See also, What's Inside a GSM cellphone

GSM 900/DCS 1800 networks use a sophisticated array of digital equipment to provide you with a seamless, hiss-free connection.

Below are some of the critical components & procedures that allow them to do so:

	Component/ Procedure	Purpose
	Base Station	GSM (Global System for Mobile Communications) uses a series of radio transmitters called Base Stations (BS) to connect you and your cellphone to your cellular network. Each BS is also termed a cell, so named because it covers a certain range within a discrete area (cell). Base Stations are all interconnected, which is why you can move from one cell to another - a process called "hand-over" - without (hopefully) losing your connection.
	Base Station Controller	A set of Base Stations is connected to a particular Base Station Controller.
	MSC	The combination of a cellphone and the SIM card creates a special digital "signature" - that includes your subscriber number - which is sent from your cellphone to the nearest BS asking that you as a subscriber of a particular network be allowed to use the network. The request is passed on along the network of BSs to the multifaceted heart of a cellular network - the Mobile Switching Center (MSC). The MSC also routes all your incoming and outgoing calls to and from the fixed-line networks or other cellular networks.
	HLR	The MSC also contains a critical component called the Home Location Register (HLR) which provides the administrative information required to authenticate, register and locate you as a that network's subscriber. Once it’s received your log-on request, the HLR immediately checks the special "signature" contained in the request against it’s special subscriber database. If your subscription is current, the MSC sends a message back to the phone via the network of BSs that indicates that you’re allowed to access the network. The name or code of that network will appear on the LCD screen of the cellphone. Once this network "name" message appears on your phone’s LCD screen, it means you’re connected to the network and able to make and receive calls. The entire log-on process usually takes only a couple of seconds.
	Polling	At the same time, the HLR also registers which BS your cellphone is currently connected to, so that when the network’s MSC needs to route an incoming call to your cellphone number, it will first check the HLR to see where you are. Every now and gain, the cellphone will send a message to the network indicating where it is, a process called Polling.
	Multiplexing	Each BS uses digital techniques to enable a number of phones to be simultaneously connected to it, as well as simultaneously allowing a number of subscribers to make and receive calls. This sophisticated digital call-juggling ability is called Multiplexing. However, the combination of the tracking function and your unique digital signature allows the MSC to route that call to the precise BS your cellphone happens to be connected to, and then exclusively to your cellphone - even if a number of other subscribers are simultaneously connected to that BS.
	Hand Over	When you "hand-over" to another cell whilst driving, the HLR is automatically updated, and continues to monitor where exactly it should route your calls should you then move within range of to another Vodacom BS. This sophisticated routing procedure means that out of hundreds of thousands of subscribers, only the correct cellphone will ring when necessary.
	VLR	When you want to make an outgoing call, another section of the MSC called the Visitor Location Register (VLR) checks whether you are actually allowed to make that call. For example, if you are barred for international dialing, a message to that effect will be generated by the VLR, sent along the network, and almost instantly back to your cellphone.
	MailBox	If you’re unavailable for some reason and your Mailbox has been activated, any incoming voice calls will be transferred to the Mail system.
	SMSC	Some VoiceMail systems are linked to a network's SMS Center (SMSC), a special facility that handles Short Messages. The SMSC generates the special SMS message that notifies you that you have mail waiting in your Mailbox. SMS messages can be received on your SMS-capable cellphone even while you’re on a voice call. That’s because they are sent on a different radio frequency - the GSM data channel - than voice calls, so that the two never interfere. These sophisticated digital facilities are the reason why GSM is now considered the de facto global cellular standard.