a-flex and a-signalling over ip in b11

8/12/2019 A-Flex and a-signalling Over IP in B11

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A-Flex and A-signalling over IP in Release B11

Alcatel-Lucent File Reference Date Edition PageFFUV7OE3.DOC 3DC 21144 0132 TQZZA 23/01/2009 03 1

All rights reserved. Passing on and copying of this document, use and communication of its contents not permitted without written authorization.

Functional Feature Description

A-Flex and A-Signalling over IP

In Release B11


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Contents

1. INTRODUCTION .................................................................................................. 31.1 Scope ...................................................................................................... 31.2 Rationale.................................................................................................. 41.3 References................................................................................................ 4

2. GENERAL DESCRIPTION......................................................................................... 52.1 End-to-End architecture................................................................................ 52.2 Feature Benefits......................................................................................... 5

3. A-FLEX DETAILED DESCRIPTION ............................................................................... 73.1 3GPP Standards.......................................................................................... 73.2 A-Flex concepts.......................................................................................... 7

3.2.1 CS pool area ....................................................................................73.2.2 Network Resource Identifier (NRI).......................................................... 8

3.3 Non Access Stratum (NAS) Node Selection Function .............................................. 93.3.1 MS Access ....................................................................................... 93.3.2 Network sending PAGING..................................................................... 9

3.4 Load Balancing function...............................................................................103.5 Load Re-Distribution function........................................................................10

4. A SIGNALLING OVER IP DETAILED DESCRIPTION ...........................................................114.1 Overview.................................................................................................11 4.2 Telecom flows and IP Endpoints.....................................................................124.3 Network Architecture for A signalling Over IP ....................................................124.4 Redundancy management.............................................................................14

5. OPERATION AND MAINTENANCE..............................................................................155.1 A-Flex Parameters......................................................................................155.2

A-Signalling over IP Configuration...................................................................16

6. HW COVERAGE AND DEPENDENCIES .........................................................................187. GLOSSARY........................................................................................................19


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1. INTRODUCTION1.1 ScopeThe present functional feature description provides detailed information about the features A-Flex

and A-signalling over IP:

15 13 24 A-Flex

15 13 22 A Signalling over IP

Both features are optionally introduced from the Alcatel-Lucent BSS release B11 onwards.

The description includes:

An overview of the features,

A description of the implementation within the BSC,

A description of end-to-end architecture between the BSS and the CS Core Network.

Preliminary notice

The information contained in this document is subject to change without notice.

Notice of proprietary information

This document contains proprietary technical information belonging to Alcatel-Lucent. By

accepting this material, the recipient agrees that this material will not be reproduced or used in

whole or part except as otherwise agreed between Alcatel-Lucent and the recipient.


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1.2 RationaleOn the core network side, Operators are transitioning to NGN architecture with separate

management of control plane (CP) and user plane (UP). The CP (i.e. signalling) is managed by the

MSC Server (MSC-S) while the UP (User traffic) is handled by the Media Gateway (MGW).

MSC Server can reach much higher capacity than legacy MSC, so the failure of an MSC Server can

have very important impacts on the network availability for a very high number of subscribers.

Therefore, the A-Flex feature is interesting for Operators migrating to NGN, as BSC can be

connected to several MSC Servers, which allows limiting capacity losses in case of MSC Server site

disaster.

The backbone of the NGN is based on IP technology mostly. The A-signalling over IP feature

extends the IP based transport on the control plane down to the BSC and supports the general

trend to IP based inter-connection layers. The feature provides a high flexibility for BSS to connect

to NGN and makes the introduction of the A-flex functionality much easier and future proof than

the combination of A-flex with TDM transport. This strategy avoids the introduction of several SS7-

MSC instances (one MTP3 instance per MSC, one MTP2 link set per MSC).

The A-signalling over IP feature is based on SIGTRAN protocol stacks already in use in the NGN

core. Therefore limited interoperability issues are expected between BSC and MSC Server. The A-

signalling over IP feature complements also the Alcatel-Lucent native IP transport in the BSS. Both

functionalities can be introduced independently.

