介绍 SMS over SGs

LTE在3GPPrelease 8中被定义为新一代无线通信标准, LTE是一个单纯的分组交换系统，不支持传统的电路交换业务，因此LTE只能以Voiceover IP的方式来提供语音业务。

从长远来看，基于IMS技术为LTE用户提供语音和短信业务是发展的方向。2010年2月，GSMA宣布发起一个VoLTE产业联盟，目前已经完成了LTE提供基于IMS的最优语音和短信业务的标准定义，包括漫游和互通性规范。
IMS（IP Multimedia Subsystem）是LTE支持语音业务的关键技术, IMS最早在3GPP release 5提出，它提供了一个框架来支持基于IP的业务。但是，由于IMS的建设大大落后于LTE，如何在LTE网络部署语音业务成为运营商面临的一大挑战，同时导致一些过渡的替代技术方案的出现，比如CSFB（circiut switched fallback），SMS over SGs。
在CSFB方案中，处于LTE网络覆盖下的用户想要发起一个语音呼叫或者接受一个语音呼叫时，用户终端会自动“倒回（fall back）”到GSM、UMTS或者CDMA 2000 1X网络。CSFB是一个比较好的不需要IMS协助的LTE语音实现技术，CSFB还支持漫游, 如下图所示。

SMSover SGs Overview

SMS over SGs is an interim solution to realize the transmission of native SMS from CS infrastructure over the LTE radio network. Without SGs supporting, when a UE is camped on the LTE network,it would need to change its radio access technology back to 2G/3G, so as to process SMS service. SG is a new interface between the MME and the MSC. The SMS is transferred via SGs to MME and carried in NAS signaling to the UE, avoiding the need to execute fallback to a 2G/3G network.

SMS over SGs allows network operators to support SMS as a circuit switched service within LTE. In October 2009, the industry initiative NGMN announced the recommendation to implement “SMS only over SGs”as a minimal requirement to enable roaming. SMS over SGs was specified in 3GPP release-8.

The SGs is the reference point between the MME of the evolved packet system and the MSC server, see the below Figure. The protocol used to connect an MME to an MSC server is SGsAP. The protocol for transferring signalling messages is the Stream Control Transmission Protocol(SCTP).

SMSover SGs Procedure

The SGs is used to handle mobility management and paging procedures between EPS and the CS domain (see Figure 1),and, for SMS, to deliver both mobile originating and mobile terminating SMS.The reference architecture for SMS over SGs for UMTS and GSM networks is shown in Figure 1. For 1xRTT networks, the mechanism to support SMS over S102 isspecified accordingly.

SMS over SGs (as well as CSFB) requires some slight modifications to the EPS attach procedure, which is based on the combined EPS/IMSI Attach procedure.

Compared to the conventional attach,additional information about CS domain capabilities is exchanged. The EPS Attach Type in the ATTACH REQUEST message indicates that the UE requests a combined EPS/IMSI attach and informs the network whether the UE requests “SMS only” services. For SMS service only (but not CSFB), the UE includes an"SMS-only" indication in the ATTACH REQUEST message.

The general procedure for a mobile terminated SMS is as follows:

The UE is camped on the LTE network, it is combined attached with EPS/IMSI, and updates MSC and MME with combined update procedure.

When the MSC receives an ingoing SMS, it will send a paging to the MME via SGs interface, then the MME will tunnel the paging message carried in NAS signaling to the UE.

The UE will respond to the paging message. Then the MSC will send the SMS via SGs interface towards the MME, which will tunnel the SMS carried in NAS signaling to the UE.

The below picture shows the delivery of a mobile terminating SMS in idle mode:

The SMS-GMSC is the “gateway MSC for SMS”capable of receiving an SMS from a service center, interrogating an HLR for routing information and SMS info, and forwarding the SMS.

The service center initiates the transfer of the SMS. The HLR is requested for the routing number for SMS services, and the SMS is forwarded to the right MSC/VLR where the UE is attached. The MSC/VLR issues a paging to the MME, and the MME initiates a paging to each LTE base station with cells in the tracking area(s) in which the UE is registered.

Upon successful paging, the UE sends a SERVICE REQUEST message to the MME who issues a service request to the MSC/VLR. The MSC/VLR builds the SMS and forwards it to the MME which encapsulates theSMS in a NAS message to the UE. The UE acknowledges the receipt of the SMS to the MSC/VLR and issues a delivery report. The delivery report is forwarded to the service center. The MSC/VLR acknowledges the receipt of the delivery report to the UE and indicates to the MME that no more NAS messages need to be tunneled.

Reference:

[1] http://wireless.agilent.com/rfcomms/refdocs/gsmgprs/gprsla_gen_bse_sms_over_sgs.html

[2]http://www.rohde-schwarz.com/en/applications/voice-and-sms-in-lte-application-note_56280-15521.html

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