Almost exactly 2 years ago, I've first seen a demonstration of a cat Cat 7/8 HSDPA device and went away very impressed after seeing downlink speeds of 4.2 MBit/s. Now I've finally gotten my hands on one myself and tested it in Vodafone's network in Germany. On the 6th floor of a building, the base station pretty close by (I assume) and my download speeds easily exceeded 5 MBit/s (= 700 kbyte/s) consistently over several days and at different times of the day. Wow, that's twice as fast as my ADSL line. But that's not the end of the line, a few operators have already rolled out HSPA+ with 21 MBit/s. Examples are Mobilkom Austria in Vienna and Telstra in Australia.
Web Browsing over DSL and 3G Compared Side by Side
Web surfing over 3G must be slower than over DSL, right? To see if this statement is correct, I used two computers side by side, one connected to to Internet over a 3 Mbit/s DSL line and the other one over a 3G dongle. The 3G network of choice was that of Vodafone Germany in Cologne with the base station at least supporting HSDPA category 8 (7.2 Mbit/s) devices.
For the first web browsing test I used a Huawei E220 3G dongle with a somewhat older but very reliable HSDPA category 6 software load. Computer screens side side by side I simultaneously clicked on links to load web pages, both visited and never visited before, to see on which computer the pages displayed first. The result: In this test, the web pages took around one second longer to be first displayed on the 3G connected computer but the difference was quite minor. Definitely less than what I expected.
In a second test, I used an HSDPA category 8 + HSUPA capable E176 which has been available on the market already for a little while. Definitely not the latest and greatest anymore but still good hardware. With this setup the side by side comparison showed no difference anymore, pages on both computers showed up almost instantly. Sometimes, the page would show on the DSL connected computer a fraction of a second earlier, sometimes it would be loaded a bit quicker over the 3G connection. Fantastic!
A little caveat: At the time of the test the network was only slightly loaded so one of these days I have to find a time at which it is a bit busier and repeat the test to see if that makes a big difference.
HSPA and TCP Window Auto-Scaling
Back in 2006 I noticed that my Windows XP machine could not fully use the bandwidth of my ADSL line and also throughput over HSDPA to some servers was less than I expected. As I found out at the time, the fixed and small TCP window size was responsible for the behavior. In Windows XP, things could be tweaked by changing the window size in the registry as I described here. When running some throughput tests with Ubuntu Linux this week with an HSPA 7.2 MBit/s 3G stick, I noticed that no tweaking was necessary to get the full speed.
A quick look with Wireshark revealed why: Unlike Windows XP that has a static window size that is set somewhere around 17 kbytes, Ubuntu sets the TCP window size dynamically. It starts with a modest 5k window and steadily increases it during the file download to over 1 megabyte. Looks like Vista has a similar algorithm as well. Very nice, no more worries about throughput limitations in the future!
The 14.4 Psychology
A little anecdote today: In the "old" days I had a 14.4 kbit/s fixed line modem for quite a number of years. Even though new and shiny 28.8 kbit/s modems came on the market, I was stuck with my '14.4' because the new modems were expensive and as a student my monetary resources forbade an upgrade. So for me, the number '14.4' has a bit of a negative touch attached to it ever since.
Fast forward to today to the "megabit" era. In wireless, HSPA 7.2 Mbit/s downlink is currently pretty much state of the art. Some network operators have announced further upgrades and in due time, top speeds of 21 MBit/s and beyond will be reached. On the way to double digit speeds, there's also a 14.4 step. No, not kbit/s, but Mbit/s. Still it kind of reminds me of my 14.4 kbit/s days and has a negative "psychological" touch to it to me.
Strange strange, because I'd really like to have this 14.4 this time around 🙂 Any numbers in telecoms that have a psychological edge for you?
The HSUPA RTD Delight
When discussing High Speed Pack Uplink Access (HSUPA), or E-DCH, to use the correct term, the major focus usually lies on the improved uplink speeds. Seldom is it mentioned, however, that E-DCH also improves the latency, i.e. the time it takes for an IP packet to be sent to a server and a response packet to arrive back to the source. But is this relevant in practice?
