BSC CONFIGURATION
The Base
Station Controller (BSC) controls and supervises the radio resources in the
Base Transceiver Station (BTS). Together with the BTS, the BSC constitutes the
Base Station System (BSS), responsible for the management and cell
configuration data of the radio network. The main functions of the BSC are:
· Administration
of BSS resources
· Supervision of
the BTS
· Connection
handling of mobile stations
· Locating and
handover
· Administration
of paging
· Transmission
network management
· Operation and
maintenance of the BSS
The unit
that performs the speech conversion from 64 kbit/s
into a total of 16k or 8k, 13+3kbit/s and
15.1+0.9kbit/s (Full rate and Enhanced speech coder, FR and EFR) or 6.5+1.5 kbit/s (Half rate speech codec, HR) per channel is called
the transcoder. This function can either be placed in
a separate node, called the Transcoder Controller
(TRC), or together with the BSC, which then becomes a BSC/TRC. The different
configuration types are illustrated in the picture below.
· TRC - a
Stand Alone transcoder controller node
The TRC
node allows a flexible location of the transcoder
resources. Typically, the TRC is located at or near the MSC. It is controlled
by the BSC. 16 BSCs can be connected to one TRC.
· BSC/TRC - a
combined BSC and transcoder controller
The
BSC/TRC is suitable for medium and high capacity BSC applications, that is,
urban and suburban area networks. This node can handle up to 1,020 Transceivers
(TRXs). 15 stand alone BSCs
can be connected to the BSC/TRC.
· BSC - a
Stand Alone BSC without transcoders
The BSC is
optimized for low and medium capacity BSS networks and is a complement to the
BSC/TRC, especially in rural and suburban areas. For GSM 900/GSM 1800, it can
handle up to 1,020 TRXs.
An
overview of the system components in the GSM system, integrated with the
circuit-switched part of the GSM System, is shown in Figure 2-2.
NETWORK NODES
Terminal Equipment (TE)
The
Terminal Equipment (TE) is the computer terminal that the end-user works on.
This is the component used for the GPRS system to transmit and receive end-user
packet data. The TE can be, for example, a laptop computer. The GPRS system
provides IP connectivity between the TE and an Internet Service Provider or
Corporate LAN, connected to the GPRS system. From the TE kbit/s
of view, it is possible to compare the MT to a modem that connects the TE to
the GPRS system.
Mobile Terminal (MT)
The Mobile
Terminal (MT) communicates with a TE, and over the air with a BTS. The MT must
be equipped with software for the GPRS functionality when used in conjunction
with the GPRS system. The MT is associated with a subscriber in the GSM system.
The MT establishes a link to an SGSN. Channel reselection is provided at the
radio link between the MT and the SGSN. The IP connection is static from the TE
kbit/s of view, that is, the TE is not aware of being
mobile and retains its assigned IP address until the MT detaches.
Mobile Station (MS)
The
combination of a TE and an MT is an MS (Mobile Station). Often in this document
as in the ETSI GSM standard for GPRS, the term MS is used when discussing the
GPRS features. It can be concluded from the context which parts relate to the
MT or the TE parts. Note that the MT and TE parts can co-exist in the same
piece of equipment. GPRS MSs can, depending on the MS
and the network capabilities, operate in three different modes:
· Class A mode of operation allows an MS to have a circuitswitched connection at the same time as it is
involved in a package transfer.
· Class B mode
of operation allows an MS to be attached to both CS and PS but it cannot use
both services at the same time. However, an MS that is involved in a package
transfer can receive a page for circuit-switched traffic. The MS can then
suspend the packet transfer for the duration of the circuit-switched connection
and afterwards resume the package transfer.
· Class C mode
of operation allows an MS only to be attached to one service at a time. An MS
that only supports GPRS and not circuit-switched traffic always works in class
C mode of operation.
Base Station System (BSS)
The Base
Station System (BSS) consists of a Base Station Controller (BSC) and a Base
Transceiver Station (BTS). The BTS is the radio equipment, which transmits and
receives information over the air to allow the BSC to communicate with MSs in the BSCs service area. A
group of BTSs is controlled by a BSC. The BTS must
contain GPRS-specific software. The BSC provides all radio-related functions.
