Download PPT, 1.7M - Feng Xia

Chapter 6: Wireless and Mobile Networks
Background:
# wireless (mobile) phone subscribers now
exceeds # wired phone subscribers!
 computer nets: laptops, palmtops, PDAs,
Internet-enabled phone 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
6: Wireless and Mobile Networks
6-1
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
6.3 IEEE 802.11
wireless LANs (“wifi”)
 6.4 Cellular Internet
Access



architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and
higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-2
Elements of a wireless network
network
infrastructure
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary
(non-mobile) or mobile

wireless does not
always mean mobility
6: Wireless and Mobile Networks
6-3
Elements of a wireless network
network
infrastructure
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
6: Wireless and Mobile Networks
6-4
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
6: Wireless and Mobile Networks
6-5
Characteristics of selected wireless link
standards
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX)
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
.384
UMTS/WCDMA, CDMA2000
.056
3G
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
6: Wireless and Mobile Networks
6-6
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
6-7
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
6: Wireless and Mobile Networks
6-8
Wireless network taxonomy
single hop
infrastructure
(e.g., APs)
no
infrastructure
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
multiple hops
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET
6: Wireless and Mobile Networks
6-9
Wireless Link Characteristics (1)
Differences 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 ad destination at
slightly different times
…. make communication across (even a point to point)
wireless link much more “difficult”
6: Wireless and Mobile Networks
6-10
Wireless Link Characteristics (2)

SNR: signal-to-noise ratio
 larger SNR – easier to
extract signal from noise (a
“good thing”)
SNR versus BER tradeoffs
 given physical layer:
increase power -> increase
SNR->decrease BER
 given SNR: choose physical
layer that meets BER
requirement, giving highest
throughput
• SNR may change with
mobility: dynamically adapt
physical layer (modulation
technique, rate)
10-1
10-2
10-3
BER

10-4
10-5
10-6
10-7
10
20
30
40
SNR(dB)
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
6: Wireless and Mobile Networks
6-11
Wireless network characteristics
Multiple wireless senders and receivers create
additional problems (beyond multiple access):
C
A
B
A
B
Hidden terminal problem
C
C’s signal
strength
A’s signal
strength
space
 B, A hear each other
Signal attenuation:
 A, C can not hear each other
 B, C hear each other
 B, C hear each other
 B, A hear each other
means A, C unaware of their
interference at B
 A, C can not hear each other
interfering at B
6: Wireless and Mobile Networks
6-12
Wireless network characteristics
Hidden Node Problem
A
B
C
6: Wireless and Mobile Networks
6-13
Wireless network characteristics
Exposed Node Problem
A
X
B
C
D
6: Wireless and Mobile Networks
6-14
Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
6.3 IEEE 802.11
wireless LANs (“wifi”)
 6.4 cellular Internet
access



architecture
standards (e.g., GSM)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and
higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-15
IEEE 802.11 Wireless LAN

