[LTE-A] Attach Request - What is Combined EPS/IMSI Attach

EPS Attach Type은 크게 EPS Attach와 Combined EPS/IMSI Attach 이렇게 두가지로 나뉘게 됩니다.

EPS Attach는 말글대로 EPS 망에만 Attach하여 EPS 서비스를 제공받겠다는 의미이고,
Combined EPS/IMSI Attach는 EPS 뿐아니라 non-EPS 서비스도 제공받겠다는 의미입니다.

여기서 non-EPS란 4G LTE의 Packet Switched망이 아닌 3G 혹은 2G의 Circuit Switched망을 의미합니다.

LGU+ 가입자의 단말은 따로 Circuit Switched망이 없으므로 VoLTE 만을 지원하고 있습니다.

떄문에 Attach Type을 항상 EPS Attach로 올리고,

KT나 SKT 단말은 WCDMA 망을 가지고 있기 때문에, 

UE가 CSFB이나 SRVCC등의 서비스를 받기위해서 Combined EPS/IMSI Attach로 올릴 수 있습니다.

단말이 Combined EPS/IMSI Attach로 올릴 경우 이를 수신한 MME는 자신과 연동되어 있는 

WCDMA망의 SGSN, GSM망의 MSC등에게 Signaling을 전송하게 됩니다.

SKT 로그를 보면, 항상 Combined EPS/IMSI attach 으로만 Attach를 시도하는 것을 확인 할 수 있습니다.

Non-Access-Stratum (NAS)PDU

    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: Attach request (0x41)

    0... .... = Type of security context flag (TSC): Native security context (for KSIasme)

    .111 .... = NAS key set identifier: No key is available (7)

    .... 0... = Spare bit(s): 0x00

    .... .010 = EPS attach type: Combined EPS/IMSI attach (2)

    EPS mobile identity

        Length: 11

        .... 0... = odd/even indic: 0

        .... .110 = Type of identity: GUTI (6)

        Mobile Country Code (MCC): Korea (Republic of) (450)

        Mobile Network Code (MNC): Unknown (05)

        MME Group ID: 32769

        MME Code: 19

        M-TMSI: 0xe82273ad

    UE network capability

        Length: 5

        1... .... = EEA0: Supported

        .1.. .... = 128-EEA1: Supported

        ..1. .... = 128-EEA2: Supported

        ...1 .... = 128-EEA3: Supported

        .... 0... = EEA4: Not Supported

        .... .0.. = EEA5: Not Supported

        .... ..0. = EEA6: Not Supported

        .... ...0 = EEA7: Not Supported

        1... .... = EIA0: Supported

        .1.. .... = 128-EIA1: Supported

        ..1. .... = 128-EIA2: Supported

        ...1 .... = 128-EIA3: Supported

        .... 0... = EIA4: Not Supported

        .... .0.. = EIA5: Not Supported

        .... ..0. = EIA6: Not Supported

        .... ...0 = EIA7: Not Supported

        1... .... = UEA0: Supported

        .1.. .... = UEA1: Supported

        ..0. .... = UEA2: Not Supported

        ...0 .... = UEA3: Not Supported

        .... 0... = UEA4: Not Supported

        .... .0.. = UEA5: Not Supported

        .... ..0. = UEA6: Not Supported

        .... ...0 = UEA7: Not Supported

        1... .... = UCS2 support (UCS2): The UE has no preference between the use of the default alphabet and the use of UCS2

        .1.. .... = UMTS integrity algorithm UIA1: Supported

        ..0. .... = UMTS integrity algorithm UIA2: Not Supported

        ...0 .... = UMTS integrity algorithm UIA3: Not Supported

        .... 0... = UMTS integrity algorithm UIA4: Not Supported

        .... .0.. = UMTS integrity algorithm UIA5: Not Supported

        .... ..0. = UMTS integrity algorithm UIA6: Not Supported

        .... ...0 = UMTS integrity algorithm UIA7: Not Supported

        00.. .... = Spare bit(s): 0x00

        ..0. .... = H.245-ASH capability: H.245 after SRVCC handover capability not supported

        ...0 .... = ACC-CSFB capability: eNodeB-based access class control for CSFB not supported

        .... 0... = LPP capability: LTE Positioning Protocol not supported

        .... .1.. = LCS capability: Location services notification mechanisms supported

        .... ..0. = 1xSRVCC capability: SRVCC from E-UTRAN to cdma2000 1x CS not supported

        .... ...0 = NF capability: Notification procedure not supported

    ESM message container

        Length: 5

        ESM message container contents: 0201d011d1

            0000 .... = EPS bearer identity: No EPS bearer identity assigned (0)

            .... 0010 = Protocol discriminator: EPS session management messages (0x02)

            Procedure transaction identity: 1

            NAS EPS session management messages: PDN connectivity request (0xd0)

            0001 .... = PDN type: IPv4 (1)

            .... 0001 = Request type: initial request (1)

            ESM information transfer flag

                1101 .... = Element ID: 0xd-

                .... 000. = Spare bit(s): 0x00

                .... ...1 = EIT (ESM information transfer): Security protected ESM information transfer required

    Tracking area identity - Last visited registered TAI

        Element ID: 0x52

        Mobile Country Code (MCC): Korea (Republic of) (450)