The BSC has the functionality to set up, supervise, and disconnect circuitswitched and packet-switched calls. It is a high
capacity switch that provides functions including handover, cell configuration
data, and channel assignment. The BSC must be equipped with GPRS hardware and
software when used for GPRS. One or several BSCs are
served by an MSC, and a number of BSCs are served by
an SGSN. The BTS separates the MS-originated circuit-switched calls from packet
data communication, before the BSC forwards CS calls to the MSC/VLR, and PS
data to the SGSN. The protocols towards the BSC are standard GSM protocols, for
the desired compatibility.
The
Serving GPRS Support Node (SGSN)
The
Serving GPRS Support Node (SGSN) is a primary component in the GSM network
using GPRS and is a new component in GSM. The SGSN forwards incoming and
outgoing IP packets addressed to/from a mobile station that is attached within
the SGSN service area. The SGSN provides
· Packet routing
and transfer to and from the SGSN service area. It serves all GPRS subscribers
that are physically located within the geographical SGSN service area. A GPRS
subscriber may be served by any SGSN in the network, all depending on location.
The traffic is routed from the SGSN to the BSC, via the BTS to the mobile
station.
· Ciphering and
authentication,
· Session
management
· Mobility
management
· Logical link
management towards the MS
· Connection to
HLR, MSC, BSC, SMS-GMSC, SMSIWMSC, GGSN
· Output of charging data. The SGSN collects charging information for each MS related to the radio
network usage.
Both the
SGSN and the GGSN collect charging information on usage of the GPRS network
resources.
Gateway GPRS Support Node (GGSN)
Like the
SGSN, the Gateway GPRS Support Node (GGSN) is a primary component in the GSM
network using GPRS and it is a new component. The GGSN provides
· The interface
towards the external IP packet networks. The GGSN, therefore, contains access
functionality that interfaces external ISP functions like routers and RADIUS
servers (Remote Access Dial-In User Service), which are used for security
purposes. From the external IP network’s kbit/s point
of view, the GGSN acts as a router for the IP addresses of all subscribers
served by the GPRS network. The GGSN, thus, exchanges routing information with
the external network
· GPRS session
management; communication set-up towards external network
· Functionality
for associating the subscribers to the right SGSNs of
traffic.
· Output of charging data. The GGSN collects charging information for each MS, related to the
external data network usage. Both the GGSN and the SGSN collect charging
information on the usage of the GPRS network resources.
Home Location Register (HLR)
The Home
Location Register (HLR) is the database that holds subscription information for
every person who has bought a subscription from the GSM/GPRS operator. The HLR
stores information for CS and for PS communication. The HLR contains
information about, for example, supplementary services, authentication
parameters, and whether or not packet communication is allowed. In addition,
the HLR includes information about the location of the MS. For GPRS, subscriber
information is exchanged between HLR and SGSN. Note that the authentication
triplets for GPRS are fetched directly from the HLR to the SGSN, that is, it
does not use the MSC/VLR like for CS GSM. For more information, see section
Visitor Location Register (VLR) Functionality in SGSN and in MSC, above. The
information going from the HLR to the SGSN is what the operator sets up for the
subscriber. This information transfer occurs when the operator changes
subscriber information, or when a new SGSN needs to have data for a subscriber
after attach or roaming. The old SGSN also receives information about the
roaming. The information going from the SGSN to the HLR is the routing
information that is transferred upon MS action, for example, attach or roaming.
For a roaming mobile, the HLR may be in a different PLMN than the SGSN serving
the mobile.
Gateway
The
Gateway Mobile services Switching Center (GMSC)
switches the circuit-switched calls between the GSM circuitswitched
network and the Public Switched Telephone Network (PSTN), that is, the fixed
telephony network. The GMSC is not changed for use by the GPRS system.
Non-GPRS-Specific System Components
Those
components that are not specific to the GPRS system will only be discussed in
terms of their involvement in the packet data services. Such components
include, for example, the Authentication Center
(AUC). Since the mobile stations use IP addresses, you can connect from the
GPRS system to Internet Protocol services, obtained from an Internet Service
Provider or from a Corporate LAN. The services can be, for example, World Wide
Web, e-mail, or telemetry.
