EPS N/W CONCEPTS (MOBILITY PRINCIPLE)

I have been away for quite sometime, I guess this is the nature of life, lots of up's and down's, anyway..that aside..............Lets us begin this discussion by taking us back in time..."A cell"

In a mobile system, A cell is the least unit. Several neighboring cells can be grouped together into a “Registration area”. 

The base station will broadcast the registration area information, and UE compares the broadcast registration area information with that previously stored, if they are different, the UE will initiate a registration area update procedure.

In 2G/3G system, the registration area for PS Services is called routing area (RA), the registration area update is called routing area update (RAU).

 In EPS or 4G, the registration area is called tracking area (TA), the registration area update is called tracking area update (TAU).

IDENTIFIERS & CORRESPONDING LEGACY ID'S:

We can classify these into 2 classes:

1). Permanent Subscriber Identifiers

2). Temporary Subscriber Identifiers

PERMANENT SUBSCRIBER IDENTIFIERS:

Subscriptions (the USIM cards) are identified with an IMSI (International Mobile Subscriber Identity). Each USIM card is assigned a unique IMSI. The IMSI is an E.212 number with a maximum length of 15 digits. 

The IMSI is constructed by 

1). MCC (Mobile Country Code) 

2). MNC (Mobile Network Code) 

3). MSIN (mobile subscriber identity). 

MCC and MNC together is used to identify a operator uniquely & worth noting is, the IMSI is subscribed in the HSS.

TEMPORARY SUBSCRIBER IDENTIFIERS:

To provide a level of privacy since the permanent identifier does not need to be sent over the radio interface, some temporary subscriber identifiers are used. This additionally provides a mechanism to find the resources where the subscriber’s temporary context is stored, for example, eNodeB can use GUTI to send signalling messgaes from UE to the correct MME.

The temporary context for the subscriber is allocated by the MME, and stored in both UE and MME.

A). GUTI: 

The GUTI can be used to identify the MME globally. GUTI contains the following parts:

• GUMMEI: uniquely identifies the MME which allocated the GUTI
• MMEI: constructed from an MMEGI (MME group ID) and an MMEC (MME Code)
• MMEGI: in most cases, MME group ID is used to identify the pool ID when MME pool is implemented
• MMEC: to identify a single MME in the specific MME pool
• M-TMSI: identifies the subscriber within the MME, the generation of M-TMSI is vendor dependent & usually random.

B). S-TMSI :

The S-TMSI is the shortened form of the GUTI to enable more efficient radio signalling procedures (e.g. paging and Service Request). For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI shall be constructed from the MMEC and the M-TMSI

LOCATION AREA IDENTIFIERS:

In the LTE network, a new location area is introduced, the tracking area. A TAI is used to identify a tracking area globally.A tracking area is very similar with routing area in GERAN/UTRAN.Basically, the TAI should be planned by operators.

TAI = MCC + MNC + TAC

MCC Mobile Country Code

MNC Mobile Network Code

TAC Tracking Area Code

TRACKING AREA IN THE EVOLVED PACKET SYSTEM:

The purpose of the GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the Evolved Packet System (EPS).

MCC and MNC shall have the same field size as in earlier 3GPP systems.

M-TMSI shall be of 32 bits length.

MME Group ID shall be of 16 bits length.

MME Code shall be of 8 bits length.

The TAC is a 16 bit integer.

TA List: A TA List consists of several Tracking Areas (TAs). When a UE moves among TAs belonging to the same TA List, the TA update procedure is not triggered. When the paging procedure is triggered, the UE is paged in all TAs belonging to one TA List. The MME sends the Attach Accept, TAU Accept, or GUTI Relocation message, carrying the TA List, to the UE.

SERVICE IDENTIFIERS:

APN:

We have discussed this before but it doesnot hurt to go over it again:

The APN is used by UE to request a connection to the selected external Packet Data Network (PDN).

When requesting a PDN connection, The UE should include the PDN in the request  to identify the external PDN which to be accessed.

APN include two parts:

1). APN-NI

2). APN-OI

APN-NI: determined by the operators to identify the destination PDN, e.g. 4GNET can be used to identify the PDN -internet, and 4GVOICE can be used to identify the PDN to be accessed is IMS network. Usually,APN-NI is subscribed in HSS

APN-OI: cause the MNC and MCC value for an operator is fixed, the APN-OI can be used to identify an operator globally, which is specially useful in inter-PLMN roaming cases

APN = APN-NI + APN-OI

APN-NI: determined by operators

APN-OI: mnc<MNC>.mcc<MCC>.gprs

An APN example:

4GNET.MNCXXX.MCCYYY.GPRS


In the Next Discussion we will look at QoS. Stay Tuned!!
#PerfectionIsANorm.