Ethernet (w2-3) Flashcards
what are the 7 OSI layers
(from top) Application, Presentation, Session, Transport, Network, Link, Physical
does an osi layer provide a service for the layer above or below?
layer above.
why is layering important?
for flexibility to modify one layer without affecting the others
What does the physical layer transfer?
bits across a physical link (ethernet)
Link Layer provides which feature? (and example of a link layer)
Addressing, for example, MAC addresses (eg Ethernet, Wi-Fi or 4g)
Network Layer provides which service?
Routing services, transfers packets across multiple links
explain the Transport Layer (and example)
transfers segments end to end from a process in one machine to a process in another (UDP or TCP)
Explain the Session Layer?
It allows information of different streams, perhaps originating from different sources, to be properly combined or synchronized.
explain Presentation Layer
Byte order, encoding of numbers, character codes and structures
Application layer incorporates which two layers?
Session and Presentation
Explain encapsulation in OSI
application data is encapsulated insude of packets with headers
CSMA/CD
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)
Half vs Full Duplex, vs Simplex
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.
Catagory 5 cable vs Coaxial Cables
Coax is heavy and expensive. Also only half duplex where catagory 5 is full duplex
Difference between hub and switch
hub broadcasts information to all ports, switch learns which nodes transmit through which port, and sends out data only to the learned ports
What is one way computers can achieve some sort of timing immunity?
computers restart their timers every character, stops two timers drifting apart.
what is the clock rate of manchester encoding
twice the data rate, (slow)
what is the point of a crossover cable?
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.
how does 4b5b help timing recovery?
by not allowing more than 3 zero’s in a row, ensuring at least two transitions, which helps the receiver in clock recover.
how does 4b5b help recognize error/corruption in transmition?
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.
multicast vs broadcast vs unicast
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.
what does a network interface card receive by default, in terms of multicast vs broadcast vs unicast
all messages for its own unicast address, and all messages for broadcast.
CSMA/CD vs CSMA/CA
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
What is twisted pair and why is it used.
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.
In Ethernet Frames, the data is encapsulated with ethernet headers and trailers. What is in these?
Header: Dest. MAC address, Src MAC address and Ether type
Trailer: Cyclic Redundancy Check 4 bytes, or 32 bits
what is the EtherType?
It is the protocol type, allowing multiple types of protocols to be carried (IPv4, IPv6)
What is CRC32
Fairly strong error detection, used to detect errors in received messages.
Before transmission of a frame, ethernet sends out a preamble for the receiver to synchronise its clock. WHat is contained in those 8 bytes
7 bytes of 10101010
final byte of 10101011
How long must the delay be (in bytes) after transmitting a frame, before the node can send again?
12 byte delay
woudl a software writer use ethernet addresses?
Mac addresses (Ethernet address) are handled by the operating systems, whereas the software writer would use IP addresses.
which protocol would an operating system use to find out the destination mac address, when sending an IP packet?
the ARP, or address resolution protocol
How does ARP work.
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)
does a computer send out an arp everytime it wants to send something?
no, it holds a cache of recent replies to arp requests. periodically, they expire
key difference in NPD vs ARP
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.
key difference in NPD vs ARP
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.
