18.1 Overview of the model

The WifiNetDevice models a wireless network interface controller based on the IEEE 802.11 standard. We will go into more detail below but in brief, ns-3 provides models for these aspects of 802.11:

• basic 802.11 DCF with infrastructure and adhoc modes
• 802.11a and 802.11b physical layers
• QoS-based EDCA and queueing extensions of 802.11e
• various propagation loss models including Nakagami, Rayleigh, Friis, LogDistance, FixedRss, Random
• two propagation delay models, a distance-based and random model
• various rate control algorithms including Aarf, Arf, Cara, Onoe, Rraa, ConstantRate, and Minstrel
• 802.11s (mesh), described in another chapter

The set of 802.11 models provided in ns-3 attempts to provide an accurate MAC-level implementation of the 802.11 specification and to provide a not-so-slow PHY-level model of the 802.11a specification.

The implementation is modular and provides roughly four levels of models:

• the PHY layer models
• the so-called MAC low models: they implement DCF and EDCAF
• the so-called MAC high models: they implement the MAC-level beacon generation, probing, and association state machines, and
• a set of Rate control algorithms used by the MAC low models

There are presently six MAC high models, three for non-QoS MACs and three for QoS MACs.

• non-QoS MACs:
  1. a simple adhoc state machine that does not perform any kind of beacon generation, probing, or association. This state machine is implemented by the ns3::AdhocWifiMac class.
  2. an active probing and association state machine that handles automatic re-association whenever too many beacons are missed is implemented by the ns3::NqstaWifiMac class.
  3. an access point that generates periodic beacons, and that accepts every attempt to associate. This AP state machine is implemented by the ns3::NqapWifiMac class.
• QoS MACs:
  1. a simple adhoc state machine like above but also able to manage QoS traffic. This state machine is implemented by ns3::QadhocWifiMac class.
  2. a station state machine like above but also able to manage QoS traffic. Implemented by ns3::QstaWifiMac.
  3. a QoS access point state machine like above implemented by ns3::QapWifiMac.

With QoS MAC models is possible to work with traffic belonging to four different access classes: AC_VO for voice traffic, AC_VI for video traffic, AC_BE for best-effort traffic and AC_BK for background traffic. In order to determine MSDU’s access class, every packet forwarded down to these MAC layers should be marked using ns3::QosTag in order to set a TID (traffic id) for that packet otherwise it will be considered belonging to AC_BE access class.

The MAC low layer is split into three components:

1. ns3::MacLow which takes care of RTS/CTS/DATA/ACK transactions.
2. ns3::DcfManager and ns3::DcfState which implements the DCF and EDCAF functions.
3. ns3::DcaTxop or ns3::EdcaTxopN which handle the packet queue, packet fragmentation, and packet retransmissions if they are needed. ns3::DcaTxop object is used by non-QoS high MACs. ns3::EdcaTxopN is used by QoS high MACs and performs also QoS operations like 802.11n MSDU aggregation.

There are also several rate control algorithms that can be used by the Mac low layer:

• ns3::ArfMacStations
• ns3::AArfMacStations
• ns3::IdealMacStations
• ns3::CrMacStations
• ns3::OnoeMacStations
• ns3::AmrrMacStations

The PHY layer implements a single model in the ns3::WifiPhy class: the physical layer model implemented there is described fully in a paper entitled "Yet Another Network Simulator". Validation results for 802.11b are available in this technical report.

In ns-3, nodes can have multiple WifiNetDevices on separate channels, and the WifiNetDevice can coexist with other device types; this removes an architectural limitation found in ns-2. Presently, however, there is no model for cross-channel interference or coupling.

The source code for the Wifi NetDevice lives in the directory src/devices/wifi.

Figure 18.1: Wifi NetDevice architecture.

This document was generated on August 20, 2010 using texi2html 1.82.