Abstract—With an alarmingly low teledensity in South Africa, just 12%, and not much hope of further wired infrastructure at the local loop level, as the costs incurred are high compared to potential revenue, wireless connectivity could be a great asset and service in South Africa. However, the use of unlicensed spectrum in building wireless networks can be comparable to
“The Tragedy of the Commons”, the result of selfish behaviour towards common and limited resources. This paper evaluates the use of 802.11 wireless technologies in building a broadband wireless network and the effects of high amounts of interference on such a network. The paper concludes that for urban areas 802.11 technologies using unlicensed spectrum is not advisable, unless used in point-to-point links, while its use in rapid rural development (where there is less interference) is very promising.*
I. INTRODUCTION
N 2002 the total teledensity in South Africa was just 12%
with most fixed line telephones located in urban, historically white, residential and business areas, while black rural areas continued to experience teledensities commensurate with the rest of rural Africa, which is around 1% [1]. According to the Genesis report published in 2005 [2], of the 2.8 million lines that Telkom rolled out in compliance with its exclusivity agreement with government, 70% have now been disconnected due to non-payment, leaving South Africa with approximately 5 million fixed lines for 42 million citizens.
In order for more South Africans to benefit from the Information Age and the empowerment that it provides, more and more wireless technologies could be employed in order to affordably connect more South Africans to the Internet and a world of information. Wireless technologies can be easily deployed in areas where teledensity is low and at lower cost than wired alternatives [3], [4]. We should begin to see more telecommunication providers making use of wireless technologies in providing their services to their customers – in both rural and urban areas. Extremely popular today is the IEEE 802.11 range of technologies, commonly referred to as
*This work was undertaken in the Distributed Multimedia Centre of Excellence at Rhodes University, with financial support from Telkom SA, Business Connexion, Comverse, Verso Technologies, Tellabs, Stortech, THRIP, and the National Research Foundation.
Wi-Fi.
II. WHAT IS WI-FI?
The IEEE 802.11 standard, commonly referred to as Wi-Fi, is part of a family of standards for local and metropolitan area networks. This family of standards deals with the Physical (PHY) and Data Link layers (also known as the Medium Access Control (MAC)) as defined by the International Organization for Standardization (ISO) Open Systems Interconnection (OSI) Basic Reference Model. These standards define the protocol and compatible interconnection of data communication equipment via the air – radio or infrared – in a local area network (LAN) using the Carrier Sense Multiple Access protocol with Collision Avoidance (CSMA/CA) medium sharing mechanism [5]. They define the PHY and MAC layers so that they are compatible with the existing standards for higher layers (Logical Link Control and higher) [6], allowing stations to move and roam freely through a wireless LAN (WLAN) and still appear stationary to the Logical Link Control (LLC) sub-layer and above. This allows existing network protocols such as TCP/IP to operate transparently over IEEE 802.11 WLANs [7].
IEEE 802.11b (1999) extends the IEEE 802.11 standard (1999) by building on the data rate capabilities to provide 5.5 Mbps and 11 Mbps payload data rates in addition to the 1 Mbps and 2 Mbps rates [8]. The IEEE 802.11g (2003) further extends the 802.11 standard and the 802.11b extension by building on the payload data rates of 1 and 2 Mbps that use Direct Sequence Spread Spectrum (DSSS) modulation and by building on the payload data rates of the 1, 2, 5.5 and 11 Mbps that use DSSS, Complementary Code Keying (CCK) and optional Packet Binary Convolutional Code (PBCC) modulations. The extended rates of 802.11g provide additional payload data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbps [9]. For more detail about the IEEE 802.11 standard, and its "b" and "g" extensions, we recommend reading [5], [8], [9].
802.11 technologies operate in the 2.4 GHz Industrial, Scientific, Medical (ISM) band. The band is divided into evenly sized portions of spectrum, referred to as channels [10].
