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SMS with LTE (SG-SMS)

 

Everybody would know what SMS (Short Messaging Service) is. So I will not talk about what SMS is. The question you are more interested in would be "Is it possible to send SMS in LTE ?" and "How to do it ?".

Yes, SMS is possible in LTE and there are two ways to implement SMS in LTE. The ideal solution would be doing SMS using IMS. IMS over LTE is specified to transfer any form of data (e.g, voice, SMS and any other form of multi media data), but the question is when every body (both network implementation and UE implementation) will seamlessly implement this IMS on their network and UE. It seems that they are not implemented fully at least for now. So they thought out a kind of interim solution called SG LTE. (It is like we have CS Fallback as an interim solution before they fully implement voice call over IMS). For the over view of SG SMS, refer to this blog. I found the explanation in the blog is short, practical and easy to understand.

 

In terms of network Architecture, SG SMS goes through a special interface marked in red below.

 

 

Followings are the topics I will deal with in this page.

 

 

Basic Signal Flow

 

The implementation logic of SG-SMS is very similar to WCDMA SMS. In WCDMA, we injected the SMS message into a DCCH channel and send it to the destination. It means that we carried the message over a control channel, not over a data channel. SG SMS is also using a similar concept, we send the message over a control channel. One example is as follows (This is for MT SMS case).

 

    i) UE <-- NW : RRC Connection Reconfiguration

    ii) UE --> NW : RR Connection Reconfiguration Comlete

    iii) UE <-- NW : dlInformationTransfer (DOWNLINK NAS TRANSPORT : SMS message - CP Data)

    iv) UE --> NW : ulInformationTransfer (UPLINK NAS TRANSPORT : embedd CP-ACK into this message)

    v) UE --> NW : ulInformationTransfer (UPLINK NAS TRANSPORT : Delivery Report)

    vi) UE <-- NW : dlInformationTransfer (DOWNLINK NAS TRANSPORT : CP-ACK)

 

Actually Step iii) carries a special NAS message called "DOWNLINK NAS TRANSPORT" and this NAS message carries CP Data in it.

 

        c1: dlInformationTransfer (1)

            dlInformationTransfer

                rrc-TransactionIdentifier: 0

                criticalExtensions: c1 (0)

                    c1: dlInformationTransfer-r8 (0)

                        dlInformationTransfer-r8

                            dedicatedInfoType: dedicatedInfoNAS (0)

                                dedicatedInfoNAS: 27d1356f030407622809012501000481999999001c040a81...

                                Non-Access-Stratum (NAS)PDU

                                    0010 .... = Security header type: Integrity protected and ciphered (2)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    Message authentication code: 0xd1356f03

                                    Sequence number: 4

                                    0000 .... = Security header type: Plain NAS message, not security protected (0)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    NAS EPS Mobility Management Message Type: Downlink NAS transport (0x62)

                                    NAS message container

                                        Length: 40

                                        NAS message container content: 09012501000481999999001c040a81214365870900003111...

                                            GSM A-I/F DTAP - CP-DATA

                                                Protocol Discriminator: SMS messages

                                                    .... 1001 = Protocol discriminator: SMS messages (0x09)

                                                    0... .... = TI flag: allocated by sender

                                                    .000 .... = TIO: 0

                                                DTAP Short Message Service Message Type: CP-DATA (0x01)

                                                CP-User Data

                                                    Length: 37

                                                    RPDU (not displayed)

                                            GSM A-I/F RP - RP-DATA (Network to MS)

                                                Message Type RP-DATA (Network to MS)

                                                RP-Message Reference

                                                    RP-Message Reference: 0x00 (0)

                                                RP-Origination Address - (999999)

                                                    Length: 4

                                                    1... .... = Extension: No Extension

                                                    .000 .... = Type of number: unknown (0x00)

                                                    .... 0001 = Numbering plan identification:

                                                                    ISDN/Telephony Numbering (Rec ITU-T E.164) (0x01)

                                                    BCD Digits: 999999

                                                RP-Destination Address

                                                    Length: 0

                                                RP-User Data

                                                    Length: 28

                                                    TPDU (not displayed)

