How File Sharing Of Others Drains Your Battery

Ever thought file sharing of others could have an impact on your mobile device’s runtime on a battery charge? If not, read on:

For DSL connections I think it is quite the norm rather than the exception that subscribers are assigned a public IP address. In wireless networks things are a bit different. While some operators also assign public IP addresses when customers establish a connection to the Internet from their mobile device, others use a pool of private IP addresses. When private IP addresses are used, which is similar to the way a DSL router at home maps several PCs and notebooks to a single public IP address, wireless clients are not directly reachable from the outside world. Thus, packets destined to ports from which a subscriber has not originated a connection are rejected. In networks that assign a public IP address, however, all packets are delivered.

From a technical point of view using public IP addresses rather than private ones for mobile terminals is the right thing to do. However, there is a big practical disadvantage: If the IP address assigned to a mobile terminal was previously used by somebody for file sharing, other hosts keep sending packets to that IP address as they are not aware the IP address is now used by someone else. In terms of bandwidth usage this is not a problem as the packets are small and only occur every couple of seconds. For mobile terminals however, this means radio resources are kept assigned instead of being removed during times of inactivity (e.g. reading a web page or simply doing nothing with the device for some time). This in turn has a drastic impact on how long the device can run with a single battery charge.

I’ve seen this happening several times now while being connected via a 3G network and running Wireshark on the PC. In addition I can also see Internet worms banging on my door every now and then trying to send messages to the Windows Messaging Service (a bug closed several years ago…)

Do We Have To Use Private IP Addresses With Future Always On Devices?

Today, this behavior is probably not an issue for most people as Internet sessions tend to be brief. In next generation wireless networks, however, mobile devices will always have an IP connection to the network because voice calls will use the IP network rather than a circuit connection like today. Further down the road when not only a few users but most will use SIP/IMS etc. clients, end user devices should be able to connect with each other without the need for a gateway. With private IP addresses and NAT (Network Address Translation) this will be rather difficult to do.

With IPv6, Network address translation, private addresses and dynamically allocated public IP addresses should hopefully be a thing of the past. So will IPv6 with static IP addresses be the solution to the problem or the beginning of the next one?

List of Countries With UMTS Prepaid Internet Access

In the past few months the list of countries in which mobile access to the Internet is now available via UMTS/HSDPA networks with prepaid pay-as-you-go SIM cards has grown considerable and I’ve been reporting about it quite a bit. Time for an overview with links to the corresponding blog entries:

Germany:

  • AldiTalk (0.23 euros per MB)
  • Vodafone (WebSessions: 1.95 euros / 15 min, 9.95 euros / 2h and 14.95 euros / 24h)

Italy:

  • TIM (volume bases, 500 MB / 30 days for 20 Euros)
  • Wind (volume based, 1 GB / 30 days for 20 Euros)
  • Vodafone (time based, 1.5 euros per 15 minutes)

Spain:

  • Yoigo (capped at € 1.20 a day)

Austria:

  • Drei (0.8 euros per MB)

U.K.:


Reports from people commenting:

Poland:

  • Plusgsm (Simdata (Plus GSM): prepaid, 0.03PLN/100KB = 0.30 PLN/MB = 0.08 EUR/MB.)

India

U.S.A.:

And for those countries in which Prepaid Internet Access via UMTS networks is not yet available, the German Vodafone WebSession offer with a German Vodafone prepaid SIM might help.

Vodafone Websessions with SFR in France

Spring is one of the best times of the year to be at the Côte d’Azur in France. While the weather and landscape is great, France is really missing attractive prices for Internet access over 3G. Not really affordable for post-paid customers, nothing is available for pre-paid customers at all. A good opportunity to use my Vodafone Germany prepaid SIM card for Internet access via the WebSessions roaming offer.

Downlink Speeds

Vodafone’s partner network in France is SFR and according to their web page, they’ve got HSDPA deployed in some parts of the network. Accordingly my speed expectations where high. And indeed, when I activated the 3G connection my data card showed that HSDPA is available in the network. Speed tests performed over several days revealed however, that the downlink speed is artificially limited to around 45 kBytes/s. The limitation is certainly not the air interface as the signal strength was good and speeds I measured in Germany and Italy were much higher.

