WirelessMoves – Page 270 – My Thoughts on the Evolution of Wireless, FOSS and the Past, Present and Future of Computing

Light Reading Webinar on Mobile Backhaul Evolution

With mobile networks getting faster and faster a growing pain for network operators is the backhaul connection between the base station sites and the next element in the network. Today, T-1 or E-1 connections are used with a line rate of 1.5 and 2 MBit/s. With HSDPA being put in place today, backhaul capacity requirements of 3G base stations now reach 10 MBit/s or more. This means putting additional T-1 or E-1 lines in place. While this might still work today for HSDPA speeds despite the associated rising costs it certainly won’t work tomorrow for WiMAX, LTE and other Beyond 3G technologies that require backhaul capacities of 60 MBit/s per base station and more.

So the big question is what comes after T-1/E-1 connections over copper, fiber or microwave!? The common answer these days seems to be:

IP over Ethernet with the capability to carry legacy GSM (TDM) and UMTS/HSDPA (ATM) links in IP pseudo-wires alongside native IP traffic generated by native WiMAX and LTE base stations.

But how do you connect the base station sites to Carrier Ethernet Networks? Can the last mile be done over copper, is fiber required or is next generation microwave an alternative? Questions over Questions 🙂

I found some answers in a recent one hour Light Reading Webinar on the topic which is available for free at this link. If you are interested in the topic take a look.

Testing a Cat-7 HSDPA Card In a Live Network

I recently got an invitation from T-Mobile’s press center to check out T-Mobile’s HSDPA network in Friedrichshafen, which has become Deutsche Telecoms open air test bed ‘T-City’. There, the invitation said the HSDPA network was upgraded for maximum performance with category 7 HSDPA cards. In theory (no interference, no neighboring cells, single user, close to the antenna), such HSDPA cards are capable of speeds up to 7.2 MBit/s. Interesting stuff, so I went there to check it out.

In the local Telecom shop they let me check things out first hand with a notebook and an Option Globetrotter GT max HSDPA card which they said they had updated with the latest software. You won’t see a big difference when web browsing once speeds are higher than about 1 MBit/s so I decided to download a large file from a high capacity server on the Internet. The average download speed was around 4.2 MBit/s. Quite impressive even when compared to my already fast category 6 Motorola V3xx HSDPA mobile with which I have reached 2.5 MBit/s in the past.

For more on HSDPA on my blog take a look here.

Jumping from EDGE to EDGE on the Train with SFR

I had an interesting wireless experience recently when I took the TGV from Stuttgart to Paris. There isn’t a lot of 3G network coverage along the railway track so while in France I used SFR’s network with a Vodafone Websession from Strasbourg to Paris. So far I thought that SFR did not have EDGE in its network. Looks like that assumption was not quite correct.

Every now and then one of the cells along the railway track was EDGE activated and data rates suddenly jumped from a meager 60 kbit/s (typical GPRS) up to 200 kbit/s, even at 300 km/h. The bad thing is just that EDGE was only available in few places so it’s not really worthwhile to go online and download eMails and do some web browsing. I can only speculate that SFR tries to cover some towns along the railway track but they certainly don’t try to do anything in terms of higher rate data for the railway track in particular. What a shame…

At first I thought it might be a mobile device problem. So I tried with a second mobile which has an engineering mode which confirmed that EDGE is only sporadically activated. The pictures on the left shows the throughput of a file download in a standard GPRS cell and with EDGE when it was available.

Have to try with Orange next time.

Nokia’s Mobile Web Server Gets A Blog

In case you haven’t heard of Nokia’s Mobile Web Server yet, go check out my blog entry on it. In short the mobile web server is a port of the Apache web server to the S60 platform with a front end to access your mobile phone via the Internet or via a local Wifi network. Not a main stream application yet but with a lot of potential for the future. For those who want to stay informed what’s going on with the project check out the mobile web server blog which has just been created over at S60.

Yesss – 50 Euros For One Year of Prepaid 3G Internet Access

2007 is definitely the year Prepaid Mobile Internet Access took off in Europe. In many countries especially, tourists, business travelers and most importantly local young people and students can now get affordable wireless 3G access to the Internet with prepaid SIM cards. Mobile Virtual Network Operator Yesss in Austria now takes the concept to the next level.

