BSC FUNCTIONS
The BSC controls a major part of the radio network. Its most important task is to ensure the highest possible utilization of the radio resources. The main functional areas of the BSC are:
• Radio Network Management
• RBS Management
• TRC Handling
• Transmission Network Management
• Internal BSC Operation and Maintenance
• Handling of MS connections
Radio Network Management
Radio network management includes the following tasks:
• Administration of radio network data including:
− Cell Description Data (e.g. cell identity, BCCH channel number, maximum and minimum output powers in the cell, RBS type, etc.)
− System information data (e.g. information about whether or not the cell is barred from access, maximum output power allowed in the cell, BCCH channel identities in neighboring cells)
− Locating data (e.g. cell rank used in HCS and high traffic load situations)
− Cell load sharing data, i.e. parameters for forcing early handovers from congested cells
• Traffic and event measurements: (e.g. number of call
attempts, congestion, traffic levels for a cell, traffic levels for an MS, number of handovers, number of dropped connections, etc.).
• Idle channel measurement:
The RBS collects statistics from the MS’s about signal strength and quality. These statistics are then used during the channel allocation process, so that a channel with low interference is allocated for a call.
RBS Management
Ericsson's RBS implementation is transceiver-orientated, ensuring good redundancy features. This means that as little as possible of the equipment is common to several transceivers. This philosophy inevitably leads to a master slave relationship between the BSC and the transceivers in the RBS. A logical model of the RBS is built up within the BSC and RBS equipment can be logically defined, connected and disconnected. The main tasks of RBS management are:
• RBS configuration:
This involves the allocation of frequencies to channel combinations and power levels for each cell according to available equipment. If equipment becomes faulty causing the loss of important channels,n reconfiguration of the remaining equipment is activated, sacrificing less important channels.
• RBS software handling:
This involves the control of program loads.
• RBS equipment maintenance:
RBS faults and disturbances are recorded and logged continuously.
TRC Handling
Although TRAUs are located in a TRC, the BSC, as controller of the radio resources of a GSM network, actually co-ordinates the sourcing of a TRAU for a call. During call set-up, the BSC instructs the TRC to allocate a TRA device to the call. If one is available, the TRC confirms the allocation of a TRA device. The TRA device is considered to be under the control of the BSC for the duration of the call.
Transmission Network Management
The transmission network for a BSC includes the links to and from MSC/VLRs and RBS’s. This involves the following tasks:
• Transmission interface handling:
This provides functions for administration, supervision, test and fault localization of the links to RBS’s. The BSC configures, allocates and supervises the 64 kbits/s circuits of the PCM links to the RBS. It also directly controls a remote switch in the RBS that enables efficient utilization of the 64 kbits/s circuits.
Internal BSC Operation and Maintenance
Operation and maintenance tasks can be performed locally in the BSC or remotely from the OSS. Internal BSC operation and maintenance involves the following features:
• TRH maintenance:
Administration, supervision and test of the TRansceiver Handler (TRH) is carried out in the BSC. The TRH consists of both hardware and software. A TRH is located on a Regional Processor for the Group switch (RPG). One RPG thus serves several transceivers. There can be several RPGs in the BSC.
• Processor load control in the BSC:
This function ensures that during processor overload situations, a large number of calls can still be handled by the BSC. If too many calls are accepted, real time requirements such as call set-up times can not be fulfilled. To prevent this, some calls need to be rejected in situations of high load. Calls already accepted by the system are given full service and are not affected by the overload situation.
Handling of MS Connections
Call Set Up
Call set up involves the following processes:
• Paging:
The BSC sends paging messages to the RBS’s defined within the desired LA. The load situation in the BSC is checked before the paging command is sent to the RBS.
• Signaling set-up:
During call set-up, the MS connection is transferred to an SDCCH allocated by the BSC. If the MS initiated the connection, the BSC checks its processor load before the request is further processed.
• Assignment of traffic channel:
After SDCCH assignment, the call set-up procedure continues with the assignment of a TCH by the BSC. As this takes place, the radio channel supervision functions in the BSC are informed that the MS has been ordered to change channels. If all TCHs in the cell are occupied an attempt can be made to utilize a TCH in a neighboring cell.
During a Call
The main BSC functions during a call are:
• Dynamic power control in MS and RBS:
The BSC calculates adequate MS and BTS output power based on the received measurements of the uplink and downlink. This is sent to the BTS and the MS every 480 ms to maintain good connection quality.
• Locating:
This function continuously evaluates the radio connection to the MS, and, if necessary, suggests a handover to another cell. This suggestion includes a list of handover candidate cells. The decision is based on measurement results from the MS and BTS. The locating process is beingb executed in the BSC.
• Handover:
If the locating function proposes that a handover take place, the BSC then decides which cell to handover to and begins the handover process. If the cell belongs to another BSC, the MSC/VLR must be involved in the handover. However, in a handover, the MSC/VLR is controlled by the BSC. No decision making is performed in the MSC because it has no real time information about the connection.
• Frequency Hopping:
Two types of hopping are supported by the BSC:
− Baseband hopping: this involves hopping between frequencies on different transceivers in a cell
− Synthesizer hopping: this involves hopping from frequency to frequency on the same transceiver in a cell
BSC IMPLEMENTATION
The BSC is implemented on a non-AM-based AXE platform consisting of standard APZ and APT subsystems and the following APT subsystems:
BSC Subsystems
BSC hardware configuration
• Regional Processor for the Group switch (RPG)/TRansceiver Handler (TRH)
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