Daryl C. Robinson

Glenn Research Center, Cleveland, Ohio

CSMA Versus Prioritized CSMA for

Air-Traffic-Control Improvement

December 2001

NASA/TM—2001-211213
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Daryl C. Robinson

Glenn Research Center, Cleveland, Ohio

CSMA Versus Prioritized CSMA for

Air-Traffic-Control Improvement

NASA/TM—2001-211213

December 2001

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

2002 Aerospace Conference

sponsored by the Institute of Electrical and Electronics Engineers

Big Sky, Montana, March 9–16, 2002
Acknowledgments

I acknowledge Mr. Robert Kerczewski for recruiting me for this area of research and for funding it.

I also acknowledge the efforts of my summer interns, Proloy Bhattacharyya and Steve Drescher. Proloy built

the network used in these simulations according to my specifications. Steve researched and generated,

according to my specifications, all of the aircraft trajectories used in the simulations.

They wrote process models and C programs to validate their very good work, thereby

allowing me to concentrate on writing all of the process models used in the simulation.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

This report contains preliminary

findings, subject to revision as

analysis proceeds.

Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-211213 1

CSMA Versus Prioritized CSMA for Air-Traffic-Control Improvement

Daryl C. Robinson

National Aeronautics and Space Administration

Glenn Research Center

21000 Brookpark Road

Cleveland, Ohio 44135

Abstract—OPNET version 7.0 simulations are presented

involving an important application of the Aeronautical Telecommunications Network (ATN), Controller Pilot Data Link

Communications (CPDLC) over the Very High Frequency Data

Link Mode 2 (VDL-2). Communication is modeled for essentially all incoming and outgoing nonstop air-traffic for just

three United States cities: Cleveland, Cincinnati, and Detroit.

There are 32 airports in the simulation, 29 of which are either

sources or destinations for the air-traffic of the aforementioned

three airports. The simulation involves 111 Air Traffic Control (ATC) ground stations, and 1,235 equally equipped aircraft—taking off, flying realistic free-flight trajectories, and

landing in a 24-hr period. Collisionless, Prioritized Carrier

Sense Multiple Access (CSMA) is successfully tested and

compared with the traditional CSMA typically associated with

VDL-2. The performance measures include latency, throughput, and packet loss. As expected, Prioritized CSMA is much

quicker and more efficient than traditional CSMA. These

simulation results show the potency of Prioritized CSMA for

implementing low latency, high throughput and efficient

connectivity.

1. INTRODUCTION

Due to a lack of surveillance and communications coverage,

in many parts of the world, aircraft are forced to fly routes and

maintain separations that are inefficient from both a fuel and

scheduling perspective. The total loss to airlines due to these

inefficiencies is measured in billions of dollars [1]. The problem is expected to rapidly mushroom given the expected user

demand for scheduled air service. The Advanced Air Transportation Technologies (AATT) Program has been instituted

to develop new technologies that enable free-flight, an operating system in which pilots have the freedom to select their

path and speed in real-time [2].

To implement free-flight, CPDLC is viewed as very important for the new aeronautical communications infrastructure.

CPDLC will eliminate voice-only communications.

In the simulations of this paper, realistic ground-to-air and

air-to-ground communications are achieved by assuming an

effective, intact terrestrial network and by treating planes as

traffic generators and sinks, in a manner analogous to the transparent usage of a traffic injector or “sniffer” in a network.

Further, the idea of Prioritized CSMA is reintroduced and successfully tested through simulation. Prioritized CSMA trades

off the use of an additional radio frequency in order to implement efficient CSMA without collisions. The benefit gained

of efficient, collision-less CSMA is that the inefficiencies

introduced by wasted time division multiple access (TDMA)

time slots may be avoided.

2. SIMULATION FOCUS

The primary focus of the simulations is to examine the behavior of ATC communications over VDL-2 in an aviation

scenario involving a substantial amount of air and communications traffic. Both weather and terrain were ignored, and the

simulation assumes a spherical earth. Indirect communication

is not implemented in this “OPNET” (network simulation software tool) simulation so two nodes may communicate only

when they are in direct line-of-sight. So extending the range

of ground stations by bouncing signals off of the ionosphere

is not permitted here. All incoming and outgoing nonstop air

traffic for three cities was simulated. Given the time constraints

for this research and the scope of this simulation, it was not

desirable to simulate the communications architecture for the

entire OSI stack. Since the media access control layer (MAC)

layer is especially important in broadcast media, largely

determining the limit of performance, heavy emphasis was

placed upon the data link layer, VDL-2. So these simulations

do not model the presentation, session, transport, or network

layers, as it was of most interest to simulate the VDL-2

data link layer, which is being deployed. Perhaps the most

important use of these simulations is to test Prioritized CSMA.
NASA/TM—2001-211213 2

3. SIMULATION OVERVIEW

As previously stated, the simulation involves 1,235 flights,

111 ATC transceivers or ground stations, and 32 airports. The

take off, arrival, and flight times for one day were based on

real flight plans obtained from the airports. Instead of actually

modeling the fact that one plane may make several flights, a

separate OPNET mobile airplane node is used for each flight.