It’s hard to tell from a users point of view exactly where the bottleneck is. It could be that Voda’s Home Location Register (HLR) in Germany and SFRs SGSN in France can not exchange the QoS profile correctly which subsequently leads to the throttling of my connection. It could also be that there is a limitation on the IP link used for forwarding my packets between the SFR network and the Vodafone Germany network. It’s also possible that the SFR SGSN or the Vodafone GGSN is unilaterally limiting my speed. In practice this means that HSDPA does not give me a great advantage in the SFR network over a 3G UMTS device as the speeds are the same. Due to this I was not sure if the connection was HSDPA at all. Subsequent tests described below showed, however, that the data card really got an HSDPA and not UMTS bearer.

Uplink Speeds

After getting a 384 kbit/s uplink bearer in Italy and Germany I was also disappointed about the ‘meager’ uplink speed of only 128 kbit/s in the SFR network. It’s likely that this is no interoperability or throttling problem but a general network limitation of the SFR radio network. Either they haven’t activated the higher bearer option or it’s not yet available in the current software version of their radio network. Whichever it is they should consider upgrading or switching on the option as the difference is remarkable.

Round Trip Delay Times

One of the indicators that the data card got an HSDPA bearer and not a UMTS bearers were the round trip delay times. With the data card I got a round trip time of about 170 ms. HSDPA usually delivers a round trip time to an external host of around 120 ms (100 ms to the first hop). The additional delay is most likely due to international roaming which means that my data is tunneled from SFR into Vodafone Germany’s network before entering the Internet via Vodafone’s GGSN. With a Nokia N93 3G ‘only’ terminal I got round trip times of around 380 ms. I am not quite sure why there are an additional 200 ms of dealy as UMTS is usually only around 50 ms slower.

Radio Ressource Management

On the positive side I noted that the HSDPA radio resource management was more advanced than what I experienced in the TIM network in Italy and the Vodafone network in Germany. While the HSDPA bearer is active, the above mentioned round trip times to an external host of about 170 ms can be observed. In Cell_FACH state, which TIM and Voda’s network in Germany might not support yet, round trip delay time s were around 360 ms. This reduced activity state was only entered after around 45 seconds. After about 60 seconds the connection is put into Idle, Cell_PCH or URA_PCH state from which it takes around 800 ms to get back into active state. This is a lot quicker then the 2 seconds observed in Vodafone’s network in Germany and TIM’s network in Italy.

Skype, VoIP and IPSec

I tried Skype and my companies VoIP client over both HSDPA and UMTS and got crystal clear connections. Also, my IPSec tunnel worked fine between the notebook and my company. Very well!

Wifi competition

Except for the artificially throttled speed, my experiences in the SFR network with the HSDPA card were very positive. I should also note, however, that some Wifi operators such as Orange have moved forward a bit as well and are now offering 10 hours online for 15 euros. The 10 hours can be distributed over 30 days. For 15 euros, one can stay online for several days if the connection is only used for a couple of hours a day. For me a Vodafone 24h WebSession for 15 Euros is still better because I am online for more than 10 hours a day. Also, I need access at different locations throughout the day which is difficult with Wifi hotspots. People with less online time and stationary use, however, might find a 10h over 30 days for the same price more attractive. Also, they are not limited to 50MB of traffic per WebSession Vodafone intends to introduce in September.

P.S. For more articles on this topic, click on the HSDPA link next to the date below

Mobile Internet Advertisment in Austria

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When it comes to mobile technology, Austria is usually the place where things happen first. Not surprising therefore, that Mobile Internet offers are not only on the front page of most mobile operators web sites but also being marketed out in the open now. Take a look at the picture on the left. When I was in Austria recently, there was an advertisement of ONE for their H.U.I. mobile Internet access offer at almost every bus stop I passed. T-Mobile is also putting large advertising banners in cities to market their latest offer of €0.- basic charge and 10 cents per Megabyte of HSDPA traffic (post paid).

A reason for promoting the mobile Internet so fiercely these days might be that the operators don’t have have much to advertise for anymore in the voice domain since Mobile Virtual Network Operators (MVNO’s) have caused a landslide in prices for voice minutes.

One of the things still missing in Austria is Internet access via prepaid SIMs. Let’s hope that will follow soon as well. Some German MVNO’s have set a good example.

Are Virtual COM Ports Fast Enough for HSDPA, WIMAX And Other Wireless Broadband Technologies?

