Author: Martin

Supplement to Surviving With Wifi In The U.S.

I am back from Boston reflecting on what’s happened over the week. One of the things I found quite interesting was to see how much Wifi Internet access is used at the two hotels I stayed during the week. In the DaysInn hotel which offered free Wifi, throughput was in the range of around 300-400 kbit/s in the evening and in the range of 1 MBit/s in the early morning hours. Looks like heavy use to me.

I detected the same behavior in the Westin hotel in Boston which charges 10 dollars a day for Internet access. During evening hours, I didn’t get speeds any higher than 300 kbit/s and the voice quality of my Skype sessions where at times a bit rough. I did a quick check with Wireshark which detected around 150 active computers in the network! It’s probably not surprising as there were around 300 telecoms people staying in the hotel 🙂 As in the previous hotel, speeds where much higher in morning hours (around 5 MBit/s).

And the moral of the story: Internet access in hotels is quickly becoming an expected basic service. For 300 bucks a night however, I wonder why the towels and soap are free while Internet has to be paid for separately!? One could argue if one stays at a hotel for 300 bucks a night 10 extra dollars will not hurt. Good point. But again, why don’t they then lower the room price and charge separately for towels, soap, water, light, air conditioning, room cleaning and TV inside the room? I just don’t see the difference…

P.S.: And of course, these findings also indicate that hotels should monitor usage and think about upgrading their DSL backhaul.

Amazed By The Wifi Mesh of MIT’s One Laptop Per Child Project

Last week I had the enormous pleasure to have been invited to MIT’s Media Lab in Cambridge, Massachusetts to take a look at their activities and in particular the One Laptop Per Child (OLPC) project. Walter Bender gave an intro and a hands on demo of the laptop and as you can see on the picture on the left he had all the eyes and ears of the audience.

Olpc_sm
It’s difficult if not impossible not be amazed by any part of the project and the green/white prototypes of the laptop and its software. As somebody who looses a word or two every now and then about communication and networks, I was especially interested in the Wifi Mesh network implementation and what kids can do with it. Communicating with each other, playing, painting pictures etc. etc. and of course get access to the Internet if a gateway is in reach of just one of the laptops forming a mesh network. Individual laptops act as a mesh node even when they are switched off. Great stuff! Details about the OLPC project in general and about the mesh implementation (complying to the current draft of 802.11s as much as possible) specifically can be found here.

The 3GSMWorldCongress Has Been Rebranded

Heike Scholz over at "Mobile Zeitgeist" is at the pulse of the time with her report that the 3GSMWorldCongress has been re-branded by the GSM Association into the "Mobile World Congress".

It’s an interesting move but I still haven’t figured out the real reasons for it… I think it’s more than just a new marketing strategy. Has there been a struggle about the rights to the name? Anyone?

Removing "3GSM" from the name of the conference opens up a number of interesting possibilities for the congress to develop in the future. The official statement for the re-branding is:

"The GSMA is rebranding the 3GSM World Congress and its sister event in
Asia to reflect their role encouraging the development of compelling
new mobile services for end-users."

I am not quite sure how the re-branding will help that goal!? Anyone? By removing "3GSM" from the name the congress might open up potentially for other technologies to be also present. We saw a bit of this already this year with a number of people showing WiMAX at their booths.

Another possibility for the name change could be that the GSM Association now considers GSM and UMTS to be the world’s dominant wireless standard implying that it’s obvious that a mobile world congress can only be about this technology!?

Whatever the reasons are I am sure it’s more than what they say in their official statement. Comments welcome.

WiMAX Waves

Lately, I’ve stumbled over statements like "Our WiMAX product supports 802.16e Wave 1 and 2 feature sets". It’s mentioned a lot but nobody goes into the details of what kind of features the different "waves", or versions, actually contain. So I’ve done some research on my own. I still don’t have all the details, but at least some clues.

From an article on "Mobilehandsetsdesignline":

"…The Wave 1 feature set focuses on single-input, single-output antenna systems and basic mobility.


On the heels of Wave 1 will be Wave 2 certification testing. In addition to Wave 1
backward compatibility, Wave 2 focuses on advanced antenna array
features such as multiple-input, multiple output (MIMO) arrays and
using space-division multiple access (SDMA)-based beam forming. Wave 2
will most likely begin testing in late 2007 or early 2008. …"

And from "Informatm":

Wave 1 mobile WiMAX certification will embrace products in the 2.3GHz and 3.5GHz profiles. Wave 2 certification will then feature 2.5GHz products, and products with smart antenna capabilities like MIMO and beamforming.

