IMSI stands for International Mobile Subscriber Identity.
IMSI is used to identify globally unique SIM or USIM card. It is comprised of 3 parts.
1. MCC: Mobile Country Code:
It consists of 3 digits. It is used to uniquely identify the country of the mobile subscriber.
MCC values are administered and allocated by international numbering plan.
2. MNC: Mobile Network Code
It consists of 2 or 3 digit code. It is used to identify the network operator with in the country.
MCC + MNC = PLMN [Public land mobile network]
3. MSIN: Mobile Subscriber Identification Number
MSIN is used to identify a subscriber within the PLMN.
Below is the pictorial representation of IMSI
Few important points on IMSI:
With the help of IMSI a mobile operator can know if a subscriber is allowed in his network or not, and to give proper QoS policy and to charge appropriately.
So how does IMSI helps in authenticating the user?
1. For a user to attach to LTE network, he should be having a mobile device and a SIM card that has IMSI in it.
2. The LTE network which he is connecting should already have the same IMSI number registered with it.
3. These IMSI numbers are stored in HSS (Home Subscription Server) and SPR (Subscription Profile Repository).
4. HSS will have a key to authenticate the subscription and a QoS profile will be attached to it.
5. So when a user tries to access the LTE network by sending an attach request, HSS will denies any user with an unregistered IMSI.
6. SPR along with PCRF (Policy and Charging Rules Function) is used to apply policy to a subscriber.
7. Below is the pictorial representation of the same.
GUTI: Globally Unique Temporary Identifier.
1. GUTI is assigned to UE during initial attach sequence by MME.
2. GUTI is used to identification of the UE so that the UE will not reveal its IMSI.
3. As IMSI is important to identify a subscriber, if the IMSI gets stolen, other person can use your network allocation.
4. To avoid such scenario, you can use GUTI where ever possible. As a new GUTI value is generated whenever it is used.
Below are the steps how GUTI is assigned to a UE:
1. When UE initially sends attach request to LTE network, IMSI number is sent to the network for authentication.
2. When the authentication is successful, the network (MME) will send a GITU value in the “attach accept” message to UE.
3. Then this GUTI value will be stored in UE and it will use this ID when it re-attaches to the network, for example when UE is turned off and on.
4. The same process is followed in TAU [Tracking Area Update] process.
GUTI format is a combination of GUMMEI, M-TMSI, MME Group ID, MME Code, MNC, MCC.
GUMMEI: Globally Unique Mobility Management Entity Identifier
MMEI: Mobility Management Entity Identifier
MMEGI: Mobility Management Entity Group
GUTI = GUMMEI + M-TMSI
GUMMEI = MCC + MNC + MME Identifier
MME Identifier = MME Group ID + MME Code
Tracking Area (TA) and Tracking Area Update (TAU)
When an User Equipment is in active state, the location is known by LTE network at the cell level.
When the UE is in idle state, the location is known by LTE at TA level.
A TA is a group of cells or eNBs.
TA is also used to paging idle mode subscribers.
Paging is a way to wakeup the UE so that it can receive the data.
UE informs MME every time it changes it’s TA via the TAU procedure or at expiration of a timer (T3412)
Access Point Name (APN)
APN represents a PDN (Packet Data Network).
APN connects UE to PDN
During the attach process an APN will be presented to UE.
UE can access any APNs.
APN is also used as a tunnel for connecting UE to a PDN such as Internet.
PDN has to have an APN and an associated P-GW.
APNs are configured by network operators.
P-GW uses rules that are set in place to determine which APN should a UE use.
APN’s usually have 2 parts:
1. Network Identifier
2. Operator Identifier.
Inside APN, bearers will be created.
A single PDN multiple APN’s can be created.
For any dedicated bearer, default bearer should be created.
List of all the identifiers in LTE