10 Direct Tunnel Signalling Procedures

1 © Nokia Siemens Networks CN3203EN01GLN00

PCNSIG

Direct Tunnel

Signalling procedures


Objectives

After the module, the participants will be able to:

• Describe the Direct Tunnel Signalling procedures and messages

• Explain the new Information Elements in Direct Tunnel Signalling procedure messages


Signalling procedures – PDP context activation (1/5)

1. Activate PDP context request

2. Create PDP context request (SGSN IP address and TEID)

8. Update PDP context response 9. Activate PDP context accept

SGSN informs GGSN about RNC IP and TEID and if the QoS attributes

are downgraded

7. Update PDP context request (RAN IP address and TEID)

4. RAB assignment request (GGSN IP address and TEID)

5. RAB establishment response (RAN IP address and TEID)

6. Invoke trace

3. Create PDP context response (GGSN IP address and TEID)

RAB establishment

SGSN GGSN RAN

node

GTP-U


Signalling procedures – Intra SGSN SRNS relocation (2/5)

SGSN GGSN Target

RAN

node

3. Relocation Request (IMSI, Cause, CN Domain Indicator, S- RNC, RAB to be set up)

2. Relocation Required (Cause, S-RNC Id, T-RNC Id, used RAB)

6. Relocation Command (RAB to be released)

10. Relocation Detect

7c. Forward SRNS Context (seq nbr of GTP-PDUs to be transmitted next in UL and DL)

7d. Forward SRNS Context (seq nbr of GTP-PDUs to be transmitted next

in UL and DL )

)

8. RRC message

11. RRC message

13a. Update PDP Context Request

14a. Iu Release Command

14b. Iu Release Complete

7. Forwarding of data (session context data, seq nbr of GTP-PDUs to be transmitted next in UL and DL )

5. Relocation Request Acknowledge (RAB id)

Source

RAN

node

1. Relocation Decision

9. UE detected

12. Relocation Complete

13b. Update PDP Context Response

)

Needed for hard handover

(No Iur available)

GTP-U

GTP-U

7a. SRNS Context Request (session context data req )

7b. SRNS Context Response (session context data provided )

4. RAB establishment


Signalling procedures – RAB (Iu) release (3/5) RNC releases PDP context (inactivity timer expires)

1. RAB release request

2. Update PDP context request (SGSN IP address and TEID)

3. Update PDP context response (GGSN IP address and TEID)

SGSN GGSN RAN

node

GTP-U

GTP-U

Note: After RAB release the GTP user plane needs to be established between SGSN and GGSN, so that the

dowlink data packets are not lost


Signalling procedures – Service request (4/5)

Subsriber in idle state

1. Service request


2. Service accept

6. Update PDP context response

SGSN GGSN RAN

node

GTP-U


4. RAB assignment response (RAN IP address and TEID)

GTP-U


Signalling procedures – Downlink data (5/5) Subscriber in idle state

3. Service request (Paging response)


4. Service accept


SGSN GGSN RAN

node

GTP-U


6. RAB assignment response (RAN IP address and TEID)

GTP-U

1. GTP PDU 2. Paging request

9. GTP PDU


Direct Tunnel – 3GPP support

DT-flag:

• DTI-flag indicates to the GGSN that this is a DT context

• In case the RNC has lost the RABs then it sends an “Error indication” of this to the GGSN. GGSN marks the tunnel unavailable and inform SGSN of the EI. SGSN starts an Update PDP context (DT flag=EI).

• If DT is used in a network where the GGSN does not support the feature, it is possible to lose the DT PDP context. This happen in uncrontrolled situation such as RAN node failure where the RAB is dropped during data transmission. N this case, GGSN receives an “Error indication” from RAN node and deletes the related PDP context.

3GPP Rel-7 introduced two new fields in the ”Update PDP context” message for the DT usage.

No-Qos-flag:

• No-QoS-flag indicates that the GGSN will accept the QoS received from SGSN. This verifies

that the Update PDP context message related to DT shall not initiate QoS re-negotiation. This will not prevent GGSN from re-negotiating the QoS later for some other reason.


