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work and they must cooperatively provide the functionality that is usually provided by
the network infrastructure (e.g., routers, switches, and servers). This approach requires
that the users™ density be high enough to guarantee the packet forwarding among
the sender and the receiver. When the users™ density is low, networking may become un-
feasible.
Even though large-scale multihop ad hoc networks will not be available in the near fu-
ture, on smaller scales, mobile ad hoc networks are starting to appear, thus extending the
range of the IEEE 802.11 technology over multiple radio hops. Most of the existing IEEE
802.11-based ad hoc networks have been developed in the academic environment but, re-
cently, even commercial solutions have been proposed (see, e.g., MeshNetworks1 and
SPANworks2).
Other than being a solution for pure ad hoc networking, the IEEE 802.11 ad hoc tech-
nology may also constitute an important and promising building block for solving the
first-mile problem in hotspots. This aspect is related to the understanding of some basic
radio frequency (RF) transmission principles. Specifically, the transmission range is limit-
ed since the RF energy disperses as the distance from the transmitter increases. In addi-
tion, even though WLANs operate in the unregulated spectrum (i.e., the users are not re-
quired to be licensed), the transmitter power is limited by the regulatory bodies (e.g., the
FCC in the United States and ETSI in Europe). IEEE 802.11a and IEEE 802.11b can op-
erate at several bit rates but, since the transmitter power is limited, the transmission range
decreases when the data rate is increased.
It is expected that the bandwidth request in hotspots will increase very fast, thus re-
quiring higher-speed access technologies. As explained in the previous chapter in this
book, channel speeds for the IEEE 802.11 family continue to increase: 802.11a operates
at 54 Mbps, and enhanced versions operating at speeds up to 108 Mbps are also under in-
vestigation. Such high-speed WLAN standards are expected to further increase the popu-
larity of wireless access to the backbone infrastructure. On the other hand, increasing the
transmission rate (while maintaining the same transmission power) produces a reduction
in the coverage area of an AP. Specifically, at the 100 Mbps rate the coverage area will
correspond to a radius of few meters around the AP. It does not seem to be a feasible solu-
tion to spread in a hotspot a large number of APs uniformly and closely spaced. A more
feasible solution may be based on the use of a relatively low number of multirate APs, and
the deployment of multihop wireless networks that provide access to the wired backbone

1
http://www.meshnetworks.com.
2
http://www.spanworks.com.
71
3.2 IEEE 802.11 ARCHITECTURE AND PROTOCOLS


via multiple wireless hops. When the population in a hotspot is low, the AP can use low
transmission rates, thus covering a large area. In this case, the users™ devices can contact
the AP directly (i.e., single-hop). When the hotspot population increases, the data rate is
increased as well and, hence, some devices can no longer directly contact the AP but must
be supported by other devices for forwarding their traffic toward the AP. By further in-
creasing the data rate, more users can be accommodated in the hotspot but, at the same
time, more hops may be necessary for user traffic to reach the AP.
Currently, the widespread use of IEEE 802.11 cards makes this technology the most in-
teresting off-the-shelf enabler for ad hoc networks. However, the standardization efforts
have concentrated on solutions for infrastructure-based WLANs, whereas little or no at-
tention has been given to the ad hoc mode. Therefore, the aim of this chapter is triple: (i)
an in-depth investigation of the ad hoc features of the IEEE 802.11 standard, (ii) an analy-
sis of the performance of 802.11-based ad hoc networks, and (iii) an investigation of the
major problems arising when using the 802.11 technology for ad hoc networks, and possi-
ble directions for enhancing this technology for a better support of the ad hoc networking
paradigm.
The rest of the chapter is organized as follows. The next section briefly describes the
architecture and protocols of IEEE 802.11 WLANs. The aim is to introduce the terminol-
ogy and present the concepts that are relevant throughout the chapter. The interested read-
er can find details on the IEEE 802.11 protocols in the standard documents [15].
The characteristics of the wireless medium and the dynamic nature of ad hoc net-
works make IEEE 802.11 multihop networks fundamentally different from wired net-
works. Furthermore, the behavior of an ad hoc network that relies upon a carrier-sens-
ing random-access protocol such as the IEEE 802.11 is further complicated by the
presence of hidden stations, exposed stations, “capturing” phenomena [28, 29], and so
on. The interaction between all these phenomena makes the behavior of IEEE 802.11 ad
hoc networks very complex to predict. Recently, this has generated an extensive litera-
ture related to the performance analysis of the 802.11 MAC protocol in the ad hoc en-
vironment, which we survey in Section 3.3. Most of these studies have been done
through simulation. To the best of our knowledge, only very few experimental analyses
have been conducted. For this reason, in Section 3.4 we extend the 802.11 performance
analysis with an extensive set of measurements that we have performed on a real test-
bed. These measurements were performed both in indoor and outdoor environments, and
in the presence of different traffic types. For the sake of comparison with the previous
studies, our analysis is mostly related to the basic IEEE 802.11 MAC protocol (i.e., we
consider a data rate of 2 Mbps). However, some results related to IEEE 802.11b are also
included in Section 3.5. In the same section, we present some problems (gray zones) that
may occur by using IEEE 802.11b in multihop ad hoc networks. Finally, in Section 3.6
we discuss some possible extensions to the IEEE 802.11 MAC protocol to improve its
performance in multihop ad hoc networks.


3.2 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 [15], with particular attention to the MAC layer. Later, in Section
3.5, we will emphasize the differences between the 802.11b standard with respect to the
original 802.11 standard.
72 IEEE 802.11 AD HOC NETWORKS: PROTOCOLS, PERFORMANCE, AND OPEN ISSUES


contention-free
services contention
services


Point Coordination
Function

Distributed Coordination
Function


Physical Layer


Figure 3.1. IEEE 802.11 Architecture.



