The Ultimate Test: The N82 In The Hands Of A Non-Techie

The constelation seems right: N-series devices becoming more mature and mobile Internet access now affordable, it’s time for the ultimate test: How will a Nokia Nseries device fare in the hands of a non-techie for more than just voice calls? I don’t have the answer yet, the ‘experiment’ has just started, but I should know soon enough. After testing the N82 in and out for a week, I finally shipped it to ‘the user’ with 4 applications preconfigured: MP3 player with 40 or so CD’s preloaded, Profimail for eMail access, picture upload to Flickr and OperMini 4.1 beta for mobile web access. In addition, the phone will be used intensively as a data modem for the PC. So let’s see how that works out. I’ll keep you posted.

More HSPA+: Enhanced Cell-FACH

HSPA+ is about more than just higher data rates, it is also about enhancing the radio interface to allow more devices to simultaneously connect to the network in a more power efficient way. I’ve described most of those features in various blog entries in the past but it seems I have missed one feature: Enhanced Cell-FACH.

One of the challenges of always on Internet connectivity is that mobile devices or PCs running instant messaging applications, Voice over IP prgrams, push eMail and other connected programs are anything but silent even while these applications are just running in the background. Even if just one of those applications is running, the device transmits and receives several IP packets per minute to keep the connection to the servers on the Internet alive. This means that in most cases, the radio link to mobile devices is not in idle state for most of the time.

As keeping the mobile in a fully connected state while only little data is transfered is quite wasteful in terms of bandwidth and battery capacity. UMTS networks therefore usually set device into the so called Cell-FACH state, once they detect that there is only little activity. In this state, the device uses the random access channel to transmit IP packets in uplink and the Forward Access Channel (FACH) in downlink to receive IP packets.

This method is quite efficient for the mobile, since no power control is performed on those channels. Hence, there is no radio layer signaling overhead in this state, which leaves more air interface capacity for other devices and also saves battery capacity. For the network, however, managing more than a few mobiles per cell on the FACH is not as efficient, since the channel was never designed to function as an always on data pipe for a high number of devices.

This is where the Enhanced Cell-FACH extension comes in. Once mobiles support this feature and they are set into Cell-FACH state, their data packets are sent on a Highspeed Downlink Shared Channel (HS-DSCH) instead of the Forward Access Channel. This improves the efficiency of downlink transmissions and also speeds up a state transmission into dedicated state once more packets are transferred again. An application note by Rhode and Schwarz goes into the details in Chapter 6.

What puzzles me a bit at this point is two things:

  • When will the feature become available?
  • In Cell-FACH state, the mobile is identified via the Cell-Radio Network Temporary ID (C-RNTI). In theory, this is a 16 bit value, i.e. up to 65536 mobiles per cell could be in Cell-FACH state simultaneously. Strangely enough, most networks only seem to increase this value up to 0xFF (256) before the being reset back to 0. Anyone got any idea why?

AT&T Starts Prepaid Internet Access For Mobile Devices

Great move from AT&T in the US to be the first GSM  / UMTS carrier in the region to start a small screen Internet access offer on prepaid SIMs. For $19.99, the ‘MediaNet Unlimited’ offer is not the cheapest I have seen, but definitely a good start! Can’t wait to try it out next month when I come to Orlando to speak at a Tech Conference. For the technical details take a look here at the Prepaid Wireless Internet Wiki.

Thanks to phonenews and Matthew Stevens, who’s blogging over at DarlaMack, for pointing me to it!

Breaking the Radio Silence with VoIP

In cellular networks, the primary rule for voice telephony is efficiency, efficiency, efficiency. Translated into practice, this means that the mobile device patiently waits till it receives a paging for an incoming call or until the user wants to establish an outgoing call. In the time between, there is complete radio silence, except for occasional short signaling exchanges once every few hours to confirm to the network that the device is still switched on. With always on mobile Internet devices, this is going to change significantly, as the following example shows:

In previous blog entries, I’ve described how well the SIP VoIP client works on the Nokia N95. It blends in very nicely with the rest of the phone’s functionality and I can’t tell the difference between a cellular call and a VoIP call over Wifi. On the radio layer, however, things could not be more different.

