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.
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!
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?
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.
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:
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.
I think everyone in the industry is pretty clear by now that the amount of data that cellular wireless networks will have to carry in the future is going to rise. In my recent book I’ve taken a closer look at theoretical and practical capacity on the cellular level in chapter 3 and I come to the conclusion that from a spectrum point of view, there is quite a lot of free space left in most parts of the world that will last for quite some time to come.
So while alternative approaches like integrating Wi-Fi and femtos into an overall solution will ultimately bring much more capacity, I think it is quite likely that network operators will over time deploy their cellular networks in ever more bands. In Europe, for example, I think it’s quite likely that operators at some point will have networks deployed on the 900, 1800, 2100 and 2600 MHz band simultaneously.
Quite an interesting challenge to solve for networks and especially for mobile devices as they have to support an ever growing number of frequency bands. Also, those bands should not also be used in tight cooperation instead of just aside each other. Ideally, the resources in the 900 MHz band could be reserved for in-house coverage as radio waves in this band penetrate walls quite well. But as soon as the network or the device detect that other bands can be received quite well, they should automatically switch over to them to leave more capacity for devices used indoors or under difficult radio conditions.
Switching between different frequencies and radio technologies during a call or a session is already done today but mostly based on deteriorating reception levels. So in the future, when using so many bands, I think this reactive mechanism has to be enhanced into a proactive mechanism and switch-overs need to be timed so that the user does not notice an interruption.
Over at LinkedIn, Eiko Seidel recently published a link to a whitepaper by Nomor research on Dual Carrier HSDPA (or Dual Cell HSDPA operation as it is called in the standards), a new feature currently worked on in 3GPP. I've been waiting for this feature to come out for quite some time now as HSPA+ has already added 64QAM modulation and MIMO to HSPA. Consequently, not much can be done anymore to improve performance in a 5 MHz channel.
In practice, dual carrier (DC) HSDPA means that two adjacent 5 MHz carriers can be bundled by the network and DC capable HSPA mobiles can be assigned resources simultaneously on both carriers. In addition to the higher throughput, the 10 MHz bandwidth can also be used to schedule mobiles more efficiently around fading conditions, which according to the paper, results in an efficiency gain of up to 7% with 32 users and up to 25% with 2 users.
By increasing transmission speeds the round trip delay time is also further reduced, good news for online gamers. I have to note, however, that current round trip delay times of around 100ms are hardly distinguishable anymore from the delay of a DSL line. What's still distinguishable are the longer delay times caused by state changes after some time of inactivity. That's addressed by another feature, though, the enhanced Cell-FACH.
The enhancements also brings a number of new terminal categories. In addition to HSDPA terminal categories 1-20 which exist today (most people these days have a category 6 (3.6 MBit/s) or a category 7/8 (7.2 MBit/s) device), category 21-24 terminals will be able to use two adjacent carriers. The conserve energy on the mobile device side, the network can dynamically instruct such terminals to only listen to a single carrier if the amount of data to be transferred is low and doesn't warrant the use of two simultaneous carriers which requires more energy for decoding.
For the moment, multicarrier HSDPA is only for the downlink direction and while 64QAM is included, MIMO is not. Theoretical peak throughput in the combined 10 MHz carrier is around 42 MBit/s. But I guess this is not the end of the story yet, I think it is quite likely that in 3GPP Release 9, uplink dual carrier and MIMO is added to the feature list. The authors go a step further and speculate that in the future the standard could include further enhancements to go beyond two simultaneous carriers and to even include simultaneous transmission on carriers not adjacent to each other, even in different bands (e.g. 900 + 2100 MHz simultaneously).
While it looks good on paper, it remains to be seen which operators will go for it in practice. Some operators are determined to squeeze out as much as possible of their 3G networks before going to LTE. By the time these features are market ready, I'd say two to three years down the line, it's quite likely that many 3G base stations will already be used with two carriers per sector. If the feature can be done in software only, I guess it could become quite popular. In that time frame, however, many of today's 3G base stations will be end of live and might be replaced with triple mode GSM, UMTS and LTE base stations. If the feature is required when LTE is also in the cabinet, well, that remains to be seen.
But one way or the other, this new round of enhancements show that there is still a lot of life left in HSPA.
And here's some background as to where the technical details can be found in the specifications: First, 3GPP TR 25.825 contains an overview of the feature. Nomor's whitepaper lists a number of Change Requests (CR) to add the functionality to the relevant specification documents (TS 25.211, 25.212 and 25.214). I've had a look at the latest versions in 3GPP Release 8 and those CRs have been approved and are part of the specs now. So it looks like Dual Carrier HSDPA will be part of Release 8 which will be finalized in this quarter.
Let's see if there is already talk about this at the MWC in Barcelona in just a couple of days from now.
Recently, a question was asked in the LTE forum on LinkedIn how LTE can reduce the cost per bit compared to todays broadband wireless systems such as HSPA. I found it quite interesting that a lot of people immediately jumped at the greater spectral efficiency as the means to reduce the overall cost. But I think there are also other innovations which will drive down cost:
Anything else you can think of?