M900/M1800 Base Station Controller

Technical Manual

M900/M1800 Base Station ControllerChapter 2

REF _Ref27738240 \* MERGEFORMAT Hardware Description

Chapter 2 Hardware Description

2.1 Overall Architecture of BSC

2.1.1 Overview of BSC Architecture

The hardware system of the M900/M1800 base station controller adopts a modular structure, and can be divided into four modular levels, as shown in Figure 2-1.

The lowest level is composed of various circuit boards. Various circuit boards are combined together to form frame units. Each frame unit accomplishes the specific functions.

Frame units with various functions are combined together to form a module, and respective modules can implement specific functions independently.

Different modules are combined together to form the base station controller.

Figure 2-1 900/M1800 BSC modular architecture

The modular design makes the installation and expansion of BSC convenient and flexible i.e., new functions and technologies can be introduced by just addition/removal of functional frames.

Application of very large scale integrated circuit (VLSI) in circuit designing gives a compact and highly reliable system with low power consumption.Hardware design is simplified due to the application of microprocessors and programmable logic chips. To enhance functions, it is only required to add corresponding hardware and software.

II. Types of BSC

BSC can be divided into multi-module BSC and single-module BSC. The functional composition and the modular composition of the BSC are shown respectively in Table 2-1.

Table 2-1 BSC types

BSC typesFunctional descriptionModules

Multi-module BSCWhen BSC has more than 128TRXs, it is called multi-module BSC, and AM/CM is required. The quantity of BMs depends on a specific capacity. 8 BMs can be configured at the most.AM/CM

BM

BAM

TCSM

CDB

Single-module BSCWithout SMUXWhen BSC has only 128TRXs or less, only one BM needs to be configured. The AM/CM is not required.

Only one basic cabinet and a extension cabinet are required for BSC without SMUX.BM

BAM

TCSM

CDB

With SMUXWhen BSC has only 128TRXs or less, only one BM needs to be configured. The AM/CM is not required.

Only one basic cabinet is required for BSC with SMUX.BM

BAM

TCSM

CDB

III. Modules

The module functions and cabinet composition are shown in Table 2-2.

Table 2-2 BSC modules

ModulesFunction descriptionFunctional frames

AM/CMDesigned only for multi-module BSC, the AM/CM, a center for BSC speech channel switching and information exchange, accomplishes inter-modular communication between BMs. Communication control frame

Transmission interface frame

Clock frame

BMBM performs mainly such functions as call handling, signaling processing, radio resources management, radio link management and circuit maintenance. Main control frame

BIE frame

Clock frame (when there is no AM/CM in BSC)

TCSMTCSM implements the transcoding / rate adaptation and sub-multiplexing functions. TCSM frame

Cell Broadcast Database (CDB)Linked with the short message center, the CDB module is a traffic processing center, supporting the broadcast short message service. CDB frame

Back Administration Module (BAM)BAM is a bridge between BSC and OMC. The latter conducts the operation & maintenance of BSC via BAM. BAM frame

IV. Functional Frames

The functional frames, their functions and circuit boards are listed in Table 2-3.

Table 2-3 BSC functional frames

Functional framesFunction descriptionCircuit boards

Clock FrameThe clock frame phase-locks upper-level MSC or BITS clock reference resources and provides the AM/CM and BM with stable clock sources.Clock Board (GCKS)

Power Control Board (PWC)

Main Control FrameThe main control frame carries out management and control of the BM, communication between AM/CM and signaling processing.Main Processing Unit (GMPU)

GMPU Switchover Board (GEMA)

Master Node Board (GNOD)

Memory Board (GMEM)

Module Communication 2 Link (GMC2)

Optic Fiber Interface Board (GOPT)

Alarm Board (GALM)

SS7 Signaling Processing Board (LPN7)

Link Access Protocol Processing Board (GLAP)

Switching Network Board (GNET)


Communication Control FrameThe communication control frame is the control center of AM/CM. The communication control unit mainly manages and controls the system.Inter-module Communication Board (GMCCS)

Signaling T-network Board (GSNT)

Central T-network Board (GCTN)

Alarm Board (GALM)


Transmission Interface FrameThe transmission interface frame implements multiplexing/demultiplexing of inter-modular speech channels and signaling links, optic-electric conversion and E1 interface driving so that the inter-modular communication messages can be transmitted on the optical fiber.Fiber Communication Board (GFBI)

Enhanced E3 Sub-multiplexer (E3M)


TCSM FrameThe TCSM frame fulfills the functions of transcoding / rate adaptation and sub-multiplexing.Transcoding Board (FTC)

Sub-Multiplexer (MSM)

Power Control Board (PWS)

BIE FrameDesigned for the BM, the BIE frame presents an Abis interface in between BSC and BTS.BS Interface Board (BIE)


Sub-Multiplexer Interface Board (SMI)

CDB FrameThe CDB frame, a traffic processing center, supports the broadcast short message service.The CDB is physically a Windows NT computer, occupying half of the frame.

Back Administrative Module Frame (BAM Frame )The BAM frame is a bridge between BSC and OMC. The latter performs the operation & maintenance of the BSC via OMC.The BAM is placed in the BAM frame as standalone equipment.

V. Circuit Boards

The circuit boards used in the BSC are shown in Table 2-4. Logic board is created by loading some software on the physical board, so varying logic boards may share the same physical boards.

Table 2-4 BSC circuit boards

Logic boardPhysical boardFunction description

BIEBIEA transmission interface board between BSC and BTS, Provides E1 interface and multiplexing/demultiplexing functions.

E3ME3MIntegrating sub-multiplexer functions, the E3M offers externally 4 E1 interfaces to connect the PCU and TCSM frames. It carries out receiving, sending, switching, HDLC link control and multiplexing/demultiplexing of 5-E1 signals.

GFBIFBIGFBI provides optical paths for inter-modular communications in collaboration with GOPT in the BM.

GALMGALMGALM provides a hardware interface for room environment alarms, collecting temperatures, humidity and fire alarms, etc.

