Tracking Area


Tracking Area is a logical concept of an area where a user can move around without updating the MME. The network allocates a list with one or more TAs to the user. In certain operation modes, the UE may move freely in all TAs of the list without updating the MME. You can think of 'Tracking Area' as 'Routing Area' in UMTS.


Each eNobe broadcasts a special tracking area code (TAC) to indicate to which Tracking Area the eNodeB belong to and the TAC is unique within a PLMN. (Since PLMN is a unique number allocated to each of the system operator and TAC is a unique in a PLMN, if you combine these two numbers you would have a globally unique number. This number (PLMN + TAC) is called Tracking Area Identity (TAI)


Tracking Area for each eNodeB is broadcast by SIB1 as follows.


    +-c1 ::= CHOICE [systemInformationBlockType1]

      +-systemInformationBlockType1 ::= SEQUENCE [000]

        +-cellAccessRelatedInfo ::= SEQUENCE [0]

        | +-plmn-IdentityList ::= SEQUENCE OF SIZE(1..6) [1]

        | | +-PLMN-IdentityInfo ::= SEQUENCE

        | |   +-plmn-Identity ::= SEQUENCE [1]

        | |   | +-mcc ::= SEQUENCE OF SIZE(3) OPTIONAL:Exist

        | |   | | +-MCC-MNC-Digit ::= INTEGER (0..9) [0]

        | |   | | +-MCC-MNC-Digit ::= INTEGER (0..9) [0]

        | |   | | +-MCC-MNC-Digit ::= INTEGER (0..9) [1]

        | |   | +-mnc ::= SEQUENCE OF SIZE(2..3) [2]

        | |   |   +-MCC-MNC-Digit ::= INTEGER (0..9) [0]

        | |   |   +-MCC-MNC-Digit ::= INTEGER (0..9) [1]

        | |   +-cellReservedForOperatorUse ::= ENUMERATED [notReserved]

        | +-trackingAreaCode ::= BIT STRING SIZE(16) [0000000000000001]

        | +-cellIdentity ::= BIT STRING SIZE(28) [0000000000000000000100000000]


UE stores a group of TAC and this group of TAC maintained in a UE is called Tracking Area List.  UE does not need to go through Tracking Area Update procedure when it moves along this TAI.


Why we need this kind of grouping, called Tracking Area ?

What would happen if we didn't do a good job in Tracking Area Design ?

What UE is supposed to do when it move from a Tracking Area to another Tracking Area ?


These are some of the questions you have to find answers. (I will talk about these later in somewhere else)