What is UMTS?

If UMTS is part of GSM, then what exactly is the difference between it and what we presently know as GSM? There is a tendency to refer to the old service as strictly GSM, but I need to clarify that calling it that is a bit of a misnomer. GSM is actually the name given to the entire system upon which a cell phone and its service are part. The term GSM encompasses everything from the air interface to the way in which the switches interact with one another, as well as to the landlines to which they connect. However, in this context we are using GSM to refer to the old 2.5G service that most North American’s know as GSM. UMTS is the new iteration of the GSM air interface.



Okay, so now that we have the nitpicks out of way, let’s delve into the differences between the old and the new air interfaces. The original GSM service was based on a TDMA (Time Division Multiple Access) scheme. This approach broke up a single channel into various “slots”, which phones took turns transmitting on in order to share the channel. The new UMTS service uses a CDMA (Code Division Multiple Access) scheme.



That immediate begs comparison to the existing networks that most North Americans refer to generically as CDMA (which, like GSM, is a bit of a misnomer). Currently the big CDMA providers include Bell Mobility, Telus, Sprint PCS, and Verizon. The comparison is certainly a valid one, but rest assured that both of these systems are quite different, though they are based on a similar air interface technology.



But getting back to GSM, a big difference between a TDMA-based system and a CDMA-based system is how the phone deals with handing off from one cell site to another. In TDMA-based systems the phone must wholeheartedly switch from one channel to another in order to switch towers. There are no halfway measures here, it’s all or nothing. As a result, all TDMA-based GSM phones suffer from slight (but rather annoying) interruptions in the audio stream whenever a handoff occurs, and if one doesn’t occur in a timely fashion, the user can experience rather devastating degradation of the call quality.