GCTNCTNThe GCTN is a speech channel switching center of AM/CM. In the multi-module BSC, GCTN is mainly designed for network switching and equipment control.

GCKSGCKSThe GCKS board, a high-level reference clock source generation board, is designed mainly to provide the equipment with a superb clock source.

GSNTSNTThe GSNT switches the inter-modular signals and the internal messages of AM/CM, and delivers loading paths for modules.

GMCCSGMCCGMCCS provides a signaling communication link between BM and AM/CM, transfers control messages from BM to BM, from BM to GMCCM and from BM to GCTN, and presents a serial port for maintenance.

GMCCMGMCCGMCCM controls the entire AM/CM and provides an interface with BAM.

GMEMGMEMLocated in the main control unit of the BM, the GMEM is a data storage board, which serves mainly for network communications.

GNETGNETThe GNET implements the function of intra-module speech channel switching.

GMPUGMPUGMPU, a central processing unit in the module, conducts active/standby switchover via GEMA and operates in hot backup mode.

GNODGNODThe GNOD is responsible for the communication of GMPU with other frames.

GEMAGEMAThe GEMA is an Emergency Message Automatic Transmission System. It communicates with two GMPUs and controls their switchover.

LPN7LAPLPN7 handles SS7 signaling on the A-interface.

GLAPGLAPThe GLAP is a LAPD protocol processing board. The LAPD signaling at the Abis interface and Pb interface is processed by the GLAP.

GMC2GMC2The GMC2 is an inter-module communication processing board of BM.

GOPTGOPTGOPT is the physical bearer for the communication between BM and AM/CM.

DRCDRCThe DRC presents E1 interfaces in collaboration with E3M, and coupling and over-voltage protection modules, etc. The DRC is plugged on the backplane.

FBCFBCFBC collaborates with GFBI to accomplish electric-optic conversion and optic-electric conversion of 40.96Mbit/s signals.

FTCFTCThe FTC is mainly designed for coding/decoding of speech signals, data format conversion and rate adaptation as well as transparent transmission of SS7 signaling.

MSM (TCSM frame)MSMThe MSM performs the multiplexing/demultiplexing function in multi-module BSC.

SMI (BIE frame)SMIThe SMI performs the multiplexing/demultiplexing function in single-module BSCs.

PWCPWCA power board, whose power is 100W, supplies power to each board in the frame.

PWSPWSA power board, whose power is 300W, supplies power to each board in the TCSM frame and bears an emergency serial port.

2.1.2 Functional Blocks of BSC

According to the functions, the BSC can be divided into control system, switching network, TCSM unit, Base Station Interface Equipment (BIE), clock synchronization system, alarm system, Back Administration Module (BAM) and Cell Broadcast Database (CDB).

A functional structure of the BSC system is shown in Figure 2-2.

Figure 2-2 Functional structure of BSC system

II. Control System

The M900/M1800 BSC works on distributed processing and centralized control principles.A single-module BSC has only one BM and no AM/CM, GOPT or inter-module communications function. In terms of structure and control system, the single-module BSC is a subset of a multi-module BSC so we will focus on the multi-module BSC, which is illustrated in Figure 2-3.

Figure 2-3 Functional blocks of control system

2) System structure

The control system is mainly composed of processor circuit, inter-module communication circuit, intra-module communication circuit, signaling switching circuit and signaling processing circuit, etc.

Main processing boards refer to the GMCCM of AM/CM and GMPU of BM.

Inter-module communication circuit includes the GMCCS in AM/CM and the GMC2 in BM.

Intra-module communication circuit: The communication within the AM/CM module is accomplished by GMCCS, and GNOD mainly accomplishes the communication within BM module.Signaling switching circuit is mainly responsible for signaling switching control, here signaling refers to various control and state information. In the AM/CM module, GSNT accomplishes the signaling switching function, and in the BM module, GNET accomplishes that function.Signaling processing circuit mainly refers to LPN7 (LAP) and GLAP.

3) Communication routes

The data channels for the communication between modules of the multi-module BSC are composed of the GMCCM and GMCCS in the AM/CM, and GMPU & GMC2 in the BM, as shown in Figure 2-4.

The communication messages among modules mainly include management data, call handling messages, maintenance & testing messages, loaded programs & data, traffic statistics, etc.

Figure 2-4 Communication between modules

As illustrated in Figure 2-4, the GMC2 of a BM is responsible for the two-channel HDLC inter-module communication, and the GMCCS of AM/CM is responsible for multi-channel HDLC inter-module communication.

All possible inter-module communication routes are shown in Figure 2-5.

Figure 2-5 Inter-module communication routes

Communication between GMPU and GMC2 in the BM and that between GMCCM and GMCCS in the AM/CM module are conducted through dual-port buffer (mail box), while the communication between GMC2 and GMCCS is through the HDLC link.GMC2 and GMCCS communicate through optical fiber. GOPT and GFBI are their respective optical fiber interfaces.Each BM houses two GMC2 boards which communicate with two GMCCS boards respectively, thus improving reliability. The two GMCCS boards communicate with corresponding GMC2 boards of BM in load sharing mode. On the failure of one link, the second link will take over the full load automatically, which ensures the system reliability.

The physical layer of inter-module communication is achieved by optical fibers and HSCX (High level Serial Communication Controller with extended feature and functionality). The data link layer is fully compliant with X.25 LAPD protocol.The transfer layer is realized by GMCCS, and the transmission layer and application layer are accomplished by GMCCM and GMPU software.

III. Switching Network

The GCTN of AM/CM and the GNET of BM provide a large-capacity T-T-T switching network, and jointly accomplish the switching of speech information, as shown in Figure 2-6.A GCTN provides 16k(16k T switching network and a GNET of BM is a single 4k(4k T switching network.

Figure 2-6 Switching network structure of multi-module BSC

The switching network of the single-module BSC is much simpler, as shown in Figure 2-7 (including TCSM). It only has 4k(4k T switching network boards (GNET) in BM, which independently implements the switching of speech information, etc.