BSC/TRC HARDWARE AND BASIC CONCEPTS
The
BSC/TRC node comprises all hardware that constitute
the stand alone nodes TRC and BSC, so this will be explained first. The
differences are briefly described later on in the chapter.
GROUP SWITCH (GS)
The GS is
the central part of the BSC/TRC. The GS connects an incoming channel with an
outgoing channel. For example, it can connect any incoming PCM timeslot and
send it out on any outgoing PCM link on any timeslot. The GS comprises Time
Switch Modules (TSM) and Space switch Modules (SPM) and can switch down to 64 kbit/s. If switching
should be done to lower bitrates, for example, 16 kbit/s, the SubRate Switch (SRS)
must be used.
Switching Network Terminal (SNT)
All
equipment connected to the group switch uses the same standardized interface,
which is called Switching Network Terminal (SNT). The SNT is a software concept
and represents the software connection of the physical hardware to the Group
Switch. The hardware is normally referred to as device hardware. Each SNT is
connected to the GS at a Switching Network Terminal Point (SNTP). In Figure
2-3, the following device hardware is shown, which will be further
explained in the chapter:
· Exchange
Terminal Circuit (ETC)
· Signaling
no.7 terminal (ST7, C7-ST)
· Transcoder
and Rate Adaptor (TRA)
· Transceiver
Handler (TRH)
· Subrate Switch(SRS)
Device (DEV)
As
previously mentioned, the hardware connected to the GS is referred to as device
hardware. A device is the resource that each SNT has connected to the GS.
Depending on what device hardware and what software is loaded the device can
have different capabilities. The devices and their names will be explained under each device hardware.
EXCHANGE TERMINAL CIRCUIT (ETC)
The ETC
board is the common hardware in the AXE to handle the PCM transmission links,
in this case between the MSC-BSC and BSC-RBS. The links can either be 1.5 Mbit/s (T1) or 2 Mbit/s (E1) PCM
links. The two link types use different hardware, that is, for BYB 501, which
is the latest building practice, the 1.5 Mbit/s uses
ETC-T1 boards and the 2 Mbit/s uses ETC5 boards. What
differs, though, between the ETC boards towards the MSC and those towards the RBSs, is that they have different software loaded. This
means that the resources are named differently. Figure 2-4 shows the
different names and concepts connected to the PCM links in an E1 system.
ETRALT and ETRBLT
Figure
2-4 illustrates two
types of SNTs: The ETRALT and the ETRBLT use the same
type of hardware (ETC), but they are loaded with different types of software.
This means that they have slightly different functions. The SNT concept
supervises everything from the connection to the GS, the SNTP, to the output
from the ETC board. The Digital Path (DIP) then takes over the supervision of
the PCM link.
Digital Path (DIP)
Digital
Path (DIP) is the name of the function used for supervision of the connected
PCM lines. ITU-T has issued recommendations which state how the PCM links
should be supervised. All these recommendations are implemented in the DIP
function and the ETC. Depending on whether the PCM link goes toward the MSC or
the RBS the DIP will have different names. RALT towards the MSC and RBLT
towards the RBSs. RBLT stands
for RTS A-Bis interface Line Terminal
whereas RTS stands for Radio Transmission & Transport Subsystem. RALT
stands for RTS A-interface Line Terminal.
RBLT Devices
Each Time
Slot (TS), which is 64 kbit/s, on
the PCM link towards the RBS is called an RBLT device. The device is a
resource that the BSC can store information on. In this case it is either LAPD signaling or speech towards the RBS. The number of RBLT
devices is 32 on an E1 PCM link and 24 on a T1 PCM link. Figure 2-4 illustrates
an E1 PCM system. The numbering of the RBLT devices starts from 1 to 31 for the
first DIP RBLT-0. This is written as -1&&-31,
where the "&&" stand for how to specify a range of numbers in
an AXE command. It should also be noted that the RBLT devices 0, 32, 64, and 96
are not used. It is TS 0 on the PCM link that is used for synchronization and
which, therefore, cannot be used for other purposes. This is not the case in a
T1 PCM link, where synchronization is performed differently. In the T1 system,
the devices are also called RBLT24 devices. There is a more detailed
explanation of what the RBLT devices can be used for in the A-bis chapter.
