BSSDIM: BSS Subsystem Network ConfigurationBSS DIM Module 01
Module Objectives
At the end of the module, the participant will be able to:
Know the main elements that compose BSS Subsystem Network
Distinguish the different interfaces used to connect such elements
* BSSDIM-BSS Network Configuration and Interfaces
Introduction
Base Station Subsystem (BSS) is the network section in charge of the radio aspects of the network
It interfaces with the MS via the air interface, with the NSS via the A and Gb interface and with the NMS for operation and management issues
MS
BSS
(GERAN)
NSS
NMS
Content
BSS Functions
Mobility Management
Synchronization
Collection of Statistical Data
BSS Architecture
BSS Network Elements
Main functionalities of BSC are:
Control of the Base Transreceiver Stations (BTS)
Configuration of TRXs and frequencies
Control of the connection with the MS
Radio channel establishment and release
Radio resource allocation
Radio network control: integration of new BTSs, collection of statistical measurements from BTS
Forwarding of messages between the NSS and the MS
* BSSDIM-BSS Network Configuration and Interfaces
BSS Network Elements
Transcoder Submultiplexer (TCSM)
For transmission over the air interface, the speech signal is compressed by the mobile station to 13Kbits/s (Full Rate) or 6.5Kbits/s (Half Rate). This compression algorithm is known as "Regular Pulse Excitation with Long Term Prediction" (RPE-LTP)
This information is transmitted through the Abis interface at 16 or 8 kbps
However, standard bit rate towards the MSC is a 64 kbps PCM interface
Transcoder and Rate Adaptation Unit (TRAU) is the element taking care of converting speech from one digital coding format to another and vice versa
* BSSDIM-BSS Network Configuration and Interfaces
BSS Network Elements
Packet Control Unit (PCU)
PCU is a new plug-in unit for BSC providing required protocols and GPRS radio resource management functions needed to support PS traffic in the BSC
Main functions of the PCU are:
GPRS radio resource management
Temporary Block Flow (TBF)
Coding scheme selection
Load sharing (uplink)
Initial timing advance (uplink, downlink)
PCU statistics
BSS Network Elements
Base Transreceiver Station (BTS) corresponds to the transceivers and antennas used in each cell of the network
Its transmitting power defines the size of a cell. The number of TRXs per BTS depends on the BTS family, but a typical one can vary from 1 to 12. The election would depend on the users density in the cell
Example of BTS configurations
BSS Interfaces
- EIR (Equipment ID Register) for tracking Stolen Mobiles...
* BSSDIM-BSS Network Configuration and Interfaces
BSS Interfaces
A interface
A interface is based on 1536 kbps (T1) PCM digital interface, composed by 24 channels of 64 kbps
Channel 0 is reserved for frame alignment
Channels 1 to 23 can be used for voice or signaling
Main functions provided by this interface are:
Radio Channel control
BSS Interfaces
Gb interface
Gb is the interface interconnecting the BSS with the SGSN
Both signalling and user data are sent through Gb, using the same physical resources
Different users can be multiplexed on the same physical resource
Access rates available per user may vary from zero data to the maximum possible bandwidth (e.g. the available bit rate of a T1)
Primary functions provided by this interface include:
In downlink, provision by an SGSN to a BSS of radio related information used by the RLC/MAC function
In uplink, provision by a BSS to an SGSN of radio related information derived from the RLC/MAC function
Provision of functionality to enable two physically distinct nodes, an SGSN and a BSS, to operate node management control functions
* BSSDIM-BSS Network Configuration and Interfaces
BSS Interfaces
Abis interface
Abis interface is defined between the BSC and the BTS, with freedom for vendors to introduce proprietary solutions
Based on the utilization of T1/E1 digital interface as physical layer. Sub multiplexing is used as a standard solution, supporting 16 kbit/s (full rate) and 8 kbit/s (half rate) portions of the 64 kbit/s timeslots
Each TRX at a BTS requires up to 8 TCH/F traffic channels, or up to 16 TCH/H traffic channels of 16 kbit/s, and a 16 kbit/s, 32 kbit/s (or 64 kbit/s) signaling link to the BSC (marked as TRXSIG)
Main functions provided by Abis:
Selection of the radio and terrestrial channel for a mobile communication
Management of the common radio
Channel encoding and decoding
Measurement collection, Ciphering, Paging
Handover control and management
Sub multiplexing is used on the Abis interface as a standard solution, supporting 16 kit's (full rate) and 8 kit's (half rate) portions of the 64 kit's timeslots So the 2 Ambit's PCM frame is actually divided to 128 (16 kit's rate) subtimeslots or to 256 (8 kit's rate) subtimeslots.
* BSSDIM-BSS Network Configuration and Interfaces
BSS Interfaces
Ater interface
Ater interface was not specified in the first releases of GSM, but it was introduced later to reflect the alternative of implementing the TRAU as a separate network element
From the logical viewpoint, the MSC-BSC interface is the only existing interface, so that BSC-TRAU and TRAU-MSC are just two physical parts of the same logical interface
Normally, Ater interface specification is proprietary. In any case, there are two kind of channels defined for this interface:
Signaling Channels:
BSSAP signaling forwarding
BSSOMAP signaling forwarding
Specific signaling between BSC and TRAU, for the configuration of the transcoder unit
Voice and data Channels
Conclusions
The main role of the BSS is to handle all the issues related with the radio aspects of the network
Base Station Subsystem is composed by three network elements:
BSC: Base Station Controller (including PCU: Packet Control Unit)
BTS: Base Transreceiver Station
TRAU / TCSM: Transcoder Units
A interface: MSC – BSC
Abis interface: BSC – BTS
Ater interface: BSC - TRAU