While the cellular telephony application remains silent while no call is ongoing, the VoIP part remains quite active on the IP layer. Per minute, there are at least 10 message exchanged between the mobile and the network for various reasons such as keeping communication ports open on NAT firewalls. While it doesn’t seem to be an issue for battery capacity, as there is still ample capacity left in the evening despite being logged into the SIP server over Wifi all day,  it does have implications for cellular networks once VoIP is used there, too. While not all messages exchanged over Wifi will appear in cellular networks, at least 6 of those 10 are relevant for that scenario as well.

Today, each cell serves about 2000 users. For the network, this is not a problem since most mobiles are dormant. In a world where most mobile devices are IP enabled and use a standard SIP VoIP client, 2000 x 6 (or even more) message exchanges per minute means 12,000 message exchanges per minute over a single cell for more or less nothing.

To stay with the SIP VoIP example, here’s an overview of what I traced with my Wifi Tracer during a typical 60 seconds time interval while the SIP client is running and the phone is connected to a Wifi network:

  • At 7 seconds into the minute, the N95 wakes up because it receives a notification from the access point that an IP packet has arrived. It sends ‘power save poll’ management frame and receives an IP packet from the STUN server or the SIP server. In total, the mobile transmits 4 frames and receives 4 frames during this message exchange (including acknowledgments at the MAC layer).
  • At 11 seconds into the minute, The N95 decides to return the polling gesture and sends 1 packet to the STUN server and 1 packet to the SIP server. The STUN server sends a confirmation. Afterwards, the mobile enters the Wifi sleep state and informs the network with a corresponding management frame. In total, the mobile transmits 4 frames and receives 3 frames.
  • At 12 seconds into the minute, the mobile has to turn on it’s transmitter again because there is some data waiting again. It sends a poll frame and receives an ARP broadcast as the access point queries all IP addresses in the subnet. The mobile answers the ARP request and goes back to sleep. In total, the mobile transmits 7 frames and receives 6 frames.
  • At 16 seconds into the minute, the mobile feels a sudden urge to check that the MAC address of the router is still valid. This is as unnecessary as the ARP request from the router at 12 seconds, but it’s happening at least once a minute. The mobile transmits 2 frames and receives 2 frames.
  • At 22 seconds into the minute, a keep alive frame is received from the SIP or STUN server. I stop counting frames at this point.
  • At 34 seconds into the minute, the router runs another ARP request for all IP addresses in the subnet.
  • At 35 seconds, the SIP/STUN server sends a keep-alive frame.
  • At 37 seconds, the mobile returns the favor.
  • At 46 seconds, the mobile returns to sleep state and signals this to the Wifi Access Point.
  • At 49 seconds, the SIP/STUN server sends a keep alive frame.
  • Silence until 4 seconds into the next minute.

And now imagine you have a push eMail client and Instant messenger running, which will create even more traffic and 2000 other mobiles in the cell doing the same.

Standards bodies seem to have become aware of this issue, at least to some degree and have started to specify radio interface enhancements to counter the challenge. In case of UMTS and HSPA, the following come to mind:

I am not sure how LTE and WiMAX handle such very low speed but persistent message exchanges on the MAC layer. If anyone can give me pointers to that, I’d really appreciate.

The Origin of the Nokia Tune

Each and every of the billions of Nokia mobile phones that has been shipped in the last decade comes with it: The now famous Nokia tune. But do you know where it originally comes from?

Believe it or not, the few notes now known to most people around the globe are from the Valse Grande by Francisco Tárrega, a Spanish composer who lived in Spain between 1852 and 1909. For the details see the Wikipedia entry on Francisco Tárrega and the Nokia Tune.

Thanks to Guy Daniels of Telecom TV, who made me aware of it in his movie ‘Mobile Planet‘, which has been released last week in London. And here’s a link to the trailer, where you can hear, of course, the Nokia tune.

Web 2.0 Community Now Also Doing The Advertising For Products They Like?

