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IS 3413 > Chapter 4 > Flashcards

Chapter 4 Flashcards

1
Q

What is the data link layer responsible for?

A
  • moving a message from one computer or network device to the next computer or network device.
  • controls the way messages are sent on the physical media
  • determines who can transmit at what time, where a message begins and ends, and how a receiver reconizes and corrects a transmission error.
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2
Q

Describe the LLC layer

A
  • data link layer’s connection to the network layer aboiv it.
  • responsible for communicating with the network layer software and putting packet inside a frame
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3
Q

Desribe the MAC layer.

A
  • controls the physical hardware.

- controls how and when the physical layer converts bits into the physical symbols that are sent down the circuit.

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4
Q

What three functions does a data link protocol perform?

A
  • controls when computers transmit(MAC)
  • detects and corrects transmission errors(error control)
  • identifies the start and end of a message by using a PDU(message delineation).
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5
Q

When is MAC unecessary?

A

-point-to-point, full-duplex connections.

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6
Q

Describe contention.

A

-computers wait until the circuit is free and then transmit whenever they have data to send.

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7
Q

Describe controlled access.

A
  • most wireless LANs use this

- WAP controls the circuit and determines which clients can transmit at what time.

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8
Q

Describe the access request technique.

A
  • client computers request to transmit to the device that is controlling the circuit.
  • controlling device grants permission to only one device at a time.
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9
Q

Describe polling.

A
  • send a signal to a client computer that gives it permission to transmit.
  • client stores all messages that need to be transmitted.
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10
Q

Describe roll-call polling.

A
  • controller works consecutively through a list of clients.

- can increase the priority of different clients.

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11
Q

Describe hub polling.

A
  • aka token passing

- one device starts the poll and passes it to the next computer.

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12
Q

Which MAC method works best?

A
  • contention: works better for small networks with low usage.
  • controlled access: better for large networks with high usage.
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13
Q

What are the two types of network errors?

A
  • corrupted data

- lost data.

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14
Q

What three things should networkds do in regards to errors.

A

-prevent, detect, and correct both corrupted and lost data.

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15
Q

What is a burst error?

A

-more than 1 data bit is changed by the error-causing condition.

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16
Q

Describe noise.

A

noise manifests itself as extra bits, missing bits, or bits that have been flipped.

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17
Q

Describe white/Gaussian noise.

A
  • familiar background hiss or static on radios and telephones.
  • caused by thermal agitation of electrons.
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18
Q

Describe impulse noise.

A
  • primary source of errors in data communications.
  • click or crackling noise.
  • voltage changes in adjacent lines, lightning flashes, fluorescent lights.
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19
Q

Describe cross talk.

A

-one circuit picks up signals in another.

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20
Q

Describe echos.

A

-result of poor connections that cause the signal to reflect back to transmitting equipment.

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21
Q

Describe attentuation.

A

-the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer.

22
Q

Describe intermodulation noise.

A
  • special type of cross talk.

- signals from two circuits combine to form a new signal that falls into frequency band reserved for another signal.

23
Q

Describe error prevention.

A
  • shielding: best way.

- moving cables from noise

24
Q

describe parity checking.

A
  • one additional bit is added to each byte in the message.
  • parity bit is set to make the total number of 1’s in the byte including the parity bit either an even number or an odd number.
25
Q

When can parity checking detect errors?

A

-when an odd number of bits have been switched.

26
Q

Describe checksum.

A
  • calculated by adding the decimal value of each character in the message, dividing sum by 255 and using the remainder as the checksum.
  • receiver calculates its own checksum.
  • detects 95% of the errors.
27
Q

Describe CRC.

A

-adds 8, 16, 24 or 32 bits to the message. message treated as one long binary number.
-crc-16 16 bit
-crc-ccitt 16 bit
crc-32, 32 bit.
-detects 100% errors for all errors of the same length as the CRC or less.

28
Q

Describe retransmission.

A

-simplest way to correct errors.

29
Q

What is an ARQ?

A
  • automatic repeat reQuest.

- receiver requests sender to retransmit the message.

30
Q

Describe stop and wait ARQ.

A
  • sender stops and waits for a responsoe from the receiver after each data packet
  • receivers sends an ack or negative ack (NAK).
31
Q

Describe continuous ARQ

A
  • sender does not wait for an acknowledgement after sending a message, immediately sends the next one.
  • Link Access Protocl for Modems(LAP-M): only packets containing errors are retransmitted.
  • Go-Back-N ARQ: retransmits first packet with error and all that followed.
32
Q

What is continuous ARQ also called?

A

sliding window

33
Q

What else does continuous ARQ provide?

A
  • flow control

- size of window is agreed upon.

34
Q

Describe forward error correction.

A
  • uses codes containing sufficient redundancy to prevent errors by detecting and correcting them at the receiving end without retransmission of the original message.
  • used in satellite transmission.
35
Q

What is message delineation?

A

-indicates where a message starts and stops.

36
Q

Describe asynchronous transmission.

A
  • start-stop transimission.
  • transmitting computer can transmit a character whenever it is convenient.
  • point-to-point and full duplex circuits.
  • each character transmitted independently of all other characters.
  • start and stop bit added to each character.
37
Q

Describe start and stop bits.

A
  • opposite of each other.

- start with 0, stop bit is a 1.

38
Q

What is synchronization?

A

-the recognition of the start and stop of each message.

39
Q

describe idle signal.

A

-the signal that is sent down the circuit when no data is being sent as the same as the stop bit.

40
Q

Describe synchronous transmission.

A
  • all the letters or data in one group are transmitted at one time as a block of data.
  • block is called a frame.
  • start and end of entire frame is marked.
  • used on point-to-point and multipoint circuits.
  • start an end is established by adding sync (SYN) characters to the start of the frame.
41
Q

What happens after the syn characters in synchronous transmissions?

A

-the transmitting computer sends a long string of characters that may contain thousands of bits.

42
Q

Describe SDLC

A
  • synchronous data link control.
  • mainframe protocol.
  • each frame begins and ends with 01111110 which is a flag.
43
Q

Describe HDLC.

A
  • high level data link control
  • formal standard developed by ISO often used in WANS.
  • same as SLC except address and control fields can be longer.
44
Q

What type of MAC protocol does HDLC use?

A

-controlled access protocol.

45
Q

Describe ethernet.

A
  • Conceived by Bob Metcalfe in 1973.
  • developed jointly by digital intel and zerox.
  • IEEE 802.3ac
46
Q

Describe Point-to-point Protocol.

A

-developed in early 1990s and is often used in WANs.

-

47
Q

What are information bits?

A

-those used to convey the user’s meaning.

48
Q

What are overhead bits?

A

-used for purposes such as erro checking and marking the start and end of characters and packters.

49
Q

Describe transmission efficiency.

A

-total number of information bits divided by the total bits in transmission.

50
Q

Describe throughput.

A

-total number of information bits received per second after taking into account the overhead bits and the need to retransmit frames containing errors.

51
Q

What is the idea frame size?

A

between 2000 and 10,000 bytes.

IS 3413 (11 decks)

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