PDN GW selection function

Source : 3GPP TS 29.401 Clause: 4.3.8

The inspiration to the post is one of the comments raised by my blog reader. What does this PDN GW selection function mean? How is PDN allocated to the user? Can a UE have multiple PDN connections? Lets take each one step by step.

UE creates attach request and sends it to eNB. eNB forwards the request to MME. During Initial attach a UE "may send" APN in attach request. If UE sends attach request without an APN then MME will contact HSS which returns a default APN. If UE sends attach request with APN then HSS will return the corresponding PDN GW identity which may contain PDN GW IP address or PDN GW FQDN. If the case is where UE already has a PDN connection ,then UE "must" send the APN in attach request. Once the PDN GW is identified then S-GW forwards the attach request to that PDN GW.

So if UE is running two applications app1 and app2 and if app2 wants to use new pdn connection, then it sends out attach request by placing the new APN address. Basic idea is a APN is bound to PDN.

Alternatively a UE may ask for dedicated bearer establishment to the PDN GW. Then app1 can run on one bearer and app2 can run other bearer, but both are pointing to the same PDN. Note that network initiated dedicated bearer can only initiated by PDN GW to which UE has attached to.

Comments/Questions are greatly welcomed.

LTE Attach Procedure

At last I found what I needed. It was always there since beginning but I never found time to read it. So I went through the spec TS 23.401 (GPRS enhancements for EUTRAN) and I found all the call flows required. I started with the initial on, EUTRAN Initial Attach procedure. Now my interests lie in GTP based interfaces (I am avoiding PMIP here) and only on IP side. So I eliminate all the radio signals, S1- MME request response and what I arrive at is the below. A simple diagram depicting initial attach procedure of UE.

  

A UE needs to register with the network to receive services that require registration. This registration is described as network attachment. The always on IP connectivity for UE of the EPS is enabled by establishing a default EPS bearer during Network attachment. The attach procedure may trigger one or multiple dedicated bearer establishment procedures to establish dedicated EPS bearer for that UE. During the attach procedure UE may request for an IP address allocation.

Now lets look at the request/responses and the IE;s that are passed in them. As I said, I am not much of radio guy I will directly jump into

3. Create Default Bearer Request: MME ----> S-GW

MME selects a S-GW and allocate an EPS bearer Identity for default bearer associated with UE. Then it sends Create Default Bearer Request with following IE;s.

• IMSI
• MSISDN
• MME TEID for Control Plane
• PDN GW address
• PDN Address
• APN
• RAT Type
• Default EPS bearer QoS
• PDN Type
• APN-AMBR
• EPS Bearer Identity (EBI)
• Protocol COnfiguration Options
• Handover Indication
• ME identity
• User Location Information (ECGI)
• MS Info Change Reporting Support Indication
• Selection Mode
• Charging Characteristics
• Trace Reference
• Trace Type
• Trigger ID
• OMC identity
• Maximum APN Restriction
• Dual Address Bearer Flag
• Protocol over S5/S8.

Subscribed APN AMBR for the APN is provided in this message. MSISDN is included if the attach type indicates handover. Selection Mode indicates that a subscribed APN was selected. Charging characteristics indicates which kind of charging the bearer context is liable for.

4. Create Default Bearer Request : S-GW -------->PDN-GW

The S-GW creates a new entry in its EPS bearer table and sends a create default bearer request message to PDN GW indicated by the PDN GW address received in the previous step. This message contains

• IMSI
• MSISDN
• APN
• S-GW address for User Plane
• S-GW TEID of the User Plane
• S-GW TEID of the Control plane
• RAT Type
• Default EPS bearer QoS
• PDN Type
• PDN Address
• Subscribed APN-AMBR
• EPS bearer identity
• Protocol Configuration Options
• Handover Indication
• ME Identity
• USer Location Information (ECGI)
• MS Info change Reporting SUpport Indication
• Selection Mode
• Charging Characteristics
• Trace Reference
• Trace Type
• Trigger ID
• OMC Identity
• Max APN Restriction
• Dual Address Bearer Flag.

5. Create Default Bearer Response : PDN-GW ------> S-GW

The P-GW creates a new entry in its EPS bearer context table and generates a charging ID. THe new entry allows the P-GW to route user plane PDU's between the S-GW and the packet data network and to start charging. The PDN-GW returns a create default bearer response with following IE's

• PDN-GW address for the user plane
• PDN GW TEID of the user plane
• PDN GW TEID of the control plane
• PDN Type
• PDN address
• EPS Bearer Identity
• EPS bearer QoS
• Protocol Configuration Options
• Charging ID
• Prohibit payload compression
• APN Restriction
• Cause
• MS Info Change Reporting Action
• APN AMBR

6. Create Default Bearer Response : S-GW --------> MME

S-GW returns a create default bearer response to MME with following IE's.

• PDN Type
• PDN Address
• S-GW address for User Plane
• S-GW TEID for User Plane
• S-GW TEID for control plane
• EPS bearer Identity
• EPS bearer QoS
• PDN GW addresses and TEID's at the PDN GW for uplink traffic
• Protocol configuration options
• Charging ID
• Prohibit payload compression
• APN restriction
• Cause
• MS Info change Reporting Action
• APN-AMBR

These are the Default bearer request and response messages. Watch out more messages.