Paging Procedures in UMTS

Introduction

Paging is a message which will be received by the User Equipment (UE) from Network.

The Paging can be either UTRAN initiated or Core Network initiated.

Following topics are covered in this post.

• Purpose of paging procedure
• Types of paging message in UMTS
• Paging Message flow from Core network to RNC to UE

Purpose of Paging Procedure

The paging procedure can be initiated from the network to inform to the User equipment (UE) through the Paging message about the following.

• Incoming call/Incoming SMS from CS domain or PS domain
• Change of BCCH information
• For releasing the RRC Connection
• ETWS information.

The Core network originated Paging message will only be for informing the UE about Incoming Call or Incoming SMS either from CS or PS Domain.

UTRAN originated paging message will be for informing the UE about the change of BCCH information and also for ETWS information. Additionally, UTRAN will initiate paging for releasing the RRC connection when the UE is in CELL_PCH or URA PCH State.

Types of Paging Messages in UMTS

There are two kinds of paging messages in UMTS, Paging Type 1 and Paging type 2.

Paging Type 1 

This message will be sent to the UE, when UE is either in Idle State, Cell PCH State or URA PCH State. This message will contain optional fields which were used to inform to the UE about the change of BCCH modification information and also about the ETWS information.

This message will have an identity of the UE, in order to make the UE to differentiate the paging for its own SIM and paging for other SIMs, since paging message for many UE’s in the cell can also be configured in the same paging occasion.

Paging type 2

This message will be sent to the UE, when UE is either in CELL DCH State, Cell FACH State or Cell PCH state((FDD and 1.28 Mcps TDD only). This message will not contain any identity value of the UE, since this message is sent only to a particular UE, instead this message will contain the type of the identity. Paging type 2 message is called as Dedicated mode Paging.

Both Paging type 1 and Paging type 2 Messages will contain Core Network domain information in the case of Core Network originated paging message, based on which the UE’s upper layers will respond with the paging response for the particular Core Network domain.

Paging Message flow from Core network to RNC to UE

In the case of Core Network originated paging, Core network sends the Paging message to RNC with the Permanent NAS UE Identity, Temporary NAS UE identity and other information elements. The permanent NAS UE identity is nothing but IMSI of the UE and Temporary NAS UE identity may be TMSI or P-TMSI.

The purpose of adding the Permanent NAS UE identity is to make RNC to decide whether to send Paging type 1 or Paging type 2.

When RNC receives the Paging message from Core network, RNC checks whether any existing RRC Connection is available mapped with the IMSI received in the Paging message, if the RRC Connection mapped with the IMSI is available, then trigger Paging type 2 Message through the existing RRC Connection, otherwise RNC triggers paging type 1 message through the PCCH.

When UE receives Paging type 1 or paging type 2 message, depending on the paging cause and type of the CN domain, UE NAS triggers Paging response to the corresponding core network domain.  

Message to Readers : If you have any doubts regarding the paging procedure in UMTS, lets discuss the same through this post.

Reference : 3GPP Specification 25.331, 25.413