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EMM Procedure 4. Service Request
February 09, 2014 | By Netmanias (tech@netmanias.com)
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Page 2 of 6




2. Cases of Service Request


When a UE is still registered at a network, but its S1 connection is released due to inactivity, the UE has no radio resources available. That is, the UE is in EMM-Registered, but ECM-Idle state. If, at this time, new traffic is generated from the UE, or from the network to the UE, the UE requests the network for services, transmitting to ECM-Connected state. Then an ECM connection (RRC + S1 signaling connections) and E-RAB (DRB + S1 bearer) are setup in the control and user planes, respectively, allowing the UE to receive or send traffic. In case the network is sending out traffic to a UE, it first informs the UE of such intent so that the user can request for services. 


When the UE has new traffic to send, or learns about the network’s intent to send new traffic, it sends the MME a Service Request message, transmitting to ECM/RRC-Connected state. Then the UE, by using the allocated radio and network resources, can receive or send traffic. Service requests can be triggered by a UE or by a network, and can be categorized as follows depending on where the new traffic is generated:


• Service Request Case 1: UE-triggered New Traffic

   When there is uplink data to be sent from UE to the network

• Service Request Case 2: Network-triggered New Traffic

   When there is downlink data to be sent from the network to UE


Figure 1 shows the connections established in the user and control planes, and the states of UE and MME, before and after the service request is made. Before the service request, the user is in EMM-Registered and ECM/RRC-Idle state. Thus, only the resources allocated by EPC are kept unreleased while those allocated by E-UTRAN are already released. In the control plane, S5 GTP-C and S11 GTP-C tunnels remain active while the ECM connection is lost. In the user plane, the S5 bearer and the uplink S1 bearer are kept active while the downlink S1 bearer and the DRB are released.


After the service request, the UE, now with the allocated E-UTRAN resources, is in EMM-Registered and ECM/RRC-Connected. Also, we can see all of the bearers and connections in the EPS bearer (i.e. DRB, S1 bearer and S5 bearer) and the signaling connection (i.e. ECM connection, S11 GTP-C and S5 GTP-C tunnels) are now established to support traffic delivery between the UE and the network (UE through P-GW).



Figure 1. Connections and States before/after Service Request


Page 2 of 6
Joseph 2014-04-21 08:04:47
Hi Netmanias,
I know, in the step "15) EPS Session Modification (optional)" in Figure 3, when P-GW forwards a CCR message to PCRF, IMSI and UE’s IP address are sent as well to inform whose session is sent. How does the P-GW know to which user the session belongs to when no user ID is included in the Modify Bearer Request message that S-GW is sending to the P-GW?
Netmanias 2014-04-21 16:39:46
Hi Joseph,

In control plane, there is a GTP-C tunnel (per user) established between S-GW and P-GW. This tunnel (or user of this tunnel) is identified by a TEID. The TEID value is carried in the GTP tunnel header while control message is carried in the GTP tunnel payload field.

In the Step 15), when S-GW sends a Modify Bearer Request message to P-GW through this GTP-C Tunnel, the P-GW checks the TEID in the tunnel header and learns whose session it is.
debasish 2014-05-27 16:16:03
Hi Netmanias team,

Please explain me a scenario where UE is establishing a second dedicated bearer(ofcourse network initiated).
gecuili 2015-01-26 17:34:14

 “At this time, the eNB allocates eNB UE S1AP ID, and includes it in the Initial UE Message sent to the MME. ”

eNB UE S1AP ID --》 eNB S1AP UE ID. Right?


azgar shaik 2015-03-22 13:40:35

Is there Any Timer defined for Inital Context Setup Request/ Response?

James 2015-06-23 21:14:29

"SGW finds out at which MME the UE is registered"  how?

Ranjith Kumar Kopperla 2015-12-14 18:51:18

Dear Netmanias team,


Thanks for your explanation, can you please explain what happens when the Service Request procedure is rejected. If possible please explain the behavior for different cause codes.



nitinatakg@gmail.com 2016-03-11 14:39:42

This site is the best

Gul 2016-06-24 12:13:50

I have a question , After establishing the internet connection Ue is sending SR for three times since Network is ignoring the Service request . But according to spec it should send for five times .

What could be the reason here ?

Harshil 2017-10-09 18:53:59

Great Explanation with a diagram

rajeevkr 2017-10-14 00:38:04


Could you please explain when this scenario happens: After the S1 Setup Procdure between eNB and MME UE is sending Service Request(RRC Connection Request cause Mo-Data) and being rejected by MME since UE didn't do an Attach Request. UE never attached before.

Bhoop Singh 2020-12-16 03:21:41

How S-GW knows that the incoming data is for which MME/UE, means how S-GW decides the MME to send the DL data Notification. Please expaln thanks in advance

vimal kumar mishra 2021-02-12 16:04:00

What kind of information related to UE subscription is kept in SPR ? Why PCRF does not use HSS for the same purpose ? 

Irfan Ali 2021-06-06 18:52:06

Excellent writeup. Thank you. One correction, Service request message is n ot sent encrypted; it is sent integrity protected, but NOT encrypted. See Clause 4.4.5 of 24.301, where the bullet says: unciphered, if it is any other initial NAS message (other than CP Service request which is sent partially ciphered).  Ciphering is turned on when the service request procedure is successful, which imu, is SMC mode command (step 7) if authentication occurs, or after DRBs are setup (step 10).

bhavya 2022-07-12 12:22:53

hi Netmanias, one small question , what is difference bwteen seesion ID and tunnel ID and is session ID stored any where .. this queary is with respect to diagram 7 above...

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