Ethernet (w2-3)

Question 1

what are the 7 OSI layers

Answer 1

(from top) Application, Presentation, Session, Transport, Network, Link, Physical

Question 2

does an osi layer provide a service for the layer above or below?

Answer 2

layer above.

Question 3

why is layering important?

Answer 3

for flexibility to modify one layer without affecting the others

Question 4

What does the physical layer transfer?

Answer 4

bits across a physical link (ethernet)

Question 5

Link Layer provides which feature? (and example of a link layer)

Answer 5

Addressing, for example, MAC addresses (eg Ethernet, Wi-Fi or 4g)

Question 6

Network Layer provides which service?

Answer 6

Routing services, transfers packets across multiple links

Question 7

explain the Transport Layer (and example)

Answer 7

transfers segments end to end from a process in one machine to a process in another (UDP or TCP)

Question 8

Explain the Session Layer?

Answer 8

It allows information of different streams, perhaps originating from different sources, to be properly combined or synchronized.

Question 9

explain Presentation Layer

Answer 9

Byte order, encoding of numbers, character codes and structures

Question 10

Application layer incorporates which two layers?

Answer 10

Session and Presentation

Question 11

Explain encapsulation in OSI

Answer 11

application data is encapsulated insude of packets with headers

Question 12

CSMA/CD

Answer 12

Used in half duplex. Carrier Sense (don’t speak over top of broadcast) Multiple Access Collision Detection (if detected, back off then begin transmitting at random times)

Question 13

Half vs Full Duplex, vs Simplex

Answer 13

Simplex: station can only either transmit or receive
Half duplex: Station can transmit and receive, but not at the same time
Full Duplex: Station can transmit and receive at the same time.

Question 14

Catagory 5 cable vs Coaxial Cables

Answer 14

Coax is heavy and expensive. Also only half duplex where catagory 5 is full duplex

Question 15

Difference between hub and switch

Answer 15

hub broadcasts information to all ports, switch learns which nodes transmit through which port, and sends out data only to the learned ports

Question 16

What is one way computers can achieve some sort of timing immunity?

Answer 16

computers restart their timers every character, stops two timers drifting apart.

Question 17

what is the clock rate of manchester encoding

Answer 17

twice the data rate, (slow)

Question 18

what is the point of a crossover cable?

Answer 18

Crossover cables allow output of one device to be the input of the connect device. used when connecting two computers directly, or to link switches and hubs together.

Question 19

how does 4b5b help timing recovery?

Answer 19

by not allowing more than 3 zero’s in a row, ensuring at least two transitions, which helps the receiver in clock recover.

Question 20

how does 4b5b help recognize error/corruption in transmition?

Answer 20

4b5b output has 32 different patterns avaialbale (5^2), but only uses 16 of them. if the output is not one of those 16 options, you know you have a transmit/recieve error.

Question 21

multicast vs broadcast vs unicast

Answer 21

Broadcast addresses all machines on the network, where multicast addresses all machines who want to listen. unicast is the usual case where only the addressee uses the message.

Question 22

what does a network interface card receive by default, in terms of multicast vs broadcast vs unicast

Answer 22

all messages for its own unicast address, and all messages for broadcast.

Question 23

CSMA/CD vs CSMA/CA

Answer 23

CD is collision detection and CA is collision avoidance. CA is used in wireless, where sender cannot detect incoming data while sending. CA involves listening if the medium is being used before sending

Question 24

What is twisted pair and why is it used.

Answer 24

It is a type of wiring in which two conductors are twisted together, reducing electromagnetic radiation from the pair and improves rejection of external interference.

Question 25

In Ethernet Frames, the data is encapsulated with ethernet headers and trailers. What is in these?

Answer 25

Header: Dest. MAC address, Src MAC address and Ether type
Trailer: Cyclic Redundancy Check 4 bytes, or 32 bits

Question 26

what is the EtherType?

Answer 26

It is the protocol type, allowing multiple types of protocols to be carried (IPv4, IPv6)

Question 27

What is CRC32

Answer 27

Fairly strong error detection, used to detect errors in received messages.

Question 28

Before transmission of a frame, ethernet sends out a preamble for the receiver to synchronise its clock. WHat is contained in those 8 bytes

Answer 28

7 bytes of 10101010

final byte of 10101011

Question 29

How long must the delay be (in bytes) after transmitting a frame, before the node can send again?

Answer 29

12 byte delay

Question 30

woudl a software writer use ethernet addresses?

Answer 30

Mac addresses (Ethernet address) are handled by the operating systems, whereas the software writer would use IP addresses.

Question 31

which protocol would an operating system use to find out the destination mac address, when sending an IP packet?

Answer 31

the ARP, or address resolution protocol

Question 32

How does ARP work.

Answer 32

A wants to talk to C, but needs to fill ethernet feilds.
A broadcasts a packet saying who has ?
C replies is at (C’s MAC address)

Question 33

does a computer send out an arp everytime it wants to send something?

Answer 33

no, it holds a cache of recent replies to arp requests. periodically, they expire

Question 34

key difference in NPD vs ARP

Answer 34

ARP utilizes broadcast. every host recieves the ARP request.
NDP uses multicast. all hosts receive frame, but only hosts configured to process frames with the specific multicast address will pass the frame on to the operating system.
only hosts with the same last 24 bits in IPv6 address, whuch is usually only in one host.

Question 35

key difference in NPD vs ARP

Answer 35

ARP utilizes broadcast. every host recieves the ARP request.
NDP uses multicast. all hosts receive frame, but only hosts configured to process frames with the specific multicast address will pass the frame on to the operating system.
only hosts with the same last 24 bits in IPv6 address, whuch is usually only in one host.