Direct Tunnel Flags IE (3GPP 29.060 section 7.7.81)

Update PDP Context Request The Direct Tunnel Flags information element is used to hold values for multiple bit flags related to the GTP direct tunnel

feature.

The Direct Tunnel Indicator (DTI) bit field is relevant only for the Update PDP Context procedure to indicate whether the

procedure is being used for the establishment of a direct GTP-U tunnel between the GGSN and the RNC.

The GPRS-CSI (GCSI) bit field is relevant only for the SRNS Relocation procedure and Inter-RAT handover procedure

and is used to indicate to the new SGSN whether the subscriber's profile in the old SGSN contained a GCSI.

The Error Indication (EI) bit is relevant only for the case when a direct tunnel is used and the GGSN receives an Error

Indication message from the RNC.

Bits marked as "Spare" shall be assigned the value 0 by the sending node and shall not be evaluated by the receiving

node.

Bits

Octet

s

8 7 6 5 4 3 2 1

1 Type=182 (Decimal)

2-3 Length

4 – n Spare Spare Spare Spare Spare EI GCSI DTI

NOTE:In the present release of the present document, n=4 in the above figure. In future releases of the present document, may be

greater.


Direct Tunnel Establishment – PDP Context Activation procedure

GGSN

8. Activate PDP Context Accept

3. Create PDP Context Response

(GGSN IP address, TEID)

2. Create PDP Context Request

(SGSN GTP-U IP address, TEID)

1. Activate PDP Context Request

SGSN RNC

7. Update PDP Context Response

6. Update PDP Context Request

(RNC GTP-U IP address and TEID, DTI flag + No QoS negot. flag)

4. RAB Assignment Request

(GGSN IP address, TEID)

5. RAB Assignment Response

(RNC GTP-U IP address and TEID)


New Information Element messages in PDP Context activation procedure

On Iu-PS Control Plane interface (between RNC and SGSN), no message or Information Element is modified, only the Transport Layer address of GGSN is used instead of SGSN.

On the Gn Control Plane interface (GTP-C between SGSN and GGSN), Update PDP Context request/response messages are added.

Update PDP Context request contains a new Information Element (Direct Tunnel Flag) and the Transport Layer address of RNC is used instead of SGSN.


Signalling procedures – RNC failure (RANAP or RNC node reset)

1. Update PDP context request (SGSN IP address and TEID)


SGSN GGSN RAN

node

GTP-U

GTP-U

Iu guard timer

SCTP link lost

Iu connection lost for that UE!

RAN failure or broken cable


Error Handling with Direct Tunnel (1/2)

GGSN

3. Update PDP context request

EI (Error Indication) flag

2. Error Indication

SGSN RNC



(SGSN GTP-U IP address and TEID, EI flag)

5. RAB Establishment

RNC failure

1. GTP PDU

6. Update PDP Context Request

(RNC GTP-U IP address and TEID, DTI flag + No QoS negot. flag)

8. GTP PDU


New Information Element messages in Error Handling procedure (2/2)

• Error Indication message is sent over the GTP-U (between RNC and GGSN)

• Update PDP Context request sent over GTP-C (between GGSN and SGSN) contains EI (Error Indication) flag.


Direct Tunnel impacts for Iu-PS Interface

• no new messages or new Information Elements on Iu-PS interface introduced by Direct Tunnel.

• different Transport Layer Address value (the GGSN IP Address instead of SGSN IP Address) is the only visible effect on Iu-PS interface.

There is no impact on Iu-PS interface.


Direct Tunnel impacts for Gn/Gp Interface

• One new Information Element (Direct Tunnel Flags) on Gn interface introduced by Direct Tunnel. The Direct Tunnel Flags IE is used in the Update PDP Context Request message.

• The signalling flow on Gn interface is affected by one additional Update PDP Context Request/Response between SGSN and GGSN during PDP Context activation (without QoS change) when Direct Tunnel is used.

The Gn interface is impacted by the Direct Tunnel feature.


Direct Tunnel PDP Context Activation messages

Create PDP Context Request message (1/2)


Create PDP Context Request message (1/2)




Create PDP Context Response message (1/2)



Create PDP Context Response message (2/2)



Update PDP Context Request message



Update PDP Context Response message

10 Direct Tunnel Signalling Procedures

Documents