Wireless and Mobile Networks Flashcards
What is the mobility challenge?
Handling mobile user who changes point of attachment to network.
What elements make up a wireless network?
Wireless hosts
Base stations
Wireless link
What is a wireless host?
Laptop, smartphone
Run applications
May be stationary or mobile.
What is a base station?
Typically connected to wired network.
Relay - responsible for sending packets between wired network and wireless host(s) in its area
EG: Cell towers, wifi access points.
What is a wireless link?
Typically used to connect mobile(s) to base stations.
Also used as backbone link
Multiple access protocol coordinates link access.
Various data rate due to distance.
What is infrastructure mode?
Base station connects mobiles into wired network.
Handoff: mobile changes base station as they move.
What is ad hoc mode?
No base stations, nodes can only transmit to other nodes within link coverage
Nodes organise themselves into a network: routing among themselves
Why is the signal weaker in wireless than in wired connections?
Path loss as it propagates through matter
Interference from other wireless network frequencies.
What is SNR?
Signal-to-noise ratio.
Larger SNR - easier to extract signal from noise - good.
What is BER?
Bit-Error Rate
the rate at which errors occur in the transmission of digital data.
What are the tradeoffs between SNR and BER?
Given physical layer: Increase power, increase SNR, decrease BER
Given SNR: Choose physical layer that meets BER requirement, giving highest throughput.
SNR may change with mobility.
What is the hidden terminal problem?
Anything that can block the signal means that hosts may not be able to see each other and therefore, not able to detect collisions.
What are the speeds of the various wireless LAN?
- 11b: 2.4-5 GHz unlicensed spectrum. up to 11Mbps
- 11a: 5-6GHz range, up to 54Mbps
- 11g: 2.4-5 GHz range, up to 54 mbps
- 11n: Multiple antennae, 2.4-5 GHz range, up to 200 Mbps
What is the architecture of Wireless LAN?
Wireless host communicates with base station.
What is the Basic Service Set (BSS)?
Cell in infrastructure mode contains
Wireless hosts Access point (AP) base station
Ad hoc mode: Hosts only.
What is association in WiFi?
802.11B: 2.4GHz-2.48GHz spectrum divided into 11 channels at different frequencies.
Access point admin chooses frequency. Interfering possible if channel is the same as neighboring AP
How do hosts associate with an access point?
Scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address.
Selects AP to associate with
May perform authentication
Will run DHCP to get IP address.
What is passive scanning?
Beacon frames sent from Access Points
Association request frame sent from host to access point.
Association response frame sent from selected access point to host.
What is active scanning?
Probe request frame broadcast from host
Probe response frames from access point.
Association request frame sent host to selected access point.
Association response frame sent from selected AP to host.
How does WiFi handle multiple access?
Use CSMA/CA - sense before transmitting
Has no collision detection, difficult to sense collisions when transmitting due to weak received signals. G
How does CSMA/CA work?
If sense channel idle for DIFS then transmit entire frame (No Collision detection)
If sense channel busy, start random backoff time. Transmit when channel idle and timer expires.
If frame received ok, return ACK after SIFS.
How else does WiFi avoid collisions?
Transmit Request to send packet to base station using CSMA
Packet is small, so shouldn’t collide.
Base station broadcasts clear-to-send in response..
CTS heard by all frames
How are 802.11 frames structured?
Frame cntrl | duration | addr 1 | addr 2 | addr 3 | seq cntrl | addr 4| payload | CRC
2 | 2 | 6 | 6 | 6 | 2 | 6 | 0-2312 | 4
What do the different addresses mean in the 802.11 frames?
addr 1: MAC address of wireless host or AP to receive this frame.
addr2: MAC address of wireless host or AP transmitting this frame.
addr3: Mac address of router interface to which AP is attached
addr4: Used only in Ad Hoc mode.