1.3 References

[1] 3DC 21144 0130 TQZZA FFD: Native IP Transport in the BSS

[2] 3GPP TS 29.202 SS7 Signalling Transport in Core Network; Stage 3

[3] 3GPP TS 23.236 Intra-domain connection of Radio Access Network (RAN) nodes tomultiple Core Network (CN) nodes

[4] 3GPP TS 23.003 Numbering, addressing and identification

[5] 3GPP TS 24.008Mobile radio interface Layer 3 specification;

Core network protocols; Stage 3

[6] RFC 4666Signalling System 7 (SS7) Message Transfer Part 3 (MTP3) - User

Adaptation Layer (M3UA)

[7] RFC 2960 Stream Control Transmission Protocol (SCTP)


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2. GENERAL DESCRIPTION2.1 End-to-End architectureThe A-Flex feature belongs to the circuit switched domain. It allows the connection of a BSC to

several MSC Servers for the CS signalling. In the Alcatel-Lucent implementation, A-Flex relies on

the feature A-signalling over IP. The end-to-end architecture is shown in the figure below.

IP Backbone

Asignalling overIP

+

A-Flex

MSC server2

9130 BSC1

MSC server1

9130 BSC2

Media Gateways

9125 TC

User CS traffic over TDMUser CS traffic over TDM

Figure 1: Alcatel-Lucent A-Flex end-to-end architecture with A Signalling over IP

Note: It is not in the scope of the figure to show the exhaustive NGN connections. Also each BSC can be connected to one or several

MGW, without any impact on the BSS.

2.1.1 Feature BenefitsThe A-Flexfeature brings the following benefits:

Reduction of the signalling load in the core network: Signalling between the MSC/VLRand HLR due to Location Update and inter-MSC handover procedure become necessary only

when MS leave the CS pool area (see 3.2.1).

Better MSC resilience: with A-Flex feature a BSC can be connected to several MSC Servers

for the handling of A-signalling. As a consequence if an MSC Server fails the remaining MSC

Servers can take over new service requests and maintain the service availability.

Better signalling load balancing between MSC Servers.


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Easier Core network expansion: if the core network capacity needs to be increased,

there is no need any more to reconfigure the radio network. The existing LA/RA

configuration can be kept. It is sufficient to add a new MSC to the CS pool area with the

same radio configuration as the other MSCs of this CS pool area.

The benefits of A-signalling over IPfeature are:

Easier configuration of signalling links on A interface using Ethernet connectivity of Alcatel-

Lucent 9130 BSC evolution

Reduced transmission OPEX

Support of multiple SS7 end pointsfacilitating A-Flex configuration.


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3. A-FLEX DETAILED DESCRIPTION3.1 3GPP StandardsA-Flex has been standardized in 3GPP Rel-5 (3GPP TS 23.236 (Ref. [3])).

3.2 A-Flex concepts3.2.1 CS pool areaA CS pool area is an area within which a MS may roam without need to change the serving MSC

Server. A CS pool area is served by one or more MSC Servers in parallel.

MSC server2

MSC server1

MSC server3

MSC server5

MSC server4

MSC server6

Area 1

9130 BSC1

Area 2

9130 BSC2

Area 3

9130 BSC3

Area 5

9130 BSC5

Area 6

9130 BSC6

Area 7

9130 BSC7

CS pool area 1CS pool area 2

MSC server2MSC server2






Area 1

9130 BSC1

Area 1

9130 BSC19130 BSC1

Area 2

9130 BSC2

Area 2

9130 BSC29130 BSC2

Area 3

9130 BSC3

Area 3

9130 BSC39130 BSC3

Area 5

9130 BSC5

Area 5

9130 BSC59130 BSC5

Area 6

9130 BSC6

Area 6

9130 BSC69130 BSC6

Area 7

9130 BSC7

Area 7

9130 BSC79130 BSC7

CS pool area 1CS pool area 2

Figure 2: A-Flex network architecture

Figure 2 gives an example of CS pool-area configuration. It contains CS pool-area 1 (area 1, 2, 5, 6

served by MSC Servers 1, 2, 3), CS pool-areas 2 (area 2, 3, 6, 7 served by MSC Servers 4, 5, 6).


