04_TM51154EN04GLA2_LTE Air Interface Overview

LTE/EPS Mobility & Session Management

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- The CellId is a System Level parameter - The PhyCellID is a Physical level parameter - UE gets the PhyCellID from the Primary and Secondary Synchronization Signals (PSS and SSS) PSS:

provides the PhyCellID sector: 0..2

SSS:

provides the PhyCellID group: 0…167

Example: •

Let's say that we are going to deploy a LTE network in a city and that city needs 1000 cells.

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Each of the 1000 cells will have their own cell ID, but, since there is only 504 physical cell IDs, we will need to repeat the physical cell IDs twice.

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The key is that that the two cell that share a physical cell ID cannot be geographically close to each other or they will interfere will each other.

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why a TAU is necessary in the connected state? The answer to that question can be found in the message sequence charts for handovers. For example: during an X2 handover, which is directly negotiated between two base stations, the Mobility Management Entity (MME) in core network is only informed of the handover after it has taken place. Also, there's no direct communication between the MME and the mobile device during the handover procedure. That means that in case the new cell is in a new tracking area, the mobile has to update its tracking area list as that information was not contained in the handover messaging. From a logical point of view that also makes sense. Tracking areas are administered by the core network (by the Non Access Stratum) while handovers are performed by the access network. Also, the signaling does not interrupt the user data transfer so there are no side effects of performing this procedure in connected mode and while transferring data.

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•

Another difference between TAU and the LAU/RAU of UMTS is that the mobile can have a list of several valid tracking areas and an update only has to be made if the new cell is in a tracking area that is not part of that list.

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This solution will avoid unnecessary tracking area updates at the tracking areas border when the UE is ping-ponging between cells belonging to different TAs.

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•USIM card can be used to access 2G networks (besides 3G and LTE Networks) •SIM card (original 2G SIM card) can not be used to access LTE Networks

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Further Reading: The GUMMEI in turn consists of the following: − PLMN Id: MCC, MNC − MME Identifier (MMEI): MME Group Id (MMEGI) and MME Code (MMEC) The MMEC provides a unique identity to an MME within the MME pool, while the MMEGI is used to distinguish between different MME pools. More details about these identifiers can be found in TS 23.003. GUTI reallocation is further described in TS 23.401 and TS 24.301.

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More about LTE Mobility and Connection States on 3GPP TS23.401

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•

For further information about the EPS Bearer, please refer to 3GPP TS 23.401, v9.2.0, section 4.7.2

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•

There is a one to one mapping between EPS Radio Bearer (RB) and EPS Bearer, and the mapping between EPS RB Identity and EPS Bearer Identity is made by E-UTRAN.

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The E-RAB ID value used at S1 and X2 interfaces to identify an E-RAB is the same as the EPS Bearer ID value used to identify the associated EPS Bearer.

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•

An E-RAB (E-UTRAN Radio Access Bearer) refers to the concatenation of an S1 bearer and the corresponding radio bearer, as defined in TS 36.300

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•

Default bearer is established during the attach phase.

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Dedicated bearers are established based on the services running between the UE and the PDN/IMS.

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A comparison can be made between the dedicated bearer in EPS and the secondary PDP context in UMTS.

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TS 29.274 defines the create bearer request message. This request is used to establish dedicated bearers but not default bearer.

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Reading from the specs, it may lead to a confusion the following sentence: “the dedicated bearers are network initiated”. Because LTE/EPS is all on IP and if you are receiving a call then network may initiate dedicated bearer to forward that call to you. This doesn't mean that UE cannot ask for dedicated bearers. UE can ask for dedicated bearers by sending out bearer modification command but UE cannot send create bearer request . Bearer modification command will make PDN trigger a dedicated bearer.

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•

A default Evolved Packet System (EPS) bearer is the bearer that is established during the attach process.

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It will give the UE an IP address and packet data resources so that the UE can do limited packet services.

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One of the best examples of a service that would be good for the default EPS bearer is an IMS registration.

