W11L3 - Wireless Flashcards
Wireless Security Issue?
all traffic is subject to being listened to
Wireless Topology Issue?
Changing dynamically as stations move
Wireless Power Issue?
Mobile stations imply batteries that have capacity limits
Wireless Spectrum Issue?
lower frequencies go further, but higher frequencies provide more bandwidth
Wireless Hidden Node Issue?
A device in the middle of two communications devices. It can see both of them, but the devices further away can’t see eachother
AM
Amplitude Modulation - Signal intensity
FM
Frequency Modulation - Keep constant amplitude, but changes the frequency
Spread Spectrum Techniques
Originally create for hidden communications. Signal is smeared across a range of frequencies. Appears as small increase in noise to other frequencies
OFDM
Orthogonal Frequency Division Multiplexing
How does OFDM work?
Split the frequency band into a series of sub channels which do not interfere with each other
Spread transmission across multiple slowly modulated signals rather than one quickly modulating one
Each sub carrier can be tested and modulated at the best rate it can support
Interference sources are often frequency selective
Coding error can be spread across channels
Can assign channels to different users for multiple access
MIMO
Multiple In Multiple Out
How does MIMO work?
Using multiple antennas, multiple paths can be deliberately created giving several channels from transmitter to receiver
What is Wireless Ethernet?
Unlicensed band of 2.4 and 5.8 Ghz used by wireless, mobile phones, bluetooth
What is CSMA/CA and what does it stand for?
A type of old school wireless ethernet.
Carrier Sense - listen before transmitting
Multiple Access - All nodes share the same medium
Collision Avoidance - optional - transmit a short warning frame first and get a response from the receiver
How does modern Wifi Access work?
Stations use random access to book ‘slots’ in frame structure - book time slots for when you’re going to send.
Can be channelized
Synchronized by the access point
