Base Station Subsystem
Encyclopedia
The base station subsystem (BSS) is the section of a traditional cellular telephone network
which is responsible for handling traffic and signaling between a mobile phone
and the network switching subsystem
. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging, transmission
and reception
over the air interface
and many other tasks related to the radio network.
The base transceiver station
, or BTS, contains the equipment for transmitting and receiving radio signals (transceivers), antennas
, and equipment for encrypting
and decrypting communications with the base station controller (BSC). Typically a BTS for anything other than a picocell
will have several transceivers (TRXs) which allow it to serve several different frequencies
and different sectors of the cell (in the case of sectorised base stations).
A BTS is controlled by a parent BSC via the "base station control function" (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an operations and maintenance (O&M) connection to the network management system (NMS), and manages operational states of each TRX, as well as software handling and alarm collection.
The functions of a BTS vary depending on the cellular technology used and the cellular telephone provider. There are vendors in which the BTS is a plain transceiver which receives information from the MS (mobile station) through the Um
(air interface) and then converts it to a TDM (PCM) based interface, the Abis interface, and sends it towards the BSC. There are vendors which build their BTSs so the information is preprocessed, target cell lists are generated and even intracell handover (HO) can be fully handled. The advantage in this case is less load on the expensive Abis interface.
The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE
-enabled networks it is GMSK and 8-PSK
.
Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found in micro and pico cells only.
Frequency hopping is often used to increase overall BTS performance; this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, and the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets.
A TRX transmits and receives according to the GSM standards, which specify eight TDMA
timeslots per radio frequency. A TRX may lose some of this capacity as some information is required to be broadcast
to handsets in the area that the BTS serves. This information allows the handsets to identify the network and gain access to it. This signalling makes use of a channel known as the Broadcast Control Channel
(BCCH).
s have a beamwidth of 65 to 85 degrees. This increases the traffic capacity of the base station (each frequency can carry eight voice channels) whilst not greatly increasing the interference
caused to neighboring cells (in any given direction, only a small number of frequencies are being broadcast). Typically two antennas are used per sector, at spacing of ten or more wavelength
s apart. This allows the operator to overcome the effects of fading
due to physical phenomena such as multipath reception. Some amplification
of the received signal as it leaves the antenna is often used to preserve the balance between uplink and downlink signal.
The base station controller (BSC) provides, classically, the intelligence behind the BTSs. Typically a BSC has tens or even hundreds of BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, and controls handovers from BTS to BTS (except in the case of an inter-BSC handover in which case control is in part the responsibility of the anchor MSC). A key function of the BSC is to act as a concentrator
where many different low capacity connections to BTSs (with relatively low utilisation) become reduced to a smaller number of connections towards the mobile switching center (MSC) (with a high level of utilisation). Overall, this means that networks are often structured to have many BSCs distributed into regions near their BTSs which are then connected to large centralised MSC sites.
The BSC is undoubtedly the most robust element in the BSS as it is not only a BTS controller but, for some vendors, a full switching center, as well as an SS7
node with connections to the MSC and serving GPRS support node (SGSN) (when using GPRS
). It also provides all the required data to the operation support subsystem (OSS) as well as to the performance measuring centers.
A BSC is often based on a distributed computing architecture, with redundancy applied to critical functional units to ensure availability in the event of fault conditions. Redundancy often extends beyond the BSC equipment itself and is commonly used in the power supplies and in the transmission equipment providing the A-ter interface to PCU.
The databases for all the sites, including information such as carrier frequencies
, frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are stored in the BSC. This data is obtained directly from radio planning engineering which involves modelling of the signal propagation as well as traffic projections.
. Specifically, GSM uses a regular pulse excited-long term prediction (RPE-LTP) coder for voice data between the mobile device and the BSS, but pulse code modulation (A-law or μ-law standardized in ITU G.711
) upstream of the BSS. RPE-LPC coding results in a data rate for voice of 13 kbit/s where standard PCM coding results in 64 kbit/s. Because of this change in data rate for the same voice call, the transcoder also has a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks.