For reasons discussed later, CPDLC messages in these simulations have a 5,000 bit mean file size. CPDLC file sizes are

chosen according to the normal distribution. CPDLC messages

have a variance of 2500. They have a mean interarrival time of

6 min, using the exponential distribution. All CPDLC transceivers operate at 136 MHz with a 10 KHz bandwidth.

Message Length

Although 5,000 bit message lengths are somewhat excessive

for CPDLC messages, they were chosen so that the effects of

message collisions could be better studied given the lower

amount of aeronautical communications traffic present in these

presently bounded simulations.

Ground Stations

It was not intended to perfectly replicate the National Aerospace System (NAS) in these simulations, but to provide a

data communications environment in the simulation similar

to that in the NAS. Consequently we did not require an exact

distribution of ground stations. Instead, for research purposes,

Figure 1: 32 airports (top) and 111 ground stations.
NASA/TM—2001-211213 3

we distributed them uniformly throughout the United States.

A 100 m ground station may maintain direct line-of-sight communication with an airplane having an average altitude of

3.43 mi. for about 300 km. So we used an average spacing of

290 km between adjacent ground stations. According to our

calculations, this spacing should be sufficient to ensure continuous air to ground and ground to air communications. The

ATC tower at Hopkins is 199 ft = 60.93 m in height. The simulation approximates the altitude of typical VDL ground stations

as half that value, 30.47 m. There are 111 ground stations in

the simulation. Additionally, there is an air traffic control tower

at each of the 32 airports. Figure 1 shows a view of the 32

airports and 111 ground stations involved in the simulation.

The ground stations are capable of detecting the presence of a

plane and only send CPDLC messages if there is a plane within

its 290 km airspace to receive them. Due to the functioning of

Prioritized CSMA, the ground stations are coordinated and

produce no uplink interference.

Details

Each airport is initially stocked with many planes, which will

take off for one of the remaining 31 airports during the course

of the 24 hr simulation. Again, all simulated flights are nonstop. Each ground station, including air traffic control towers,

consists of a CPDLC transceiver. Each airplane has identical

communications architecture. CPDLC exists only between

aircraft and ground stations. The CPDLC transmission node

architecture is shown in Fig. 2.

In Fig. 2, “gen” is a clocked generator of packets. “q_1” is a

queue to buffer the packets. “p_0” is a processor module, which

decides whether to leave the packets in the queue or to forward them on to the radio transmitter through pt_0.

Airline officials provided us with typical flight altitudes as a

function of distance traveled for various ranges. A typical plot

of a trajectory profile is shown in Fig. 3.

Cruise altitudes used in the simulation depend on the range of

the flight. The histogram, in Fig. 4, of the number of planes in

flight, as a function of simulation time in minutes is based on

the actual data from the airports and is not an output of simulation. This histogram can be used to understand traffic loading in the simulation. Air traffic begins 1 hr 10 min into the

simulation and continues throughout the 24 hr simulation.

pt– p 0 – q 0 – gen Count 1

rr–1

Figure 2: CPDLC node architecture.

0 100

30 000

35 000

25 000

20 000

15 000

10 000

5000

0

200 300 400 500

Figure 3: Flight trajectory profile: Cleveland to Albany.

Altitude (ft) vs. time (min).
NASA/TM—2001-211213 4

From the airport data, the average number of planes flying

is 90.8. So the expected number of CPDLC messages originating from aircraft is 90.8 (1,440/6) ≈ 21,792, where 1,440 is

the number of minutes in a day. Additionally, each of these

90.8 aircraft is in view of approximately 2.25 unique ground

stations, bringing the total number of expected CPDLC

messages to 2.25 (21,792) = 49,032, which is almost exactly

what is observed later in the presented results. The peak

traffic is at (60 s/min) (910 min) = 54,600 s or 3:10 p.m.

4. CSMA DISCUSSION

A single communications frequency is used for radio frequency

conservation. Just as in CB radio, one party communicates at

a time. But as east coast truckers may talk to their east coast

neighbors while west coast truckers may simultaneously talk

to their west coast neighbors—on the same frequency as their

east coast counterparts—without interference, so in the simulations here, different line-of-sight groups can communicate

on the same frequency simultaneously without interference.

CSMA is contention-based. All parties listen to the channel.

When the channel is free, many parties contend for it until

after a random back-off time. Eventually, one party gains control of the channel for uninterrupted usage. Because of the

contention process, collisions can be inefficient.

