1. 60GHz POINT TO POINT NETWORKS – DENIAL OF SERVICE
INTRODUCTION
Sub10 Systems define the phrase ‘denial of service’ for circumstances when radio link traffic is
disrupted 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’ at
60GHz is very low, owing to propagation characteristics in the band and antenna design of the Sub10
Liberator.
Deliberate denial of service does present a slightly higher risk, with physical damage or disruption of
link equipment most likely. Attempting to use a 60GHz radio ‘jammer’ to interrupt link
communication is seen as complex to do and unlikely to succeed.
LINK LOCATIONS AND PHYSICAL SECURITY
In order to achieve optimum performance in the network, all links should be mounted on rooftops
or high points of buildings. Either on poles fixed to a high point on the roof, or on stand-off
mountings fixed to an outside wall, just below a parapet. In most cases we expect that access to the
roof of any building, used for an element of the network, will be strictly controlled, ensuring good
physical 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 requiring
equipment 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 of
training in climbing techniques.
Combine good physical security with relatively inaccessible mounting points and it can be seen
someone with malicious intent would find it difficult to interfere with one or more of the Sub10
Liberator links. In this sense, “interfere” is used in the physical, rather than electronic sense. After
discussion with our specialist RF team, the general consensus is that someone wanting to sabotage a
network could do this very easily by simply cutting one or more of the power/data ‘drop’ cables
feeding an ODU. The recommendation? If the access to a rooftop is not secure and strictly
controlled, put procedures into place to make it so. Additionally, run power and data drop cables
inside steel conduit in order to protect them from accidental or deliberate damage.
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2. ACCIDENTAL ‘DENIAL OF SERVICE’
Considering the accidental situation. The propagation of radio-waves at 60GHz is affected very
significantly 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 Band
The principle effect of high levels of attenuation, due to oxygen absorption, is to restrict ranges of
60GHz radio systems to below 2km. Add the effects of this range limitation to the very narrow radio
beam produced by a Sub10 Liberator as shown in Figure 2 and it can be seen the 60GHz band is very
resistant to interference, regardless of how many 60GHz radios are deployed in a small geographic
area.
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3. Fig. 2 – Narrow antenna beam significantly reduces chances of interference
A 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 in
support of a city wide mobile telephone network; using the links to transport data from mobile
telephone base-stations back into the network. Prior to the roll-out, a detailed interference analysis
was conducted; the analysis showed, theoretically, there would be no interference despite, in many
cases, 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 deployment
and prove that theory and practice were one and the same. The 60GHz element of the whole
network has been operating, interference free, providing carrier grade levels of service, for well over
2 years. The diagram at Figure 3 above illustrates the extensive use of various types of radio links in
one area of the network. The light blue paths are a small proportion of the total of 90 Liberator links
deployed in the network.
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4. DELIBERATE ‘DENIAL OF SERVICE’
Moving on to a scenario where, despite your best efforts to secure the roof of a building, someone
with malicious intent is able to gain access to it. As stated earlier, the simplest thing for them to do
would be to tamper with the drop-cable providing power and data to each ODU. Alternatively, they
could obstruct the antenna face with cardboard or some similar material. Actions of this type
should immediately alert those monitoring the network that service had ceased and would trigger an
inspection and rectification visit.
The Sub10 RF team have considered the challenge of building a device able to ‘jam’ radio signals at
60GHz. In order for this jammer to be effective, it would need a relatively large (60cm) directional
antenna, the transmitter itself and a portable power supply. There is then the difficulty of getting
this whole device onto a rooftop without attracting suspicion. Once there, the only way an
individual 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 a
building, above a sheer drop, this would be extremely difficult.
Using the same device to try to ‘attack’ a remote Liberator ODU positioned on another building
some hundreds of metres away would be defeated owing to the significant degradation of the
transmitted 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 the
antenna of the Sub10 Liberator ODU, would be of insufficient strength to disrupt service.
SUMMARY
Accidental denial of service has, through deployments such as those in Vienna, been shown to be
unlikely.
Deliberate denial of service is possible and would most likely be via physical damage to drop cables
or ODU. The best protection against this is to ensure best in class security for those rooftops where
the Sub10 Liberator will be deployed.
The probability of disruption using a 60GHz ‘jammer’ is perceived to be very low.
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