Figure 2-7 Switching network structure of the single-module BSC

IV. TCSM Unit

Generally, TRAU and SMUX are integrated in one unit called TCSM, its position is shown in Figure 2-8. For single-module BSC where sub-multiplexing is not needed, TCSM is often used although it has no SMUX.

The TCSM unit accomplishes the function of transcoding/rate adaptation and sub-multiplexing.In PSTN, Pulse Code Modulation (PCM) is used for normal speech, with a rate of 64kbit/s. In GSM system, RPE-LTP or CELP coding with much lower rate (16kbit/s) is used due to the limitation of radio resources. If a subscriber of fixed network wants to access a GSM subscriber, then there is a need of code conversion and this conversion is done by TRAU.Since the rate of each channel in existing terrestrial lines is 64kbit/s, it is a waste if one channel is used to carry one 16kbit/s GSM channel. To save terrestrial line resources, sub-multiplexer (SMUX) is used between MSC and BSC to multiplex 4(16kbit/s channels to transmit four speech channels through one terrestrial line channel.

Figure 2-8 Position of TCSM in the system

When the multiplexing mode is adopted between BSC and MSC, the TCSM unit is put on the MSC side physically to save the transmission lines between BSC and MSC by multiplexing the lines between E3M (or SMI) and TCSM.When the multiplexing mode is not introduced between BSC and MSC (in the case of single-module BSC), the TCSM unit is put on the BSC side.BSC delivers a standard A-interface to MSC via the TCSM unit. The A-interface, a standard E1 interface physically, can interconnect with MSCs of other manufacturers.V. Base Station Interface Equipment (BIE)

The interface between BTS and BSC is called BIE. It provides a standard E1 interface, and mainly accomplishes functions like BTS access, channel multiplexing on Abis interface, etc. Each E1 interface can supports up to 15 TRXs (15:1).

The position of the base station interface equipment in system is shown in Figure 2-9.

Figure 2-9 Position of BIE in system

VI. Back Administration Module (BAM)

BAM serves as a communication bridge between BSC and OMC. Via BAM, OMC can perform operation and maintenance over BSC.

BAM communicates with the control system through HDLC link, and forms Local Area Network (LAN) or Wide Area Network (WAN) together with the OMC system. When BSC and OMC are in the same premises, BAM and OMC can be connected through LAN, and in case of long distance, these can be connected through WAN with the help of network adapter, router and transmission equipment.

The position of BAM in the system is shown in Figure 2-10.

Figure 2-10 Position of the BAM in the system (WAN configuration)

VII. Clock Synchronization System

The BSC clock synchronization system phase-locks the upper-level MSC or BITS clock as reference source, and provides a stable clock source for the AM/CM and BM.

1) System features

The BSC clock synchronization system has the following features:

The clock can be synchronized by Phase-lock Loop (PLL) and by software, so that the clock of the system can follow the MSC or BITS clock reliably.

BSC clock uses international stratum 3 clock which provides a reliable clock source for the system.

Clock system is equipped with perfect display, alarm, maintenance and operation system, and internal parameters of the clock can be set through OMC directly.

2) System structure

Both small and multi-module BSCs extract, "purify", and synthesize the clock synchronization signals from the MSC/BITS reference sources. But they have quite different clock synchronization system structures, as shown in Figure 2-11 and Figure 2-12.

Figure 2-11 Clock synchronization system structure of the multi-module BSC

In a multi-module BSC, the synthesized clock synchronization signals are sent to GCTN and GSNT, and then to other units/parts of the AM/CM. The BM's GOPT extracts clock signals from optical signals and generates required clock synchronization signals. These signals are sent to GNET, which will forward the signals to other parts of the BM.

Figure 2-12 Clock synchronization system structure of single-module BSC

In a single-module BSC, the synthesized clock synchronization signals are directly sent to GNET, which then sends these signals to other parts of the BM.

3) System control

The clock synchronization system is configured in the clock frame that contains two GCKS boards in hot backup mode.

In multi-module BSC, the OMC communicates with GMCC through BAM, and the GMCCM implements the maintenance and operation over 2 GCKS boards via two serial ports. In this way, the OMC can operate and maintain the clock synchronization system.

In single-module BSC, the OMC communicates with GMPU through BAM, the GMPU communicates with GALM through HDLC link, and GALM communicates with GCKS through serial port. In this way, the OMC can implement the operation and maintenance of the clock synchronization system.

The clock control methods for the clock synchronization systems in multi-module and single-module BSCs are shown in Figure 2-13 and Figure 2-14 respectively.

Figure 2-13 Clock synchronization control of multi-module BSC

Figure 2-14 Clock synchronization control of single-module BSC

VIII. Alarm System

The M900/M1800 BSC alarm system collects various alarm messages and forwards them to the GMPU for classification and processing, then these alarms are sent to the alarm box and OMC alarm console respectively.

The whole alarm system is composed of the alarm box, OMC alarm console, alarm communication board, etc.

There are two alarm boxes connected to the BSC, one is for the BSC and the other is centralized alarm box for the BTS, responsible for the centralized audible and visible alarms of all BTSs managed by that BSC.

The structures of the multi-module and single-module BSC alarm systems are shown in Figure 2-15 and Figure 2-16 respectively.

Figure 2-15 Alarm system structure of multi-module BSC

Figure 2-16 Alarm system structure of single-module BSC

In a multi-module BSC, the BM's GMPU and AM/CM's GMCCM collect alarm information of the system software/hardware, which is sent to the OMC alarm console and alarm box.

In a single-module BSC, the BM's GMPU collects alarm information of the system software/hardware, which is sent to the OMC alarm console and alarm box.

GALM provides the hardware interfaces for equipment room environmental alarms. It collects alarms including temperature, humidity, fire, and secondary power supply alarms. These alarm messages are also sent to the OMC alarm console and alarm box.

IX. CDB

Cell Broadcast Database (CDB) is a traffic processing center, responsible for providing the interface between the Short Message Center (SMC) and BSC, and supporting short message broadcast service. Its server communicates with the GMEM boards of the modules through the Ethernet interface.

In M900/M1800 BSC, CDB is a centralized database. Each BM communicates with CDB via Ethernet interface provided by a GMEM board, as shown in Figure 2-17.