So far, I used an HSDPA 3.6 non E-DCH capable 3G stick and my round trip delay times (RTD) to a number of web sites I visit most were around 110 milliseconds. Over a DSL link, the same sites can be reached with an RTD of around 45 ms. In other words, a difference of 55 ms. In practice this can be felt especially during web browsing, as web sites take a bit longer to load over 3G compared to a DSL link with a similar bandwidth. Not that this is a showstopper but it can definitely be felt.
A few days ago, I ran some tests with a Cat 8 HSDPA + HSUPA 3G stick and was quite surprised that the RTD times to those web sites were just around 65 ms. In other words, that's only 20 ms more compared to DSL. The difference to the HSDPA only 3G stick are quite remarkable. I compared backwards and forwards with my DSL line but I couldn't "feel" the difference anymore. Stunning!
The one thing E-DCH does not do away with, however, are the delays incurred when radio states are switched. The 300 ms or so delay when switching between a full DCH and the less power and resource intensive FACH are still there. In practice, however, background traffic from applications such as my Instant Messengers usually keep the link in DCH state so I rarely come into contact with it anyway.
Ericsson says 1GB can be had for 1 Euro
Here's an interesting article from Ericsson on the business case of mobile broadband. Taking CAPEX, OPEX for both access and core network into account, the article comes to the conclusion that once an economy of scale is reached in terms of the number of broadband subscribers, the network can deliver 1GB of data for one Euro.
While this is the main outcome of the paper, there are a number of other pieces of information in there on which the calculation is based which are quite interesting. Here are some which I noted:
- 20% of the cells carry 50% of the traffic. I've heard of similar numbers before and I think it's a good thing because the network operator can focus on upgrading a subset of all cells rather than having to work on the whole network simultaneously.
- 3-5% of the cells carry very heavy load. The article doesn't say where such cells are usually located. It would be interesting if this load is mostly generated in-house, for example in shopping centers, train stations, airports, etc. and if femtos would provide a cheap future capacity extension for those places.
- The technical evolution of 3G networks is all about keeping pace with higher user demand for capacity. Fully agree to that.
- Going from 7.2 MBit/s to 21 MBit/s adds a cost of around 10-15% but increases capacity around 70%. An interesting statement because 7.2 -> 21 MBit/s is about a 3x theoretical speedup while from a practical point of view it is much less. The article says its 70% or 0.7x.
- 70% of overall CAPEX is spent on base stations.
- 50% of overall OPEX is spent on base stations.
- The €1 per GB seems to be a number over a 5 year period. At the end of the article it is stated that the networks that Ericsson looked at for the study are not quite there yet. However, the first network, after 2 years of operation, reported that they have reached €2 per GB.
- For the study, a base station price of €40k was assumed. Looks like they have gotten quite a bit cheaper than what was calculated with just a couple of years ago.
And just to get a bit more aggressive, this Ericsson presentation states that mobile broadband is even cheaper than DSL (cp slide 13 ff.). I assume that leaves IPTV delivery out of the equation, but still it's an interesting way of looking at things.
HSDPA now with 28 Categories and Speeds up to 84 Mbit/s
Almost exactly two years ago, I wrote a post in which I reported my first sighting of new HSPA+ device categories. The top at the time were category 14 and 16 with 64-QAM modulation and MIMO respectively and speeds up to 28 Mbit/s. This was Release 8 of the 3GPP standards. Now in Release 9, 28 device categories are listed in 3GPP TS 25.306 (see table 5.1a) with top speeds under ideal radio coverage of up to 84 Mbit/s if everything is combined, i.e. 64-QAM, MIMO and Dual Carrier. For almost every combination of options there's a category now. Breathtaking…
As I found no good overview of which device category goes up to which speed, I took the liberty of updating the HSDPA article on Wikipedia and add device classes 16-28 in the table. A screenshot of it can be seen on the left.
Now where can I get a Category 28 device and a suitable network please? 🙂
P.S. 1 – Important: Note that all indicated speeds are top speeds under ideal signal conditions. See here for further details and a reality check!
