Wireless digital cellular telephone have several type, but now, there are two kind of type base on freq and how it work. Known as GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). This digital cellular system is use at many country in the world. Every country have different rule about both of it.
Now, I would like to write about GSM at my country. At year 1996 the first digital telepon cellular introduced to market, before that, the cellular phone use analog signal. The digital cellular base using GSM type. There was a difference in the type of phone, from the previous use of the phone handset which size was big into using the headset that size is relatively small and brief.
When using the analog signal, and talking with the recipient, the speaking often become get noise, sometimes a small voice, or sometimes obvious, such as loosing the signal like listen to the FM radio. But that problems not happen at the digital system. GSM sound quality more clearly than the analog (AMPS), and also the encryption system for the GSM make it more secure, so we can feel more safe in communicating.
Until now, most of GSM in the world have three kind of frequency: 900, 1,800, or 1,900 Mhz. Each have special headset with the same frequency, but currently the GSM headset already covering all frequency, so when we are move from one to other type of base station with different frequency, we still can communicate without interference.
The GSM handset usually have to have a slot for simcard. Simcard is small chip that put in a card, this chip contain the information about phone number and any encryption data which use to that distinguishes between one number to other number. So with the single handset, it can serve every number depend on the simcard. Simcard have amount of memory size. From 32kb to 64kb, maybe will be more. The memory of the simcard use to record phone number and also sms, but it's record with a limited number of sms/phonenumber depend on the memory size in the card.