ECM 1413 network layer

Question 1

There are 2 sorts of multiple access control protocol

Answer 1

• Carrier sense, multiple access/collision detection (CSMA/CD), for wired networks, such as ethernet
• Carrier sense, multiple access/collision avoidance (CSMA/CA), for wireless networks, such as wifi

Question 2

In a CSMA/CD wired network:

Answer 2

• All computers are attached to a shared cable - multiple access (MA)
• Any computer may transmit if the cable is unused - carrier sense (CS)
• While transmitting, a computer may detect that it is receiving a message - Collision Detection (CD)
• After detecting a collision, a computer waits for a random interval (chosen from an exponentially-doubling range), and then tries again
• By using a random interval we avoid repeated collisions#an exponentially doubling range enables computers to adapt to the network load
• CSMA/CD is seldom used today since modern ethernet variants avoid collisions by operating in full-duplex

Question 3

Half duplex vs Full duplex

Answer 3

Full-duplex enables simultaneous two-way communication, which allows data to flow freely bidirectionally. Half-duplex enables two-way communication, but not simultaneously. Half-duplex requires switching between a sending and receiving mode.

Question 4

CSMA/CA wireless network:

Answer 4

• All computers use a shared frequency - multiple access (MA)
• Any computer may transmit if the frequency is clear - carrier sense (CS)
• A computer finding the frequency clear, transmits after an instant
• A computer finding the frequency busy counts down from a random value (chosen from an exponentially doubling range) while the channel is clear; otherwise, the count is frozen - collision avoidance (CA)
• A computer then transmits and receives an acknowledgement

Question 5

The Hidden Node problem (CSMA/CA)

Answer 5

A computer may be in range of the base station, but out of range of another computer - the hidden node problem. This may be solved by reservation using ready to send and clear to send messages

Question 6

Collision-Free Protocols
There are protocols which are completely collision-free

Answer 6

1- Token ring
2- Bit-map protocols
3- Binary countdown

Question 7

Token ring

Answer 7

In a token ring wired network, a token continuously circulates, to which messages may be attached, and from which they can be removed

In a token bus, the token is passed around a virtual ring

Question 8

Bit-map protocol

Answer 8

In a bit-map protocol, host n may announce that it has a frame to send by inserting a 1 bit into slot n. Following this, hosts begin to transmit frames in numerical order

Question 9

Binary countdown

Answer 9

a collision-free protocol that operates in the Medium Access Control (MAC) layer of the OSI model

each station is assigned a binary address. The binary addresses are bit strings of equal lengths. When a station wants to transmit, it broadcasts its address to all the stations in the channel, one bit at a time starting with the highest order bit.

In order to decide which station gets the channel access, the addresses of the stations which are broadcasted are ORed. The higher numbered station gets the channel access.

Question 10

Collision

Answer 10

2 or more devices on a network attempting to send data at the same time. the data would then become garbled