Each channel is 22 MHz wide and its starting frequency is separated from the next channel’s starting frequency by only 5MHz. Thus, adjacent channels overlap and can interfere with
Wi-Fi as a Last Mile Access Technology and The Tragedy of the Commons
Ingrid Brandt, Alfredo Terzoli, Cheryl Hodgkinson-Williams Departments of Computer Science and Education, Rhodes University
Tel: 046 603-8291 Fax: 046 636-1915
I.Brandt@ru.ac.za, A.Terzoli@ru.ac.za, C.Hodgkinson@ru.ac.za
I
T. Sobh et al. (eds.), Innovative Algorithms and Techniques in Automation, Industrial Electronics and Telecommunications, 175–180.
© 2007 Springer.
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one another. There are only 3 channels that do not overlap and therefore can be used in networks that are adjacent [10]. Figure 1, taken from [10], depicts available channels for 802.11b and 802.11g networks.
There are several advantages and disadvantages to using 802.11 technologies in connecting South Africans to telecommunication services. The advantages of using the IEEE 802.11 standards are low cost, standardisation and interoperability. Standardisation and interoperability make such networks cost-effective and easy to deploy and manage.
The greatest disadvantage of 802.11 as a WAN or MAN wireless network is that it is specifically designed to be a LAN network standard and thus not particularly suited to covering distances longer than 100 m [5]. As a result of this another important disadvantage of 802.11 emerges, namely interference. Interference problems within the 802.11 radio frequency range tend to be substantial as many other household appliances operate in the same range [11] such as microwaves, electric door openers, car alarms, cordless phones and Bluetooth devices. More importantly, in building wider area networks, other telecommunication operators also using 802.11 technologies use the ISM band [10]. This can result in large amounts of interference and an unreliable signal, especially in non-point-to-point communication links.
As a result of 802.11 technologies making use of unlicensed spectrum and being so affordable, interference on 802.11 networks tends to be the most obvious obstacle to providing reliable broadband connectivity because it is so widely used. One is reminded of “The Tragedy of the Commons” with respect to the use of 802.11 technologies in unlicensed spectrum.
III. THE TRAGEDY OF THE COMMONS
“The Tragedy of the Commons” is a well known paper written by Garrett Hardin in 1968 and published in Science [12]. Hardin extended the original parable published by William Forster Lloyd in his 1833 book on population. The parable illustrates how unrestricted access to resources such as pastures, the ocean or, we believe, radio frequency (RF) spectrum eventually dooms these resources because of over-exploitation [13]. This over-over-exploitation occurs because the benefits of exploitation accrue to individuals, while the costs of exploitation are distributed between all those exploiting the
resource. While Hardin, like William Forster Lloyd, was primarily interested in population and especially in the problem of human population growth, he also commented more generally on the use of resources such as the atmosphere and oceans.
Hardin uses the example of the pasture that is shared by local herders in order to make his point clear and understandable. In this example, he assumes that the herders wish to maximize their yield and so will increase their herd size whenever it is possible to do so. The utility of each additional animal has both a positive and a negative component:
• Positive: the herder receives all the proceeds from each additional animal
• Negative: the pasture is slightly degraded by each additional animal
The crucial aspect is that these two components are unequally divided. The individual herder gains all of the positive aspect while the negative aspect is shared between all the herders using the pasture [12], [13]. Thus for an individual herder weighing up these aspects, the rational course of action is to add the extra animal as the positive outweighs the negative.
Whenever faced with this choice the herder will always add yet another animal. However, since all the herders reach the same conclusion, over-grazing and degradation of the pasture will be the long-term result. Nonetheless, the rational response for any individual will remain the same each time the decision is faced because the gain is always greater to an individual than the distributed cost. Therein lies the tragedy: the inevitability of it all (unless the “rational” response considers the whole system and not just the individual interest). Thus each herdsman is locked into a system that compounds him to increase his herd without limit, in a resource that is limited [12].
One method of preventing over-use is through regulation.
However, Hardin suggests that this “policing of the commons”
tends to favour selfish individuals over those whom are more far-sighted [13]. In order to avoid over-exploiting the commons Hardin concludes by quoting Hegel, “Freedom is the recognition of necessity [12].” He suggests that when we interpret freedom as the freedom to do as we please, we create the tragedy of the commons. However, if we recognise resources as commons in the first place and relinquish our freedom to exploit them, realising that they require management then “we can preserve and nurture other and more precious freedoms [12].” Recognising that our commons
Fig. 1. Channels and center frequencies for 802.11b and 802.11g. Note that channels 1, 6, and 11 do not overlap.
