Class3-Shared_Medium_Access - EE/CS652: EE/CS652: Fall2007...

EE/CS 652: Wireless Sensor Networks
EE/CS 652: Wireless Sensor Networks
Fall 2007
Fall 2007
Bhaskar Krishnamachari
Assistant Professor, EE-S, CS
University of Southern California
Autonomous Networks Research Group
http://ceng.usc.edu/~bkrishna/
[email protected]
Lecture 3: Shared Medium Access
Lecture 3: Shared Medium Access

Aloha, Slotted Aloha
•
The most basic schemes for wireless MAC
are Aloha, Slotted Aloha
•
Easiest schemes to analyze.
•
Weakness: no attempt to avoid collisions.

•
Classical CSMA-CD (wired Ethernet) does not work due
to (a) Hidden Terminal and (b) Exposed Terminal
Problems
CSMA in Wireless Networks
CSMA in Wireless Networks
C B
A D
C B
A D
(a) Channel free at sender,
busy at receiver
(b) Channel busy at sender,
free at receiver

•
MACA: A way of virtually sensing the channel at
receiver. “request-to-send”/“clear-to-send”
messages:
–
If a node has packet to send, it first transmits a RTS
packet.
–
If available receiver responds with CTS.
–
After sender receives CTS, it transmits data.
–
Nodes that hear RTS wait long enough for transmitter
to receive CTS.
–
Nodes that hear CTS back off long enough to allow
receiver to receive data.
Collision Avoidance
Collision Avoidance
P. Karn, MACA: A new channel access method for Packet Radio, AARL Computer Networking Conference, 1990.

IEEE 802.15.4

IEEE 802.x.x
•
IEEE 802.11
–
WLAN
•
IEEE 802.16
–
Broadband Wireless Access Standards
•
IEEE 802.15
–
IEEE 802.15.1: Bluetooth
•
High data rate High power consumption small network
–
IEEE 802.15.4
•
For Low power low rate Wireless Personal Network
•
IEEE 802.11: an “overkill technology”
•
Bluetooth: high data rate for multimedia applications, small
size network, high power consumption

802.15.4 Applications
LR-WPAN
Low Rate
Wireless Personal
Area Network

802.15.4 Applications
•
Low-Rate Low-Power Wireless Networks