Scheduler Algorithm

1








WIreless Systems providing high QUality Services

Introduction
Scheduler Algorithm
The contribution is based on a WLAN platform with
802.11e functionality. Services are prioritized
based on a ToS (Type of Service) value that is
contained in the packet header. The novel
contribution here is the addition of a scheduling
algorithm which possesses the knowledge of the
channel condition for that user, and which
thereby manipulates the prioritization of the
service. The functioning of the SCHEDULER that
was developed in partnership between Wavecom and
I.T. is discussed under System Description of
this contribution. The aggregate and user
throughputs are used as metrics to compare the
performance of the WLAN platform, with and
without the proposed scheduler.
The scheduler presents an algorithm that
evaluates the channel quality for each station
(user) and gives a high packet priority for the
user with a better channel quality (the tests
were performed with 2 stations and a hardware
channel simulator). The scheduler is based on
some features of the 802.11e standard. This
standard enhances the support of new services
that need QoS.
Scheduler It defines four classes, each one with
different priority values, as we can see on the
diagram.
Hardware The platform comprises the
following Access Point (AP) PC - Pentium 4
3.40GHz, 1.00 GB of RAM O.S.- Ubuntu
(Debian GNU Linux) Interface wireless- 3Com
wireless 11a/b/g PCI adapter
compatible with 11e
Users (STAs) Hardware- Geode SC1100 266 MHz CPU
(x86 architecture), 128MB-Ram O. S.- Based on
Debian GNU Linux Interface wireless- Atheros
WMIA 123AG Mini PCI Card compatible
with 11e Other specifications- 10/100 Mbps
Ethernet - 3W (idle), 7W (full
load) - CF-Socket -
I2C - LPC - RS232
The scheduler algorithm can be divided in three
parts -Channel Measurement -Read and
re-inject the packet into network. -Packet
manipulation.
These figures represent the block diagrams of
the AP and the scheduler respectively and how the
system works on the AP.
Results
Conclusion
The results presented were obtained using the
scheduler algorithm that selects which AC the
packet must be sent to according the SNR
threshold values.

• The scheduler algorithm improves the overall
  throughput because it removes the fairness
  transmission defined by the 802.11 standard.
• In stations (users) which have better channel
  conditions the transmissions are done faster due
  to the automatic link adaptation. In stations
  with a low channel quality, an average throughput
  is maintained.
• The results were obtained only 2 stations but it
  can be extended for several stations.