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require management would hopefully result in commons not being over-exploited but rather utilised for the common good.
By managing our commons well and stipulating in what manners they can be used it is possible to avoid over-exploitation without having to stop making use of the commons altogether.
IV. WI-FI IN THE LAST MILE
At Rhodes University a research team has been working on the problem of integrating ICT into the secondary schools and their curricula. One of the problem areas that schools in South Africa face today is the issue of how they connect the computers in their schools to the Internet. A solution to this problem is to use wireless technologies. To this end the Telkom Centre of Excellence in the Computer Science Department at Rhodes University has been investigating the use of IEEE 802.11b and 802.11g based wireless LAN technologies as a means of connecting these schools to the University and then to the rest of the Internet [14], [15].
The investigation of IEEE 802.11 standards began in 2003 with 802.11b and changed in 2004 to the faster 802.11g. We wanted to cover distances much greater than 100 m as we wished to be able to connect schools that were as far as 10 km from Rhodes University. In order to achieve this we placed an access point with an omni-directional antenna on the 1820 Settlers Monument. The Monument stands approximately 620 m above sea-level (much higher than the buildings in the
town) and is situated on the south western hills, relative to the centre of town (see Figure 2). For the experiment, we replaced the access point's (AP) standard 2 - 3 dB antenna with an 8 dB omni-directional antenna and used 12 dB directional antennas connected to wireless Ethernet bridges at client sites.
The schools in Grahamstown East are unable to see the Monument and thus connections were impossible. We thus decided to experiment with building a wireless repeater station on top of a water tower at one of the schools which could see the Monument (RS in Figure 2, which is 5 km away from the Monument). The repeater consisted of a high gain 22 dB directional antenna aimed at the Monument, on the south-western side of town, connected to a wireless Ethernet access point configured in bridging mode and another access point with an 8 dB omni-directional antenna. This repeater station allowed clients in Grahamstown East to access the Monument via the repeater. A more detailed explanation of this wireless network can be found in [15]. The results of all the experimentation were very promising and interesting in terms of what we learnt about the possibilities of using Wi-Fi technologies in providing last mile access.
During initially testing phases we were able to obtain transfer rates of 2.6 Mbps at site A (1.2 km from the Monument), a client site within the city bowl, and 4 Mbps at site C (see Figure 2). After time these promising results began to taper off as we began to notice a marked increase in the amount of interference in the town. As discussed in section II
Fig. 2. Grahamstown with wireless network nodes. RU: Rhodes University; M: 1820 Settlers Monument; A: site A; B: site B; C: site C; RS: repeater station
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we are aware that other telecommunication service providers making use of the same 2.4 GHz ISM band will interfere with each other. As interference in Grahamstown increased, we experienced a marked decrease in the quality of the wireless network. On the links closer to the Monument, such as that of site A, we noticed degradation in the signal with slower transfer rates, increased delay and increased packet loss.
On longer distance links, such as that from the Monument to site C and the repeater station, the effects of interference were much more severe. We found that the repeater station was able to make an association with the AP at the Monument on layers one and two of the IEEE 802.11 protocol, but at layer three, IP traffic could not be transferred as the usable signal was too weak. In Figure 4 it can be seen that there was a 100% packet loss when pinging the repeater station. While trying to associate with the Monument AP from the repeater station we were able to “see” wireless AP beacons from the additional wireless networks creating the increased interference. Figure 5 shows the results of a scan for beacons run from the repeater station. The increase in the number of wireless networks resulted in an increased interference for our original wireless network, which explained the degradation in the network performance.
While the connection at site A was not as good as it had been before, the link from the repeater station to the Monument was unusable and thus we needed a method to work around the interference. To that end we replaced our original AP equipment (which had a signal output of 63 mW) with newer equipment whose signal transmission strength was 100 mW. Furthermore, we added a 22 dB directional antenna at the Monument in order to build a direct, point-to-point connection from the Monument to the repeater station. These hardware improvements allowed us to overcome the increased interference in the town.
