I mentioned wireless operating systems: analog cellular, PCS, GSM, and so on. This page gives you an overall look at cellular radio before we concentrate on their details. Bookmark this page and go to the next topic if you don't find it relevant right now.
Wireless systems share many things in common. Here's a short pictorial of basic wireless elements:
The MS or Mobile Station
1. The mobile makes a call . . .
The Cell Site Antenna
2. A nearby cell site's antenna picks up the call from the mobile . . .
THE MSC OR MTSO
4. The mobile switching center or mobile telecommunications switching office gets the call next. This switch can be a normal landline switch like a 5ESS or an AXE or a dedicated one like a Motorola. Each MSC manages dozens to scores of cell sites and their attendant base stations. Large systems may have two or more MSCs. . .
THE HLR. VLR, AC, EIR
5. The mobile switch queries several databases before permitting a call. A dedicated server associated with the switch houses these databases. The Home Location Register (HLR), The Visited Location Register (VLR), the Authentication Center (AC), and the Equipment Identity Register (EIR) are some of these databases. . .
6. The call is processed and routed next to the telephone network at large, also known as the Public Switched Telephone Network. The switch communicates, too, with distant databases over the PSTN.
This silly little spinning globe is supposed to represent the Public Switched Telephone Network at large.
7. At all times an Operations and Maintenance Center monitors the network.
Simple block diagram of network elements
Now that we've seen the elements, let's put it into a block diagram and discuss some terms. We'll look at more complicated diagram after the terminology discussion below. Again, if this is more than you need to know about cellular radio, bookmark this page and move to the next topic.
The elements in depth
The Home Location Register and the Visitor Location Register work together – they permit both local operation and roaming outside the local service area. You couldn't use your mobile in San Francisco and then Los Angeles without these two electronic directories sharing information. Most often these these two directories are located in the same place.
The HLR and VLR are big databases maintained on computers called servers, often UNIX workstations. Companies like Tandem and DSC make the servers, which they simply call HLRs. These servers maintain more than the home location register, but that's what they call the machine. Many mobile switches use the same HLR.
The HLR stores complete local information. It's the main database. Signed up for cellular service in Topeka? Your carrier puts your information on its nearest HRL, or the one assigned to your area. That info includes your international mobile equipment identity number or IMEI, your directory number, and the class of service you have. It also includes your current city and your last known "location area", the place you last used your mobile.
The VLR or visitor location registry contains roamer information. Passing through another carrier's system? Once the visited system detects your mobile, its VLR queries your assigned home location register. The VLR makes sure you are a valid subscriber, then retrieves just enough information from the now distant HLR to manage your call. It temporarily stores your last known location area, the power your mobile uses, special services you subscribe to and so on. Though traveling, the cellular network now knows where you are and can direct calls to you.
The AC or AUC is the Authentication Center, a secured database handling authentication and encryption keys. (GSM, PCS 1900, and certain cellular systems support these features.) As we'll see later, authentication verifies a mobile customer with a complex challenge and reply routine. The network sends a randomly generated number to the mobile. The mobile then performs a calculation against it with a number it has stored and sends the result back. Only if the switch gets the number it expects does the call proceed. The AC stores all data needed to authenticate a call and to then encrypt both voice traffic and signaling messages.
The EIR or Equipment Identity Register is another database.The EIR lists stolen phones, fraudulent telephone identity numbers, and faulty equipment. It's one tool to deny service or track problem equipment.
The OMC or operation maintenance center is network control. It monitors every aspect of a cellular system. A maintenance center may monitor several carrier's systems. Every OMC is staffed twenty four hours a day.
Now let's look at the more complicated block diagram below. It's from Ericsson and it details a GSM or PCS system. I'm discussing this to familiarize you with block diagrams; to show network elements don't have to be mysterious.
A more complicated model
Much is familiar in the diagram below. Ericsson divides a system into several parts, such as a switching system, base station system, network management system and so on. Here's my quick guide below:
Base station system
Made up of a base station controller (BSC) and the individual base transceiver stations (BTS), which most people just call base stations. The radio base station 2000 (RBS) is Ericsson's newest base station. AXE stands for Automatic Exchange Electric, Ericsson's digital switch. Seeing AXE in a box means that element is tied or linked to the switch.
The service order gateway (SOG) means a service desk, where clerks access network databases. Operators enter and cancel accounts and do administrative chores. The billing gateway (BGW) is where customer and administrative billing information contacts the individual carrier.
"Stores and forwards voice, fax and electronic mail, as well as short texts from paging networks."
MIN stands for mobile intelligent network. The service control point (SCP), The service management system (SMAS) provides service management functions. "800-number lookup services, calling card services, calling number identification, short message service, message waiting indicator, and debit card services" are all provided through databases linked to the cellular system by the much larger, countrywide MIN.
Operations support system
Operations support system (OSS) is another word for the operation and maintenance center we discussed above. EET stands for Ericsson engineering tool, a network planning device.
AXE: Automatic Exchange Electric: Ericsson's digital switch. They operate as either a landline or wireless switch. OSS: Operations support system EET: Ericsson engineering tool, network planning software. SOG: service order gateway BGW: billing gateway. MIN: Mobile intelligent network. SCP: service control point.
Familiar now or at least more comfortable with block diagrams? Good, let's end this discussion and move on. And those who want more, well, download this outstanding .pdf file of Levine, far more details on GSM and PCS and network elements than I will ever write.