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
6-1
Elements of a wireless network
network
infrastructure
Wireless, Mobile Networks
6-2
Elements of a wireless network
wireless hosts



network
infrastructure
laptop, smartphone
run applications
may be stationary (nonmobile) or mobile
 wireless does not always
mean mobility
Wireless, Mobile Networks
6-3
Elements of a wireless network
base station


network
infrastructure
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
Wireless, Mobile Networks
6-4
Elements of a wireless network
wireless link


network
infrastructure


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
Wireless, 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
Mesh
802.11a,g point-to-point
4G: LTWE WIMAX
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
802.15
.384
2.5G: UMTS/WCDMA, CDMA2000
.056
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
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
6-7
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
6-8
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 network
Sensor network
6: Wireless and Mobile Networks
6-9
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
6: Wireless and Mobile Networks
6-10
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
Wireless, Mobile Networks
6-11
Wireless Link Characteristics
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 at destination at slightly different times
…. make communication across (even a point to point) wireless
link much more “difficult”
6: Wireless and Mobile Networks
6-12
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)
C
A
B
A
B
C
C’s signal
strength
A’s signal
strength
space
6: Wireless and Mobile Networks
6-13
IEEE 802.11 Wireless LAN
802.11a
802.11b
 5-6 GHz range
 2.4-5 GHz unlicensed
 up to 54 Mbps
spectrum
802.11g
 up to 11 Mbps
 2.4-5 GHz range
 direct sequence spread
 up to 54 Mbps
spectrum (DSSS) in physical
802.11n: multiple antennae
layer
 2.4-5 GHz range
 all hosts use same
 up to 200 Mbps
chipping code


all use CSMA/CA for multiple access
all have base-station and ad-hoc network versions
Wireless, Mobile Networks
6-14
802.11 LAN architecture

Internet
wireless host
communicates with base
station
 base station = access point
(AP)
hub, switch
or router

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
Wireless, Mobile Networks
6-15
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
6-16
802.11: passive/active scanning
BBS 1
BBS 1
BBS 2
BBS 2
1
AP 1
1
1
2
AP 2
AP 1
2
3
2
3
AP 2
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
from selected AP to H1
(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
Wireless, Mobile Networks
6-17
IEEE 802.11 MAC Protocol: CSMA/CA
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)
sender
receiver
DIFS
increase random backoff interval, repeat 2
data
else /* received ack */
return back to 2 (why?) to transmit next
frame
802.11 receiver
- if frame received OK
SIFS
ACK
return ACK after SIFS (ACK needed due to
hidden terminal problem) (no ack in ethernet!!)
DIFS: distributed inter-frame spacing, SIFS: short inter-frame spacing
6: Wireless and Mobile Networks
6-18
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 collisions
using small reservation packets!
6: Wireless and Mobile Networks
6-19
Collision Avoidance: RTS-CTS exchange
A
B
AP
DIFS
reservation collision
CIFS
CIFS
DATA (A)
time
defer
CIFS
6: Wireless and Mobile Networks
6-20
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
6-21
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
6-22
802.11 frame: addressing
R1 router
H1
Internet
AP
R1 MAC addr AP 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
6-23
802.11 frame: more
duration of reserved
transmission time (data, 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
6-24
802.11: mobility within same subnet
 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
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
6: Wireless and Mobile Networks
6-25
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
6-26
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
6-27
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
6-28
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
6-29
CDMA example
6: Wireless and Mobile Networks
6-30
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
6-31
Why extract correctly
By each user?
A: user codes are
orthogonal
6: Wireless and Mobile Networks
6-32
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?
On the 2-D space, if we add noise
To the X-axis, it does not affect
The node’s Y-axis position value
6: Wireless and Mobile Networks
6-33
Components of cellular network architecture
MSC
connects cells to wired tel. net.
 manages call setup (more later!)
 handles mobility (more later!)

cell
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

Mobile
Switching
Center
Public telephone
network
Mobile
Switching
Center
wired network
Wireless, Mobile Networks
6-34
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
frequency
access
time slots
bands
Wireless, Mobile Networks
6-35
2G (voice) network architecture
Base station system (BSS)
MSC
BTS
G
BSC
Public
telephone
network
Gateway
MSC
Legend
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Wireless, Mobile Networks
6-36
3G (voice+data) network architecture
MSC
G
radio
network
controller
Gateway
MSC
G
SGSN
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
GPRS: Generalized Packet Radio Service
Public
telephone
network
Public
Internet
GGSN
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Wireless, Mobile Networks
6-37
3G (voice+data) network architecture
MSC
G
radio
network
controller
Public
telephone
network
Gateway
MSC
G
SGSN
Public
Internet
GGSN
radio interface
(WCDMA, HSPA)
radio access network
Universal Terrestrial Radio
Access Network (UTRAN)
core network
General Packet Radio Service
(GPRS) Core Network
public
Internet
Wireless, Mobile Networks
6-38