Effects of exceptional demand on mobile phone networks

Nick Patrick-Gleed, a cell site expert at CCL-Forensics, looks at the effect of a large number of people gathering in one place and how the networks compensate for it. He’ll also talk about just how this may affect what cell site analysts see on call data records, and the potential challenges which may come up in court.

Q: Let’s take, for example, a large festival being held in a rural location – where the usual network configuration would be unable to cope with such an influx. What happens when all that phone traffic hits the network?

A: A number of things – three main ones. The first is that, as these events are obviously planned well in advance, the network can introduce temporary cells to cover the area in question. These are usually vehicle/trailer mounted, and are located overlooking the site to ensure maximum coverage – and are quickly removed afterwards.

Q: Doesn’t that pose a problem when surveying if a cell which shows up on a call data record no longer exists?

A: It’s not a problem, as they are labelled on the CDR (call data record) as being a temporary cell, and they usually say why – for example “temporary cell – Glastonbury”. If that is the case, then a survey doesn’t add anything to the process. But these cells tend to have a very small coverage area to avoid interference with the existing network, so as long as you know it’s a temporary cell, you can (even as part of a desktop exercise) locate a phone with reasonable precision.

By that, I mean that the phone is clearly in the vicinity of the festival and not, say, 100 miles away, as claimed by their alibi.

Also, the caller may use neighbouring cells around the time of the incident. They will be detected during a survey.

Q: Is there any way that extra capacity can be introduced to the existing network?

A: Absolutely, and that brings me on to points two and three, following on from point number one above.

One is that extra “kit” can be installed at the mast site, and the other is that software add-ons can be used to handle more calls.

The first of those is a relatively simple procedure, which sees an engineer adding (usually) a piece of rack-mounted equipment in the hut or box at the foot of the cell tower. This takes about an hour – and gives the cell a significant number of extra traffic channels, and therefore the ability to handle more calls.

The benefit of this to cell site analysts, over the temporary cell scenario, is that it has no effect on what you see on the CDRs. The cell ID already exists, and can be surveyed if required.

Q: And the final method?

A: This is the software solution, and basically means that different codecs (audio compression) are used between the phone and the cell. It reduces the bandwidth used to code the callers’ voices into the bits and bytes that are carried by the network. This means that the same frequency can handle more calls, but they are of a slightly lower quality. The quality is still good enough to hold a conversation; it’s just not to the normal standard. Again, this has no effect on the CDRs, and the cell can be surveyed as normal.

This tends to be used as a temporary technique, as it costs the network in software licensing fees… although in theory, they should make it back by handling more calls. It can be a useful method to add capacity as a short term solution or on a daily basis.

Q: Even with these techniques, there are still bound to be scenarios where call volumes are too great, and even they can’t cope.

A: Yes – it is possible that demand can outstrip capacity… and this is possible where large unexpected events happen like a major pile-up or a large scale incident occurs.

When this happens, a person wanting to make a call will be forced to use a neighbouring mast, as their “first choice” is unavailable. The handset has a list of cells that it can detect. It prioritises these, to give a preferred cell, and a list of others which can provide service (seven in total). It also knows other cells that it can detect, but they are not considered to provide service. This list changes dynamically, so the phone always has options.

If the handset is in an area of non-dominance more than one cell may already provide service. When the handset tries to use its “number one” cell, and it is congested with other calls, a process known as “directed retry” happens. This is where it uses one of the other masts in its list, which can provide service.

Q: How does this impact on the way the event appears on a call data record?

A: That’s something of an irrelevant question. If we were to survey the area, we would also see the list of cells which can provide service. Just because the phone doesn’t use the number one cell, it isn’t a problem. We would detect during our survey other cells which provide, or are considered to provide, service – which would undoubtedly include the cell which was used.

Cell site analysis is about much more than just turning up with a piece of kit and surveying one cell. The skill and knowledge of the analyst is key in handling any of these challenges – and the science of cell site analysis is robust enough to rebut them.

Q: Is network congestion a common occurrence?

