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A base transceiver station or cell site (BTS) is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. UEs are devices like mobile phones (handsets), WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc. The network can be that of any of the wireless communication technologies like GSM, CDMA, WLL, WAN, WiFi, WiMAX etc. BTS is also referred to as the radio base station (RBS), node B (in 3G Networks) or, simply, the base station (BS). For discussion of the LTE standard the abbreviation eNB for enhanced node B is widely used.So, to understand BTS we have to go through the basics of GSM system because BTS is a part of a GSM system.
Various Subsystems of GSM
Base Station Subsystem (BSS)
From this figure we can say that BTS is a part of BSS(Base station subsystem)
The Base Station Subsystem (BSS)
The Base Station Subsystem consists of the following elements:
1. BSC (Base Station Controller)
2. BTS (Base Transceiver Station)
3. TC (Transcoder)
The Base Station Controller (BSC) is the central network element of the BSS and itcontrols the radio network. This means that the main responsibilities of the BSC are: Connection establishment between MS and NSS, Mobility management, Statisticalraw data collection, Air and A interface signalling support.
The Base Transceiver Station (BTS) is a network element maintaining the Air
interface. It takes care of Air interface signalling, Air interface ciphering and speechprocessing. In this context, speech processing refers to all the functions the BTSperforms 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 iscapable of converting speech from one digital coding format to another and vice versa.
The BTS, BSC and TC together form the Base Station Subsystem (BSS) which is a
part of the GSM network taking care of the following major functions:
Radio Path Control
In the GSM network, the Base Station Subsystem (BSS) is the part of the network
taking care of Radio Resources, i.e. radio channel allocation and quality of the radioconnection. For this purpose, the GSM Technical Specifications define about 120different parameters for each BTS. These parameters define exactly what kind of BTSis in question and how MSs may "see" the network when moving in this BTS area.The BTS parameters handle the following major items: what kind of handovers (whenand why), paging organization, radio power level control and BTS identification.
BTS and TC Control
Inside the BSS, all the BTSs and TCs are connected to the BSC(s). The BSCmaintains the BTSs. In other words, the BSC is capable of separating (barring) a BTSfrom the network and collecting alarm information. Transcoders are also maintainedby the BSC, i.e. the BSC collects alarms related to the Transcoders.SynchronizationThe BSS uses hierarchical synchronization which means that the MSC synchronizesthe BSC and the BSC further synchronizes the BTSs associated with that particularBSC. Inside the BSS, synchronization is controlled by the BSC. Synchronization is acritical issue in the GSM network due to the nature of the information transferred. Ifthe synchronization chain is not working correctly, calls may be cut or the call qualitymay 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 and NSS
The BSS is located between two interfaces, the Air and the A interface. From the callestablishment point of view, the MS must have a connection through these twointerfaces before a call can be established. Generally speaking, this connection may beeither a signallingtype of connection or a traffic (speech, data) type of connection.
Network Switching Subsystem (NSS)
The elements of Network Switching Subsystem that have been described so far are:
•MSC (Mobile Services Switching Centre)
•VLR (Visitor Location Register)
•HLR (Home Location Register)
The MSC is responsible for controlling calls in the mobile network. It identifies the
origin and destination of a call (either a mobile station or a fixed telephone in both
cases), as well as the type of a call. An MSC acting as a bridge between a mobile
network and a fixed network is called a Gateway MSC. An MSC is normallyintegrated with a VLR, which maintains information related to the subscribers who arecurrently in the service area of the MSC. The VLR carries out location registrationsand updates. The MSC associated with it initiates thepaging process.
A VLR database is always temporary (in the sense that the data is held as long as thesubscriber is within its service area), whereas the HLR maintains a permanent registerof the subscribers. In addition to the fixed data, the HLR also maintains a temporarydatabase which contains the current location of its customers. This data is required forrouting calls.
In addition, there are two more elements in the NSS: the Authentication Centre (AC)and the Equipment Identity Register (EIR). They are usually implemented as part ofHLR and they deal with the security functions.
