16.3 802.11 MAC protocol

Prior to release ns-2.33, there was only one main-tree 802.11 model, although other researchers were maintaining third-party patches on the web. Starting with ns-2.33, there are multiple choices in the main distribution.

The first extension described below (infrastructure mode) extends the legacy model to include infrastructure mode. However, the last two items (802.11Ext and dei802mr) are complete replacements for the legacy model.

Therefore, researchers now have a choice of 802.11 models, and should carefully read the documentation and code of each one to understand which is the best fit for the job.

802.11 DCF from CMU

This model has been the only model available in the main ns source tree prior to release ns-2.33. See ~ns/mac-802_11.{cc,h} for implementation details. It uses a RTS/CTS/DATA/ACK pattern for all unicast packets and simply sends out DATA for all broadcast packets. The implementation uses both physical and virtual carrier sense. The Mac802_11../ns-2/mac-802_11.h is implemented in ~ns/mac-802_11.{cc,h}.

802.11 infrastructure extensions

Ilango Purushothaman from the University of Washington has implemented infrastructure extensions to the above 802.11 model, and fixed some bugs along the way. The extensions include passive and active scanning, authentication, association, inter-AP communications, and mobility support (handoff). Please note that this model still supports single-channel scenarios only.

• Documentation: http://ee.washington.edu/research/funlab/802_11/report_80211_IM.pdf
• Example script: tcl/ex/infra.tcl
• Test suite: tcl/test/test-suite-wireless-infra-mobility.tcl tcl/test/test-suite-wireless-infra.tcl

802.11Ext

A team from Mercedes-Benz Research and Development North America and from University of Karlsruhe have collaborated to develop a completely new 802.11 Mac and Phy model, called Mac802_11Ext and WirelessPhyExt, respectively. The new model contains the following features:

• Structured design of MAC functionality modules: transmission, reception, transmission coordination, reception coordination, backoff manager, and channel state monitor
• Cumulative SINR computation
• MAC frame capture capabbilities
• Multiple modulation scheme support
• Packet drop tracing at the PHY layer
• Nakagami fading model

This model should be used as a replacement for the existing models. The example scripts show how to do this.

• Key files: apps/pbc.{cc,h}, mac/mac-802_11Ext.{cc,h}, mac/wireless-phyExt.{cc,h}, mobile/nakagami.{cc,h}
• Documentation: http://dsn.tm.uni-karlsruhe.de/Overhaul_NS-2.php
• Example scripts: tcl/ex/802.11/ directory: IEEE802-11a.tcl IEEE802-11p.tcl broadcast_validation.tcl unicast_validation.tcl
• Test suite: tcl/test/test-suite-wireless-lan-newnode-80211Ext.tcl

dei80211mr

The dei80211mr library - nicknamed 'multirate' for short - provides an 802.11 derived from the CMU implementation. This library depends on the Dynamic Library (Chapter 28) and is included in the ns-allinone distribution only (see the top-level dei80211mr directory in the ns-allinone distribution or see http://www.dei.unipd.it/wdyn/?IDsezione=5091). For step-by-step installation instructions, please refer to the tutorial at http://www.dei.unipd.it/%7Ebaldo/nsmiracle-dei80211mr-howto.html The following functionalities are provided by the dei80211mr library:

• support for multiple PHY modes is included; in particolar, dei80211mr simulation of the different transmission rates, modulation and coding schemes defined in the IEEE802.11b/g standards.
• a SINR-based packet level error model is introduced:
  • the RX Threshold variable which was used in the 802.11 implementation included in standard NS to determine successful receptions has been removed. Instead, Packet Error Rate (PER) is used to determine random packet losses.
  • PER is calculated using pre-determined curves (PER vs SINR and packet size); the curves can be specified by the user via TCL. Some default curves for both 802.11g and 802.11b are provided.
  • SINR is calculated using received signal strength, noise and interference
  • interference is calculated using a gaussian model to account for all transmissions which happen simultaneously to the one which is considered for reception
  • noise power is set via TCL
• the capture model, i.e. the determination of whether a packet can be received when there are other concurrent transmissions are simultaneously ogoing, is now embedded in the above mentioned interference model (no more Capture Threshold)
• In the wireless channel, the affected nodes distance is no more determined using the CS threshold, but we used a fixed value in meters which can be set at the beginning of the simulation. The reason is that, since we use a gaussian interference model, nodes well below the CS threshold often still provide a non-negligible contribution to interference. The default value for the affected nodes distance is very conservative, so that all nodes are considered for interference calculation. This default value therefore yields accurate but computationally intensive simulations. The value can be adjusted via TCL to achieve different trade-offs between computational load and simulation accuracy.

• Documentation: http://www.dei.unipd.it/%7Ebaldo/nsmiracle-dei80211mr-howto.html
• Example script: dei80211mr-1.1.4/samples/adhoc_tcp.tcl
• Test suite: None

In addition, a patch (relating to the CMU implementation) improving ns-2 802.11 wireless support is available at http://www.telematica.polito.it/fiore/. The patch introduces realistic channel propagation, concurrent multiple data transmission rates among stations and ARF mechanisms, has been tested with ns-2.29, and features the following contributions:

• channel propagation improvements by Wu Xiuchao
• ricean propagation model by Ratish J. Punnoose
• SNOOPy calendar scheduler by David X. Wei
• 802.11 bug fixes by Felix Schmidt-Eisenlohr
• multiple data transmission rates support by Marco Fiore
• Adaptive Auto Rate Fallback (AARF) by Marco Fiore.