Another week, another Carnival! This time the Carnival has stopped over at Wap Review and Denis has done an outstanding job to give an overview of the best people in the blog sphere have written on mobile topics in the past week. So if you haven’t done already, head over and enjoy!
GPRS and EDGE Multislot Classes
The upload and download speeds that can be reached with a GPRS and EDGE mobile phones depends on a number of factors. First, the capacity available in the cell. Usually, voice calls take precedence over GPRS traffic and in case not enough time slots are available for both, some GPRS timeslots are sacrificed for voice calls. Second, the capability of the mobile, or it’s multislot class. Third, the network has to match the abilities of the mobile device. And fourth, the reception conditions experienced by the mobile.
The most comment GPRS/EDGE mobile station class today seems to be multislot class 10. Mobiles of this class can use 4 timeslots in downlink direction and 2 timeslots in uplink direction with a maximum number of 5 simultaneous timeslots. Depending on the amount of data to be transfered in the uplink the network will automatically configure the ongoing data stream for either 3+2 or 4+1 operation.
Some high end mobile, usually also supporting UMTS also support GPRS/EDGE multislot class 32. According to 3GPP TS 45.002 (Release 6), Table B.2, mobile stations of this class support 5 timeslots in downlink and 3 timeslots in uplink with a maximum number of 6 simultaneously used timeslots. If data traffic is concentrated in downlink direction the network will configure the connection for 5+1 operation. When more data is transferred in the uplink the network can at any time change the constellation to 4+2 or 3+3. Under the best reception conditions, i.e. when the best EDGE modulation and coding scheme can be used, 5 timeslots can carry a bandwidth of 5*59.2 kbit/s = 296 kbit/s. In uplink direction, 3 timeslots can carry a bandwidth of 3*59.2 kbit/s = 177.6 kbit/s.
While the numbers are quite impressive it should not be forgotten that a single carrier has 8 timeslots of which two are reserved for signaling on the first carrier. Using 6 timeslots for a single mobile means almost the complete carrier is used for the data transfer of a single mobile. Under realistic conditions it’s unlikely a cell has that much free capacity to offer next to voice calls.
WiMAX Base Station Prices and Coverage Ranges
The past week saw an interesting meeting of WiMAX operators, consulting companies and equipment vendors in Germany. Not a lot was reported from it except for this very interesting article on Heise. The technical details given in the report are quite interesting:
WiMAX Range
Haven’t we heard of incredible WiMAX base stations coverage ranges of 70 kilometers before!? Now that first networks are being rolled out the numbers suddenly sound a lot more realistic. The presenter of DBD, one of the WiMAX operators in Germany was saying that the coverage radius of a typical base station in urban areas is around 500 to 900 meters. In rural areas they are planing cells with a radius of 4 km. I think that this is quite a realistic number now and quite similar to the coverage areas of GSM and UMTS/HSDPA base stations today!
Customers per Base Station
The DBD presenter is quoted in the Heise article that the company is aiming for 200 users per base station at network launch and that they are seeking 400-500 users per base station in the long run. Again, numbers which are similar to GSM/UMTS networks. In a typical GSM deployment, each base station servers about 2000 subscribers. This number is much higher than the one given by DBD but I think they are nevertheless comparable as lot of those people are not using their phone a lot and thus do not require a lot of bandwidth and do not spend a lot of money. The 400-500 users DBD says it would like to have per base station would probably not be occasional users but such with a broadband Internet contract for,let’s say, 20-40 euros a month similarly like DSL or cable customers.
Base Station Prices
Heise quotes Alvarion saying that their next generation Micro WiMAX base station will be available for less than 10.000 euros. Prices of UMTS base stations are reported by Unstrung to be less than $24.000 these days. It’s a bit difficult to compare the two numbers number as the report says nothing about how many transceivers, bandwidth, capacity such a base station would offer. Also, I found a comment in the UMTS base station price article linked above quite interesting which mentions that today the cost for installation, site acquisition costs, etc. is much more dominant than the cost of the base station itself.
Market Strategies
For areas in which DSL and cable are already available, WiMAX operators will have to think hard about what they can offer customers that DSL and cable can’t to make people come over to them. The Heise article once more quotes the DBD manager saying that they will try doing this by providing telephony service on their network. While this is certainly a good approach it’s nothing new and nothing that isn’t already offered by DSL and cable operators today. So from my point of view they should concentrate more on the mobility of their solution and market that accordingly. Especially for students, commuters and mobile workers, a high speed Internet access both at home and while on the go in one package and for one price will make more sense than DSL at home and WiMAX/HSDPA/EVDO on the go. This limits the competition to UMTS and EVDO networks.