5. PRIORITIZED CSMA

In Prioritized CSMA, each communications party is assigned

a priority for transmission, based on its need to transmit. In

these simulations, transmission priority is effectively granted

on a first come, first serve basis. Effectively, if the medium is

busy, each transmitter receives a waiting ticket; when its

number comes up, the transmitter takes its turn. When the channel is free, instead of a random back-off time elapsing before

one node gains usage of the channel, in Prioritized CSMA, the

node with the next higher priority begins uninterrupted transmission immediately in an orderly fashion, without contention.

By choosing to study Prioritized CSMA, we simultaneously

accomplish two purposes. We can test this new idea and also

obtain the upper bound for performance of VDL-2 with the

given traffic of the simulation. Because of its random backoff time, VDL-2 should not perform as well as Prioritized

CSMA.

Details

It is assumed that in a real implementation of the idea of

Prioritized CSMA, both planes and ground stations include a

connection transmission (cnctrans) transmitter. Much like an

Automatic Dependent Surveillance-Broadcast Mode

(ADS-B) transmitter, this transmitter would broadcast cnctrans

packets at regular intervals. The cnctrans packets are nearly

length zero and contain the unique source identification code

(srcid) of the transmitting node. They also contain a time stamp

and the transmission time remaining, ending, or beginning of

that node. When a node receives a cnctrans packet, it updates

an array of cnctrans information from its neighbors. If a

cnctrans packet has not been received from a node in ∆t, it is

assumed unreachable. When a node seizes the channel, all

nodes wait until it is finished. Each node waits until the farthest neighbor of the last transmitting node has received the

transmission. When the transmission is finished, the next node

begins orderly transmission. The cnctrans packets do not

collide since they are small and each node is assigned a unique

phase lag with which to broadcast them.

200

150

100

50

0 250 500 750 1750 1000 1250 1500

0

Figure 4: Number of planes aloft vs. time (min).
NASA/TM—2001-211213 5

6. SIMULATION RESULTS AND ANALYSIS

Results

There were four simulation runs, which used either no CSMA

(no access scheme—transmit at will) or Prioritized CSMA,

and which either used transceivers and queues with data rates

and service rates of 31.5 Kbps or 1.544 Mbps:

I: no CSMA, 31.5 Kbps, [3]

II: no CSMA, 1.544 Mbps

III: Prioritized CSMA, 31.5 Kbps.

IV: Prioritized CSMA, 1.544 Mbps

The number of CPDLC messages received out of those transmitted for each of the Runs I to IV is respectively

(43,673/49,904), (45,267/49,334), (49,926/49,931), and

(49,759/49,762). Plots of CPDLC transmitted and received

packets for Runs I to IV are shown in Figs. 5 to 8. Included in

those figures are plots of end-to-end delays for each run.

Only the runs using Prioritized CSMA successfully transmitted all CPDLC packets with negligible packet loss. These

results show that this implementation of the idea of prioritized, collision-less CSMA works. Moreover, a comparison

between the performance latencies in these simulations and

the 95th percentile end-to-end delay requirement of 3 sec [4]

shows that Prioritized CSMA is remarkably quick and

efficient.

0.0

0.1

0.2

0.3

0.4

0

10 000

20 000

30 000

50 000

0

10 000

20 000

30 000

50 000

40 000

0 4 8 12

h

Figure 5: CPDLC packet reception and delay, I.

16 20 24

40 000

CPDLC trans pkts

CPDLC received pkts

Traffic sink 1. CPDLC end-to-end delay (seconds)

0.000

0.002

0.004

0.006

0.010

0

10 000

20 000

30 000

50 000

0

10 000

20 000

30 000

50 000 CPDLC trans pkts

CPDLC received pkts

Traffic sink 1. CPDLC end-to-end delay (seconds)

40 000

0 4

h

8 12

Figure 6: CPDLC packet reception and delay, II.

16 20 24

0.008

40 000

0.0

0.1

0.2

0.4

0

10 000

20 000

30 000

50 000

0

10 000

20 000

30 000

50 000

40 000

0 4

h

8 12

Figure 7: CPDLC packet reception and delay, III.

16 20 24

0.3

40 000

CPDLC trans pkts

CPDLC received pkts

Traffic sink 1. CPDLC end-to-end delay (seconds)

0.000

0.002

0.008

0.010

0

10 000

20 000

30 000

50 000

40 000

0

10 000

20 000

30 000

50 000 CPDLC trans pkts

CPDLC received pkts

Traffic sink 1. CPDLC end-to-end delay (seconds)

40 000

0 4

h

8 12

Figure 8: CPDLC packet reception and delay, IV.

