Mobile Communications

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Concepts and Terms


IMSI International Mobile Subscriber Identity is an unique number, usually fifteen digits, associated with Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UTMS) network mobile phone users, identifying a GSM subscriber. The number has two parts: first six digits (North American standard) identify the network operator in a specific country with whom the subscriber holds an account. The second part is allocated by the network operator to uniquely identify a subscriber.

LTE Architecture

LTE Long Term Evolution.


User Equipment.


System Architecture Evolution (SAE) is the core network architecture of 3GPP's LTE wireless communication standard.


Evolved packet core is the main component of the SAE.


The S1 Application Protocol (S1AP) describes the communication between UE and EPC.


Mobility Management Entity is the main signaling node in the EPC, responsible for initiating paging and authentication of the mobile device. MME retains location information at the tracking area level for each user and then selects the appropriate gateway during the initial registration process. MME connects to the evolved node b (eNB) through the S1-MME interface and connects to S-GW through S11 interface. Multiple MMEs can be grouped together in a pool to meet increasing signaling load in the network. MME is also the main control node in the LTE access network. It has the following functions:

  • Network Access Control: MME manages authentication and authorization for the UE. It also facilitates UE access to the network to gain IP connectivity.
  • Radio Resource Management: MME works with the HSS and the RAN to decide the appropriate radio resource management strategy (RRM) that can be UE-specific.
  • Mobility Management: provide seamless inter-working.
  • Roaming Management.
  • UE Reachability. MME manages communication with the UE and HSS to provide UE reachability and activity-related information.
  • Tracking Area Management: allocates and reallocates a tracking area identity list to the UE.
  • Lawful Intercept
  • Load Balancing between S-GWs: Directs UEs entering an S-GW pool area to an appropriate S-GW. This achieves load balancing between S-GWs.
Network Attach Procedure


Serving Gateway


PDN Gateway


Home Subscriber Server is a central database that contains user-related and subscription-related information. Functions:

  • mobility management
  • call and session establishment support
  • user authentication and access authorization.

Radio Resource Control RRC

The Radio Resource Control (RRC) protocol is used in LTE on the Air interface. This is a layer that exists between UE and eNB, and exists at the IP level.