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The characteristics of the default EPS bearer will be defined by the subscription and established by the Mobility Management Entity (MME) upon receiving the attach message based on the subscriber profile in the Home Subscriber Server (HSS).

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Default bearers are created on a per PDN basis. So if a UE is connecting to two PDNs it will need to establish two default bearers.

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•

Schedulers in eNB, SAE GW and PDN GW must respect the QoS of each individual SAE bearer.

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Limits coming from a user’s subscription must be taken into account when a new SAE bearer is set up or one is modified. This is one task of the MME.

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B as ic G uideline: The LTE/EPS Bearer and QoS management has to be

improved in comparison to the way it is done in existing 3GPP system. •

The main reason is that it has not been easy for operators to implement QoS attributes in GSM/WCDMA networks, as they were somehow disconnected from the application layer. This problem was even getting worse by the fact that the UE was responsible for setting the QoS attributes for a Bearer.

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It was therefore agreed that only a reduced set of QoS parameters and standardized attributes would be specified for the EPS bearer.

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For every EPS bearer the following QoS parameters are available: • Dedicated or default EPS bearer • Guaranteed Bit Rate (GBR) or Non-Guaranteed Bit Rate ( N-GBR) • Maximum Bit Rate (MBR)

• Traffic Flow Control (UL/DL-TFT): • Integer number indicating QoS category: Label or QoS Class identifier (QCI) • Allocation/Retention Priority (ARP) For all bearers together for one user, following QoS parameter is available: • Aggregate Maximum Bit Rate (AMBR)

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GBR identifies the bit rate that will be ensured to the bearer.

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ARP Parameter Notes from the Specs (3GPP TS 23.401, v9.2.0, section 4.7.3) regarding the ARP parameter: → The ARP should be understood as "Priority of Allocation and Retention"; not as "Allocation, Retention, and Priority". → Video telephony is one use case where it may be beneficial to use EPS bearers with different ARP values for the same UE. In this use case an operator could map voice to one bearer with a higher ARP, and video to another bearer with a lower ARP. In a congestion situation (e.g. cell edge) the eNB can then drop the "video bearer" without affecting the "voice bearer". This would improve service continuity. UE-AMBR Notes from the Specs (3GPP TS 23.401, v9.2.0, section 4.7.3) regarding the UE AMBR parameter: → The UE-AMBR limits the aggregate bit rate that can be expected to be provided across all Non-GBR bearers of a UE (e.g. excess traffic may get discarded by a rate shaping function). → Each of those Non-GBR bearers could potentially utilize the entire UE-AMBR, e.g. when the other Non-GBR bearers do not carry any traffic. → GBR bearers are outside the scope of UE AMBR. → The E-UTRAN enforces the UE-AMBR in uplink and downlink.

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Nine pre-configured classes have been specified in 2 categories of Bearers: GBR and N-GBR. In addition, Operators can create their own QoS class identifiers (QCI) The QoS attributes associated with the QCI parameter are: •

Priority: used to define the priority for the Packet Scheduler function in the eNB.

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Delay Budget: helps the packet scheduler to ensure that users are scheduled sufficiently often to guarantee the delay requirements of the Bearer.

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Loss Rate tolerance is primarily intended for setting the RLC protocol settings (e.g. number of RLC retransmissions). The label will most likely also include a priority parameter, which the packet scheduler can use for differentiation.

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•

The figure shows a UE with three applications running: e-mail, SIP user agent and VoIP call. The voice over IP call was initiated via the SIP user agent. In this example we have three applications running, although for the user the SIP UA and the VoIP call belong together and form one service component.

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First let us analyze how many different QoS requirements we have. If we don’t want to make a too fine split, we can say, that SIP signaling and e-mail is not so time sensitive. So both could share a single SAE bearer with NGBR behavior and this could be the default EPS bearer created when the user attached to the system.

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On other hand the VoIP call is obviously time critical, as speech codecs do not tolerate a high delay or delay jitter. Thus for the speech call we would have to setup a SAE bearer providing a minimum bit rate equal to the minimum useful bit rate the codec requires.

