PDN Gateway Overview
▀ Features and Functionality - Base Software
▄ Cisco ASR 5x00 Packet Data Network Gateway Administration Guide
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pool and preserves IP addresses assigned to the UE during inter-technology handover. In the current release, the native
LTE (GTP-based) P-GW service address is IPv4-based, while the e-HRPD (PMIP) address is an IPv6 service address.
During the initial network attachment for each APN that the UE connects to, the HSS returns the FQDN of the P-GW
for the APN. The MME uses DNS to resolve the P-GW address. When the PDN connection is established in the P-GW,
the P-GW updates the HSS with the IP address of the P-GW on PDN establishment through the S6b authentication
process. When the mobile user roams to the e-HRPD network, the HSS returns the IP address of the P-GW in the P-GW
Identifier through the STa interface and the call ends up in the same P-GW. The P-GW is also responsible for initiating
the session termination on the serving access connection after the call handover to the target network.
During the handover procedure, all dedicated EPS bearers must be re-established. On LTE- handovers to a target e-
HRPD access network, the dedicated bearers are initiated by the mobile access terminal. In contrast, on handovers in the
opposite direction from e-HRPD to LTE access networks, the dedicated bearers are network initiated through Gx policy
interactions with the PCRF server.
Finally, in order to support the inter-technology handovers, the P-GW uses common interfaces and Diameter endpoint
addresses for the various reference points:
S6b: Non-3GPP authentication
Gx: QoS Policy and Charging
Rf: Offline Charging
All three types of sessions are maintained during call handovers. The bearer binding will be performed by the HSGW
during e-HRPD access and by the P-GW during LTE access. Thus, the Bearer Binding Event Reporting (BBERF)
function needs to migrate between the P-GW and the HSGW during the handover. The HSGW establishes a Gxa session
during e-HRPD access for bearer binding and releases the session during LTE access. The HSGW also maintains a
limited context during the e-HRPD <-> LTE handover to reduce latency in the event of a quick handover from the LTE
RAN back to the e-HRPD network.
Important:
For more information on handoff interfaces, refer to the
Supported Logical Network Interfaces
(Reference Points)
section in this chapter.
Online/Offline Charging
The Cisco EPC platform offers support for online and offline charging interactions with external OCS and CGF/CDF
servers.
Online Charging
Gy/Ro Reference Interfaces
The StarOS 9.0 online prepaid reference interface provides compatibility with the 3GPP TS 23.203, TS 32.240, TS
32.251 and TS 32.299 specifications. The Gy/Ro reference interface uses Diameter transport and IPv6 addressing.
Online charging is a process whereby charging information for network resource usage must be obtained by the network
in order for resource usage to occur. This authorization is granted by the Online Charging System (OCS) upon request
from the network. The P-GW uses a charging characteristics profile to determine whether to activate or deactivate
online charging. Establishment, modification or termination of EPS bearers is generally used as the event trigger on the
PCRF to activate online charging PCC rules on the P-GW.
When receiving a network resource usage request, the network assembles the relevant charging information and
generates a charging event towards the OCS in real-time. The OCS then returns an appropriate resource usage
