plane or data plane message is important for the successful operation of a mobile communications network. It is also important from the interworking and interoperation point of view. We then closed the chapter with the method of piggybacking of a signaling message over another signaling message, thus reducing signaling overhead, be it over the air interface or between CN elements.
5 Network Elements : Identities and Its Addressing
Introduction
This chapter covers the various identities that are used to identify and address different network elements or logical objects of mobile communications systems and networks, i.e. Global System for Mobile Communication (GSM), General Packet Radio Service (GPRS), Universal Mobile Telecommunication System (UMTS), and Long‐Term Evolution (LTE) system. Network identities used in the 5G system are described in Chapter 16. Every network element has its own identity using which the peer network element can identify the source of data received over a particular interface.
We begin with the network identities that are used at the radio access and core network (CN) domain along with their nature. A network element identity has several other aspects such as its persistency, which may be either permanent or temporary; also, an identity may have a local or global presence. A network element can have several identities, especially if it supports multiple radio access technologies (RATs). We then present the fundamental or native and mapped identity of a network element. Mapped identities are used in case of an interworking scenario where a user and its User Equipment (UE) move from one RAT to another and vice versa.
A network identity may be used by a network element to keep track of the resources allocated to another network element. Apart from the network elements, network identities are also assigned to identify and address other logical objects such as the GSM location area, GPRS routing area, and LTE/Evolved Packet System (EPS) tracking area. Such network identities are further described in Chapter 18.
5.1 Network Elements and Their Identities
A mobile communications system and network comprise numerous physical network elements or resources as well as logical objects or resources that are used as part of the design, development, implementation, and field deployment. Network elements or resource identities are logical in nature. An identity may represent a logical connection and association between two network elements. A network element, say eNodeB, may assign an identity to another network, say UE, to uniquely identify it over a particular logical interface. The network element's identities are divided into the following categories:
UE/Mobile Station (MS) Identities
Radio Access Network (RAN), UMTS Terrestrial Radio Access Network (UTRAN), Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN), 5G Next‐Generation Radio Access Network (NG‐RAN) Identities
CN Identities
Figure 5.1 illustrates the above network identities along with their natures.
Figure 5.1 Network identities, their persistency, and presence.
A network element, or a subscriber, or a logical link between peer protocols or other logical resources are identified and addressed with their corresponding identities, which can be a permanent, temporary, static, or dynamic in nature, as shown in Figure 5.1. For example, the core network may allocate a temporary identity to a mobile device for paging purposes. The CN and the access network allocate a temporary identity to an MS/UE as a result of events such as the power off/on or changes in the current location of the mobile device. This is required to protect the real identity of a mobile device while communicating with the network over the air interface.
A permanent identity is assigned to a network element as part of its administrative provision, network planning, and configuration process. For example, an International Mobile Subscriber Identity (IMSI) is allocated to uniquely identify each mobile subscriber in the GSM, UMTS, LTE/EPS, and 5G systems and may be also used to page an MS/UE by the CN.
A network identity can be a local one, i.e. visible to a particular network element such as GSM Visitor Location Register (VLR), GPRS Serving GPRS Support Node (SGSN), and LTE/EPS Mobility Management Entity (MME), or it can be used/visible across the end‐to‐end network element, i.e. from the mobile device to base station controller (BSC), UTRAN, E‐UTRAN, and 5G NG‐RAN to Core Network.
A network identity can be a global one that is exchanged across an interface connecting two peer network elements. For example, in the LTE/EPC system, a UE and MME S1AP identity is exchanged between the eNodeB and the MME over the S1 interface to uniquely identify a UE. Note that the network identities have different lengths.
5.2 Permanent Identities
There are network identities that are permanent and unique in nature. Examples of permanent network element identities are mentioned below:
An MS/UE is assigned an International Mobile Equipment Identity (IMEI) by its manufacturer that is burnt into the firmware of an MS/UE.
An IMSI is assigned by an operator to a subscriber during its provisioning in the database of the Home Location Register (HLR). An IMSI is stored in the SIM card of an MS/UE.
A Public Land Mobile Network (PLMN) identity of a network consists of the Mobile Country Code (MCC) and Mobile Network Code (MNC). A PLMN uniquely and globally identifies the operator of a particular network as an MCC is allocated by the International Telecommunication Union (ITU).
An operator‐configured network identity also can be a permanent one as long as its configuration does not change.
5.3 Temporary Identities Assigned by CN
In an end‐to‐end mobile communications network, one important function of the CN is to protect the real identity of an MS/UE while exchanging information over the air interface. The confidentiality of an MS/UE identity over the air interface is accomplished by allocating a temporary identity to it by the concerned CN element. Also, the temporary network identity is provided by the concerned network element which controls a particular service area. In the case of the GSM system, a VLR controls a location area; in the case of the GPRS system, an SGSN controls a routing area; and in the case of the LTE/EPS system, an MME controls a tracking area. Some of the temporary identities assigned to a mobile device by the concerned CN elements of GSM, GPRS, UMTS, and LTE networks are described below. For more information on these temporary identities, refer to TS 23.003 [30] and TS 23.060 [31].
5.3.1 GSM System Temporary Identities
Temporary Mobile Subscriber Identity (TMSI)
A TMSI is allocated by the CS domain VLR to protect an MS/mobile user's identity while communicating over the air interface.
