Tuesday, 21 January 2020

How MOCN RAN-Sharing Works

Shared RAN deployment scenarios are an excellent opportunity for mobile network operators to lower their investments on both, network hardware and operational costs by sharing resources.

The MORAN approach where each operator continues to have its dedicated spectrum (= radio network cells) is easy to understand.

However, the Multi-Operator Core Network (MOCN) is a bit more complex, especially if one of the involved operators asks for service assurance KPIs that apply to its - and only its - subscribers. In this case it is a prerequisite to find out which "call" belongs to which core network operator to enable further KPI correlation and aggregation.

The figure below illustrates how this works:

(click on picture for larger version)

In the System Information Block (SIB) 1 of the cell a list of PLMN-IDs is broadcasted followed by a single Tracking Area Code (which can be combined each of the PLMN-IDs to get multiple TAIs) and a single Cell Identity.

Encoding is specified in 3GPP 36.331 (RRC) as follows:

SystemInformationBlockType1 ::=     SEQUENCE {
    cellAccessRelatedInfo              SEQUENCE {
       plmn-IdentityList                 PLMN-IdentityList,
       trackingAreaCode                  TrackingAreaCode,
       cellIdentity                      CellIdentity,

The spec further defines that the ECGI is the CellIdentiy combined with the first (!!!) PLMN-ID from the PLMN-ID List:

CellGlobalIdEUTRA field descriptions
Identity of the cell within the context of the PLMN.
Identifies the PLMN of the cell as given by the first PLMN entry in the plmn-IdentityList in SystemInformationBlockType1.

So there is one and only 1 ECGI per radio cell in the network, but multiple PLMN-IDs and hence, multiple TAI, one fore each core network operator, are broadcasted.

During RRC establishement a particular UE signals on behalf on the selected PLMN-ID information element in the RRC Connection Setup Complete message to which core network operator shall be used.

This information is "translated" by the eNB into ECGI and TAI with different PLMN-IDs. While the ECGI displays the PLMN-ID of the operator that owns the RAN equipment the TAI shows the selected PLMN-ID of the UE's core network operator. 


Anonymous said...

Ah - realised how it works. There can be a maximum of 12 different PLMN identities across the up to 12 sets of up to 12 identities. So presumably, valid examples would be: 3 operators each with 4 PLMN codes, 1 operator with up to 12 PLMN codes, or 12 operators with only one PLMN code each.

Ralf Kreher said...

Your scenario is valid in case that the max number of PLMN Identities is 12. They all could belong to the same or different operators. At the end of the day it is a matter how the transport network and/or signalling traffic routing is designed between RAN and core.

According to 3GPP 36.331 the max number of PLMN-IDs in the PLMN-ID-List is 15 since Rel. 11. See ASN.1 encoding:

PLMN-IdentityList3-r11 ::=SEQUENCE (SIZE (1..16)) OF PLMN-Identity