6 Signalling Protocols and Interfaces

3GPP43.059Functional stage 2 description of Location Services (LCS) in GERANRelease 16TS

6.1 Protocol layering in A/Gb mode

6.1.1 Generic Signalling Model

Figure 4 shows the generic signalling model applicable to LCS for signalling interaction in which an SMLC forms at least one of the signalling end points.

Figure 4: Generic Model for LCS Signalling to an SMLC

The functions performed by each protocol layer are as follows:

a) LCS application protocol – this depends on the other signalling end point (e.g. whether a target MS or LMU) and may be absent if supported in the BSSAP-LE layer. The application protocol supports specific LCS functions (e.g. positioning measurements, assistance measurements) and is independent of lower protocol layers.

b) BSSAP-LE – this is an extension of BSSAP and carries the LCS application protocol signaling units. Necessary functions include identification of the LCS application protocol and identification, where not provided by the network layer, of the two end points. This layer can be relayed by an intermediate entity or mapped into an equivalent layer 3 protocol used by the other signaling end point. This layer supports segmentation of LCS application layer protocols.

c) Network Layer – provides signaling transport between the SMLC and either the other end point or some intermediate entity at which the BSSAP-LE layer is relayed or mapped. The network layer may support connection oriented or connectionless signaling. For second generation circuit oriented applications, the network layer is provided using either MTP or M3UA/SCTP and SCCP. This layer supports segmentation of LCS application layer protocols.

d) Physical Layer – When MTP is used as transport protocol, SS7 signaling links are supported by the physical layer. It is recommended that when IP transport (via M3UA/SCTP) is used, the data link layer is implemented using Ethernet. A node using IP transport having interfaces connected via low bandwidth PPP links like E1/T1 shall also support IP Header Compression [37] and the PPP extensions ML/MC-PPP [38], [39]. In this case, the negotiation of header compression [37] over PPP shall be performed according to [40].

e) L3 – a protocol layer compatible with or the same as BSSAP-LE.

f) L2 – logical link layer for the other endpoint

g) L1 – physical layer for the other end point.

6.1.2 Message Segmentation in A/Gb mode

Message segmentation is needed to transport any large LCS message that exceeds the message size limitation supported by any GSM interface over which transport is needed.

6.1.2.1 Network Level Segmentation

Segmentation and reassembly of large SMLCPP messages at the network (e.g. SCCP) level may be supported. For message transfer over any interface where network level segmentation is not supported, segmentation at the application level shall be used. This may require support of both network and intermediate level segmentation by certain intermediate entities.

6.1.2.2 Intermediate Level Segmentation

The segmentation of SMLCPP 3GPP TS 48.031[19], messages is supported by segmentation mechanisms defined in 3GPP TS 48.008 [18], and 3GPP TS 49.031[22]. The sending, receiving and all intermediate entities supporting segmentation shall ensure reliable and sequenced delivery of the message segments by appropriate use of the capabilities supported by lower transport and network level protocols.

For support of legacy (Rel4 and older) MS and GERAN, there are segmentation mechanisms defined in the RR layer (3GPP TS 44.018 [14]) and BSSAP-LE layer (3GPP TS 49.031 [22]) for segmentation of RRLP (3GPP TS 44.031) messages, which need to be supported by the MS, BSS, and SMLC in the uplink direction in the CS domain and by the MS and BSS for the downlink direction in the CS domain.

6.1.2.3 RRLP Pseudo-Segmentation

The use of several RRLP messages to deliver a large amount of information is called "RRLP pseudo-segmentation". If more information than what fits in the RRLP maximum PDU size needs to be delivered, the RRLP pseudo-segmentation shall be used. (For Rel 4 or older MS and GERAN, may also use intermediate level segmentation).

6.1.3 Signalling between an SMLC, MSC and BSC

An SMLC can either be separate logical entity or integrated functionality in the BSC. If the SMLC is a separate logical entity, the LCS signalling between SMLC and MSC is accomplished through the A and Lb interfaces. If the SMLC is integrated, the LCS signalling is accomplished through the A interface only. Figure 5 shows the protocol layers used to support LCS signaling between the SMLC, MSC and BSC.

Figure 5: Signalling Protocols between SMLC, MSC and BSC

6.1.3a Signalling between an SMLC, SGSN and BSS

An SMLC can either be separate logical entity or integrated functionality in the BSS. If the SMLC is a separate logical entity, the LCS signalling between SMLC and SGSN is accomplished through the Gb and Lb interfaces. If the SMLC is integrated, the LCS signalling is accomplished through the Gb interface only. Figure 5a shows the protocol layers used to support LCS signaling between the SMLC, SGSN and BSS. Notice that the Network Service layer may be based on frame relay or IP (see 3GPP TS 48.016 [24]).

