chapter 7 - wireless and mobile network Flashcards
(46 cards)
What is a base station and what does it do?
Connected to the wired network
relays packets between wireless and wired parts
if gets a wireless pckt -> sends to base station -> sends to wired place (webserver)
What are the two main challenges in wireless networking?
- interference: signals vulnerable to interference from other devices
- security : signals broadcast through the air can potentially be accessed by anyone within range
What is ad hoc mode?
Devices connect directly without base station; peer-to-peer
What makes wireless links different from wired links?
- Signal attenuation
- wireless: radio signals
- wired: cables
- interferance - only in wireless
What is the hidden terminal problem?
Two nodes can’t hear each other but collide at a common receiver
* a-b-c
* a nd c dont know about each other and sends data to b
* collision at b -> data loss
* cant use CSMA bc the other devices are “hidden” and cant hear
what does handoff/handover mean
change the base station to another as a user moves through a network
- eks: on a phone call while walking through a city. As you move:
- Your phone switches from one cell tower to the next without dropping the call
- That switch is called a handoff.
Single-hop, infrastructure-based
base station that is connected to a larger wired network
Single-hop, infrastructure-less
no base station that is connected to a wireless network
one of the nodes in this single-hop network may coordinate the transmissions of the other nodes (ad-hoc)
Multi-hop, infrastructure-less
No base station; nodes relay data trough other nodes
Multi-hop, infrastructure-based
hops through multiple base stations
What is a Basic Service Set (BSS)?
devices communicate trough one access point (AP) in infrastructure mode
BSS = 1 Access Point + all associated wireless devices
when does multipath propagation occurs?
when a wireless signal takes multiple different paths to reach the receiver
wireless signal - can be reflected, bounce off walls -> receiver may get multiple copies of the same signal at slightly different times
What is signal attenuation in wireless links?
Signal gets weaker as it travels through matter (path loss)
Why is interference a problem in wireless networks?
Many devices share the same frequencies, causing collisions and noise
What does a higher SNR mean? (signal-to-noise ratio)
Easier to extract signal from noise
What’s the tradeoff between SNR and BER(bit error rate)?
Higher SNR → lower BER → better throughput
How does a device connect to a Wi-Fi network?
- Scans for beacons (Each beacon contains network name)
- → selects AP (Based on signal strength)
- → authenticates
- → associates(sends association request to AP)
- → gets IP via DHCP
What’s the difference between passive and active scanning?
Passive: device listens for beacon frames
Active: device sends probe requests and waits for responses
How does CSMA/CA work?
Sense channel
If busy: backoff time -> sends when time = 0
If idle: wait short time ->transmit
If no ACK: retry with longer backof (assumes collision happened)
wait the short time -> Interframe Space (IFS)
What is passive scanning in 802.11?
- Device listens for beacon frames that AP sends out
- assosiaction request: device finds a network it want to connect to
- Association response: AP accepts and connects device
What is active scanning in 802.11?
- Device SENDS probe requests to all AP nearby
- AP that fulfill criteria -> probe response
- Association req from device to AP
- Association response from AP to device
How does a device choose which AP to associate with?
Typically selects AP with strongest signal
What is the association process in 802.11?
Device sends association request →
AP replies with association response → device may request IP via DHC
How is a device authenticated in 802.11?
via MAC address or username/password
