Adeel Akram, Shahbaz Pervez, Shoab A. Khan University of Engineering and Technology, Taxila, Pakistan.
Email: {adeel, shahbaz, shoab}@uettaxila.edu.pk Abstract—In this paper we present an optimal combination of
various key factors in CSMA(CD) that affect the performance of 802.11 ad hoc networks for outdoor long range communication.
These factor not only improve performance but also help in extending the possible range of connection.
Keywords; 802.11, Outdoor Communication, CSMA(CD), Multimedia over Ad hoc
I. INTRODUCTION
The 802.11 standard was originally designed to provide indoor communication. Its main focus was to provide low cost solution to small office SOHO LAN deployment with allowance of mobility for client nodes.
With the passage of time, the technology has matured and much work has been done on improvement in standard and removal of shortcomings of 802.11 Protocol.
Today’s work requirements especially in educational campus like setups emphasize on deployment of 802.11 based networks for Outdoor use. Students and faculty members can roam around the various buildings but still want to get connectivity with their Office/LAN network.
Outdoor deployment of 802.11 was limited by inherent problems in the design of the standard. In outdoor deployment, timeouts and retries were encountered frequently, which caused instability and poor reliability. Specifically, extending the range of 802.11devices with antennas and amplifiers has its limitations at the communications level.
As 802.11 medium access control is carried out by CSMA-CD, A device does not transmit when it senses any another devices transmitting on the channel. Occasionally, two or more devices may try to send packets at the same time. In order to prevent collision between simultaneous uses of the medium, “CTS”
(Clear-To-Send) is used to signal to one of the sender that the receiver is ready to receive.
In long range communication, when distances are extended between two points, the packets have to travel a longer distance. The longer distance leads to an increase in transit time and therefore the packets may not reach the other end within the timeout window.
For long-range applications using the 802.11 standard, CTS has to be increased to prevent timeouts.
During normal communication over 802.11 networks, “ACK”
(Acknowledgement) packets are sent from sender to receiver, and a time limit is set for obtaining a reply, failing which the sender assumes packet loss and resends. An ACK timeout of
20 μsec is defined for 802.11b and 9 μsec is defined for 802.11a/g standards by IEEE.
Under the 802.11 standards, packets are retransmitted if ACK is not received within the allowed timeout duration.
Continuous loss of ACK packets leads to network instability and poor reliability.
Furthermore collisions in the medium will cause the sender to wait a certain amount of time before retransmitting. This is known as the “slot-time”. The sender is informed of collision by other device on the network, and the time taken to do so is added to calculate the slot-time. In long-range applications, the slot time has to be increased in order to prevent further collisions due to timeouts.
Following are the key factors that inhibit the performance of 802.11devices:
• ACK timeout was too small to work correctly over long distance links.
• The contention window slot-time needed to be increased to adapt to the longer distances.
• CTS timeout values must be increased to allow longer distance communication
II. EXPERIMENTAL SCENARIO:
We deployed an 802.11b outdoor access point with 16db directional antenna at one end, while on the other end we used a Laptop with Atheros chipset and 802.11b compliant (HP IPAQ 6365) PDAs (Figure 1) with internal antennas to make an ad hoc network.
T. Sobh et al. (eds.), Innovative Algorithms and Techniques in Automation, Industrial Electronics and Telecommunications, 23–25.
© 2007 Springer.
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Figure1. A roof top outdoor access point with directional Antenna was setup at the office building to communicate with a variable distance ad hoc network. The setup equipment used Atheros Chipset that allows modification of key CSMA(CD) performance parameters to enhance the distance between the two communicating peers.
We ensured that the Laptop and the roof top access point have clear line of sight connectivity.
We increased the distance between the two communicating devices and varied the slot time, ACK timeout and CTS timeout values for best performance. We started with the values specified by the IEEE 802.11 standard. According to the standard, the default values of Slot time, ACK Time and CTS Timeout are 9, 18 and 18 μsec respectively. We increased the distance between the transmitter AP and the receiving laptop in increments of 20 meters. The values provided by the standard worked perfectly till the 90 meter mark after which the connectivity deteriorates significantly. We then started to increase the values of Slot Time, ACK Time and CTS Timeout gradually to find suitable combination for these values.
The following table shows these values according to distance variation (Table 1).
Distance (meters)
SlotTime (μsec)
ACKTime (μsec)
CTSTimeOut (μsec)
10 9 18 18
30 9 18 18
50 9 18 18
70 9 18 18
90 9 18 18
100 10 23 23
300 11 25 25
600 12 27 27
900 13 29 29
1200 14 31 31
1500 15 33 33
1800 16 35 35
2100 17 37 37
2400 18 39 39
802.11 CSMA(CD) Parameters variation with Distance
Table 1: Distance vs. 802.11 Parameter values We tested the connectivity as well as voice communication using “Teamtalk” software SDK. The software incorporates a configurable audio encoder that allows reduction of codec complexity for use on less resourceful devices and low bandwidth networks.
The following equations represent the relation of 802.11b Parameter values with the variation of distance.
SlotTime = 8.6802x6+0.0092x5-0.00003x4 ACKTime = 16.6438x6+0.0433x5-0.0001x4 CTSTimeOut = 16.6438x6+0.0433x5-0.0001x4
802.11 CSMA(CD) Parameters
0 5 10 15 20 25 30 35 40 45
10 30 50 70 90 100 300 500 900 1200 1500 1800 2100 2400 Distance (meters)
802.11 Timeout Parameters (μsec)
SlotTime (μsec) ACKTime (μsec) CTSTimeOut (μsec)
These values can be further fine tuned to further improve performance according to the environmental conditions. In our case, the values are appropriate for clear line of site communication without any foliage.
Wireless AP 2A
Variable Distance
3C .
2A
5 . 3C . 3C .
Remote Ad hoc Network
Voice Server
Office Building
AKRAM ET AL.
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To confirm our calculations, we setup a peer to peer ad hoc network at the remote side using the laptop and PDAs. In the office, we connected the outdoor access point to a Wireless router that connects other indoor wireless clients to it using the IEEE 802.11b standard. Using the table parameter values on the laptop, we used the PDAs on remote side to perform voice communication with the Voice Server, Laptop and the PDA in the office building.
III. CONCLUSION
This setup was done as a proof of concept; it would be very useful in connecting different ad hoc networks when the distance between them is too large for small devices to remain in range.
Multiple such setups providing cell like coverage in a particular area can also be used during relief work and military scenarios.
The same parameter values can be used to extend the range of peer to peer ad hoc networks provided the devices have high gain antennas installed on them.
IV. FUTURE WORK
The current setup didn’t utilize any QoS support from the network. We are planning to perform the same setup for video communication using QoS.
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