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▀ How the PDN Gateway Works
Step
Description
3
If the UE is unknown in the MME, the MME sends an Identity Request to the UE to request the IMSI.
4
The UE responds with Identity Response (IMSI).
5
If no UE context for the UE exists anywhere in the network, authentication is mandatory. Otherwise this step is optional.
However, at least integrity checking is started and the ME Identity is retrieved from the UE at Initial Attach. The
authentication functions, if performed this step, involves AKA authentication and establishment of a NAS level security
association with the UE in order to protect further NAS protocol messages.
6
The MME sends an Update Location (MME Identity, IMSI, ME Identity) to the HSS.
7
The HSS acknowledges the Update Location message by sending an Update Location Ack to the MME. This message also
contains the Insert Subscriber Data (IMSI, Subscription Data) Request. The Subscription Data contains the list of all APNs
that the UE is permitted to access, an indication about which of those APNs is the Default APN, and the 'EPS subscribed
QoS profile' for each permitted APN. If the Update Location is rejected by the HSS, the MME rejects the Attach Request
from the UE with an appropriate cause.
8
The MME selects an S-GW using "Serving GW selection function" and allocates an EPS Bearer Identity for the Default
Bearer associated with the UE. If the PDN subscription context contains no P-GW address the MME selects a P-GW as
described in clause "PDN GW selection function". Then it sends a Create Default Bearer Request (IMSI, MME Context ID,
APN, RAT type, Default Bearer QoS, PDN Address Allocation, AMBR, EPS Bearer Identity, Protocol Configuration
Options, ME Identity, User Location Information) message to the selected S-GW.
9
The S-GW creates a new entry in its EPS Bearer table and sends a Create Default Bearer Request (IMSI, APN, S-GW
Address for the user plane, S-GW TEID of the user plane, S-GW TEID of the control plane, RAT type, Default Bearer
QoS, PDN Address Allocation, AMBR, EPS Bearer Identity, Protocol Configuration Options, ME Identity, User Location
Information) message to the P-GW.
10
If dynamic PCC is deployed, the P-GW interacts with the PCRF to get the default PCC rules for the UE. The IMSI, UE IP
address, User Location Information, RAT type, AMBR are provided to the PCRF by the P-GW if received by the previous
message.
11
The P-GW returns a Create Default Bearer Response (P-GW Address for the user plane, P-GW TEID of the user plane, P-
GW TEID of the control plane, PDN Address Information, EPS Bearer Identity, Protocol Configuration Options) message
to the S-GW. PDN Address Information is included if the P-GW allocated a PDN address Based on PDN Address
Allocation received in the Create Default Bearer Request. PDN Address Information contains an IPv4 address for IPv4
and/or an IPv6 prefix and an Interface Identifier for IPv6. The P-GW takes into account the UE IP version capability
indicated in the PDN Address Allocation and the policies of operator when the P-GW allocates the PDN Address
Information. Whether the IP address is negotiated by the UE after completion of the Attach procedure, this is indicated in
the Create Default Bearer Response.
12
The Downlink (DL) Data can start flowing towards S-GW. The S-GW buffers the data.
13
The S-GW returns a Create Default Bearer Response (PDN Address Information, S-GW address for User Plane, S-GW
TEID for User Plane, S-GW Context ID, EPS Bearer Identity, Protocol Configuration Options) message to the new MME.
PDN Address Information is included if it was provided by the P-GW.
14
The new MME sends an Attach Accept (APN, GUTI, PDN Address Information, TAI List, EPS Bearer Identity, Session
Management Configuration IE, Protocol Configuration Options) message to the eNodeB.
15
The eNodeB sends Radio Bearer Establishment Request including the EPS Radio Bearer Identity to the UE. The Attach
Accept message is also sent along to the UE.
16
The UE sends the Radio Bearer Establishment Response to the eNodeB. In this message, the Attach Complete message
(EPS Bearer Identity) is included.
17
The eNodeB forwards the Attach Complete (EPS Bearer Identity) message to the MME.
▄ Cisco ASR 5x00 Packet Data Network Gateway Administration Guide
96
PDN Gateway Overview
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