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All the cells controlled by a BSC belong to the same (one or more) CS pool area(s). For

example, all the cells controlled by BSC 1 belong to the CS pool-area 1. The possibility to

configure overlapping pool-areas is shown by the CS pool-areas 1 and 2: the BSC 2 and 6

belong to CS pool-areas 1 and 2.

The serving MSC Server is allocated by the BSC when the MS enters the CS pool-area (see 3.3 for

details). An MS is served by the same MSC Server of a CS pool-area as long as it remains in the

radio coverage of this CS pool-area.

Notes:

In the Alcatel-Lucent implementation, a BSC can be connected to up to 16 MSC.

If a physical MGW is connected to more than one MSC Server, the MGW is required to

support the feature Virtual MGW.

3.2.2 Network Resource Identifier (NRI)The Network Resource Identifier (NRI) identifies uniquely an individual MSC out of all MSC which

serve in parallel a CS pool-area. In areas where CS pool-areas overlap the NRI identifies uniquely a

MSC out of all MSC which serve all these overlapping pool-areas, i.e. an NRI identifies uniquely a

MSC within a BSC.

When A-Flex is used, the NRI is included in the TMSI. The TMSI consists of 4 octets as shown below:

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

CS/PS VLR-restart & TIME NRI

31 30 29 28 27 26 25 24 23 2 2 21 20 19 18 17 16 1 5 1 4 13 12 11 10 9 8 7 6 5 4 3 2 1 0

CS/PS VLR-restart & TIME NRI

3131 3030 2929 2828 2727 2626 2525 2424 232323 222222 212121 202020 191919 181818 171717 1616 1515 1414 1313 1212 1111 1010 99 88 77 66 55 44 33 22 11 00

CS/PSCS/PS VLR-restart & TIMEVLR-restart & TIME NRINRINRI

The standards allow Operators to define the NRI length and the NRI values.

The Operator defines the NRI length according to the number of MSC Servers in an MSC pool: each

MSC Server in the pool must have a unique NRI. The NRI has a configurable length of 0 to 10 bits (a

length of 0 bits indicates that the NRI is not used and A-Flex feature is not activated in the MSC).

The length of the NRI is the same in all MSC of a given CS pool area. In case of overlapping pool-

areas the NRI length is the same in all the MSC of related overlapping pool-areas. The bits of the

TMSI that are significant for the NRI are configured in the OMC-R. Once the NRI length set to n bits

for instance, the number of available NRI values is 2n-1 (one value is reserved for NULL_NRI).

It may happen that for a defined NRI length, a given MSC Server reaches TMSI shortage. In that

case, it is possible to define several NRI values for this MSC Server to increase the number of

available TMSI. In Alcatel-Lucent implementation, one MSC Server can handle up to 8 NRI.


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One uniqueNRIcalled NULL_NRIis reserved per network (i.e. per PLMN). This NRI is not linked to

any MSC Server. It is used by the load re-distribution procedures as explained in 3.5.

3.3 Non Access Stratum (NAS) Node Selection FunctionThe NAS Node Selection Function is a new routing function implemented in the BSC. Its purpose is

to assign a specific MSC Server to serve an MS and to route its signalling messages to this MSC

Server as long as the MS remains in the same CS pool-area.

The NAS Node Selection Function examines the content of the Layer 3 message (e.g. 24.008 CM

SERVICE REQUEST) coming from MS or MSC Servers.

3.3.1 MS AccessAn MS may access the network to send Layer 3 messages such as: PAGING RESPONSE; LOCATION

UPDATING REQUEST; IMSI ATTACH/DETACH INDICATION; CM SERVICE REQUEST; CM Re-

Establishment Request; etc.

When an MS has already been assigned a TMSI by the Core Network, the MS sends Layer 3 messages

with TMSI included. The BSC derives the NRI from the TMSI and routes the Layer 3 messages

according to the NRI to the relevant MSC. The association between NRI values and MSC addresses

(MSC Signalling Point Code) is configured in the BSC.

