Author: Martin

Today, we don’t think a great deal when we pick up a mobile phone to call somebody while in the car, or a train or moving otherwise. The network, if properly designed, ensures that the connection is handed over from cell to cell as we move. With most phones, this works rather well, probably because the protocol stacks have been refined for this over many years and it is tested intensly. After all, it is the main purpose of the device.

With the rising use of mobile Internet connectivity, one would assume the same would be true for devices that have an IP connection established while being carried around. However, over the past few years, I have observed now that this is not really the case and I have to admit that I am a bit frustrated by hanging and rebooting mobiles while being connected to the net while moving. At first I thought this was due to the immaturity of the technology. However, UMTS networks are around for several years now and the situation has not really improved. Here are some examples:

My Sony-Ericsson V800: This is the first 3G mobile I bought several years ago and I never managed to get hold of a firmware that would not reboot when the mobile was used in a train as a data modem. O.k., it was one of the first usable 3G phones so I give it the benefit of doubt.

The Motorola V3xx: I use this device exclusively as a 3.5G data modem and in general it works quiet nicely. It’s quite stable even in a train or other moving vehicle but does reset at least once an hour, too when moving.

The Nokia N95-8GB: This one is a notorious ‘rebooter’. As soon as I launch OperaMini and move, even when just on foot, it reboots…

So this rebooting while connected to the packet network during cell reselections is not a one company, one device issue, it seems to be a universal phenomenon. It shows that comlexitiy has risen beyond a point where such things can be tested well enough before devices are shipped. Or is it just the companies that don’t (yet) care?

But there is a little hope: Today, I tested the Nokia N82 and OperaMini while in the car and it didn’t reboot like the N95. That’s good since this device just went into hands which are even less forgiving when things don’t work.

This February at the Mobile World Congress, Intel invited me to experience their WiMAX network that they had set up in the city during the event. Of course I had lots of technical questions and one of them was on the backhaul equipment they used to connect the base stations to the network. I was told that they were using 50 Mbit/s microwave links with equipment from Dragonwave. Impressive I thought at the time and was thus happy to see that Dragonwave attended a recent WiMAX event in Munich. An excellent opportunity to get more details. Turns out that the 50 MBit/s, hyperfast compared to the 2 MBit/s E1 links used today in most 3G networks, is just the entry level speed. According to the data sheets, the system is capable of doing about 800 MBit/s with dual polarization and 256QAM modulation over a 56MHz channel and 1.6 GBit/s when using 2 channels. The range of the system at this speed is several kilometers. That's much faster than what you need even for LTE and WiMAX in the short and medium term per base station. However, base stations are usually daisy chained in a ring configuration so if you chain five base stations together that each require 100 MBit/s (e.g. three sectors), that's already 500 MBit/s.

The longest distance they have bridged with their kit so far at 50 MBit/s hiltop to hiltop was 75km. Another impressing number.

Equally impressing is the power output required. I always thought microwave equipment operates at very high power. Looks like I was wrong, the output power of the system is a mere half watt. Of course the gain of the antenna is very high but still…

As the system transports Ethernet/IP frames natively, most LTE and WiMAX base station can use the system right away. Current 3.5G base stations, however, are still using TDM links. For these, a pseudo wire box can be used to tunnel such connections over Ethernet.

I also learnt a lot about Microwave frequency license costs. Looks like the cost differs widely. In the U.S., I was told, a basic license for a channel (can't remember the exact bandwidth anymore) is around 1800 dollars for ten years. Compare that to France where the cost for the same license is around a thousand Euros  for a single year. Channel sizes that can be licensed start around 6 MHz and go up to 28 MHz. Again depending on the country, the microwave spectrum is anywhere between 11and 38 GHz.

Like all wireless systems, microwave links don't like things such as rain and snowfall. E1 based microwave links therefore need a security margin in order not to fail under such conditions. In the world of Ethernet, however, this margin can be translated into higher speeds when conditions are fine and the automatic link adaptation during  rain and snowfall by automatically lowering the data rate. By prioritizing VoIP packets, it can be ensured that voice calls are not impacted by this while background priority traffic such as web browsing gets slower during such times. I know, easier said than done in practice, IMS definitely needed for this. But that's another topic.

When asked on the price point of the system I was quoted a mid four digit number for an end to end link, depending of course on the quantity purchased. Compared to the price of a single 2 MBit/s E1 link, which easily reaches several hundred Euros per month, that's very competitive. Comforting to know that backhaul prices scale well with rising consumer demand for wireless access.

Thanks again to Dragonwave for the interesting interview, I learnt a lot!

Almost exactly a year ago I first reported on WiMAX Mobile Multihop Relay (MMR). At the time, I didn't have a lot of details of how it was supposed to work. In the meantime, however, the IEEE has published an advanced draft of the spec that answer a lot of questions. Also very helpful to get an idea of MMR is this slideset. So, here's an overview of the functionality:

Especially in rural areas, there are often little or no possibilities to backhaul high bandwidths connections via a fixed line copper or fiber links. The two possibilities are then either dedicated high bandwidth microwave connections per base station or a concept in which the base stations themselves form a mesh like network to forward traffic between base stations with no dedicated backhaul connection.