Figure 2-17 CDB networking structure

2.2 Types of BSC

As BSC is the central part of BSS, it acts as a concentrator for the links between the Abis- and A- interfaces.2.2.1 Single-module BSC

One of the most powerful features of M900/M1800 BSC is its modular approach. If only 128 TRXs or 64 BTSs are required, then there is no need to install Administration Module / Communication Module (AM/CM) along with related equipment. Single Basic Module (BM) is enough, as illustrated in Figure 2-18.

Figure 2-18 Hardware structure of single-module BSC

A single-module BSC has only one BM and no AM/CM, GOPT or inter-module communications function. In terms of structure and control system, the single-module BSC is a subset of a multi-module BSC, which is our next topic for discussion.

A standard 2100(800(550 mm cabinet can hold six frames and is used to install BM and other related equipment. A BM cabinet has six frames, numbered 0-5 from bottom to top, including main control frame (frames 1 & 2), clock frame (frame 3) and BIE. BAM is installed in the frame 0 of the main BM cabinet.

If there is no SMUX configured in the single-module BSC then two cabinets, basic and extension cabinets are needed. And if it contains SMUX, only one basic cabinet is required. If CDB is configured, it can be put in the extension cabinet.

II. Single-module BSC without SMUX

When there is no multiplexing equipment between MSC and BSC, the basic cabinet holds one clock frame, one TCSM frame and one BIE frame in addition to the main control frame. If one TCSM frame is insufficient, it is necessary to install an extension cabinet where the additional TCSM frame is placed. If BSC is to implement the cell broadcast function, a CDB frame shall also be added to the extension cabinet. For the configuration, refer to Figure 2-19.

Only FTC board but not MSM board is plugged in the TCSM frame.

Figure 2-19 Configuration of single-module BSC cabinet (without SMUX)

III. Single-module BSC with SMUX

When there is multiplexing equipment between MSC and BSC, the basic cabinet takes one clock frame and two BIE frames in addition to the main control frame. If BSC is to implement the cell broadcast function, it is necessary to add an extension cabinet where the CDB server is placed, as shown in Figure 2-20.

The SMI is plugged in the BIE frame, connecting to the MSM in the TCSM frame. The two BIE frames serve to accommodate respectively the BS interface equipment and SMUX. The TCSM unit is configured on the MSC side, occupying a whole cabinet.

Figure 2-20 Configuration of single-module BSC cabinet (with SMUX)

2.2.2 Multi-module BSC

For multi-module BSC which supports more than 128 TRXs, AM/CM module is required. The hardware structure of multi-module BSC is shown in Figure 2-21.

Figure 2-21 Hardware structure of multi-module BSC

In multi-module BSC, 8 BMs can be configured at the most. Each BM can support 64 BTSs or 128 TRXs, i.e. M900/M1800 multi-module BSC can support up to 512 BTSs or 1024 TRXs at the most, which is the ultimate solution for large cellular networks.

A multi-module BSC has multiple BMs and one AM/CM. Eight BMs can be installed in four BM cabinets, and AM/CM is configured in AM/CM cabinet. Each cabinet has six frames, numbered 0-5 from bottom to top. AM/CM cabinet contains clock frame (frame 5), communication control frame (frame 4), transmission interface frame (frames 3&2), CDB (frame 1) and BAM in frame 0. Since clock frame, BAM and CDB are installed in AM/CM cabinet, the only equipment to be installed in BM cabinet is main control frame and BIE.

For alarm prompts, external alarm boxes, including BSC alarm box and BTS centralized alarm box, shall be installed.

Figure 2-22 Configuration of multi-module BSC cabinet

2.3 Modules of BSC

2.3.1 AM/CM

AM/CM module is the center of speech channel switching and message switching of multi-module BSC.

AM/CM module is mainly composed of communication control unit, central switching network, transmission interface unit, clock synchronization system and alarm system. The structural diagram is shown in Figure 2-23.

Figure 2-23 Functional blocks of AM/CM system

II. System composition

AM/CM module is mainly composed of communication control unit, central switching network, transmission interface unit, clock synchronization system, alarm system and back administration module.

The communication control unit manages and controls the whole system. It is mainly composed of GMCCM (GMCC0-1), GMCCS (GMCC2-11) and GSNT.

The central switching network mainly handles speech channel switching between BMs. The function of central switching network is accomplished by GCTN board.

Transmission interface unit mainly responsible for multiplexing/demultiplexing of inter-module speech channels and signaling links, optic-electric conversion and E1 interface driving, so that inter-module communication messages can be transmitted over optical fibers. Transmission interface unit is mainly composed of GFBI and E3M. GFBI provides the optical interface from AM to BM module, E3M provides E1 interface from BSC to TCSM unit.

Clock synchronization system provides standard stratum 3 clock for the whole BSC system. Functions of clock synchronization system are mainly accomplished by the GCKS in clock frame.

Alarm system collects alarms and drives the alarm box. The alarm system of AM/CM is mainly composed of the GALM and the alarm box.

III. Communication modes

Three kinds of communication modes are used in AM/CM: mailbox, serial port, and HDLC.

Mailbox mode is employed for the communication among GMCC boards via the data bus.

Each GMCC (including GMCCM and GMCCS) can provide 2 serial ports. GMCCM communicates with GCKS, and GMCCS communicates with GFBI through these 2 serial ports. GALM communicates with BSC alarm box through RS422 serial port.

GMCCM communicates with GCTN, GSNT, GALM boards and BAM through HDLC link.

GMCCS communicates with E3M, GCTN boards and GMC2 (BM) through HDLC link.

GMCCS communicates with GMC2 in the BM through HDLC link.

2.3.2 BM

BM is the basic unit of M900/M1800 BSC. It handles most of the functions of call handling, signaling processing, radio resources management, radio link management and circuit maintenance.

I. System structure

BM is mainly composed of main control unit, switching network, base station interface equipment and alarm system, as shown in Figure 2-24. When BSC does not have AM/CM, the clock synchronization unit is also installed in the BM.

