4G/LTE - Protocol







To be honest, I don't know how to clearly define this word "Redirection" even though we pretty often use this terminology. Just based on my personal understanding, "Redirection" is the mechanism of chaning cells from one (let's call this 'serving/source cell') to another (let's call this 'target cell') at a specific point in the call process where RRC Session has initiated but it is before initiating Radio Bearer Setup.


Actually there are so many different cases where UE has to select or change cells in various points during the call processing and the terminology for the cell changes gets different depending on when the cell change happens. For example, if the cell change (in this case I would say 'picking up a cell among various candiate', so the term 'change' would not be the right one) happens when you power on a UE, we call the process 'Cell Selection'. If the cell change happens while UE is in idle mode, we call it 'Cell Reselection'. and if the cell change happens while UE is in communication mode after radio bearer setup, we call it 'Handover'.


Actually 'Redirection' is not the unique cocept in LTE, we have been using this concept almost all the technology.


The typical procedure for Redirection in LTE is as follows : (Note that the initial state is "Connected Mode in source cell" and the final state is the "Idle mode in target cell")





UE --> NW

Extended Service Request


UE <-- NW

RRC Connection Release

Indicate which cell UE has to connect

UE <--> NW

< Registration to Target Cell >



< IDLE in Target Cell >



I think just showing the one example of the contents of these message would be worth several pages of explanation.


< Extended service request >


NAS_LTE:EMM,Extended service request

Extended service 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]

  +-Extended service request message identity ::= V

  | +-Message type ::= MSG [4C]

  +-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 0]

  +-Service type ::= V

  | +-Service type value ::= CHOICE [mobile originating CS fallback or 1xCS fallback]

  +-M-TMSI ::= LV

  | +-Octet1 ::= DIVISION

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

  | +-Octet2 ::= 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 [No Identity]

  | +-Octet3-Octet6 ::= DIVISION

  |   +-Identity digit p ::= OCTETARRAY SIZE(0..4)

  +-CSFB response ::= TV OPTIONAL:Exist

    +-Octet1 ::= DIVISION

      +-CSFB response IEI ::= IEI [B-]

      +-spare ::= FIX [0]

      +-CSFB response value ::= CHOICE [CS fallback accepted by the UE]


< RRC Connection Release >




  +-message ::= CHOICE [c1]

    +-c1 ::= CHOICE [rrcConnectionRelease]

      +-rrcConnectionRelease ::= SEQUENCE

        +-rrc-TransactionIdentifier ::= INTEGER (0..3) [0]

        +-criticalExtensions ::= CHOICE [c1]

          +-c1 ::= CHOICE [rrcConnectionRelease-r8]

            +-rrcConnectionRelease-r8 ::= SEQUENCE [100]

              +-releaseCause ::= ENUMERATED [other]

              +-redirectedCarrierInfo ::= CHOICE [cdma2000-1xRTT] OPTIONAL:Exist

              | +-cdma2000-1xRTT ::= SEQUENCE

              |   +-bandClass ::= ENUMERATED [bc2]

              |   +-arfcn ::= INTEGER (0..2047) [0]

              +-idleModeMobilityControlInfo ::= SEQUENCE OPTIONAL:Omit

              +-nonCriticalExtension ::= SEQUENCE OPTIONAL:Omit