2. Media Access Methods MAC Functionality CSMA/CA with ACK
CSMA/CA with RTS/CTS protocol
Automatic data-rate fall-back
In cell relay operations
Ad-Hoc mode of operation

3. Accessing the Medium CSMA/CD (Standard Ethernet)
In a CSMA/CD system the media is sensed to verify that it is not in use
In CSMA/CD a station will:
Defer if carrier is in use
Transmit if the carrier is not in use
When collision is detected station all stations involved will
back off and
retransmit

4. CSMA/CA with MAC - level Acknowledgment
In a wireless environment the network adapters cannot detect collisions:
Carrier Sensing - listen to the media to determine if it is free
If media is free, then one of two methods is used to verify proper transmission
MAC - Level Acknowledgment
Request To Send / Clear To Send (RTS/CTS)

5. CSMA/CA MAC - level Acknowledgment
Collisions still can occur (interference; incapability of sensing other carrier)
IEEE defines “low-level” ACK protocol
Provides faster error recovery
Makes presence of high level error recovery less critical
Following a transmission, the receiving station must transmit an ACK signal back to the originating station within a specified time frame to verify proper frame transmission
Message
ACK

6. CSMA/CA MAC - level Acknowledgment
An ACK signal
will only be generated upon reception of a valid frame
will allow the transmitting station to know that the frame was not involved in a collision or corrupted
is a low overhead approach to verifying proper transmission
ACK is the recommended mode of operation in a small wireless network. The alternative would be using the RTS/CTS mode in a larger cell size which is discussed in the next few slides.
Message
ACK

7. The Problem “Hidden stations”
Situation that occurs in larger physical areas
(typically outdoors)
Loss of performance
Error recovery required B A C
A sends to B
C doesn’t detect that, so C might also start sending a transmission
Collision of messages occurs: both messages lost

8. The Solution CSMA/CA With Request To Sent/Clear To Send Protocol
In this scenario station A wants to transmit to station B
A transmits a Request To Send to B which tells B how long the transmission will take B A C
A to B
RTS: I want to send for 10 micro seconds

9. CSMA/CA With Request To Sent/Clear To Send Protocol
In this scenario station A wants to transmit to station B
B receives the RTS and transmits a Clear To Send to A which all other stations receive. This cause them to defer all attempts to transmit B A C
RTS: I want to send for 10 micro seconds
A to B
B to A (all other stations take note)
CTS: OK A, go ahead send for 10 micro seconds. Everybody else be quiet for 10 microseconds

10. CSMA/CA With Request To Sent/Clear To Send Protocol
In this scenario station A wants to transmit to station B
A now transmits to B and since all other stations are aware of the transmission, via the previous CTS, no collisions should occur. B A C
RTS: I want to send for 10 micro seconds
A to B
CTS: OK A, go ahead send for 10 micro seconds. Everybody else be quiet for 10 microseconds
B to A (all other stations take note)
A to B
Transmit: 10 micro seconds

11. CSMA/CA With Request To Sent/Clear To Send Protocol
In this scenario station A wants to transmit to station B
Following the transmission B will reply with an ACK signal which all other stations regard as meaning the media is clear B A C
RTS: I want to send for 10 micro seconds
A to B
CTS: OK A, go ahead send for 10 micro seconds. Everybody else be quiet for 10 microseconds
B to A (all other stations take note)
Transmit: 10 micro seconds
A to B
B to A
ACK: I received the transmission

12. RTS/CTS Issues Defaults to the off state
Can be switched on when a problem arises with hidden nodes
Increased overhead
No interruptions when large files are transmitted
Big picture:
Since most communication is from the AccessPoint (during normal operations) hidden nodes even when present will probably NOT be an issue. This is why the RTS/CTS mode should only be used when absolutely required.

13. Message Fragmentation (not available yet but planned for the future)
MAC level function to fragment large messages into smaller frames (user definable)
Improves performance in RF polluted environments
Can be switched off to avoid the overhead in RF clean environments
interference
Interference in a large frame requires re-transmission of the frame.
Fragmenting reduces the frame size and the required time to re-transmit since only a portion of the original frame will need to be retransmitted.

14. Multi-rate operations
Fall back to lower data-rate when communications quality decreases
Out of range situations
Interference
Operating bandwidth
High 11Mbps
Medium 5.5Mbps
Standard 2Mbps
Low 1Mbps

15. Why the Need for Data Rate Fall Back?
As the Signal to Noise Ratio falls to unacceptable levels, to support the data rate that is being used, the system will automatically fall back to the next lower speed.
Operating with a lower data rate makes the system better able to deal with a poor signal quality.

16. Data Rate Fall Back
802.11B 11Mbps High Speed device will fall back in the following order:
High 11Mbps
Medium 5.5Mbps
Standard 2Mbps
Low 1Mbps
Mbps device will fall back in the following order:

17. Advantages of Multiple Data Rates
User configurable option to use a fixed data rate if desired.
Wireless Network adapter in AccessPoint is capable of supporting different data-rates “simultaneously”
e.g. operates at “High” speed in communication to nearby station and at “Low” speed to station that is further away
This provides a simple reliable method to automatically regulate the wireless network!!!

18. Issues With Multiple Data Rates
High Data Rate 11Mbps
Low Date
Rate
2Mbps
Note: both stations are in the same cell but are operating at different speeds. The AccessPoint is capable of communication to each station. Stations can communicate with each other through the AccessPoint.

19. What is In-cell relay
AccessPoint acts as wireless repeater to relay all intercell station-to-station communication
Communication from one station to another station in the same wireless cell is via the AccessPoint.
Implications of all intercell traffic going to the AccessPoint:
Effective size of the cell increases Because Stations only need to be in the range of the AccessPoint and not in the range of all other stations in the wireless network
Reduced performance in peer to peer communication within one cell since all communications have to involve the AccessPoint.

20. Station A wants to communicate with Station B
Using Cell Relay
Station A wants to communicate with Station B
A transmits to B but the communication goes to the AccessPoint
Station D
Station B
Station E
RoamAbout
AccessPoint
Station A

21. Station A wants to communicate with Station B
Using Cell Relay
Station A wants to communicate with Station B
A transmits to B but the communication goes to the AccessPoint
Station D
Station B
Station E
RoamAbout
AccessPoint
Station A

22. Cell Relay
In the previous example, the AccessPoint relayed the transmission from the source to all the other stations in the cell
Note that even though station B was closer to A than was the AccessPoint was used to relay the transmission.

23. Ad-Hoc Mode of Operation
In the Ad-Hoc mode of operation, the wireless network is comprised of a peer-to-peer network without the aid of an AccessPoint.
Ad-Hoc network

24. Using the AccessPoint
Wired LAN (Ethernet)
AccessPoint network

25. Building to Building
Used to connect two separate locations with a wireless bridge connection
Link is dedicated to the point-to-point connection
Directional antennas are used for extended distances