SRNS Relocation in UMTS Network

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SRNS relocation is a procedure used during mobility scenarios when Control of the Serving Radio Network Subsystem (SRNS) is changed to another Radio Network Subsystem (RNS)

image 1 SRNS Relocation in UMTS Network

Radio Network Subsystem (RNS): The RNS controls the allocation and release of radio resources to establish a connection between a UE and the UTRAN.

A single UE may be associated with one or more Radio Access Bearers (RABs). For instance a UE may simultaneously use one RAB for voice and one RAB for a packet call.

Radio Access Bearer (RAB): RAB is a logical connection between the UE and the UMSC/SGSN.

Iu Bearer: The connection between the RNC and the Core network is referred to as the Iu bearer.

Radio Bearer (RB): The connection between the RNC and the UE is referred to as the Radio Bearer (RB).

image 2 SRNS Relocation in UMTS Network

During Hard Handover when the UE moves away from the area that is covered by one RNS to a new RNS, there is a requirement to perform an inter-RNC handover between the Serving RNC (S-RNC) to the Target RNC (RNCT) of the new RNS.

There are two ways to change the Serving RNC:

  • One possibility is that the Target RNC may immediately become the Serving RNC referred as the Combined Hard Handover and SRNS Relocation Procedure
  • In the other case known as the Serving RNS Relocation Procedure the user plane from the S-RNC extends to the Target RNC, in this situation the Target RNC is referred as the Drift RNC. The Interface between the Serving and the Drift RNC is the Iur interface described in the 3GPP TS 25.420.

NOTE: In either case the Serving RNC initiates the SRNS relocation procedure.

Relocation Procedure

image 3 SRNS Relocation in UMTS Network

RELOCATION REQUIRED RELOCATION REQUEST
Information Element Information Element
  • Message Type
  • Relocation Type UE involved/UE not involved
  • Cause
  • Source ID
  • Target ID
  • Source RNC to Target RNC transparent Container
  • Message Type
  • Relocation Type UE involved/UE not involved
  • Cause
  • Source RNC to Target RNC transparent Container
  RAB to be Setup
 
  • RAB ID
  • RAB Parameters
  • User Plane Mode
  • Transport Address
  • Iu Transport Association RAB linking

 

Serving RNS Relocation Procedure

This procedure is only performed for a UE in CONNECTED state where the Iur interface carries both the control signaling and the user data.

The Serving SRNS Relocation procedure is used to move the connection between the RAN and the CN for the source SRNC to the RAN for the target RNC, from a "standing still position". In the procedure, the Iu links are relocated.

If the target RNC is connected to the same SGSN as the source SRNC, an Intra-SGSN SRNS Relocation procedure is performed.

 

If the routing area is changed, this procedure is followed by an Intra-SGSN Routing Area Update procedure. The SGSN detects an Intra-SGSN routing area update by noticing that it also handles the old RA. In this case, the SGSN has the necessary information about the UE and there is no need to inform the HLR about new location of the UE.

image4 SRNS Relocation in UMTS Network

 

Before the SRNS Relocation procedure and RA update, the UE is registered in the old SGSN. The source RNC is acting as a serving RNC (SRNC).

Serving SRNS Relocation procedure Signalling
image5 SRNS Relocation in UMTS Network

NOTE: The sequence is valid for both intra-SGSN SRNS relocation and inter-SGSN SRNS relocation.

image6 SRNS Relocation in UMTS Network

After the SRNS Relocation procedure and RA update, the UE is registered in the new SGSN. The UE is in the state CONNECTED Mode towards the new SGSN, and the target RNC is acting as the serving RNC.

Combined Hard Handover and SRNS Relocation Procedure

This procedure is only performed for an UE in CONNECTED state in case the Iur interface is not available.

The Combined Hard Handover and SRNS Relocation procedure is used to move the connection between the RAN and the CN for the source SRNC to the RAN for the target RNC, while performing a hard handover decided by the RAN.

In the procedure, the Iu links are relocated.

If the target RNC is connected to the same SGSN as the source SRNC, an Intra-SGSN SRNS Relocation procedure is performed.

If the routing area is changed, this procedure is followed by an Intra-SGSN Routing Area Update procedure. The SGSN detects that it is an intra-SGSN routing area update by noticing that it also handles the old RA. In this case, the SGSN has the necessary information about the UE and there is no need to inform the HLR about the new UE location.

If the target RNC is connected to a different SGSN than the source SRNC, an Inter-SGSN SRNS Relocation procedure is performed. This procedure is followed by an Inter-SGSN Routing Area Update procedure.

image7 SRNS Relocation in UMTS Network

Before the SRNS Relocation and Routing Area Update the UE is registered in the old SGSN and in the old MSC/VLR. The source RNC is acting as serving RNC.

Combined Hard Handover and SRNS Relocation Procedure Signalling
image8 SRNS Relocation in UMTS Network

The sequence is valid for both intra-SGSN SRNS relocation and inter-SGSN SRNS relocation. Furthermore, this signaling flow is also applicable for BSS to RNS relocation and vice-versa, as well as BSS to BSS relocation.

image9 SRNS Relocation in UMTS Network

 

After the SRNS relocation and RA update, the UE is registered in the new SGSN and in the new MSC/VLR. The UE is in CONNECTED mode towards the new SGSN and in MM IDLE state towards the new MSC/VLR. The target RNC is acting as serving RNC.

References:

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Naveed Mumtaz

Network Vendor Coordinator at ST Ericsson in Sweden.