GSM Q&A

All answers have been held as short as possible and require an understanding and study of the corresponding chapter of the book.

Answer 1:
In a circuit switched digital telecommunication network a speech channel usually uses a 64 kbit/s timeslot. The pulse code modulation (PCM) algorithm is used to convert an analog voice signal for digital transmission.

Answer 2:
The GSM NSS consists of at least of the following network components:

  • MSC: The Mobile Switching Center, which includes the Visitor Location Register, is responsible for connecting calls and mobility management.
  • The Home Location Register, which stores subscriber information
  • Service Control Points, which handle services like prepaid or location based billing applications
  • SMS Service Center
  • Voice Mail Systems

Answer 3:
The GSM BSS consists of the following network components:

  • The Base Station Controller, which controls the channels on the air interface. It is also responsible for power control, timing advance control and handovers.
  • Transcoding and Rate Adaptation units convert the speech codecs used in the BSS into 64 kbit/s PCM coded channels used in the core network. (Note: The TRAU is usually co-located with the MSC in order to minimize the number of required transmission links between the MSCs and the BSCs).
  • The Base Transceiver Stations (BTS): A high number of base stations are connected to a base station controller. They are responsible for transferring data streams over the air interface.

Answer 4:
Three methods are used: Several calls can be handled on the same frequency. This is done by splitting a channel into 7 timeslots. This is called Time Division Multiple Access (TDMA). Capacity can be further increased by using several carrier frequencies per base station. Typical GSM base stations today use one to three carriers. This is called Frequency Division Multiple Access (FDMA). Additionally, a further increase can be achieved by splitting the coverage area of a base station into several sectors and by using different carrier frequencies in each sector. Typical base stations today use either two or three sectors.

Answer 5:
The first step is to digitize the voice signal recorded by the microphone by using the PCM codec. This digital signal is then used as input for the speech coder (Full Rate, Enhanced Full Rate, AMR) which compresses the data generated by the PCM codec. This data stream is then sent to the channel coder which adds redundancy (error detection and error correction bits). Afterwards, the Interleaver changes the order of the bits and spreads consecutive bits over a wider area of the data stream. This allows spreading transmission errors which usually affect several consecutive bits over a larger area which helps the receiver to detect and correct errors. To protect the communication from eavesdropping and tampering the resulting data stream is then ciphered. Finally the now encrypted data stream is modulated onto an analog carrier frequency and transmitted.

Answer 6:
A handover changes the routing of an ongoing voice call from one cell to another. This is necessary if a subscriber leaves the coverage area of a cell and enters the coverage area of another. Network elements involved in the handover are the mobile station, the old and the new base station and the base station controller. If the current base station controller is not responsible for the new cell, the mobile switching center and the BSC controlling the new cell are also part of the handover procedure.

Answer 7:
For an incoming call the Gateway MSC queries the home location register of the subscriber to ask for its location (Send Routing Information). The HLR is aware of the current MSC/VLR (Visited MSC) of the subscriber and requests a temporary identifier, the Mobile Station Roaming Number (MSRN). This number is returned by the HLR to the Gateway MSC. From there the call can then be forwarded to the Visited MSC. At this stage of the call routing process, standard ISUP signaling is used which is already known from fixed line networks. Instead of using the phone number as identifier, the MSRN is used to route the call. On the Visited MSC, the temporary MSRN is used to correlate the call to the correct subscriber. As the V-MSC is only aware of the subscriber’s current location area, the subscriber has to be paged in order to establish the cell to which the call has to be forwarded to.

Answer 8:
In order to authenticate a subscriber, the MSC queries the Authentication Center for the Authentication Triplets of the subscriber. Elements of each triplet are a random number (RAND) and a response value (SRES) which is generated from the random number with an authentication algorithm. The MSC then sends the random number to the mobile station. The MS forwards the random number to the SIM card which computes the SRES. This value is then sent back to the network. The MSC then compares the SRES computed by the authentication center and the SRES computed by the SIM card. The two values can only match if the authentication center and the SIM card have used the same secret key to generate the signed response from the same random number.

Answer 9:
A mobile phone always sends an SMS to the SMS Service Center (SMSC) which is a network node in the core network. The SMSC then uses the phone number contained in the short message to query the HLR for the current MSC of the destination subscriber. It then forwards the message to this MSC. If the subscriber is not reachable, the Message Waiting Flag is set in the MSC/VLR and HLR, and the SMS is stored in the SMSC. Once the subscriber becomes reachable again the SMSC is informed and another delivery attempt is undertaken.

