CS代考 IEEE 802.11 Wireless LA Fi – cscodehelp代写

Advanced Network Technologies Wireless 2
Dr. | Lecturer School of Computer Science
IEEE 802.11 Wireless LA Fi
IEEE 802.11 WiFi
IEEE 802.11 standard
Max data rate
802.11n (WiFi 4)
2.4, 5 Ghz
802.11ac (WiFi 5)
802.11ax (WiFi 6)
2020 (exp.)
2.4, 5 Ghz
35 – 560 Mbps
unused TV bands (54-790 MHz)
§ all use CSMA/CA for multiple access, and have base-station and ad- hoc network versions
Wireless and Mobile Networks: 7- 3
802.11 LAN architecture
hub, switch or router
v wireless host communicates with base station
§ base station = access point ( AP)
v Basic Service Set (BSS) (aka
“cell”) in infrastructure
mode contains:
§ wireless hosts
§ access point (AP): base station
§ ad hoc mode: hosts only
802.11: Channels, association
› 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
– AP admin chooses frequency for AP
– interference possible: channel can be same as that chosen by neighboring AP! › host: must associate with an AP
– scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address – selects AP to associate with
– may perform authentication
– will typically run DHCP to get IP address in AP’s subnet
802.11: passive/active scanning
AP1 1AP2 AP12 2
passive scanning:
(1) beacon frames sent from APs
(2) association Request frame sent: H1 to
selected AP
(3) association Response frame sent from
selected AP to H1
active scanning:
(1) Probe Request frame broadcast
(2) Probe Response frames sent
(3) Association Request frame sent:
H1 to selected AP
(4) Association Response frame sent
from selected AP to H1
spatial layout of nodes collisions
› collisions can occur: propagation delay means two nodes may not hear each other’s transmission
› collision: frame transmission time wasted
Wired Networks: CSMA/CD (collision detection)
– collisions detected within short time
– colliding transmissions aborted, reducing channel wastage
› collision detection:
– wired LANs: measure signal strengths, compare transmitted, received
– Can transmit and sense at the same time
– wireless LANs: received signal strength overwhelmed by local
transmission strength
– CSMA-CD cannot be used in wireless LAN
CSMA/CD (collision detection)
spatial layout of nodes
IEEE 802.11: multiple access
› 802.11: no collision detection!
– difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)
– can not sense all collisions in any case: hidden terminal, fading – goal: avoid collisions: CSMA/C(ollision)A(voidance)
AB A’s signal
C’s signal strength
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for DIFS (Distributed inter-frame space) then
transmit entire frame (no CD)
2 if sense channel busy then
start random backoff time
timer counts down while channel idle transmit when timer expires
802.11 receiver
– if frame received OK
return ACK after SIFS (Shorter inter-frame spacing)
Sender: if no ACK, increase random backoff interval, repeat 2
sender DIFS
Avoiding collisions (more)
idea: allowsenderto“reserve”channelratherthanrandomaccess of data frames: avoid collisions of long data frames
› sender first transmits small request-to-send (RTS) packets to BS using CSMA
– RTSs may still collide with each other (but they’re short)
› BS broadcasts clear-to-send CTS in response to RTS
› CTS heard by all nodes
– sender transmits data frame
– other stations defer transmissions
avoid data frame collisions completely using small reservation packets!
Collision Avoidance: RTS-CTS exchange
reservation collision
Please think: How does A (B) know that RTS collide?
RTS(A) RTS(A)
802.11: advanced capabilities
Rate adaptation
› base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)
operating point
10-1 10-2 10-3 10-4 10-5 10-6 10-7
10 20 30 40 SNR(dB)
1. SNR decreases, BER increase as node moves away from base station
2. When BER becomes too high, switch to lower transmission rate but with lower BER
Exposed Terminal
Source: Wikipedia
Exposed Terminal
Ideal: S1->R1 and S2->R2 simultaneously
However: S2 can sense the carrier of S1 so that it keeps silence
Can RTS-CTS fail? Yes
Source: http://www.cs.jhu.edu/~cs647/mac_lecture_3.pdf
Can RTS-CTS fail? Yes
Source: http://www.cs.jhu.edu/~cs647/mac_lecture_3.pdf
Cellular Internet Access Architecture and standards
Components of cellular network architecture
v covers geographical region
v base station (BS) analogous to 802.11 AP v mobile users attach to network through BS vair-interface: physical and link layer protocol between mobile and BS
v connects cells to wired tel. net. v manages call setup (more later!) v handles mobility (more later!)
Mobile Switching Center
Public telephone network
wired network
Mobile Switching Center
Cellular networks: the first hop
Two techniques for sharing mobile- to-BS radio spectrum
› combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots
› CDMA: code division multiple
time slots
frequency bands
2G (voice) network architecture
Base station system (BSS)
Public telephone network
Gateway MSC
Base transceiver station (BTS)
Base station controller (BSC) Mobile Switching Center (MSC)
Mobile subscribers
3G (voice+data) network architecture
radio network controller
Gateway MSC
Public telephone network
Key insight: new cellular data network operates in parallel (except at edge) with existing cellular voice network
Public Internet
v voice network unchanged in core v data network operates in parallel
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN) General Packet Radio Service
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
1. Evolved packet core (EPC): simplified all-IP core network that unifies the cellular circuit-switched voice network and the packet switched cellular data network.
Public telephone network
Public Internet
Evolved Packet Core (all-IP)
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
2. LTE Radio Access Networks: uses a combination of orthogonal frequency-division multiplexing (OFDM) and time division multiplexing.