L5 Flashcards
Types of multi-access protocols
- Random access: pure ALOHA, slotted ALOHA
- Controlled access: reservation, polling
- Channelized access: FDMA, TDMA, CDMA, SDMA
Explain time division multiple access (TDMA) and draw schematic (L5 4)
- Each user/node shares the whole channel bandwidth
- Each user/node is assigned a different timeslot to access the channel
Explain frequency division multiple access (FDMA) and draw schematic (L5 5)
- Each user/node shares the whole transmission time
- Each user/node is assigned a different frequency band to access the channel
Code/space division multiple access (C/SDMA)
- Each user/node shares the whole channel bandwidth and transmission time
- Each user/node is assigned a different code to access the channel
Write main ideas of pure ALOHA access protocol
- Each node has freedom to transmit whenever it receives a packet
- When 2 frames arrive within the vulnerable period a collision occurs. Data packets are corrupted and marked by CRC process.
- Nodes have to wait a random period and try again.
Draw a schematic of transmission with pure ALOHA (L5 8)
write on paper
What is the probability of no collision in pure ALOHA protocol and is it sufficient? (L5 10)
- 18%
- enough for IoT systems since data exchange rate is usually low
Main ideas of slotted ALOHA
- modifies pure ALOHA to minimize collision rates
- transmission attempts of all nodes are synchronized to a certain time point where the channel is slotted
- vulnerable time is only Tf instead of 2Tf, which increases probability of no collision to 36%
Main ideas of carrier sense multiple access (CSMA) protocol
- To minimize collision rate, nodes in CSMA need to first sense the channel before transmitting
- If channel is idle, node can start transmission. Otherwise, must wait a random time and try again.
- Collision rate depends on channel propagation delay. Vulnerable time is channel propagation delay.
Basic ideas of 1-persistent CSMA (draw an schematic) (L5 15)
- Node X is always sensing the channel
- When channel is free node X initiates transmission immediately
- Collisions might still occur, because more than one node can initiate transmission immediately
Basic ideas of non-persistent CSMA (draw schematic and flowchart) (L5 17,18)
- Node X is always sensing the channel, if busy it waits a random time and sense again
- Node X senses the channel if idle it starts transmission right away
- Chances of collision are less probable because each node senses channel with different random period, but sacrifices throughput
Basic ideas of P-persistent CSMA (draw schematic and flowchart) (L5 19,20)
- Channel is slotted similar to that of slotted ALOHA with equal time slots
- Node keeps sensing the channel until it is idle.
- Then it calculates a probability P and starts transmission
Main functions of network layer
- IP addressing
- routing
Types of network topologies
- Ring topology
- Bus topology
- Star topology
- Mesh topology
Main ideas of ring topology (draw schematic) (L5 22)
- nodes are arranged in circular manner
- token ring protocol is used so that each node is passing data token to the neighbouring one
- no collisions can occur
Main ideas of bus topology (draw schematic) (L5 23)
- simplest and most common. Good for small or simple networks.
- passive topology, nodes only listen for data being sent, they do not move data from one compute to the next
- common for coaxial-cable Ethernet network
Main ideas of start topology (draw schematic) (L5 24)
- Each node is connected to the other one through a hub or host.
- Host addresses incoming packet to desired destination.
Main ideas of mesh topology (draw schematic) (L5 25)
- Every device has a point-to-point link to every other device
- fully connected mesh has n(n-1)/2 physical channels and n devices
- each device needs n-1 input/output ports