RALT Devices
Each Time
Slot (TS), which is 64 kbit/s, on
the PCM link towards the MSC is called an RALT device. The device is a
resource that the BSC can store information on. In this case, it is either C7 signaling or speech towards the MSC. The numbering principle of the RALT devices are the same as for the RBLT
devices.
ETC 155 MBIT/S
ETC 155 hardware can be used for connecting different switches
to the SDH transport network. The interface may be optical fibers
or electrical cables.
DESCRIPTION
The SDH
(Synchronous Digital Hierarchy) standard was originally introduced into
transmission networks (now called transport networks). Now, the BSC can be
connected via SDH to the MSC. ETC 155 is an SDH interface, supporting both
electrical (155.52 MHz) and optical (1310 nm) communication. The ET155
terminates an STM1 (Synchronous Transfer Mode) and contains 63 E1/T1. The ETC
155 is not a part of the SDH network but is connected to the SDH network.
TRANSCODER AND RATE ADAPTOR (TRA)
The TRA is
the function responsible for the speech coding and rateadaption
of incoming speech and data from the MSC and the RBS. The hardware where the
function is implemented is called Transcoder and Rate
Adaption Board (TRAB). It has the following basic functions:
· Transcoding of speech information. Speech at 64 kbit/s to/from the MSC is transcoded
to 13 kbit/s to/from the RBS enabling four compressed
channels to be multiplexed onto one 64 kbit/s
channel. This is if Full Rate (FR) or Enhanced Full Rate (EFR) is used, which
have a bit rate of 13/15.1 kbit/s. For Half Rate (HR)
speech is transcoded to 6.5 kbit/s
· Additional
control information, 3 kbit/s for FR, 0.9 for EFR,
and 1,5 kbit/s for HR, is
added to the transcoded rate towards the RBS giving a
final output of 16 kbit/s or 8 kbit/s.
The control information which is called in-band signaling,
basically tells what type of information the information contains, for example,
speech, data.
· Rate adaptation of data information. The maximum data rate supported at present in
GSM is 14.4kbit/s per TS. With High Speed Circuit Switched Data (HSCSD) it is
possible to have higher bit rates, since then the MS will be assigned more than
one TS.
· Discontinuous
Transmission (DTX) functions on both uplink and downlink. This reduces the
interference in the network and saves mobile batteries.
Figure
2-6 illustrates how
the TRA works.
The
incoming 64 kbit/s is sent through the GS to the TRA.
Four 64 kbit/s channels are transcoded
to 16 kbit/s (FR and EFR) and multiplexed onto one 64
kbit/s. They are then sent out via the GS to the RBS
on the Abis interface on an RBLT device.
Multiplexing and Demultiplexing of Channels
The transcoder multiplexes a number of transcoded
channels into one 64 kbps channel, used between the BSC and BTS. The number of
multiplexed channels depends on the type of speech codec:
· Four traffic
channels for FR or EFR.
· Eight traffic
channels for HR.
In terms
of hardware, a TRA-EM consists of 32 devices, requires 32 GS inlets, and can
handle 24 TCHs.
In an FR
or EFR TRA-EM:
· Six MUXs handle 24 multiplexed channels towards the BTS.
· 24 DEMUXs handle the demultiplexed
channels towards the MSC.
· Four DEMUXs are statically connected to each MUX device.
In an HR
TRA-EM:
· Three MUXs handle 24 multiplexed channels towards the BTS.
· 24 DEMUXs handle the demultiplexed
channels towards the MSC.
· Eight DEMUXs are statically connected to each MUX device.
In both
configurations, two TRABs are used (for TRAU type TRA
R4). The relation between a TRA-EM and an SNT is one to one. The connection and
disconnection of a transcoder device to and from an
SNT is performed by command. Furthermore, a printout of transcoder
device states and transcoder-SNT connections can be
obtained by command. Before the transcoder equipment
can be seized for a connection towards the BTS, it must be physically and
logically connected, and manually deblocked. The transcoder equipment requested can be either
semi-permanently connected or seized in a transcoder
pool:
· Transcoder
devices can be semi-permanently connected through the GS for FR only. Once the
connection is established, it is possible to use it for traffic as soon as
synchronization is established between the transcoder
and the BTS.
· Pooled transcoder devices are seized according to TRA capability
and availability. The connections through the GS, for a transcoder
device seized in a transcoder pool, are set up on a
per call basis.