                                            GSM SMS TPDU (GSM 03.40) SMS-DELIVER

                                                0... .... = TP-RP: TP Reply Path parameter is not set in this SMS SUBMIT/DELIVER

                                                .0.. .... = TP-UDHI: The TP UD field contains only the short message

                                                ..0. .... = TP-SRI: A status report shall not be returned to the SME

                                                .... .1.. = TP-MMS: No more messages are waiting for the MS in this SC

                                                .... ..00 = TP-MTI: SMS-DELIVER (0)

                                                TP-Originating-Address - (1234567890)

                                                    Length: 10 address digits

                                                    1... .... :  No extension

                                                    .000 .... :  Type of number: (0) Unknown

                                                    .... 0001 :  Numbering plan: (1) ISDN/telephone (E.164/E.163)

                                                    TP-OA Digits: 1234567890

                                                TP-PID: 0

                                                    00.. .... :  defines formatting for subsequent bits

                                                    ..0. .... :  no telematic interworking, but SME-to-SME protocol

                                                    ...0 0000 :  the SM-AL protocol being used between the SME and the MS (0)

                                                TP-DCS: 0

                                                    00.. .... = Coding Group Bits: General Data Coding indication (0)

                                                    Special case, GSM 7 bit default alphabet

                                                TP-Service-Centre-Time-Stamp

                                                    Year 13, Month 11, Day 06

                                                    Hour 10, Minutes 05, Seconds 55

                                                    Timezone: GMT + 5 hours 0 minutes

                                                TP-User-Data-Length: (11) depends on Data-Coding-Scheme

                                                TP-User-Data

                                                    SMS text: mt sms test

 

 

Actually Step iv) carries a special NAS message called "UPLINK NAS TRANSPORT" and this NAS message carries CP ACK in it.

 

        c1: ulInformationTransfer (9)

            ulInformationTransfer

                criticalExtensions: c1 (0)

                    c1: ulInformationTransfer-r8 (0)

                        ulInformationTransfer-r8

                            dedicatedInfoType: dedicatedInfoNAS (0)

                                dedicatedInfoNAS: 27602658f6040763028904

                                Non-Access-Stratum (NAS)PDU

                                    0010 .... = Security header type: Integrity protected and ciphered (2)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    Message authentication code: 0x602658f6

                                    Sequence number: 4

                                    0000 .... = Security header type: Plain NAS message, not security protected (0)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    NAS EPS Mobility Management Message Type: Uplink NAS transport (0x63)

                                    NAS message container

                                        Length: 2

                                        NAS message container content: 8904

                                            GSM A-I/F DTAP - CP-ACK

                                                Protocol Discriminator: SMS messages

                                                    .... 1001 = Protocol discriminator: SMS messages (0x09)

                                                    1... .... = TI flag: allocated by receiver

                                                    .000 .... = TIO: 0

                                                DTAP Short Message Service Message Type: CP-ACK (0x04)

 

 

Step v) can be an optional. This is for delivery report.

 

        c1: ulInformationTransfer (9)

            ulInformationTransfer

                criticalExtensions: c1 (0)

                    c1: ulInformationTransfer-r8 (0)

                        ulInformationTransfer-r8

                            dedicatedInfoType: dedicatedInfoNAS (0)

                                dedicatedInfoNAS: 27608e1cd405076309890106020041020000

                                Non-Access-Stratum (NAS)PDU

                                    0010 .... = Security header type: Integrity protected and ciphered (2)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    Message authentication code: 0x608e1cd4

                                    Sequence number: 5

                                    0000 .... = Security header type: Plain NAS message, not security protected (0)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    NAS EPS Mobility Management Message Type: Uplink NAS transport (0x63)

                                    NAS message container

                                        Length: 9

                                        NAS message container content: 890106020041020000

                                            GSM A-I/F DTAP - CP-DATA

                                                Protocol Discriminator: SMS messages

                                                    .... 1001 = Protocol discriminator: SMS messages (0x09)

