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60GHz POINT TO POINT NETWORKS – DENIAL OF SERVICEINTRODUCTIONSub10 Systems define the phrase ‘denial of service’ for circumstances when radio link traffic isdisrupted or corrupted, temporarily or permanently, by physical or electronic means.We have looked at the issue from a practical viewpoint, taking into consideration link locations,probable RF propagation conditions and likely network topography.It will be shown later that the probability of accidental denial of service due to RF ‘pollution’ at60GHz is very low, owing to propagation characteristics in the band and antenna design of the Sub10Liberator.Deliberate denial of service does present a slightly higher risk, with physical damage or disruption oflink equipment most likely. Attempting to use a 60GHz radio ‘jammer’ to interrupt linkcommunication is seen as complex to do and unlikely to succeed.LINK LOCATIONS AND PHYSICAL SECURITYIn order to achieve optimum performance in the network, all links should be mounted on rooftopsor high points of buildings. Either on poles fixed to a high point on the roof, or on stand-offmountings fixed to an outside wall, just below a parapet. In most cases we expect that access to theroof of any building, used for an element of the network, will be strictly controlled, ensuring goodphysical security, requiring keys and or passes to get onto the roof at all.Sub10 Liberator Outdoor Units (ODU) on the roof top will be mounted in positions requiringequipment such as a ladder to reach them; alternatively, ODU fixed to the outside wall of a building,below the parapet, will be hard to reach without specialist equipment and the correct level oftraining in climbing techniques.Combine good physical security with relatively inaccessible mounting points and it can be seensomeone with malicious intent would find it difficult to interfere with one or more of the Sub10Liberator links. In this sense, “interfere” is used in the physical, rather than electronic sense. Afterdiscussion with our specialist RF team, the general consensus is that someone wanting to sabotage anetwork could do this very easily by simply cutting one or more of the power/data ‘drop’ cablesfeeding an ODU. The recommendation? If the access to a rooftop is not secure and strictlycontrolled, put procedures into place to make it so. Additionally, run power and data drop cablesinside steel conduit in order to protect them from accidental or deliberate damage. 1 60 GHz - Denial of Service - 200112
ACCIDENTAL ‘DENIAL OF SERVICE’Considering the accidental situation. The propagation of radio-waves at 60GHz is affected verysignificantly by oxygen absorption. Figure 1 illustrates that the oxygen absorption peak, of 16dB/Km,is coincident with virtually the whole 60GHz band. Fig 1 –Oxygen Absorption Peak of 16dB/km over 60GHz BandThe principle effect of high levels of attenuation, due to oxygen absorption, is to restrict ranges of60GHz radio systems to below 2km. Add the effects of this range limitation to the very narrow radiobeam produced by a Sub10 Liberator as shown in Figure 2 and it can be seen the 60GHz band is veryresistant to interference, regardless of how many 60GHz radios are deployed in a small geographicarea. 2 60 GHz - Denial of Service - 200112
Fig. 2 – Narrow antenna beam significantly reduces chances of interferenceA practical illustration of excellent frequency reuse, with no adverse interference, is in Vienna,Austria, where a mobile telephone service provider planned to deploy 90 Sub10 Liberator links insupport of a city wide mobile telephone network; using the links to transport data from mobiletelephone base-stations back into the network. Prior to the roll-out, a detailed interference analysiswas conducted; the analysis showed, theoretically, there would be no interference despite, in manycases, little physical separation between radio units. Fig 3. Vienna – (Light Blue & Green showing some Liberator paths)This study gave the integrator and operator sufficient confidence to go ahead with the deploymentand prove that theory and practice were one and the same. The 60GHz element of the wholenetwork has been operating, interference free, providing carrier grade levels of service, for well over2 years. The diagram at Figure 3 above illustrates the extensive use of various types of radio links inone area of the network. The light blue paths are a small proportion of the total of 90 Liberator linksdeployed in the network. 3 60 GHz - Denial of Service - 200112
DELIBERATE ‘DENIAL OF SERVICE’Moving on to a scenario where, despite your best efforts to secure the roof of a building, someonewith malicious intent is able to gain access to it. As stated earlier, the simplest thing for them to dowould be to tamper with the drop-cable providing power and data to each ODU. Alternatively, theycould obstruct the antenna face with cardboard or some similar material. Actions of this typeshould immediately alert those monitoring the network that service had ceased and would trigger aninspection and rectification visit.The Sub10 RF team have considered the challenge of building a device able to ‘jam’ radio signals at60GHz. In order for this jammer to be effective, it would need a relatively large (60cm) directionalantenna, the transmitter itself and a portable power supply. There is then the difficulty of gettingthis whole device onto a rooftop without attracting suspicion. Once there, the only way anindividual link could be affected is to focus the jamming signal directly into the operating beam.Considering the link ODU are going to be well above head height, or on the outside wall of abuilding, above a sheer drop, this would be extremely difficult.Using the same device to try to ‘attack’ a remote Liberator ODU positioned on another buildingsome hundreds of metres away would be defeated owing to the significant degradation of thetransmitted signal caused by oxygen absorption coupled with losses due to free space. Effectively,the jamming signal once it had travelled a few hundred metres, even if pointed directly at theantenna of the Sub10 Liberator ODU, would be of insufficient strength to disrupt service.SUMMARYAccidental denial of service has, through deployments such as those in Vienna, been shown to beunlikely.Deliberate denial of service is possible and would most likely be via physical damage to drop cablesor ODU. The best protection against this is to ensure best in class security for those rooftops wherethe Sub10 Liberator will be deployed.The probability of disruption using a 60GHz ‘jammer’ is perceived to be very low. 4 60 GHz - Denial of Service - 200112