For 50 euros they are selling prepaid SIMs in supermarkets which come with 3GB worth of data volume to be used over ONE’s HSDPA network within 12 months. A 1GB extension afterwards, again valid for up to 12 months, costs 20 Euros. The data rate is ‘limited’ to 1 MBit/s but quite frankly that’s a limit that is not too difficult to live with.

Need an HSDPA data card with it? No problem, they’ll give you a Option 1.8 MBit/s HSDPA PCMCIA card for 39 Euros or a Huawei E220 USB modem for 149 Euros. The offer seems to be a rampant success, their online shop is currently sold out. In ‘Hofer’ supermarkets, and there seems to be one around every corner in Austria, the SIM cards are still available but they also have run short on the hardware.

Since I am in Austria every now and then I recently got myself a SIM card from one of the Hofer shops. Plug and play, no registration, nothing. Just buy the card, put it into your UMTS or HSDPA mobile/modem and off you go.

As always there is at least one catch 🙂 The SIM can only be used for Internet access, no voice or SMS allowed. As a consequence the SIM is only good for Internet access with Notebooks and other ‘non voice’ devices (Note: The SIM works fine in a mobile phone used as a modem for the notebook but voice calls are rejected). Makes sense from an operator point of view I guess. Otherwise nobody would buy pocket Internet access offers for 10 euros a month for a couple of megabytes anymore.

The other slight catch is that the ONE network used by Yesss does not have EDGE in areas where they have no 3G coverage. Therefore it’s kind of all or nothing.

For the details check out the Prepaid Wireless Internet Access Wiki.

DSL Gets ‘Base Stations’

While doing some research on how capacity will grow in fixed line and wireless networks in the future I stumbled over the following thing right in my neighborhood: Future bandwidth increases on the last
mile to the subscriber come with an additional cost in comparison with today’s
standard ADSL or ADSL2+ deployments because of extra hardware that has to be installed close the the subscriber.

For ADSL2+ the DSLAM is usually installed
in the telephone exchange and the cable length to the subscriber can be up to 8
km for a 1 MBit/s service. For VDSL, which offers data rates of 50 MBit/s
in downlink, the cable length must not exceed 500m. Thus, DSLAMs can no longer
be only installed in central telephone exchanges but equipment has to be
installed in cabinets on the street. The cabinets themselves are quite large (about half the size of a GSM or UMTS base station),
require power, active cooling, and create noise. For the installation
of the cabinets earthworks are necessary to lay the additional fiber
and power
cables required to backhaul the data traffic. The picture on the left shows a VDSL DSLAM cabinet that
has been installed alongside a ‘legacy’ small telecom cabinet as part of the current VDSL build out in my region.

To connect a
new subscriber a technician is required to manually rewire the customer’s line
to one of the ports. Different sources currently
specify the maximum capacity of such cabinets from about 50 to 120 VDSL
ports. To support 500 VDSL connections per km², several cabinets are thus
required. I wonder what happens when 5 different companies put such DSL ‘base stations’ in place!?

What are Secondary PDP Contexts Good For?

I often wondered in the past what ‘Secondary PDP Contexts’ are good for in UMTS networks. I had a vague idea but never had the time to look up the details. These days I had and here’s a short explanation:

‘Secondary PDP contexts’ can be used to separate the real time data traffic from background or signaling traffic into different streams on the air interface while keeping a single IP address on the mobile device. This is done by an application providing the network with a list of IP addresses in a Traffic Flow Template. The mobile device and gateway router (GGSN) in the network will then screen all incoming packets and handle packets with the specified IP addresses differently, like not repeating them on the RLC layer after an air interface transmission error. This is transparent to the IP stack and the applications on both ends of the connection.

External providers of speech services such as Skype, however, do not have access to this functionality. A big advantage for operator controlled IMS services when things get wild on the air interface!?

