Computer Networking Lecture Notes
11 March 2013 • Wireless Medium Access
Outline
Wireless
Propagation
Reservation
IEEE 802.11, Wi-Fi.
Topologies and architecture
Frames
Shared Channels
Shared (multiaccess, broadcast, random access) channels.
What’s so special about wireless channels?
Why haven’t we solved this problem already?
Wireless Properties
Three important wireless properties:
Half duplex
operation; a host can send or receive, not both.
A time varying, complex channel.
Frequent (1 in 10
3
BER) and bursty errors.
A wired connection is full duplex, consistent and simple, and exhibits low, random errors.
Wireless MAC protocols are correspondingly more complicated.
Propagation
Assume radio signals propagate uniformly in all directions.
Not even close to being true.
Eventually signals become too weak to receive (
fade
).
Distance squared
drop-off.
Hidden Senders
Both
A
and
C
want to send to
B
.
The
hidden sender
problem.
Good-Bye CSMA/CD
Half-duplex transmission eliminates collision detection (CD).
No listening while sending.
Carrier sense (CSMA) is still possible.
But hidden senders reduce its effectiveness.
This is not the propagation-delay collision window problem.
Although that problem exists too.
Exposed Senders
A
wants to send to
B
,
C
wants to send to
D
.
The
exposed sender
problem.
Also the
discontinuous LAN
problem.
Capture
A
and
C
sending to
B
, C’s signal is stronger than
A
’s signal.
C
captures
B
.
Sometimes capture is a problem, sometimes it’s a feature.
Other Factors
Mobility and power are two more important factors.
No wired tether leads to hosts that can move.
Network boundaries are not human friendly.
Mobile devices tend to be small, and small batteries are not power dense.
Radio I-O requires power.
ALOHA
What about ALOHA?
Pure ALOHA is a simple fully distributed algorithm.
Slotted ALOHA too, but less so.
However, even under modest contention ALOHA isn’t efficient.
And it degrades rapidly under load.
Collisions are expensive in a mobile wireless environment.
Wireless MAC Protocols
Particular services and infrastructure need particular MAC protocols.
Bluetooth (IEEE 802.15) - personal-area networks (PANs).
Wi-Fi (IEEE 802.11) - local-area networks (LANs).
WiMax (IEEE 802.16) - metropolitan area networks (MANs), wireless last miles.
Other contenders include radio-frequency ID (RFID) and Iridium-like satellite networks.
Wi-Fi Protocols
Some of the Wi-Fi IEEE 802.11 standards.
802.11
GHz
Mbit/sec*
original
1997
2.4
2
a
1999
5
54
b
1999
2.4
11
g
2003
2.4
54
n
2009
2.4 or 5
600
*Maximum; actual bit rates vary adaptively.
Development continues.
Wi-Fi CSMA
Carrier sense can be helpful, but avoiding collisions is even more helpful.
Non-slotted
p
-persistent CSMA provides the starting point.
Don’t flip a coin on an idle channel, randomly backoff.
No collision detection, so expect acknowledgments.
This is known as
multiple access collision avoidance
(
MACA
, also
CSMA/CA
).
CSMA/CA
Backoff time is adjusted to channel characteristics.
Reservation
Carrier sense is helpful, and reservation is even more helpful.
Use small frames to reserve clear channels around sender and receiver.
Sender emits
ready to send
(
RTS
).
Receiver emits
clear to send
(
CTS
).
Data follows.
RTS-CTS reservation is an optional CSMA/CA feature.
CSMA/CA Reservation
RTS and CTS frames are small (10s bytes).
Less likely to collide, less lost when they do.
Sender’s and receiver’s neighbors hear the reservation frames and mute to avoid collisions.
RTS and CTS contain expected channel hold time.
Neighbors compute the mute-period size.
CSMA/CA Example
802.11 Topologies
A collection of communicating systems is a
basic service set
(
BSS
)
BSS
Independent BSS
Basic Service Systems
An independent BSS is also known as an
ad-hoc
or
mesh network
.
Hosts communicate directly (or indirectly) with each other.
All communication in a BSS goes through a distinguished system called the
access point
or
base station
.
All communication not with the access point is indirect.
802.11 Architecture
Now imagine the hosts moving around.
Distribution Systems
Two or more BSSs can be connected via a
distribution system
(
DS
) or a
backbone
.
The backbone is not part of 802.11, although it could be wireless.
The backbone can be connected to other networks too (the Internet, for example).
An
infrastructure
is a collection of BSSs interconnected with distribution systems.
Finding APs
A host
scans
for an access point:
Host broadcasts a
Probe frame
.
APs reply with a
Probe Response frame
.
Host chooses an AP and sends an
Association Request frame
.
The AP replies with an
Association Response frame
.
A host scans for an AP when it doesn’t have one, or it wants a better one.
Scanning
Host initiated scanning is called
active scanning
.
Access points also provide
passive scanning
.
APs periodically broadcast
Beacon frames
.
A Beacon frame advertises an AP and its capabilities.
A
hand-off
occurs when a host changes APs.
802.11 Frames
Other Wi-Fi Protocols
There are a number of other Wi-Fi protocols in place or under development:
802.11e - Quality of service specifications.
802.11i - Security enhancements (replacing WEP).
802.11k - Access-point selection.
802.11r - Mobile management and hand-off.
802.11s - Mesh (ad hoc) networks.
Bluetooth
Bluetooth, a wireless technology to replace data cabling.
Also a 10th century Danish king.
An example of a personal-area network (PAN).
Distance usually between 1 and 10 m.
Data rates between 1 and 3 Mbit/sec.
Developed by an industrial consortium.
Standardized by IEEE 802.15.
Bluetooth Frequencies
Bluetooth operates in an unlicensed ISM band from 2.4 to 2.4853 GHz.
Which conflicts with the lower Wi-Fi frequency band.
The band is divided into 79 1 MHz channels and two approximately 2 MHz guard bands.
The Bluetooth physical layer uses frequency hopping spread spectrum.
A form of frequency-division multiplexing.
Bluetooth Architectures
Piconet
Scatternet
All hosts are mobile.
Masters and Slaves
A Bluetooth host
discovers
nearby hosts and
pages
responders to form a piconet.
The discoverer is the piconet master, the responders are the slaves.
A master has up to seven
active slaves
, and any number of
parked slaves
.
The master activates and parks slaves as necessary.
Slaves can request master status.
Scatternets
A host in more than one piconet is a
bridge host
.
The bridge is a master in at most one piconet.
A bridge time-division multipexes among piconets.
A group of hosts can form several piconets to get a scatternet.
This approximates peer-to-peer communication among hosts.
WiMAX
WiMAX (Worldwide Interoperability for Microwave Access).
An example of a metropolitan-area network (MAN).
Distance up to 10-30 miles.
Data rates between 30 and 40 Mbit/sec.
A set of IEEE 802.16 standards.
Summary
Carrier sense is less of an advantage for wireless.
And direct collision detection is impossible.
IEEE 802.11 (Wi-Fi) is one of many wireless specifications.
Others include Bluetooth, WiMax, and G3-G4-LTE.
There are many 802.11 specs, and more are on the way.
This page last modified on 2011 October 9.