Author: Martin

MWC: 160 Characters Is Too Long, Network Load and Pictures

Fira1
Sunday, 15th of February and I am in the midst of the Fira to get things up and running for the Mobile World Congress. Not only physically but also virtually.

This year we are using Twitter to organize ourselves and by following the #mwc tag its fun to see what others are doing and experiencing. I noticed that while for other purposes 160 characters per SMS or Twitter message is far to short, for the current purpose it's actually too long. 160 characters are too long, that's a thing to think about in itself. When skimming through the #mwc messages on the mobile or the PC, the 100+ character "long" messages are just too difficult to read and I mostly skip them.

It's also interesting to observe the network load at the Fira. While the fixed line Internet worked quite well this morning, throughput is now quite erratic now in the afternoon. Vodafone's public 3G network on the other hand is still doing quite well. Let's see how that changes tomorrow with several thousands of people here in addition.

In case you are interested in pictures of what's going on here, have a look at Andrew Grill's Flickr Stream.

MWC: Blogging and Staying Connected with Less is More

Like every year I will post live from the Mobile World Congress in Barcelona again and provide my own angle on the show off the beaten path. Last year, I had a Nokia N93 and a Nokia N800 Internet tablet with me for the job. This year I was thinking about replacing the N800 with my eeePC as the web browser of the N800 is just too slow, i.e. very painful to use. However, after some deliberation, I decided against it, it is just too heavy to carry around all day. The benefit from having a full web browser and spell checker is just not big enough. So I decided that less is more and I will exclusively use my Nokia N95 and my Bluetooth keyboard to keep me connected during the week. Let the show begin.

MWC – Connectivity in Barca – Sometimes Even Plan C Fails

I arrived in Barcelona yesterday for the Mobile World Congress which does not start until next Monday (or Sunday, depending whether you count the pre-congress parties or not…) but it's nice to be here a couple of days early to relax a bit. I am usually used to just replace my SIM card with a local one or at least with one that has acceptable roaming rates and get connected in a couple of minutes. What I experienced yesterday, however, reminds me of days that I thought were long gone.

After having arrived at the airport I replaced my French SIM with one from German MVNO "Medion – Alditalk" to check my e-mails on the mobile before I could top up my local SIM card or buy a new one after baggage claim. Unfortunately, while the SIM card booked into the circuit switched part of two networks o.k., I could not get a data connection on the packet switched side (GPRS attach fail). O.k. Medion – Alditalk is not really known for their stable network operation so I moved to plan B.

From last year I still have two local SIM cards from Yoigo. One was not working anymore, while the other still booked into the network (both CS and PS attach ok). No idea why one is still activated and the other is not!? Anyway, so I went to the next store at the airport to put some money on it again (the balance was at zero since they deduct 6 euros per month if not used). Unfortunately I had to find out that you have to know the phone number to top up. Eh, sorry, can't remember, I am used to scratch card top ups…

All right, time for plan C, by now already at the hotel. My German Vodafone SIM card with the Websession option gives me 50 MB of traffic for 15 euros / 24h while roaming. While it worked fine on the Vodafone Spain 2G network, my N95 refused to work on their 3G network (PS attach ok, PDP context activation ok, but only spurious ping replies). Network/mobile incompatibility? Hm, so I put the SIM card in my E220 3G USB dongle but the effect was still the same. So either the 3G cell near the hotel is having problems or there is a more serious problem up the line.

So plan D for the moment is to use the Vodafone SIM on the slow GPRS network for mobile use and the crappy hotel Wi-Fi for the notebook. In the meantime the Medion – Alditalk SIM is booking into the GPRS network of Orange again, so I could now also use that.

Well, I guess that was not really my connectivity day… Let's hope things work out better today. All this and the 50.000 people coming to the congress haven't even started yet to put load on the networks in Barcelona.

Dual Carrier HSDPA – The Push Beyond 5 MHz

Over at LinkedIn, Eiko Seidel recently published a link to a whitepaper by Nomor research on Dual Carrier HSDPA (or Dual Cell HSDPA operation as it is called in the standards), a new feature currently worked on in 3GPP. I've been waiting for this feature to come out for quite some time now as HSPA+ has already added 64QAM modulation and MIMO to HSPA. Consequently, not much can be done anymore to improve performance in a 5 MHz channel.

In practice, dual carrier (DC) HSDPA means that two adjacent 5 MHz carriers can be bundled by the network and DC capable HSPA mobiles can be assigned resources simultaneously on both carriers. In addition to the higher throughput, the 10 MHz bandwidth can also be used to schedule mobiles more efficiently around fading conditions, which according to the paper, results in an efficiency gain of up to 7% with 32 users and up to 25% with 2 users.

