3213 Quiz 2

Question 1

MAC Address

Answer 1

Header to frame specifies address of receiver
48 bits = 6 bytes // repped as 12 hexadecimal digits

Question 1

What is bit stuffing
What are steps

Answer 1

Used to avoid confusion / help manage
Insert 0 after a string of 5 - 1’s in frame
If next bit 0, delete bit stuffing
if 10 - flag
if 11 - error

Question 2

Three types of MAC Address

Answer 2

Unicast - To one user in LAN (2nd digit needs to be even)
Multicast - To group of users in LAN (2nd dig odd)
Broadcast - To all users in LAN // FF:FF…:FF

Question 3

What is ARP

Answer 3

Address Resolution Protocol
Service of getting MAC address for a specified IP address

Question 4

ARP Table

Answer 4

IP/MAC address mappings for LAN nodes
<IP address, MAC address, Time-to-Live>
TTL = time after map is forgotten (20-60mins)

Question 5

Describe ARP example

Answer 5

Node A wants to send frame to B
A broadcasts ARP request containing B’s IP
All machines receive req, B receives ARP req
B replies to A with MAC Address via unicast
A saves IP-to-MAC pair in ARP table

Question 6

Frame Error in DLL
2 types of error
Probability?

Answer 6

Lost Frame: not received
Damaged Frame: bits are corrupted
Prob of Bit Error (BER) :
wireless - pb = 10^-3
copper - pb = 10^-6
fiber - pb=10^-9

Question 7

Types of error (2)

Answer 7

Single bit errors: one bit in frame corrupted

Burst errors: two or more bits in frame corrupted

Question 8

Error control method 1:
Detect + Automatic repeat reQuest

Answer 8

ARQ: waste of bandwidth + return channel required

Question 9

Key idea of error control?

Answer 9

Redundancy bits: add extra bits for detect + correct

Question 10

Block coding

Answer 10

Data stream split into blocks of datawords which then converted to code word of larger size. Error detected if code word got corrupted and is no longer apart of possible codewords in dictionary

Question 11

Hamming distance

Answer 11

number of distance between corresponding bits
found by XORing two codewords and counting # of 1’s in result

Question 12

Minimum hamming distance for detection

Answer 12

dmin = s + 1
Such that s errors are guaranteed to be detected in all cases

Question 13

Minimum hamming distance for correction

Answer 13

dmin = 2t + 1
Such that t errors are guaranteed to be corrected in all cases

Question 14

Example [ Hamming distance]
A code scheme has a Hamming distance dmin=4. What is the error detection and
error correction capability of this scheme?

Answer 14

detection - s = 3
correction - t = 1.5
Floor the results

Question 15

Single bit parity

Answer 15

Given k bits, append a 1 such that total number of 1 bits are even
- Receiver can detect errors if there are odd numbers of 1 bits
- Position of error unknown

Question 16

Error vector

Answer 16

w(e) represents number of 1’s in error vector s.t
1 = error
0 = no error

Question 17

Probability of k bit errors

Answer 17

p(w(e)=k) = (1-pb)^n-k * pb^k

Question 18

Probablity that single parity check fails

Answer 18

P(error detection failure) =
(n choose k) * pb^k(1-pb)^n-k + “increment k” + …increment again

k = # of bit errors