Eventually the marked increase in interference tapered off again down to a fairly reasonable level and we were then able to add other test sites to our network. With the then decreased interference in the 2.4 GHz band we were able to add site B which is 3.5 km from the Monument to the network. The average throughput to site A was 2.4 Mbps, site B was 1.8 Mbps and site C was 1.9 Mbps. Figure 3 depicts the throughput at site C and provides an example of the graphs generated by throughput testing. This test site was connected to the Monument via the repeater station. Its improved throughput over site B is probably due to the fact that site C is approximately 50 m from the repeater station and the link between the repeater station to the Monument employs two high gain, 22 dB directional antennas. While site B also has a directional antenna of 22 dB, it is communicating with an omni-directional antenna of only 8 dB and thus radio conditions will be worse than the repeater-to-Monument link.
When the interference caused by the additional networks were removed, the original network again was able to successfully provide broadband connectivity in the same way it had prior to the increased interference. These results depict how easily a wireless network operating in unlicensed spectrum can be affected by interference from other networks operating in the same spectrum, especially when employing the use of omni directional antennas in a point-to-multipoint network. Further complications result when the technology has not been designed to overcome such interference, as is the case with 802.11 technologies.
Furthermore, from these results, we can see that when there is little or no interference it is possible to obtain good quality broadband connectivity using 802.11 technologies and that it is possible to overcome signal degradation due to interference by employing directional, point-to-point links only.
Fig. 3. Throughput from the Monument AP to site C, via the repeater station, in Mbps. The solid black line indicates the average throughput on this wireless link
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V. SOLUTIONS TO THE TRAGEDY OF THE COMMONS
According to Gardin [12] the solution to the tragedy of the commons is that we should relinquish our freedom to exploit the commons. Does this mean then that we should not make use of 2.4 GHz spectrum and other unlicensed spectrum in providing wireless broadband? We believe the answer depends on the geo-social context. In urban areas, where there is a high density of people, unlicensed spectrum should not be used for long distance connectivity because there will more than likely be a higher concentration of service providers who will want to use such spectrum in order to save on costs. This higher concentration will result in a marked increase in interference which will degrade the signal and network quality for all whom use the unlicensed spectrum.
That is not to say that it is impossible to circumvent interference issues. When using 802.11 (Wi-Fi) technologies it is possible to have more than one operator in the same area if those operators are using point-to-point links in order to connect two sites together. Point-to-point links are more immune to interference than point-to-multipoint links. There
are also some technologies that are designed in order to operate in unlicensed spectrum, in such a way that they are immune to higher interference ratios, such as is claimed for the Motorola Canopy product [16]. Furthermore, as technological advances take place in the fields of software radios and intelligent antennas, it is conceivable that systems may eventually be trained to know what is signal and what is interference, making wireless networks more robust and immune to interfering sources [17], [18].
802.11 technologies can however be extremely useful as a first phase in providing broadband connectivity to people living in rural areas with only one (or more often none) network operator. Here interference sources will mostly likely be relatively limited and thus allow a service provider to connect local communities to a broadband network that can provide them with telephony, data and possibly video services.
VI. CONCLUSION
The use of unlicensed spectrum, such as the 2.4 GHz ISM band which is used in 802.11 (Wi-Fi) technologies, has its place in WAN applications, but we believe that place is in
Fig. 4. Returned ICMP ping packets from the repeater station, note there were no ping packets returned (100% packet loss)
Traffic will be disrupted during the channel scan => BSS'es from the selected wireless mode
BSS Type Channel RSSI BSSID WEP SSID Ad-hoc 2.412 (1) 19 02:02:36:b4:2c:0e ON *Mk7lnK#
AP BSS 2.412 (1) 5 00:0f:3d:df:c5:dc OFF Rhodes AP BSS 2.422 (3) 39 00:0f:3d:9f:f6:e7 OFF scw2 AP BSS 2.437 (6) 14 00:02:6f:35:62:db ON amcctai
AP BSS 2.412 (1) 5 00:0f:3d:df:c5:dc OFF Rhodes AP BSS 2.422 (3) 39 00:0f:3d:9f:f6:e7 OFF scw2 AP BSS 2.437 (6) 14 00:02:6f:35:62:db ON amcctai