The IEEE 802.11 standard specifies both the MAC layer and the Physical Layer (see
Figure 3.1). 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 cannot be adopted in the ad hoc mode, it will not be considered hereafter.

3.2.1 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 3.2). 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; it is used to initialize the back-
off 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 3.2 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 the Contention Window. At the first
transmission attempt, CW = CWmin, and it is doubled at each retransmission up to CWmax.

3
To guarantee fair access to the shared medium, a station that has just transmitted a packet and has another pack-
et ready for transmission must perform the backoff procedure before initiating the second transmission.
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3.2 IEEE 802.11 ARCHITECTURE AND PROTOCOLS



FRAME
Station 1

FRAME
Station 2

FRAME
Station 3

DIFS DIFS
DIFS

Figure 3.2. Basic access mechanism.


In the standard, CWmin and CWmax values depend on the physical layer adopted. For exam-
ple, for the FHSS Phisical Layer CWmin and CWmax values are 16 and 1024, respectively
[15].
Obviously, it may happen that two or more stations start transmitting simultaneously
and a collision occurs. In the CSMA/CA scheme, stations are not able to detect a collision
by hearing their own transmissions (as in the CSMA/CD protocol used in wired LANs).
Therefore, an immediate positive acknowledgement scheme is employed to ascertain the
successful reception of a frame. Specifically, upon reception of a data frame, the destina-
tion station initiates the transmission of an acknowledgement frame (ACK) after a time
interval called the Short InterFrame Space (SIFS). The SIFS is shorter than the DIFS (see
Figure 3.3) in order to give priority to the receiving station over other possible stations
waiting for transmission. If the ACK is not received by the source station, the data frame
is presumed to have been lost, and a retransmission is scheduled. The ACK is not trans-
mitted if the received packet is corrupted. A Cyclic Redundancy Check (CRC) algorithm
is used for error detection.
After an erroneous frame is detected (due to collisions or transmission errors), a sta-
tion must remain idle for at least an Extended InterFrame Space (EIFS) interval before it
reactivates the backoff algorithm. Specifically, the EIFS shall be used by the DCF when-
ever the physical layer has indicated to the MAC that a frame transmission was begun that
did not result in the correct reception of a complete MAC frame with a correct FCS value.


Packet Arrival

FRAME
Source Station

ACK
Destination Station

DIFS SIFS

Figure 3.3. Interaction between the source and destination stations. The SIFS is shorter than the
DIFS.
74 IEEE 802.11 AD HOC NETWORKS: PROTOCOLS, PERFORMANCE, AND OPEN ISSUES


Reception of an error-free frame during the EIFS resynchronizes the station to the actual
busy/idle state of the medium, so the EIFS is terminated and normal medium access (us-
ing DIFS and, if necessary, backoff) continues following reception of that frame.

3.2.2 Common Problems in Wireless Ad Hoc Networks
In this section, we discuss some problems that can arise in wireless networks, mainly in
the ad hoc mode. The characteristics of the wireless medium make wireless networks fun-
damentally different from wired networks. Specifically, as indicated in [15]:

The wireless medium has neither absolute nor readily observable boundaries out-
side of which stations are known to be unable to receive network frames.
The channel is unprotected from outside signals.
The wireless medium is significantly less reliable than wired media.
The channel has time-varying and asymmetric propagation properties.

In a wireless (ad hoc) network that relies upon a carrier-sensing random-access proto-
col, like the IEEE 802.11 DCF protocol, the wireless medium characteristics generate
complex phenomena such as the hidden-station and exposed-station problems.
Figure 3.4 shows a typical “hidden-station” scenario. Let us assume that station B is in
the transmitting range of both A and C, but A and C cannot hear each other. Let us also as-
sume that A is transmitting to B. If C has a frame to be transmitted to B, according to the
DFC protocol, it senses the medium and finds it free because it is not able to hear A™s
transmissions. Therefore, it starts transmitting the frame but this transmission will result
in a collision at the destination Station B.
The hidden-station problem can be alleviated by extending the DCF basic mechanism
through a virtual carrier sensing mechanism (also referred to as a floor acquisition mech-
anism) that is based on two control frames: Request To Send (RTS) and Clear To Send
(CTS). According to this mechanism, before transmitting a data frame, the station sends a
short control frame, named RTS, to the receiving station announcing the upcoming frame
transmission (see Figure 3.5). Upon receiving the RTS frame, the destination station




Figure 3.4. The “hidden-station” problem.
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3.2 IEEE 802.11 ARCHITECTURE AND PROTOCOLS


DIFS SIFS

FRAME
RTS
Source Station

CTS ACK
Destin. Station
SIFS SIFS
Backoff Time
Another Station
NAV RTS
NAV CTS DIFS



Figure 3.5. Virtual career sensing mechanism.


replies by sending a CTS frame to indicate that it is ready to receive the data frame. Both
the RTS and CTS frames contain the total duration of the transmission, that is, the overall
time interval needed to transmit the data frame and the related ACK. This information can
be read by any listening station that uses this information to set up a timer called the Net-
work Allocation Vector (NAV). When the NAV timer is greater than zero, the station must
refrain from accessing the wireless medium. By using the RTS/CTS mechanism, stations
may become aware of transmissions from hidden stations and learn how long the channel
will be used for these transmissions.
Figure 3.6 depicts a typical scenario in which the “exposed-station” problem may oc-
cur. Let us assume that both Station A and Station C can hear transmissions from B, but

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