Figure 2-24 Functional blocks of BM system (Multi-module BSC)

The main control unit mainly accomplishes the management and control over BM module, communication with AM/CM module and signaling processing. It is mainly composed of GMPU, GNOD, GMEM, GMC2, GOPT, GALM, LPN7 and GLAP.

The switching network accomplishes the switching of timeslots in the module, which is mainly handled by GNET board.

Base station interface equipment (BIE) can multiplex/de-multiplex the transmitted signals.

The alarm system is designed to collect alarms and drive the alarm box. The collected alarm messages can either be reported to OMC or sent directly to the external alarm box (used in single-module BSC). The BM alarm system consists mainly of the GALM boards.

II. Communication modes

There are three major communication modes within the BM, which are mailbox, serial ports and HDLC.

The GMPU communicates with other boards in the main control frame through the bus in mailbox mode.

The GMPU communicates with GNOD through mailbox, while each GNOD provides 4 serial ports for the communication with non-main control frame devices such as BIE board. GMC2 communicates with GMCCS in AM/CM module via HDLC link.

Note: This mode applies to the multi-module BSC only, a single-module BSC has no AM/CM.

2.3.3 TCSM Unit

TRAU and SMUX are usually integrated in one unit called TCSM, i.e. TCSM handles both rate adaptation and multiplexing.

In multi-module BSC, functions of TRAU are accomplished by FTC boards, functions of SMUX are accomplished by MSM and E3M together. The frame to insert FTC board and MSM is called TCSM frame. Four MSM boards and sixteen FTC boards can be inserted in 1 TCSM frame. In real application, the configuration of TRAU is necessary while SMUX is optional.

I. TRAU

Pulse Code Modulation (PCM) is used for normal speech in PSTN, at a rate of 64kbit/s whereas in GSM, RPE-LTP or CELP coding with much lower rate (16kbit/s) is used due to the limitation of radio channel resources. If a subscriber of PSTN network wants to access a GSM subscriber, then there is a need of code conversion. This conversion is completed by Transcoder & Rate Adapter Unit (TRAU).

The main functions of TRAU are, to perform coding/decoding on speech signal and rate adaptation to realize the communication between GSM subscribers and PSTN subscribers. In addition, TRAU can also accomplish the rate adaptation of digital signals and transparent transmission of SS7 signaling on A-interface.

The position of the TRAU in the GSM system is shown in Figure 2-25.

Figure 2-25 TRAU in the GSM system

In M900/M1800 BSC, the functions of TRAU are accomplished by FTC board.

2) Speech service

The most fundamental function of TRAU is to encode and decode voice. Regular Pulse Excitation Long Term Prediction (RPE-LTP) algorithm is used. TRAU frames the speech signals received from MSC in one frame per 20 ms. One frame of speech data includes 160 PCM sampling points, 1280 bits in total, the encoded output parameters are 260 bits altogether (EFR service adopts CELP algorithm, the encoded parameters are 244 bits altogether). After the addition of synchronization bits and command words, TRAU frame has 320 bits. The reverse process of coding is called decoding. After receiving TRAU frame from BSC, TRAU will restore it to speech data by decoding algorithm and send to the MSC.

TRAU adopts discontinuous transmission (DTX) technology to minimize the power consumption of BTS and MS, and to reduce the co-channel interference of radio interface.

Voice activity detection (VAD) is used together with SID (Silence Descriptor) technique in the discontinuous transmission (DTX) mode of GSM.

If TRAU detected that there is no speech information in the data received from MSC through VAD functional module, it will clear voice flag in the encoded TRAU frame. After BTS identifies this flag bit, downlink transmission will be disconnected till the flag resets.

In the same way, TRAU will also identify SID flag at the reception of uplink frame. When SID flag is reset, it indicates that MS is in the interval of emission.

To make the subscribers feel that GSM network is still in service, TRAU adopts the substitution technology to insert comfortable noise in uplink to avoid the impression of interrupted communication.

In MS-MS conversation, the encoding/decoding function of TRAU may be omitted, as it causes the degrading of voice quality.By canceling the encoding\decoding function (i.e. Tandem Free Operation, TFO), the voice quality can be improved.The TFO function is implemented by FTC board through inner signaling communication to reduce the times of encoding\decoding during MS-MS conversation.

3) Data service

GSM system provides various services for subscribers, which are defined and classified into telephony and data services. For telephony services, the transferred information is speech signals within audio range, for data services, signals other than voice are transferred, e.g. text, image, fax, various messages, computer files, etc.

TRAU determines current service operation type by detecting the TRAU frame format command word sent from base station.

During data service communication, TRAU accomplishes the format converting of data frame and rate adaptation without transcoding transferred data.4) Signaling timeslot

In TRAU, each FTC board is responsible for one PCM stream (32 timeslots in each PCM stream), where timeslot 0 is for transferring frame synchronization signals. Signaling timeslot may be assigned through OMC randomly.

FTC board forwards the content of signaling timeslot transparently so that signaling information will not be affected.

II. SMUX

To save terrestrial line resources, Sub-multiplexer (SMUX) is used between MSC and BSC to multiplex 4(16kbit/s channels to carry four speech channels through one terrestrial line channel. No matter speech signals or data, they are transferred with a rate of 16kbit/s between the BSC and TRAU.The position of SMUX in the system is illustrated in Figure 2-26, where TCSM consists of MSM and FTC boards.In multi-module BSC, the functions of SMUX are accomplished by MSM and E3M board. While in a single-module BSC, this function is implemented in the MSM plugged in the TCSM frame and the SMI plugged in the BIE slot.

Figure 2-26 Position of SMUX in the system

SMUX has the following functions.

Multiplexing/demultiplexing speech channels: SMUX can multiplex 4 channels into 1 standard E1 link and demultiplex 4 channels from 1 standard E1 link.

Transparent transmission of signaling: SMUX can transparently transfer signaling.

Operation and maintenance link: MSM and E3M boards can communicate with each other through HDLC link, which occupies the last two bits of 31st timeslot on E1 link. BSC can operate and maintain the remote TCSM units through this HDLC link.