        Mobile Network Code (MNC): Unknown (05)

        Tracking area code(TAC): 0x0805

    DRX Parameter

        Element ID: 0x5c

        DRX Parameter

            Split PG Cycle Code: Reserved, interpreted as 1 (8)

            0000 .... = CN Specific DRX cycle length coefficient: CN Specific DRX cycle length coefficient / value not specified by the MS (0)

            .... 0... = SPLIT on CCCH: Split pg cycle on CCCH is not supported by the mobile station

            .... .010 = Non-DRX timer: max. 2 sec non-DRX mode after transfer state (2)

    MS Network Capability

        Element ID: 0x31

        Length: 3

        1... .... = GEA/1: Encryption algorithm available

        .1.. .... = SM capabilities via dedicated channels: Mobile station supports mobile terminated point to point SMS via dedicated signalling channels

        ..1. .... = SM capabilities via GPRS channels: Mobile station supports mobile terminated point to point SMS via GPRS packet data channels

        ...1 .... = UCS2 support: Mobile station supports mobile terminated point to point SMS via GPRS packet data channels

        .... 01.. = SS Screening Indicator: capability of handling of ellipsis notation and phase 2 error handling (0x01)

        .... ..0. = SoLSA Capability: The ME does not support SoLSA

        .... ...1 = Revision level indicator: Used by a mobile station supporting R99 or later versions of the protocol

        1... .... = PFC feature mode: Mobile station does support BSS packet flow procedures

        .110 000. = Extended GEA bits: 0x30

            .1.. .... = GEA/2: Encryption algorithm available

            ..1. .... = GEA/3: Encryption algorithm available

            ...0 .... = GEA/4: Encryption algorithm not available

            .... 0... = GEA/5: Encryption algorithm not available

            .... .0.. = GEA/6: Encryption algorithm not available

            .... ..0. = GEA/7: Encryption algorithm not available

        .... ...0 = LCS VA capability: LCS value added location request notification capability not supported

        0... .... = PS inter-RAT HO from GERAN to UTRAN Iu mode capability: PS inter-RAT HO to UTRAN Iu mode not supported

        .0.. .... = PS inter-RAT HO from GERAN to E-UTRAN S1 mode capability: PS inter-RAT HO to E-UTRAN S1 mode not supported

        ..1. .... = EMM Combined procedures capability: Mobile station supports EMM combined procedures

        ...1 .... = ISR support: The mobile station supports ISR

        .... 0... = SRVCC to GERAN/UTRAN capability: SRVCC from UTRAN HSPA or E-UTRAN to GERAN/UTRAN not supported

        .... .1.. = EPC capability: EPC supported

        .... ..0. = NF capability: Mobile station does not support the notification procedure

        .... ...0 = GERAN network sharing capability: Mobile station does not support GERAN network sharing

    Location area identification - Old location area identification

        Element ID: 0x13

        Location Area Identification (LAI) - 450/05/8305

            Mobile Country Code (MCC): Korea (Republic of) (450)

            Mobile Network Code (MNC): Unknown (05)

            Location Area Code (LAC): 0x2071 (8305)

    Mobile station classmark 2

        Element ID: 0x11

        Length: 3

        0... .... = Spare: 0

        .10. .... = Revision Level: Used by mobile stations supporting R99 or later versions of the protocol (2)

        ...1 .... = ES IND: Controlled Early Classmark Sending option is implemented in the MS

        .... 0... = A5/1 algorithm supported: encryption algorithm A5/1 available

        .... .111 = RF Power Capability: RF Power capability is irrelevant in this information element (7)

        0... .... = Spare: 0

        .1.. .... = PS capability (pseudo-synchronization capability): PS capability present

        ..01 .... = SS Screening Indicator: Capability of handling of ellipsis notation and phase 2 error handling  (1)

        .... 1... = SM capability (MT SMS pt to pt capability): Mobile station supports mobile terminated point to point SMS

        .... .0.. = VBS notification reception: no VBS capability or no notifications wanted

        .... ..0. = VGCS notification reception: no VGCS capability or no notifications wanted

        .... ...0 = FC Frequency Capability: The MS does not support the E-GSM or R-GSM band

        1... .... = CM3: The MS supports options that are indicated in classmark 3 IE

        .0.. .... = Spare: 0

        ..1. .... = LCS VA capability (LCS value added location request notification capability): LCS value added location request notification capability supported

        ...1 .... = UCS2 treatment: the ME has no preference between the use of the default alphabet and the use of UCS2

        .... 0... = SoLSA: The ME does not support SoLSA

        .... .0.. = CMSP: CM Service Prompt: Network initiated MO CM connection request not supported

        .... ..1. = A5/3 algorithm supported: encryption algorithm A5/3 available

        .... ...0 = A5/2 algorithm supported: encryption algorithm A5/2 not available

    Voice domain preference and UE's usage setting

        Element ID: 0x5d

        Length: 1

        0000 0... = Spare bit(s): 0

        .... .0.. = UE's usage setting: Voice centric

        .... ..00 = Voice domain preference for E-UTRAN: CS Voice only (0)

    GUTI type - Old GUTI type

        1110 .... = Element ID: 0xe-

        .... 000. = Spare bit(s): 0x00

        .... ...0 = GUTI type: Native GUTI