I’ve been having a lot of fun lately with my Sierra Wireless HSDPA data card for my notebook which reaches speeds I still have difficulties to believe that they are possible over a wireless network. While many people probably prefer data cards for wireless Internet access I’d rather like to use my mobile phone to connect the notebook to the Internet. This is because I use connected applications both on the mobile phone and the notebook and don’t want to have two subscriptions for mobile data access. While the data card acts like a network adapter and thus has no speed limitation on the interface between the data card and the notebook, mobile phones simulate a modem over a serial port, also called a COM port. The question I thus had was if those virtual COM ports have the same speed limitations as their real counterparts. If so, this could be a problem for mobile phones that support HSDPA, WiMAX and other future wireless broadband technologies which can deliver higher data rates than real COM ports can transport.

The maximum data rate that can be set for a real or a virtual COM port is 921.600 bits/s. This is far too slow for modern HSDPA mobiles that support downlink speeds of up to 3.6 MBit/s. For real serial ports this limit is real as the speed setting in the modem dialog translate into commands to a UART chip to send bits over a serial wire at exactly this speed. Most if not all high end phones today, however, have no serial connector anymore. Instead they use USB for communicating with a PC. In order not to modify the dial up networking software of operating systems, serial port emulations over USB are used. To the dial up network softwae it looks like the ‘modem’ in the mobile phone is connected via a serial port. The question now is whether the same speed limitations apply to these virtual COM ports over USB as well. At least in the dial up networking menus, the maximum speed that can be set for them is also 921.600 bits/s. USB is much faster than HSDPA or any other wireless wide area network. Even USB 1.1 can already handle a speed of up to 12 MBit/s, so physically no limitation exists.

Virtual_com_speed_192_kbits
As I don’t have an HSDPA capable mobile phone yet I did the next best thing and used a Nokia 6680 UMTS mobile. Instead of setting the COM port speed to 921.600 bit/s, which is twice as high as what UMTS can deliver, I set it to 19.200 bit/s which equals a maximum transfer rate of 2.4 kBytes/s. This is much less than what a UMTS connection can deliver, about 40 kBytes/s. Then, I established a UMTS connection, started a download of a big file and monitored the throughput. As can be seen in the picture on the left, the COM port speed is set to 19.2 kbit/s = 2.4 kBytes/s. The download rate, however is 40 kByte/s.

This proves that the speed setting for a virtual COM port has no influence on the actual speed of the data transfer over the virtual connection. Thus, if both the driver on the PC and the mobile phone support higher speeds over the virtual COM port, HSDPA and WiMAX speeds will be supported without a need for modifying the dial up networking software of the operating system. Very well, so I am looking forward to testing things with a real HSDPA mobile phone!

U.K. Operators Now Offer Mobile Access To The Internet With Prepaid SIMs

The Times ran an interesting article this week about new data tariffs introduced by mobile operators. While mostly highlighting prices for contracts, they also mention at the end of the article that Orange and T-Mobile U.K. are also offering web access via their prepaid pay as you go SIMs.

That’s great news for three reasons: First, prices are now low enough and the terms and conditions are now easy enough to understand for allowing people to experiment with mobile Internet access. Second, especially young people are often on pay-as-you-go SIMs and were so far not permitted access at all or only at ridiculously high cost. And third, mobile access to the Internet via prepaid cards now also offers international travelers the opportunity to connect whenever and wherever they are in the U.K.

T-Mobile U.K.

Pay-As-You-Go Web’n’Walk: 0.73 pence per kB capped at 1 pound a day. The fine print here says that the transfer volume should not regularly exceed an average of 40 MB per day. That should be good enough for most purposes. They do not mention port blocking or other restrictions such as a restriction on IM and VoIP.

Orange U.K.:

According to the Times article the prepaid offer is called "Internet snacks". The price is 40p for 15 mins with daily cap of £2
or a daily £1 bundle. I tried to find the details on the Orange U.K. page but they have hidden the offer very well. I searched for 10 minutes but could not find anything so I have no idea what the restrictions of the offer are.