Although transcoding (compressing/decompressing) functionality is defined as a base station function by the relevant standards, there are several vendors which have implemented the solution outside of the BSC. Some vendors have implemented it in a stand-alone rack using a proprietary interface. In Siemens
' and Nokia
's architecture, the transcoder is an identifiable separate sub-system which will normally be co-located with the MSC. In some of Ericsson
's systems it is integrated to the MSC rather than the BSC. The reason for these designs is that if the compression of voice channels is done at the site of the MSC, the number of fixed transmission links between the BSS and MSC can be reduced, decreasing network infrastructure costs.
This subsystem is also referred to as the transcoder and rate adaptation unit (TRAU). Some networks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its rate adaptation unit function to give compatibility between the BSS and MSC data rates.
The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site. In most of the cases, the PCU is a separate node communicating extensively with the BSC on the radio side and the SGSN on the Gb side.
: The air interface between the mobile station
(MS) and the BTS. This interface uses LAPDm protocol for signaling, to conduct call control, measurement reporting, handover, power control
, authentication
, authorization
, location update and so on. Traffic and signaling are sent in bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.
Abis: The interface between the BTS and BSC. Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM subchannels for traffic (TCH), LAPD protocol
for BTS supervision and telecom signaling, and carries synchronization from the BSC to the BTS and MS.
A: The interface between the BSC and MSC. It is used for carrying traffic channels and the BSSAP user part of the SS7
stack. Although there are usually transcoding units between BSC and MSC, the signaling communication takes place between these two ending points and the transcoder unit doesn't touch the SS7 information, only the voice or CS data are transcoded or rate adapted.
Ater: The interface between the BSC and transcoder. It is a proprietary interface whose name depends on the vendor (for example Ater by Nokia), it carries the A interface information from the BSC leaving it untouched.
Gb: Connects the BSS to the SGSN in the GPRS core network
.
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
which is responsible for handling traffic and signaling between a mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
and the network switching subsystem
Network Switching Subsystem
Network switching subsystem is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations...
. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging, transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
and reception
Reception
Reception is a noun form of receiving, or to receive something, such as information, art, experience, or people. It is often used in the following contexts:...
over the air interface
Air interface
In mobile or wireless communication, the air interface is the radio-based communication link between the mobile station and the active base station...
and many other tasks related to the radio network.
Base transceiver station
Base Transceiver Station
A base transceiver station or cell site is a piece of equipment that facilitates wireless communication between user equipment and a network. UEs are devices like mobile phones , WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc...
, or BTS, contains the equipment for transmitting and receiving radio signals (transceivers), antennas
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
, and equipment for encrypting
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
and decrypting communications with the base station controller (BSC). Typically a BTS for anything other than a picocell
Picocell
A picocell is a small cellular basestation typically covering a small area, such as in-building , or more recently in-aircraft...
will have several transceivers (TRXs) which allow it to serve several different frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and different sectors of the cell (in the case of sectorised base stations).
A BTS is controlled by a parent BSC via the "base station control function" (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an operations and maintenance (O&M) connection to the network management system (NMS), and manages operational states of each TRX, as well as software handling and alarm collection.
The functions of a BTS vary depending on the cellular technology used and the cellular telephone provider. There are vendors in which the BTS is a plain transceiver which receives information from the MS (mobile station) through the Um
Um Interface
The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station and the Base transceiver station . It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of...
(air interface) and then converts it to a TDM (PCM) based interface, the Abis interface, and sends it towards the BSC. There are vendors which build their BTSs so the information is preprocessed, target cell lists are generated and even intracell handover (HO) can be fully handled. The advantage in this case is less load on the expensive Abis interface.
The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE
Enhanced Data Rates for GSM Evolution
Enhanced Data rates for GSM Evolution is a digital mobile phone technology that allows improved data transmission rates as a backward-compatible extension of GSM...
-enabled networks it is GMSK and 8-PSK
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
.
Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found in micro and pico cells only.
Frequency hopping is often used to increase overall BTS performance; this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, and the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets.
