In last few years mobile communications have dramatically increased in popularity and usage. This growth has inspired a development of advanced communication protocols offering higher throughput and reliability over wireless links.
Much of wireless technology is based on the principle of direct point-to-point communication, where participating nodes “speak” directly to a centralized access point.
However, there is an alternative, “multi-hop” approach, where the nodes communicate to each other using other nodes as relays for traffic if the endpoint is out of direct communication range.
Mobile Ad hoc NETwork (MANET), described here, uses the multi-hop model.
Wikipedia describes MANET (Mobile Ad Hoc Network) as a continuously self-configuring, infrastructure-less network of mobile devices connected wirelessly.
All the nodes (devices) are wireless, mobile and equal (no access points, base stations, or any other kind of infrastructure).
The best comparison could be a cellular network WITHOUT base stations, where all the phones need to create multi-hop mesh network.
Infrastructure-based Network (i.e. Cellular):
Infrastructure-less Network (MANET – Mobile Ad hoc Network):
These networks are self-configuring and can be set up randomly and on-demand. Such networks can have dynamically changing multi-hop topologies, composed of, likely, bandwidth-constrained wireless links.
The concept of the mobile ad-hoc network suggests the incorporation of routing functionality into mobile nodes, in other words all nodes should be able to act as routers for each other.
Since an infrastructure-based network is always a better solution than an infrastructure-less network in the meaning of network performance, MANET is relevant only in cases when laying the infrastructure is impossible or is not practical:
- Natural disasters: for rescue forces
- Remote areas / difficult terrain, i.e. pit mines, tunnels, mountains, deserts, so on
- Military, paramilitary, rescue, anti-terror forces
- Others: Vehicular ad hoc networks, distributed sensor network, smartphones ad hoc network…
MANET – Layer-3 Routing Core
Ad-hoc networks are not restricted to special hardware or a certain link layer. MANET is a routing core (Layer-3 routing protocols) running on top of any possible Layer-2 wireless medium that is able to provide connectivity between the neighboring (1-hop) nodes:
It is important to note a difference between MANET routing and traditional IP routing. Routing in fixed networks is based on aggregation combined with best matching. When a packet is to be forwarded, the routing table is consulted and the packet is transmitted on the interface registered with a route containing the best match for the destination, i.e. all hosts within the same subnet are available on a single one-hop network segment via routers. However, in MANETs nodes route traffic by transmitting packets on the interface it has arrived from.
Aggregation is not required in MANETs, as all routing is host based and for all destinations within MANET, a sender has a specific route.
There are two principal approaches for route maintenance in MANET – reactive and proactive:
- Reactive routing protocols set up traffic routes on-demand (examples – Ad hoc On-demand Distance Vector, Dynamic Source Routing)
- Proactive routing protocols dynamically maintain a full understanding of the topology (examples – Optimized Link State Routing Protocol, Babel)
Northforge implemented the Optimized Link State Routing Protocol (OLSR). OLSR is an IP routing protocol optimized for mobile and wireless ad hoc networks. The protocol was integrated in a commercial routing stack suite. OLSR is documented in RFC3626 and uses the link-state scheme in an optimized manner to propagate topology information. The optimization is based on a technique called MultiPoint Relaying.
OLSR operation mainly consists of updating and maintaining information in routing tables. The data in these tables is based on received control traffic and the control traffic is generated based on information retrieved from the tables.
A general MANET network is illustrated below:
Key to Success: Network Simulation
Network simulation is designed for characterizing, creating and validating the communication solutions, computer networks and distributed or parallel systems. It enables the prediction of network behavior and performance. One can create, run and analyze any desired communication scenario.
Generally, a simulation is the only method that allows continuous development, testing and debugging of a network comprised of hundreds and thousands of mobile MANET nodes, since a standard lab won’t do, and field tests are expensive, difficult to operate and non-deterministic.
One of the challenges during the development was testing OLSR with various topologies, e.g. two nodes, three 1-hop neighbors or 2-hop neighbor. In order to validate correct behavior of OLSR, it was important to emulate the dynamic nature of MANET, where nodes can roam around, come up and come down. To address these challenges, we decided to deploy a virtualized test environment, based on Linux containers (LXC), thus enabling execution of multiple OS instances on a single x86 machine.
In addition, Network Simulation has a very important practical usage: it can be supplied as a Network Planning System add-on product for MANET core.
This application supplies the following abilities:
- Planning of node movements
- Showing the dynamic status of network topology and connectivity (map or canvas based)
- Operational planning based on communication conditions
- Planning of the communications infrastructure
- Verification & comparison of communication solutions
- “What if” analysis of real actual situation by changing the scenario
- Real terrain, realistic radio and propagation models
We learned a lot during this MANET project, specifically the ability to interpret and implement RFCs in MANET situations and how network simulation based on LXC is a key to success. We’ll be taking this knowledge to future MANET projects as mobile ad-hoc networks grow in use in this growing mobile network environment.
Authors: Oleg P. and Sasha I.