Closed Subscriber Groups (CSG / VCSG)
This document describes how OmniMSC handles Closed Subscriber Groups (CSG): the CSG subscription data held per subscriber, the access-control decision made when a subscriber attempts to use a CSG cell, and the MAP interaction with the CSG Subscriber Server (CSS) that keeps that subscription data current. For the general subscriber data model and how subscription data is downloaded from the HLR, see MAP Operations. For Operator Determined Barring, a related access-control mechanism, see Supplementary Services.
Overview
A Closed Subscriber Group identifies a set of subscribers permitted to access one or more CSG cells (for example a Home NodeB / femtocell serving a specific home, office, or campus). Each CSG is identified by a CSG Identity (a 27-bit value, coded per 3GPP TS 23.003). A CSG cell broadcasts its CSG Identity in system information; a subscriber camping on that cell receives normal service only when the subscriber is a member of that CSG and the membership has not expired (TS 22.011 Section 8).
The set of CSGs a subscriber may access is the VCSG subscription data (also called the Allowed CSG list). This data is held by the CSG Subscriber Server (CSS) and provided to the serving node (VLR / SGSN). OmniMSC stores it on the subscriber record and evaluates access at the point of use, in the same way Operator Determined Barring is enforced by the serving node rather than the HLR.
CSG Subscription Data
Each subscriber record carries a CSG membership list. Every entry describes one Closed Subscriber Group the subscriber belongs to and, optionally, when that membership expires.
| Field | Type | Description |
|---|---|---|
csg_id | Integer | CSG Identity broadcast by the CSG cell. A 27-bit value coded per TS 23.003. |
expiry | DateTime / null | Membership expiry. A membership with null expiry never expires. Once the current time reaches or passes the expiry instant, the membership is treated as expired and access is denied. |
The membership list is populated from the VCSG subscription data supplied by the CSS and is cleared by MAP cancelVcsgLocation (see MAP Interaction). Removing only the CSG subscription data keeps the subscriber registered; other subscription data (basic services, supplementary services, CAMEL, Operator Determined Barring) is untouched.
Access Control
When a subscriber attempts to access a cell that broadcasts a CSG Identity, OmniMSC evaluates the subscriber's membership list against the broadcast CSG Identity. A cell that does not broadcast a CSG Identity is a normal (non-CSG) cell and is always accessible; the check below applies only to CSG cells.
The decision has three outcomes:
| Outcome | Condition | Meaning |
|---|---|---|
| Allowed | The subscriber has a membership for the broadcast CSG Identity and it has not expired. | The subscriber receives normal service on the CSG cell. |
| Denied (not a member) | The subscriber has no membership for the broadcast CSG Identity (including the case where the subscriber has no CSG subscription data at all). | Access to the CSG cell is rejected. |
| Denied (expired) | The subscriber has a membership for the broadcast CSG Identity, but its expiry has passed. | Access to the CSG cell is rejected. The stale membership remains until the CSS updates or cancels it. |
The expiry boundary is inclusive: when the current time equals the expiry instant, the membership is treated as expired.
MAP Interaction with the CSS
The CSG Subscriber Server maintains the master copy of a subscriber's VCSG subscription data and keeps the serving node in step using MAP operations (3GPP TS 29.002). OmniMSC handles the following.
| Operation | Opcode | Direction | Effect |
|---|---|---|---|
| updateVcsgLocation | 53 | Serving node to CSS | Registers the serving node for VCSG service and retrieves the subscriber's CSG subscription data. |
| cancelVcsgLocation | 36 | CSS to serving node | Cancels this serving node's VCSG registration for the subscriber. OmniMSC removes all CSG subscription data while keeping the subscriber registered. |
cancelVcsgLocation is sent by the CSS when the subscriber's VCSG registration is to be removed from this node (for example the subscriber's CSG membership was revoked, or the subscriber moved to another node). The argument carries the subscriber identity (IMSI). On receipt, OmniMSC clears the subscriber's CSG membership list and acknowledges the operation.
CSG subscription data can also be withdrawn as part of MAP DeleteSubscriberData (the CSG data category), which removes the CSG membership list while leaving the subscriber registered and other data intact. See MAP Operations for the general DeleteSubscriberData procedure.
3GPP Specification References
| Specification | Title | Relevance |
|---|---|---|
| TS 23.060 | General Packet Radio Service (GPRS); Service description | CSG access control and VCSG location handling in the packet domain |
| TS 22.011 | Service accessibility | CSG concept, Allowed CSG list, CSG membership and expiry |
| TS 23.003 | Numbering, addressing and identification | CSG Identity coding (27-bit) |
| TS 29.002 | Mobile Application Part (MAP) specification | updateVcsgLocation, cancelVcsgLocation, CSG subscription data, CSS interaction |