The topology of a Mobile Ad
hoc Network (MANET) is inherently unstable, due to mobility of the nodes
that make up the network. However, several works have indicated that this
issue is actually complicated by the fact that the IEEE 802.11 Medium Access
Control (MAC) layer is poorly suited for use in these networks. Contending
nodes often interfere with each other, causing increased instability to the
network topology. These frequent changes in topology wreak havoc upon the
A large number of works exist
relating to routing for the MANET environment. Although each protocol has a
slightly different focus within the overall problem, the dynamic topology is
necessarily always central. Every protocol must find a way to deal with
managing the large number of changes in the network links, and therefore the
changes in the network paths. While working protocols have been developed,
they all suffer from one or more significant weaknesses, particularly delay
or overhead expense.
In this project, we are
working to draw benefit from the interactions of the MAC and routing in a
MANET. Rather than having the MAC negatively impact on the routing protocol,
we intend to exploit some of this closeness. We are developing an improved
MAC layer for ad hoc networks. The added facilities within the new MAC can
then be utilized in order to streamline the routing process.
A MANET is a collection of
mobile devices (nodes) equipped with transmitters and receivers for wireless
communication. These nodes can communicate with each other, however, they
are limited by their transmitting and receiving capabilities (and network
characteristics). Therefore, they cannot directly reach all of the nodes in
the network as most of the nodes are outside of direct range.
In order to overcome this,
the network operates in a multihop fashion. Nodes route traffic for each
other. Therefore, in a connected ad hoc network, a packet can travel from
any source to its destination either directly, or through some set of
intermediate packet forwarding nodes. However, the mobility of the nodes can
cause the network to change rapidly, resulting in a high volatility of the
paths between any two nodes. This is the central problem in MANET routing.
Despite the large volume of
work that has been published on routing protocols for ad hoc networks, no
outstanding solution has been found. These protocols can basically be
divided into three classifications: proactive (table-driven), on-demand
(reactive), or hybrid (a combination of the previous two). Proactive
protocols suffer from heavy overhead demands incurred by sending frequent
updates in order to maintain consistent routing tables under the highly
dynamic conditions. On-demand protocols often require a significant delay,
as a path is discovered only when a source decides it needs to send to a
particular destination. Hybrid protocols try to combine the features of each
type of protocol, in order to minimize the drawbacks.
This hybrid category is the
newest class of routing protocols for ad hoc networks. Likely the most
recognized protocol of this type is the Zone Routing Protocol (ZRP). ZRP
makes use of proactive routing in order to keep, maintain, and utilize the
paths to nearby nodes (nodes within the local zone). Due to their proximity,
the paths to these nodes can be kept relatively easily and inexpensively.
For more distant nodes, source-initiated on-demand routing is used. Although
this requires additional delay in order to discover the route, it eliminates
wasted routing effort to these nodes.
Our work also falls into the
hybrid category. It is dependent on several characteristics of an ad hoc
network. First, the extreme volatility of paths within the network means
that pre-building routes can be an expensive and wasted operation. Paths can
break even before the process of on-demand route discovery is completed, or
the usable lifespan may be very short. In addition, as longer paths include
a greater number of links, their average lifetime also tends to be shorter.
This means the added effort in building a longer path often needs to be
performed more often.
Second, because all of the
nodes in the network are capable of routing traffic, a number of different
paths are available between source and destination pairs. Some of these
paths may exist for a very short time, but some may be longer lived.
Unfortunately, it is difficult to predict which will be the longest
surviving paths at the time of route determination. However, being able to
make use of these change-resistant paths may have great advantage over the
more volatile paths.
Our approach has two key
features. First, it keeps network-wide routing information, but only
information that is less volatile than path information. It also does not
require the information to remain precisely consistent. Plus, it is
information that is often easily gathered, with minimal overhead. Second,
route determination, based on this information, is left until the last
possible moment, on a per-packet basis. This eliminates the possibility for
paths to become stale, as well as allowing packets to follow the
best-available path at the time they are being sent.
In order to allow this type
of routing to be performed in an efficient manner, we have realized that we
need some additional facilities at the MAC layer. We are attempting to alter
the 802.11 MAC, adding these features while maintaining compatibility with
the existing protocol. These additional capabilities basically permit the
routing protocol to select an available next hop at the time of packet
We are currently working on
simulating these ideas using the NS-2 simulator.