程序代做CS代考 scheme database chain DHCP algorithm Real-time Conversational Applications – cscodehelp代写
Real-time Conversational Applications
Advanced Network Technologies
Wireless 2
School of Computer Science
Dr. | Lecturer
1
IEEE 802.11 Wireless LA Fi
IEEE 802.11 WiFi
Wireless and Mobile Networks: 7- 3
IEEE 802.11 standard Year Max data rate Range Frequency
802.11b 1999 11 Mbps 30m 2.4 Ghz
802.11a 1999 54 Mbps 30m 5 Ghz
802.11g 2003 54 Mbps 30m 2.4 Ghz
802.11n (WiFi 4) 2009 600 Mbps 70m 2.4, 5 Ghz
802.11ac (WiFi 5) 2013 3.47Gpbs 70m 5 Ghz
802.11ax (WiFi 6) 2020 (exp.) 14 Gbps 70m 2.4, 5 Ghz
802.11af 2014 35 – 560 Mbps 1 Km unused TV bands (54-790 MHz)
802.11ah 2017 347Mbps 1 Km 900 Mhz
all use CSMA/CA for multiple access, and have base-station and ad-hoc network versions
3
802.11 LAN architecture
wireless host communicates with base station
base station = access point (AP)
Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains:
wireless hosts
access point (AP): base station
ad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switch
or router
4
BSS Base station system
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
5
Channel overlap, except 1, 6, 11.
802.11: passive/active scanning
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
3
1
passive scanning:
beacon frames sent from APs
association Request frame sent: H1 to selected AP
association Response frame sent from selected AP to H1
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
2
3
4
active scanning:
Probe Request frame broadcast from H1
Probe Response frames sent from APs
Association Request frame sent: H1 to selected AP
Association Response frame sent from selected AP to H1
6
Beacon: to provide illumniation guidance.
collisions
collisions can occur: propagation delay means two nodes may not hear each other’s transmission
collision: frame transmission time wasted
spatial layout of nodes
7
Wired Networks: CSMA/CD (collision detection)
CSMA/CD:
collisions detected within short time
colliding transmissions aborted, reducing channel wastage
collision detection:
wired LANs: measure signal strengths, compare transmitted, received signals
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
8
CSMA/CD (collision detection)
spatial layout of nodes
9
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)
space
A
B
C
A
B
C
A’s signal
strength
C’s signal
strength
10
Carrier sense multiple access (CSMA)
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
receiver
DIFS
data
SIFS
ACK
11
DIFS: Distributed inter-frame space: period of time waited before transmitting its frame
SIFS: Shorter inter-frame spacing
As opposed to CSMA/CD, 802.11 sends the entire frame and tries to avoid collision rather than aborting.
Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random access 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!
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Clear-to-send CTS, request-to-send RTS
Collision Avoidance: RTS-CTS exchange
AP
A
B
time
RTS(A)
RTS(B)
RTS(A)
CTS(A)
CTS(A)
DATA (A)
ACK(A)
ACK(A)
reservation collision
defer
Please think: How does A (B) know that RTS collide?
13
Rate adaptation
base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
802.11: advanced capabilities
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
10
20
30
40
SNR(dB)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
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
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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
Mobile
Switching
Center
Public telephone
network
Mobile
Switching
Center
Components of cellular network architecture
connects cells to wired tel. net.
manages call setup (more later!)
handles mobility (more later!)
MSC
covers geographical region
base station (BS) analogous to 802.11 AP
mobile users attach to network through BS
air-interface: physical and link layer protocol between mobile and BS
cell
wired network
20
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 access
frequency
bands
time slots
21
Frame division/ time division/ code division FDMA/TDMA/CDMA
BSC
BTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
MSC
Public
telephone
network
Gateway
MSC
G
2G (voice) network architecture
radio
network
controller
MSC
SGSN
Public
telephone
network
Gateway
MSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public
Internet
GGSN
G
Key insight: new cellular data
network operates in parallel
(except at edge) with existing
cellular voice network
voice network unchanged in core
data network operates in parallel
3G (voice+data) network architecture
General Packet Radio Service
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
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)
VoIP issue: IP is best effort whereas voice requires timely constraints.
OFDM: orthogonal frequency division multiplexing [Rhode 2008; Ericsson 2011]
24
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
LTE Radio Access Networks: uses a combination of orthogonal frequency-division multiplexing (OFDM) and time division multiplexing.
