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COS 332 Chancellors > Chapter 8 > Flashcards

Chapter 8 Flashcards

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1
Q

What does a layer 2 protocol include?

A

MAC
Data delineation
Error Control (transmission)

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2
Q

What does MAC do?

A

It decides whether a node is allowed to transmit data (on the medium that connects it to another node)

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3
Q

What is the MAC’s decision based on? (whether a node can transmit or not)

A

Whose turn it is to transmit data

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4
Q

What are the 3 solutions for MAC?

A

1: Access is controlled by a controlling node (aka master-slave)
2: Access is controlled by possession of a token (token passing protocols)
3: Any node transmit data whenever he want to(Multiple access protocols)

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5
Q

What is the difference between point to point and multipoint connections in master slave protocols?

A

Point-to-point = connection between 2 nodes
Multipoint = connection between “smart” node and 2+ “dumb” nodes

Multipoint needs to identify slave it wants to send messages to.

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6
Q

How does master slave protocols work?

A

Master polls salve to see if it wants to send a message. Slave either responds with data or negative ack.
If master wants to send something it just sends it to the destination

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7
Q

Problem with master slave protocols?

A

Latency ( slave has to be polled before it can send anything )
Single point of failure

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8
Q

What is the best example of master slave protocols?

A

SDLC (Synchronous Data Link Control)

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9
Q

How does token ring protocols differ from master slave protocols?

A

All nodes in token ring is seen as equals

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10
Q

When can a node send a message in a token ring protocol?

A

When it has access to the token

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11
Q

Does all nodes have to be equal in a token ring protocol?

A

Technically no, since you can implement a priority scheme such that more important nodes can get the token before less important nodes. Can lead to starvation?

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12
Q

How is data transferred in the token ring?

A

It is sent all around the ring until it reaches the destination node, which copies it and sends it to layer 3.

Data goes around back to sender and then it takes the data off the network

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13
Q

In a priority schema token ring protocol. How does a node know if it can “take” the token?

A

It checks the priority on the token. If priority on token is higher it sends it along without data.

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14
Q

What happens if 2 nodes have the same priority level in a token ring protocol??

A

The one at which the token arrives first will get the token

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15
Q

What is the problem with token ring protocol?

A

Token may be lost ( use active monitor node)

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16
Q

What does a active monitor node do in a token ring protocol?

A

Checks that the token regularly passes it to ensure it stays active and isnt lost

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17
Q

What happens when the active monitor goes down in a token ring protocol?

A

Get an election for new monitor. Uses bully algorithm

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18
Q

How does the bully algorithm work to get a new active monitor?

A

1: call an election
2: Pass a frame with the highest MAC address inside. Check if your address is higher. If it is place yours inside the frame.
3: Highest MAC address wins

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19
Q

What should you do to the token ring protocols if the network becomes large enough?

A

Use multiple tokens aka empty slots, to put data in.

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20
Q

The ….. is an example of a protocol in the large token ring protocols

A

FDDI ( Fibre Distributed Data Interface)

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21
Q

What is IBM’s token ring standardized as?

A

IEEE 802.5

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22
Q

What is the standard example of a token bus?

A

IEEE 802.4

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23
Q

When does Multiple access networks work?

A

On a quiet network

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24
Q

How does Multiple access protocols work?

