Cisco Cisco ASR 5000
MOCN for 2G SGSN
▀ How It Works
▄ SGSN Administration Guide, StarOS Release 18
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The BSC receives the LLC frame with foreign [or random] TLLI =X.
The BSC works in a Shared RAN MOCN, and, therefore, forwards the message in a BSSGP ULUNITDATA message with
an additional redirect attempt flag set. The flag indicates that the SGSN shall respond to the attach request with a BSSGP DL-
UNITDATA message providing when relevant a redirection indication flag set to inform the BSC that a redirection to
another CN must to be performed. The selection of a CN node is based on NRI (valid or invalid) or random selection. The
mechanism defined for Gb-Flex in TS 23.236 [8] is used.
The BSC works in a Shared RAN MOCN, and, therefore, forwards the message in a BSSGP ULUNITDATA message with
an additional redirect attempt flag set. The flag indicates that the SGSN shall respond to the attach request with a BSSGP DL-
UNITDATA message providing when relevant a redirection indication flag set to inform the BSC that a redirection to
another CN must to be performed. The selection of a CN node is based on NRI (valid or invalid) or random selection. The
mechanism defined for Gb-Flex in TS 23.236 [8] is used.
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The SGSN receives the BSSGP UL-UNITDATA message with the redirect attempt flag set. It then knows it may have to
provide the BSC with a redirection indication flag set or a redirection completed flag set.
provide the BSC with a redirection indication flag set or a redirection completed flag set.
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The SGSN needs the IMSI of the UE retrieves it either from the old SGSN or from the UE as in this example. By comparing
the IMSI with the roaming agreements of the CN operator, SGSN A discovers that roaming is not allowed or that roaming is
allowed but CS/PS coordination is required. The Attach procedure is aborted.
the IMSI with the roaming agreements of the CN operator, SGSN A discovers that roaming is not allowed or that roaming is
allowed but CS/PS coordination is required. The Attach procedure is aborted.
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5a) A BSSGP DL-UNITDATA message is sent back to the BSC with a redirection indication flag set containing the reject
cause, the attach reject message, and the original attach request message received from the UE. The V(U) shall also be
included in the message. The IMSI is also included in the message. The BSC selects a SGSN B in the next step. The already
tried SGSN A is stored in the BSC during the redirect procedure so that the same node is not selected twice.
5b) The BSC makes a short-lived binding between the TLLI =X and SGSN ID so that it points to SGSN B.
cause, the attach reject message, and the original attach request message received from the UE. The V(U) shall also be
included in the message. The IMSI is also included in the message. The BSC selects a SGSN B in the next step. The already
tried SGSN A is stored in the BSC during the redirect procedure so that the same node is not selected twice.
5b) The BSC makes a short-lived binding between the TLLI =X and SGSN ID so that it points to SGSN B.
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The BSC sends a new BSSGP UL-UNITDATA to the next selected SGSN B with the original attach request message (for
CS/PS coordination the BSSGP UL-UNITDATA may also be sent back to the first SGSN depending on the outcome of the
coordination). Redirect attempt flag is set and IMSI is included to avoid a second IMSI retrieval from the UE or old SGSN
and to indicate that PS/CS domain coordination has been done in BSC (if enabled in BSC). The V(U) shall also be included
in the message. The SGSN B receiving the message starts its attach procedure.
CS/PS coordination the BSSGP UL-UNITDATA may also be sent back to the first SGSN depending on the outcome of the
coordination). Redirect attempt flag is set and IMSI is included to avoid a second IMSI retrieval from the UE or old SGSN
and to indicate that PS/CS domain coordination has been done in BSC (if enabled in BSC). The V(U) shall also be included
in the message. The SGSN B receiving the message starts its attach procedure.
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SGSN B does support roaming for the HPLMN of the IMSI; authentication is done and RAN ciphering is established. The
value of V(U) in SGSN-B is set according to the received value from BSC. Uplink LLC frames are routed to SGSN B despite
the NRI of the TLLI=X pointing to SGSN A.
value of V(U) in SGSN-B is set according to the received value from BSC. Uplink LLC frames are routed to SGSN B despite
the NRI of the TLLI=X pointing to SGSN A.
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SGSN B updates the HLR and receives subscriber data from HLR Subscriber data allows roaming, and the SGSN B
completes the attach procedure.This includes the assignment of a new P-TMSI with an NRI that can be used by BSC to route
subsequent signalling between UE and the correct SGSN (Gb-Flex functionality).
completes the attach procedure.This includes the assignment of a new P-TMSI with an NRI that can be used by BSC to route
subsequent signalling between UE and the correct SGSN (Gb-Flex functionality).
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A BSSGP DL-UNITDATA Attach accept message is sent to BSC with the Redirection Completed flag set. The BSC knows
that the redirect is finished and can forward the Attach Accept message to the UE and clean up any stored redirect data.
SGSN B is allowed to reset the XID parameter only after the Attach Request is accepted.
that the redirect is finished and can forward the Attach Accept message to the UE and clean up any stored redirect data.
SGSN B is allowed to reset the XID parameter only after the Attach Request is accepted.
10 The Attach Accept is forwarded to the UE. The UE stores the P-TMSI with the Gb-Flex NRI to be used for future signalling,
even after power off.
11 UE responds with an Attach Complete message (P-TMSI [re-]allocation if not already made in Attach Accept). The Attach
Complete uses the new TLLI. After this, the BSS releases the binding between TLLI=X and SGSN B.
If the BSC finds no SGSNs to redirect to after receiving a BSSGP DL-UNITDATA message with the Redirection
Indication flag set, it compares the cause code with cause codes from other BSSGP DL-UNITDATA messages it has
previously received for this UE. A cause code ranking is done and the “softest” cause code is chosen. The corresponding
saved Attach Reject message is returned to the UE.
Indication flag set, it compares the cause code with cause codes from other BSSGP DL-UNITDATA messages it has
previously received for this UE. A cause code ranking is done and the “softest” cause code is chosen. The corresponding
saved Attach Reject message is returned to the UE.
Each CN node that receives a BSSGP UL-UNITDATA, runs its own authentication procedure. This may in some rare
situations cause the UE to be authenticated more than once. However, the trust-model used is that one CN operator shall
not trust an authentication done by another CN operator. This is not an optimal usage of radio resources, but given the
rare occurrence of this scenario, the increased signalling is insignificant.
situations cause the UE to be authenticated more than once. However, the trust-model used is that one CN operator shall
not trust an authentication done by another CN operator. This is not an optimal usage of radio resources, but given the
rare occurrence of this scenario, the increased signalling is insignificant.
During the redirect procedure the BSC keeps a timer, which corresponds to the UE timer for releasing the RR
connection (20 seconds). If the BSC when receiving a BSSGP DL-UNITDATA message with the Redirection
Indication flag set finds that there is insufficient time for another redirect, further redirect attempts are stopped (for this
connection (20 seconds). If the BSC when receiving a BSSGP DL-UNITDATA message with the Redirection
Indication flag set finds that there is insufficient time for another redirect, further redirect attempts are stopped (for this