Wireless network architecture
Question: I'm interested in what a network engineer considers when purchasing or obtaining switching equipment. In the wireless/cellular network equipment world, what kind of common questions would likely need to be answered? I'm also confused about where the cellular switch is located with respect to the base stations.
Answer: Mark van der Hoek here. The switch sits at some telephone company central office, not necessarily even in the same state as the cell sites. The sites themselves have some intelligence. For digital technologies, there's an additional box called a BSC or base station controller that handles most of the low level call processing. It sits (logically, not geographically) between the sites and the switch. Typically there will be multiple BSCs on a switch, each BSC having up to a couple of hundred sites that talk to it. BSCs have pretty much the same power, AC, and maintenance requirements as a switch, so are often located in the same building. (Unlike everyone else, Lucent CDMA and AMPS switches incorporate the BSC function in the switch.)
(Network element description of above is here AND, with nice little pictures, here.)
Other than that, the switch dimensioning is pretty much the same with cellular. Number of cell sites and subscribers is the main factor. In other words, capacity. One big difference is that cellular switches are not necessarily interchangeable. It depends on the technology. GSM systems have excellent interoperability. Buy a site from Ericsson, a BSC from Siemens, and a switch from Nortel. No problem. Build a network with Siemens switches and add a couple of Nortel switches down the road a few years. No problem.
With other technologies, such as CDMA or AMPS, you buy a complete system. Buy a Lucent system, or buy a Nortel system, or buy a Motorola system. But you can't mix and match. It's all or nothing, from cell site to switch. The only part that can be interchanged is the HLR. That can be just about anything, as it interfaces via an industry standard protocol, IS-41.
Was that all wonderfully confusing?
Mark van der Hoek
Editor's note: I have diagrams and easy to follow text on these pages which outline different GSM architectures. Kind of pHun to puzzle over. See the example below and these different pages:
Generic GSM architecture by Scourias / Lucent GSM architecture / Ericsson GSM architecture / Nokia architecture / Siemen's GSM architecture
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.