I am a bit late to the Carnival of the Mobilists this week, probably because of some violent stomach problems and night shifts to get some equipment running in a wireless network in Italy. Anyway, here’s the link to this week’s edition of the Carnival of the Mobilists, this time hosted by Xen Mendelsohn. Great job I very much enjoyed reading it and many of the articles you point to. Next week, I’ll have the honor of hosting the Carnival, so stay tuned here!
The Timing Advance Is Back with LTE and WiMAX
In the high times of GSM, mobile enthusiasts equipped with mobile phones with an engineering menu had a lot of fun finding base stations by taking a closer look at the timing advance parameter. This parameter implicitly contains the distance to the base station the mobile currently communicates with. A GSM mobile requires this parameter as it has to start sending data in it’s timeslot earlier the farther it is away from the base station. This is necessary as radio waves only travel at the speed of light. If no adjustment is made, transmissions of a far away mobile tramples over transmissions in the next time slot of another mobile as they would arrive too late.
With UMTS things got a bit difficult as due to the CDMA approach of the radio interface a timing advance parameter was not necessary anymore. Unfortunately this makes finding specific UMTS base stations quite difficult. But don’t despair, LTE and WiMAX will require a timing advance parameter again since these systems are based on OFDMA and timeslots. This means that the network has to send timing advance information to the mobiles again to ensure their data always arrives at the instant it is supposed to. So network tracking should get easier again in the future!
Orange France Promo For UMA – Do You Get It?
Here’s a picture of an Orange France advertisement for their UMA service called "Unik" for which they now also offer a Nokia 6086. I like advertisement that is clear, easy to understand, that makes me want something and that gives me all the facts. But this one!? It tells people nothing… How should people know that this panel promotes a fixed/mobile convergence product and that you need a France Telecom DSL connection and Wifi access point to use it? Why should people want to buy this phone after seeing the pannel? There are so many other 1 Euro (with a * to the fine print) ads out there. I am really puzzled. Maybe my French readers can enlighten me?
802.16j: WiMAX For WiMAX Backhaul
One of the main OPEX (operational expenditure) drivers in mobile networks today is the requirement to connect each base station directly to the network. In most cases this means installing a cable or microwave connection at each base station site. For fixed line connections, UMTS/HSPA base stations mostly use a number of E-1 or T-1 links today, each being capable of transmitting about 2 MBits/s. With rising bandwidths, however, using several E-1 links for each base station will soon become difficult for both cost and availability of wires. Alternatives are fiber or using a transmission technology which can get more out of a pair of copper cables. Nevertheless, it will remain expensive.
For WiMAX networks, the IEEE has started in 2006 to work on 802.16j "Mobile Multihop Relay" (MMR). The basic idea behind MMR is to allow WiMAX base stations which do not have a backhaul connection to communicate with base stations that do. On the one hand this will of course reduce the bandwidth available to users in the cells involved in relaying packets. On the other hand it’s an elegant way to save costs and extend network coverage into areas where connecting a base station directly to the network via a fixed line connection is economically or technically not feasible.
Here are some ressources with more information:
Collaborative MIMO for WiMAX and LTE
In two previous blog entries I focused on the limited uplink power of mobile stations and how WiMAX, UMTS/HSDPA and LTE overcome this hurdle by allowing several mobiles to transmit simultaneously. In the future, however, limited transmission power might not be the only limitation.
WiMAX and LTE will probably both use a technology called MIMO (Multiple Input / Multiple Output) which makes use of multiple antennas at both the transmitter and the receiver to transmit independent data streams on the same frequency via different directions. Especially small hand held devices, however, might not be equipped with several antennas due to their small size or due to the additional cost incurred. Thus, they can not make use of MIMO. This reduces both their own speed as well as the overall speed of the network.
The solution to this problem is called "uplink collaborative MIMO" or multi user MIMO (MU-MIMO). Here, the network can instruct, for example, two mobiles to transmit simultaneously, each on an independent MIMO path. Even though both signals are sent on the same frequency, a MIMO capable base station will still be able to pick up the signals independently from each other if the main energy of each signal arrives from a different direction. This in effect creates a MIMO channel, just that the two or more antennas do not belong to one terminal but to several. Interesting approach!
From what I can read in the press, only Nortel has so far picked up on this and has stated that it will implement collaborative MIMO in the uplink direction for both WiMAX (here and here) and LTE (here).