By increasing transmission speeds the round trip delay time is also further reduced, good news for online gamers. I have to note, however, that current round trip delay times of around 100ms are hardly distinguishable anymore from the delay of a DSL line. What's still distinguishable are the longer delay times caused by state changes after some time of inactivity. That's addressed by another feature, though, the enhanced Cell-FACH.

The enhancements also brings a number of new terminal categories. In addition to HSDPA terminal categories 1-20 which exist today (most people these days have a category 6 (3.6 MBit/s) or a category 7/8 (7.2 MBit/s) device), category 21-24 terminals will be able to use two adjacent carriers. The conserve energy on the mobile device side, the network can dynamically instruct such terminals to only listen to a single carrier if the amount of data to be transferred is low and doesn't warrant the use of two simultaneous carriers which requires more energy for decoding.

For the moment, multicarrier HSDPA is only for the downlink direction and while 64QAM is included, MIMO is not. Theoretical peak throughput in the combined 10 MHz carrier is around 42 MBit/s. But I guess this is not the end of the story yet, I think it is quite likely that in 3GPP Release 9, uplink dual carrier and MIMO is added to the feature list. The authors go a step further and speculate that in the future the standard could include further enhancements to go beyond two simultaneous carriers and to even include simultaneous transmission on carriers not adjacent to each other, even in different bands (e.g. 900 + 2100 MHz simultaneously).

While it looks good on paper, it remains to be seen which operators will go for it in practice. Some operators are determined to squeeze out as much as possible of their 3G networks before going to LTE. By the time these features are market ready, I'd say two to three years down the line, it's quite likely that many 3G base stations will already be used with two carriers per sector. If the feature can be done in software only, I guess it could become quite popular. In that time frame, however, many of today's 3G base stations will be end of live and might be replaced with triple mode GSM, UMTS and LTE base stations. If the feature is required when LTE is also in the cabinet, well, that remains to be seen.

But one way or the other, this new round of enhancements show that there is still a lot of life left in HSPA.

And here's some background as to where the technical details can be found in the specifications: First, 3GPP TR 25.825 contains an overview of the feature. Nomor's whitepaper lists a number of Change Requests (CR) to add the functionality to the relevant specification documents (TS 25.211, 25.212 and 25.214). I've had a look at the latest versions in 3GPP Release 8 and those CRs have been approved and are part of the specs now. So it looks like Dual Carrier HSDPA will be part of Release 8 which will be finalized in this quarter.

Let's see if there is already talk about this at the MWC in Barcelona in just a couple of days from now.

Moving From Being Networked to Being Connected

In his book "Mobile as the 7th of the Mass Media" Tomi Ahonen has an interesting description of how the cellphone (I would call it 'the mobile device') changes the way people use the Internet: It moves the experience from 'being networked' to 'being connected'. It totally applies to me.

When I think back to the early days of the Internet, I was indeed networked: I specifically sat down in front of a computer to access the Internet, i.e. to get to my e-mails and to search for information. Once I got up and left the place, I was disconnected. Yes, I was networked, but not all the time.

Today, I don't go to a specific place anymore to access the network. Today, I have a mobile device and it is always connected to the Internet. I no longer open an Internet session and close it, it's just there all the time. I don't log on to check for my e-mails, an indication is already waiting on the idle screen when I look at it. When I search for something, I don't log onto the Internet and start the browser. The phone is already connected to the Internet and the browser is waiting in the background to be used again. And for that matter, so is my social network, the news and all those web 2.0 applications that allow me to see things created by my friends at the other end of the planet just seconds ago and let me put my pictures and content online as well for them to see. And all without sitting down and logging in. For me, the 'net' has become omnipresent, I am logged on 24h a day.

I haven't mentioned voice telephony so far specifically but that's because for me it's just an application running over the Internet as well these days. While I still use the circuit switched cellular network a great deal for voice calls, I most mintues are via Voice over IP now. Thanks Nokia for the great SIP implementation in the N95, it saves me a ton of money for those international calls.

But beware, being connected 24h a day does not mean that I am reachable for everyone 24h a day as well. It's a big mistake people make on both sides of the equation. Some think that once your are always connected you must be reachable. Others think you are forced into it and are thus trying to avoid it. I don't think so and I don't live it that way. With profiles that can be changed with the press of a button I decide who can reach me and who can't.

So I guess here's the difference for me between today and the past: In the past I had to decide when to connect. Today, I have to decide who can reach me at what time. I rather prefer it that way.

Breaking HTTPS Connections in Two Parts Considered Harmful

Last week, About Mobility and Masabists ran a story on the difficulties mobile transcoders have with secure HTTP connections and how they can put themselves into the the connection and thereby breaking end to end security. I've done some research on my own and since I am quite opinionated on the topic I wanted to post my results and thoughts here as well.