                                                    1... .... = TI flag: allocated by receiver

                                                    .000 .... = TIO: 0

                                                DTAP Short Message Service Message Type: CP-DATA (0x01)

                                                CP-User Data

                                                    Length: 6

                                                    RPDU (not displayed)

                                            GSM A-I/F RP - RP-ACK (MS to Network)

                                                Message Type RP-ACK (MS to Network)

                                                RP-Message Reference

                                                    RP-Message Reference: 0x00 (0)

                                                RP-User Data

                                                    Element ID: 0x41

                                                    Length: 2

                                                    TPDU (not displayed)

                                            GSM SMS TPDU (GSM 03.40) SMS-DELIVER REPORT

                                                .0.. .... = TP-UDHI: The TP UD field contains only the short message

                                                .... .0.. = TP-MMS: More messages are waiting for the MS in this SC

                                                .... ..00 = TP-MTI: SMS-DELIVER REPORT (0)

                                                TP-Parameter-Indicator

                                                    0... .... :  No extension

                                                    .000 0... :  Reserved

                                                    .... .0.. :  TP-UDL not present

                                                    .... ..0. :  TP-DCS not present

                                                    .... ...0 :  TP-PID not present

 

Step vi) is the CP-ACK for Step v)

 

        c1: dlInformationTransfer (1)

            dlInformationTransfer

                rrc-TransactionIdentifier: 0

                criticalExtensions: c1 (0)

                    c1: dlInformationTransfer-r8 (0)

                        dlInformationTransfer-r8

                            dedicatedInfoType: dedicatedInfoNAS (0)

                                dedicatedInfoNAS: 27b0d94c40050762020904

                                Non-Access-Stratum (NAS)PDU

                                    0010 .... = Security header type: Integrity protected and ciphered (2)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    Message authentication code: 0xb0d94c40

                                    Sequence number: 5

                                    0000 .... = Security header type: Plain NAS message, not security protected (0)

                                    .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                    NAS EPS Mobility Management Message Type: Downlink NAS transport (0x62)

                                    NAS message container

                                        Length: 2

                                        NAS message container content: 0904

                                            GSM A-I/F DTAP - CP-ACK

                                                Protocol Discriminator: SMS messages

                                                    .... 1001 = Protocol discriminator: SMS messages (0x09)

                                                    0... .... = TI flag: allocated by sender

                                                    .000 .... = TIO: 0

                                                DTAP Short Message Service Message Type: CP-ACK (0x04)

 

 

Addional Example - International Number / Message Class / Message Time Stamp

 

These information is specified in specific information elements in CP-DATA as shown in color below. Red indicate 'International Number Type', Green indicates 'Message Class' and Blue indicate 'Message Time Stamp'

 

DLT: 147, Payload: lte-rrc.dl.dcch (LTE Radio Resource Control (RRC) protocol)

DL-DCCH-Message

    message: c1 (0)

        c1: dlInformationTransfer (1)

            dlInformationTransfer

                rrc-TransactionIdentifier: 0

                criticalExtensions: c1 (0)

                    c1: dlInformationTransfer-r8 (0)

                        dlInformationTransfer-r8

                            dedicatedInfoType: dedicatedInfoNAS (0)

                                dedicatedInfoNAS: 27d05511ee0507622c090129010004819999990020a40a90...

                                    Non-Access-Stratum (NAS)PDU

                                        0010 .... = Security header type: Integrity protected and ciphered (2)

                                        .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                        Message authentication code: 0xd05511ee

                                        Sequence number: 5

                                        0000 .... = Security header type: Plain NAS message,

                                                                          not security protected (0)

                                        .... 0111 = Protocol discriminator: EPS mobility management messages (0x07)

                                        NAS EPS Mobility Management Message Type: Downlink NAS transport (0x62)

                                        NAS message container

                                            Length: 44

                                            NAS message container content:

                                                GSM A-I/F DTAP - CP-DATA

                                                    Protocol Discriminator: SMS messages

                                                        .... 1001 = Protocol discriminator: SMS messages (0x09)

