HSPA State Change Measurements

A1-state-changes Last week I did some measurements to get an idea of the time required when switching between different HSPA air interface states. While data is transferred, the mobile is usually in Cell-DCH state on a High Speed Shared Channel. When only little or no data is transferred, the connection transferred to the Forward Access Channel, which is slow but has little overhead for both the network and the mobile device in terms of control measurements and power adjustments commands. If no data is transmitted for a longer duration (e.g. 30 seconds) the connection is put into Idle state. While the IP address is retained, the physical connection between the mobile and the network is severed.

As can be seen in the picture on the left, the round trip time to the first hop in the network of a ping packet is around 100 to 120 milliseconds while the mobile is using a high speed shared channel. While on the slower forward access channel, round trip time increases to 240 to 260 milliseconds. Moving from the high speed shared channel to the forward access channel is relatively quick, it takes around 550 to 600 ms (minus the actual round trip time of the packet itself). Going back to the high speed shared channel takes a little bit more time, around 1000 to 1500 milliseconds.

When using a 3G dongle with a notebook, a connection is rarely set into idle state as there is always one program or another such as an instant messenger, VoIP client, etc., that feels it needs to send a keep alive message to a server in the network before the idle time can expire. Therefore I haven't measured it this time. In the past, I've seen values around 2500 to 2800 milliseconds.

Some say that the effect of this state switching is that web browsing feels a bit more sluggish over HSPA than over a DSL line, which always offers Internet connectivity at full speed without the need of state switching. I use 3G connectivity a lot and quite frankly, while I can feel a difference, it's absolutely no problem to work and live with it.

And here's a quick overview of the test setup: Mobilkom Austria 3.5G HSPA network, a notebook connected via Wi-Fi to a D100 Wi-Fi/3G gateway, connected to a Huawei E220 3G USB stick, HSDPA category 6, no HSUPA.

3G Network Stability: 8h of Continuous Voice, IM and Remote Desktop

This week, I've ventured far beyond my 'normal' 3G use by giving remote support to someone being connected with a notebook over a 3G link for over 8h at a time. During that time, we had a Skype voice session established with excellent audio quality, used Instant Messaging and e-mail to send and receive documents and I had a remote desktop session open to see what is going on and to directly lend a hand when necessary. All sessions were open simultaneously and there was not a single glitch with a single application or the 3G connection.

That's what I call network stability! During that time, around 300 Mbyte of data were exchanged. It's impressive to see that both networks and devices have matured to such a level. On the network side, Mobilkom Austria (A1) has to be congratulated for the stability and performance of their HSPA network and for offering Internet access with prepaid SIMs. On the terminal side, the Huawei E220 modem did it's part. Congratulations to all companies involved, it was a truely great experience!

Telstra to Upgrate to HSPA+

A tip from a reader brought me to this article on Telstra in Australia saying that they intend to upgrade their 3.5G network in Australia first to 21 MBit/s in 2009 and later on to 42 MBit/s. The step to 21 MBit/s seems logical. According to the 3GPP standards, that's an upgrade to 64QAM modulation. If they have the latest base stations from Ericsson, they might be able to do this without a hardware upgrade.

Concerning the 42 MBit/s, that sounds like the 28 MBit/s one gets with MIMO plus 64QAM modulation on top. When I last had a look at the standards document referenced above, there was not yet a terminal class for this maximum speed.

A note of caution: Such speeds can only be reached under very special circumstances, i.e. no other subscribers in the cell and the base station antenna very close by.

HSDPA Alongside A CS Voice Call

Back a year ago I noticed that an incoming circuit-switched voice call during a 3.5G HSDPA packet-switched data session forced the packet connection to go back to 64 kbit/s dedicated bearer while the call was ongoing. After the call the bearer was upgraded to 384 kbit/s but was only put back on the High Speed Shared Channels once the download was finished. Looks like the software on the network side has advanced a bit in the meantime as I recently noticed that even during a phone call an ongoing download continued at HSDPA speeds. Very nice!

Note: The test a year earlier was performed in the German Vodafone network while my latest observation is from the Orange France network. The RAN vendors might not necessarily be the same and it's even likely that they are not.

