INVITED
COMMENTARY
One
of
the most
vibrant and active
“new“ fields today
is
that
of
ad hoc
networks. Within the
past few years the
field has seen
o
rapid expansion
of
visibility and work
due to the proliferation
of
inexpensive,
widely available
wireless devices and
the network
community’s interest
in mobile computing.
A
BRIEF OVERVIEW
OF
AD
Hoc
NETWORKS:
CHALLENGES
AND
DIRECTIONS
RAM
RAMANATHAN
AND
JASON
REDI
BBN
TECHNOLOGIES
ne of the most vibrant and active
“new” fields today is that of ad hoc
networks. Significant research in this
area has been ongoing for nearly 30
years, also under the names packet
radio
or
multi-hop networks. Within
the past few years, though, the field
0
has seen a rapid expansion
of
visibility
and work due to the proliferation of inexpensive,
widely available wireless devices and the network
community’s
interest in mobile computing.
An
ad hoc network is a (possibly mobile) col-
lection of communications devices (nodes) that
wish to communicate, but have no fixed infra-
structure available, and have no pre-determined
organization of available links. Individual nodes
are responsible for dynamically discovering which
other nodes they can directly communicate with.
A key assumption is that not all nodes can direct-
ly communicate with each other,
so
nodes are
required to relay packets on behalf of other
nodes in order to deliver data across the net-
work. A significant feature
of
ad hoc networks is
that rapid changes
in
connectivity
and
link
char-
acteristics are introduced due to node mobility
and power control practices. Ad hoc networks
can be built around any wireless technology,
including infrared and radio frequency (RF).
Ad hoc networking is a multi-layer problem.
The physical layer must adapt to rapid changes
in link characteristics. The multiple access con-
trol (MAC) layer needs to minimize collisions,
allow fair access, and semi-reliably transport
data over the shared wireless links in the pres-
ence of rapid changes and hidden or exposed
terminals. The network layer needs to determine
and distribute information used to calculate
paths in a way that maintains efficiency when
links change often and bandwidth is at a premi-
um. It’also needs to integrate smoothly with tra-
ditional, non ad hoc-aware internetworks and
perform functions such as auto-configuration in
this changing environment. The transport layer
must be able to handle delay and packet loss
statistics that are very different than wired net-
works. Finally, applications need to be designed
to handle frequent disconnection and reconnec-
tion with peer applications as well as widely
varying delay and packet loss characteristics.
Ad hoc networks are suited for use in situa-
tions where infrastructure is either not available,
not trusted, or should not be relied on in times
of emergency. A few examples include: military
solders in the field; sensors scattered throughout
a city for biological detection; an infrastructure-
less network of notebook computers in a confer-
ence or campus setting; the forestry or lumber
industry; rare animal tracking; space exploration;
undersea operations; and temporary offices such
as campaign headquarters.
HISTORY
The history of ad hoc networks can be traced
back to 1972 and the DoD-sponsored Packet
Radio Network (PRNET), which evolved into
the Survivable Adaptive Radio Networks
(SURAN) program in the early 1980s [l]. The
goal
of
these programs was to provide packet-
switched networking to mobile battlefield ele-
ments in an infrastructureless, hostile
environment (soldiers, tanks, aircraft, etc., form-
ing the nodes in the network).
The PRNET used a combination of ALOHA
and CSMA approaches for medium access, and
a kind of distance-vector routing.
SURAN
signif-
icantly
improved upon the radios (making them
smaller, cheaper, power-thrifty), scalability
of
algorithms, and resilience to electronic attacks.
The routing protocols were based on hierarchi-
cal link-state and were highly scalable.
In the early 1990s a spate
of
new develop-
ments signaled a new phase in ad hoc network-
ing. Notebook computers became popular,
as
did open-source software, and viable communi-
cations equipment based
on
RF and infrared.
The idea of an infrstructureless collection of
mobile hosts was proposed in two conference
papers [2,3], and the IEEE 802.11 subcommit-
tee adopted the term “ad hoc networks.” The
concept of commercial (non-military) ad hoc
networking had arrived. Other novel non-mili-
tary possibilities were suggested (as mentioned
in the introduction), and interest grew.
At around the same time, the DoD continued
from where it left off, funding programs such as
the Global Mobile Information Systems
(GloMo), and the Near-term Digital Radio
(NTDR). The goal of GloMo was to provide
office-environment Ethernet-type multimedia
connectivity anytime, anywhere, in handheld
devices. Channel access approaches were now in
the CSMA/CA and TDMA molds, and several
novel routing and topology control schemes were
developed. The NTDR used clustering and link-
state routing, and self-organized into a two-tier
20
IEEE
Communications Magazine
50th
Anniversary Commemorative Issue/May
2002
