

What is the equation for propagation delay?

A

distance / velocity

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19
Q

What is the equation for transmission delay?

A

Length of frame / data rate

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20
Q

What is an example if a centralised round-robin technique?

A

Polling

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21
Q

What is an example of a distributed round-robin technique?

A

Token Bus
Token Ring

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22
Q

What is an example of a centralised reservation technique?

A

Centralised reservation

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23
Q

What is an example of a distributed reservation technique?

A

Distributed reservation

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24
Q

What is an example of a centralised contention technique?

A

There are none

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25
Q

What is an example of a distributed contention technique?

A

ALOHA, CSMA, CSMA/CD

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26
Q

What is polling?

A

Master node invited slave nodes to transmit in turn

27
Q

What is token passing?

A

Control token is passed form one node to the next sequentially

28
Q

What is a random access MAC protocol specify?

A
  • how to detect collisions
  • how to recover from collisions
29
Q

What are examples of a random access MAC protocol?

A

CSMA, CSMA-CD, CSMA-CA, ALOHA, slotted ALOHA

30
Q

What is CSMA?

A
  • carrier sense multiple access
  • LAN protocol
  • if channel is busy then wait until becomes idle
  • if collision occurs wait a random time and start over
31
Q

What are the 6 CSMA-CD performance assumptions?

A
  1. N independent stations
  2. only 1 medium channel
  3. Normalised transmission time (=1)
  4. if overlapping -> collision -> frame loss (only source of error)
  5. continuous or slotted time
  6. carrier sense or no carrier sense
32
Q

For 1 persistent CSMA-CD what is the probability that exactly one station attempts transmission in a slot?

A

A = NP(1-P)^(N-1)

N = no of stations
P = prob station transmits during an available slot
Slot = twice the end to end propagation = 2a

33
Q

What is the utilisation for 1 persistent CSMA-CD?

A

U = 1/[(1+2a)/A]

34
Q

What does a bus ethernet cable have?

A

coaxial cable

35
Q

What does a star ethernet cable have?

36
Q

What is the ethernet frame structure?

A

Preamble
Destination address
Source address
Type
Data
CRC

37
Q

What is a token ring?

A
  • point to point interfaces
  • token circulates round ring
  • seize token ot transmit
38
Q

What are the token ring states?

A
  • listen
  • transmit
39
Q

When is the token released on a token ring?

A

after the station finishes transmitting and the receipt of the physical transmission edge

40
Q

What are the performance assumptions for the token ring?

A
  • normalised throughput to
    system capacity
  • normalised transmission time
  • propagation delay = a
  • N stations ready to transmit
  • all stations equidistant
41
Q

If a>1 what time is the transmission finished for a token ring?

42
Q

if a>1 what time does all the the frame return back to the start for a token ring?

43
Q

If a<1 for a token ring, what is the transmission process?

A
  • frame started to be sent
  • still transmitting when the
    frame starts to be received
  • waits for all of frame to be
    received until sending ack
44
Q

What is the token ring utilisation for a<1?

A

U = 1/(1+a/N)

45
Q

What is the token ring utilisation for a>1?

A

U = 1/(a+a/N)

46
Q

For a broadcast token ring mechanism, what is the transmission process?

A
  • if have data, listen
  • free token then append data
  • transmit token
  • information goes round ring
  • one station reads the data
  • full frame transmitted back
    round to start
  • station assumes data read
    and extracts data back
  • token free again
47
Q

What are the 2 flow control protocols?

A

stop and wait
sliding window

48
Q

What are the 3 error control protocols?

A

stop and wait ARQ
selective reject ARQ
go back n ARQ

49
Q

What is the purpose of flow control?

A

assure sender doesn’t overwhelm receiver

50
Q

What is the principle of flow control?

A

do not accept frames from sender until receiver grants permission

51
Q

What is the half duplex stop and wait protocol?

A
  • send frame
  • receives ok
  • acknowledge
  • send new frame
52
Q

What is the utilisation of the stop and wait protocol for a half duplex?

A

U = 1/(1+2a)

53
Q

What is the sliding window protocol?

A
  • sender keeps list of
    sequence number of frames
    sent
  • keep frames in memory until
    ack
  • if buffer full, stop transmitting
  • receievr keeps window size
    with number of frame
    sequence
  • frames falling out of window
    discarded
54
Q

What is the utilisation of the sliding window protocol?

A

U = W/(2a+1)

55
Q

What is the stop and wait ARQ?

A

once frame acknowledged transmit next frame

56
Q

what is the go back n ARQ?

A
  • send series of frames
    determined by window size
  • if error detected discard all
    frames after corrupted one
  • sender retransmits from
    corrupted onwards
57
Q

What is the selective repeat ARQ?

A
  • only one frame transmitted
    due to corrupted frame/time-
    out
  • minimizes amount of transmission
  • receivers contain storage and
    logic to order frames
58
Q

What is the utilisation of the ARQ protocols?

59
Q

What are the assumptions when analysing ARQ protocols?

A

acks and negative acks are error free

60
Q

What is the equation for expected number of transmission of one frame (Nr)?

A

Nr = 1/(1-P)

61
Q

What is the utilisation for stop and wait protocol?

A

(1-P)/(1+2a)

62
Q

What is the utilisation for selective repeat protocol?

A

W>2a+1: U = 1-P

W<2a+1: U = [W(1-P)]/(1+2a)

63
Q

What is the utilisation for go back n ARQ protocol?

A

W>2a+1: U = (1-P)/(1+2aP)

W<2a+1: U = [W(1-P)]/[(1+2a)
(1-P+WP)]

Communication Networks (8 decks)

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