picoChip’s recent press release about smart signaling for femtocells is worthy of a bit more analysis than I’ve seen elsewhere. Here’s a slightly more indepth discussion of what’s new, different and how quickly it might make an impact. Not only does it relieve congestion, but it can speed up data access by over 60% for web browsing and similar smartphone applications.

What’s the problem?

Smartphones, such as Apple iPhone, RIM Blackberry and Google Andoid are selling in huge numbers, although Nokia still ship by far the most. Some analysts report these already account for some 40% of handset sales in the US, while European operators are said to be seeing figures grow from something like 25% recently towards an anticipated 40% in the near future.

Lower cost smartphones are expected to become even more popular, with a target cost as low as $120 being suggested for Android-based devices by some.

These have enabled a wide range of internet related applications (here's a breakdown for the US market), ranging from email through video, leading to dramatic growth in data traffic volumes overall. Monthly usage of several hundred Mbytes per user is not uncommon, with several users consuming many GBytes.

However, a separate capacity problem relates not to overall data transfer usage, but instead to the frequent status checks that many smartphone applications make. For example, your facebook and/or several email accounts could each be checked several times per hour as a background process.

These status checks transfer very little usable data – typically only a few bytes are needed to ask if anything has changed and be told no. However, the overhead of opening up a data connection and closing it down immediately after each status check can be considerable. This can take as much network capacity as sending a text message.

It wouldn’t make sense to keep these data connections permanently active all the time. Even with little or no data transfer, there would be continual load on both the smartphone (killing the battery) and the nearby cellsite (congesting use for others).

The industry solution

The technical experts who devise and continuously improve the design and operation of 3G networks designed an improved way to solve this problem. This was introduced in the 3GPP Release 6 standard published about 3 years ago.

This introduces an additional state between connected and disconnected, somewhat like your car being stopped at the lights with the engine running. The network remembers your data connection so that your smartphone can be sent any new data without the need for a full data setup sequence. The network knows which cell you are connected to (in this case your femtocell), so when data needs to be sent it can page only this one cell rather than a whole region. This feature is known as the Cell_PCH  state and is fully documented in 3GPP standards document 25.331, which covers Radio Resource Management.

In order to work effectively, both the smartphone and the local basestation need to have this feature built in and working.

What are the benefits?

Four I can think of are:

1. Web browsing and related data applications will work much faster. Instead of taking 2.5 seconds for a one-off round trip status check, it can be done in less than a second. Martin Sauter reports measurements comparing the difference here.
2. Congestion in basestations which use this feature is likely to be much reduced, increasing the capacity and service received for everyone.
3. Battery life and performance of smartphones using the feature should be improved – they don’t have to send as many radio transmissions to achieve the same purpose
4. Core network resources are also much less heavily loaded. Each packet data session passes through a GPRS Gateway (GGSN) and often one or more Deep Packet Inspection (DPI) boxes which shape traffic and/or handle real-time data charging and related IP routers. Continuously opening and closing data connections can cause heavy strain on these systems, reducing overall performance and increasing costs.

Which vendors support this feature?

Up to now, only Nokia Siemens Networks has included it in their basestations. It is believed that most of the latest smartphones sold today also have it built in. Typically, it takes around 12 to 18 months for new standards features to be included in the latest phones. This varies depending on the commercial demand (e.g. how much the operators are prioritising it against other features), and is often more rapid on the more expensive/high end devices. Nokia-Siemens have been running a strong marketing campaign promoting their unique support for the feature over recent months. They claim that status checks from smartphones can generate the same signalling traffic as 1,000 voice calls per day.

picoChip has now developed support for this within their latest PC3x3 system-on-chip too. Femtocell vendors  who have based their designs on the picoChip devices can therefore easily incorporate it in their next hardware revision. I’d anticipate it might take 6-12 months before it’s in volume commercial production from one of picoChip’s existing customers.

What about the large numbers of users per femtocell?

Femtocells have commonly been sized by the number of active, concurrent users they can support. Each user can have both a voice and data session active at the same time.

Typically residential femtocells have been sized around 4 active users, with enterprise femtocells being 8 to 32 channels. The most recent designs cater for up to 64.

These numbers don’t include those users who are inactive, but still camped onto a femtocell. Existing femtocell chipsets, such as picoChip, can already handle many inactive users

The Cell_PCH feature will help the femtocell scale to cope with these larger traffic capacities without becoming swamped by smartphone signalling traffic.

What this might mean for femtocells?

Femtocells have often been promoted as a very cost effective solution for offloading data traffic, particularly for indoor use. Where used, they would offload the signalling traffic from smartphones.

What this latest development ensures is that femtocells can scale up to really quite large numbers of smartphone users in an efficient way. They won’t get swamped by the status checks, speed up response time for data applications, further improve smartphone battery life and reduce the strain on the operator’s core network.

Users taking advantage of this scheme with a femtocell will find their mobile data service works even better than when outdoors – extending the high quality voice experience already enjoyed by femtocell users across to fast efficient access to their data services.

Comments (0)