quartz/content/notes/23-wireless-mobile.md

title	aliases	tags	sr-due	sr-interval	sr-ease
23-wireless-mobile		cosc203 lecture	2022-10-23	3	250

More wireless phones that wired phones. More mobile broadband than fixed broadband

two different challenges

• communication over wireless link
• mobility: handling the mobile user to changes point of attachment

elements

• wireless hosts
  • does not always mean mobile
• base station
  • connected to wired network
  • relay: sendig packets between wired network and wireless hosts in its "area"
    • e.g., cell towers
• wireless link
  • used to connect mobile to base station, also used as backbone link
  • various transmission rates, distances, frequency bands

architecture

• infrastructure mode
  • base station connect mobiles to wired network
  • handoff: mobile changes base station
• ad hoc mode
  • no base stations
  • can only transmit to other nodes within link coverage
• nodes organize themselves into a network: route among themselves

wireless link

• deccreased signal strength: radio signal attentuates as it travels
• interference from other sources: frequency shared by other technologies
• multipath propagation: signal reflects of objects arriving at sightly different times

SNR BER tradeoffs

• SNR - signal to noise ratio
  • larger SNR is easire to extract signal
• BER - bit error rate
• larger SNR => smaller BER
• many parameters at physical level which alter SNR

hidden terminal problem

• multiple wireless sender, recievers create additional problems (beyond multiple access):
• 
• 

WLAN and wireless link

Standards created and maintaned by IEEE

• covers data link layer and physical layer
• all use CSMA/CA for multiple access, and have base station and ad-hoc versions

WLAN architecture

• wireless hosts communicates with base station (Access point AP)
• Basic service set (BSS): set of devices that share the same service identifier
  • wireless hosts
  • AP
• in ad hoc mode: hosts only, no AP

channels and association

• spectrum divided into channels at different frequencies
  • admin chooses frequency for AP
  • interference possible: channel can be the same as a neighboring AP
• a new host must assiciate with the AP (connect)
  • listens for beacon frames
    • contain AP name (SSID) and MAC
  • may perform authentication
  • run DHCP to get IP in AP's subnet

CSMA/CA

• avoid collisions
• CSMA sense before transmitting
• wifi: no collision detection
  • hard due to: hidden terminal, fading,
  • instead: CA collision avoidance

DCF (distributed coordination function)

• CSMA/CA with binary exponential backoff algorithm

sender

• if sense channel idle for DIFS (DCF interframe space) then send data (no CD)
• if sense channel busy:
  • start random backoff time
  • timer counts down while channel idle
  • transmit when timer expires
  • if no ACK, increase random back of interval, repeat 2

reciever

• if frame recieved ok
  • return ACK after SIFS (short interframe space)
  • ACK needed due to hidden terminal problems

collision avoidance

• handshake to "reserve" channel use for data frames using request-to-send (RTS) and clear-to-send (CTS) packets
• 

Cellular networks

solution for wide-area mobile internet

basic concepts

• base stations: a.k.a., cell tower
• cell: geographical area covered by a BS
• frequency reuse principle
  • set of freqs is limited
  • neighboring cells cannot use the same set of freqs
• a freq reuse pattern is a configuration of N cells where N is the reuse factor

handoff

• transfer ongoing call or data connection from one BS to another
• methods
  • hard handoff
    • break before make
  • soft handoff
    • make before break
• roaming
  • users can use cell services when traveling outside the coverage area of home network by using a visited network
  • roaming agreement bettween home at visited network

4G LTE

similarity to wired

• 

difference:

• 

Long-Term Evolution (LTE), 4G standard

• traffic recieved by wirless network is routed using the internet

mobile device

• 64-bit international mobile subscriber id (IMSI) stored on SIM (subscriber identity module) card
• UE (user equipment) - any that has a sim card and connects to a cell tower

base station

• at "edge" of carrier network
• 

Mobility management entity

• 

data plane protocol stack

• 

associating with a BS

• 

5G

increase data date by 10x, 10x latency decrease, 100x increase in traffic capacity

5g NR

• two freq bands
• not back compatible
• massive MIMO

millimeter wave freq

• higher rates
• over shorter distances
• dense deployment of new base stations