802.11b




2.4-5 GHz unlicensed spectrum
up to 11 Mbps

direct 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
6: Wireless and Mobile Networks
6-16
802.11 LAN architecture
 wireless host communicates
Internet
AP
hub, switch
or router
BSS 1
AP
BSS 2
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
6: Wireless and Mobile Networks
6-17
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
6: Wireless and Mobile Networks
6-18
802.11: passive/active scanning
BBS 1
AP 1
BBS 2
1
1
2
AP 2
BBS 1
BBS 2
AP 1
AP 2
1
2
3
2
3
4
H1
H1
Passive Scanning:
Active Scanning:
(1) beacon frames sent from APs
(2) association Request frame sent:
H1 to selected AP
(3) association Response frame sent:
H1 to selected AP
(1) Probe Request frame broadcast
from H1
(2) Probes response frame sent from
APs
(3) Association Request frame sent:
H1 to selected AP
(4) Association Response frame sent:
H1 to selected AP
6: Wireless and Mobile Networks
6-19
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)
C
A
B
A
B
C
C’s signal
strength
A’s signal
strength
space
6: Wireless and Mobile Networks
6-20
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for DIFS then
sender
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
receiver
DIFS
data
SIFS
ACK
802.11 receiver
- if frame received OK
return ACK after SIFS (ACK needed due
to hidden terminal problem)
6: Wireless and Mobile Networks
6-21
CSMA/CA in IEEE 802.11
 Physical carrier sense, and
 Virtual carrier sense using Network Allocation
Vector (NAV)
 NAV is updated based on overheard
RTS/CTS/DATA/ACK packets, each of which
specified duration of a pending transmission
 Backoff intervals used to reduce collision
probability
6: Wireless and Mobile Networks
6-22
Backoff Interval
 When transmitting a packet, choose a backoff
interval in the range [0,cw]
 cw
is contention window
 Count down the backoff interval when medium
is idle
 Count-down
busy
is suspended if medium becomes
 When backoff interval reaches 0, transmit
RTS
6: Wireless and Mobile Networks
6-23
DCF Example 1
B1 = 25
wait
B1 = 5
data
data
B2 = 20
cw = 31
wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
6: Wireless and Mobile Networks
6-24
DCF Example 2
DIFS
DIFS
DIFS
6: Wireless and Mobile Networks
6-25
Backoff Interval
 The time spent counting down backoff
intervals is a part of MAC overhead
 Choosing a large cw leads to large backoff
intervals and can result in larger overhead
 Choosing a small cw leads to a larger number of
collisions (when two nodes count down to 0
simultaneously)
6: Wireless and Mobile Networks
6-26
Backoff Interval
 Since the number of nodes attempting to
transmit simultaneously may change with time,
some mechanism to manage contention is
needed
 IEEE 802.11 DCF: contention window cw is
chosen dynamically depending on collision
occurrence
6: Wireless and Mobile Networks
6-27
Binary Exponential Backoff in DCF
 When a node fails to receive CTS in response
to its RTS, it increases the contention window
 cw
is doubled (up to an upper bound)
 When a node successfully completes a data
transfer, it restores cw to Cwmin
 cw follows a sawtooth curve
6: Wireless and Mobile Networks
6-28
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!
6: Wireless and Mobile Networks
6-29
Collision Avoidance: RTS-CTS exchange
A
B
AP
reservation collision
DATA (A)
defer
time
6: Wireless and Mobile Networks
6-30
RTS/CTS/DATA/ACK
6: Wireless and Mobile Networks
6-31
802.11 frame: addressing
2
2
6
6
6
frame
address address address
duration
control
1
2
3
Address 1: MAC address
of wireless host or AP
to receive this frame
2
6
seq address
4
control
0 - 2312
4
payload
CRC
Address 4: used only
in ad hoc mode
Address 3: MAC address
of router interface to
which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame
6: Wireless and Mobile Networks
6-32
802.11 frame: addressing
R1 router
H1
Internet
AP
R1 MAC addr H1 MAC addr
dest. address
source address
802.3 frame
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.11 frame
6: Wireless and Mobile Networks
6-33
802.11 frame: more
frame seq #
(for RDT)
duration of reserved
transmission time (RTS/CTS)
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
Protocol
version
2
4
1
Type
Subtype
To
AP
6
2
1
seq address
4
control
1
From More
AP
frag
1
Retry
1
0 - 2312
4
payload
CRC
1
Power More
mgt
data
1
1
WEP
Rsvd
frame type
(RTS, CTS, ACK, data)
6: Wireless and Mobile Networks
6-34
CSMA/CD


仅采用物理监听
不要求接收方给予应答
仅当监听到碰撞时进入
Backoff过程
 Backoff过程中不对信
道进行监听
 Backoff过程结束后重
新监听信道
 如果信道空闲则发送帧

CSMA/CA
 采用物理监听和虚拟监





听
要求接收方给予应答
信道忙或传送失败时进
入Backoff过程
Backoff过程中保持对
信道的监听
Backoff过程结束后立
刻开始帧发送
信道持续空闲DIFS时间
后才可发送帧
35
Chapter 6 Summary
Wireless
 wireless links:






IEEE 802.11 (“wi-fi”)


capacity, distance
channel impairments
CDMA
Mobility
 principles: addressing,
routing to mobile users
CSMA/CA reflects
wireless channel
characteristics
cellular access


architecture
standards (e.g., GSM,
CDMA-2000, UMTS)


case studies



home, visited networks
direct, indirect routing
care-of-addresses
mobile IP
mobility in GSM
impact on higher-layer
protocols
6: Wireless and Mobile Networks
6-36