DBD will start deploying 802.16e kit beginning in 2008 and will also retrofit their existing network. While this will require exchanging customer terminals it will also provide them with the ability to offer notebook adapters and (hopefully) even notebooks with built in WiMAX chips from Intel. This should also lower site installation costs as mobile WiMAX will depend less on installing external directional antennas at the customer premises.
Other recent blog entries on WiMAX:
Using Wireshark to Peek Into the UMTS Air Interface
Since having arrived in Italy I’ve enjoyed using UMTS and HSDPA networks to connect to the Internet for my day to day work for reasonable prices. It has also allowed me to run a number of network traces to get some more insight into the performance of the UMTS and HSDPA air interface. In the first part of this mini series, I’ve been looking at the data transfer rates and packet delay that I could get on some of the networks. While having been used to UMTS speeds for quite some time now I was even more positively surprised by HSDPA speeds of up to 1.5 MBit/s which I was able to reach in the network of Telecom Italia Mobile (TIM). For the details take a look here. Part two of the mini series focuses on how the network tracing tool Wireshark can be used to chase after some UMTS specific air interface phenomena.
Wireshark is a great tool to analyze any kind of network traffic over any kind of PC interface. The picture on the left (click to enlarge) shows a trace which was generated using Wireshark’s "TCP Round Trip Time Graph" statistics module and commented by me. In my opinion the name of this module is quite misleading as the graph does not show the TCP round tip time evolution during a transmission but rather the reception time delta between TCP packets over time. A better name for the analysis module would thus be TCP inter-packet spacing diagram. The y-axis of the diagram shows the time in seconds that has elapsed between two TCP packets while the x-axis represents the time. To generate the diagram I downloaded a large file from an FTP server.
The graph shows that at the beginning of the transfer the inter-packet time was around 0.1 seconds or 100 ms. The TCP packet size of the download was 1500 bytes. These two values can now be used to calculate the transmission speed. One packet every 0.1 seconds means that on average 10 packets are received per second. As each packet has a size of 1500 bytes, 10*1500 bytes = 15.000 bytes are received per second. As each byte has 8 bit the resulting transmission rate is 15.000 bytes/s * 8 bits = about 120 kbit/s. This corresponds quite nicely to the maximum transmission rate that can be reached with a 128 kbit/s radio bearer with a CDMA spreading factor of 16.
At about the middle of the diagram the inter-packet spacing suddenly becomes much smaller, about 35 milliseconds. Doing the same calculation again results to 1/0.035 = 28.5 packets/s. 28.5 packets/s * 1500 bytes = 42.857 bytes/s. 42.857 bytes/s * 8 bits = 342.857 bits/s. This is close to the maximum transmission rate of a 384 kbit/s radio bearer with a CDMA speading factor of 8. In practice this means that at this point there were fewer people using the UMTS cell which in turn reduced the load of the cell. The network then decided to upgrade my radio bearer from a spreading factor of 16 to 8. As can be seen in the diagram the fun didn’t last long as the bearer was reduced again to 128 kbit/s for a short time. Then, I was upgraded again to 384 kbit/s for the remainder of the trace.
The same behavior could also have been achieved by physically moving through the network during the download and thus change reception conditions. As I was not moving with the mobile during the test the effect was definitely caused by changing load and changing interference conditions in the cell. Whether the load was caused by other people doing data transfers or by voice calls can’t be told from the diagram. What can be told however is that the network at this time was highly loaded as the network always assigns the best possible radio bearer.
So much for now. In the next entry of this mini series I am going present the results of the same test performed in a 3.5G HSDPA network. I can already promise spectacular and thought provoking results! Watch this space!
German Railway Extends Wifi On Trains Coverage
It looks like German Railways (Deutsche Bahn, DB) has had some positive feedback from their Wifi on Train pilot on the track from Dortmund to Cologne and has decided to expand the offer. Since 2005, seven high speed trains are equipped with on train Wifi which regularly run back and forth between the two cities. To connect to the Internet, Wifi to UMTS bridges were installed in the trains which used the 3G network of T-Mobile to backhaul the Internet traffic.