A

Any node may transmit data whenever it wants

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25
What is the problem with multiple access protocols?
Collisions. Results in both frames of data being lost
26
How do we decrease collisions?
Use a CSMA to detect if a node is already transmitting Carrier Sense Multiple Access
27
How can you decrease collisions without a CSMA?
Divide transmission opportunities into slots
28
Problems with CSMA?
Collisions between 2 nodes after 1 node has finished sending ( both has sensed that there are no longer transmission from node 1 so they both send at same time
29
How can you fix CSMA?
Wait for transmission line to become idle. Dont let nodes wait while listening. aka persistent CSMA
30
What is a non-persistent CSMA?
Node stops listening if they want to transmit and the medium is busy
31
When wont non-persistent CSMA work?
When the network is really busy
32
What is p-persistent CSMA?
Take a probability that the node will wait for the line to become idle
33
What is CSMA/CD
CSMA + Collision detection. Listen while transmitting. If collision, abort transmission and try again later.
34
Most common CSMA/CD protocol?
Ethernet
35
By what has ethernet been standardized?
IEEE 802.3
36
What is the hidden node problem?
When 2 nodes are out of range of each other but in range of a central node
37
What is CSMA/CA?
CSMA + collision avoidance. Basically non-persistent CSMA but nodes sending at the same time is less probable
38
What solves the hidden node problem partially?
MACA ( Multiple access with collision avoidance )
39
How does MACA work?
Node that wants to transmit sends a RTS frame (request to send) Reply with CTS ( clear to send)
40
What is the time it takes to send RTS known as?
Slots. Number of slots to send RTS == CTS
41
How does MACAW (Multiple access with collision avoidance for wireless) solve wasted bandwidth?
it uses a CTS-RTS-DS sequence to initiate communication (DS = Data Sending)
42
What uses MACAW
Wifi aka IEEE 802.11
43
How can a frame be delineated?
Mark start and end of frame. ( transparency problem) Mark start and tell how long the frame is
44
What is the transparency problem?
Prematurely ending a segment. if X is used to mark beginning and end, X in the middle will end the segment to early
45
3 ways to mark the start and end of a frame
Special character, bit pattern, non-bit pattern
46
Oldest data link protocol?
Bisync ( binary sync communications aka BSC) Uses special character to indicate start and end
47
Bisync characters:
STX ETX SOH EOT ENQ ACK NAK DLE SYB
48
With what does a bisync frame start?
SYN followed by SOH and then STX then payload then ETX Later changed to DLE-STX and DLE-ETX
49
What is SDLC?
Synchronous Data Link Control
50
How is SDLC different from bisync?
Bisync = character oriented SDLC = bit oriented (uses flag to indicate start and end of a fame)
51
What is SDLC's start and end flag sequence?
01111110
52
What is bit stuffing in SDLC?
If it finds a sequence of 5 1's in the data it auto inserts a 0 in the data. This means that after seeing 5 ones you need to remove the 0
53
How was the token ring standardized?
IEEE 802.5
54
Starting + ending delimiter of token ring
Starting :JK0JK000 Ending: JK1JK1xx (xx is not important)
55
What does a ethernet frame consist of?
7 octet preamble 1 octet start of frame delimiter 6 octet destination address 6 octet source address 2 octet length field 46-1500 octets of data 4 octet CRC field (error detection)
56
How does ethernet frames solve the transparency issue?
All header fields are a fixed size. Length has a indicated size. and CRC occurs at the end with specific length
57
What is the max data that may be sent by a single frame called?
MTU ( max transmission unit )
58
How can we detect errors?
Parity Checksum CRC ( cyclic redundancy code ) (most popular ) Cryptographic hash functions Digital signatures
59
How can we detect errors + correct it?
n-modular redundancy Hamming code Reed-solomon codes
60
What is the problem with Parity checking?
Very limited and can only detect errors in the simplest cases
61
How does parity checking work?
Even = makes it such that all 1's must add up to even number Odd = Makes it such that all 1's must add up to a odd number It detects a change in a odd number of bits Can also use 1 or 0 parity.
62
How does longitudial parity work?
10101010 11111111 11110000 --------------- 10100101 (Parity octet using even parity)
63
What are checksums?
