24-09-2012, 03:05 PM
IEEE 802.11 Ad Hoc Networks: Protocols, Performance and Open Issues
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Introduction
The previous chapter has presented the activities of the different task groups within the IEEE
802.11 project [IEE], and has highlighted that the IEEE 802.11 is currently the most mature
technology for infrastructure-based Wireless LANs (WLANs). The IEEE 802.11 standard
defines two operational modes for WLANs: infrastructure-based and infrastructure-less or ad
hoc. Network interface cards can be set to work in either of these modes but not in both
simultaneously. The infrastructure-based is the mode commonly used to construct the so
called Wi-Fi hotspots, i.e., to provide wireless access to the Internet. The drawbacks of an
infrastructure-based WLAN are the costs associated with purchasing and installing the
infrastructure. These costs may not be acceptable for dynamic environments where people
and/or vehicles need to be temporarily interconnected in areas without a pre-existing
communication infrastructure (e.g., inter-vehicular and disaster networks), or where the
infrastructure cost is not justified (e.g., in-building networks, specific residential communities
networks, etc.). In these cases, a more efficient solution can be provided by the infrastructureless
or ad hoc mode.
IEEE 802.11 Architecture and Protocols
In this section we will focus on the IEEE 802.11 architecture and protocols as defined in the
original standard [IEE99], with a particular attention to the MAC layer. Later, in Section 5,
we will emphasize the differences between the 802.11b standard with respect to the original
802.11 standard.
The IEEE 802.11 standard specifies both the MAC layer and the Physical Layer (see Figure
1). The MAC layer offers two different types of service: a contention free service provided by
the Distributed Coordination Function (DCF), and a contention-free service implemented by
the Point Coordination Function (PCF). These service types are made available on top of a
variety of physical layers. Specifically, three different technologies have been specified in the standard: Infrared (IF), Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence
Spread Spectrum (DSSS).
The DCF provides the basic access method of the 802.11 MAC protocol and is based on a
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme. The PCF is
implemented on top of the DCF and is based on a polling scheme. It uses a Point Coordinator
that cyclically polls stations, giving them the opportunity to transmit. Since the PCF can not
be adopted in ad hoc mode, it will not be considered hereafter.
Distributed Coordination Function (DCF)
According to the DCF, before transmitting a data frame, a station must sense the channel to
determine whether any other station is transmitting. If the medium is found to be idle for an
interval longer than the Distributed InterFrame Space (DIFS), the station continues with its
transmission3 (see Figure 2). On the other hand (i.e., if the medium is busy), the transmission
is deferred until the end of the ongoing transmission. A random interval, henceforth referred
to as the backoff time, is then selected, which is used to initialize the backoff timer. The
backoff timer is decreased for as long as the channel is sensed as idle, stopped when a
transmission is detected on the channel, and reactivated when the channel is sensed as idle
again for more than a DIFS (for example, the backoff timer of Station 2 in Figure 2 is
disabled while Station 3 is transmitting its frame; the timer is reactivated a DIFS after Station
3 has completed its transmission). The station is enabled to transmit its frame when the
backoff timer reaches zero. The backoff time is slotted. Specifically, the backoff time is an
integer number of slots uniformly chosen in the interval (0, CW-1). CW is defined as the
Backoff Window, also referred to as Contention Window. At the first transmission attempt
CW=CWmin, and it is doubled at each retransmission up to CWmax.
Ad Hoc Networking Support
In this section we will describe how two or more 802.11 stations set up an ad hoc network. In
the IEEE 802.11 standard, an ad hoc network is named Independent Basic Service Set (IBSS).
An IBSS enables two or more 802.11 stations to communicate each other without the intervention of either a centralized AP, or an infrastructure network. Hence, the IBSS can be
considered as the support provided by the 802.11 standard for mobile ad hoc networking.4
Due to the flexibility of the CSMA/CA protocol, to receive and transmit data correctly it is
sufficient that all stations within the IBSS are synchronized to a common clock. The standard
specifies a Timing Synchronization Function (TSF) to achieve clock synchronization between
stations. In an infra-structured network the clock synchronization is provided by the AP, and
all stations synchronizes their own clock to the AP’s clock. In an IBSS, due to the lack a
centralized station, clock synchronization is achieved through a distributed algorithm. In both
cases synchronization is obtained by transmitting special frames, called beacons, containing
timing information.