TRA Devices and SNT
Each SNT
in Figure 2-6 has 30 devices, for example, SNT -> RTTF1S1-0 has
devices RTTF1D1-2&&-31. From this you can deduce, using Figure 2-9 below,
that this is TRA R5 hardware, with an FR speech version. The number of 64kbit/s
that can be transcoded on this type of TRA is 24.
They are called demultiplexed (DEMUX) devices. The
other six devices are called multiplexed (MUX) devices. One MUX device is
16kbit/s (FR and EFR) and a DEMUX device is 64 kbit/s.
Transcoders in Pool and
Semi-permanently Connected Transcoders
The transcoder devices can either be in a pool or be semipermanently connected. If they are semi-permanently
connected, the transcoder device is always connected
to the same TS in the RBS. This means that the resource is not accessible for
others, even if there is no ongoing traffic. One TRA device is required for
each air TS, which will require a lot of TRA boards. To put the transcoders in a pool, transcoders
are seized on a per call basis leading to better utilization of the installed transcoder hardware. Figure 2-10 illustrates how the
TRA in pool generally works and the hardware that is involved.
In this
configuration of the transcoder, the TRA resources
can be set to be “pools”. In one BSC/TRC there can be different pools, for
example, one pool with EFR devices, one with FR devices, and one with HR
devices. Depending on what MS equipment should be connected, the BSC/TRC seizes
a device that is dependent on each MS's capabilities,
for example, not all MSs can handle EFR, and releases
the device when the call is terminated. This results in less hardware being
required, since all people in the BSC area will not call simultaneously. There
is seldom congestion, due to no available TRA devices in the pool. To be able
to handle semi-permanently connected transcoders, there
is no need for extra hardware. However, if "transcoders
in pool" are going to be used, the BSC/TRC must have a Subrate
Switch (SRS). The reason for this is that different TRA resources, for example,
FR and EFR, are mixed onto the same 64 kbit/s, and the GS (as previously mentioned) cannot switch
lower than 64kbit/s. The SRS can switch down to 8 kbit/s and can then put different 16 kbit/s devices on the same 64 kbit/s.
The SRS function is explained below.
TRANSCEIVER HANDLER (TRH)
The TRH
performs the activities that are required to control the RBS and the
transceivers, and is responsible for a multitude of functions including:
· Handling of signaling on the Link Access
Protocol on the Dchannel (LAPD) link between BSC-BTS.
· Handling of logical channel addressing part of signaling
to/from the BTS and mobile stations (MS).
· Processing of
measurement data from the BTS and MSs
· Operation and maintenance of the BTS.
Figure
2-11 illustrates
the principle of the TRH.
TRH Devices and SNT
Each SNT
in Figure 2-11 has 32 devices. The SNT, in this case, is called RHSNT
and it handles the TRH devices, named RHDEV. Each transceiver in the RBS must
have a signaling connection towards the BSC. The
device handling the signaling connection towards the
RBS is the RHDEV. One RHDEV is semipermanently
connected to one transceiver in the RBS.
As
previously mentioned, the RHSNT has 32 devices, but in reality only 24 of them
are usable (RPG2). This is due to the fact that one TS
is used for test purposes and the others are excluded so as not to load the TRH
with tasks. That is why the numbering in the picture states
RHDEV-1&&-24 and
RHDEV-33&&-56.
The TRH
that is explained above, is the latest TRH which uses
Regional Processor Group (RPG) hardware. The older hardware that uses Regional
Processor Device (RPD) hardware has only eight RHDEVs
per board, seven of which can be used. The LAPD protocol is explained further
in the A-bis chapter.
SUBRATE SWITCH (SRS)
Subrate
switching allows for the connection of rates lower than 64 kbit/s. The rates allowed are n*8 kbit/s (where n=1->7). An example of how the SRS can be
used to switch calls to different destinations using only one TRA resource is
illustrated in Figure 2-12. Four 64 kbit/s
timeslots that contain speech arrive at the BSC from the MSC. The TRH controls
the call set-up and determines whether the SRS should be used, which TRA should
be used, the call type, destination BTS, etc. The GS sets up connections to the
TRA which transcodes the four 64 kbit/s
channels into four 16 kbit/s. The 4x16 kbit/s channels are then multiplexed into one 64 kbit/s channel which is returned to the GS. In this
example, the destination of two of the calls is BTS1, and of the other two
calls, BTS2. The TRH has this information and determines that it is necessary
to set up a connection towards the SRS.