Answer 10:
In downlink direction (network to mobile phone) the DSP is used for the analysis of the incoming data stream. The training sequence of a burst is used to calculate a channel approximation which is then applied to the main parts of each burst. This improves compensation of external effects on the transmission. Furthermore, the DSP compresses and decompresses the speech data by using the Full Rate / Enhanced Full Rate / AMR codec. The RISC processor on the other hand deals with channel coding and decoding, interleaving/de-interleaving and ciphering/de-ciphering of the incoming and outgoing data stream. Additionally, the RISC processor also manages the user interface, the overall control of a connection (Mobility Management / Session Management), user programs and external interfaces (Bluetooth, USB, etc.).

Answer 11:
Data is stored on the SIM card in a non-volatile, re-writable memory. To the outside, the microcontroller on the SIM card presents the memory as a directory tree and files. While PCs use filenames to identify files and directories, the SIM card instead uses 4 digit hexadecimal numbers. Each file has its set of individual read and write permissions. Some files are readable only, such as the file that contains the IMSI of the user. Some files can neither be read nor written to from the outside, like for example the file which contains the secret key used for authentication. Directories are referred to as ‚dedicated files’, ordinary files are called ‚elementary files’.

Answer 12:
CAMEL is the abbreviation for Customized Applications for Mobile Enhanced Logic. It describes how databases and applications can communicate over network boundaries and interfaces via MSCs, SGSNs and GGSNs. CAMEL is used for services such as prepaid or location based services. As CAMEL is not restricted to the home network of a user, foreign MSCs are able to contact databases in the home network of a subscriber for services such as prepaid billing when a subscriber is abroad. CAMEL specifies both the protocol between the network nodes and a state model which describes the different phases of a call by using Detection Points. If a certain detection point is activated for a subscriber, the treatment of a call is suspended at this point and the database and service responsible for the subscriber is contacted for further instructions of how to proceed with the call.

Nokia Open Studio: Webcast Now Available

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Many people have reported from the recent introduction of three new Nokia phones in Berlin. What’s even more exciting for me than this is that the whole event was recorded and is now available via webcast. To view the webcast, click on ‘webcast’ in the top right corner of the linked page.

I like presentations like this which do not push the products too plainly but rather the ideas behind them. Here are the main buzzwords that were used throughout the presentation and what I think about them:

  • "Multi Radio Device" to indicate that some of the NSeries phones (N80, N93) now include WiFi on top of GSM, EDGE, UMTS and Bluetooth to share information, interact with other people and upload and download content such as videos, music, etc. For me including Wifi in the mobile phone will open up completely new dimensions (see for example my post about the personal mobile web server) as the ‘phone’ is not longer an isolated device but becomes a part of the network at home.
  • "Multimedia Computer": Nokia see their Nseries products not as ‘phones’ anymore but as ‘multimedia computers’. The telephony functionality is just one of the many functions. They use the word computer to indicate that NSeries phones are smartphones with many applications. Most important of all, they are programmable and thus not rigid like other multimedia devices such as digital cameras or camcorders. Personally I haven’t yet gotten used to this term. For me a computer is still something you don’t carry around with you all the time. Well, maybe that is part of the transformation process.
  • "Citizen Journalism": Justin Dyche of the BBC did an impressive presentation of how the news media today gets pictures and videos from events just minutes after they’ve happened, much faster then they can be there themselves. He also talked about how the BBC makes use of 3G phones and networks to be there first.
  • Content Creation by the User: While citizen journalism described above is a many (citizen) to few (news organizations) application, User Content Creation on the mobile phone for others goes one step further and many people create content for many others.
  • 3rd Party Applications: While in the past Nokia mainly focused like most others to push their own brand it seems that for the multimedia age and the ‘multimedia computer’ they have changed tactics and realized that a rich ecosystem of applications is the path to success. So the presentation was full of talk about 3rd party applications that run on Nokia Nseries phones (sorry for using the ‘p’ word again, it’s still to deeply embedded within me…) like for example a native Flickr Application that will come prepackaged with the new phones and how important this is. Indeed, I have to agree with them, this is the main fact that differentiates a ‘multimedia computer’ from other mobile multimedia devices today. Also, it opens the platform for applications that do not even exist when the phones are first shipped.

For me, the key is the device’s ability to communicate with the Internet. Without that link, the multimedia computer is just another fancy gadget in line with mobile cameras and gameboys. To me, Nokia’s multimedia and communication strategy makes a lot of sense. Can’t wait to get an N80 or N93 in my hands, with hopefully a bundle of extra software on top of what’s already there today.

