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VII GSM Call Processing: (1) Introduction / (2) The Radio elements/ (3) The Network or Switching elements / (4) Main discussion
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5. THE MSC OR MTSO
"Once received from the base station controller our call is next passed to a mobile switching office. . . ."
Mobile switches go by many names: mobile switch (MS), mobile switching center (MSC), or mobile telecommunications switching office (MTSO). They all do the same thing, however, and that is to process mobile telephone calls. This switch can be a normal landline switch like a 5ESS, a Nokia, an Alcatel, or an Ericsson AXE (Automatic Exchange Electric) or a dedicated switch, built just to handle mobile calls. Each mobile switch manages dozens to scores of cells. In GSM the mobile switch handles cell sites by first directing the base station controllers. Large systems may have two or more MSCs. It's easy understand what a switch does. What is harder to understand is the role the switch has to do with other network resources.
6. THE HLR. VLR, AuC, and the EIR
"The mobile switch communicates with local and distant databases before allowing a call . . ."
The mobile switch queries several databases before permitting a call. Why give a customer access if their account isn't valid? No need to let Mr. Jones on the network if he hasn't paid his bill. To do this account lookup and other vital pre-call processing tasks, a number of databases are consulted. A dedicated server associated with the switch houses these databases. To explain, a server is really just a large computer, often using the UNIX operating system. The Home Location Register (HLR), The Visited Location Register (VLR), the Authentication Center (AuC), and the Equipment Identity Register (EIR) are some of these databases. These cumbersome names and abbreviations will become clear soon.
(An important aside. In America the national emergency number is 9-1-1. In theory all mobiles have access to this number, whether or not they have a valid account.)
Simple block diagram of network elements
Remember the friendly block diagram on the previous page? Let's look at it again and go into more detail. I describe most of the main components below. Don't worry if there are too many things to remember. Bookmark this page or make it one of your favorites so you can use it as a reference.
As John states, he presents a generic GSM architecture. Lucent, Ericsson, Nokia, and others feature their own vision in their own diagrams. But they all share the same main elements and parts from different vendors should work together. The links below show how these vendors picture the GSM architecture. You can remember the different terms much better by looking at all of these diagrams.
Lucent GSM architecture/ Ericsson GSM architecture / Nokia GSM architecture / Siemen's GSM architecture
Two page .pdf file on the network subsystem by Nokia. It's a glossy product brochure but it does mention all the important elements. (363k in .pdf)
HLR and VLR: Working together for you
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, often on the same computer.
The HLR and VLR are big databases maintained on computers called servers, often UNIX workstations. Companies like Tandem, now folded into Compaq (external link), make the servers, which they call HLRs when used for cellular. These servers maintain more than the home location register, but that's what they call the machine. Many mobile switches use the same HLR. So, you'll have many Home Location Registers. To operate its nationwide cellular system, iDEN, Motorola uses over 60 HLRs nationwide.
The HLR stores complete local customer 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 AuC and EIR: Knowing you and your mobile
The AC or AuC is the Authentication Center, a secured database handling authentication and encryption keys. 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 in its SIM 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 Equipment Identity Register (EIR) is a standard GSM network element that allows a mobile network to check the type and serial number of a mobile device and determine whether or not to offer any service." The EIR or equipment identity register is yet another database. It's first purpose is to deny stolen or defective mobiles service. Good mobiles are allowed on the network, of course, as is faulty but still serviceable equipment. In the latter case such mobiles are flagged for the cellular carrier to monitor.
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The A, Um, Abis, and Ater interfaces
The most cryptic names in cellular radio belong to these interface terms. A telecom interface means many things. Newton says it is a mechanical or electrical link connecting two or more pieces of equipment together. The cable between your printer and your computer could thus be an interface.
An interface is also the boundary between systems, such as between the base station system and the network subsystem. Interface "A", remember? To be more specific, Smith says "A" is the signaling link between the two subsystems.
Secondly, we have the A-bis, which Tektronix says "is a French term meaning 'the second A Interface.' " Good grief! In most cases this is a T1 line or in Europe its equivalent, the E1.
What about the mysterious UM? That's the radio link between a mobile and a base station. Um are the actual radio frequencies that calls are put on. Um may stand for User Mobile. R.C. Levine clears up the interface name game nicely,
"Interface names (A, Abis, B, C, etc.) were arbitrarily assigned in alphabetical order. The Um label is taken from the customer-network U interface label used in ISDN. Although mnemonics have been proposed for these letters, they are after-the-fact."
Note that the interface between the MSC and the Public switch telephone network isn't labeled. Macario calls this, I think, interface "E." The mobile switch communicates with the telephone network using Signaling System Seven, an internationally agreed upon standard. Using SS7 is a big difference between conventional cellular and GSM. IS-136 and IS-95 use a standard called IS-41 to communicate with the wireline telephone network. SS7 is perhaps faster and more efficient, especially when dealing with telephone companies outside the United States.
.pdf file on SS7 and mobile networking -- Good reading!
Excellent GSM tutorial:
http://www.ee.surrey.ac.uk/Personal/
L.Wood/constellations/tables/gsm.html
Let's now look at the more complicated block diagram below. It's from Ericsson and it details their GSM architecture. It's really for completeness, now that you're familiar with the main GSM elements. Skip ahead for the call processing.
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A more complicated model -- more perhaps than you need to know
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.
Much is now familiar in the diagram above. We saw it on the first page and I add it only, as I say, for completeness. 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. AXE stands for Automatic Exchange Electric, Ericsson's digital switch. Seeing AXE in the outlined box means that element is tied or linked to the switch.
Gateway products
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.
Message Center
"Stores and forwards voice, fax and electronic mail, as well as short texts from paging networks."
MIN Network
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
Every cellular system has an OMC or operation maintenance center. Some call it 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. In the case of Ericsson terminology, Operations support system (OSS) is the word for the operation and maintenance center. EET stands for Ericsson engineering tool, a network planning device.
Familiar now or at least more comfortable with block diagrams? Good, let's move to call processing. 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.
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