A: Service providers monitor their network performance, routinely recording data such as the number of dropped calls, the distances of user from mast, and congestion – among others. Their performance is also monitored by OFCOM. They will compare the data against the planned performance and optimise the network accordingly. If a problem is routinely observed, for instance congestion on a cell, additional capacity would be added.

Nick Patrick-Gleed

Cell Site Expert

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How can geographical features affect cell site analysis?

Matthew Tart, one of CCL-Forensics’ cell site analysts looks at the effects that height and landscape have on cell site analysis techniques. He also looks at a scenario where, on the face of it, a suspect seems to be moving in a particular direction – but could actually be doing the opposite.

Q: It seems obvious that the landscape can affect the coverage area of cells, but just what sort of effect does it have?

A: There are two main effects. The first is that height limits coverage area by preventing the signal reaching areas in a “shadow” caused by the particular geographical feature (hills, mountains, that kind of thing). And the other is that being at height affects which mast (cell) you are able to connect to, compared to the cells which would serve at a lower level.

Q: Taking the first of those two, this is presumably about “line of sight”?

A: It is – but don’t be misled by the word “sight”. You don’t physically have to see a cell for it to provide service where you are – reflection, diffraction and other factors take care of that. Let’s consider a simplified example. The signal from the mast reduces when it travels through or around “stuff”. Whereas the walls of a building contain a relatively small amount of stuff, hills and mountains (clearly) contain a lot of stuff. It stands to reason that, if a hill is in the way, the signal will not be received on the other side.

Q: That sounds pretty obvious, but how do cell site analysts cope with this?

A: It’s all about the preparation: about knowing the landscape and the potential effects before you go out to survey (if, indeed, surveying is relevant to the investigation in question). It’s obviously wrong to be doing all your preliminary work in two dimensions, when the scene itself is in three.

Q: In terms of the other issue, i.e. height affecting which cell you connect to… just how high are we talking about?

A: We’re talking more about “artificial” height here… tower blocks and so on, rather than natural features. It’s feasible that if you’re higher than four storeys from the ground, a whole new range of cells may come into view. When you get above the rooftops of neighbouring buildings you have a whole new vista, meaning new cells can be seen. The “stuff” is no longer in the way.

A good example of this is on an estate, where a localised picocell provides coverage at street level, but twelve storeys up, a more remote macrocell provides coverage, despite being a number of miles away. Simply mapping this as a desktop exercise, you could incorrectly infer that the phone had moved a considerable distance.

Q: How misleading could this be?

A: Let’s look at an extreme example: of a valley with a mast situated high on each side, and one in the town below.

If an individual moves in the direction of the arrows shown in the diagram, i.e. from right to left through the town, he may use (in this example) mast A first, as it provides line-of-site coverage, but he is in the shadow of mast C. Next, he may use mast B as he moves through the town, as this provides localised coverage for the town only, and then as he travels back up the other side of the valley, he could use mast C, as he is in the shadow of mast A.

Plotting these on a map, and giving no consideration to the terrain, makes it look like he did the opposite. Simply looking at the mast location, and saying he used A, then B, then C, does seem to imply he is moving from left to right, rather than vice versa.

Looking at this from above…

Of course, in reality, you would more than likely be using a much larger set of data, so this is an extreme example, but it does illustrate the problems faced in not appreciating the effect of height.

In another example, let’s look at the other effect of height – that of providing service at height, which you wouldn’t expect at ground level.

In this example, the purple mast is designed to provide service at ground level (let’s say in this example it’s a picocell on the front of a nearby shop) but the orange mast would easily provide coverage at the top of the tower block, even though it is a considerable distance away. This demonstrates the importance of carrying out a survey which is in line with the requirements of the investigation.

Was the suspect likely to have been on the high levels of a building at the time of the crime? Was height a potential factor? Would a “walk survey”, using hand-held equipment, be more appropriate than a drive survey using vehicle-mounted equipment at ground level?

In summary, it’s worth remembering that a lot of network planning is focused on providing coverage at ground level, and it’s important therefore to consider the likelihood of more remote cells (at greater altitude) also providing service.

Matthew Tart

Cell Site Analyst