Leliwa Technical Bulletin
Date:
Date:
15.07.2009
Revision:
004/MIP/009
Author:
Jakub Bluszcz
1
Copyright ©2010 Leliwa. All Rights Reserved.
MSC in Pool
Leliwa Technical Bulletin
MSC in Pool
Table of contents
Topic
Page
Introduction.....................................................................................................3
Network Resource Identification .....................................................................5
Node Selection Function.................................................................................6
Mobility Management....................................................................................10
Acronyms and Abbreviations ........................................................................12
References ...................................................................................................13
Disclaimer.....................................................................................................14
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Introduction
The MSC in Pool, as a part of Intra Domain Connection of RAN Nodes to
Multiple CN Nodes solution, overcomes the strict hierarchy, which restricts
the connection of a BSC node to just one MSC. This restriction results from
routing mechanisms in the BSC which differentiate only between information
to be sent to the MSC (CS domain) or to the SGSN (PS domain) and which
do not differentiate between multiple CN nodes in each domain. The MSC in
Pool solution introduces a routing mechanism and other related functionality,
which enables the BSC to route information to different CN nodes within the
CS or PS domain, respectively.
MSC1
BSC1
BSC2
MSC2
BSC3
MSC3
BSC4
BSC5
BSC6
Figure 1 MSCs in Pool (logical view)
The MSC in Pool solution introduces further the concept of ‘pool-areas’ which
is enabled by the routing mechanism in the BSC. An MSC pool-area is
comparable to an MSC service area as a collection of one or more BSC
service areas. In difference to an MSC service area a pool-area is served by
multiple MSCs in parallel which share the traffic of this area between each
other. Furthermore, pool-areas may overlap which is not possible for the
MSC service areas. From a BSS perspective a pool-area comprises all LAs
of one or more BSC that are served by a certain group of MSC nodes in
parallel. One or more of the MSCs in this group may in addition serve LAs
outside this pool-area or may also serve other pool-areas. This group of
MSCs is also referred to as MSC pool.
The MSC in Pool enables a few different application scenarios with certain
characteristics. The service provision by multiple MSCs within a pool-area
enlarges the served area compared to the service area of one MSC. This
results in reduced inter MSC LA updates and it reduces the HLR update
traffic. The configuration of overlapping pool-areas allows to separate the
overall traffic into different MS moving pattern, e.g. pool-areas where each
covers a separate residential area and all the same city centre. Other
advantages of multiple MSCs in a pool-area are the possibility of capacity
upgrades by additional MSCs in the pool-area or the increased service
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MSC in Pool
availability as other MSCs may provide services in case one MSC in the poolarea fails.
MSC2
MSC7
MSC4
MSC1
MSC6
MSC3
MSC5
BSC3
BSC2
BSC4
BSC5
BSC8
BSC6
BSC1
Pool-area 1
BSC7
Pool-area 2
Pool-area 3
Figure 2 Pool area configuration example
An MS is served by one dedicated MSC node of a pool-area as long as it is in
radio coverage of the pool-area. Fig. 11-2 shows most of the possible poolarea configurations. It contains Pool-area 1 (BSC area 1, 2 and 3 served by
MSCs 1 and 2), Pool-area 2 (BSC area 4 and 5 served by MSCs 3 and 4) and
Pool-area 3 (BSC area 5, 6 and 7 served by MSCs 5, 6 and 7). In addition the
BSC areas 8 is served by MSC 7 without any usage of the MSCs in Pool
feature. The possibility to configure overlapping pool-areas is shown by the
Pool-areas 2 and 3. The Pool-area 1 is configured non-overlapping with any
other Pool-area. The number or capacity of MSCs is configured
independently for each pool-area. The usage of MSCs in Pool may be
configured in parts of the network only and can co-exists with other areas not
using this feature.
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Leliwa Technical Bulletin
Network Resource Identification
Identification
An MSC pool-area is an area within which an MS roams without a need to
change the serving MSC. A pool-area is served by one or more MSCs nodes
in parallel. The complete service area of a BSC belongs to the same one or
more pool-area(s). An BSC service area may belong to multiple pool-areas,
which is the case when multiple overlapping pool-areas include this BSC
service area. If LAs span over multiple BSC service areas then all these BSC
service areas have to belong to the same pool-area.
BSC3
MSC3
MSC2
BSC1
MSC1
BSC2
An MSC pool-area is an area within which an MS roams
without a need to change the serving MSC.
Figure 3 Pool-area definition
The Network Resource Identifier (NRI) identifies uniquely an individual MSC
out of all MSCs, which serve in parallel a pool-area. The length of the NRI is
the same in all MSCs in one pool-area. In areas where pool-areas overlap the
NRI identifies uniquely an MSC out of all MSCs, which serve all these
overlapping pool-areas, i.e. an NRI identifies uniquely an MSC within a BSC.