The FACH Power Consumption Problem

In UMTS and HSPA, there are a number of different activity states on the air interface while data is exchanged with the network. During phases of high activity, the mobile device is usually put into dedicated state (Cell_DCH) and transmits/receives data on the high speed downlink shared channels and a dedicated uplink channel. During times of lower activity or to keep a physical connection open to resume data transfers quickly (e.g. the user clicks on a link after some time of inactivity) the network puts the connection into Cell_FACH (Forward Access Channel) state. While the FACH is quite slow, it reduces power consumption somewhat. However, not enough for all kinds of applications.

eMail Polling in 3G mode

While in Austria recently, I noticed that when using 3's UMTS network and Profilmail with a POP3 eMail polling interval of 5 minutes, my battery ran dry within 6 hours. Quite devastating and very short compared to GSM/GPRS/EDGE where the battery easily lasts a full day under the same conditions. With the help of Nokia's Energy Profiler I dwelled down to the bottom of the problem. It turns out that 3 leaves the air interface in DCH state for 20-25 seconds after the last data packet has been sent before putting it into the Cell_FACH state for 1 minute and 45 seconds. Afterwards, the air interface connection is put into Idle state. In Cell_DCH state, even if no data is transmitted, power consumption is around 1.5 watts. In Cell_FACH state, power consumption is still around 0.8 watts, while in idle state and backlight off, power consumption is "almost zero". Even if no eMail is sent/received, these values result in the radio being active for almost half the time of each 5 minute interval, resulting in an average power consumption "in the pocket" (i.e. backlight always off) of 0.5 watts on average. As the battery capacity is 4.4 Wh (that is watt hours), the result is that the battery is empty in just a couple of hours.

If noticed this behavior in 3G networks before but never in such an extreme. This is because most other 3 G networks I usually use have different activity timers. In most other networks, the Cell_DCH state is left after about 15 seconds and Cell_FACH after about 30-45 seconds. This of course decreases the browsing comfort because it often takes longer than 30-60 seconds to read a web page in which case the transition to from idle to Cell_DCH state takes longer than from Cell_FACH to Cell_DCH. On the other side, however, it increases the autonomy on a single battery charge.

eMail Polling in 2G mode

Polling eMails every 5 minutes while the mobile is locked to GPRS is much more efficient. Here, the mobile takes about 1.5 watts while communication is ongoing. However, power consumption goes down almost immediately after no data is sent or received. As a result the average power consumption is only 0.1 watts or only a fifth of the power consumption while in 3G mode.


Reducing the 3G timers to lower values is no option since it would have a negative impact on the users experience. Maybe the enhanced FACH, which is not yet implemented in devices and networks, will help somewhat in the future. When looking at the specifications, however, it looks like it mainly addresses capacity and not so much mobile device power consumption. So that remains to be seen. 

Another possibility is to switch from the POP3 pull approach to a push approach where the server starts communicating with the device only when a new eMail has been received or very infrequently to keep the TCP session open. Not sure how Blackberries receive their email, but it would be interesting to experiment a bit. IMAP push would be another option but unfortunately, Profimail does not support that extension.


An interesting case in which the 2G air interface is superior to 3G. How LTE and WiMAX fare in the same scenario is also in interesting question. LTE, for example, has a different air interface state model compared to 3G. Here, only active and idle state exist and active mode timers can be set by the network dynamically in a way to reduce the mobile's average radio activity time to almost the same values as when being in idle state. That should reduce power consumption somewhat if the base stations are clever enough to adapt the timers based on the traffic pattern observed. We shall see…

4G and Peak-Rate Marketing

Moray Rumney, Lead Technologist at Agilent Technology is quite outspoken about the negative impact of what he calls "Peak-Rate marketing in telecommunications", i.e. the gap between proclaimed (theoretical) data rates of wireless systems and realistic data rates and capacity achieved in practice. I fully agree with his arguments and will also discuss this topic in my next book. In the latest Agilent Measurement Journal, Moray looks at the topic again, this time from the point of view of how Femto cells could positively influence the data rate and capacity equation in the future. His argument is that the effect of adding a femto layer (Wifi or 3G/4G femto cells) in an overall network architecture increases throughput and overall capacity by orders of magnitude while increasing theoretic peak data rates of macro cells does relatively little in comparison. An article not to be missed, it starts on page 52! Also interesting from a wireless point of view is the article starting on page 25 about resolving design issues in HSPA mobile devices. Earlier issues of the journal can be found here.

3G FACH capacity

With the rising number of push eMail devices in 3G networks and mobile applications such as instant messengers and voice over IP clients the number of small IP packets to keep the connections of such applications alive through network address translation routers is rising. For the network this means of lot of radio layer signaling and waste of bandwidth. For the mobile device, keep alive messaging means significantly increased battery consumption.