The SRS
switches the 16 kbit/s subrate
channels to two 64 kbit/s channels that are returned
to the GS. Hereafter, the GS can set up connections towards BTS1 and BTS2,
which contain the correct subrate channels.
The SRS is
required when TRA in pool is used. In addition, it is needed when utilizing
LAPD multiplexing, which occurs when the speech and signaling
towards the RBS is multiplexed onto the same 64 kbit/s.
This will be explained further in the A-bis chapter.
SIGNAL TERMINAL NO.7 (ST7)
The MSC
must have the ability to signal with the BSC. This is done using Signaling Terminals (ST). The signaling
devices are called, for example, C7ST2C for E1 PCM links. The signaling between the MSC and BSC is slightly different in
a T1 network, since T1 has a separate signaling
network. This means that there is no connection between the GS and the ST.
Generally, there are two signaling TSs between the BSC and MSC. Whereas one is sufficient for
all signaling, the second is installed for redundancy
purposes.
PROCESSORS (RP AND CP)
The RPs are designed to execute simple
high-repetition functions and are mainly used for the direct control of the
hardware units of the application systems. These hardware-units offer the
traffic devices of the exchange, for example, ETC, TRA. The CPs
execute complex and data demanding tasks. The standard RPs are called either
RPM6A or RP4, but there are a few other types.
Regional Processor Device (RPD)
The RPD
device hardware can supply TRH or C7 signaling and is
integrated with the RP.
Regional Processor Group (RPG)
The RPG
has the same functionality as the RPD, but it has higher capacity than the RPD.
The RPG, with different software loaded, can, in the BSC, serve as TRH, C7, or
STC terminal.
STAND ALONE TRC AND BSC
TRC
The Transcoder Controller (TRC) node contains the
pooled transcoder resources and is a stand alone node.
The TRC is
connected to the MSC via the A-interface and to the BSC via the A-Ter Interface. The TRC node has the ability to support up
to 16 BSCs over the A-ter
interface. The transcoders in the various TRAnscoder (TRA) pools in a TRC can be shared between all BSCs, associated with the TRC. One of the connected BSCs may be residing on the same physical platform as the
TRC, that is, in a combined BSC/TRC network element. One TRC can be connected
to up to four MSCs. This makes it possible to build
rather large TRCs supporting several MSCs. One BSC is still controlled by one specific MSC. The
TRC can contain several transcoder resource pools,
one pool per type of transcoder resource, for
example, Full Rate, Enhanced Full Rate, and Half Rate. The A-interface signaling remains unchanged in the new system structure.
For the communication between the TRC and a remote BSC, a C7 based Ericsson
proprietary communication protocol is used. Concerning a combined BSC/TRC,
internal signaling between the TRC and BSC part is
used.
The TRC
node handles the A-ter transmission interface
resources. The operation and maintenance signaling
and the handling of the A-ter interface are similar
to the current implementation on the A-interface. At call set-up and after signaling connection set-up, an assignment request is sent
via the MSC to the BSC. The request is sent directly to the BSC and can pass
transparently through the TRC. The BSC receives the assignment request and
requests a transcoder device from the TRC, also
indicating the Ainterface Circuit Identification
(CIC) to be used for this specific call. The TRC allocates a transcoder device and the time slot on the A-ter interface, which is connected to the A-interface CIC,
specified by the MSC. The TRC replies to the BSC, which establishes the
connection to the mobile.
BSC
The stand
alone BSC has been developed and optimized especially for rural and suburban
areas and is a complement to the BSC/TRC node in the BSC product portfolio. The
BSC contains the SRS and TRH, as previously explained. The BSC, however, does
not contain any transcoders. It utilizes transcoder resources from a central BSC/TRC, or from a TRC
node. The BSC is connected to the BSC/TRC, or the TRC via the A-ter interface.
PCM LINK DEVICE TYPES
Figure
2-15 illustrates
the different names of the PCM link devices in the three types of BSS
implementation.