When the BSC receives a message with a NULL_NRI it uses the load balancing function for re-

routing the received message to one of the MSC servers connected to the BSC.

If the NRI derived from the TMSI is unknown by the BSC, it uses the load balancing function to

select an MSC Server.

The MSC servers being off loaded (see 3.4) are excluded from the load balancing by the BSC.

3.3.2 Network sending PAGINGThe network may send PAGING Layer 3 messages to an MS with its IMSI included.

There are two cases:

If PAGING (with only IMSI) is received from the MSC Server via A-interface, then the BSC

saves the MSC address (MSC Signalling Point Code MSC SPC) and IMSI.

If CS PAGING (with only IMSI) is received from the SGSN via Gs-interface, then the BSS

saves MSC address (MSC_CN_ID) and IMSI.


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When the BSC receives a PAGING RESPONSE (with only IMSI) from the MS, the BSC checks its

internal mapping table (IMSI/SPC; or IMSI/MSC_CN_ID) to find the MSC address corresponding to

the IMSI. If no such MSC is found, the BSC uses the load balancing function to assign an MSC Server

to the MS.

If there is neither TMSI nor IMSI included, the BSC selects an MSC Server using the load balancing

function.

3.4 Load Balancing functionWhen the BSC does not know to which MSC Server an MS is assigned, it chooses an MSC Server with

the target to keep a fair load balancing among all its connected MSC Server.

In the Alcatel-Lucent implementation, each MSC Server is assigned a weight, configurable from

OMC-R (MSC_WEIGHTparameter). The different MSC Servers connected to the BSC are selected

proportionally to their weight. For instance, an MSC Server with a weight 10 is selected twice

more often than an MSC Server with a weight 5.

A MSC Server is not selected in the following cases:

The MSC Server is not operational; or

The MSC Server is off loaded (i.e. a parameter configurable from the OMC-R

(MSC_OFFLOAD_STATE) allows the Operator to set an MSC-S to off loaded or on loaded.

This is used for load re-distribution function as explain in 3.5); or The MSC Server is on loaded & operational & overloaded, and there is at least one MSC

Server connected to the BSC who is on loaded & operational & normal load.

3.5 Load Re-Distribution functionThis function allows the Operator to remove load from one MSC Server (this means preventing this

MSC Server to handle new calls), in order to remove definitively an MSC Server from an MSC pool

for instance. The Operator triggers the Load Re-Distribution function by O&M actions both on BSC

and MSC.

From a BSS perspective, the Operator sets the MSC_OFFLOAD_STATEparameter of the MSC Server

to off loaded from the OMC-R.

During the Load Re-distribution, the MSC Server being off-loaded sends messages with new TMSI

with NULL_NRI embedded. Upon reception of its new TMSI, the MS sends message with NULL_NRI

to the BSC. The BSC then uses the load balancing function to assign a new MSC Server to the MS.

When the MSC Server is free of traffic (i.e. all MS have been reallocated to other MSC Servers in

the MSC pool), the Operator can safely remove it.


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4. A SIGNALLING OVER IP DETAILED DESCRIPTION4.1 OverviewThe purpose of A-signalling over IP is to replace the legacy SS7 network by an IP network for the

transport of signalling on A interface, between the BSC and the MSC Server. Figure 3 illustrates the

usage of A-signalling over IP (with the example of a TDM based BSS and A user plan over TDM).

The A-signalling over IP feature also applies to BSS network where IP transport in BSS is activated.

M3UAM3UA

SCCPBSC

Eth.

Eth.

A itf

TDM MSC Server

Eth.

TDM

IP backbone

User Plane

A-Signalling over IP

M3UASCCP M3UA SCCP

MGWTC

Figure 3 TDM BSS with A-signalling over IP

The protocol stack to transfer BSSAP messages over IP is shown in Figure 4.