Modular(mod function) total of all values that have been transmitted If 8 bit octets are used = D1 + D2 + D3 ....... /256
64
Problem with checksum?
If a wire is broken it still wont detect an error ( if expected checksum is 0 and nothing gets sent in == "no problem")
65
How does CRC work?
Polynomial arithmetic. 10011 == X^4 + x^1 + X^0 Divide the padded data and divide it by the CRC polynomial.
66
CRC steps:
Generation: 1: Find length of divisor L 2: append L-1 bits to original message 3: perform binary division (XOR) 4: remainder = CRC thus CRC must be L-1 bits 5: append crc to data
67
Example of CRC generation
https://www.youtube.com/watch?v=A9g6rTMblz4
68
Example of CRC checking:
Divide transmitted data and if the remainder is 0 then its correct
69
How does n-modular redundancy work?
Assume most of data is sent correctly. If data is transmitted multiple times, value of majority of the messages is probability the correct value and the other may be corrected. Any value that occurs over int(n/2) + 1 times are assumed to be correct
70
problem with n-modular redundancy?
Large overhead ( n-1 ) * s where s = message size
71
n-modular redundancy example:
Buy ZAR 1 000 000 shares in Acme ASAP. Buy ZAR 1 900 000 shares in Acme ASAP. Buy ZAR 1 000 000 shares in Acme ASAP. Correct message is thus Buy ZAR 1 000 000 shares in Acme ASAP. And the 9 may be corrected
72
How does Hamming code work?
Split data into n-bit quantities (usually octets) Determine the number of parity bits: 2^r >= m + r + 1 r = number of parity bits, m = number of data bits Place parity bits ay positions of power ( 1,2,4,8,...) Use the parity bit's position to see what it "covers" 7|6|5|4(p)|3|2(p)|1(p) The associated data bits for the parity bit are chosen by the parity bit's exponentiation F.e. P1 = 2^0 = 1, so you check & skip bits always with 1 step. CHECK, SKIP, CHECK, SKIP etc. All your check-bits are the ones that will be used for parity later. F.e. P2 = 2^1 = 2, this would result in CHECK, CHECK, SKIP, SKIP, CHECK, CHECK, SKIP, SKIP etc. F.e. P3 = 2^2 = 4, this would result in 4 times CHECK, 4 times SKIP, 4 times CHECK, 4 times SKIP etc.
73
How do you correct a error in hamming code?
Find the intersections where it occurs (which parity bits is now wrong and fix that one)
74
What are the 2 layer 2 connection-oriented services?
SDLC (synchronous Data Link Control) and HDLC (High-level data link control based on SDLC)
75
What is the format for I frames in SDLC?
8 bits 7-5 = N(r), 4 = P/F 3-1 = N(S), 0 = 0 ( indicates an I frame) N(s) = number frames that has been sent N(r) = number frames that have been received p/f = poll / final
76
IF I frames can only number from 0-7 in SDLC, how does it handle more packets?
It uses the frame numbers in a cyclical manner 6,7,0,1,2...
77
How does A acknowledge that it has received a frame from B in SDLC?
It sends the "next" number of what was sent B sends N(s) = 1,2,3 then A replies N(r)=4 and if B sends N(s) = 6,7,0 A sends N(R) = 1
78
What happens if A doesnt acknowledge the packets/iframes sent by B in SDLC?
B retransmits
79
What is another type of frame besides I frames in SDLC?
S-frames ( supervisory frames )
80
Difference between I frames and S frames in SDLC?
S frames dont carry data Thus no N(S) field only N(R) to indicate which packets are missing
81
S frame format in SDLC:
7-5 = N(r), 4 = P/F, 3 + 2 = type, 1= 0, 0 =1
82
Different types of types in SDLC?
REJ (reject a frame if frame is faulty) RR ( receiver ready) RNR ( receiver not ready )
83
Difference between SDLC and HDLC?
HDLC has 1 more S frame type SREJ ( selective reject ) HDLC can also expand control field to 16 bit ( can cater for 2^7 -1 unacknowledged frames )
84
What type of ARQ protocol is HDLC?
Selective repeat
85
What type of ARQ protocol is SDLC?
go back n
86
What is the goal of P/F bits in SDLC frames?
Indicate end of a series of frames ( initially sent to 0 until 1 to mark the final frame )
87
What are U frames in SLDC and HDLC?
Unnumbered frames. Dont contain data
88
Format of a U frame?
7-5 = type 1, 4 = P/F, 3-2 = Type 2, 1 = 1, 0=1
89
How are U frames distinguished from other frames?
ending in 11
90
What are the "2" sub layers in layer 2?
LLC and MAC ( logical link control )
91
Example of LLC packet
8 bits = DSAP (Destination and Source Service Access Points) 8/16 bits = SSAP 8/16 bits = Control Several octets = Info

COS 332 Chancellors (10 decks)

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