2.3.4 BAM

I. Functions

Back Administration Module (BAM) helps customers to maintain and operate BSC through OMC. It forwards the maintenance and operation commands from OMC to BSC system and sends back the system response to the corresponding OMC terminal. It also stores and forwards alarm messages, traffic statistics data, etc.

BAM keeps normal communication with GMPU during operation. In case of any abnormality in BAM software, it can restart within preset time.

BAM communicates with control system through HDLC link, and communicates with OMC directly or indirectly via network adapter. When BSC and OMC are in the same premises, then BSC can communicate with OMC directly through network adapter. When BSC and OMC are not in the same premises, they communicate through network adapter, router and transmission equipment.

II. System structure

BAM is connected with the BSC through 2Mbit/s HDLC link and with the O&M terminal via the LAN or WAN. A structural diagram is shown in Figure 2-27.

Figure 2-27 BAM structure

The BAM is composed of three major parts, which are: Peripheral Interface (PI), Terminal Network Interface (TNI) and MCP.

Through Peripheral Interface (PI), various devices can be handled such as dual CD-ROMs, hard disk array, printer and tape drive used to dump or hard copy of data.

With TNI, terminal systems (maintenance, test, traffic statistics and data setting systems) can form a LAN attached with network servers to provide 10Mbit/s to 100Mbit/s transmission links, and to extend the network through devices such as network bridge/router, achieving data sharing in a larger scope. In M900/M1800 BSC, this interface is directly or indirectly connected to OMC.

MCP is the PC card for the communication between BAM and BSC. Each card provides two 2Mbit/s HDLC links to connect with BSC, serving as the message paths between BSC and BAM.

III. Structure features

When BAM software is abnormal, BAM will reset and restart automatically, thanks to BAM self-restoring capability.

All components have passed the electromagnetism compatibility test.

-48V standard industrial power supply is used, in consideration that BAM is installed on the cabinet in actual application. -48V power supply is highly reliable, stable and safe. The power supply has passed the electromagnetic compatibility test.

BAM can be installed inside the cabinet. The outer surface of the cabinet is painted, while the inner surface is not, so as to make sure good grounding effect. There are ventilation openings at the front of the cabinet, together with various indicators, buttons, keyboard and monitor ports.

2.3.5 CDB

Cell Broadcast Database (CDB) is a traffic processing center, responsible for providing the interface between the Short Message Center (SMC) and BSC, and supports short message broadcast service. Its server communicates with the GMEM boards of the modules through Ethernet. CDB can communicate with CBC through either TCP/IP or X.25 interface. To support X.25 interface, a X.25 card should be added to CDB for communication with CBC.

I. Cell broadcast system

Short Message Service Cell Broadcast (SMSCB) allows short message to be broadcast to all mobile stations in certain areas. These areas may be one or several cells, even the entire PLMN area. Short message from cell broadcast center (CBC) is sent to the CDB of BSC which manages the message. BSC then sends the received message to BTS. BTS can make load control.

The functions of cell broadcast system is briefly described as follows:

Able to explain and response to the message primitives from CBC.

Able to report to CBC about CBCH channel state and the conditions of message sending.

Reporting error information to CBC when received message primitives can not be understood or executed.

Able to report cell fault to CBC.

BSC sends overload indication of related cell to CBC when the frequency of CBC message is beyond the load of BSC.

Storage and management of cell broadcast short message.

Supporting DRX mode.

Arrangement of cell broadcast short messages in CBCH channel and sending them to BTS.

II. Database structure

CDB contains three parts, which are message library, cell data table and general control table.

Message library mainly stores the cell broadcast short message sent from CBC and currently being broadcast in BSC, including message flag, message serial number, message coding method, transmitting frequency, message sending request, message contents etc.

Cell data table mainly stores broadcast channel configuration message and message related to broadcast short message for each cell of current BSC, including cell state, state and configuration of CBCH channel, storage arrays of broadcast short message, sending queue of broadcast short message etc.

General control table mainly stores, controls and records related information about cell broadcast of current BSC, including connection information with BM module, connection information with CBC, parameters of BSC cell broadcast, etc.

III. CDB features and performance

CDB supports the storage and management of 300 broadcast short messages.

Each cell can hold 60 broadcast short messages.

CDB supports the flow control of broadcast short message between CBC and itself.

CDB supports message flow control between BTS and itself.

CDB supports DRX mode.

CDB supports the forwarding of broadcast short messages among several modules. If there is some error in GMEM of a BM, CDB can send the message to another BM through another working GMEM.

For more information about CDB, please refer to M900/M1800 Cell Broadcast System User Manual.

2.4 Functional Frames of BSC

2.4.1 Clock Frame

The clock synchronization system of BSC operates in the clock frame.

The clock frame phase-locks the upper-level MSC or BITS clock reference sources and provides the AM/CM and BM with stable clock sources. The clock stratum of the clock frame can set flexibly to Stratum 2 clock or Stratum 3 clock through data configuration. M900/M1800 BSC uses Stratum 3 clock system.

Clock frame configuration is shown in Figure 2-28.

Configured with active/standby GCKS boards in hot backup (two boards), one clock frame outputs active/standby clocks (two clocks) and sends them to GCTN and GSNT.

In a single-module BSC, the GCKS communicates with the GMPU via the GALM board. In multi-module BSC, GCKS communicates with the GMCCM directly.

Figure 2-28 Clock frame in full configuration

The clock reference source is input via the backplane interface of the clock frame to the GCKS board. GCKS locks and pulls-in the reference source by software phase-locking and generates clock signals identical in frequency and phase with the reference source.In a multi-module BSC, the synthesized clock synchronization signals are sent to GCTN and GSNT, and then to other units/parts of the AM/CM. The BM's GOPT extracts clock signals from optical signals and generates required clock synchronization signals. These signals are sent to GNET, and then forwarded to other parts of the BM.

In a single-module BSC, the synthesized clock synchronization signals are directly sent to GNET, which then sends these signals to other parts of the BM.

