As 2G is a digital network you would expect little trouble in making a digital connection but given that the technology was designed and optimised for voice communication it isn’t quite as simple.
The most basic is to use a “modem” built into the phone to dialup another modem and make a Circuit Switched Data CSD connection. In theory a single time slot should be capable of carrying 34Kbits/s but the need for error correction and encryption reduces this by at least a third producing a usable data rate of around 9.6Kbits/s. It also takes longer for a call to be set up because the cell handles a data call differently. Instead of routing the call in the usual way it actually dials the number that you supplied and connects to your ISP’s modem. This also adds a latency of up to one second.
An improved CSD connection, High-Speed Circuit Switched Data (HSCSD), is offered by some operators. This works by using multiple, up to eight, TDMA slots per user and can achieve, taking count of some inefficiencies in using multiple slots, up to 57.6Kbits/s. In practice the operator limits the number of slots that can be used so as to allow other users to make calls. You also need a phone that supports HSCSD.
CSD and HSCSD are both point-to-point connections which make sense for dialling a private network but not for connecting to the Internet.
GPRS or General Packet Radio Service is a direct connection to the Internet.
Your phone sends and receives IP (Internet Protocol) data packets via the cell to the Internet. In this sense the mobile phone company is acting as your ISP. Unlike CSD there is no setup and data packets can be sent when required and received at any time. That is GPRS is an “always on” service.
As in the case of HSCSD GPRS can also use multiple slots to send a packet and with improved coding also increases the useful data that can be set in a slot from 9.6 to 13.4Kbits/s to give a maximum data rate of approximately 100Kbits/s.
In practice, due to limitations in cell hardware, GPRS uses a maximum of four slots in the downlink direction giving a transfer rate of around 40Kbits/s and as few as one slot for uplink providing around 10Kbits/s. GPRS is idea for burst data transfer whereas HSCSD is better for bulk connections. It is also worth knowing that HSCSD is generally charged by connection time whereas GPRS is charged by amount of data transferred.
Some operators also prioritise voice traffic so that GPRS is only allocated time slots when they would otherwise be unused.
EDGE, Enhanced Data rates for GSM Evolution, is the final attempt to push more data through the existing 2G network.
It is essentially an improved GPRS and is often referred to as Enhanced GPRS or EGPRS. What EDGE does is to improve the coding used to transmit the binary data. Standard GSM transmission codes bits using a fairly simple frequency shift coding (Gaussian Minimum Shift Keying or GMSK) which essentially allocates two tones to represent a zero and a one.
EDGE uses octal Phase Shift Keying (8-PSK) which can cram more bits into the same frequency allocation. In fact 8-PSK triples the data capacity of each channel, to around 40 to 70Kbits/s, but it also makes it more sensitive to noise and signal strength and so it also defines nine variations of coding to allow speed to be traded for reliability if the user is on the edge of a cell. Using multiple time slots and with a good signal strength EDGE can provide nearly 400Kbits/s. Of course you need a phone and a phone company that supports EDGE. At the moment there are none in the UK.
At this point you might be wondering what makes 3G so much better?
This is indeed a question that has been asked many times in the light of 2.5G services such as EDGE. Indeed both GPRS and EDGE are sometimes referred to as 3G services even though this is misleading.
There is one thing that no amount of tinkering can change about 2G services – they all use FDMA and TDMA – both of which are known not to be the best way to share a block of frequencies. The revolution in most wireless technologies such as WiFi has been the use of spread-spectrum techniques. Instead of transmitting on a single frequency, spread-spectrum makes use of all of the available frequencies either by frequency hopping or the even more efficient approach of spreading the signal at low power across the available bandwidth.
When GSM and other 2G services were being designed spread-spectrum was in its infancy but one phone system did use a form of it as Code Division Multiple Access or CDMA which currently has about 10% of the market mostly in the US.
CDMA works by mixing the data bits with a pseudo random bit sequence that makes the resulting signal look like noise – it sounds like background static if you listen to it on a standard radio. To recover the data the same pseudo random bit sequence has to be subtracted from the received data.
The phone and cell transmitter have to negotiate which of the possible bit sequences to use during their initial setup. After this, different phones using different bit sequences can transmit using the same frequency allocation at the same time and their data can be recovered.
The European 3G standard – Universal Mobile Telecommunications System UMTS – which is used in 3GSM, makes use of Wideband CDMA. In this case the channel used is 5MHz which is four times wider than the original CDMA.
As you can guess 3G data transmission is a packet based system which is essentially GPRS without the need for time slots. It also uses an adaptive coding system that trades transmission speed for reliability. Hence the actual data rate varies according to the strength and quality of the signal.
At its best it allows operators to offer data rates as high as 2.4Mbits/s. 3GSM is designed to interwork with existing GSM networks allowing calls to be made on whatever system is available 2G voice, CSD, HSCSD, GPRS, EDGE, 3G voice and 3G data. If you are mobile then calls can be passed between GSM and 3GSM cells and the protocols adjusted to what is available without you knowing anything is happening at all.
CDMA works by mixing the data with a faster stream of coding bits. This spreads the transmission across the entire bandwidth. When the receiver subtracts the same coding stream the original data is recovered. (click for larger image)
Different counties have adopted different versions of 3G CDMA technologies, for example in the US existing CDMA networks have upgraded to CDMA 2000 rather than WCDMA or 3GSM.
Given the lack of a single 3G standard how can we expect to roam with our phones?
The answer is that microelectronics is so advanced that it is possible to build not just multi-band phones but multi-standard phones.
In the future the “softradio” approach will allow changed to be made simply by updating the software in the phone as you move from one system to the next.
So where does 3G go to become 4G?
This is a difficult question to answer because the story isn't ended. Currently there are two alternative technologies in use LTE and WiMax. WiMax is just an extension of the WiFi standards to a longer range and a higher power. While WiMax is being used to bring the Internet to remote locations or to locations that don't have good ADSL speeds it is almost certainly not going to be the winner in the 4G stakes. Various companies have produced WiMax phones and more are on their way to market but LTE hardware seems much more advanced.
If WiMax is a development on WiFi then LTE is a development on GSM/UMTS. It uses advanced signal processing techniques to increase the data rate and reliability. It also works in a different area of the wireless spectrum because its radio is incompatible with 3G signals. Even though LTE is relatively new, standardized in 2008, there is already an improved version - LTE Advanced standardized in March 2011.
So what does LTE do that is so much better?
The simple answer is that it is all in the radio. It makes use of a better coding scheme, OFDM - Orthogonal Frequency Division Multiplexing, to cram more data into the same space and it uses a bigger range of radio frequencies to transfer more data. The use of OFDM makes the transmission scheme more like that used in ADSL than conventional radio modulation. You can think of it as applying the best 21st century signal processing to the radio spectrum available.