A TRX transmits and receives according to the GSM standards, which specify eight TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
timeslots per radio frequency. A TRX may lose some of this capacity as some information is required to be broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
to handsets in the area that the BTS serves. This information allows the handsets to identify the network and gain access to it. This signalling makes use of a channel known as the Broadcast Control Channel
Broadcast Control Channel
The broadcast control channel is a point to multipoint, unidirectional channel used in the Um Interface of the GSM cellular standard....
(BCCH).
Sectorisation
By using directional antennae on a base station, each pointing in different directions, it is possible to sectorise the base station so that several different cells are served from the same location. Typically these directional antennaDirectional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
s have a beamwidth of 65 to 85 degrees. This increases the traffic capacity of the base station (each frequency can carry eight voice channels) whilst not greatly increasing the interference
Co-channel interference
Co-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....
caused to neighboring cells (in any given direction, only a small number of frequencies are being broadcast). Typically two antennas are used per sector, at spacing of ten or more wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s apart. This allows the operator to overcome the effects of fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
due to physical phenomena such as multipath reception. Some amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
of the received signal as it leaves the antenna is often used to preserve the balance between uplink and downlink signal.
Base station controller
Concentrator
In telecommunication, the term concentrator has the following meanings:# In data transmission, a functional unit that permits a common path to handle more data sources than there are channels currently available within the path...
where many different low capacity connections to BTSs (with relatively low utilisation) become reduced to a smaller number of connections towards the mobile switching center (MSC) (with a high level of utilisation). Overall, this means that networks are often structured to have many BSCs distributed into regions near their BTSs which are then connected to large centralised MSC sites.
The BSC is undoubtedly the most robust element in the BSS as it is not only a BTS controller but, for some vendors, a full switching center, as well as an SS7
Signalling System 7
Signalling System No. 7 is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls...
node with connections to the MSC and serving GPRS support node (SGSN) (when using GPRS
General Packet Radio Service
General packet radio service is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications . GPRS was originally standardized by European Telecommunications Standards Institute in response to the earlier CDPD and i-mode...
). It also provides all the required data to the operation support subsystem (OSS) as well as to the performance measuring centers.
A BSC is often based on a distributed computing architecture, with redundancy applied to critical functional units to ensure availability in the event of fault conditions. Redundancy often extends beyond the BSC equipment itself and is commonly used in the power supplies and in the transmission equipment providing the A-ter interface to PCU.
The databases for all the sites, including information such as carrier frequencies
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
, frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are stored in the BSC. This data is obtained directly from radio planning engineering which involves modelling of the signal propagation as well as traffic projections.
Transcoder
The transcoder is responsible for transcoding the voice channel coding between the coding used in the mobile network, and the coding used by the world's terrestrial circuit-switched network, the Public Switched Telephone NetworkPublic switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
. Specifically, GSM uses a regular pulse excited-long term prediction (RPE-LTP) coder for voice data between the mobile device and the BSS, but pulse code modulation (A-law or μ-law standardized in ITU G.711
G.711
G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
) upstream of the BSS. RPE-LPC coding results in a data rate for voice of 13 kbit/s where standard PCM coding results in 64 kbit/s. Because of this change in data rate for the same voice call, the transcoder also has a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks.
Although transcoding (compressing/decompressing) functionality is defined as a base station function by the relevant standards, there are several vendors which have implemented the solution outside of the BSC. Some vendors have implemented it in a stand-alone rack using a proprietary interface. In Siemens
Siemens
Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...
' and Nokia
Nokia
Nokia Corporation is a Finnish multinational communications corporation that is headquartered in Keilaniemi, Espoo, a city neighbouring Finland's capital Helsinki...
's architecture, the transcoder is an identifiable separate sub-system which will normally be co-located with the MSC. In some of Ericsson
Ericsson
Ericsson , one of Sweden's largest companies, is a provider of telecommunication and data communication systems, and related services, covering a range of technologies, including especially mobile networks...
