data link layer

Question 1

what are some services that the datalink layer provides

Answer 1

framing
reliable delivery between adjacent nodes
flow control
error detection and correction
full and half duplex

Question 2

framing

Answer 2

encapsulating datagrams from the network layer into a frame and adding a header and trailer

Question 3

MAC address

Answer 3

added in the frame header
id the source and destination of each hop

Question 4

how are mac addresses portable

Answer 4

when you change networks you dont need a new one

Question 5

ARQ; automatic repeat request

Answer 5

asks the previous hop to resend the packet if theres an error

Question 6

flow control

Answer 6

the pacing between adjacent sending and receiving hops

Question 7

half-duplex

Answer 7

both the sender and receiver cant transmit data at the same time

Question 8

how is the link layer implemented

Answer 8

on a network interface card

Question 9

what is the NIC a combination of

Answer 9

firmware; software thats hardware specific
software; the kernel has modules for the link layer
hardware; the nic

Question 10

ep roam

Answer 10

non volatile memory that stores firmware

Question 11

what does the edc include

Answer 11

could be the entire frame including the header or just the data

Question 12

what does the node do if theres too much corruption compared to if theres only a little

Answer 12

too much; asks to resend
little; corrects and sends

Question 13

parity bit

Answer 13

single bit that detects single errors
with even parity you set the bit so theres an even amount of 1s

Question 14

how do we alter parity bit to correct errors and how does this work

Answer 14

use two-dimensional parity
theres a parity bit for each row and column
when an error is detected the intersection of the row and column shows which bit it is which can then be flipped to correct it

Question 15

what is an issue with 2-d parity

Answer 15

can only detect one error per row(odd number of errors )

Question 16

what are two issues with regular parity bit

Answer 16

only detect an odd number of errors
doesnt correct

Question 17

what effects do a larger edc have

Answer 17

better edc however decreases the efficiency of data throughput

Question 18

cycle redundancy check(crc)

Answer 18

uses field theory to compute semi unique values for a given message

Question 19

why does crc only compute semi-unique values

Answer 19

depends on d and g and some values may have the same remainder

Question 20

what type of errors does crc detect

Answer 20

single and duplicate
errors with odd bit numbers

Question 21

what happens on the sender side of crc

Answer 21

add r 0s to the data bits
divide the new sequence by g using binary division and get the remainder
transmit the originial data followed by the computed remainder(crc)

Question 22

what happens on the receiver side of crc

Answer 22

divide the transmitted sequence by g
if the remainder is 0 then there are no errors

Question 23

what is the generator value in crc

Answer 23

r + 1

Question 24

what are the 2 types of multiple access links

Answer 24

point-to-point
shared broadcast

Question 25

multiple access protocol

Answer 25

a distributed algorithm that determined how nodes share a channel

Question 26

collision

Answer 26

when two or more nodes transmit simultaneously and their signals interfere with each other

Question 27

distributed algorithm

Answer 27

a protocol that determines how and when each node can transmit on a shared channel to avoid collisions

Question 28

what are two mac protocols

Answer 28

channel partitioning
random access

Question 29

channel partitioning

Answer 29

dividing the channel into smaller pieces and allocating each one to a node

Question 30

tdma; time division multiple access

Answer 30

each node gets a fixed time slot which is when the node can send data if its available

Question 31

what is the issue with tdma

Answer 31

not efficient as unused slots go idle

Question 32

fdma; frequency division multiple access

Answer 32

the channel is divided into different frequency bands and each node gets assigned one where they can transmit data at any time

Question 33

what is the issue with fdma

Answer 33

lacks efficiency as a band is wasted if a device is not ready to transmit

Question 34

what are the two channel partitioning protocols

Answer 34

tdma
fdma

Question 35

random access protocols

Answer 35

channels arent divided and allow collisions but can recover from them

Question 36

what are some principles of sorted ALOHA

Answer 36

all frames are the same size and time slots are divided equally
code is transmitted at the beginning of the time slot
the nodes are synchronised and if 2 or more nodes transmit at the same time they detect the collision

Question 37

in ALOHA how are nodes able to detect a collision

Answer 37

nodes can hear their own signal so if they hear one that isnt them then they know that theres a collision

Question 38

how does ALOHA work

Answer 38

when a node gets a fresh frame it transmits in the next slot
if theres no collision then it can send a new frame in the next slot
if there is a collision then the node either backs off or re transmits until the frame goes through based on an algorithm

Question 39

what is a negative of aloha

Answer 39

theres a lot of collision and wasted resources
37% max efficiency

Question 40

simple csma; carrier sense multiple access

Answer 40

listens before transmission
if channel is empty then transmits entire frame
if busy then defer transmission

Question 41

csma/cd; with collision detection

Answer 41

when a collision is detected then the node backs off for a certain amount of time
if its repeated then the time to back off increases
collisions are detected within a short amount of time which reduces the amount of time wasted on them

Question 42

how does an nic detect a collision

Answer 42

comparing the signal theyre transmitting with the signal theyre receiving on the network

Question 43

what does the nic do when it detects another transmission whilst sending

Answer 43

sends a jam