Null-NRI based 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
The operator wants
to carry out a scheduled maintenance.
The operator wants
to perform a load re-distribution.
To avoid an overload.
Offloading has to be
performed with minimum impact on the end users.
Types of MS Offloading:
IMSI based offloading
Null-NRI based offloading
is carried out in the following three phases:
UEs performing a
RAU or Attach are moved to other SGSN in the pool.
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.
A new Routing Area
Update is triggered by setting the periodic routing area update timer
to a sufficiently low value in the accept message.
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.
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").
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.
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".
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 off-loaded in the same manner. The move operation will not
overload the network, as throttling is supported for both Attach
and Inter SGSN RAU procedures.
Target NRI based offloading
was primarily introduced so that subscribers can be offloaded to
a chosen SGSN. In the case of NULL-NRI based offloading there is
no control on which SGSN the subscribers are offloaded to. SGSN
offloads subscribers by assigning NB-RAI, stamping Target-NRI in
PTMSI and reducing periodic routing area update timer during Attach/RAU
is carried in the following three phases:
With Target-NRI based
method of offloading though there is control on the SGSN to which the
subscribers are offloaded, there is no control on the subscribers
being offloaded to the SGSN. IMSI-based offloading enhancement allows
the operator to choose the subscribers to be offloaded to a particular
When a Attach accept
or a RAU accept is issued, the offloading configuration is verified
and if offloading is enabled, the corresponding NRI is issued (if
it is not issued earlier). In case the specific IMSI based offloading
configuration is configured, the configured target-nri is used. When
offloading is enabled, if ptmsi allocation configuration is absent,
a ptmsi is allocated to the subscriber in Attach/RAU accept.
On receiving an activation
trigger from the MS, the subscriber is detached and the re-attach required
is set to true. The MS will return an attach in due time, after
which the MS is offloaded to another SGSN by setting the Target-NRI
and NB-RAI appropriately.
The subscriber is cleared
unconditionally and a detach is sent by setting the re-attach required
to true. The subscriber is lost at this stage. In the next attach,
the subscriber is offloaded to the configured SGSN.
For information on the
procedure to configure MS-Offloading, refer to the section "Configuration
of SGSN Pooling - Procedure to configure MS-Offloading".