I am well aware that such shows are marketing events run by marketing people. However, I just wonder how close marketing, R&D and design work together to produce such a show and in creating new products in general!? I am a strong advocate in letting ideas flow between those groups. Having a strong R&D background I don’t really want to work in marketing which is only focused on selling the product. Marketing for me also means to listen to customers, have your own ideas, and be in constant communication with design and R&D to let ideas flow in all directions. I wonder how much of this is done in Nokia?

So you guys from Nokia and S60: How about some comments or trackbacks?

Muni Wifi: How many Access Points are necessary to cover a city?

There’s not a single day in which I do not read about Municipal Wifi on a web site or blog. However, most do not say how many access points are required for city wide coverage and how that compares to cellular networks that offer similar services. Maybe I can add some numbers here.

Dailywireless has recently featured an interesting article on Muni Wireless in Annapolis and San Francisco. In this article they state that about 25 access points are required per square mile. If you do the maths you end up with one access point about every 300 meters. That sounds realistic as I can imagine that communication between access points over 300 meters without directional antennas in the open is possible. Communication between the nodes is required for the meshed network architecture most Muni Wifi projects use in order to minimize the number of fixed line Internet connections required.

25 doesn’t sound like a lot at first. However, a square mile is not much either. The article goes on to say that San Fransisco’s Muni Wifi network will cover 54 square miles. With 25 access points per square mile as described in the blog above, 1.350 access points need to be distributed in an area of 7.5 x 7.5 miles.

Let’s compare this number to the number of cellular towers of a UMTS/HSDPA or CDMA/1xEV-DO network required for the same area: Let’s say such a network operator aims for reasonable indoor coverage (which by the way the Wifi Muni network can not do with an access point every 300m). In such a configuration, the cell radius would be about one mile. This gives you a coverage area of a single base station of around three square miles, or 3 times 25 = 75 wireless LAN access points. 75 that’s quite a ratio. For a 54 square mile area, this would result in 18 cellular sites vs 1.350 wireless lan access points. I know, a wireless lan access point is a lot cheaper than a cellular base station but it has to be a lot cheaper to make up for this. (Note: Since my initial posting I changed this paragraph twice to take the good information given in the comments below into account. Thanks for posting!)

Looks like these numbers are no fiction. Take a look at this article about the Wifi Network in Taipei. It says they will (or already have?) distribute(d) 10.000 Wifi access points in the city. Quite a challenge!

From a consumer perspective I hope there will be enough competition to have a positive effect on prices.

What can we learn from Japanese FOMA phones?

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Over the last few days, I’ve had an interesting discussion with Denis of Wapreview on mobile phone experiences during his recent trip to Japan. There are a number of very interesting links in the blog entry as well, including one to download the English manual of the NEC N902i, which seems to be a quite popular phone in Japan. I had a look through the manual to see what kind of functionality it offers that I haven’t seen so far on phones in Europe or in the U.S. and found a number of quite interesting features.

Camera with stabilization: The built in 4 mega pixel camera has an auto focus, digital (only) zoom, and a stabilization/anti shake feature.

Lock phone: In case your phone gets stolen you can lock the phone from a payphone or any other phone. This is done by calling the phone a certain number of times within a certain time frame. I wonder why I haven’t seen this one before, it’s so simple to implement. Does NEC hold a patent on this one and it’s unaffordable to license it?

Bar Code Reader: A hot topic in the blogsphere. It looks like in Japan this is already used very much in everyday life. Bar codes (one and two dimensional) are scanned with the camera and can contain contact addresses and phone numbers to be put into the phone book, eMail addresses and URLs that can be bookmarked or used to go directly to a page. Wap review mentions that these bar codes are at many places in Japan these days, including posters and business cards. I wonder how long it takes for this to become popular in Europe and elsewhere!? A typical critical mass problem.

Remote Monitoring: Incoming video calls from pre-programmed numbers can be accepted automatically (auto answer).

Change between Voice and Video Call: A call can be started as a
voice call and upgraded to a video call later on (by the calling party
only). An interesting feature, I don’t think other UMTS networks and
phones support this today!?

Mobile Wallet (FeliCa): A chip on the back of the phone can be used for contact less micro payment, e.g. for train tickets. The chip also communicates with the phone where an application can used to see transaction / current balance, etc.  The chip can also be used to receive text information like for example phone book entries or coupons in conjunction with the application on the mobile phone.

Text Scanner: The camera can be used to make pictures of business cards, URLs, mail addresses and free memos. Once the picture is taken the phone scans the picture and extracts the information for the phone book, the browser etc. Cool for business cards without a 2 dimensional bar code 🙂

Speech output of Text: The phone can read eMails to the user (I wonder how that sounds like)

So Nokia’s, Motorola’s and Sony-Ericsson’s of this world, it’s time to play catch up ! (and to open your pockets for license payments?)

