30-03-2012, 02:30 PM
Adaptive Packet Reservation Protocols
adaptive packet reservation protocols.ppt (Size: 1.43 MB / Downloads: 40)
OUTLINE
An Overview of IEEE 802.11
Legacy 802.11:
DCF ( Distributed coordination function )
PCF ( Point coordination function )
QoS Support Mechanisms of 802.11e:
EDCF ( Enhanced DCF )
HCF controlled channel access
AN OVERVIEW OF IEEE 802.11
IEEE 802.11 advantages:
Broadband bandwidth capability
Low deployment cost
Internet services access anytime, anywhere
Mobility and connectivity
IEEE 802.11 disadvantages:
Best effort services
No build in QoS modification of existing standards required
Shared medium
Figure 2. PHY components
PLCP ( Physical Layer Convergence Procedure )
PMD ( Physical Medium Dependent )
Types of Networks
Independent BSS (IBSS)
IBSSs are sometimes referred to as ad hoc BSSs or ad hoc networks.
Infrastructure BSS
An infrastructure BSS is defined by the distance from the access point.
Access points in infrastructure networks are in a position to assist with stations attempting to save power.
MAC Access Modes
Access to the wireless medium is controlled by coordination functions.
Ethernet-like CSMA/CA access is provided by the distributed coordination function (DCF).
If contention-free service is required, it can be provided by the point coordination function (PCF), which is built on top of the DCF.
Contention-free services are provided only in infrastructure networks.
Interframe Spacing
Varying interframe spacings create different priority levels for different types of traffic.
The high-priority traffic doesn't have to wait as long after the medium has become idle.
To assist with interoperability between different data rates, the interframe space is a fixed amount of time, independent of the transmission speed.
Short interframe space (SIFS)
The SIFS is used for the highest-priority transmissions, such as RTS/CTS frames and positive acknowledgments.
PCF interframe space (PIFS)
The PIFS is used by the PCF during contention-free operation.
Stations with data to transmit in the contention-free period can transmit after the PIFS has elapsed and preempt any contention-based traffic.
DCF interframe space (DIFS)
The DIFS is the minimum medium idle time for contention-based services.
Stations may have immediate access to the medium if it has been free for a period longer than the DIFS.
Extended interframe space (EIFS)
It is not a fixed interval.
It is used only when there is an error in frame transmission.
Distributed Coordination Function (DCF)
The DCF is the basis of the standard CSMA/CA access mechanism.
Like Ethernet, it first checks to see that the radio link is clear before transmitting.
To avoid collisions, stations use a random backoff after each frame, with the first transmitter seizing the channel.
In some circumstances, the DCF may use the CTS/RTS clearing technique to further reduce the possibility of collisions.
Most traffic uses the DCF, which provides a standard Ethernet-like contention-based service.
The DCF allows multiple independent stations to interact without central control, and thus may be used in either IBSS networks or in infrastructure networks.
Backoff with the DCF
A period called the contention window or backoff window follows the DIFS.
This window is divided into slots.
Stations pick a random slot and wait for that slot before attempting to access the medium; all slots are equally likely selections.
When several stations are attempting to transmit, the station that picks the first slot (the station with the lowest random number) wins.
As in Ethernet, the backoff time is selected from a larger range each time a transmission fails.
Contention window sizes are always 1 less than a power of 2 (e.g., 31, 63, 127,255).
Each time the retry counter increases, the contention window moves to the next greatest power of two.
When the contention window reaches its maximum size, it remains there until it can be reset.
The contention window is reset to its minimum size when frames are transmitted successfully, or the associated retry counter is reached, and the frame is discarded.