Cisco Cisco ASR 5000
Serving GPRS Support Node (SGSN) Overview
▀ Features and Functionality
▄ SGSN Administration Guide, StarOS Release 18
The MS initiates the Modification procedure by sending a MODIFY PDP CONTEXT REQUEST message to the SGSN.
The SGSN validates the received message and sends out a BEARER RESOURCE COMMAND message to the S-GW
with a valid PTI value which is then sent to the PGW. On accepting the modification, the P-GW sends out an Update
Bearer Request with the PTI copied from the received BEARER RESOURCE COMMAND message. Upon successful
completion of the modification, the SGSN replies with the MODIFY PDP CONTEXT ACCEPT message.
The SGSN validates the received message and sends out a BEARER RESOURCE COMMAND message to the S-GW
with a valid PTI value which is then sent to the PGW. On accepting the modification, the P-GW sends out an Update
Bearer Request with the PTI copied from the received BEARER RESOURCE COMMAND message. Upon successful
completion of the modification, the SGSN replies with the MODIFY PDP CONTEXT ACCEPT message.
P-GW-Initiated PDP Context Modification
The Packet Data Node Gateway (P-GW) initiated PDP context modification procedure is used in cases when:
One or several of the EPS Bearer QoS parameters are to be modified
To add/modify/delete the TFT related to the PDP Context or BCM-Mode change
To modify the APN-AMBR
The P-GW can request the modification procedure by sending an UPDATE BEARER REQUEST message without a
PTI field to the S-GW, and the S-GW will forward the request to SGSN. The SGSN validates the request and initiates a
MODIFY PDP CONTEXT REQUEST message to the MS. On successful completion of the procedure, the SGSN will
send an UPDATE BEARER RESPONSE with an appropriate cause value.
PTI field to the S-GW, and the S-GW will forward the request to SGSN. The SGSN validates the request and initiates a
MODIFY PDP CONTEXT REQUEST message to the MS. On successful completion of the procedure, the SGSN will
send an UPDATE BEARER RESPONSE with an appropriate cause value.
HSS Initiated PDP Context Modification
The Home Subscriber Server (HSS) initiated PDP context modification procedure is used when the HSS decides to
modify the subscribed QoS, where typically QoS related parameters are changed. The parameters that may be modified
are UE-AMBR, APN-AMBR QCI and Allocation/Retention Policy.
modify the subscribed QoS, where typically QoS related parameters are changed. The parameters that may be modified
are UE-AMBR, APN-AMBR QCI and Allocation/Retention Policy.
The HSS initiates the modification by sending an Insert Subscriber Data (IMSI, Subscription Data) message to the
SGSN. The Subscription Data includes EPS subscribed QoS (QCI, ARP) and the subscribed UE-AMBR and APN
AMBR.
SGSN. The Subscription Data includes EPS subscribed QoS (QCI, ARP) and the subscribed UE-AMBR and APN
AMBR.
The S4-SGSN then updates the stored Subscription Data and acknowledges the Insert Subscriber Data message by
returning an Insert Subscriber Data Ack (IMSI) message to the HSS and sends the Modify Bearer Command (EPS
Bearer Identity, EPS Bearer QoS, APN AMBR) message to the S-GW. The S-GW forwards the Modify Bearer
Command (EPS Bearer Identity, EPS Bearer QoS, APN AMBR) message to the P-GW. Note that the EPS Bearer QoS
sent in the Modify Bearer Command does not modify the per bearer bit-rate. It is sent to carry only a change in the ARP
/ QCI received from subscription. Also, the Modify Bearer Command can be sent only for the default bearer (primary
PDP) in a PDN connection.
returning an Insert Subscriber Data Ack (IMSI) message to the HSS and sends the Modify Bearer Command (EPS
Bearer Identity, EPS Bearer QoS, APN AMBR) message to the S-GW. The S-GW forwards the Modify Bearer
Command (EPS Bearer Identity, EPS Bearer QoS, APN AMBR) message to the P-GW. Note that the EPS Bearer QoS
sent in the Modify Bearer Command does not modify the per bearer bit-rate. It is sent to carry only a change in the ARP
/ QCI received from subscription. Also, the Modify Bearer Command can be sent only for the default bearer (primary
PDP) in a PDN connection.
The P-GW modifies the default bearer of each PDN connection corresponding to the APN for which subscribed QoS
has been modified. If the subscribed ARP parameter has been changed, the P-GW shall also modify all dedicated EPS
bearers having the previously subscribed ARP value unless superseded by PCRF decision. The P-GW then sends the
Update Bearer Request (EPS Bearer Identity, EPS Bearer QoS [if QoS is changed], TFT, APN AMBR) message to the
S-GW.
has been modified. If the subscribed ARP parameter has been changed, the P-GW shall also modify all dedicated EPS
bearers having the previously subscribed ARP value unless superseded by PCRF decision. The P-GW then sends the
Update Bearer Request (EPS Bearer Identity, EPS Bearer QoS [if QoS is changed], TFT, APN AMBR) message to the
S-GW.
The S-GW sends the Update Bearer Request (EPS Bearer Identity, EPS Bearer QoS [if QoS is changed] APN-AMBR,
TFT) message to the SGSN. On completion of modification S4-SGSN acknowledges the bearer modification by sending
the "Update Bearer Response (EPS Bearer Identity)" message to P-GW via S-GW. If the bearer modification fails, the
P-GW deletes the concerned EPS Bearer.
TFT) message to the SGSN. On completion of modification S4-SGSN acknowledges the bearer modification by sending
the "Update Bearer Response (EPS Bearer Identity)" message to P-GW via S-GW. If the bearer modification fails, the
P-GW deletes the concerned EPS Bearer.
Fallback from the S4 Interface to the Gn Interface
The S4-SGSN supports fallback the S4 interface and selects the Gn interface for the 1st PDP context activation if the
APN DNS-SNAPTR resolution returns only a Gn address. This functionality allows the PDP context request to be
completed when DNS resolution returns a GGSN address instead of a P-GW address.
APN DNS-SNAPTR resolution returns only a Gn address. This functionality allows the PDP context request to be
completed when DNS resolution returns a GGSN address instead of a P-GW address.