Now, DB has decided to extend the partnership with T-Mobile who will invest in new wireless base stations and tunnel coverage in 2007 on the tracks between Frankfurt – Hanover – Hamburg and Frankfurt – Stuttgart – Munich. The press release does not say if T-Mobile will use it’s UMTS network again or if they will deploy a new network based on Flarion’s Flash OFDM technology (now part of Qualcom) on the 450 MHz band as some rumors had it in the past (see here and here).
Deutsche Bahn will also increase the number of Wifi equipped trains to 50. Good news for me since I regularly use high speed trains between Munich and Stuttgart. Can’t wait to test it. After announcing to install power sockets at every seat this is yet another sign that DB has understood how to get new customers.
Carnival of the Mobilists over at AAS
This week, the Carnival of the Mobilists has arrived at All About Symbian. In it’s 66th edition, it’s as always a great roundup of what mobile bloggers had to say in the past week. Great job, Rafe, thanks a lot! So don’t wait and head over!
A Real Life Comparison of HSDPA and UMTS
These days I am totally unplugged but still always connected as I am staying in Italy for the moment, far away from my ADSL line back home. UMTS has kept me connected to the world in the past two weeks and I’ve been writing about my general experiences over at m-trends. So far, I’ve used a prepaid card from Wind to keep me connected. As Wind is not offering HSDPA (High Speed Data Packet Access) in their network yet the HSDPA card remained in the suitcase. Over the weekend, however, I’ve bought a prepaid card of Telecom Italia Mobile (TIM) where HSDPA is available. Their offer is not as good as Wind’s, giving me ‘only’ 500 MB for 20 euros over 30 days and an additional 1GB for an extra 20 euros should that not be enough. Still, for my purposes it should be more than enough. I am pretty much impressed by the speed increase HSDPA brings over plain UMTS. Also, responsiveness when clicking on a link in the web browser has noticeably increased as well. For the technical details read on.
The Hardware
HSDPA was standardized in a flexible way allowing data rates to grow as end user devices and networks become more capable. For my test, I used a Sierra Wireless Aircard 850, which supports HSDPA category 12 (inter-TTI of 1, QPSK only), i.e. a top speed of 1.8 MBit/s. Note that there are already category 6 mobiles and data cards available today promising speeds of up to 3.6 MBit/s by using 16QAM modulation in good coverage situations. However, my card is not capable of doing this yet. I am looking forward to compare the speeds of these two categories in a real network once I can get a hand on one. Enough about networks and terminal categories for the moment. For details you might want to take a closer look at my book 😉
Top Speed on Sunday Morning
There can be a big difference between theoretical maximum speeds and speeds that can be reached in a real environment. As I woke up early on Sunday morning I gave it a try when most other people were probably still sleeping, i.e. low overall radio network load from other people making phone calls and accessing the Internet. I was quite positively surprised in my first download test as the average speed for downloading a large file from the internet was about 1.5 MBit/s. Hey, that’s faster than my DSL line in Germany! It looks like TIM has not only upgraded their base stations to HSDPA but also ensured that the backhaul connection from the base station does not become the bottleneck.
I also downloaded the same file via the Wind UMTS network to be able to compare the behavior. As expected, the network load was also low and the download reached the highest possible UMTS download speed of 384 kbit/s. Also very nice but four times slower than via HSDPA.
The image above on the left shows a graph of the download as it happens. I started the download inside the apartment where radio coverage was far from ideal. Nevertheless, it can be seen in the graph that the download speed exceeded 1 MBit/s. Going to the balcony with the notebook after about half the download was finished improved the radio environment and the download speed even further.
Speeds at Other Times
To see how the network load impacts download speeds I ran the same test again at around noon on Sunday. This time my download speed was about 750 kbit/s or about 90 kbyte/s. The corresponding graph for the download is shown in the third image on the left. Note that I did not download the whole file which is why the download graph is not as long as in the previous image. Not quite as high in the morning but still quite respectable.
Web Browsing
The next test on my list was web browsing. I connected one notebook to the Internet via TIM’s HSDPA network and another one via Wind’s UMTS network. Then I surfed to a number of graphics intensive pages such as those from Nokia, CNN and a couple of German news magazines to compare first page display times and overall download times. While UMTS is by all means capable of delivering a good web browsing experience, HSDPA is by far quicker. All pages I tried always started to be shown a couple of seconds earlier on the notebook with the HSDPA connection than on the notebook with the UMTS connection. Needless to say that the time until the complete page is downloaded is also faster. I have to try again when at home with an ADSL connection in reach but I am pretty sure I would not be able to tell the difference between a DSL line and an HSDPA connection for web surfing except for the channel establishment delay described below.