Wireless Broadband in Rural Austria

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While UMTS operators are starting to upgrade their networks for high speed Internet access (HSDPA) it seems the competition is already a step ahead in some areas. During a walk in the countryside in Austria (to be exact: Gaspoltshofen, 3000 inhabitants, about 60 km from Linz) I saw an interesting antenna tower on a hill (see pictures). Turns out that the antenna belongs to Flashnet, a local company providing fast wireless Internet access to the local population.

It looks like they are using Wireless LAN as technology and directional antennas which are installed on the rooftops of their customers houses. Prices are quite interesting, €30.- a month for 2 gigabyte data traffic and 1 MBit/s up- and downlink. Their website also provides a map of the coverage area and they say on their website that they are now serving over 1000 customers in the area. Too bad that they don’t specify how many sites they use to cover the area.

Great stuff!!!

Audible on S60 revisited

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Back in January I expressed my frustration with the Audible player for S60 that would only download audio content over the air (via GPRS, UMTS, etc.) but would not play audio content that I’ve already downloaded to my PC and from there to the phone. Well, it looks like it was worth writing the blog entry because Bruno Santos kindly enough left me a message with a solution:

The audible player for Series 60 can only decode category 2 and 3 files but not the highest quality category 4 encoding with which (of course) I’ve downloaded all my audio files to the PC. After downloading one of my audio books again with category 3 encoding to the PC and then to the mobile phone, the audible player just asked once to connected to the Internet to verify that I have the rights to listen to the content. Since then no more costly connections to the Internet. The player works fine even if used over several hours at a time. Thanks very much Bruno, your tip will save me a lot of work (see blog entry from back in January).

Wireless VoIP Demystified – Part 4: Skype

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This blog entry is part four in my mini series of looking at the different Voice over IP systems (VoIP) that can be used over wireless networks such as UMTS or CDMA 1xEV-DO. Part 1 focused on UMA, part 2 on SIP, part 3 on IMS, and this part will take a look on the use of Skype over wireless.

UMA, SIP and IMS are all centralized systems. That means that they use a centralized server which is responsible for authenticating users, for establishing connections between users for voice, video, instant messaging or any other kind of media transfer, and also for billing. Skype uses a fundamentally different architecture as it does not rely on a centralized server for most of these tasks. Skype is a Peer to Peer network in which end points of the network help out each other to establish and maintain a connection.

A peer to peer network has a number of advantages over a centralized approach:

  • Centralized servers are costly to buy, maintain and operate. The more people use the service the bigger the server has to become. In a peer to peer network such as Skype, however, signaling load at a central point does not increase in the same way as in centralized systems.
  • Individual peers help out each other to establish a connection. This is especially important as many users are behind firewalls or network address translation (NAT) routers typically used at home. Thus, they can not communicate directly with each other. Skype peers that have no such restrictions help out peers that do and forward traffic between such users. This is the main reason why Skype is so easy to set up on PCs and other devices compared to other technologies like for example SIP. For those of you who would like to find out more about Skype, here’s a link to an analysis of how Skype works which has been published by Philippe Biondi and Fabrice Desclaux of EADS.

While most other VoIP systems use legacy voice codecs to transport the media stream over IP, Skype uses its own resource efficient codecs which which on top even have a superior voice quality. Thus, Skype works quite well over UMTS and I use it on a regular basis when traveling. It should also work quite well over EV-DO as well, as bandwidth is also sufficient. Personally I’ve never tried so this is just a speculation.

Many operators (carriers) are scared of Skype and other VoIP systems as they are afraid that such services will decrease their revenue on traditional voice minutes. I think there is no such risk in the near future as there is still a PC required to run Skype over a wireless link. However there are first signs that Skype is also moving to mobile devices. A beta client for Windows Mobile is already available and a non official beta of Skype on a Nokia S60 6680 has also been spotted by the author (see picture above). So operators should hurry up and develop strategies to integrate such innovative applications into their concepts. Some have already done so, like for example E-Plus in Germany. They even offer a UMTS flatrate together with the Skype software and a headset. An interesting first step, certainly not made too soon as new devices such as the Nokia N80 with built in WLAN will spur the interest of a wider audience to cheap VoIP over wireless.

At this point I close my wireless VoIP mini series for now. Four different VoIP systems, four different basic ideas and four ways for every one in the industry and of course the users to benefit. I think it will still take several years before Wireless VoIP becomes mainstream but the first signs are already here.