1. Wireless Communication and Networks
Applications of Wireless Communication
Wireless Communication Technologies
Wireless Networking and Mobile IP
Wireless Local Area Networks
Student Presentations and Research Papers
Wireless Medium Access

2. Outline Medium Access Control Random Contention Access
CSMA
CSMA-CD
CSMA-CA
Random Contention Access
Multiple Access schemes
PCF
DCF
HCF
BA and WMM
Virtual Carrier Sense
Physical Carrier Sense
Multiple Access Problems

3. Multi-transmitter Interference Problem
Similar to multi-path or noise
Two transmitting stations will constructively/destructively interfere with each other at the receiver
Receiver will “hear” the sum of the two signals (which usually means garbage)

4. Medium Access Control Protocol required to coordinate access
i.e. transmitters must take turns
Similar to talking in a crowded room
Also similar to hub based Ethernet

5. Carrier Sense Multiple Access (CSMA)
Procedure
Listen to medium and wait until it is free (no one else is talking)
Wait a random back off time then start talking
Advantages
Fairly simple to implement
Functional scheme that works
Disadvantages
Can not recover from a collision
(inefficient waste of medium time)

6. Medium Access Control
Media Access Control (MAC) describes how the media (wired or wireless) is used
Polling, token passing, contention based
CSMA/CD is for ethernet wired networks
CSMA/CA for wireless
Stations using either access method must first listen to see whether any other device is transmitting. If another device is transmitting, the station must wait until the medium is available.

7. Carrier Sense Multiple Access with Collision Detection
CSMA-CD Procedure
Listen to medium and wait until it is free
Then start talking, but listen to see if someone else starts talking too
If a collision occurs, stop and then start talking after a random back off time
This scheme is used for hub based Ethernet
Advantages
More efficient than basic CSMA
Disadvantages
Requires ability to detect collisions

8. Collision Detection Problem
Transmit signal is MUCH stronger than received signal
Due to high path loss in the wireless environment (up to 100dB)
Impossible to “listen” while transmitting because you will drown out anything you hear
Also transmitter may not even have much of a signal to detect due to geometry

9. Carrier Sense Multiple Access with Collision Avoidance
CSMA-CA Procedure
Similar to CSMA but instead of sending packets control frames are exchanged
RTS = request to send
CTS = clear to send
DATA = actual packet
ACK = acknowledgement

10. Carrier Sense Multiple Access with Collision Avoidance
Advantages
Small control frames lessen the cost of collisions (when data is large)
RTS + CTS provide “virtual” carrier sense which protects against hidden terminal collisions (where A can’t hear B) A B

11. Carrier Sense Multiple Access with Collision Avoidance
Disadvantages
Not as efficient as CSMA-CD
Doesn’t solve all the problems of MAC in wireless networks

12. CSMA-CD vs CSMA-CA The difference is when the coast is clear
CSMA/CD node can immediately begin transmitting.
If a collision occurs while a CSMA/CD node is transmitting, the collision will be detected and the node will temporarily stop transmitting.
wireless stations are not capable of transmitting and receiving at the same time, so they are not capable of detecting a collision during their transmission.
wireless networking uses CSMA/CA instead of CSMA/CD to try to avoid collisions.

13. CSMA-CA
IF CSMA/CA station sees no transmissions it will wait a random interval
Keep watching the medium
If still clear after interval, transmit
If not, start over
Because only 1 station can use the frequency at a time
Half duplex

14. CSMA-CA
Standard Distributed Coordination Function (DCF) is defined to allow access of multiple stations
Checks and balances to keep the line clear
Minimize chances of collision

15. CSMA-CA
Since stations cannot detect collisions, how do they know when they happen?
Every unicast frame requires an acknowledgement
Broadcast and multicast don’t
ACK Frame
When ACK Frame is received, original station knows the frame is received

16. Collision Detection Receiving device will also check the CRC
If frame is corrupt, no ACK frame
IF no ACK frame, sender assumes delivery failure

17. Random Contention Access
Slotted contention period
Used by all carrier sense variants
Provides random access to the channel
Operation
Each node selects a random back off number
Waits that number of slots monitoring the channel
If channel stays idle and reaches zero then transmit
If channel becomes active wait until transmission is over then start counting again

18. Distributed Coordination Function (DCF)
Fundamental method of communication
Mandatory access method
Point Coordination Functions (PCF) is optional
also has Hybrid Coordination Function (HCF)
Four Main parts
Interframe Space
Virtual Carrier Sense
Physical Carrier Sense
Random back-off timer
Pg 254

19. Interframe Space (IFS)
The amount of time between transmissions
Actual length of time depends on network speed
Short interframe space (SIFS), highest priority
PCF interframe space (PIFS), middle priority
DCF interframe space (DIFS), lowest priority
Arbitration interframe space (AIFS), used by QoS stations
Extended interframe space (EIFS), used with retransmissions
Pg 254

20. Interframe Space (IFS)
Only certain types of frames are sent after certain interframe spaces
Only ACK and CTS after SIFS
Pg 254