's systems it is integrated to the MSC rather than the BSC. The reason for these designs is that if the compression of voice channels is done at the site of the MSC, the number of fixed transmission links between the BSS and MSC can be reduced, decreasing network infrastructure costs.
This subsystem is also referred to as the transcoder and rate adaptation unit (TRAU). Some networks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its rate adaptation unit function to give compatibility between the BSS and MSC data rates.
Packet control unit
The packet control unit (PCU) is a late addition to the GSM standard. It performs some of the processing tasks of the BSC, but for packet data. The allocation of channels between voice and data is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes full control over that channel.The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site. In most of the cases, the PCU is a separate node communicating extensively with the BSC on the radio side and the SGSN on the Gb side.
BSS interfaces
UmUm Interface
The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station and the Base transceiver station . It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of...
: The air interface between the mobile station
Mobile Station
The mobile station comprises all user equipment and software needed for communication with a mobile network.The mobile station refers to global system connected to the mobile network, i.e. mobile phone or mobile computer connected using a mobile broadband adapter. This is the terminology of 2G...
(MS) and the BTS. This interface uses LAPDm protocol for signaling, to conduct call control, measurement reporting, handover, power control
Power control
Power control, broadly speaking, is the intelligent selection of transmit power in a communication system to achieve good performance within the system. The notion of "good performance" can depend on context and may include optimizing metrics such as link data rate, network capacity, geographic...
, authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
, authorization
Authorization
Authorization is the function of specifying access rights to resources, which is related to information security and computer security in general and to access control in particular. More formally, "to authorize" is to define access policy...
, location update and so on. Traffic and signaling are sent in bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.
Abis: The interface between the BTS and BSC. Generally carried by a DS-1, ES-1, or E1 TDM circuit. Uses TDM subchannels for traffic (TCH), LAPD protocol
Link Access Procedures, D channel
Link Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...
for BTS supervision and telecom signaling, and carries synchronization from the BSC to the BTS and MS.
A: The interface between the BSC and MSC. It is used for carrying traffic channels and the BSSAP user part of the SS7
Signalling System 7
Signalling System No. 7 is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls...
stack. Although there are usually transcoding units between BSC and MSC, the signaling communication takes place between these two ending points and the transcoder unit doesn't touch the SS7 information, only the voice or CS data are transcoded or rate adapted.
Ater: The interface between the BSC and transcoder. It is a proprietary interface whose name depends on the vendor (for example Ater by Nokia), it carries the A interface information from the BSC leaving it untouched.
Gb: Connects the BSS to the SGSN in the GPRS core network
GPRS Core Network
The GPRS core network is the central part of the General Packet Radio Service which allows 2G, 3G and WCDMA mobile networks to transmit IP packets to external networks such as the Internet...
.
See also
- Network switching subsystemNetwork Switching SubsystemNetwork switching subsystem is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations...
- GPRS core networkGPRS Core NetworkThe GPRS core network is the central part of the General Packet Radio Service which allows 2G, 3G and WCDMA mobile networks to transmit IP packets to external networks such as the Internet...
- Cell siteCell siteA cell site is a term used to describe a site where antennas and electronic communications equipment are placed, usually on a radio mast, tower or other high place, to create a cell in a cellular network...
- U.S. Federal Communications CommissionFederal Communications CommissionThe Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) - Base stationBase stationThe term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
- Cellular repeaterCellular repeaterA cellular repeater, cell phone repeater, or wireless cellular signal booster, a type of bi-directional amplifier as commonly named in the wireless telecommunications industry, is a device used for boosting the cell phone reception to the local area by the usage of a reception antenna, a signal...
- Telecom infrastructure sharingTelecom infrastructure sharingDue to economy of scale property of telecommunication industry, sharing of telecom infrastructure among telecom service providers is becoming the requirement and process of business in the telecom industry where competitors are becoming partners in order to lower their increasing investments...
- OpenBTSOpenBTSOpenBTS is a software-based GSM access point, allowing standard GSM-compatible mobile phones to make telephone calls without using existing telecommunication providers' networks...
External links
- OpenBSC - open source Base Station Controller implementation