Ad-hoc networks also do not assign any static routing role to any node in the network. In
contrast to TCP/IP where distinct nodes are tasked with being routers and gateways, ad-
hoc networks require that all nodes be minimally capable of performing some routing
function on demand. Typically the minimal roles of an ad-hoc network node are to
forward data packets intended for some other node and to pass addressing information
throughout the network.
There exists considerable discussion concerning ad-hoc networking over wireless media,
but the main focus of these concern relatively high-performance "line of sight" networks.
These types of networks are fast, typically able to support network speeds of 16 kilobits
per second and higher. “Line of sight” networks normally have low latency, which
allows them to implement channel access using channel partitioning methods such as
TDMA (Time Division Multiple Access) [TDMA] or CDMA (Code Division Multiple
Access) [CDMA]. These have the advantage of providing constant bit-rate channels to
each network participant. However, these mechanisms depend on the available channel
bandwidth to be sufficiently large to divide it among all network participants. Relatively
low-bandwidth, low-speed, high-latency channels such as 3 kilohertz HF radio channels
are not given the same discussion priority as high-bandwidth, high-speed, low-latency
HF radio channels possess certain properties that are conducive to ad-hoc network
operation. These radio waves propagate extremely well in two conditions. HF radio
deployments at sea exhibit “ground wave” propagation in which the HF radio waves
follow the surface of the water over the horizon while maintaining consistently high
signal quality. In this manner, ships within a fleet can communicate effectively without
any infrastructure. Additionally, HF radio waves exhibit “sky wave” propagation in
which the waves bounce off the Earth's ionosphere and surface at obtuse angles. “Sky
wave” propagation allows land or sea based deployments to transmit over thousands of
miles. However, “sky wave” channels are of much lower quality than “ground wave”
channels and are subject to greater interference due to changing atmospheric conditions
and solar activity.
Data communication across an HF radio channel is accomplished by means of an HF
modem. HF modems can implement several distinct waveforms. The more advanced
waveforms, such as MIL-STD-188-110B, allow bit rates up to 12800 bits per second for
certain environments. Additionally, the waveforms can provide data interleaving, which
assists in error correction but adds up to 9.6 seconds to every transmission. Due to the
speed limitations and latency issues, protocols that are optimized for HF traffic exhibit
relatively few long transmissions that contain a low overhead-to-data ratio. Similarly,
media access control must avoid extraneous transmissions.
The Dynamic Source Routing (DSR) algorithm described by Johnson and Maltz
[Johnson+, 2001] provides some features of ad-hoc network routing that scale well to
operation over HF radio networks. DSR primarily sends network control information on
demand. Nodes do not initiate route discovery to build up routes that may not be used.
This is appropriate in HF networks since bandwidth is extremely limited. In addition,