Figure 5a: Signalling Protocols between SMLC, SGSN and BSS

6.1.4 Signaling between SMLC and MS

SMLC Signalling to a target MS is accomplished through the Um interface. Figure 6 shows the protocol layers used to support signaling between an SMLC and target MS in the CS domain.

Figure 6: Signalling between an SMLC and Target MS in CS domain

Figure 6a shows the protocol layers used to support signaling between an SMLC and target MS in the PS domain. The signalling is routed through the core network to utilize standard GPRS functions. BSSGP is specified in 3GPP TS 48.018 [25] and RLC/MAC is specified in 3GPP TS 44.060 [28]. The TOM and LLC protocols are specified in 3GPP TS 44.064 [26]. The TOM Protocol Header for RRLP is specified in 44.031 [15]. For an overview of GPRS, see 3GPP TS 23.060 [27].

NOTE*: The network layer for the Gb interface may be based on IP or frame relay.

Figure 6a: Signalling between an SMLC and Target MS in PS domain

6.1.5 SMLC Signalling to a Type A LMU

Notice that the signalling to a Type A LMU is using CS signalling over the Um interface. A Type A LMU can be used to support positioning of mobile stations both in the CS and the PS domain.

6.1.5.1 Circuit Switched Signalling using an SDCCH

Figure 7 shows the protocol layers used to support signaling between an SMLC and a Type A LMU, using an SDCCH on the Um interface.

Figure 7: Signalling between an SMLC and Type A LMU using an SDCCH

6.1.5.2 Signalling using a TCH

Figure 8 shows the protocol layers that can be used to support signaling between an SMLC and a Type A LMU using a TCH on the Um interface. The TCH is assumed to support either transparent or non-transparent synchronous data and may be provided in a multislot configuration. The main usage would be for O&M data and SW download – e.g. during offpeak hours.

Figure 8: Signalling between an SMLC and a Type A LMU using a TCH

6.1.6 SMLC signalling to a Type B LMU

The protocol layers employed to enable signaling between the SMLC and a Type B LMU are shown in figure 9. Notice that the signalling to a Type B LMU can be used to support positioning of mobile stations both in the CS and the PS domain.

Figure 9: Signalling between an SMLC and Type B LMU

NOTE*: Abis interface is beyond the scope of the present document.

6.1.7 SMLC Signalling to a peer SMLC

The protocol layers used for SMLC to SMLC signalling are shown in figure 10, where it is assumed that both SMLCs have connections to an intermediary signalling network (or there is a direct link between the SMLCs).

Figure 10: SMLC to SMLC Signalling via an intermediary signalling network

In the absence of either a direct link or links to an intermediary signalling network, signalling can go via attached BSCs and MSCs as shown in figure 11 for signalling between the SMLCs sharing the same MSC.

Figure 11: SMLC to SMLC Signalling via associated BSCs and MSC

6.2 Protocol layering in Iu mode

6.2.1 Signalling between SMLC and MSC/SGSN

A SMLC can either be a separate logical entity or integrated functionality in the BSC. If the SMLC is a separate logical entity, the LCS signalling between SMLC and MSC/SGSN is accomplished through the Iu and Lb interfaces. If the SMLC is integrated, the LCS signalling is accomplished through the Iu interface only. Figure 11a shows the protocol layers used to support LCS signaling between the SMLC, BSC and MSC/SGSN.

NOTE: * Iu protocol stack is inherited from UMTS specifications [29-32]. See GERAN Stage 2 specification [11] for additional transport layer options.

Figure 11a: Signalling Protocols between SMLC, BSC and MSC/SGSN

6.2.2 Signalling between SMLC and MS

SMLC Signalling to a target MS is accomplished through the Um interface. Figure 11b shows the protocol layers used to support signaling between an SMLC and target MS.

Figure 11b: Signalling between an SMLC and Target MS

6.2.3 SMLC signalling to a Type A LMU

The signalling to a Type A LMU is defined using A/Gb Mode CS signalling over the Um interface, see Clause 6.1.5. A Type A LMU operating in A/Gb Mode CS Domain can be used to support positioning of mobile stations operating in Iu Mode. Signalling to a Type A LMU in Iu Mode is FFS.

6.2.4 SMLC signalling to a Type B LMU

The signalling to a Type B LMU is defined in Clause 6.1.6. A Type B LMU operates independently of the mode of the MS (A/Gb Mode or Iu Mode) and signalling to a Type B LMU as defined in Clause 6.1.6 can be used to support positioning of mobile stations operating in Iu Mode.

6.2.5 SMLC signalling to a peer SMLC

The signalling to a peer SMLC is defined in Clause 6.1.7. A peer SMLC connection and signaling is independent of the mode of the MS (A/Gb Mode or Iu Mode) and signalling to a peer SMLC as defined in Clause 6.1.7 can be used to support positioning of mobile stations operating in Iu Mode.