In addition to rural backhauling, forwarding traffic between wireless network nodes is also an interesting method to fill coverage holes and to improve in building coverage. At first, it might seem illogical that sending a data packet over the air interface more than once actually increases the data rate. In practice, however, transmitting the packet over two or more links with a high signal to noise ratio is better than only transmitting it once but very slowly because the signal quality is low.

The 802.16j amendment to the standard covers the following points to achieve these goals without increasing the number of base stations with expensive backhaul links:

Backwards Compatibility

MMR has been specified in a way that does not require mobile devices to be aware of relay nodes. This is important as introducing relaying would otherwise not be possible in already deployed networks.

Multi Hop Capability

The standard is designed in a way that allows a packet to traverse several hops until it reaches a base station that has a backhaul connection.

Relay Station Implementation Options

From the point of view of mobile stations, relays without a dedicated backhaul connection look like a standard base station and have their own base station ID. The specification allows two kinds of relay stations (RS). A simple RS relays everything up to a real base station, including even simple messages such as ranging requests and leaves the processing of all messages to the base station. Such simple relays are also referred to as transparent relays as all links to mobile devices via relay stations are controlled by a base station. More complex relays, referred to as non transparent relays, are able to locally manage the link to the subscriber and only forward user data packets to a base station and higher layer signaling information.

Summary

An ambitious spec! Let's see if and when we see this in practice

Today a little bit of self advertisement which might be of interest to those of you in the WiMAX business: On June 12 and 13, Chris Beardsmore of Intel, John Edwards of PicoChip and myself will host a 2 day WiMAX course at the University of Oxford’s Department of Continuing Education.

I am very proud to be part of this trio and each of us has a different angle on the technology. Chris has been working on the WiMAX business development side for many years and has a lot of information to share of how WiMAX is doing from that point of view. John is the air interface expert and will share his knowledge about the in’s and out’s from basic OFDM to MIMO. And myself, being the core and radio network guy will concentrate on the WiMAX core network, the radio network, authentication, mobility and session management and lots of things around it.

So, if I have caught your interest, head over to the course’s web site for the details. During this week, there are also a number of other wireless courses given by industry experts ranging from mobile web to the latest on HSPA and LTE. So if you have a bit more time to spend, it’s very well worth to stay a few extra days!

I am glad to have the Internet in my pocket to help in situations when the organization of others make success without pain highly unlikely. Take the follwoing scenario that happend to me recently: A theater visit is organized (note the passive in the sentence) in a city none of us have visited before. Turns out instructions concerning how to find the theater are practically worthless. People in the town are helpful but have different opinions on where the theater is ranging from ‘just aroundd the corner’ to ‘at the other end of town’. So here the Internet and navigation on my Nokia N95 comes to the rescue. Googeling for the town’s and theater’s name reveals the address and phone number of the theater immediately. Ah, the theater has three stages throughout the city. That explains the different opinions of the locals. A quick look into a local newspaper reveals the theater for the play. O.k., since Google has found the address, a quick look with Nokia Maps shows us the exacct place and guides us to the theater. How nice, how painless 🙂 Needless to say that the bus driver of the other group was one hour late since he had no one finding the details with his phone.

I use OperaMini day in and day out as my prefered browser and love the experience. Well, except for one thing, the frequent network selection dialog boxes. Some say it is an S60 feature but in my opinion this is just a big bug in the software. Anyway, Constantine from Intomobile now provided a great solution to the problem: In the S60 application manager, it is possible to define the behavior for each Java program. To get rid of the stupid dialog boxes, open the application manager, select OperaMini, select Open, and then set ‘Network Access’ to ‘Ask first time’. Afterwards, no more stupid dialog boxes. Thanks, Constantine!

The only drawback seems to be that one has to select a default access point when the setting is changed. This makes it difficult to use OperaMini over the Wifi network at home when the default access point has been set to the cellular network. Well, I guess you can’t have it all.

In an earlier post , I reported on how I am using my 3 UK SIM card in 3’s Italian network for Internet access without roaming charges. This works well in the bigger cities such as Catania, Siracusa, etc. In smaller cities on the countryside, however, coverage is a mess. Often, there are at least three GSM networks and usually also some UMTS network while outdoors. But once inside somewhere, most networks just fade away. Or even worse, your mobile shows average coverage but as soon as you try to do something, the coverage indication goes away… In my apartment for example, there is complete radio silence except for 3’s UMTS network at the window. At another place where I stay sometimes, only TIM has reasonable coverage. It almost looks like in those smaller towns they just dropped a couple of base stations and never did any proper radio planning and interference analysis. So since SIM cards are cheap and Internet access to be had for a couple of Euros, I now have a SIM card for 3, one from TIM and another one from WIND. At least one works in any place. Not ideal, but pragmatic…