Mobile Data

An Introduction

Mobile data is a growth market. Whether WAP browsing or connecting to the internet with a laptop or PDA, mobile data use is growing at a phenomenal rate. Traditionally limited to 9600 baud connections, newer developments such as HSCSD (High Speed Circuit Switched Data) and GPRS (General Packet Radio Service) are making connection speeds roughly equivalent to traditional landline analogue modems possible.

Circuit Switched Vs. Packet Switched

There are two main types of data connection available over GSM now - circuit switched and packet switched. Circuit switched gives you a radio timeslot to yourself, with the full data rate of that timeslot (9600 baud) available exclusively to you. The downside to this is that you are then paying for your connection per second. If you are doing an activity such as web browsing, your link may be idle for a significant period whilst you read the contents of a page - time that you are paying for at mobile rates.

HSCSD takes the circuit switched paradigm one step further. It modifies the error coding of a radio timeslot to give 14k4 per timeslot, but then allows you to use more than one timeslot. If you use two timeslots, you can get up to 28k8 data. HSCSD can be symmetrical or asymmetrical - you can use two downlink and one uplink timeslot to give 28k8 downloads but 14k4 uploads. There are three reasons why you might want to do this - handset limitations, billing purposes, or the conservation of battery power. Every transmit timeslot takes a significant amount of power, so if you are more interested in downloading data than uploading, it can be worthwhile only using one uplink timeslot.

HSCSD handsets typically are able to use a certain number of timeslots - for example 4 on the Nokia 6210 and Card Phone II. These slots can be split as needed between up and download timeslots, allowing you to use 2 up/2 down (28k8 in both directions) or 3 down and 1 up (43k2 down 14k4 up). The actual speeds and timeslots available will depend upon your network.

From a billing point of view, the more timeslots you use, the more your network may charge you. Some networks simply make a flat rate per-month charge then charge multi-slot connections the same as single slot. Other networks may charge you twice as much (or even more!) for using multiple timeslots. In the UK only Orange support HSCSD.

Packet switched networks (such as GPRS) use a different data connection method. Your network will allocate a certain number of timeslots per base station for GPRS data. Rather than having exclusive use of that timeslot, you share it with all other users in your area. You send and receive data in short packets instead of maintaining a permanent link.

The advantage is that you can have a permanent connection, and are only billed for the amount of data transferred, not your connection time. If there is no-one else using GPRS on your cell, you will get very fast data rates. The disadvantage comes if there are many other users on your cell - you have no guaranteed minimum data rate, and may well find that you get data time-outs and extremely low download speeds.

Configuration

Circuit Switched Data AT Strings

Using a single timeslot for circuit switched data, you can choose to make analogue or ISDN connections. ISDN has the advantage of much faster call setup, but GSM only supports V110 at 9600 baud - not all ISPs will accept this type of incoming connection.

To choose to use ISDN, you should use one of the following initialisation strings:-

ATS35=0 (for older Nokia phones)

AT+CBST=71,0,1 (for newer handsets)

Note that if you are using a Palm PDA, you may have to add a semi-colon to the end of the string, as these machines add additional commands after the user-defined string (i.e. use AT+CBST=71,0,1; ).

The +CBST string is defined as "select bearer service type" in GSM 07.07. The full definition of this is:-

AT+CBST=<speed>,<name>,<ce>

The most common values for <speed> are:-

The most common values for <name> are:-

The valus for <ce> are:-

HSCSD AT Strings

HSCSD configuration invloves telling the handset how many timeslots you want to use, and what speeds. Whilst HSCSD may allow you to use 14k4 rather than 9k6 on each timeslot, if the radio signal is poor the network may force you to use 9k6.

The most common configurations will be:-

AT+CHSN=2,1,1,8; want single slot 14k4

AT+CHSN=2,1,1,12; want single slot 14k4 but will accept 9k6

AT+CHSN=4,2,2,8; want double slot 28k8

AT+CHSN=4,2,2,12; want double slot 28k8 but will accept 19k2

Again, the semi-colons are optional, but may be required by some handsets and some Palm devices.

The actual format of the command is defined in GSM07.07 as:-

AT+CHSN=<wAiur>,<wRx>,<topRx>,<codings>

The most common values of wAiur (wanted air interface user rate) are:-

The <wRx> and <topRx> are the wanted and maximum number of receive timeslots. <wRx>=0 means that the handset should calculate the correct value from <wAiur> and <codings>. <topRx>=0 means that the handset will not request a change in timeslots allocated during the call.

The values for <codings> is a sum of the following:-

These are additive, so for example setting <codings> to 12 means 9600 or 14400 are acceptable.