Wireless and Mobile Networks
Background:
# wireless Internet-connected devices equals # wireline Internet-connected devices
• laptops, Internet-enabled phones promise anytime untethered Internet access
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# wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to- 1)!
two important (but different) challenges
• wireless: communication over wireless link
• mobility: handling the mobile user who changes point of attachment to network
Elements of a wireless network
network infrastructure
Elements of a wireless network
network infrastructure
wireless hosts
laptop, smartphone
run applications
may be stationary (non-mobile) or mobile
• wireless does not always mean mobility
Elements of a wireless network
base station
typically connected to wired network
network infrastructure
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 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
Characteristics of selected wireless links
802.11a,g point-to-point
4G: LTWE WIMAX
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
2.5G: UMTS/WCDMA, CDMA2000 2G: IS-95, CDMA, GSM
Indoor 10-30m
Outdoor 50-200m
outdoor 200m – 4 Km
Long-range
outdoor 5Km – 20 Km
Data rate (Mbps)
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
Wireless Link Characteristics (1)
important differences from wired link ….
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
decreased signal strength: radio signal attenuates as it propagates through matter (path loss)
multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times
…. make communication across (even a point to
point) wireless link much more “difficult”
Wireless Link Characteristics (2)
SNR: signal-to-noise ratio
• larger SNR – easier to extract signal from noise (a “good thing”)
10-1 10-2 10-3
SNR versus BER tradeoffs
• given physical layer: increase power -> increase SNR->decrease BER
• given SNR: choose physical layer that meets BER requirement, giving highest thruput
SNR may change with mobility: dynamically adapt physical layer
QAM256 (8 Mbps) QAM16 (4 Mbps)
BPSK (1 Mbps)
10 20 30 40
(modulation
Wireless network characteristics
Multiple wireless senders and receivers create additional problems (beyond multiple access):
AB A’s signal
Signal attenuation:
B, A hear each other
B, C hear each other
A, C can not hear each other interfering at B
C’s signal strength
Hidden terminal problem
B, A hear each other
B, C hear each other
A, C can not hear each other means A, C unaware of their interference at B
IEEE 802.11 Wireless LAN
2.4-5 GHz unlicensed
spectrum
up to 11 Mbps
direct sequence spread spectrum (DSSS) in physical layer
• all hosts use same chipping code
5-6 GHz range up to 54 Mbps
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
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
hub, switch or router
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
802.11: passive/active scanning
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
1112 AP2 AP1 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
IEEE 802.11: multiple access
avoid collisions: 2+ nodes transmitting at same time
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 A’s signal
C’s signal strength
IEEE 802.11 MAC Protocol:
802.11 sender
1 if sense channel idle for DIFS 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
if no ACK, increase random backoff interval, repeat 2
802.11 receiver
– if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender DIFS
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!
Collision Avoidance: RTS-CTS exchange
A AP B reservation collision
RTS(A) RTS(A)
802.11 frame: addressing
2 2 6 6 6 2 6 0 – 2312 4
frame control
seq control
Address 1: MAC address of wireless host or AP
to receive this frame
Address 4: used only in ad hoc mode
Address 2: MAC address
which AP is attached
of wireless host or AP transmitting this frame
Address 3: MAC
of router interface to
802.11 frame: addressing
R1 MAC addr
H1 MAC addr
dest. address
source address
802.3 frame
AP MAC addr
H1 MAC addr
R1 MAC addr
802.11 frame
802.11 frame: more
duration of reserved transmission time (RTS/CTS)
frame seq # (for RDT)
2 2 6 6 6 2 6 0 – 2312 4
frame control
seq control
2411111111
Protocol version
frame type
(RTS, CTS, ACK, data)
802.11: mobility within same
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
802.11: advanced capabilities
Rate adaptation
10-1 10-2 10-3 10-4 10-5 10-6 10-7
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 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
802.11: advanced capabilities
power management
node-to-AP: “I am going to sleep until next beacon frame”
beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent
AP knows not to transmit frames to this node
node wakes up before next beacon frame
node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame
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