TAG is a new geographic routing protocol that advances the state of the art by (1) supporting topologies that are not unit disk graphs, (2) using local (potentially unreliable) topology information to make better
geographic forwarding decisions, and (3) fully supporting dynamic topologies without the risk of forming routing loops. In this chapter, the routing protocol was detailed, and then simulated comparisons of TAG in a theoretical network and a real-world large-scale mobile airborne network were presented. TAG outperforms GOAFR+, GFG, and OLSR in the simulations.
CHAPTER 5: AIRBORNE NETWORK SIMULATIONS AND RESULTS
Having now examined the topology management framework and topology aware geographic routing in the previous chapters, this chapter presents the results of a series of simulations that demonstrate the capabilities of these protocols working together to manage and route traffic through the airborne network. In addition, this chapter gives an overview of the implementation of the simulation models developed in the ns-3 simulator to simulate these airborne networks and the supporting protocols.
5.1 Simulation Environment
The simulations described in Chapter 4, and those to be discussed in this chapter, use the ns-3 network simulator. Ns-3 is an open-source discrete event network simulation tool used for education and research. Several new ns-3 models had to be designed and implemented to support the simulation of airborne net- works. In this section, the various components added to ns-3 to support our airborne network simulations are described.
5.1.1 Aircraft Mobility
Most other mobile networking simulations use some sort of mobility model to determine the movements of nodes in a simulation. Rather than use such a model to simulate aircraft mobility, the simulations in this work use previously collected flight path data to model the movements of aircraft in the network. This makes the simulations all the more realistic but also requires that on solve all the issues accompanying the use of real-world data. To accommodate incorporating this data into our simulations a new ns-3 model was created.
As mentioned in Section 2.1.2.2, the Federal Aviation Administration (FAA) makes available to some industry and research partners, aircraft position information for most of the aircraft in the United States National Airspace System (NAS). This data, called Aircraft Situation Display to Industry (ASDI) [Vol00], includes not only the position and velocity of each aircraft but also other information such as the flight call sign and flight plan information. The simulations in this work make use of the ASDI data recorded during the month of July 2015. The ASDI data set includes position updates about once per minute for each aircraft,
though more precision is available in real-time. Since the time resolution required for networking is much finer (milliseconds or less), linear interpolation is used to obtain the expected positions of aircraft between position samples in the data set.
The Tracktable open-source library [RW15] is used in a new aircraft mobility model to read and parse the ASDI data files. These files contain a list of trajectories each of which is made up of a list of potentially many trajectory points. Each trajectory point contains at least an aircraft’s call sign, a time stamp (including the date), and the geodetic position of the aircraft (latitude, longitude, altitude) at the given time. This geodetic position is also translated into a Cartesian coordinate system called Earth-centered Earth-fixed (ECEF).
The ECEF coordinate system has its origin (0,0,0) at the center of the earth’s mass and is fixed to the earth (rotating with the earth as it spins). Calculating and storing the ECEF positions allows for quickly determining the distance between two aircraft, using normal Cartesian methods. When determining the distance between nodes in the simulation, the ECEF positions are used, but when computing the topology, the latitude and longitude values are used.
The mobility model uses ns-3’s existing constant velocity mobility model to schedule the mobility of the nodes. It first sets appropriate initial positions for each aircraft. Then, as a pre-processing step before a simulation begins, the mobility model schedules velocity changes at the appropriate times. Ns-3’s constant velocity mobility model handles efficiently determining the interpolated node positions as the simulation runs. To accurately determine what velocity to set when the aircraft arrives at waypoint n, waypointn+1 is used. The velocity (including both speed and direction) required to travel from waypointn and reach waypointn+1 at exactly the time associated with waypointn+1 is determined. An event is then scheduled at the time associated with the arrival at waypointnthat updates the node’s velocity to the calculated value. The velocities are computed and scheduled in this way for each waypoint of each flight. The aircraft mo- biltiy model also handles the extra processing required to accommodate nodes that enter and/or leave the simulation.
5.1.2 Wireless Point-to-Point Channel
Ns-3 has built-in support for wired point-to-point links and for various wireless link types, but not necessarily for directional wireless data links. These links have some attributes of wireless links. For example, they may communicate with any other node within range (assuming they are pointed correctly).
However, they also share attributes with point-to-point connections in that network devices (links) are paired and communication is generally limited such than only paired network devices may exchange data. What is needed is a wireless point-to-point link type combining some of the characteristics of both point-to-point links and wireless links.
For this work, a new model was added to ns-3 to simulate the unique characteristics of these directional wireless connections. Unlike omnidirectional channels that send packets to all the Network Devices attached to the channel, this new channel model need only send a packet to the Network Device whose antenna is connected with, and pointing at, this device. To accomplish this, the channel stores a connection map that indicates which Network Devices are connected at the current time.
Figure 5.1: Diagram showing the relationship of the components that enable simulating wireless point to point links.
Each time a packet enters the channel it determines which network device, if any, is connected with (pointing at) the source Network Device. The receipt of the packet is then scheduled on the destination network device at some future time, where the future time accounts for the transmission time and the prop- agation delay. Figure 5.1 shows a diagram of how these components work together. Many wireless point- to-point network devices can be connected to a single wireless point-to-point channel. When two network devices are connected (as net devices A and B or C and D are in the figure), packets sent from one device arrive at the other, and vice versa. This mapping can change over time as commanded by the topology management application (described shortly), with network devices disconnecting and re-connecting in the channel as the network evolves. Packets sent by network devices that are not connected to another network device are dropped. For these simulations, the topology management application explicitly controls the con-
nection map in the wireless point-to-point channel. A more realistic model could be employed in the future where the current antenna pointing direction could be taken into account, and packets could be dropped unless antennas are correctly aligned.
5.1.3 Wireless Point-to-point Network Device
The wireless point-to-point network device model is much the same as ns-3’s traditional point-to-point network device except that the wireless point-to-point protocol header contains two extra fields. These fields are the sending and receiving media access control (MAC) addresses. When sending a packet the network device enqueues the packet in the transmit queue. Then, if there are no other packets in the queue, the network device begins simulating the transmission of the packet, sending it to the channel for distribution to the connected network device, if any. When receiving a packet the network device determines whether the packet was corrupted, given the error model (bit error rate). If the packet was corrupted it is discarded, and otherwise, it is forwarded to the next level in the stack.
5.1.4 Topology Management Application
The topology management application is run on each node to control the topology of the network in a distributed manner. Each node periodically, but in sync with other nodes, generates an opinion topology and points its links accordingly, as described in Chapter 3.
5.1.5 Steerable Directional Antenna Model
Steerable directional antennas often employ a tracking system to automatically adjust the pointing of the antennas once connected. These simulations assume the existence of such a tracking system. Upon creation of a connection between two nodes, where the connection did not exist in the previous iteration, each node explicitly informs the channel of its intention to connect to the remote node. Once both nodes have connected, the connection table in the channel is updated to reflect the connection, and allow packets to flow.