1. Elements of a wireless network
wireless hosts
laptop, PDA, IP phone
Run applications
may be stationary (non-mobile) or mobile
wireless does not always mean mobility
network
infrastructure
6: Wireless and Mobile Networks

2. Elements of a wireless network
base station
typically connected to wired network
relay - responsible for sending packets between wired network and wireless host(s) in its “area”
cell towers
access points
network
infrastructure
6: Wireless and Mobile Networks

3. Elements of a wireless network
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
network
infrastructure
6: Wireless and Mobile Networks

4. Characteristics of selected wireless link standards
mesh
54 Mbps
802.11{a,g}
802.16e or Mobile WiMAX
5-11 Mbps
.11 p-to-p link
802.11b
1 Mbps
802.15 3G
384 Kbps
UMTS/WCDMA, CDMA2000 2G
56 Kbps
IS-95 CDMA, GSM
Indoor
10 – 30m
Outdoor
50 – 200m
Mid range
outdoor
200m – 4Km
Long range
5Km – 20Km
6: Wireless and Mobile Networks

5. WiMax Vs. Wireless Mesh WiMax Wireless Mesh
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 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, QoS
6: Wireless and Mobile Networks

6. Elements of a wireless network
infrastructure mode
base station connects mobiles into wired network
handoff: mobile changes base station providing connection into wired network
network
infrastructure
6: Wireless and Mobile Networks

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
Wireless active research area:
Ad hoc network
Sensor network
6: Wireless and Mobile Networks

8. 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
Military battlefield, civil engineering, environmental monitoring
6: Wireless and Mobile Networks

9. 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

10. IEEE Wireless LAN
802.11b
GHz unlicensed radio spectrum
up to 11 Mbps
direct sequence spread spectrum (DSSS) in physical layer
all hosts use same chipping code
widely deployed, using base stations
802.11a
GHz range
up to 54 Mbps
802.11g
GHz range
Use OFDM in physical layer
All use CSMA/CA for multiple access
All have base-station and ad-hoc network versions
6: Wireless and Mobile Networks

11. 802.11 LAN architecture wireless host communicates with base station
Internet
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 AP
hub, switch
or router AP
BSS 1
BSS 2
6: Wireless and Mobile Networks

12. 802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
11 channels are partial overlapping (1, 6, 11 non-overlapping)
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

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) A B C
A’s signal
strength
space
C’s signal
6: Wireless and Mobile Networks

14. IEEE 802.11 MAC Protocol: CSMA/CA
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
else /* received ack */
return back to 2 (why?) to transmit next frame
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

15. 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

16. Collision Avoidance: RTS-CTS exchangeB AP
DIFS
RTS(A)
RTS(B)
reservation collision
RTS(A)
CTS(A)
CIFS
CIFS
DATA (A)
ACK(A)
defer
time
CIFS
6: Wireless and Mobile Networks

17. 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 Max frame length
6: Wireless and Mobile Networks

18. frame: addressing
frame
control
duration
address 1 2 4 3
payload
CRC 6
seq
Address 4: used only in ad hoc mode
Address 1: MAC address
of wireless host or AP
to receive this frame
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

19. 802.11 frame: addressing Internet router H1 R1 AP
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
frame H1 R1
R1 MAC addr AP MAC addr
dest. address
source address
802.3 frame
6: Wireless and Mobile Networks

20. 802.11 frame: more duration of reserved
transmission time (data, RTS/CTS)
frame
control
duration
address 1 2 4 3
payload
CRC 6
seq
Type
From AP
Subtype To
More
frag
WEP
data
Power
mgt
Retry
Rsvd
Protocol
version 2 4 1
frame type
(RTS, CTS, ACK, data)
6: Wireless and Mobile Networks

21. 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

22. 802.15 MAC and Bluetooth 802.11 MAC 802.15 MAC 11 Mbps – 54 Mbps
Up to 100 meters range
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

23. 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 ( is 11 Mbps to 54 Mbps)
6: Wireless and Mobile Networks

24. 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

25. 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

26. CDMA example
6: Wireless and Mobile Networks

27. Working with multiple users
How to extract data when multiple users transmit at the same time?
Assumptions:
Interfering signals are additive
Signal (-1) = 2
New signals in the air (N senders):
Same decoding formula!
6: Wireless and Mobile Networks

28. Why extract correctly By each user? A: user codes are orthogonal
6: Wireless and Mobile Networks