Interface in GSM Network

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June 1, 2010 by Isybel Harto

Open Interfaces of GSM

The purpose behind the GSM specifications is to define several open interfaces, which then are limiting certain parts of the GSM system. Because of this interface openness, the operator maintaining the network may obtain different parts of the network from different GSM network suppliers. Also, when an interface is open it defines strictly what happening theough the interface is and this in turn strictly defines what kind of actions\procedures\functions must be implemented between the interfaces.

Nowadays, GSM specifications define two truly open interfaces. The first one is between the Mobile Station. This open air interface is appropriately named the “Air Interface”. The second one is between the Mobile Services Switching Center – MSC & the Base Station Controller (BSC). This interface is called the “A Interface”. These two network elements will be discussed in greater detail in later part. The system includes more than two defined interfaces but they are not totally open as the system specifications had not been completed when the commercial systems were launched.

When operating analogue mobile network, experience has shown that centralized intelligence generated excessive load in the system, thus decreasing the capacity. For this reason, the GSM specifications, in principle, provides the means to distribute intelligence throughout the network. Referring to the interfaces, the more complicated the interfaces in use, the more intelligence is required between the interfaces in order to implement all the functions required. In a GSM network, this decentralized intelligence is implemented by dividing the whole network into three separate subsystems:

• Network Switching Subsystems (NSS)
• Base Station Subsystems (BSS)
• Network Management Subsystems (NMS)

The actual network needed for establishing calls is composed of the NSS and the BSS. The BSS is responsible for radio path control and every call is connected through the BSS. The NSS takes care of call control functions. Calls are always connected by and through the NSS. The NMS is the operation and maintenance related to the network and it is needed for the control of the whole GSM network. The network operator observes and maintains network quality and service offered through the NMS. The three subsystems in a GSM network are linked by the Air, A and O&M interfaces

The MS (Mobile Station) is a combination of terminal equipment and subscriber data. The terminal equipment as such is called ME (Mobile Equipment) and the subscriber’s data is stored in a separate module called SIM ( Subscriber Identity Module).

Therefore, ME + SIM = MS


Base Station Subsystem (BSS)
The BSS consists of the following elements:

• BSC ( Base Station Controller)
• BTS ( Base Transceiver Station)
• TC ( Trans Coder)

The Base Station Controller (BTS) is the central network element of the BSS & it controls the radio network. This means that the main responsibilities of the BSC are: Connection establishment between MS & NSS, Mobility Management, Statistical raw data collection, Air and A interface signaling support.
The Base Transceiver Station (BTS) is a network element maintaining the AIR interface. It takes care of the Air interface signaling, Air interface ciphering and Speech processing. In this context, speech processing refers to all the functions the BTS performs in order to guarantee an error-free connection between the MS and the BTS.

The Transcoder (TC) is a BSS element taking care of speech transcoding, i.e. it is capable of converting speech from one digital coding format to another and vice versa. Also TC is known as Multiplexer. It changes speech from 13 Kpbs from 64 Kpbs & vice versa.

The main function of the BSS is as follows:

• Radio Path Control
In the GSM network, the BSS is the part of the network taking care of Radio Resources, i.e. radio channel allocation and quality of the radio connection. For this purpose, the GSM Technical specification defines about 120 different parameters for each BTS. These parameters define exactly what kind of BTS is in question & how MSs may “see” the network when moving in this BTS area. The BTS parameter handle the following major items: what kind of handovers, paging organization, radio power level control and BTS identification.
• BTS and TC Control
Inside the BSS, all the BTSs & TCs are connected to the BSCs. The BSC maintains the BTSs. In other words, the BSC is capable of separating a BTS from the network & collecting alarm information. Transcoders are also maintained by the BSC, i.e. the BSC collects alarms related to the Trancoders.
• Synchronisation
The BSS uses hierarchical synchrinisation which means that the MSC synchronises the BSC & the BSC further synchronises the BTSs associated with that particular BSC. Synchronisation is a critical issue in the GSM network due to the nature of the information transferred. If the synchronization chain is not working properly, calls may be cut or the call quality may not be the best possible. Ultimately, it may even be impossible to establish a call.
• Air & A Interface Signalling
In order to establish a call, the MS must have a connection through the BSS. This connection requires several signalling protocols.
• Connection Establishment between MS & NSS
The BSS is located between two interfaces, the Air & A interface. From the call establishment point of view, the MS must have a connection through these two interfaces before a call can be established. Generally speaking, this connection may be either a signaling type of connection or a traffic (speech, data) type of connection.
• Mobility Management and Speech Transcoding
BSS Mobility Management mainly covers the different cases of Handovers.
• Collection of Statistical Data
The BSS collects a lot of short-term statistical data that is further sent to the NMS for post processing purpose. By using the tools located in the NMS the operator is able to create statistical “views” & thus observe the network quality.