                                                        0... .... = TI flag: allocated by sender

                                                        .000 .... = TIO: 0

                                                    DTAP Short Message Service Message Type: CP-DATA (0x01)

                                                    CP-User Data

                                                        Length: 41

                                                        RPDU (not displayed)

                                                GSM A-I/F RP - RP-DATA (Network to MS)

                                                    Message Type RP-DATA (Network to MS)

                                                    RP-Message Reference

                                                        RP-Message Reference: 0x00 (0)

                                                    RP-Originator Address - (999999)

                                                        Length: 4

                                                        1... .... = Extension: No Extension

                                                        .000 .... = Type of number: unknown (0x00)

                                                        .... 0001 = Numbering plan identification:

                                                                    ISDN/Telephony Numbering

                                                                    (ITU-T Rec. E.164 / ITU-T Rec. E.163) (0x01)

                                                        BCD Digits: 999999

                                                    RP-Destination Address

                                                        Length: 0

                                                    RP-User Data

                                                        Length: 32

                                                        TPDU (not displayed)

                                                GSM SMS TPDU (GSM 03.40) SMS-DELIVER

                                                    1... .... = TP-RP: TP Reply Path parameter is set

                                                                        in this SMS SUBMIT/DELIVER

                                                    .0.. .... = TP-UDHI: The TP UD field contains only

                                                                         the short message

                                                    ..1. .... = TP-SRI: A status report shall be returned

                                                                        to the SME

                                                    .... .1.. = TP-MMS: No more messages are waiting

                                                                         for the MS in this SC

                                                    .... ..00 = TP-MTI: SMS-DELIVER (0)

                                                    TP-Originating-Address - (1234567890)

                                                        Length: 10 address digits

                                                        1... .... :  No extension

                                                        .001 .... :  Type of number: (1) International

                                                        .... 0000 :  Numbering plan: (0) Unknown

                                                        TP-OA Digits: 1234567890

                                                    TP-PID: 0

                                                        00.. .... :  defines formatting for subsequent bits

                                                        ..0. .... :  no telematic interworking,

                                                                        but SME-to-SME protocol

                                                        ...0 0000 :  the SM-AL protocol being used

                                                                        between the SME and the MS (0)

                                                    TP-DCS: 17

                                                        00.. .... = Coding Group Bits:

                                                                        General Data Coding indication (0)

                                                        00.. .... :  General Data Coding indication

                                                        ..0. .... :  Text is not compressed

                                                        ...1 .... :  Message class is defined below

                                                        .... 00.. :  Character set: GSM 7 bit default alphabet

                                                        .... ..01 :  Message Class: Class 1 Default meaning:

                                                                         ME-specific

                                                    TP-Service-Centre-Time-Stamp

                                                        Year 15, Month 04, Day 07

                                                        Hour 13, Minutes 41, Seconds 28

                                                        Timezone: GMT + 5 hours 0 minutes

                                                    TP-User-Data-Length: (16) depends on Data-Coding-Scheme

                                                    TP-User-Data

                                                        SMS text: MT SMS -  Class1

 

 

Tips from 3GPP Sepcification

 

For further information on these special NAS TRANSPORT Message, you can refer to 24.301

 

[TS 24.301, clause 5.6.3.1]

The purpose of the transport of NAS messages procedure is to carry SMS messages in an encapsulated form between the MME and the UE. The procedure may be initiated by the UE or the network and can only be used when the UE is attached for EPS services and IMSI attached for non-EPS services and is in EMM-CONNECTED mode.

 

[TS 24.301, clause 5.6.3.3]

The network initiates the procedure by sending a DOWNLINK NAS TRANSPORT message. When receiving the DOWNLINK NAS TRANSPORT message, the EMM entity in the UE shall forward the contents of the NAS message container IE to the SMS entity.

 

[TS 24.301, clause 9.9.3.22]

This information element is used to encapsulate the SMS messages transferred between the UE and the network. The NAS message container information element is coded as shown in figure 9.9.3.22.1 and table 9.9.3.22.1.

The NAS message container is a type 4 information element with a minimum length of 4 octets and a maximum length of 253 octets.