Chapter 4 Flashcards
What is the data link layer responsible for?
- 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.
Describe the LLC layer
- 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
Desribe the MAC layer.
- controls the physical hardware.
- controls how and when the physical layer converts bits into the physical symbols that are sent down the circuit.
What three functions does a data link protocol perform?
- 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).
When is MAC unecessary?
-point-to-point, full-duplex connections.
Describe contention.
-computers wait until the circuit is free and then transmit whenever they have data to send.
Describe controlled access.
- most wireless LANs use this
- WAP controls the circuit and determines which clients can transmit at what time.
Describe the access request technique.
- client computers request to transmit to the device that is controlling the circuit.
- controlling device grants permission to only one device at a time.
Describe polling.
- send a signal to a client computer that gives it permission to transmit.
- client stores all messages that need to be transmitted.
Describe roll-call polling.
- controller works consecutively through a list of clients.
- can increase the priority of different clients.
Describe hub polling.
- aka token passing
- one device starts the poll and passes it to the next computer.
Which MAC method works best?
- contention: works better for small networks with low usage.
- controlled access: better for large networks with high usage.
What are the two types of network errors?
- corrupted data
- lost data.
What three things should networkds do in regards to errors.
-prevent, detect, and correct both corrupted and lost data.
What is a burst error?
-more than 1 data bit is changed by the error-causing condition.
Describe noise.
noise manifests itself as extra bits, missing bits, or bits that have been flipped.
Describe white/Gaussian noise.
- familiar background hiss or static on radios and telephones.
- caused by thermal agitation of electrons.
Describe impulse noise.
- primary source of errors in data communications.
- click or crackling noise.
- voltage changes in adjacent lines, lightning flashes, fluorescent lights.
Describe cross talk.
-one circuit picks up signals in another.
Describe echos.
-result of poor connections that cause the signal to reflect back to transmitting equipment.
Describe attentuation.
-the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer.
Describe intermodulation noise.
- special type of cross talk.
- signals from two circuits combine to form a new signal that falls into frequency band reserved for another signal.
Describe error prevention.
- shielding: best way.
- moving cables from noise
describe parity checking.
- 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.
When can parity checking detect errors?
-when an odd number of bits have been switched.
Describe checksum.
- 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.
Describe CRC.
-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.
Describe retransmission.
-simplest way to correct errors.
What is an ARQ?
- automatic repeat reQuest.
- receiver requests sender to retransmit the message.
Describe stop and wait ARQ.
- sender stops and waits for a responsoe from the receiver after each data packet
- receivers sends an ack or negative ack (NAK).
Describe continuous ARQ
- 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.
What is continuous ARQ also called?
sliding window
What else does continuous ARQ provide?
- flow control
- size of window is agreed upon.
Describe forward error correction.
- 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.
What is message delineation?
-indicates where a message starts and stops.
Describe asynchronous transmission.
- 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.
Describe start and stop bits.
- opposite of each other.
- start with 0, stop bit is a 1.
What is synchronization?
-the recognition of the start and stop of each message.
describe idle signal.
-the signal that is sent down the circuit when no data is being sent as the same as the stop bit.
Describe synchronous transmission.
- 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.
What happens after the syn characters in synchronous transmissions?
-the transmitting computer sends a long string of characters that may contain thousands of bits.
Describe SDLC
- synchronous data link control.
- mainframe protocol.
- each frame begins and ends with 01111110 which is a flag.
Describe HDLC.
- 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.
What type of MAC protocol does HDLC use?
-controlled access protocol.
Describe ethernet.
- Conceived by Bob Metcalfe in 1973.
- developed jointly by digital intel and zerox.
- IEEE 802.3ac
Describe Point-to-point Protocol.
-developed in early 1990s and is often used in WANs.
-
What are information bits?
-those used to convey the user’s meaning.
What are overhead bits?
-used for purposes such as erro checking and marking the start and end of characters and packters.
Describe transmission efficiency.
-total number of information bits divided by the total bits in transmission.
Describe throughput.
-total number of information bits received per second after taking into account the overhead bits and the need to retransmit frames containing errors.
What is the idea frame size?
between 2000 and 10,000 bytes.