Transmission between BSC and BTS

There are three alternative methods to provide the connections between a BSC & several BTS. They are:

• Point to Point connection
• Multi drop Chain
• Multi Drop Chain

Point to Point connection indicates that the BSC is connected directly to every BTS whit 2 Mbit/s PCM line. If the BSC–BTS distance is few kilometers were as the distance between a group of BTS is much shorter it does not make sense to draw a Point to Point connection to every BTS. One PCM line is capable to transfer data to several BTs simultaneously. There for it possible to draw just one BSC-BTS connection and link.The BTs as a chain. This technique is called Multi Drop Chain. The BSC sends all the data in one 2 Mbit/s PCM line and each BTS in turn analyses the signal, collect the data from correct time slots assign for itself and passes the signal to next BTS. But there is a one problem with Multi Drop Chain. Consider if there is a break in chain more BTS are isolated from BSC. This problem can be solved by using Multi Drop Loop technique. In Multi Drop Loop if there is a break the signal split in two directions. So if there is a break in one side than BTS can also able to receive signal from the other side.

Network Switching Subsystems (NSS)

The GSM network is divided into three subsystems: NSS , BSS , NMS. The concept of NSS is introduced in this section.The elements of Network Switching Subsystem (NSS) are as follows:

• MSC (Mobile Service Switching Center)
• VLR (Visitor Location Register)
• HLR (Home Location Register)
• AC (Authentication Center)
• EIR (Equipment Identity Register)

The MSC is responsible for controlling calls in the mobile network. It identifies the origin & destination of a call as well as a type of call. An MSC acting as a bridge between a mobile network and a fixed network is called a “GATEWAY MSC” (GMSC). An MSC is normally integrated with a VLR, which maintains information related to the subscribers who are currently in the service area of the MSC. The VLR carries out location registration & updates. The MSC associated with it initiates the Paging process. A VLR database is always temporary whereas the HLR maintains a permanent register of the subscribers. In addition to the fixed data, the HLR also maintains a temporary database which contains the current location of its customers. This data is required for routing the calls.

In addition there are two more elements in the NSS. They are AC & EIR. They are usually implemented as a part of HLR & they deal with the security functions

So, the main functions of NSS are as follows:

• Call Control
This identifies the subscriber, establishing a call and clears the connection after the conversation is over
• Charging
This collects the charging information about a call as the members of the caller and the called subscriber, the time & type of the transaction, etc. , and transfers it to the Billing Center (BC).
• Mobility Management
This maintains information about the location of the subscriber.
Signalling with other network & the BSS This applies to interfaces with the BSS and PSTN (Public Switch Telephone Network)
• Subscriber Data Handling
This is the permanent data storage in the HLR and temporary storage of relevant data in the VLR.

Network Management Subsystems (NMS)

The NMS is the third subsystem of the GSM network in addition to NSS & BSS. The purpose of the NMS is to monitor various functions and elements of the network. These tasks are carried out by the NMS/2000 which consists of a number of Work Stations, Servers & a Router which connects to a “Data Communications Network”(DCN).

The functions of the NMS are as follows:

• Fault Management
The purpose of Fault Management is to ensure the smooth operation of the network & rapid correction of any kind of problems that are detected. This provides the network operator with information about the current status of the Alarm events and maintains a history database of Alarms.
• Configuration Management
The purpose of Configuration Management is to maintain up to date information about the operation & configuration status of network elements. Specific configuration functions include the management of the radio network, software & hardware management of the network elements, time synchrinisation and security operations.
• Performance management
In Performance Management, the NMS collects measurements data from individual network elements and stores it in a database. On the basis of these data, the network operator is able to compare the actual performance of the network with the planned performance & detect both good and bad performance areas within the network.

GSM Network Architecture

Whole GSM architecture containing all the three systems is drawn as under The operator work stations are connected to the database and communication servers via a Local Area Network (LAN). The database servers store the management information about the network. The communications servers take care of the data communication between the NMS and the equipment in the GSM network known as “Network Elements”. These communications are carried over a Data communication Network (DCN) which connects to the NMS via a Router. The DCN is normally implemented using an X.25 Packet Switching Network.


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