Both PWC and GCKS operate in 1+1 redundant mode to ensure the reliable operation of the clock frame.2.4.2 Main Control Frame

I. Functional Blocks

The main control frame is designed to implement management and control of the BM, communications with AM/CM, signaling processing, etc.

The main control unit is mainly composed of the processor circuits, signaling processing circuits, inter-module communication circuits and database interface circuits.

Three-level distributed control is adopted in the BM, with GMPU, GNOD and slave nodes (CPUs) from top down, as illustrated in Figure 2-29.

Figure 2-29 Hierarchical structure of the main control unit

For internal communication, mailbox mode is employed between the first and the second level CPUs, while the master node/slave node high-speed serial communication mode of point-to-point or point-to-multipoint is employed between the second and third level CPUs.

GMPU is the central processor in the main control unit of the BM. To improve the system reliability, two GMPU are used in hot backup mode.

The GEMA is used to help GMPU data backup and to control the GMPU switchover.

Active/standby GMPUs are determined by GEMA, forming the first level control system.

GMPU directly controls GNET via the bus, and exchanges messages with GNOD, LPN7, GMEM and GLAP via mailbox communication mode. These boards in the main control unit constitute the second level control system.

GMPU sets up the connection with respective functional slave nodes via GNOD. Here, slave node refers to the microprocessor on functional circuit board (such as BIE board). GNOD communicates with CPUs on related circuit boards via serial ports and controls respective CPUs in master/slave node communication mode.

The CPUs accommodated in respective control interface ports in the BM cooperate with each other, forming a functional multi-processor control system. The inter-processor communication is conducted through the mailbox by using memory mapping technology, which greatly reduces the overhead for internal communication.

The processor circuit mainly consists of GMPU, GEMA and GNOD. Among them, GMPUs are the central processing units in the module, whose active/standby state is controlled by the GEMA. Both GMPUs work in redundant mode and communicate with slave nodes via GNOD.

The processing of SS7 signaling on A-interface is implemented by LPN7. GLAP is responsible for signaling on Abis interface and Pb interface.

Inter-module communication circuit mainly consists of GMC2 and GOPT. (Note: There is no inter-module communications circuit in the single-module BSC.)

The BSC is connected with CDB through GMEM.

II. Frame Configuration

The main control frame in full configuration is shown in Figure 2-30. The boards that can be installed in it are as follows:

GMPU

GNOD

GMEM

GMC2

GOPT

GALM

LPN7

GLAP

PWC

GEMA

GNET

CKV

Figure 2-30 Main control frame in full configuration

In multi-module BSC, two GOPTs and two GMC2s should be configured in the main control frame.

The GOPT connects with the AM/CM via optical fiber.

The GMEM works only when the cell broadcast service is in operation.

2.4.3 Communication Control Frame

The communication control frame is the control center of the AM/CM. The communication control unit manages and controls the overall system.


The functional blocks of the communication control frame are shown in Figure 2-31.

Figure 2-31 Functional blocks of communication control frame

The GMCCM communicates with GCTN, GALM and BAM through GSNT. It provides signaling communication links for the BM and AM/CM, and transfers control messages from BM to BM, from BM to GMCCM, from BM to GCTN and from BM to TCSM unit. The GMCCM also processes the maintenance messages of all the boards in the AM/CM and clock frame. It also controls the GSNT in its provision of loading paths for the BM and AM/CM, but it is not responsible for the switching control of the overall system.

The GMCCS communicates with GCTN and BM via the HDLC link. The GMCCS provides signaling communication links for the BM and AM/CM and transfers control messages from BM to BM, from BM to GMCCM, from BM to GCTN and from BM to TCSM unit.

The GSNT, a signaling switching center of AM/CM, performs switching of signaling messages between boards in the AM/CM, and provides loading paths to the modules.


The communication control frame in full configuration is shown in Figure 2-32. The boards that can be installed in it are as follows:

GMCC

GSNT

GALM

PWC

Figure 2-32 Communication control frame in full configuration

The communication control frame occupies one frame space and accommodates 10 GMCC boards in full configuration. The GMCC boards are numbered from right to left. GMCC0 can only be plugged in Slot 16 and GMCC1 only in Slot 15. The right-most two GMCC slots hold GMCCM boards. The other GMCC slots hold GMCCS boards (at most 8 GMCCS boards can be configured).

2.4.4 Transmission Interface Frame

Transmission interface frame mainly accomplishes the functions of multiplexing/ demultiplexing of inter-module speech channels and signaling links, optic-electric conversion and E1 interface driving, so that inter-module communication messages can be transmitted over optical fibers.

Transmission interface unit is mainly composed of GFBI/FBC, GCTN, E3M and DRC.


The functional blocks of the transmission interface frame are shown in Figure 2-33.

Figure 2-33 Functional blocks of transmission interface frame

The transmission interface frame uses GCTN as the center for speech channel switching.

Each BM connects with the GFBI via two pairs of optical fibers. The GFBI extracts and separates 32Mbit/s speech channel signals from the optical path signals and sends them to the GCTN, and then separates 2.048Mbit/s signals and sends them to the GSNT of the communication control frame for processing. In addition, it combines the speech channel signals from the GCTN and the link signals from the GSNT into 40.96Mbit/s stream and sends them to the FBC.

The E3M connects with GCTN via 32Mbit/s HW. It fulfils the switching from super HW (512 timeslots) to 16 E1s, compresses these 16 E1s into 4 E1s by 4:1, thus greatly reduces transmission lines. It provides 4 Pb ports to the PCU. The speech channel signals are sent to the MSC after switching by GCTN and E3M.

The DRC board, in collaboration with the E3M, provides the external E1 interface coupling and over-voltage protection modules. The DRC is installed on the backplane.


The boards that can be installed in the transmission interface frame are as follows:

GFBI

E3M

PWC

GCTN

The transmission interface frame in full configuration is shown in Figure 2-34.

Figure 2-34 Transmission interface frame in full configuration

The transmission interface frame occupies two frame spaces.

GCTN, a central speech channel switching system of AM/CM, occupies two slots. The two GCTNs work in active/standby mode and implement 16k(16k speech channel switching.The FBC and GFBI are used in pairs. The FBC is plugged in the socket on the backplane of the AM/CM interface frame, in one-to-one correspondence with the GFBI board.

