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Telecom Gyan - GSM Overview

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We have a Telecom revolution of some sorts going on. Its touted to be the next big thing. Infact it already is. All you can hear about these days is 2G scams, 3G, 4G and what not. And when you go for technical jobs, you will realize that most of your company's clients will be telecom operators like Airtel, Vodafone, BT etc. Infact many of you will be working for telecom companies; Companies like Wipro, TechMahindra, Aricent are heavily into telecom.

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So we decided that while 2G (or 2.5G) is still there, lets have some basic tutorial (or gyan sessions) on telecom. How it evolved, what it is and how do people communicate apart from the new technologies that are upcoming. Let me start explaining a GSM architecture here. And don't be confused. As more articles pour in, the understanding will deepen.

GSM Overview

A basic GSM system looks like this

GSM Architecture

The Base Station System (BSS):

All radio-related functions are performed in the BSS, which consists of BTSs and the BSCs

  • Base Transceiver Station (BTS) — BTSs  are the radio equipments (transceivers and antennas) which service each cell in the network (more on this later). The BTS handles the radio or the air interface to the mobile station. 
  • Base Station Controller (BSC) — A BSC controls a group of BTSs. It is a high-capacity switch that provides functions such as handover, cell configuration, and control of radio frequency (RF) power levels in BTSs. It also provides all the control functions and physical links between the MSC and BTS. A number of BSCs are controlled by an MSC.

The Switching System:

The switching system (SS) is responsible for call switching and processing, and performing subscriber-related functions. It consists of the following functional units.

  • Home Location Register (HLR) — The HLR is a database which stores permanent data about subscribers, including a subscriber's location information, service profile and current activity status. When a person buys a subscription from any mobile operator, he or she is registered in the HLR of that operator.
  • Mobile services Switching Center (MSC) — The MSC is responsible for all the telephony switching functions in the call flow. It controls calls to and from other telephones and mobile systems. All calls which originate in the system have to pass through an MSC to be correctly routed (although, these days, modern BSCs have been designed to route calls inside their serving areas). The MSC also performs functions like toll ticketing, network interfacing, common channel signaling, and others.
  • Visitor Location Register (VLR) — The VLR is another database. It contains temporary information about subscribers an MSC requires in order to service visiting subscribers (for example in case of roaming). The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC requests data about the roaming mobile station from the HLR. Later, if the roaming mobile makes a call, the VLR will have the information needed for the call setup without the need to interrogate the HLR every time.
  • Authentication Center (AUC) — A unit called the AUC provides authentication and encryption parameters to verify the user's identity and ensure the confidentiality of each call. It also protects networks from frauds and hacks.
  • Equipment Identity Register (EIR) — The EIR is a database that contains information about the identity of mobile equipment (the cell phone itself) that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The Operation and Support System (OSS):

There is also an operations and maintenance center (OMC) which is connected to all equipments in the Switching System and to the BSC. The implementation of OMC is called the operation and support system (OSS). The network operator monitors and controls the system from the OSS. Its purpose is to provide a network overview and a platform for local operational and maintenance activities that are required for a GSM network. 

Note: While a Mobile Equipment refers to the cell phone itself, the Mobile Station generally refers to the SIM (Subscriber Identity Module) inside it. When authenticating, both the phone identity and the SIM identity are checked. The phone identity is called IMEI (International Mobile Equipment Identity) while SIM identity is the IMSI (International Mobile Subscriber Identity)

Reference: burnsidetelecom GSM Whitepaper

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