Resources:

Secondary PDP Context Activation: 3GPP TS 23.060, Chapter 9.2.2.1.1 (Rel 6)
Traffic Flow Template Description: 3GPP TS 24.008, Chapter 10.5.6.12 (Rel 6)

Wifi Layer 1 Tracing with Wi-Spy – Part 3

For those interested in getting a feeling of how Wifi works on the physical layer, Metageek’s Wi-Spy is the ideal tool. I’ve already reported about my first experiences here and here. Wifi has become very popular in Paris due to DSL being quite cheap. So it should thus probably not be surprising that I can see 13 access points in my Paris apartment. As there are only three non-overlapping Wifi channels on the 2.4 GHz ISM band the question is what kind of impact such a high number of access points has on throughput.

Matters are made worse by the fact that some of the networks I can see are used for TV and video streaming. This is quite popular in Paris as this is offered by most DSL ISP’s and one even offers video streaming over Wifi to a remote set top box. The picture on the left shows a trace taken with Wi-Spy under these conditions. The lower graph in the figure shows the frequency range of the ISM band between 2400 and 2480 MHz. Instead of showing the frequency in MHz the x-axis labs show the 13 available Wifi channels. On the y-axis the amplitude of the signal received over the band is shown. The color of the peak depends on the intensity of the signal received during 60 minutes. Bright color indicates high activity. The graph shows five partially overlapping networks with their center frequency on channel 1 (only little traffic so the arch is not very well visible), channel 3, 5,6 and 11.The most activity can be observed in the wireless network that is centered around channel 11.

The upper graph shows a time graph over the frequency range. On the y-axis I’ve chosen a resolution of 60 minutes to show the activity in the ISM band in the course of one hour. The Wifi networks on channel 5 and 11 were most likely used from streaming as there is uninterrupted activity throughout the test period. The Wifi networks on 3 and 6 were also used for streaming. Streaming was stopped on the Wifi network on channel 6 after about 12 minutes while streaming was started on the Wifi network on channel 3 about 40 minutes into the trace.

To see what the impact of that streaming has on throughput in my network I used two notebooks, one connected via Ethernet, the other via Wifi and Iperf, a UDP and TCP throughput measurement tool. With a fully overlapping Wifi network which is used for TV streaming, capacity of the Wifi network under test was reduced to 72%. Partial overlapping entails an even bigger speed penalty and performance was reduced to 59%.

Here are the absolute values:

No interference: 22.5 MBit/s
Full Overlapping: 16.3 MBit/s
Partial Overlapping: 13.4 MBit/s

Looks like it is time equipment manufacturers are taking the 5 GHz band a bit more seriously…

For more traces take a look at my previous traces or head over to Metageek where you can download the software and check out some sample traces yourself.

T-Mobile activates EDGE in Southern Germany

T-Mobile has announced a couple of months ago that it will upgrade most parts of its GSM/GPRS network in Germany to EDGE. Since then I’ve seen reports that EDGE has been activated here and there but not where I live. Today, I noticed that T-Mobile has also switched to EDGE in the South of Germany near the Lake of Constance where I live. Not that we wouldn’t have excellent 3G coverage here already by all network operators but since the first iPhone won’t be 3G capable and we’ve still got a good number of rural spots in this part of Germany without 3G coverage it’s a welcome move.

Looks like T-Mobile is quite active of continuously improving their 2.5G network.

Wifi Ueber-Geek Question Result

Back in mid September I reported on using my Linksys WRT54 Access Point in "Access Point Client Mode" to create a wireless link to another access point for a number of notebooks which are connected via Ethernet to the Linksys. The traces which I took on the Linksys and on the notebooks indicated that the Linksys replaces the MAC addresses of the notebooks with its own before it sends the packets over the wireless link. Equally it replaced its own MAC address in incoming packets with the MAC address of the real recipient. This is neither layer 2 bridging nor layer 3 IP switching but something in between. I couldn’t quite believe it.

In the meantime thanks to the suggestions I received I made some further tests and I can now confirm that the Linksys really does replace the MAC addresses. Take a look on the picture on the left which shows the ARP table of a PC connected wirelessly to the real access point. The notebooks connected to the Linksys Client AP both have the same MAC address. The MAC address is that of the access point! Quite sophisticated! (Note: All devices in the network are in the same IP subnet)

I am not sure how this feature should be called. It’s not really ‘Layer 3 switching’ which is already a highly overloaded term anyway. I’d prefer the term ‘MAC masquerading’ although the term is also already used for something else as well.

Thanks to all who sent their comments and suggestions!