3G UMTS networks are thus putting devices that only send little data on the Forward Access Channel (FACH) which requires much less radio channel signaling overhead than if the device is instructed to remain or use the High Speed Downlink/Uplink channels for such kind of traffic. As more always-on devices are used in the networks, this will quickly become an issue since the total capacity of the FACH of a cell is limited to 32 kbit/s today. With the bandwidth so small I think most operators will be very thankful for the enhanced-FACH extension which reserves some capacity on the high speed downlink channels for FACH operation. Despite using the high speed channels, no additional radio layer signaling will be used so overhead and battery consumption remains limited at the expense of spectral efficiency. While networks and mobile devices do not support this feature today I expect that this is definitely a feature that will be implemented in the future.

For more on radio interface optimization for future devices and services have a look at my previous entries on continuous packet connectivity (here, here and here), some more background on enhanced FACH (here) and some thoughts on upcoming capacity issues due to keep alive messaging (here).

It seems there is now also an initiative in Release 8 of the 3GPP standards to improve the uplink behavior of the system while a device in in Cell_FACH state. More about that once I have taken a look at the details.

Beyond 3G: The Manuscript is Ready

Some of you might have noticed that recent blog entries haven grown in size again and speculated that I have a bit more time at hand again. Well you have guessed right. Over the past months, I spent most of my free time working on my next book, to be published by John Wiley and Sons by the end of the year. Finally, the manuscript is ready and the title will be

"Beyond 3G: Bringing Networks, Terminals and the Web Together"

For the moment, people at Wiley's are now working on the cover, they are proof-reading the manuscript, and typesetting is starting soon. After that, I'll have to work a bit on the index, on the glossary, etc., etc. But that's a bit later in the year. I always find it amazing how many steps are necessary from finalizing the manuscript to having the finished book finally being shipped to the bookstores.

It's a long process. However, I strongly feel that all of this work is necessary and justified to produce something outstanding that has something which is missing in day to day online publishing: Depth and broadness.

That doesn't mean that online sources are less valuable, they are just different. I very much like my blog for example, because it catches spontaneous thoughts, thoughts about a clearly defined single topics, ideas, it's great for posting the latest news, for responding to someone else publishing something, and it is thus a great complement to my offline writing activities. Together, online and offline are hard to beat!

So what exactly will the book be about? Here's the current version of the back cover text:

Giving a sound technical introduction to 3GPP LTE and SAE, this book explains the decisions taken during standardization while also examining the likely competition for LTE such as HSPA+ and WiMAX. As well as looking at next generation network technologies, Beyond 3G – Bringing Networks, Terminals and the Web Together describes the latest mobile device developments, voice and multimedia services and the mobile web 2.0. It considers not only how the systems, devices and software work but also the reasons behind why they are designed in this particular way. How these elements strongly influence each other is discussed as well as how network capabilities, available bandwidth, mobile device capabilities and new application concepts will shape the way we communicate in the future.

  • Examines current and next-generation network technologies such as UMTS, HSPA+, WiMAX, LTE and Wifi
  • Analyses and explains performance and capacity in practice as well as future capacity requirements and how they can be fulfilled.
  • Introduces the reader to the current cellular telephony architecture and to voice over IP architectures such as SIP, IMS and TISPAN
  • Looks at mobile device hardware and mobile operating system evolution
  • Encompasses all major global wireless standards for application development and the latest state of the mobile web 2.0

As I said above, it's going to take until the end of the year until the book is finally shipped. If you would like to be informed when it's available, please send an eMail to "gsmumts at gmx.de", I'll be happy to keep you informed.

Nokia Research Center on Impact of Keep-Alive Messaging on Power Consumption

With always on applications (think mobile eMail, IM, VoIP, etc.) on wireless devices, power consumption inevitably increases due to the constant exchange of TCP and UDP keep-alive messages to keep NAT firewalls open. Gone are the days in which wireless devices only communicated when there was really something to say. Pasi Eronen of the Nokia Research Center has taken a closer look at the issue and has measured and compared the impact of keep-alive messaging in 2G, 3G, 3.5G and Wifi networks. In the second part of the paper, Pasi then takes a look at how current VPN
security products could be enhanced to avoid frequent UDP keep-alive
messaging and thus increase the operating time of mobile devices. An interesting read, highly recommended!