Here is an interesting blog entry on the Nokia Beta Labs Blog: Tommi reports about the recent coverage of Nokia SportsTracker (which I like very much by the way) in Newsweek and about a YouTube advertising video that came out of the blue. Looks like it was not being done by Nokia!? Is the web 2.0 community now also coming up with the advertising for products they like? Incredible!

Sniffing Wifi Packets and Exploring Retransmission Behavoir

Since I figured out how to configure my eeePC for Wifi tracing with Wireshark, I’ve gained a number of interesting insights which go far beyond what you can read in literature on the topic. Standards are one thing, how they are implemented in devices are quite another sometimes. Here are some results I thought I’d share with you:

Wifi_retransmit
As a wireless medium is prone to transmission errors, each frame has to be acknowledged by the receiver. If the frame is not received it is automatically retransmitted. So how do devices do this in practice and how often does it happen? I’ve been able to do some pretty interesting traces with a Siemens Access point and a Nokia N95. The two devices are quite different in their radio characteristics as the antenna in the mobile device is far from ideal, especially if there is a wall separating the two devices as was the case in my test. Also, the behavior on the air interface is quite different, which has nothing to do with the antenna configuration.

The Siemens Access point is quite conservative concerning the use of modulation and coding schemes for a packet. The majority of packets are never sent with the maximum speed of 54 MBit/s, but instead with 36 MBit/s or even less. As a consequence, most packets are received correctly and are immediately acknowledged. In case there is no acknowledgement, the access point immediately reduces the modulation and coding by a step for the retransmission. As a result I haven’t seen many cases in which a frame has not been acknowledged after the second try.

The N95 on the other hand is pretty aggressive concerning modulation and coding scheme. It usually always starts with 54 MBit/s and even uses this setting for the first retry. If the first transmission has failed, the second usually does as well. Only for the third transmission is the modulation and coding scheme lowered to 48 MBit/s, then to 36 MBit/s and finally to 24 MBit/s. For subsequent frames, 36 MBit/s seems to be used for about 500 ms before the N95 Wifi chipset tries to go back to 54 MBit/s.

Also interesting is the repetition time in case no acknowledgment is received. On average, a packet is resent after around 0.5 ms. In case a packet is retransmitted 3 times, the resulting delay is about 2 ms. As a single frame usually carries 20 ms voice data, this time is negligible on the application layer and is far less than the jitter introduced on the way through the public Internet.

If you think three repetitions is a lot, you are right. However, I’ve seen cases in which the frame was transmitted 7 times before it was finally acknowledged. In this case, the backoff algorithm even allowed other stations to send their frame before the device could try all retransmissions. Pretty impressive.

The figure on the left shows how such a trace looks like in practice. Frame 6420 is sent but no acknowledgment is received. It is then resent as frame 6421 and then as 6422, after which an ack. is finally received. 6426, 6427 and 6430 are all the same frame, only acknowledged after the second retry. In between is 6428, a frame from another device together with its acknowledgment, 6429.

So that’s how it works in practice 🙂

Are We Going to See a Shootout between DVB-H and DVB-T?

Once the Nokia N96 hits the shelf it will probably be one of the first DVB-H (Digital Video Broadcast – Handheld) devices being shipped in large numbers. Not that DVB-H capable handsets haven’t sold for about two years now. However, DVB-H is only available in a few European countries such as Italy, and reception is not free. Maybe it is this fact coupled with licensing issues and access to the required spectrum that prevents mobile TV from taking off?

T-Mobile and Vodafone might think just that and have decided to launch DVB-T (Digital Video Broadcast – Terrestrial) capable handsets before the European football championship this year. The advantage: The DVB-T receiver in the mobile receives the non encrypted standard digital television signal for TVs. No subscription is required and there are no doubts concerning the programming, since users know it from their TV set at home.

Note that opening up the mobile platform to receive standard terrestrial programming is nothing new. In Japan, mobile TV seems to be quite popular, maybe just because among other things, there is also no subscription required to receive the program via the 1seq, the technology used there.

Critics say the DVB-T receiver chip is likely to consume more energy than the mobile optimized DVB-H chip. That’s probably true but the big question will be if it really matters…