BSSAP

SCCP

M3UA

SCTP

BSSAP

SCCP

M3UA

SCTP

IP

BSS MSC

Figure 4: Protocol Stack for A Signalling Over IP

A signalling over IP uses the Alcatel-Lucent 9130 BSC Ethernet connections.


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4.2 Telecom flows and IP EndpointsIn the BSC, the IP endpoints for A signalling over IP are managed by the OMCP.

IP Backbone

BSC

OMCP

SSW

A signalling over IP

MSC ServerMSC Server

Figure 5: Telecom flow and IP endpoints for A signaling over IP

Note: In the case of A-signalling over IP, the O&M flow between the BSC and the OMC-R cannot be

conveyed on timeslots of Ater interface. Like for the A-signalling flow, the Ethernet connectivity of the

BSC is used.

4.3 Network Architecture for A signalling Over IPA-signalling over IP introduces the following new layers below the SCCP layer:

Message Transfer Part 3 User Adaptation (M3UA) [6]. Implemented at the same level as

MTP3, M3UA defines a protocol for supporting the transport of any SS7 MTP3 user signalling

(e.g. ISUP or SCCP messages) and provides the equivalent set of primitives as used by MTP3

to its local MTP3 users at an SS7 signalling endpoint.

The Stream Control Transport Protocol (SCTP) [7] is an IP transport protocol

implemented at the same level than TCP or UDP. The basic service offered by SCTP is a

reliable transfer of user messages between peer SCTP users (i.e. signalling messages

between BSC and MSC Server). SCTP is connection oriented. It establishes first a

connection (called SCTP association) between two endpoints before transmitting data.

M3UA elements definition:

An Application Server (AS)can be a BSC or an MSC Server.


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TheIP Server Process (IPSP) is the physical entity managing the SCTP associations. One AS can

have one or several IPSP.

The SCTP Association is an association established between two IPSP belonging to different AS

(i.e. BSC and MSC Server).

There is one IPSP in the BSC side per MSC. A BSC can be connected to more than one MSC Server.

Each MSC Server can have more than one IPSP to handle a given BSC. On BSC side, all the IPSP have

the same IP address. Different port numbers distinguishes the IPSP. Figure 6 illustrates the

connections between the BSC and the MSC Server.

Figure 6: Connections between BSC and MSC Server

The way the signalling is transferred on the IPSP depends on the transport mode of the AS. The

possible transport modes used by one AS are defined by TRAFFIC_MODEparameter and can

be:

o Broadcast mode: the IPSP receives the same messages as any other currently

active IPSPs.


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o Override mode: only one of the IPSPs takes the traffic whatever the number of

IPSPs.

o Load-Sharing mode: the IPSP shares the traffic with any other currently active

IPSPs. The parameter MIN_NB_ACTIVE_IPSP defines the minimum number of active

IPSPs in the MSC Server required to handle the traffic with that MSC in load-sharing

mode.

4.4 Redundancy managementThe BSC can be connected to two routers connected with VRRP (Virtual Router Redundancy

Protocol). The BSC has two switch boards, each of which is connected to one router. There is only

one external IP address for the BSC. In case of router failure, there is a take over by the other

router. The IPSP also benefits from the internal redundant BSC architecture, so that the IPSP is

always on the active OMCP board (if an OMCP fails, the IPSP is moved to the stand-by OMCP). Please

refer to [1] for more details.

When the MSC SCTP endpoint has more than one IP address, multi-homing can be applied on MSC

side. Alcatel-Lucent BSC can make use of MSC Server multi homing capability to establish a SCTP

association using two different IP addresses at MSC Server side. Only one path (i.e. one IP address) is

active at a time. SCTP supervises both paths. When the active path fails, SCTP switches path

without breaking the SCTP association.


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5. OPERATION AND MAINTENANCE5.1 A-Flex Parameters

Parameter name Definition Instance Range

EN_A_FLEX Enables or disables the A-Flex

feature.

BSC 0: disabled

1: enabled

MSC_CN_ID Core Network identifier:

PLMN-Id + CN-Id.

MSC 0 to 4095

MSC_SPC MSC Signalling Point Code. MSC 0 to 16383

MSC_WEIGHT Weight assigned to the MSC for load

balancing function.