Some of the findings:

  • NAT timeouts for UDP are anywhere between 30 and 180 seconds
  • NAT timeouts for TCP is anywhere between 30 and 60 minutes
  • Sending a keep-alive packet every 20s increases power consumption by a factor of 10 and more
  • The paper suggests that VPN products use a TCP connection to reestablish the UDP connection used for encrypted packets after a long timeout instead of sending frequent UDP keep-alives. Works well as long as no IM or VoIP client uses the VPN tunnel.

Efficiency: LTE vs. HSPA

Yesterday, I've been looking at how LTE Radio Bearers are established and how data is transferred over the air interface. It's now time to draw a conclusion with a comparison of how data is transferred over the HSPA air interface to see if there are improvements concerning the time it requires to establish a radio bearer and how efficient the interface is for transporting small amounts of data.

Establishment Time

In current UMTS/HSPA networks, mobile devices are put into RRC_idle state after about 30 seconds of inactivity. If data transfer resumes, e.g. because the user clicks on a link on a web page, it takes about 2-3 seconds to re-establish the full bearer on the High Speed Downlink Shared Channels. This is quite different, to say a fixed line DSL connection, which has no such noticeable delay. The 30 seconds delay to put the mobile into idle state is thus a compromise to give the user a good browsing experience in most situations at the expense of power consumption, as the device requires a significant amount of power to keep the radio connection open, even if no data is transferred (more about that in one of the next blog entries).

In LTE on the other hand, state switches between idle and connected (on the air interface) have been designed to be very short. The requirements list a time of 0.1 seconds. Given the air interface structure as described in the previous article and only a single inquiry by the base station to the access gateway node to get the subscribers subscription profile and authentication information, it is likely that this target can be fulfilled. This means there will be almost no noticeable delay when the mobile moves back from a mostly dormant air interface state to a fully active state.

Another advantage of LTE is that base stations manage the radio interface autonomously. This is a significant difference to current HSPA networks, in which centralized Radio Network Controllers (RNCs) manage the air interface activity state on behalf of the base station. As more mobile devices are connected to the packet switched part of the network, the signaling load keeps increasing for the RNC and might well become a limiting factor in the future.

Transferring Small Amounts of Data

Even while in active state on the air interface, the network can react
to reduced activity and instruct the mobile to only listen at certain
intervals for incoming packets. During all other times, the mobile can
switch off its transceiver and thus conserve power. This enables
resumption of data transfer from the mobile even quicker than going
through a full state change. This method is called Discontinuous
Reception (DRX) and is likely to be an important tool to conserve
battery power and reduce signaling. Again, the base station is free to set and change the DRX period for an ongoing connection at any time and no information has to be exchanged with other nodes in the network. 

In UMTS networks, the closest tool availble to handle connections in which only little data is exchanged is the Forward Access Channel (FACH). The FACH is also a a shared channel, but no power control is required. This saves a lot of energy at the mobile side as there is no dedicated channel for uplink power control required while data is exchanged on the FACH. With a capacity of only 32 kbit/s, however, the channels ability to handle many simultaneous connections is very limited. Furthermore, there are no DRX cycles, so the mobile has to continuously listen on the downlink for incoming data. Despite requiring much less energy than observing the high speed shared control channels and keeping a power control channel open in uplink direction, power requirements are still significant (as I will discuss in another blog post soon).

Why not sooner?

With all these advantages over UMTS, the question arises why UMTS has not been designed in this way in the first place? One of the answers probably is that when UMTS was specified back in the late 1990's, the world was still a circuit switched place for the end user. Most people still used dial-up modem connections and voice over IP up to the end user was not a hot topic. So the design of the radio network was strongly influenced by circuit switched thinking. HSPA was the first step into a fully packetized world with the introduction of the high speed downlink shared channels.

Today, however, even HSPA is still embedded into the concept of persistent radio channels and a hierarchical network structure in which control of the radio network is not fully at the base station but with radio network controllers. Also, there is still a high degree of communication with core network nodes such as the SGSN, e.g. whenever the RNC decides to put radio connection to the mobile into idle state. With HSPA, changes were made to give the base station more autonomy as data transfer over the high speed shared channels is controlled by the base station and no longer by the RNC.

To improve things further, companies organized in 3GPP have made many additional enhancements to the UMTS/HSPA air interface in recent versions of the standard. These are sometimes referred to as "Continued Packet Connectivity" CPC, and I reported about these new features here, here and here. Most of the CPC features are likely to be implemented and introduced in the next few years while LTE matures to help HSPA cope with the rising traffic until LTE can take over.