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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