11 MSC in pool
Chapter 11 MSC in Pool Topic
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Introduction .................................................................................................... 281 Network Resource Identification ................................................................... 283 Node Selection Function ................................................................................ 284 Mobility Management .................................................................................... 288
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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 11-1 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 281
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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 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
BSC7
BSC8
BSC6
BSC1 Pool-area 1
Pool-area 2
Pool-area 3
Figure 11-2 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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Network Resource 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 11-3 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.
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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 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 11-4 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 poolarea 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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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 11-5 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
pool
BSC Paging Res. (IMSI)
MSC
IMSI – MSC ID
Figure 11-6 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
pool MSC
Paging Res. (IMSI) Paging Req. (IMSI)
BSC
Paging Req. (IMSI)
Paging (IMSI + MSC/VLRidentity)
SGSN
Figure 11-7 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.
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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, 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.
Null-NRI, non-broadcast LAI
MSC (NRI=1) O&M
LA Upd. (periodic) LA Upd. Accept ( )
BSC
MSC (NRI=2)
LA Upd. (periodic)
MSC (NRI=3)
Figure 11-8 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.
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Null-NRI, non-broadcast LAI TMSI re-allocation ( )
BSC
MSC (NRI=2)
LA Upd.
MSC (NRI=3)
Figure 11-9 Load Re-Distribution (phase 2) 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 (TMSI, old LAI)
Send Identification / Send Parameters
MSC NRI & old LAI â–ş MSC/VLR number
Figure 11-10 MSC change (new MSC outside old pool)
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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. 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 â–ş default MSC number
Figure 11-12 Default MSC
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