Author: Martin

Back in June last year I wrote a post on my very positive experiences with the satellite based Internet access on board the Thayls high speed trains. Multi-megabit speeds, it was a joy to use it. Now, only a little bit more than half a year later the system is not much more than a shadow of its former self. During my past 4 trips, the system was down twice. And the other two times when it was available, speeds could be measured in bits per second and it was impossible to use it. What's going on Thalys? Have you reduced your overall bandwidth to reduce the monthly bill or has the service become so popular you are chocked by the demand? Either way, it's unusable and not worth paying for…

Oh and by the way, I thought long before writing this post but I couldn't let my very positive review stand on it's own, it just doesn't reflect reality anymore.

Interesting to see how the bottleneck of my Internet connection at home has been shifting recently. With my new 25 MBit/s VDSL connection I am having difficulties shuffling peak data rates over my Wi-Fi network. So far with a 6 MBit/s ADSL connection that was no problem even for my notebook with a 802.11g adapter and a throughput of 21 MBit/s under the best of conditions.

For a 25 MBit/s downlink and 5 MBit/s uplink, however, that's not enough anymore. I've upgraded to 802.11n but even so I find it difficult to really squeeze the 25 MBit/s out of the wireless link. The 2.4 GHz band is quite crowded and the Wi-Fi access point is in another room so I don't get the signal conditions necessary for the highest transmission rates. I've tried the 5 GHz band that both the access point and the notebook support but the attenuation of the walls between my office and the router make the throughput even worse.

On the other hand, you first need to find a host on the Internet that delivers data at such high speed. In practice to really saturate the line several things need to go on in parallel. Not too difficult with kids using the connection, too I suppose…

Even when a mobile device just sits there and seemingly doesn't doesn't do a whole lot it is nevertheless quite busy every now and then internally to check the signal strength and quality of the current cell and those around it to make sure it never looses contact to the network even if the user roams around. I've recently came across a good description of which values are actually measured in this book and thought I'd put together an ultra brief summary:

GSM

Here, the world is still quite simple. Once the mobile has found a cell it keeps monitoring it's received signal power and computes the Received Signal Strength Indicator (RSSI) over the complete carrier frequency (200 kHz). Neighboring cells use different frequencies and the same value is calculated for those as well.

UMTS

Here, things get a bit more complicated as all cells use the same frequency. As a result not only the received signal strength is important but also the general noise level on the carrier. In the end, what good does it do to you when the overall signal level is strong but most of it is interference? That's why there are three values: 

• RSCP: The Received Signal Code Power: That's the power level the pilot channel of a cell is received with and usually expressed in dBm (mW on a logarithmic scale). With this parameter, different cells using the same carrier can be compared and handover or cell reselection decisions can be taken.

• RSSI: In UMTS that's the signal power over the complete 5 MHz carrier which includes all components received, including the signals from the current and neighboring cells on the same frequency.

• EcNo: That's the received energy per chip (Ec) of the pilot channel divided by the total noise power density (No). In other words the EcNo is the RSCP divided by the RSSI. And again in other words: The better this value the better can a signal of a cell be distinguished from the overall noise. The EcNo is usually expressed in dB as it's a relative value. The value is negative as the RSCP is smaller than the total received power. As the RSCP this value can be used to compare different cells on the same carrier and handover or cell reselection decisions can be taken.

LTE

LTE networks also use the same carrier frequency in neighboring cells so again the signal level and interference are important:

• RSRP: That's the Received Power of the Reference Signal.

• RSSI: That's the total received power including Interference from neighboring cells and other sources.

• RSRQ: That's the Reference Signal Received Quality and equals the RSRP divided by the RSSI. The better this value the better can the signal of the cell be received against the interference generated by other cells.

Quite a lot of similarities between UMTS and LTE from a high level point of view even though the values are measured in quite a different way due to the different air interface natures (WCDMA vs. OFDMA).

This is the strangest setup of a conference call I have been in for a while: A Skype video session with high quality audio over speakers and a third person, also on the other end, part of the conversation over a mobile phone as part of the call via loudspeaker/hands-free mode. The person on the other end of the line didn't even realize I was not in the room but a thousand kilometers away. On my end, the speech quality of the hands-free mode on the other end was also pretty much the same as if the phone was on my end. Speaks for Skype's audio codec and the echo cancellation of all devices.

Observation of the day:

In the past, I 'had to' do many things at home because I could not do them while commuting, because I couldn't take a sufficiently small computer with me and had no Internet connection.

Today, I 'don't do' the same things at home because I can as well do them while commuting with a netbook and a 3G Internet connection.

In other words a total reversal of thinking and behavior. And to the better I think!

And here a quick not to myself and to all of those who sometimes need an overview of all the functionalities that have been changed and added in the different 3GPP Releases since Release 99. Here's a link to one document per Release which contains a 'short' one page description of each new work item and the updated TS and TR documents behind it. I know, there are also whitepapers available on the net describing the different 3GPP releases and they are probably much easier to read. However, they usually don't contain references to the specification documents and they don't contain all the features.