O.k. so first of all, what is the fuzz all about in simple words: Today, when somebody uses his mobile browser to connect to his bank, a secure HTTP (HTTPS) web connection is established to the mobile portal of his bank. HTTPS means that before any data is exchanged the banking portal sends a certificate the browser can cross check to ensure that the browser really talks to the server of the bank and has not been redirected to another site. After the certificate has been received, an encrypted end to end connection is established that no one, not even a mobile transcoder in the network can put itself into.

So for the user this is good since he can trust HTTPS to verify that he is really connected to the bank and he can also trust that all data that is sent and received is encrypted from end to end. For the transcoder this is bad since it has no chance to transcode the content and do other things with it.

So some smart people came up with what is called link rewriting to circumvent this 'issue', if you want to call it that way. With link rewriting, the transcoder doesn't forward a web page to the user with an original HTTPS link but with an HTTPS that points to the transcoder itself. When this HTTPS connection is established a secure connection is only established to the transcoder itself. The transcoder then establishes a second HTTPS link to the original server.

This means that the user no longer has a end to end encrypted protection but has to trust that the transcoder keeps his data secret. Also, the user can no longer verify if the transcoder contacts the original server, as the certificate that can be queried in the browser is that of the transcoder and not that of the original site.

In addition, this is totally transparent to the user as he will still see the "lock" icon that suggests an end to end secure and encrypted connection. Only when the user actively looks at the certificate will he actually see that the connection is terminated at the transcoder.

Counter measures: The only way for the user to ensure that this does not happen is to save the original https link as a bookmark. This way the transcoder has no possibility to rewrite the URL and hence can not put itself in the transmission chain.

From a user point of view I consider breaking a HTTPS connection in two parts as very harmful. It only takes one incident where data is stolen via a leak in the transcoder to damage HTTPS' reputation. Also, if it is suddenly acceptable to break HTTPS connections in two parts for reformatting purposes, why not also use it for statistics or to ensure no content the provider does not approve of traverses the network!? No way, the data inside an HTTPS connection only belongs to the user and the server at the end and to no one in between, no matter how good the intentions of the party in the middle are.

My Latest Book is Now Also Available at Amazon in the US

I've noticed today that after the launch of my new book last month in Europe, it is now also available on Amazon in the US. It took a bit for the bulk shipment from the UK to arrive but now that it's available, sales seem to have picked up quite quickly. At the moment, Amazon.com says only a few copies are left and more are on the way. Thanks to all who have already ordered a copy! So if you live in the US or Canada and have considered getting a copy from Amazon.com, now's the time. At some point it will go back to "shipping in 7-10 days" status but it's usually restocked much quicker now that it is available from a local distribution center.

Opera Mini for Android

As an avid user of Opera Mini I was very happy to see that the version v4.2 is now available for Android. Alphas and betas have been available since last October/November so announcing a non beta version probably means it's quite stable by now. Good news for G-Phone users and mobile Internet access in general. Kudos to Opera, looking forward to see it on the G-Phone in Barcelona at the MWC!

Vodafone Germany Now Earns More With Data Than With SMS

Interesting trend to observe these days in Europe: The revenue generated from data services in mobile networks is now close or even surpasses the revenue generated by SMS services. Recently, Vodafone Germany reported their numbers of the previous quarter compared to the quarter a year ago here and the table shows how data service revenue is now slightly higher than SMS revenue. Only a year ago, data revenues were still significantly behind.

What the table does not show, however, is that from an earnings point of view, SMS is probably still far ahead. After all, transferring 160 character messages through the network at a price of around 10 cents makes a far better bottom line than the megabytes of data transferred at a flat rate. But nevertheless, things are changing and it shows how mobile Internet access continues to increase in importance to mobile network operators.

SAE Review Part 2: Mobility and Connection Management

LTE and SAE are making big steps forward and the major specification documents are nearing completion. In part 1 of this mini-series, I've started taking a closer look at 3GPP TS 23.401, the main SAE (System Architecture Evolution) specification document and reported about the flexible meshed like architecture design. In this part, I'll have a look at Evolved Packet System Mobility Management (EMM) and EPS Connection Management (ECM) and their differences to Mobility- and Session Management of UMTS.

Before taking a look at the features in SAE, let's have a look at how similar things work today with UMTS as many of you will be familiar to that. Here, the SGSN at the border between the radio access network and the core network has two management tasks:

UMTS – Packet Mobility Management

The first is called Packet Mobility Management (PMM) and deals with keeping track of the whereabouts of mobile devices. There are three states: A mobile device is PMM detached when it is switched off or if it is not connected to the packet switched part of the UMTS network. That's the case for example if the device has been set to connect to the circuit switched part right at power on but not to the packet switched part unless it becomes necessary, i.e. the user wants to establish a data session. When a data session is established, the connection state changes to PMM connected. Afterwards, if the mobile is connected but hasn't exchanged data with the network for some time, the radio network controller (RNC) can ask the SGSN to release the mobility management connection. The connection then enters PMM idle state and the mobile only reports to the SGSN when it changes a routing or location area. If an IP address was assigned, it is kept. From the application layer point of view (e.g. the web browser) there is no difference between PMM connected and PMM idle.