Uplink Speed
TIM has also upgraded its radio network to support uplink speeds of 384 kbit/s. Note that this is not HSUPA (High Speed Uplink Packet Access) yet but plain 3G standards pushed to the limit. Even under average reception conditions, sending an eMail with a 2MB attachment was very quick with an average uplink data rate of about 350 kbit/s. Compare that to most 3G only networks today which usually support 64 kbit/s or 128 kbit/s at the most. 1 MBit/s ADSL connections usually have a 128 or 180 kbit/s uplink. So in this respect, current HSDPA even have a speed advantage in the uplink over a typcial 1 MBit/s DSL line.
Round Trip and Channel Establishment Delay
Round trip delays have also decreased a bit. While 3G connections usually have around 120-130 ms round trip delay times, I measured 90-100 ms to the first hop in the network over the HSDPA connection.
During the test it was also interesting to see that there is still a noticeable delay of 2.1 seconds in ping times or web page access time when no packets were transferred for some time. This is due to the fact that the network releases the HSDPA radio connection after some time of inactivity to reduce the power drain on the mobile’s battery and also the channel usage on the air interface. I experimented a bit and it seems TIM has set the transition timer to 15 seconds. Unless TIM has a stupid network implementation which drops the user to PMM IDLE state after this time, the 2.1 seconds are the time required for the transition from the FACH to HSDPA (DCH).
Summary
I am very impressed by the performance of HSDPA. Even my first generation category 12 data card exceeds a download speed of 1.5 MBit/s in a lightly loaded network and still over 700 MBit/s under normal network load conditions during the day. Uplink speeds beyond 350 kbit/s are very impressive as well. With further enhancements like category 6,7 and 8 handsets in the near future, multiple antennas in end user devices, enhanced receivers, improved signal processing, etc., etc., both end user speeds and overall wireless network capacity will continue to grow over the next couple of years. And beyond that, 3GPP Long Term Evolution is already in the pipe which ensures speeds will continue to rise. After all, the You-Tube generation needs as much bandwidth and speed as they can get!
Note: Click on the "HSDPA" category link below next to the date to see all articles on further tests which have followed afterwards.
Call Us Free By Skype
This is not a very mobile blog entry today unless you are using Skype via one of the third party clients now available. For many years I’ve heard people in the industry talking about how to integrate telephony functionality into web pages. Never saw a web site that actually did it until today. Take a look at the picture on the left. It’s part of a web page of Hotel Morgana in Rome, Italy. A Skype call button and the invitation to call them via Skype for free is placed neatly in the middle of the page. It makes total sense for them as they have guests from all over the world who would not be able to call national 0800 numbers. For them it also makes sense as the cost of offering the service for them is either zero as well or only very small in case they use Skype call forwarding to their fixed line phone. This is free for the caller and only costs a couple of cents to the hotel owner.
VoIP capacity over HSDPA
One of the main issues with VoIP over 3G networks is that the number of possible simultaneous calls per cell is much lower today than the number of calls that can be transported over 3G networks in circuit switched mode. This is due to the fact that the radio interface has been optimized on every layer to squeeze through as many circuit switched voice calls as possible. VoIP calls on the other hand are transported over IP which makes it impossible to specifically adapt each layer of the air interface for the application as each protocol layer is independent from the one above and below.
Another disadvantage to transport voice over IP is it’s requirement for real time data transmission. As voice data can be compressed quite well, the required bandwidth is quite small. In order to keep the delay acceptable a single IP packet only carries around 20 milliseconds of speech data. At this rate, the additional information generated by the air Interface, IP, UDP and RTP headers is almost the same as the actual voice data. This doubles the bandwidth required to transport a voice call over IP compared to transporting it over optimized circuit switched channels over the air interface.
As if this was not enough there is yet another problem that plagues VoIP over wireless: While most other IP applications benefit from retransmission of lost or damaged air interface frames, this is most unwelcome for VoIP as it’s better to loose a couple of frames rather than to wait for the retransmission. As the lower layers are not application aware, however, it’s not possible to carry voice and data of other applications over the same connection and treating them differently on the air interface.