MSC 1 to 255

NRI_LIST List of NRI. MSC 0 to 1024

NULL_NRI Please refer to 3.2.2 BSC 0 to 1023

NRI_LENGTH Length of NRI inside TMSI. BSC 1 to 10

MSC_OFFLOAD_STATE State assigned to a given MSC,

connected to the BSC, for the load

redistribution function.

MSC 0: off loaded

1: on loaded


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5.2 A-Signalling over IP Configuration

Parameter name Definition Instance Range

EN_ASIG_OVER_IP Enables or disables the A Signalling

Over IP feature.

BSC 0: disabled

1: enabled

TRAFFIC_MODE MSC traffic mode.

override mode (i.e. only one of the

IPSPs handle the traffic)

broadcast mode (i.e. all the active

IPSPs receive the same messages)

load sharing mode (i.e. the traffic isdistributed over all the active IPSPs)

MSC 0: override mode

1: broadcast

mode

2: loadsharing

mode

MIN_NB_ACTIVE_IPSP The minimum number of active IP

server process (IPSP) in the MSC

server required to handle the traffic

with that MSC. (Significant when MSC

is in load sharing mode)

MSC 1 to 4

LOCAL_ASIG_SCTP_END

POINT_IP_Address_1

Local primary IP adress of a SCTP

endpoint (used by M3UA protocol) of

the BSC.

BSC 0 to 4294967295

LOCAL_ASIG_SCTP_END

POINT_PORT

Local TCP port of a SCTP endpoint

(used by M3UA protocol) of the BSC.

BSC 61953 to 61999

ASIG_SCTP_ENDPOINT_L

IST

List of the SCTP endpoints (used by

M3UA protocol) of the MSC.Eachentry is an SCTP endpoint, composed

of

ASIG_SCTP_ENDPOINT_IP_Address_1,

ASIG_SCTP_ENDPOINT_PORT and

ASIG_SCTP_ENDPOINT_IP_Address_2

MSC 0 to 4294967295


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ASIG_SCTP_ENDPOINT_I

P_Address_1

Primary IP address of a SCTP endpoint

(used by M3UA protocol) of the MSC.

SCTP

EndPoint

0 to 4294967295

ASIG_SCTP_ENDPOINT_I

P_Address_2

Secondary IP address of a SCTP

endpoint (used by M3UA protocol) of

the MSC.

It is an optional address used in case

of SCTP multi Homing in MSC side.

SCTP

EndPoint

0 to 4294967295

ASIG_SCTP_ENDPOINT_

PORT

Port of a SCTP endpoint (used by

M3UA protocol) of the MSC.

SCTPEndPoint

0 to 65535


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6. HW COVERAGE AND DEPENDENCIESThe A-Flex and A-signalling over IP features are supported on 9130 BSC evolution. They are not

supported on 9120 BSC G2. They have impacts on the Core Network.

A-Flex is available with A-signalling over IP. A-Flex over TDM is not supported.


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Alcatel-Lucent File Reference Date Edition PageFFUV7OE3 DOC 3DC 21144 0132 TQZZA 23/01/2009 03 19

7. GLOSSARY

End of Document

BSS Base Station Subsystem

CIC (A interface) Circuit Identification Code

CN Core Network

CNE Core Network Element (MSC server, SGSN, etc)

GPRS General Packet Radio Service

GSM Global System for Mobile communications

MFS Multi-BSS Fast packet Server

NAS Non Access Stratum

NASNSF Non Access Stratum Node Selection FunctionNGN Next Generation Network

NRI Network Resource Identifier

OPC Originating Point Code (ITU-T Q.701 - Q.704)

SCCP Signalling Connection Control Part

SCTP Stream Control Transmission Protocol (RFC 2960)

SGSN Service GPRS Support Node

SIGTRAN SIGnalling TRANsport

SPC Signalling Point Code (ITU-T Q.701 - Q.704)

STP Signalling Transfer Point

TC TransCoder

a-flex and a-signalling over ip in b11

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