Ad Hoc Mode
By Jim Geier
Most installed wireless LANs today utilize "infrastructure" mode
that requires the use of one or more access points. With this configuration,
the access point provides an interface to a distribution system
(e.g., Ethernet), which enables wireless users to utilize corporate
servers and Internet applications.
an optional feature, however, the 802.11 standard specifies "ad
hoc" mode, which allows the radio network interface card (NIC) to
operate in what the standard refers to as an independent basic service
set (IBSS) network configuration. With an IBSS, there are no access
points. User devices communicate directly with each other in a peer-to-peer
hoc applications provide truly wireless solutions
hoc mode allows users to spontaneously form a wireless LAN. For
example, a group of people with 802.11-equipped laptops may gather
for a business meeting at their corporate headquarters. In order
to share documents such as presentation charts and spreadsheets,
they could easily switch their NICs to ad hoc mode to form a small
wireless LAN within their meeting room. Another example is when
you and your associates are waiting for a flight at the airport,
and you need to share a relatively large PDF file. Through ad hoc
mode, you can easily transfer the file from one laptop to another.
With any of these applications, there's no need to install an access
point and run cables.
ad hoc form of communications is especially useful in public-safety
and search-and-rescue applications. Medical teams require fast,
effective communications when they rush to a disaster to treat victims.
They can't afford the time to run cabling and install networking
hardware. The medical team can utilize 802.11 radio NICs in their
laptops and PDAs and enable broadband wireless data communications
as soon as they arrive on the scene.
product vendors are beginning to base their solutions on ad hoc
mode. As an example, MeshNetworks offers a wireless broadband network
system based on 802.11 ad hoc mode and a patented peer-to-peer routing
technology. This results in a wireless mesh topology where mobile
devices provide the routing mechanisms in order to extend the range
of the system. For example, a user on one side of the building can
send a packet destined to another user on the far side of the facility,
well beyond the point-to-point range of 802.11, by having the signal
hope from client device to client device until it gets to its destination.
This can extend the range of the wireless LAN from hundreds of feet
to miles, depending on the concentration of wireless users.
and cons to consider
making the decision to use ad hoc mode, you should consider the
savings. Without the need to purchase or install access
points, you'll save a considerable amount of money when deploying
ad hoc wireless LANs. Of course this makes the bean counters
happy, but be sure you think about all of the pros and cons
before making a final decision on which way to go.
setup time. Ad hoc mode only requires the installation of
radio NICs in the user devices. As a result, the time to setup
the wireless LAN is much less than installing an infrastructure
wireless LAN. Obviously this time savings only applies if the
facility you plan to support wireless LAN connectivity doesn't
already have a wireless LAN installed.
performance possible. The question of performance with ad
hoc mode is certainly debatable. For example, performance can
be higher with ad hoc mode because of no need for packets to
travel through an access point. This assumes a relatively small
number of users, however. If you have lots of users, then you'll
likely have better performance by using multiple access points
to separate users onto non-overlapping channels to reduce medium
access contention and collisions. Also because of a need for
sleeping stations to wake up during each beacon interval, performance
can be lower with ad hoc mode due to additional packet transmissions
if you implement power management.
network access. Because there is no distribution system
with ad hoc wireless LANs, users don't have effective access
to the Internet and other wired network services. Of course
you could setup a PC with a radio NIC and configure the PC with
a shared connection to the Internet. This won't satisfy a larger
group of users very well, though. As a result, ad hoc is not
a good way to go for larger enterprise wireless LANs where there's
a strong need to access applications and servers on a wired
network management. Network management becomes a headache
with ad hoc networks because of the fluidity of the network topology
and lack of a centralized device. Without an access point, network
managers can't easily monitor performance, perform security audits,
etc. Effective network management with ad hoc wireless LANs requires
network management at the user device level, which requires a
significant amount of overhead packet transmission over the wireless
LAN. This again leans ad hoc mode away from larger, enterprise
wireless LAN applications.
hoc operates differently
of the 802.11 standard defines a common operation whether you're
using ad hoc or infrastructure mode. The use of ad hoc mode only
affects the protocols, so there is no impact on the Physical Layers
(i.e., 802.11a and 802.11b). Within the MAC Layer, all of the carrier
sensing and most of the frame types and corresponding usage are
the same regardless of which mode you choose. The absence of an
access point, however, means that an ad hoc wireless LAN must take
on more of the MAC Layer responsibilities.
first ad hoc station (radio NIC) active establishes an IBSS and
starts sending beacons, which are needed to maintain synchronization
among the stations. (With infrastructure mode, only the access point
sends beacons.) Other ad hoc stations can join the network after
receiving a beacon and accepting the IBSS parameters (e.g., beacon
interval) found in the beacon frame.
stations that join the ad hoc network must send a beacon periodically
if it doesn't hear a beacon from another station within a very short
random delay period after the beacon is supposed to be sent. The
random delay minimizes the transmission of beacons from multiple
stations by effectively reducing the number of stations that will
send a beacon. If a station doesn't hear a beacon within the random
delay period, then the station assumes that no other stations are
active and a beacon needs to be sent.
receiving a beacon, each station updates their local internal clock
with the timestamp found in the beacon frame, assuming the timestamp
value is greater than the local clock. This ensures that the all
stations are able to perform operations, such as beacon transmissions
and power management functions, at the same time.
with infrastructure networks, an ad hoc sleeping station (i.e.,
power management "on") indicates that they're entering sleep state
by setting the power management bit in the control field of any
frame. All other stations learn of this by monitoring the frame
control fields of all frames. Stations will then hold off transmitting
to the sleeping station and buffer the corresponding packets locally.
all sleeping stations wake up at the same time during the announcement
traffic indication map (ATIM) window, which corresponds with each
beacon transmission. If a station is holding packets for a sleeping
destination, the station will send an ATIM frame to the sleeping
station indicating that packets are awaiting transmission. The station
that had been asleep then knows to stay awake through the next beacon
interval, which is hopefully long enough for the station buffering
the packet to send it successfully. After receiving and acknowledging
reception of the packet, the station can go back to sleep.
you can see, ad hoc mode offers enough advantages to consider when
deploying wireless LANs. The thought of saving the cost on access
points is certainly a compelling reason to strongly consider this
configuration. Unless you implement routing among the wireless users,
however, you'll find that ad hoc mode mostly applies to smaller,
spontaneous networks when there isn't a strong need for interfacing
with a wired network.
Geier provides independent consulting services to companies developing
and deploying wireless network solutions. He is the author of the
book, Wireless LANs (SAMs, 2001), and regularly instructs workshops
on wireless LANs. Join Jim for discussions as he answers questions
in the 802.11 Planet Forums.
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