Cisco Cisco ASR 5000
SGSN Pooling
▀ How it Works
▄ SGSN Administration Guide, StarOS Release 18
4. The designated default SGSN relays the GTP message to the source SGSN in the pool, which is located using
the NRI in the P-TMSI and hence the DNS query with NRI, LAC and RAC.
5. In the implementation above both SGSN-1 and SGSN-2 are designated as default SGSNs to load share the GTP
signalling traffic.
6. For every LAC/RAC in the pooled areas the DNS resolves the query into two IP addresses pertaining to the Gn
loopback addresses of SGSN-1 and SGSN-2 respectively.
MS Offloading
MS offloading is a procedure of offloading the subscribers from one SGSN in the pool to another SGSN within the
same pool. Offloading is performed during the following scenarios:
same pool. Offloading is performed during the following scenarios:
1. The operator wants to carry out a scheduled maintenance.
2. The operator wants to perform a load re-distribution.
3. To avoid an overload.
Offloading has to be performed with minimum impact on the end users.
Types of MS Offloading:
1. Null-NRI based.
2. Target-NRI based.
3. IMSI based offloading
Null-NRI based Offloading
Null-NRI based offloading is carried out in the following three phases:
Phase - 1
1. UEs performing a RAU or Attach are moved to other SGSN in the pool.
2. When the SGSN receives the Routing Area Update or Attach request, it returns a new P-TMSI with the null-
NRI, and non-broadcast LAC and RAC in the accept message.
3. A new Routing Area Update is triggered by setting the periodic routing area update timer to a sufficiently low
value in the accept message.
4. The UE sends a new Routing Area Update, the BSC then routes this RAU to a new SGSN due to the presence of
a null-NRI. The BSC uses a round robin mechanism to allocate an SGSN for this UE.
Phase - 2
1. All PDP context activation requests are rejected and the UEs are requested to detach and re-attach (Detach
request sent from the network with cause code “reattach required”).
2. When the UEs re-attach, the SGSN moves them as described above in “Phase 1”, that is, by sending the null-
NRI and non-broadcast LAC and RAC and triggering a periodic RAU update.
Phase -3
This phase includes scanning through the remaining UEs and initiating a detach procedure for them. The UEs are
requested to detach and re-attach, this results in the UEs moving as described in “Phase 1”.
requested to detach and re-attach, this results in the UEs moving as described in “Phase 1”.
UEs being moved from one SGSN can be stopped from registering to the same SGSN again by issuing a CLI command
in BSCs connected to the pool. UEs moving into a pool area may also be stopped from registering into a SGSN being
in BSCs connected to the pool. UEs moving into a pool area may also be stopped from registering into a SGSN being