Featuring optical interface and conversion functions, the GFBI splits the optical fiber signals between BM and AM/CM into 32Mbit/s super HW signaling and 2Mbit/s HW signaling. The GFBI collaborates with the GOPT in the BM to provide paths for inter-modular communications.

The E3M performs the timeslot switching of 2k network and E1 multiplexing function.

Four PWCs are configured, fixed in positions.

2.4.5 BIE Frame


Located in the BM cabinet, the BIE frame provides Abis interface between BSC and BTS. The BIE of BSC includes the BS interface device BIE and the transparent transmission BIE (responsible for transmitting SS7 signaling transparently to E3M). The two boards are identical in hardware and compatible in slot, with the only difference of DIP switch settings.

The BIE boards installed in the BIE frame work in active/standby group. There is no association between different working groups.

The functional blocks of the BIE frame are shown in Figure 2-35.

Figure 2-35 Functional blocks of BIE frame

BIE performs transcoding, re-timing, control, hot backup and signal multiplexing. The BIE is a transmission interface device between BSC and BTS. Operating in active/standby (1+1) mode, the BIE provides the largest convergence ratio (15:1) of the Abis interface and supports star, chain, tree and hybrid topologies for BTS networking. It connects BTS to BSC in a flexible way to minimize the E1 links between BSC and BTS.


The boards in the BIE frame are:

BIE

PWC

The BIE frame in full configuration is shown in Figure 2-36.

Figure 2-36 BIE frame in full configuration

There are two kinds of BIE boards in the BIE frame: one is the BIE that transmits transparently SS7 signaling and the other is the general BIE that establishes connection between BSC and BTS. The only difference of these boards is their DIP switch settings.

The BIE boards are numbered from left to right starting from 0. The two adjacent BIE boards operate in active/standby state. The number of active/standby groups depends on the number of configured boards. There are a variety of BIE active/standby combinations in 1+1 redundancy mode:

Slot 2 and Slot 3 (group 0)








Slot 23 stands alone with no active/standby relationship, but its active/standby group number is still defined as 8.

When the quantity worked out by BIE is N, the total number of slots required is 2(N-1.

2.4.6 TCSM Frame


The FTC and MSM can be plugged in the TCSM frame. The Transcoder & Rate Adapter Unit (TRAU) and Sub-Multiplexer (SMUX) are jointly called TCSM unit. Every 4 FTC boards and 1 MSM make up a TCSM unit. A TCSM frame can hold 4 TCSM units. Each unit works independently without any correlation. One TCSM frame can hold 4 MSM boards and 16 FTC boards. Configuration of TRAU is compulsory while SMUX is optional.

In multi-module BSC, the functions of TRAU are implemented by the FTC and multiplexing is accomplished by MSM and E3M in the transmission interface frame.

The functional blocks of the TCSM frame are shown in Figure 2-37.

Figure 2-37 Functional blocks of TCSM frame


On the backplane of the TCSM frame there are TCB boards. The boards that can be installed in the frame are as follows:

FTC

MSM

PWS

The frame in full configuration is shown in Figure 2-38.

Figure 2-38 TCSM frame in full configuration

The TCSM frame can be placed on the MSC side (with multiplexing) or on the BSC side (without multiplexing).2.4.7 CDB Frame


The Cell Broadcast Database (CDB), a traffic processing center, supports cell broadcast short message service.

The CDB connects with the Cell Broadcast Center (CBC) via LAN or WAN for command interactivity and response transceiving. In addition, the CDB connects to the GMEM corresponding to the BSC and implements such procedures of BTS as CBCH channel query, CBS message transmission and flow control via the BSC.

The CDB network interface is shown in Figure 2-39.

Figure 2-39 CDB network interface

The major software functional modules of the CDB are comprised of CBC command interface module, GMEM interface module, CBS message storage module, CBS message scheduling module, CBS message transmission module, flow control module, network interface module, and protocol conversion module. When TCP/IP is adopted for the communication between CDB and CBC, the protocol conversion module is not needed, as shown in Figure 2-40. When X.25 protocol is adopted for the communication between CDB and CBC, the protocol conversion module is used to convert different protocols, as shown in Figure 2-41.

Figure 2-40 CDB functional blocks (using TC/IP)

Figure 2-41 CDB functional blocks (using X.25 protocol)

The CBC command interface module handles command interactivity between CDB and CBC.

The CBS message storage module is designed to store the CBS messages to be sent or not sent completely.

The CBS message scheduling module is designed to process the majority of operation requests of CBC, schedule CBS messages and generate schedule messages under the discontinuous reception (DRX) mode.

The CBS message transmission mode serves to send the CBS messages to BTS.

The GMEM interface handles command interactivity between CDB and BSC, forwards the internal operation commands of the CDB to BSC, which in turn transmits transparently these messages to the BTS.

The flow control module exercises flow control over the CBCH.

The network interface module, which establishes connections directly with external network, is responsible for receiving and transmitting messages.

The protocol conversion module converts the TCP/IP data packets sent from CDB to CBC into X.25 data packets, and converts the packets received by X.25 card into TCP/IP data packets and sends them to CDB.


There is no backplane in the CDB frame. The CDB, a sub-module of BSC, is physically a computer running on Windows NT, occupying a half frame. Installed generally in the lower part of the AM/CM cabinet of BSC, it fulfills mainly the cell broadcast functions supported by BSC.

The position of CDB in the AM/CM cabinet is shown in Figure 2-42.

Clock Frame5

Communication Control Frame4

Transmission Interface Frame3

Transmission Interface Frame2

CDB1

BAM Frame0

Figure 2-42 Position of CDB in AM/CM cabinet

2.4.8 BAM Frame

Refer to the description of BAM.

2.5 Circuit Boards of BSC

In this section we will briefly discuss the circuit boards of M900/M1800 BSC to have an overall understanding. For details, refer to M900/M1800 BSC Hardware Description Manual.

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M900/M1800 Base Station Controller