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171 lines
4.9 KiB
Markdown
171 lines
4.9 KiB
Markdown
---
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title: "23-wireless-mobile"
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aliases:
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tags:
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- cosc203
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- lecture
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sr-due: 2022-10-23
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sr-interval: 3
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sr-ease: 250
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---
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More wireless phones that wired phones.
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More mobile broadband than fixed broadband
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two different challenges
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- communication over wireless link
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- mobility: handling the mobile user to changes point of attachment
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elements
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- wireless hosts
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- does not always mean mobile
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- base station
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- connected to wired network
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- relay: sendig packets between wired network and wireless hosts in its "area"
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- e.g., cell towers
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- wireless link
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- used to connect mobile to base station, also used as backbone link
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- various transmission rates, distances, frequency bands
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architecture
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- infrastructure mode
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- base station connect mobiles to wired network
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- handoff: mobile changes base station
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- ad hoc mode
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- no base stations
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- can only transmit to other nodes within link coverage
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- nodes organize themselves into a network: route among themselves
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wireless link
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- deccreased signal strength: radio signal attentuates as it travels
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- interference from other sources: frequency shared by other technologies
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- multipath propagation: signal reflects of objects arriving at sightly different times
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SNR BER tradeoffs
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- SNR - signal to noise ratio
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- larger SNR is easire to extract signal
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- BER - bit error rate
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- larger SNR => smaller BER
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- many parameters at physical level which alter SNR
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hidden terminal problem
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- multiple wireless sender, recievers create additional problems (beyond multiple access):
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- 
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- 
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# WLAN and wireless link
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Standards created and maintaned by IEEE
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- covers data link layer and physical layer
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- all use CSMA/CA for multiple access, and have base station and ad-hoc versions
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WLAN architecture
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- wireless hosts communicates with base station (Access point AP)
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- Basic service set (BSS): set of devices that share the same service identifier
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- wireless hosts
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- AP
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- in ad hoc mode: hosts only, no AP
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channels and association
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- spectrum divided into channels at different frequencies
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- admin chooses frequency for AP
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- interference possible: channel can be the same as a neighboring AP
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- a new host must assiciate with the AP (connect)
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- listens for *beacon frames*
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- contain AP name (SSID) and MAC
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- may perform authentication
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- run DHCP to get IP in AP's subnet
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# CSMA/CA
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- avoid collisions
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- CSMA sense before transmitting
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- wifi: no collision detection
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- hard due to: hidden terminal, fading,
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- instead: CA collision avoidance
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DCF (distributed coordination function)
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- CSMA/CA with binary exponential backoff algorithm
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sender
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- if sense channel idle for DIFS (DCF interframe space) then send data (no CD)
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- if sense channel busy:
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- start random backoff time
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- timer counts down while channel idle
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- transmit when timer expires
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- if no ACK, increase random back of interval, repeat 2
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reciever
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- if frame recieved ok
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- return ACK after SIFS (short interframe space)
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- ACK needed due to hidden terminal problems
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collision avoidance
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- handshake to "reserve" channel use for data frames using request-to-send (RTS) and clear-to-send (CTS) packets
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- 
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# Cellular networks
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solution for wide-area mobile internet
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basic concepts
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- base stations: a.k.a., cell tower
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- cell: geographical area covered by a BS
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- frequency reuse principle
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- set of freqs is limited
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- neighboring cells cannot use the same set of freqs
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- a freq reuse pattern is a configuration of N cells where N is the reuse factor
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handoff
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- transfer ongoing call or data connection from one BS to another
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- methods
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- hard handoff
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- break before make
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- soft handoff
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- make before break
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- roaming
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- users can use cell services when traveling outside the coverage area of home network by using a visited network
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- roaming agreement bettween home at visited network
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# 4G LTE
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similarity to wired
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- 
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difference:
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- 
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Long-Term Evolution (LTE), 4G standard
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- traffic recieved by wirless network is routed using the internet
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mobile device
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- 64-bit international mobile subscriber id (IMSI) stored on SIM (subscriber identity module) card
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- UE (user equipment) - any that has a sim card and connects to a cell tower
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base station
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- at "edge" of carrier network
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- 
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Mobility management entity
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- 
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data plane protocol stack
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- 
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associating with a BS
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- 
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# 5G
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increase data date by 10x, 10x latency decrease, 100x increase in traffic capacity
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5g NR
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- two freq bands
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- not back compatible
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- massive MIMO
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millimeter wave freq
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- higher rates
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- over shorter distances
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- dense deployment of new base stations
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