4G/LTE - Call Processing

Combined Attach

In UMTS, we had a terminology called 'Combined Registration'. It means that UE simultaneously performs registration for CS network and PS network. LTE is bascially packet only (PS only). Then what is the purpose of this 'Combined Attach" ? What does it mean ?

It means that UE performs registration for LTE network and Non-LTE network (e.g, WCDMA or GSM) simultaneously.

Why do we need this kind of simultaneous registration for two different network ? It is mainly for CS-Fallback. Unless a system operator/UE use Voice Over IMS, we use legacy network (e.g, WCDMA, GSM, C2K) for voice call. In this case, UE has to go through the registration for the legacy network. The idea of Combined Attach is to perform the attach process for LTE and Legacy Network simultaneously.

This attach method is specified in Attach Request and Attach Accept as follows.

Attach request ::= DIVISION

  +-Security header type ::= V

  | +-Security header type ::= CHOICE [Plain NAS message, not security protected]

  +-EPS mobility management protocol discriminator ::= V

  | +-Protocol discriminator ::= PD [7]

  +-Attach request message identity ::= V

  | +-Message type ::= MSG [41]

  +-NAS key set identifier ::= V

  | +-TSC ::= CHOICE [native security context (for KSI ASME)]

  | +-NAS key set identifier ::= CHOICE [possible values for the NAS key set identifier 1]

  +-EPS attach type ::= V

  | +-Spare ::= FIX [0]

  | +-EPS attach type value ::= CHOICE [Combined EPS/IMSI Attach]

UE send its attach type information in at EPS attach type value IE of Attach Request. This IE has two options as follows.

• EPS Attach
• Combined EPS/IMSI Attach

To Complete the Combined Attach, Network has to specify Attach Type as CombinedEPS/IMSI attach and LAI and TMSI as shown below.

Attach accept ::= DIVISION

  +-Security header type ::= V

  | +-Security header type ::= CHOICE [Plain NAS message, not security protected]

  +-EPS mobility management protocol discriminator ::= V

  | +-Protocol discriminator ::= PD [7]

  +-Attach accept message identity ::= V

  | +-Message type ::= MSG [42]

  +-Spare half octet ::= V

  | +-Spare half octet ::= FIX [0]

  +-EPS attach result ::= V

  | +-Spare ::= FIX [0]

  | +-EPS attach result value ::= CHOICE [combined EPS/IMSI attach]

  .............

  +-Location area identification ::= TV OPTIONAL:Exist

  | +-Octet1 ::= DIVISION

  | | +-Location Area Identification IEI ::= IEI [13]

  | +-Octet2 ::= DIVISION

  | | +-MCC digit 2 ::= INT (0..15) [0]

  | | +-MCC digit 1 ::= INT (0..15) [0]

  | +-Octet3 ::= DIVISION

  | | +-MNC digit 3 ::= INT (0..15) [15]

  | | +-MCC digit 3 ::= INT (0..15) [1]

  | +-Octet4 ::= DIVISION

  | | +-MNC digit 2 ::= INT (0..15) [1]

  | | +-MNC digit 1 ::= INT (0..15) [0]

  | +-Octet5 ::= DIVISION

  | | +-LAC ::= INT (0..255) [0]

  | +-Octet6 ::= DIVISION

  |   +-LAC (continued) ::= INT (0..255) [0]

  +-MS identity ::= TLV OPTIONAL:Exist

  | +-Octet1 ::= DIVISION

  | | +-Mobile Identity IEI ::= IEI [23]

  | +-Octet2 ::= DIVISION

  | | +-Length of mobile identity contents ::= LEN (0..255) [5]

  | +-Octet3 ::= DIVISION

  | | +-Identity digit 1 ::= INT (0..15) [0]

  | | +-Odd/even indication ::= CHOICE [even number of identity digits and also when the TMSI/P-TMSI is used]

  | | +-Type of identity ::= CHOICE [TMSI/P-TMSI/M-TMSI]

  | +-Octet4-Octet10 ::= DIVISION

  |   +-Identity digit p ::= OCTETARRAY SIZE(0..7) [00000000]