Network Management Subsystem
The Network Management Subsystem (NMS) is the third subsystem of the GSM
network in addition to the Network Switching Subsystem (NSS) and Base Station
Subsystem (BSS). The purpose of the NMS is to monitor various functions andelements of the network. These tasks are carried out by the NMS/2000 which consistsof a number of Work Stations, Servers and a Router which connects to a DataCommunications Network (DCN).
The operator workstations are connected to the database and communication serversvia a Local Area Network (LAN). The database server stores the managementinformation about the network. The communications server takes care of the datacommunications between the NMS and the equipment in the GSM network known as Network Elements These communications are carried over a Data CommunicationsNetwork (DCN) which connects to the NMS via a router. The DCN is normallyimplemented using an X.25 Packet Switching Network.
The functions of the NMS can be divided into three categories:
These functions cover the whole of the GSM network elements from the level of individual BTSs, up to MSCs and HLRs.
The purpose of Fault Management is to ensure the smooth operation of the network
and rapid correction of any kind of problems that are detected. Fault management
provides the network operator with information about the current status of alarm
events and maintains a history database of alarms. The alarms are stored in the NMSdatabase and this database can be searched according to criteria specified by thenetwork operator.
The purpose of Configuration Management is to maintain up to date information aboutthe operation and configuration status of network elements. Specific configurationfunctions include the management of the radio network, software and hardwaremanagement of the network elements, time synchronization and security operations.
In performance management, the NMS collects measurement data from individual
network elements and stores it in a database. On the basis of these data, the networkoperator is able to compare the actual performance of the network with the plannedperformance and detect both good and bad performance areas within the network.
Various Interfaces in BTS
The Radio Interface( Um)
Radio interface (between MS and base transceiver stations [BTS]) is the mostimportant in any mobile radio system, in that it addresses the demandingcharacteristics of the radio environment. The physical layer interfaces to the data linklayer and radio resource management sub-layer in the MS and BS and to other
functional units in the MS and network subsystem (which includes the BSS and MSC)for supporting traffic channels. The physical interface comprises a set of physicalchannels accessible through FDMA and TDMA.
Each physical channel supports a number of logical channels used for user traffic andsignaling. The physical layer supports the functions required for the transmission ofbit streams on the air interface. Layer 1 also provides access capabilities to upperlayers. The physical layer is described in the GSM Recommendation 05 series (part ofthe ETSI documentation for GSM). At the physical level, most signaling messagescarried on the radio path are in 23-octet blocks. The data link layer functions aremultiplexing, error detection and correction, flow control, and segmentation to allowfor long messages on the upper layers.The radio resource layer manages the dialog between the MS and BSS concerning themanagement of the radio connection, including connection establishment, control,release, and changes (e.g., during handover).
Abis Interface (BTS to BSC)
The interconnection between the BTS and the BSC is through a standard interface,Abis (most Abis interfaces are vendor specific). The primary functions carried over
this interface are traffic channel transmission, terrestrial channel management, and
radio channel management. This interface supports two types of communications
links: Traffic channels at 64 kbps carrying speech or user data for a full- or half-rateradio traffic channel and Signaling channels at 16 kbps carrying information forBSC-BTS and BSC-MSC signaling. The BSC handles the LAPD channel signalingfor every BTS carrier. The first three layers are based on the following OSI/ITU-T
Physical layer: ITU-T Recommendation G.703 and GSM Recommendation 0-8.54
Data link layer: GSM Recommendation 08.56 (LAPD)
Network layer: GSM Recommendation 08.58
There are two types of messages handled by the traffic management procedure part ofthe signaling interface— transparent and nontransparent. Transparent messages arebetween the MS and BSC-MSC and do not require analysis by the BTS.Nontransparent messages do require BTS analysis.The Abis interface lies within the base station subsystem (BSS) and represents thedividing line between the BSC function and the BTS. The BSC and BTS can beconnected using leased lines, radio links or metropolitan area networks (MANs).
Basically, two channel types exist between the BSC and BTS:
Traffic channels (TCH): Can be configured in 8, 16 and 64 kbit/sformats and transport user data
Signaling channels: Can be configured in 16, 32, 56 and 64 kbit/sformats and are used for signaling purposes between the BTS and BSC.Each transceiver (TRX) in a BSC generally requires a signalingchannel on the Abis interface.