In 2.3GHz the Plugfest tested channelization in the 8.75MHz range
for the Korean WiBro profile and 5MHz and 10MHz for the rest of the
world.

In 2.5GHz and 3.5GHz frequencies the channelization was 5MHz and 10MHz and in 3.5GHz the 7MHz profile was also utilised.

And finally, here’s a link to an Intel presentation where physical layer (only) functionality of wave 1 and wave 2 devices are described on the final two pages.

Is anyone aware of an official and published full list of wave 1 and wave 2 functionalities on all layers? If so, please let me know.

Impact Of 3G Traffic On HSDPA

Most HSDPA tests in live networks presented on this blog so far were always performed in lightly loaded cells. In this blog entry I take a closer look at HSDPA performance in a cell that servers other non-HSDPA 3G users simultaneously.

For this test I’ve used one HSDPA terminal (a Sierra Wireless 850 PCMCIA card) to download a file and a second 3G only terminal (a Nokia 6680) to create "interference" by also downloading a file. The 3G only terminal can not use the HSDPA channels and is thus assigned a dedicated channel (DCH) by the network. In the test, the network assigns a 64 kbit/s DCH first, quickly followed by 128 kbit/s and 384 kbit/s bearer upgrades. From a CDMA code point of view, a 384 kbit/s bearer with a spreading factor of 8 takes similar resources than 16 simultaneous circuit switched voice calls (spreading factor of 128). Thus, the test not only shows how fast DCH and HSDPA bearers work together in practice but also what kind of impact a fair number of voice calls have on HSDPA throughput in the cell.

Vodafone_d_simultaneous_384_dch_2_t
The first figure on the left shows the throughput of a file download via HSDPA. At the beginning no other users are in the cell. At about 20 seconds into the file transfer I started another file transfer via the 3G only terminal. The network then seems to modify resources in the cell between 25s and 45s. There are a number of characteristic downward spikes during that time which I was able to reproduce during each re-run of the test. As I can’t look into the network, I have no explanation for them. Finally at about 45s the throughput stabilizes at a slightly lower level than at the beginning where no interference was present.

Vodafone_d_simultaneous_384_dch_2_p
The second figure on the left shows an inter-packet spacing graph (for an intro on this kind of graph see here) of the same download. For about half the download there is only a significant line at 10ms inter-packet spacing. Then, when the file transfer of the 3G mobile sets in the lines at 20 and 30ms get more dots. Lines at 20 and 30ms can have the following reasons:

  • Retransmission because of faulty packets
  • The network has started to use a different coding scheme for the packets due to interference, so packets take longer to be transmitted
  • Other HSDPA users in the network with whom a terminal has to share the capacity.

In this test, the third reason can be discounted as there as no other HSDPA capable terminal in the cell at the time. So either the interference caused by the 3G mobile forced the network to increase the coding scheme or to accept more retransmissions. Both results in slower throughput for the HSDPA terminal. Whether it is one or the other for this test or a combination of both is difficult to say from a terminal perspective.

In total, this test shows that even a fair number of voice calls by other users in parallel to HSDPA has only little impact on throughput as long as reception conditions for the voice calls and HSDPA is good. In such conditions the noise generated by other users is still low enough for the HSDPA terminal to cope with. 

When I have time and opportunity I will run the same test again in an area with weaker coverage. Also, impact of DCH users on HSDPA networks supporting 16QAM modulation, which requires a higher signal to noise ratio than QPSK modulation used by the network for this test should be fun testing.

Network Behavior For Several Simultaneous Users In An HSDPA cell

After some basic tests to see how HSDPA performs in a number of different networks I’ve now moved to a number of somewhat more complicated scenarios. The last article in this technical series on HSDPA focused on network and terminal HSDPA behavior for an incoming call. This article shows how the network behaves when several mobiles communicate simultaneously in a single cell.

In general, Internet applications usually have bursty traffic requirements. Web surfing is a good example. The time required to load a page is typically much shorter than the reading time during which no data is transfered. When downloading large chunks of data, think YouTube, podcast download, etc. the radio channel is occupied for a much longer time which increases the chances that the radio resources have to be shared with several other users. Sharing the radio channel with other users is a strength of HSDPA as users are not assigned dedicated channels anymore as in UMTS. Instead, the cell assigns short bursts on several High Speed Shared Channels. The duration of a burst is 2 ms and the system can flexibly assign those bursts to different users.