UMTS – Session Management

The Mobility Management only deals with the whereabouts and tracking of the mobile device, so this state machine knows nothing about assigned IP addresses and contexts. This is task of Session Management (SM). Here there are only two states, either a device has a session and an IP address or it hasn't. 

And now to SAE / LTE

In SAE things work a bit different and I guess that's the reason why the mechanisms had to change as well. The biggest difference in SAE is that once a mobile device is switched on it always has at least a default bearer. In other words, it always has an IP address when it is switched on. And again in other words it's not possible for a mobile device to be attached to the network and not have an IP address. Hence, session managements makes no sense in LTE/SAE. To reflect this, the following two state machines are used in LTE/SAE:

EPS – Mobility Management

This EMM state machine only has two states. When a mobile is switched off or uses a different radio access network technology (e.g. GPRS or UMTS) it's state is EMM deregistered. That's simple. There's an optional feature referred to as Idle-mode Signaling Reducation (ISR) described in Annex J of 23.401 that changes that rule somewhat but let's ignore it for now. Once the mobile sees an LTE network it tries to register and if successful it's state is changed to EMM registered. At the same time the mobile is also assigned an IP address. As a consequence mobile devices in EMM registered state always have an IP address. But the EMM state machine does not care about that fact, it is only influenced by mobility management procedures such as Attach, Detach and Tracking Area Updates. While in EMM registered, the network knows the location of the mobile device either on a cell level or a tracking area level. Which of the two depends on the connection management state machine described right below.

EPS – Connection Management

When a mobile device is registered (EMM state = registered) it can be in two connection management (ECM) states. While a data transfer is ongoing the device is in ECM connected state. For the mobile device this means that on the radio link a Radio Resource Control (RRC) connection is established. For the network, ECM connected means that both the Mobility Management Entity (MME) and the Serving (User Data) Gateway (SGW) have an connection to the mobile device via the S1 interface (the physical and logical link between the core network and the radio access network). in ECM connected state, the location of the mobile is known to the cell level and cell changes are controlled by handovers.

If there is no activity for some time, the network can decide that it is no longer worthwhile to keep a logical and physical connection in the radio network. The connection management state is then changed to ECM idle. Note the use of the term 'idle'. It doesn't mean the connection completely goes away. Logically, it is still there but the RRC connection to the mobile is removed as well the S1 signalling and data link. The mobile continues to be EMM registered and the IP address it has been assigned remains in place. In ECM idle state the location of the mobile is only known down to the tracking area level and cell changes are performed autonomously by the device without any signaling exchanges with the network

Interactions With the Radio Interface

From the base station and mobile device point of view there is a lot of room for maneuvering between ECM connected and ECM idle. While a lot of data is exchanged, the air interface can be fully activated for a device so it has to continuously listen for incoming data. In times of lower activity or even no activity at all, the base station can activate a discontinuous reception (DRX) mode so mobile devices can power down their transcievers for some time. The power down cycles range from milliseconds to seconds. In fact, the longest DRX cycle is as long as the paging interval. So from a mobile point of view the main difference between being in ECM connected state with a DRX cycle the length of a paging intervall and being in ECM idle state without a radio interface connection is how it's mobility is controlled. In ECM connected state, handovers are performed, in ECM idle state, it can change its serving cell autonomously and only has to report to the network when it leaves the current tracking area. In other words, the base station is likely to keep the mobile device in ECM connected state for as long as possible by using DRX so data transfers can be resumed very quickly before cutting the link entirely and setting the state to ECM idle.

Summary

Quite difficult to make a summary as Mobility Management, Connection Management and air interface DRX control are in theory independent from each other but have to be looked at in common to make sense. In a rough generalization I would say that during normal operation:

  • a mobile is always in EMM registered state because it's identiy is known to the network and, implicity, an IP address has been assigned;
  • a mobile transfering data is always in ECM connected state;
  • a mobile not transfering data is also in ECM connected state but DRX is activated on the air interface;
  • only mobile whith very long periods of inactivity are in ECM idle state while staying EMM registered.

I hope this look at EMM and ECM from different points of view have made the concepts a bit clearer. In the next part of this mini-series, I'll have a look at the different handover variants the SAE architecture supports to ensure the mobile device is always best connected. As always, comments are welcome.