HSDPA Performance Of Vodafone’s 3G Network in Italy Has Me Puzzled
Back I am in Italy for a while. I’ve grown quite accustomed to the great performance of the TIM HSDPA network, which I’ve described in a number of previous posts. This time around, I set out to test the Vodafone HSDPA network in Rome and to compare it with the results achieved in TIM’s (Telecom Italia Mobile) network. The results were quite a surprise.
I had two SIM cards to test the network. For the first tests, I used my German Vodafone SIM card and a Roamer WebSession, described in more detail here, to establish an Internet connection. As already experienced in the SFR network in France, file download speeds were capped at around 45 kBytes/s. While already quite good it falls far short of 160 kBytes/s that are reachable with my category 12 Sierra Wireless 850 HSDPA card in the TIM network.
In France I was quite uncertain if and where the speed was throttled down. With the help of the Vodafone Italy network, I can now add a further piece to the puzzle which unfortunately raises more questions then it answers. To find out more, I bought a local Vodafone prepaid SIM card for direct access to the Internet and not via the GGSN of Vodafone in Germany used by the German Vodafone SIM card. To my great surprise the download speed of the file was almost the same as with the German SIM card. In the IP packet inter-spacing diagram (for an introduction of how to interpret the diagram see here), however, the download of the same file with the two different SIM cards in the same network looks completely different. As can be seen in the first graph on the right side, the file download via the German Web Session in the Italian Vodafone network shows IP packet inter-spacing mostly around the 30 ms line. A clear but not yet conclusive indication for throttling. With the Italien SIM card however, most packets of the same file are transmitted with a packet inter-spacing time of 10 ms as can be seen on the left side of the graph. So the transmission would be much faster if it were not for the randomly distributed packet inter-spacing of quite a lot of packets between 50 ms and 200 ms. To be honest, I have no idea why some packets take such a long time to arrive. I don’t think it can be RLC retransmissions as the automatic retransmission of packets discarded by the Node-B’s HARQ process usually takes around 80 to 100 milliseconds. Also these inter-spacings were not caused by IP layer retransmissions.
More clues
I then went on to do a direct comparison of the performance of the TIM network and the Italian Vodafone network by downloading two files from different servers to exclude the possibility that the Vodafone network has a problem with the connection to one file server. The result is shown in the second graph. On the left, the download speed for file 1 and file 2 are shown for the Vodafone network. Note the constantly changing top speeds. Afterwards I replaced the Vodafone SIM card with the TIM SIM card in the wireless card and performed the same downloads in the TIM network. The result is shown on the right side of the graph. The throughput is fairly constant and much higher than in the Vodafone network. When looking into the Wireshark trace the Vodafone throughput suffers from two things. First, the random packet inter-spacing times described above. Second, I have observed IP layer retransmissions every couple of seconds which also greatly reduce the download speed. The TIM network does not suffer from any of those.
Conclusions
As I repeated the tests over several days and at different times of the day a temporary error or network overload can be excluded as the reason. There are two likely causes for the problems observed in the Vodafone network. The most probable one is that there is an incompatibility between my Sierra Wireless 850 HSDPA card and Vodafone’s HSDPA network. It’s still early days for HSDPA so I would not be surprised if this were the case. Another possible cause could be that Vodafone has a big IP routing problem somewhere in the network. A good way to verify this would be to repeat the tests with a different HSDPA card or mobile phone. If the situation improves it’s an interoperability issue. If not, well, then it could still be both.
Carnival of the Mobilists 72 over at 3-Lib
Another week another Carnival of the Mobilists, this time written by Steve Litchfield over at 3-Lib. In case you haven’t heard of the Carnival of the Mobilists before it’s the collection of the best blog entries of the previous week from the world of wireless. So don’t hesitate, head over and enjoy.
Music Phones Are (Audibly) Getting More Popular
Ricky Cadden lists a number of good points when it comes to why Music Phones still have some way to go before they reach the same usability as Apple’s iPod. Nevertheless, people have definitely started to use their phones for listening to music. There’s no better proof than to take a ride in the Paris metro these days. Today, I took the metro twice and each time I was in a carriage in which a person listened to the music on his phone without headphones.I am slightly annoyed at this behavior. I hope this will not become a new trend…
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.:
- STI Mobile, Boost, Amp’d (beware all non GSM/UMTS!)
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.