HSDPA And Intelligent Scheduling Come To The Rescue
While I knew all this for some time now and was thus a bit pessimistic about mid-term success of VoIP over 3G and WiMAX networks, Harri Holma and Antti Toskala describe in their book about HSDPA, or 3.5G as sometimes called in the press, that VoIP capacity is not necessarily lower than 3G circuit switched capacity per cell. Compared to an average of around 64 simultaneous circuit switched calls per cell as referenced in their book, they present a study which results in at least equal or even higher VoIP capacity in an HSDPA enabled cell. So how’s this possible with all the difficulties mentioned before? Here are the main principles they used for their calculations:
HSDPA Capacity
Due to the use higher order modulation for mobile stations with good reception conditions, better error coding and fast re-transmission, total capacity of an HSDPA cell is twice as high compared to a 3G UMTS only cell.
Use of AMR
Many VoIP implementations today use the G.711 codec for digital voice transmission which requires a bandwidth of 64 kbit/s. For HSDPA cell capacity, the authors used the AMR codec instead, which is also used for circuit switched wireless calls today, which only requires around 12 kbit/s to achieve the same voice quality.
Header Compression
Compressing IP headers of VoIP frames is absolutely essential for capacity. Thus the authors have assumed the use of Robust Header Compression (ROHC) for their simulation. This is quite realistic for the future as ROHC between the mobile station and the RNC is already in the 3GPP standards.
Intelligent Scheduling
HSDPA packets have a transmission duration of 2 milliseconds. A 2ms packet, however, can hold several VoIP packets. To achieve the highest cell capacity the traffic scheduler has to hold enough packets destined for a user to fill up a full air interface frame before they are sent. While this increases the total VoIP capacity of the cell it also has the disadvantage to introduce unwanted speech delay. For their simulation the authors did not queue more than three VoIP packets for a single user. This introduces a maximum additional delay of 60 milliseconds.
Fast HSDPA retransmission
The retransmission problem for VoIP described above is reduced by HSDPA by it’s fast retransmission scheme. A faulty packet can be retransmitted within 10 milliseconds. If air interface parameters are used to ensure that at most two retransmissions are required before the packet can be deciphered correctly on the other end, a maximum additional delay of 20 milliseconds can appear.
Summary
Based on the assumption that an additional latency of 80 milliseconds is acceptable to the user, the authors show that HSDPA network can have the same or even better voice capacity than 3G networks have today for circuit switched calls. It’s still some way to go until we are at this point as enhancements have to be made on all parts of the network. But this study impressively guides the way forward!
Anti-Creativity: Let’s Help Some MVNO’s To Invent Additional (Roaming) Fees
This year has started with a lot of positive creativity on the operator side in Germany and elsewhere. Examples: Affordable prepaid mobile data is now available in a number of countries (Germany, Austria, Italy for example). Also, Vodafone has introduced a €15 euro a day roaming for data on prepaid SIMs and Three has even scrapped roaming charges for mobile voice and data altogether. It seems, however, that there is also some anti-creativity on the operator side these days when it comes to inventing new fees for so far free services. As reported by Teltarif in this article, Arcor and 1&1, two Mobile Virtual Network Operators (MVNO) of Vodafone Germany, have silently put fees for incoming SMS message while roaming into their fine print. Prices for incoming SMS messages are foreseen to range between 59 cents and 1.49 euros depending on the country. Charging for incoming SMS messages will start by mid of this year.
The problem with charging for incoming SMS messages is that the user can not stop receiving SMS messages while roaming today. So if these MVNO’s really start doing this, then the least I would expect of them is to offer the possibility for the user to block incoming SMS while roaming. Also, I would expect them not to charge users for sending SMS messages which are then blocked by the receiver. Well, let’s see what happens, I am still not convinced that they will prevail with this.
I am not sure yet if this is a move intended as a threat against current activities of the EU to lower roaming charges or if this will be another nail in the roaming coffin which will inflame decision makers in the EU even more and push them to reduce roaming charges even further.
More Anti-Creativity Suggestions
It seems some operators are getting desperate these days to compensate for falling prices for voice minutes. Here are some suggestions from my side in case you need more destructive ideas for your business:
- Charge users for switching their mobiles on and off
- Charge users making a call which is never answered
- Charge users for switching voice mail or call forwarding on and off.
- Charge users for the time their mobile is switched on per day
- Charge users for mobility, i.e. charge them when they don’t leave the phone at home but instead carry it in the pocket while roaming out and about.
All these events require network interaction and all of them are recorded. No problem to forward these records to the billing server. I am getting goose bumps…
So all jokes aside dear operators. Instead of this roaming SMS anti-creativity why are you not considering some positive creativity like your competitors as shown at the beginning of this entry to increase your revenue by offering interesting services at affordable prices?