Wireless, Mobile Networks 6-1 Ch. 6: Wireless and Mobile Networks Background: # wireless (mobile)...
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Transcript of Wireless, Mobile Networks 6-1 Ch. 6: Wireless and Mobile Networks Background: # wireless (mobile)...
Wireless, Mobile Networks 6-1
Ch. 6: Wireless and Mobile Networks
Background: # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers (5-to-1)! # wireless Internet-connected devices equals
# wireline Internet-connected devices laptops, Internet-enabled phones promise anytime
untethered Internet access
two important (but different) challenges wireless: communication over wireless link mobility: handling the mobile user who changes
point of attachment to network
Wireless, Mobile Networks 6-3
wireless hosts laptop, smartphone run applications may be stationary
(non-mobile) or mobile wireless does not
always mean mobility
Elements of a wireless network
network infrastructure
Wireless, Mobile Networks 6-4
base station typically connected to
wired network relay - responsible for
sending packets between wired network and wireless host(s) in its “area” e.g., cell towers,
802.11 access points
Elements of a wireless network
network infrastructure
Wireless, Mobile Networks 6-5
wireless link typically used to
connect mobile(s) to base station
also used as backbone link
multiple access protocol coordinates link access
various data rates, transmission distance
Elements of a wireless network
network infrastructure
6: Wireless and Mobile Networks 6-6
Characteristics of selected wireless link standards
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m – 4 Km
Long-rangeoutdoor
5Km – 20 Km
.056
.384
1
4
5-11
54
2G: IS-95, CDMA, GSM
2.5G: UMTS/WCDMA, CDMA2000
802.15
802.11b
802.11a,g
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
4G: LTWE WIMAX
802.11a,g point-to-point
200 802.11n
Dat
a ra
te (
Mbp
s)
Mesh
6: Wireless and Mobile Networks 6-7
WiMax Vs. Wireless Mesh
WiMax Similar to cellular network infrastructure Use licensed spectrum 10 Mbit/s at 10 km in good environment Is under development by many companies
Wireless Mesh Extension of 802.11 Wireless LAN Use unlicensed public spectrum 802.11’s access routers interconnect together
• Ad Hoc (usually non-mobile) networking and routing Currently used in some places
• Town & small city’s government agents (firefighter, police)
– More popular in Europe than in US• Challenges: complex routing, high error rate over
multi-hop wireless links, bad QoS
6: Wireless and Mobile Networks 6-8
Elements of a wireless network
network infrastructure
infrastructure mode base station
connects mobiles into wired network
handoff: mobile changes base station providing connection into wired network
6: Wireless and Mobile Networks 6-9
Elements of a wireless network
Ad hoc mode no base stations nodes can only
transmit to other nodes within link coverage
nodes organize themselves into a network: route among themselves
Wireless active research area:
Ad hoc networkSensor network
6: Wireless and Mobile Networks 6-10
Ad Hoc Vs. Sensor Networks
Ad Hoc network Challenge Mobility of nodes Good features: Plenty of power, computation
resource Applications
• Mostly mobile laptops or PDAs• Vehicular network
Sensor network Challenge limited power, computing resource Good features:
• Usually stationary, dense network Applications
• Military battlefield, civil engineering, environmental monitoring
Wireless, Mobile Networks 6-11
Wireless network taxonomy
single hop multiple hops
infrastructure(e.g., APs)
noinfrastructure
host connects to base station (WiFi,WiMAX, cellular)
which connects to larger Internet
no base station, noconnection to larger Internet (Bluetooth,
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger Internet: mesh net
no base station, noconnection to larger Internet. May have torelay to reach other a given wireless node
MANET, VANET
6: Wireless and Mobile Networks 6-12
Wireless Link CharacteristicsDifferences from wired link ….
decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”
6: Wireless and Mobile Networks 6-13
IEEE 802.11: multiple access 802.11: CSMA - sense before transmitting
don’t collide with ongoing transmission by other node
802.11: no collision detection! difficult to receive (sense collisions) when transmitting
due to weak received signals (fading) can’t sense all collisions in any case: hidden terminal,
fading goal: avoid collisions: CSMA/C(ollision)A(voidance)
AB
CA B C
A’s signalstrength
space
C’s signalstrength
Wireless, Mobile Networks 6-14
IEEE 802.11 Wireless LAN
802.11b2.4-5 GHz unlicensed spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
all hosts use same chipping code
802.11a 5-6 GHz range up to 54 Mbps
802.11g 2.4-5 GHz range up to 54 Mbps
802.11n: multiple antennae
2.4-5 GHz range up to 200 Mbps
all use CSMA/CA for multiple access all have base-station and ad-hoc network
versions
Wireless, Mobile Networks 6-15
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, switchor router
Wireless, Mobile Networks 6-16
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 [Chapter 8] will typically run DHCP to get IP address in
AP’s subnet
Wireless, Mobile Networks 6-17
802.11: passive/active scanning
AP 2AP 1
H1
BBS 2BBS 1
1
23
1
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
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
active scanning: (1) Probe Request frame broadcast
from H1(2) Probe Response frames sent
from APs(3) Association Request frame sent:
H1 to selected AP (4) Association Response frame sent
from selected AP to H1
6: Wireless and Mobile Networks 6-18
IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif (no ACK)
increase random backoff interval, repeat 2
else /* received ack */return back to 2 (why?) to transmit next frame
802.11 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem) (no ack in ethernet!!)
sender receiver
DIFS
data
SIFS
ACK
DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing
6: Wireless and Mobile Networks 6-19
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 RTS heard by all nodes
sender transmits data frame other stations defer transmissions
Avoid long data frame collisionsusing small reservation packets!
