Flashcards
Wireless and Mobile networks
Cards in this deck (26)
- 1Wireless - differences from wired network?1) Decreased signal strength
2) Interference
3) Multipath propagation - 2Higher SNR =?Lower BER
- 3SNR vs BER tradeoffs If given physical layer...Increase power -> Increase SNR
Hence : Lower BER - 4SNR vs BER tradeoffs If given SNR...Choose phy layer which meets BER requirement (highest throughput)
- 5Problems with wireless networks?1) Hidden terminal
2) Signal attenuation - 6CDMA stands for?Code Division Multiple Access
- 7CDMA working principleAll users share same freq.
Each user - own "chipping" sequence.
Encoded signal = ori_data x chipping seq - 8Problems with wireless networks?1) Hidden terminal
2) Signal attenuation
- 9CDMA Advantages1) Multiple users can coexist
2) Users can tx simultaneously
3) Minimal interference from other users - 10802.11b uses ___ sequence in physical layer.DSSS - Direct Sequence Spread Spectrum
(Hosts - same chipping code) - 11802.11b How do hosts associate with AP?1) Scans channels
-beacon frames with SSID and MAC addr
2) selects AP
3) may perform authentication
4) Run DHCP to get IP address in AP's subnet - 12802.11b What are the two types of scanning?Passive scanning
Active scanning - 13Explain how Passive Scanning works1) APs send beacon frames
2) H1 -> AP : Association request
3) AP -> H1 : Association Response - 14Explain how Active Scanning works1) H1 broadcast: Probe Req frame
2) APs -> H1 : Probe Response frame
3) H1-> AP : Association Request
4) AP -> H1 : Association Response - 15MAC Protocol: CSMA/CA - Sender1) If sense channel idle for DIFS
- TX entire frame
2)Else if channel busy
-start rand backoff time
- Timer counts down while channel idle
- timer expires -> TX frame
- if no ACK -> increase rand backoff time, repeat 2) - 16MAC Protocol: CSMA/CA - ReceiverIf frame received is OK,
send ACK after SIFS - 17RTS-CTS exchange Protocol to avoid collision1) sender sends RTS packets to BS using CSMA
2) BS broadcast CTS (specific to sender, but everyone can 'see' it)
3) Sender TX data frame,
others defer transmissions. - 18802.11 : Rate Adaptation hint (SNR, distance, BER)As SNR decreases, BER increases.
When BER too high, switch to lower TX rate w/ lower BER. - 19802.11 : Power Management1) node->AP: "Im sleeping!"
2) AP won't TX frames to this node
3) Node wakes up b4 nx beacon frame
- If got AP-to-mobile frames, stay awake
- else, sleep till nx beacon frame. - 20Cellular Internet Access Functions of MSC ?- connect cell to wide area net
- manage call setup
- handle mobility - 21What is Mobility?User is able to pass through multiple APs while maintaining ongoing connections
- 22Define Home AgentEntity which performs mobility functions on behalf of mobile when mobile is remote
- 23Define Care-of-addressAddr in visited networks.
- 24Define foreign agentEntity in visited nw which performs mobility functions on behalf of mobile
- 25Explain indirect routingmobile moves to new network
1)registers with new foreign agent
2) new FA registers with HA
3) HA update c.o.addr for mobile - 26Explain direct routingFA in the 1st visited nw : Anchor Foreign Agent
- All data are routed to Anchor FA
- when mobile moves, new FA arranges for data to be forwarded from AFA (chaining)
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- Flashcards on Wireless and Mobile networks, created by Jo-Anne Tan on 02/11/2014.
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- Jo-Anne Tan
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