2. Identifiers for Network Equipment (NE IDs)

This chapter describes IDs related to LTE network equipment (NE). IDs that belong to the NE ID group are GUMMEI and MMEI for MMEs, Global eNB ID and eNB ID for eNBs, ECGI and ECI for cells, and P-GW ID for P-GWs. LTE NE IDs are classified into two groups - one with PLMN ID or one without PLMN ID - depending on whether they are uniquely identified within a PLMN only or globally.

2.1 IDs to Identify MME: GUMMEI, MMEI, MMEGI and MMEC

An MME is located between E-UTRAN and EPC. It is the key control equipment that allows, on behalf of an HSS, a UE to attach to an LTE network by exchanging control signals with the HSS which has subscription information of the UE. It also supports bearer management and UE’s mobility by exchanging control signals with an S-GW (or S/P-GW) on EPC side and with an eNB on E-UTRAN side. As such, to UEs, MMEs serve as brain for the LTE network. In general, LTE operators group multiple MMEs into a group and operate them as MME groups. An MME ID for identifying an MME is allocated by the operator when the MME is installed.

An MMEI (MME Identifier) is used when identifying an MME in the network of an operator (e.g. when a network operating personnel at “A” LTE operator needs to identify an MME in “A” LTE network). However, a GUMMEI (Globally Unique MME Identifier), combination of a PLMN ID and an MMEI, is used when identifying an MME outside of the network of the operator (e.g. when “C” is operating the networks of “A” and “B” LTE operators, and “C” needs to identify an MME in “A” LTE network). In case of an MME group formed by the operator, an MMEI consist of an MMEGI that represents an MME group, and an MMEC that represents a particular MME in the MME group. Figure 2 shows the MME-related IDs and their format.

Figure 2. IDs to identify MME and format

2.2 IDs to identify eNB: eNB ID and Global eNB ID

An eNB ID is used for identifying an eNB within an operator’s network only, whereas a Global eNB ID, combination of a PLMN ID and an eNB ID, is used for identifying one outside the network. Figure 3 shows eNB/cell-related IDs and their format. To identify a cell belonging to an eNB, an ID created by adding a cell ID to the combination of an eNB ID and a Global eNB ID is used.

eNB IDs and cell IDs are allocated by the network operator when an eNB is installed. Once installed, the eNB begins a procedure for setting up an S1 link between an MME and itself. When it requests the MME for S1 link setup, it reports its Global eNB ID and supported TAs (See Chapter III for more information about TAs) and a CSG (Closed Subscriber Group) ID if CSGs are supported. A CSG is a cell open to only a certain group of subscribers, and is made up of a single cell or a collection of cells. The MME sends served GUMMEIs to the eNB as a response to the S1 link setup request, providing MME pool information.

Figure 3. IDs to identify eNB and cell

2.3 ID to Identify P-GW: P-GW ID

When a UE attempts to attach an LTE network, the LTE network provides the UE with PDN connection (See LTE Identification III for more information about PDN connection). To set up PDN connection between the UE and a PDN, the MME has to know the PDN to connect the UE and the P-GW to attach the UE through. The default PDN for the UE has already been provisioned in the HSS as a subscribed profile. So, the MME can simply use this information downloaded from the HSS. For the P-GW, there are two ways of allocating – fixed allocation in which the network operator provisions a P-GW ID as a subscribed profile in the HSS, and dynamic allocation in which the MME selects a P-GW according to the P-GW selection policy set by the operator. In case of fixed allocation, the HSS informs the MME of the P-GW ID, so that the MME can request the P-GW for PDN connection.

P-GW IDs are used to identify P-GWs, and can be in either IP address or FQDN (Fully Qualified Domain name) forms. Figure 4 illustrates the P-GW ID assigned according to the fixed allocation policy and its format. For example, there can be a UE whose default PDNs are allocated as PDN 1 (Internet) for Internet services and PDN2 for voice services. When the UE initially accesses an LTE network, the MME requests the HSS for subscription information of the UE. The HSS then informs the MME that (i) the default PDNs of the UE are PDN 1 (Internet) and PDN 2 (IMS), and that (ii) the P-GW connecting PDN 1 is P-GW 1 and the one for PDN 2 is P-GW 3. The MME then establishes PDN connection between the UE and the Internet through P-GW 1, and one between the UE and IMS through P-GW 3.

For P-GW ID assigned according to the dynamic allocation policy, the MME obtains a P-GW IP address list through DNS query, selects one from the list, and requests the P-GW to establish PDN connection (See “LTE Identification III: Session/Bearer ID” for more information about PDN connection and EPS bearers.). If a P-GW list is provided by DNS, the MME selects one from the list in accordance with the P-GW selection policy.

Figure 4. ID to identify P-GW