In my previous blog I had motivated the idea behind the Registered State/Deregistered State and the Connected Mode/Idle Mode. I would like to continue to build on those ideas and briefly talk about five key Identifiers assigned to the UE by different entities during the UEs lifetime. This is not a comprehensive list of all the identifiers but their discussion broadly covers the end to end data connectivity concepts in LTE.

The IMSI (International Mobile Subscriber Identity) and IMEI (International Mobile Equipment Identity) are permanent identifiers assigned to the USIM card and the Mobile Equipment, respectively. They are permanently associated with the subscriber and stored in a permanent provider database like the HSS (Home Subscriber Server) and will be used by other nodes in the network to identify the user. Similar to 2G and 3G technologies, for reasons of security, efficiency and practicality - the LTE network minimizes the exchange of these two identifiers with the UE.

During the Initial Attach procedure between the UE and the LTE Network the UE is assigned three additional dynamic identifiers by different LTE Network nodes that have varying scopes of use.

The eNodeB (Evolved Node B) assigns the UE a C-RNTI (Cell Radio Network Temporary Identifier) to identify the UE during exchange of all information over the air. The C-RNTI is assigned during the setup of the RRC Connection (Idle Mode à Connected Mode transition) between a UE and an eNodeB and is valid only for that RRC Connection. Once the UE leaves the coverage area of an eNodeB the RRC Connection must be moved (Inter-eNodeB Handover) and the "new" eNodeB will assign a "new" C-RNTI to the UE. The C-RNTI is an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) specific identifier and the EPC (Evolved Packet Core) Network has no visibility to it.

The MME (Mobility Management Entity) assigns the UE a GUTI (Globally Unique Temporary Identifier) to identify the UE during all message exchanges and procedures with the EPC. The GUTI is assigned during the Attach procedure (Deregistered State à Registered State transition) between the UE and the MME and is valid only as long as the UE is attached to the MME that assigned the GUTI. Once the UE leaves the Tracking Area(s) of an MME the "Attachment" has to be moved (Inter-MME handover) and the "new" MME will assign a "new" GUTI to the UE. Embedded within the GUTI are the PLMN ID of the service provider and the MME Identity. Thus, the GUTI uniquely and globally identifies a UE attached to a specific MME in a specific Service Providers LTE Network in a specific Country. The MME may choose to periodically re-assign a "fresh" GUTI to a UE that is attached to it.

The PGW (Packet Data Network Gateway) assigns the UE an IP address to facilitate data connectivity between the UE and any internal or external PDN (Packet Data Network). This could be an IPv4, IPv6 or Dual Stack IP address and the PGW could use a variety of IP address allocation schemes associated with the type of IP Address. The UE may set up PDN Connections with more than one PGW and may be assigned more than one IP address. The first IP address is assigned to the UE during the Initial Attach procedure and it stays with the UE as long as the UE is attached to the LTE Network. Unlike the other temporary identifiers the IP address is more "persistent" or "sticky" and does not change as long as the UE is attached - thus uninterrupted IP connectivity is provided to the UE. For all practical purposes, the UE is assigned an IP address when it powers on and loses its IP connectivity when it powers off. It is important to recognize that the eNodeB, MME and the SGW do not have any use for this UE IP address for connectivity purposes. It is used for IP forwarding decisions by the PGW and all nodes "north" (between the PGW and the PDN) of the PGW.

These ideas have been captured in the picture below.

 

 

To summarize the above:

1.    IMSI - International Mobile Subscriber Identity:

  • The IMSI is a permanent identity assigned by the Service Provider
  • It is valid as long as the Service is Active with the Service Provider
  • It is stored on the USIM card and on the HSS (Home Subscriber Server)
  • It globally and uniquely identifies a user on any 3GPP PLMN (Public Land Mobile Network)

2.    IMEI - International Mobile Equipment Identity

  • The IMEI is a permanent identity assigned by the Device Manufacturer
  • Valid as long as the Device is in Use
  • Stored on the Device hardware and on the HSS (Home Subscriber Server)

3.    C-RNTI - Cell Radio Network Temporary Identity

  • Dynamic Identity assigned by the eNodeB
  • Valid as long as the UE is Connected to the eNodeB that assigned the C-RNTI
  • Stored in the UE and the eNodeB

4.    GUTI - Globally Unique Temporary Identity

  • Dynamic Identity assigned by the MME (Mobility Management Entity)
  • Valid as long as the UE is Registered with the EPC (Evolved Packet Core) and Attached to the MME that assigned the GUTI
  • Stored on the UE and the MME

5.    IP Address

  • Dynamic Identity assigned by the PGW
  • Valid as long as the UE is Registered with the EPC (Evolved Packet Core)
  • Stored in the UE and the PGW and any other node "north" of the PGW