slides08

Question 1

how is wireless interference prevented?

Answer 1

bodies manage the airwaves, allocating various frequencies to various purposes, ensuring minimal interference between the competing concerns for parts of the spectrum

Question 2

Why does wireless need to do more than csma/cd?

Answer 2

In principle, much like CSMA/CD over wireless, but with some extra problems unique to wireless
The shared medium is now all around, not just within a wire
So signals from multiple networks can interfere; not just the hosts within one network

Question 3

describe the hidden host prob

Answer 3

Hosts A can B can “see” each other; B and C can see each other, but A cannot see C, so A cannot tell if its packets to B are colliding with C’s to B

Question 4

why does csmacd not work wireles

Answer 4

But the limited ranges mean that CSMA/CD will not work for wireless

Question 5

Spatial multiplexing

Answer 5

Spatial multiplexing is the process where the same informaiton is placed across 2 or more of the available antennas in an AP or Client device.

Question 6

Beamforming

Answer 6

Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception

Question 7

dsss

Answer 7

In telecommunications, direct-sequence spread spectrum (DSSS) is a spread spectrum modulation technique used to reduce overall signal interference. The spreading of this signal makes the resulting wideband channel more noisy, allowing for greater resistance to unintentional and intentional interference.

Question 8

what are wifi channels

Answer 8

For Wi-Fi, the allocated frequency band (2.4GHz) is split into 14 overlapping 22MHz channels each centred on specified frequencies
The number of channels available depends on the country

Question 9

how much do channels interfere

Answer 9

• Separate channels by at least 2 (e.g., use 1 and 4) to reduce interference
• Separate by 4 (e.g., use 1 and 6) to have no interference at all
• This means we can have three non-interfering co-located networks on channels 1, 6 and 11

Question 10

how is collision avoidance achieved?

Answer 10

Waiting for the contention period is the collision avoidance

Question 11

howdoes a wifi network know if the message sent was received?

Answer 11

Thus, on successful receipt of a packet, a host will broadcast an acknowledgement (ACK) packet
This is just a packet to inform the sender that everything worked well and there was, in fact, no collision
If the sender never gets the ACK, it will resend, starting from the CA again