Article written by John Scourias, with comments in maroon by Tom Farley
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3.Architecture of the GSM network
A GSM network is composed of several functional entities, whose functions and interfaces are specified. Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber. The Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center (MSC), performs the switching of calls between the mobile users, and between mobile and fixed network users. The MSC also handles the mobility management operations. Not shown is the Operations and Maintenance Center, which oversees the proper operation and setup of the network. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile services Switching Center across the A interface.
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 all 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 these diagrams.
Lucent GSM architecture/ Ericsson GSM architecture / Nokia GSM architecture / Siemen's GSM architecture
3.1. Mobile Station
The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.
GSM phones use SIM cards, or Subscriber information or identity modules. Memory modules. They're the biggest difference a user sees between a GSM phone or handset and a conventional cellular telephone. With the SIM card and its memory the GSM handset is a smart phone, doing many things a conventional cellular telephone cannot. Like keeping a built in phone book or allowing different ringtones to be downloaded and then stored. Conventional cellular telephones either lack the features GSM phones have built in, or they must rely on resources from the cellular system itself to provide them. Let me make another, important point.
With a SIM card your account can be shared from mobile to mobile, at least in theory. Want to try out your neighbor's brand new mobile? You should be able to put your SIM card into that GSM handset and have it work. The GSM network cares only that a valid account exists, not that you are using a different device. You get billed, not the neighbor who loaned you the phone.
This flexibility is completely different than AMPS technology, which enables one device per account. No swtiching around. Conventional cellular telephones have their electronic serial number burned into a chipset which is permanently attached to the phone. No way to change out that chipset or trade with another phone. SIM card technology, by comparison, is meant to make sharing phones and other GSM devices quick and easy.
On the left above: Front of a Pacific Bell GSM phone. In the middle above: Same phone, showing the back. The SIM card is the white plastic square. It fits into the grey colored holder next to it. On the right above. A new and different idea, a holder for two SIM cards, allowing one phone to access either of two wireless carriers. Provided you have an account with both. :-) The Sim card is to the left of the body.
3.2 Base Station Subsystem
The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the standardized Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.
An explanation of the Abis interface is here
The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.
- The BTS or Base Transceiver Station is also called an RBS or Remote Base station. Whatever the name, this is the radio gear that passes all calls coming in and going out of a cell site.
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- The base station is under direction of a base station controller so traffic gets sent there first. The base station controller, described below, gathers the calls from many base stations and passes them on to a mobile telephone switch. From that switch come and go the calls from the regular telephone network.
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- Some base stations are quite small, the one pictured here is a large outdoor unit. The large number of base stations and their attendant controllers, are a big difference between GSM and IS-136.
Want to read more about a base station? Download this product brochure from Siemens. It's about 228K in .pdf
The Base Station Controller
The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).
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- Another difference between conventional cellular and GSM is the base station controller. It's an intermediate step between the base station transceiver and the mobile switch. GSM designers thought this a better approach for high density cellular networks. As one anonymous writer penned, "If every base station talked directly to the MSC, traffic would become too congested. To ensure quality communications via traffic management, the wireless infrastructure network uses Base Station Controllers as a way to segment the network and control congestion. The result is that MSCs route their circuits to BSCs which in turn are responsible for connectivity and routing of calls for 50 to 100 wireless base stations."
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- Want to read more about a base station controller? Download this product brochure from Siemens. It's about 363K in .pdf 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)
Many GSM descriptions picture equipment called a TRAU, which stands for Transcoding Rate and Adaptation Unit. Of course. Also known as a TransCoding Unit or TCU, the TRAU is a compressor and converter. It first compresses traffic coming from the mobiles through the base station controllers. That's quite an achievement because voice and data have already been compressed by the voice coders in the handset. Anyway, it crunches that data down even further. It then puts the traffic into a format the Mobile Switch can understand. This is the transcoding part of its name, where code in one format is converted to another. The TRAU is not required but apparently it saves quite a bit of money to install one. Here's how Nortel Networks sells their unit:
"Reduce transmission resources and realize up to 75% transmission cost savings with the TCU."
"The TransCoding Unit (TCU), inserted between the BSC and MSC, enables speech compression and data rate adaptation within the radio cellular network. The TCU is designed to reduce transmission costs by minimizing transmission resources between the BSC and MSC. This is achieved by reducing the number of PCM links going to the BSC, since four traffic channels (data or speech) can be handled by one PCM time slot. Additionally, the modular architecture of the TCU supports all three GSM vocoders (Full Rate, Enhanced Full Rate, and Half Rate) in the same cabinet, providing you with a complete range of deployment options."
(PCM? To read more about that click here.)
Voice coders or vocoders are built into the handsets a cellular carrier distributes. They're the circuitry that turns speech into digital. The carrier specifies which rate they want traffic compressed, either a great deal or just a little. The cellular system is designed this way, with handset vocoders working in league with the equipment of the base station subsystem.
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Figure 1. General architecture of a GSM network
