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UCSMA

Rate Control

🔗 GitHub Repository

Overview

The rate control protocol is the key part of the ultimate goal of UCSMA, which attempts to maintain fairness among different nodes in an ad-hoc wireless network. There are two parts of the rate control protocol:

  1. buffer enqueue delay: a packet will need to wait a certain time interval (positively proportional to the current buffer size) before being enqueued into the transmission buffer;
  2. contention window adjustment: we set a lower CW for nodes with fuller buffer to allow their packets to be send out quicker.

Getting started

Compiling

  1. Run build.sh from the main repository.

  2. Copy the modifed ath9k files to the OpenWRT-14.07-JS9331/build_dir/target-mips_34kc_uClibc-0.9.33.2/linux-ar71xx_generic/compat-wireless-2014-05-22/drivers/net/wireless/ath/ath9k/ directory.

  3. Change directory to the OpenWRT root (OpenWRT-14.07-JS9331) and compile with make package/compile.

  4. Copy the built ath9k.ko from OpenWRT-14.07-JS9331/staging_dir/target-mips_34kc_uClibc-0.9.33.2/root-ar71xx/lib/modules/3.10.49 to the development board and load the kernel module:

rmmod ath9k
insmod ath9k.ko
./init_mon0.sh  # you may need to change phy0

Technical details

Buffer enqueue delay

In the current implementation, the rate control feature calculates a wait time which equals to a scalar multiple of the size of the current size of the buffer (mentioned in Network Driver (ath9k)).

The system then delays for the calculated time during the transmission (ath9k_tx). The current implementation uses udelay from linux/delay.h. Note that udelay uses the busy-wait mechanism and does not yield the CPU to others tasks. However, udelay is still chosen as it is recommanded for atomic contexts (ath9k_tx is atomic) and the delay is typically very short.

Contention window adjustment

The current implementation modifies the contention window of the hardware once we successfully send out a packet and a free buffer node is send back to the buffer linked list, i.e. during the execution of ath_tx_return_buffer.

After a new contention window being calculated, we set the CWmin and CWmax parameters to values from 1, 3, 7, 15, 31, 63, 127, 255, 511, and 1023 given by Cisco documentation. In this step, the system calls the hardware update function to update the contention window parameter registers.

Main functions

ath_cw_update() (836-865)

  • Set up contention window based on the buffer size.
  • Larger buffer -> larger contention window.
  • Some of the conditions are redundant under the current settings for the CW_max and CW_min. (subject to change after testing)

ath_tx_default_wait() (878-880)

  • Functionality: set the default waiting time.
  • Input buf_size: the size of the buffer.
  • Output: the waiting time based on buf_size. Currently the waiting time is set linearly to the buffer size: 100 miliseconds per buffer size. (subject to change after testing)