16 20 24

0.004

0.006
NASA/TM—2001-211213 6

The numbers displayed above for Run III and IV show that

there are between 3–5 packets lost. We have strong reason to

believe that the loss is not due to a faulty implementation of

Prioritized CSMA. We have determined that this is, instead,

an error with the custom-built line-of-sight closure model that

we used. We have observed in other runs that the packets that

were lost, in both runs, did not reach any receiver in the entire

network. These losses could be the result of our closure model

determining that line-of-sight exists while OPNET’s closure

model determines otherwise. Nevertheless, we assert that our

closure model is highly accurate, but slightly imperfect, and

is inspired by OPNET code. We also note that these aberrant

losses are rare.

7. CONCLUSIONS

One thing is obvious from a comparison of Runs I and II with

III and IV: Prioritized CSMA works. Prioritized CSMA would

serve the same purpose for aeronautical communications

traffic as the traffic light does for automobile traffic—to

prevent collisions.

In the event that it is critical to receive messages without many

retransmissions or with minimum latency, Prioritized CSMA

may be very useful. Acknowledgments and retransmissions

increase the amount of traffic, increasing the number of collisions and worsening communications throughput.

Forecasts suggest that air traffic will triple over the next

20 years. Simulation studies have been performed that show

that there is an upper limit to the number of aircraft that may

be supported using VDL-2, i.e., traditional CSMA [4]. The

limitation exists because of the inherent inefficiencies present

in contentious, disorderly CSMA. Plans are underway to

replace VDL-2 (which has barely been deployed) as the

national aviation data link scheme with VDL-3, referred to as

NEXCOM, based on time division multiple access (TDMA).

This transition may be most expensive and somewhat sudden.

However, small add-on modules could be manufactured to mate

with existing VDL-2 radios to implement Prioritized CSMA,

thereby extending the lifetime of VDL-2.

It appears as though this simulation method could be used to

obtain an upper limit for the performance of CSMA or as justification for further research into the use of Prioritized CSMA.

Plans are underway to improve this simulation and to use it as

the basis for future research.

The simulation of communication was effected without the

complexity involved in the aeronautical telecommunications

network. It is desirable to identify communications systems

that work and can be proven through simulation. Presently,

there is not that much simulation research supporting the

envisioned ATN. In this research, continuous communication

was achieved in a realistic aviation scenario. It is difficult to

even begin to convincingly do this for communications based

on the ATN stack.

REFERENCES

[1] Final Report of RTCA Task Force 3 on Free Flight Implementation, October 26, 1995.

[2] Tekla S. Perry, “In Search of the Future of Air Traffic

Control,” IEEE Spectrum, August 1997.

[3] Daryl C. Robinson, “Air Traffic Control Improvement

Using Prioritized CSMA,” IEEE Aerospace Conference,

January 2000.

[4] Brian T. Hung, “Modeling and Simulation of an

Aeronautical CSMA Subnetwork,” OPNETWORK 2001,

August 2001.

Daryl C. Robinson received his BS

in Mathematics from Case Western

Reserve University (CWRU), MSEE

from Cleveland State University, and

is pursuing his doctorate at CWRU.

Currently he is pursuing research for

the AATT program. He has expertise

in mathematical modeling and simulation and in his nonwork hours

enjoys reading and writing thoughtful literature.
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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E–13065

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12

CSMA Versus Prioritized CSMA for Air-Traffic-Control Improvement

Daryl C. Robinson

CSMA; Prioritized CSMA; VDL-2; CPDLC; Air traffic control

Unclassified -Unlimited

Subject Categories: 04 and 32 Distribution: Nonstandard

Prepared for the 2002 Aerospace Conference sponsored by the Institute of Electrical and Electronics Engineers, Big

Sky, Montana, March 9–16, 2002. Responsible person, Daryl C. Robinson, organization code 5610, 216–433–3553.

OPNET version 7.0 simulations are presented involving an important application of the Aeronautical Telecommunications

Network (ATN), Controller Pilot Data Link Communications (CPDLC) over the Very High Frequency Data Link Mode 2

(VDL-2). Communication is modeled for essentially all incoming and outgoing nonstop air-traffic for just three United

States cities: Cleveland, Cincinnati, and Detroit. There are 32 airports in the simulation, 29 of which are either sources or

destinations for the air-traffic of the aforementioned three airports. The simulation involves 111 Air Traffic Control (ATC)

ground stations, and 1,235 equally equipped aircraft—taking off, flying realistic free-flight trajectories, and landing in a

24-hr period. Collisionless, Prioritized Carrier Sense Multiple Access (CSMA) is successfully tested and compared with

the traditional CSMA typically associated with VDL-2. The performance measures include latency, throughput, and

packet loss. As expected, Prioritized CSMA is much quicker and more efficient than traditional CSMA. These simulation

results show the potency of Prioritized CSMA for implementing low latency, high throughput and efficient connectivity.