21. 802.11 DCF Example B1 = 25 B2 = 20 B1 = 5 B2 = 15 data wait data wait
B1 and B2 are backoff intervals
at nodes 1 and 2
B1 and B2 are backoff intervals
at nodes 1 and 2
cw = 31
cw = 31

22. 802.11 Contention Window Random number selected from [0,cw]
Small value for cw
Less wasted idle slots time
Large number of collisions with multiple senders (two or more stations reach zero at once)
Optimal cw for known number of contenders & know packet size
Computed by minimizing expected time wastage (by both collisions and empty slots)
Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic

23. 802.11 Adaptive Contention Window
adaptively sets cw
Starts with cw = 31
If no CTS or ACK then increase to 2*cw+1 (63, 127, 255)
Reset to 31 on successful transmission
adaptive scheme is unfair
Under contention, unlucky nodes will use larger cw than lucky nodes (due to straight reset after a success)
Lucky nodes may be able to transmit several packets while unlucky nodes are counting down for access
Fair schemes should use same cw for all contending nodes (better for high congestion too)

24. 802.11 PCF (CSMA-CA) Optional access control method
Polling
AP is point coordinator
Only work in a BSS
Not in ad-hoc/IBSS
Both AP and client station must support PCF
AP will switch between DCF and PCF
PCF time is contention free period (CFP)
DCF is contention period (CP)

25. 802.11 DCF (CSMA-CA) Full exchange with “virtual” carrier sense
(called the Network Allocation Vector) A B
Sender
Receiver
Sender
RTS
DATA
CTS
ACK
Receiver A
NAV (RTS)
NAV (CTS) B B

26. Hybrid Coordination Function (HCF)
Added in e
Enhanced Distributed Channel Access (EDCA)
HCF Controlled Channel Access (HCCA)
DCF and PCF require contention for each frame
HCF defines ability to send multiple frames
Transmit Opportunity (TXOP)
During TXOP period, client station can send a frame burst
Uses Short Interframe Space (SIFS)
Pg 259

27. Enhanced Distributed Channel Access (EDCA)
Provides differentiated access using eight user priority levels
Extension of DCF
Uses frame tags similar to 802.1D standard
QoS standard at MAC (layer 2) level
Define priority values
With priority queuing
Data waiting in higher priority queues transmits before lower priority queues
Pg 259

28. Enhanced Distributed Channel Access (EDCA)
Pg 259

29. Enhanced Distributed Channel Access (EDCA)
Defines four access categories base on User Priority level
From lowest to highest priority access category:
AC_BK (Background)
AC_BE (Best Effort)
AC_VI (Video)
AC_VO (Voice
For each category, Enhanced Distributed Cannel Access Function (EDCAF) is used
Frames with higher category have lower back off values and are more likely to get a TXOP
Pg 259

30. Block Acknowledgement (BA)
Part of e
Allows for a single acknowledgement for multiple frames
Reduces overhead
Two Types
Immediate
For low latency traffic
Delayed
For latency tolerant traffic
Pg 261

31. Block Acknowledgement (BA)-Immediate
Originator sends a block of QoS data frames to station
Originator requests acknowledgement of all outstanding QoS data
Block AckReq frame
Recipient can send a single ack frame for all received frames
Can request a single frame from block be retransmitter
Pg 261

32. Block Acknowledgement (BA)-Immediate
Pg 261

33. W i-Fi Multimedia (WMM)
For latency sensitive data
Real time voice and video
Voice, video, audio, have less tolerance of latency (cumulative delay)
802.11e had layer 2 MAC methods to meet QoS requirements
WiFi Alliance created WMM
Because WMM is based on EDCA mechanisms, 802.1D priority tags from the Ethernet side are used to direct traffic to four access-category priority queues. The WMM certification provides for traffic prioritization via four access categories
Pg 262

34. W i-Fi Multimedia (WMM)
Pg 262

35. Virtual Carrier Sense Provided by RTS & CTS
Designed to protect against hidden terminal collisions (when C can’t receive from A and might start transmitting)
However this is unnecessary most of the time due to physical carrier sense
RTS
CTS A B C

36. Physical Carrier Sense Mechanisms
Energy detection threshold
Monitors channel during “idle” times between packets to measure the noise floor
Energy levels above the noise floor by a threshold trigger carrier sense
DSSS correlation threshold
Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal
Triggers carrier sense if the correlation peak is above a threshold
More sensitive than energy detection (but only works for transmissions)
High BER disrupts transmission but not detection

37. Physical Carrier Sense Range
Carrier can be sensed at lower levels than packets can be received
Results in larger carrier sense range than transmission range
More than double the range in NS simulations
Long carrier sense range helps protect from interference
Receive Range
Carrier Sense Range

38. Hidden Terminal Revisited
Virtual carrier sense no longer needed in this situation
RTS
CTS A B C

39. RTS CTS Still Useful Sometimes
Obstructed hidden terminal situation
Fast collision resolution for long data packets A B

40. Exposed Terminal Problem
Hidden terminal is not the only challenge for a distributed wireless MAC protocol
A blocks B, and C doesn’t know what is happening (B is exposed) D A B C

41. Double Exposure Problem
If A and C are out of phase, there is NO time D can transmit without causing a collision B A C D

42. Q&A ?