In case of overlapping pool-areas the NRI length is configured to be the same
in all the nodes serving these pool-areas. More than one NRI may be
assigned to an MSC.
The NRI is part of the TMSI, which is assigned by the serving MSC to the MS.
The TMSI allocation mechanism in the MSC generates TMSIs which contain
a configured NRI in the relevant bit positions. The NRI has a flexible length
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MSC in Pool
between 10 and 0 bits (0 bits means the NRI is not used and the feature is
not applied).
The NRI is coded in bits 23 to 14 of TMSI. Regardless of the NRI length the
most significant bit of the NRI is always in bit 23 of TMSI.
octet 4
octet 3
octet 2
octet 1
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
NRI
NRI - Network Resource Identification
Figure 4 Structure of TMSI
The BSC node derives the NRI from any initial signalling message. The BSC
masks the significant bits out of the TMSI to determine the NRI, which
identifies the MSC. It is configured in the BSC which bits out of TMSI
provided by the MS are significant for the NRI.
The change of a pool-area is not visible to the MS. In general there is no
need to detect a pool-area change. It may be advantageous for load
balancing purposes to detect pool-area changes in the network to distribute
MSs entering a pool-area to MSCs with an appropriate load status. MSs
changing a pool-area may be detected by configuration of different NRI
values for adjacent pool-areas.
Node Selection Function
This function is used in BSC and potentially in MSCs. In the BSC the function
selects the specific MSC to which initial signalling messages are routed. The
NRI identifies the specific MSC. If the Node Selection Function has an MSC
address configured for the NRI derived from the initial signalling message is
routed to this address. If no MSC address is configured for the derived NRI or
if no NRI can be derived (e.g. the MS indicated an identity which contains no
NRI) then the Node Selection Function selects an available MSC (e.g.
according to load balancing) and routes the message to that MSC.
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Leliwa Technical Bulletin
MSC1
(NRI=1)
TMSI (NRI=2)
BSC1
MSC2
(NRI=2)
TMSI (NRI=2)
BSC2
MSC3
Node Selection Function (NSF)
TMSI/NRI allocation
NRI routing
(NRI=3)
Figure 5 Use of NRI
In case an MSC/VLR sends a paging with IMSI (i.e. the paging message
does not contain a TMSI), the node selection function in the BSC upon
reception temporarily stores the identity of the MSC that issued the paging
message. If the node selection function receives a paging response with an
IMSI then it checks the temporarily stored identities of the MSCs on entries
matching this IMSI and forward the paging response to the node identified by
this identity.
Paging Req.
(IMSI)
MSC
Paging
pool
BSC
MSC
Res. (IMSI)
IMSI – MSC ID
Figure 6 Paging with IMSI
If the MSC/VLR initiates the paging procedure via Gs-interface the SGSN has
to add the MSC/VLR-identity to the paging message.
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MSC in Pool
MSC
pool
MSC
Paging Res. (IMSI)
Paging
BSC
Req. (IMSI)
Paging Req. (IMSI)
Paging
SGSN
(IMSI + MSC/VLRidentity)
Figure 7 CS paging via Gs interface
Load balancing
The Node Selection Function in the BSC balances the load between the
available MSCs. This is performed by an appropriate selection of the MSC for
an MS:
•
which was not yet assigned to an MSC, i.e. when there is no MSC
configured for the NRI indicated by the MS,
•
when no NRI can be derived,
•
in exceptional cases, e.g. when the MSC corresponding to an NRI
cannot be reached.
The load-balancing algorithm is implementation specific.
In case of handover into a pool-area a load balancing between all the target
MSC nodes serving this pool-area is gained by configuration. Source MSCs
which support MSC in Pool may be configured with all possible target MSCs
for each handover target. Source MSC which do not support MSC in Pool can
configure only one target MSC per handover target. In this case each of
source MSCs which handover to the same pool-area may be configured with
another target MSC out of all target MSCs serving the same handover target.
The mechanism for distribution of the traffic between the handover target
MSCs is implementation specific. This load balancing is complemented by
the Node Selection Function in the BSS, which distributes MSs between the
MSCs when these MSs enter the pool-area in idle mode.
Load ReRe-Distribution
There are situations where a network operator wishes to remove load from
one MSC in an orderly manner (e.g. to perform scheduled maintenance, or,
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Leliwa Technical Bulletin
to perform load re-distribution to avoid overload) with minimal impact to end
users and/or additional load on other entities. The re-distribution procedure
does not require any new functionality in the terminal, that is, all terminals can
be moved.
