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.
"Unless TIM has a stupid network implementation which drops the user to PMM IDLE state after this time" actually if TIM have an Ericsson RAN this would be exactly what would happen. I work for an operator that uses Ericsson and was suprised to find out this is how they implemented state transitions initially (to save on development costs I assume). This has been rectified in their subsequent release but as far as I am aware this is not operational in any country yet.
Posted by: Mark | March 31, 2007 at 12:41 AM