Vodafone_d_v3xx_two_mobiles_2_throu
Figure one on the left shows the throughput graph of a file transfer. In the first part, the terminal is the only one in the cell. The throughput is constant and at its maximum value for the current radio conditions at around 1.2 MBit/s. At about the middle of the file transfer, I’ve started a second file transfer from a different notebook with a different HSDPA data card. Immediately the network reacts to the second independent download and reduces the speed for the first user. Quite interesting to see that the throughput is no longer constant but varies quite a bit. It’s likely that this is the effect of changing radio conditions (fading) to which the network reacts by assigning more bandwidth to the user with better radio conditions at a given moment.

Vodafone_d_v3xx_two_mobiles_2_inter
Figure two on the left is also quite interesting. It shows the inter-packet spacing graph for the file transfer which was introduced here. In summary, this graph shows the time distance between two packets. HSDPA has typical lines at 10, 20 and 30 ms, explained in more detail here. In the second part of the graph where the second user comes in, inter-packet distance suddenly jumps to 20 and 30 milliseconds. In this case this is not due to a deteriorating radio channel but due to the file transfer of the second user.

The graphs were produced with a HSDPA category 6 Motorola V3xx terminal with a theoretical maximum speed of 3.6 MBit/s. As the top speed reached in the cell at the beginning of the file transfer is 1.2 MBit/s it’s likely that the cell of the Vodafone Germany network used for the test has not yet been upgraded for 16QAM modulation.

So that’s it for today. In case your are looking for an introduction to HSDPA, have a look at chapter 3 of my book. And for more real live measurement scenarios, click on the HSDPA tag next to the date below.

Broadband Was Yesteray – Wideband’s The Future

HSDPA, EVDO, WIMAX, LTE, you name them, they all go advertising these days with "mobile can now do broadband, too". I think this is true to a certain extent, if one keeps in mind that overall capacity that can be delivered by a mobile system in a densely populated area can not match capacity of DSL or cable. But that ‘s not the aim, anyway. However, DSL and cable have already moved on.

I just listened to a tech show on C-Span 2 where the CEO of Comcast introduced his new Docsis 3.0 modems of Arris that can do 120 MBit/s for a single subscriber. Sure, that bandwidth usually has to be shared with other households on the same coax cable. Nevertheless, the speed has already moved far beyond of what we know as broadband today. DSL has also not stood still and projects like in Paris (Fiber to the Curb, Fiber to the Home) have begun to offer similar speeds over telephone cable these days. And it certainly doesn’t stop here. According to the Paris section of this Wikipedia entry, GBit connections to homes are already in the trail phase.

So who should this next generation of broadband be called? Though thing… Comcast has decided to call it Wideband. Bad choice in my eyes, since in wireless, the term wideband is already used in 3G (Wideband-CDMA, W-CDMA)… But agreed, it’s certainly better than to call it Ultra Mega Broadband 😉

So trying to sell 3.5G and 4G networks around the "mobile can now do mobile broadband, too" slogan will not work much longer anymore. Lucky are those operators who have both fixed and wireless assets and make good use of both by combining them.

Blackberry meets Lederhosen

Starting the week in Boston, I flew over to Graz in Austria for the second half of the week to attend a wedding. It was a traditional wedding which means people were asked to attend in traditional costumes. Interesting to see that some (I spotted at least three) brought their Blackberries along to check their eMail every now and then. Blackberry meets Lederhosen (traditional leather pants)! I have to admit I was quite surprised by this, as people using mobile Internet access and people wearing Lederhosen usually have quite opposing views when it comes to technology. Interesting how the Internet and tradition start to melt.

Yes, Virtual COM Ports Are Fast Enough For Broadband Wireless Internet

Virtual_com_port
A couple of weeks ago, I speculated if virtual COM ports for mobile phones connected to a notebook via a USB cable are fast enough for broadband wireless Internet connections over HSDPA or EvDO Rev. A, B, etc. While some tests suggested that it could work, I wasn’t able to deliver final proof as I didn’t have a broadband mobile at the time. Now I’ve had the opportunity to run a real test with a Motorola V3xx, a category 6 HSDPA device capable of speeds up to 3.6 MBit/s. With download speeds around 2 MBit/s at the location I tested it, it shows quite clearly that virtual com ports do not have the speed limitations of their serial ancestors. The Motorola driver even announces a maximum transmission speed of 12 MBit/s. As shown in the initial post linked above, however, this value is pretty much meaningless. Anyway, it’s good to see we haven’t hit a limit here in a similar way as in Bluetooth, which can barely keep up.