Re-distribution of MSs is initiated via an O&M command in the MSC, which
needs to be off-loaded.
In a first phase (a couple of Periodic LA Update periods long), MSs doing LA
Update are moved to other MSCs in the pool. When the MSC receives the,
LA Update request, it returns a new TMSI with a null-NRI, and a nonbroadcast LAI in the accept message. The non-broadcast LAI causes the MS
to immediately send a new Location Update, which the BSC node then routes
to a new MSC due to the null-NRI.
MSC
Null-NRI,
non-broadcast LAI
(NRI=1)
O&M
LA Upd. (periodic)
BSC
LA Upd. Accept ( )
MSC
(NRI=2)
LA Upd. (periodic)
MSC
(NRI=3)
Figure 8 Load Re-Distribution (phase 1)
A second phase includes scanning through remaining MSs and initiating a
move of them to other MSCs by allocating to these MS a new TMSI using the
TMSI re-allocation procedure (with null-NRI and non-broadcast LAI) so that a
Location Update is triggered when the ongoing CM transaction ends, which
will cause them to be moved.
Null-NRI,
non-broadcast LAI
TMSI re-allocation ( )
BSC
MSC
(NRI=2)
LA Upd.
MSC
(NRI=3)
Figure 9 Load Re-Distribution (phase 2)
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MSC in Pool
The MSs being moved from one MSC are stopped from registering to the
same MSC again by an O&M command in BSCs connected to the pool. The
MSs moving into a pool area are also stopped from registering into an MSC
being off-loaded in the same manner.
Mobility Management
An MS performs LA Updates, which may result in a change of the serving
MSC. In these procedures the new MSC requests from the old MSC MS
specific parameters. If multiple MSCs are configured in the new MSC for the
old LA indicated by the MS then the new MSC derives the NRI from the old
TMSI indicated by the MS. The new MSC uses the old LA together with the
NRI to derive the signalling address of the old MSC from its configuration
data.
old pool
MSC
(NRI=1)
MSC
(NRI=2)
LA Update Request
Send Identification /
Send Parameters
(TMSI, old LAI)
MSC
NRI & old LAI ► MSC/VLR number
Figure 10 MSC change (new MSC outside old pool)
If the network contains MSCs that cannot derive the old MSC from LAI and
NRI a default MSC for each LA is used to resolve the ambiguity of the
multiple MSCs serving the same area.
Default MSC and backwards compatibility
MSCs that can only derive one MSC from the LAI (e.g. because they do not
support the MSC in Pool feature, or no detailed knowledge of the NRIs is
configured) are not aware, that multiple MSCs may serve a LA. These nodes
can therefore contact only one MSC (a default MSC) per LA.
A default MSC resolves the ambiguity of the multiple MSCs per LA by
deriving the NRI from the TMSI. The default MSC relays the signalling
between the new MSC and the old MSC.
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Note that the default MSC is configured per LA. So different MSCs in a
network might have configured different default MSCs for a LA. With this
approach more than one of the MSCs that serve a pool-area can be used as
default MSC, so load concentration on one MSC and a single point of failure
can be avoided.
old pool
MSC
(default)
MSC
Send Identification
/ Send Parameters
LA Update Request
(TMSI, old RAI)
MSC
LAI ► default MSC number
Figure 12 Default MSC
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MSC in Pool
Acronyms and Abbreviations
BSC
BSC
BSS
CM
CN
CS
CS
HLR
IMSI
LA
LAI
MS
MSC
NRI
PS
RAI
RAN
SGSN
TMSI
VLR
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Base Station Controller
Base Station System
Connection Management
Core Network
Circuit Switching
Convergence Sublayer
Home Location Register
International Mobile Subscriber Identity
Location Area
Location Area Identity
Mobile Station
Mobile Switching Center
Network Resource Identifier
Packet Switch
Routing Area Identity
Radio Access Network
Serving GPRS Support Node
Temporary Mobile Subscriber Identity
Visitor Location Register
MSC in Pool
Leliwa Technical Bulletin
References
This section contains the locations of various specifications, document
references and useful information where you can learn more about this
subject.
[1]
23.236 Intra-domain connection of Radio Access Network (RAN)
nodes to multiple Core Network (CN) nodes
[2]
23.002 Network architecture
[3]
23.060 General Packet Radio Service (GPRS); Service description;
Stage 2
[4]
48.008 Mobile Switching Centre - Base Station system (MSC-BSS)
interface; Layer 3 specification
[5]
23.003 Numbering, addressing and identification
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MSC in Pool
Disclaimer
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assumes no responsibility for any errors that may appear in this document.
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