6: Wireless and Mobile Networks 6-20
Collision Avoidance: RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
DIFS
CIFS
CIFS
CIFS
6: Wireless and Mobile Networks 6-21
RTS/CTS in Practice
RTS/CTS introduces delay, consume channel resource. Benefit when the data frame is much larger
than RTS/CTS. APs set threshold of data frame length
in order to use RTS/CTS If > threshold, use RTS/CTS
Many APs skip RTS/CTS by using a threshold larger than the Maximum frame length
6: Wireless and Mobile Networks 6-22
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
802.11 frame: addressing
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 4: used only in ad hoc mode
6: Wireless and Mobile Networks 6-23
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr AP MAC addr
dest. address source address
802.3 frame
802.11 frame: addressing
6: Wireless and Mobile Networks 6-24
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
TypeFromAP
SubtypeToAP
More frag
WEPMoredata
Powermgt
Retry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
802.11 frame: moreduration of reserved transmission time (data, RTS/CTS)
frame type(RTS, CTS, ACK, data)
6: Wireless and Mobile Networks 6-25
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
802.11: mobility within same subnet
router H1 remains in same
IP subnet: IP address can remain same
switch: which AP is associated with H1? self-learning (Ch. 5):
switch will see frame from H1 and “remember” which switch port can be used to reach H1
AP2 broadcast H1’s MAC to switch
6: Wireless and Mobile Networks 6-26
802.15 MAC and Bluetooth
802.11 MAC 11 Mbps – 54 Mbps Up to 100 meters range
802.15 MAC Wireless personal area network (WPAN) < 10 meters range Simple (cheap) device, low power
assumption Cable, wire replacement
• E.g., mouse, keyboard, headphone Example: Bluetooth
6: Wireless and Mobile Networks 6-27
Bluetooth
Physical layer properties: 2.4GHz unlicensed spectrum Frequency-hopping spread spectrum
• 79 channels with different frequencies• TDM transmit: jump among channels with preset
sequences (coding) Up to 721bps (802.11 is 11 Mbps to 54
Mbps)
6: Wireless and Mobile Networks 6-28
Bluetooth Ad hoc network
structure One master, <=7
slaves Odd time slot: master Even time: slaves
Parked: inactive devices
Problem: slow speed can be achieved by RF device Much cheaper, simpler
6: Wireless and Mobile Networks 6-29
CDMA Principle (6.2.1)
Code Division Multiple Access Wide spectrum technique All users use the full spectrum Users with different codings not interfere
Each bit is encoded by much high rate signal (code) Receiver can recover the bit with the
corresponding code
6: Wireless and Mobile Networks 6-31
Working with multiple users
How to extract data when multiple users transmit at the same time?
Assumptions: Interfering signals are additive Signal 1+1+1+(-1) = 2
New signals in the air (N senders):
Same decoding formula!
6: Wireless and Mobile Networks 6-32
Why extract correctlyBy each user?
A: user codes are orthogonal
Orthogonal Explanation
We can use the orthogonal vectors in 3-D or 2-D space to explain why signal from another user will not affect your received data?
6: Wireless and Mobile Networks 6-33
On the 2-D space, if we add noiseTo the X-axis, it does not affect The node’s Y-axis position value
Wireless, Mobile Networks 6-34
Mobile Switching
Center
Public telephonenetwork
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
Wireless, Mobile Networks 6-35
Cellular networks: the first hopTwo 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
frequencybands
time slots
Wireless, Mobile Networks 6-36
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSC
Public telephonenetwork
GatewayMSC
G
Wireless, Mobile Networks 6-37
3G (voice+data) network architecture
radionetwork controller
MSC
SGSN
Public telephonenetwork
GatewayMSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public Internet
GGSN
G
Key insight: new cellular datanetwork operates in parallel (except at edge) with existing cellular voice network voice network unchanged in core data network operates in parallel
GPRS: Generalized Packet Radio Service
Wireless, Mobile Networks 6-38
radionetwork controller
MSC
SGSN
Public telephonenetwork
GatewayMSC
G
Public Internet
GGSN
G
radio access networkUniversal Terrestrial Radio Access Network (UTRAN)
core networkGeneral Packet Radio Service
(GPRS) Core Network
publicInternet
radio interface(WCDMA, HSPA)
3G (voice+data) network architecture