P.S. 2 – I left out cat 17 and 18 as they are a bit special and I am not sure that they will be relevant. If you have an opinion on this one, please let me know. Also, feel free to add them to the table on the Wiki yourself and while you are at it, have a go at the coding rates for the higher categories as well.
MIMO Testing Challenges
Over at Betavine Witherwire there's an interesting post on the challenges of consistently testing multi-antenna devices which will shortly appear on the market. The author of the post mentions that even without MIMO, 3G network capacity could increase by 50% if all devices are equipped with multiple receive antennas and sophisticated noise cancellation algorithms. Obviously that also translates in higher throughput per device. Consequently, network operators are likely to be very interested in these developments and accurate testing of the performance enhancements is a must.
While many tests with mobile devices today are performed with the air interface simulated over a cable, that won't work that easily anymore for MIMO and receive diversity as the antennas in the device are effectively bypassed. It's the antennas and their location and shape inside the device, however, that will make the big difference. More details in the post linked to above.
So I wonder if it's possible to model the impact of the antennas by simulating their characteristics in addition to the signal path with a simulator box that sits on the cable between a real base station and the mobile device)!?
A formidable challenge and I look forward to what the guys in 3GPP RAN4 come up with.
HSPA+ Speeds in Which Areas
I keep mentioning in posts on HSPA data speeds that one has to be very careful when using such numbers as they represent the theoretical maximum that is only reached very close to the base station. In various presentations, one can often see graphs where speeds are shown over the percentage of users, i.e. what speeds 95% of the users are experiencing to the speed only experienced by 5% of the users. But what does that mean when plotted over a geographical area?
Ericsson has done a great job of visualizing this in their 1/2009 edition of the Ericsson review on page 8 figure 2. I can't reproduce it here, but I encourage you to follow the link and check it out for yourself, it's very insightful! As you can see in the 3 pictures, which represent various stages of the HSPA+ evolution, the direct benefit of adding higher top speeds gets more and more limited the higher the speed.
What the pictures don't show, however, is that subscribers not enjoying the best radio coverage do also benefit from higher speeds, in a different way though. By having at least some users in the zones where data can be transmitted faster, users under ordinary conditions benefit because the overall cell capacity has increased, as some uses can transfer their data faster, thus leaving more time to serve other users.
Great visualizations, thanks to Stefan Ström, Dirk Gerstenberger, Johan Bergman and Fredrick Gunnarsson for the article!
HSPA Downlink From 1 to 80 MBit/s and Beyond
With the addition of ever more features into HSPA by 3GPP, peak data rates keep rising and one can find many different peak data rate numbers in the media. To bring some order into this, I've decided to put together a table with shows which features bring which rough peak data rates:
- 3.6 MBit/s : Baseline HSPA with 16QAM modulation
- 7.2 MBit/s : 16 QAM, more simultaneous channels)
- 14.4 MBit/s : 16 QAM, even more simultaneous channels
- 21 MBit/s : 64 QAM modulation
- 28 MBit/s : MIMO (Multiple Input Multiple Output = 2 antennas) + 16 QAM, (3GPP Rel 7)
- 42 MBit/s : MIMO + 64QAM (3GPP Release 8)
- 42 MBit/s : 64QAM + Dual Carrier (3GPP Rel 8)
- 82 MBit/s : MIMO + 64QAM + Dual Carrier (i.e. 2 x 5MHz) (3GPP Release 9)
- + more in case 3GPP decides to increase the number of carriers that can be bundled in Rel 9 or beyond.
In the near future, operators that can upgrade their base station to 64QAM modulation without a hardware replacement are probably tempted to do so. If they were smart with the backhaul in previous upgrades, they might already have the capacity at the base station to support the added traffic. All further steps require new hardware at the base station so operators will probably think a bit about it before actually deploying it.
Important: These are peak rates, i.e. they are only achieved under very favourable coverage circumstances and microcell environments. In most macro radio environments, speeds are much lower due to interference and less then optimal signal strength.