How LTE Deals With Limited Uplink Power

In a previous blog entry I’ve been looking at how WiMAX and HSDPA allow several mobiles to simultaneously use the uplink. This is necessary as the power output of a terminal is much lower than that of a base station. Due to this restriction a single terminal can not use the total uplink bandwidth of a channel. The only way to compensate for this is to allow several mobiles to transmit at the same time. After writing this article a reader asked how LTE (Long Term Evolution), the successor of HSDPA/HSUPA, deals with this. So here we go:

In downlink direction, LTE is based on Orthogonal Frequency Division Multiplexing (OFDM) technology, quite similar to WiMAX (802.16e). While WiMAX uses OFDMA (Orthogonal Frequency Division Multiple Access) modulation in uplink direction, it was decided by 3GPP to go a different way for LTE. Here, SC-FDMA (Single Carrier – Frequency Division Multiple Access) will be used. It took me quite a while to figure out the basics of SC-FDMA but I think I’ve finally got the basics right and have posted the results of my research here.

So why doing it differently?

It looks like while OFDMA has many advantages it suffers from bad Peak to Average Power Ratio (PAPR). Again, I didn’t find an easy to understand explanation of PAPR and it’s implications on the web. Therefore I decided to do some of my own research and I am very thankful to a number of readers who have helped in the process. The results are presented here.

Deep Inside The Network: How UMTS And WiMAX Deal With Limited Uplink Power

Lately, I’ve been thinking a bit how different wireless systems deal with the fact that the power output of a mobile phone is much lower than the power output of the base station. In practice this means that uplink data rates per mobile phone can not reach the same level as in the downlink. Most systems today use a different frequency ranges for uplink and downlink (FDD, frequency division duplex) with the same bandwidths. This means that if only a single mobile can transmit in uplink direction at a time, bandwidth is wasted due to the power limitation.

UMTS / HSUPA / E-DCH

3G networks use Code Division Multiple Access (CDMA) in both uplink and downlink. This means that several mobile phones can send their data at the same time to the base station, each with a different code. The base station knows the code of each terminal and is thus able to extract the simultaneous data streams from the single incoming signal. This way, the data rates of all mobiles can be added up and the uplink is used very efficiently, despite the limitation in uplink power. A single mobile is not able to fully use the available bandwidth due to the power limit. If several terminals communicate with the base station, however, as is usually the case, the uplink frequency band can be used to its limit. This method applies to both 3G UMTS and 3.5G HSUPA (aka E-DCH) as they both use dedicated bearers.

WiMAX

The WiMAX air interface uses Orthogonal Frequency Division Multiplexing (OFDM) in both uplink and downlink direction. Basically, the OFDM approach splits the total available bandwidth into independent sub-channels and data is sent simultaneously over these sub-channels. As UMTS/HSPA terminals, WiMAX terminals are also power limited and therefore face the same problem. Contrary to the code division approach described above, WiMAX assigns different sub-channels in the uplink to different terminals. Thus, each terminal can focus it’s power on fewer sub-channels. In other words a terminal can put more power in a sub-channel if it doesn’t have to use all of them. Other sub-channels not used by the terminal are assigned to other terminals. This means that several terminals in effect communicate with the base station in uplink direction simultaneously.

The comparison shows that both UMTS and WiMAX have interesting ways to ensure that several mobile terminals can communicate with a base station simultaneously in the uplink direction to counter the restricted power output and to use uplink resources efficiently. The way it is done, however, is quite different.

Vodafone WebSessions Tested With A Nokia N93

In the previous blog entry I’ve taken a look at how to use the Vodafone Germany WebSession offer for data roaming with a PC. As a ultra-mobile roamer, however, I use my mobile phone almost as much as my PC when traveling. So the natural question is, are WebSessions also usable with the N93 on-board applications?

The simple answer is yes, but with a slight quirck 😉 As described in the previous entry a web session starts and is restarted by pressing the accept button on the WebSession portal in the web browser. This enables an "almost" transparent tunnel for IP traffic between the terminal and the Internet. Activating a web session with Nokia’s built in browser is easy and works flawlessly. Afterwards, all other applications in the phone can also use the connection so sending and receiving eMails for example works well.

So here’s the catch: It’s also possible to connect to the network from the eMail program. However, no data can then be transferred because each time the web session is entered again the portal web page has to be touched first. In order to use any other application the browser thus has to be started first to touch the portal page. Afterwards all other applications work. While at least one application is running to keep the connection to the network open the browser can be closed. As soon as the last application that requires Internet access closes, however, the only way to restart the session later on is to start the browser again first. Not very convenient, but practicable.

Another good thing is that a Web Session initially opened on the N93 can be re-used on the PC later on. This is very important to me since I usually use both devices throughout the day.