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So we end up with two SAE bearers, the default one for the e-mail application and the SIP user agent. The second SAE bearer is a dedicated one and is used for the transfer of the VoIP speech packets (usually IP/UDP/RTP datagrams).

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For the dedicated bearer we have to specify a DL and UL TFT to support the system in its decision which IP datagrams will be transferred via which SAE bearer. In the simplest from the TFT specify the IP addresses of the UE and the opposite VoIP client and their allocated UDP port numbers for the VoIP call.

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•

SAE bearers consist of three segments: radio bearer, S1-U bearer and S5/S8 bearer.

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For the S5/S8 bearer between SAE GW and PDN GW t here are two options mentioned. The first one is based on the 2G/3G protocol GTP which is also used on S1-U. The second option for S5/S8 is based on Mobile IPv6 (MIPv6). As the latter is not completed yet, we discuss here only the GTP based S5/S8 interface.

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On the radio interface the SAE bearer is uniquely associated with one radio bearer RB. The radio bearer is by the radio scheduler dynamically mapped to the available physical layer resources, this means, that a RB does not allocate resources in a fixed manner for a long t ime. This provides the required flexibility for resource re-assignments which WCDMA introduced with HSDPA.

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Between eNB and SAE GW the SAE bearer is tied to a single GTP-U tunnel. A GT P-U tunnel is identified by a TEID (Tunnel Endpoint IDentifier) allocated by both endpoints - in this c ase one from eNB TEIDeNB and one from SAE GW TEID-SG1. It is a task of the MME to exchange both TEID s between eNB and SAE GW during setup of the tunnel. Packets in the downlink will be sent in GTP -U frames (T-PDU) and will carry the TEID-eNB in its header. The eNB must c onnect its TEID-eNB internally with the radio bearer. This also works for uplink, where all data from the associated radio bearer will have to be sent on S1-U with the TEID-SG1 in the GTP-U header.

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If the S5/S8 interface is based on GTP option, then we will also here find a GTP-U t unnel for the SAE bearer. Again exactly one tunnel will be provided for the SAE bearer. The setup of the tunnel requires two new TEID - one from SAE GW TEID-SG2 (usually different from TEID-SG1) and one from the PDN GW TEID-PG. The communication principle is the same as on S1-U interface. But this time SAE GW and PDN GW handle the exchange of their TEIDs for themselves. Therefore they use the control part of the GTP protocol which provides messages to setup such tunnels. [NOTE: W hich changes in GTP are required for this is currently under investigation.]

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The SAE gateway is responsible to link the S1 GTP-U tunnel and the S5/S8 GTP-U tunnel with each other to allow efficient forwarding of data between PDN GW and eNB. The PDN GW on the other hand must link its tunnel to the external network and to the IP address of the UE inside this network. The DL TFT packet filters support the PDN GW in the task to select the right GPT-U tunnel of a UE for an incoming IP datagram. The UL TFT on the other hand is used at t he UE side for the same task.

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It is important to note, how and when these tunnels and bearer segments are available. When a new SAE bearer is setup usually a radio bearer, a S1 GTP-U tunnel and a S5/S8 GTP-U tunnel is created. The latter will only be released, when the SAE bearer is released. Radio bearer and S1 GTP-U t unnel on the other hand will be released when the UE enters an idle state. T his state can be triggered due to inactivity. When this happens the radio bearer is removed and the eNB will also clear the TEIDs from its memory for the UE (to be true, the eNB will delete everything). The SAE GW therefore also must delete the TEID-eNB, but will usually keep its own TEID-SG1. If there should be data to be sent later on, the UE must send a SERVICE REQUEST to the MME to demand the re-establishment of the S1 GTP-U tunnel and the radio bearer. In short words, the S5/S8 tunnel is rather permanent, whereas radio bearer and S1 tunnel are dynamic with respect to the life time of a SAE bearer.

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The attach procedure in LTE/EPS is quite similar to the GPRS attach in 2G/3G. It brings the UE from EMM_DEREGISTERED state to EMM_REGISTERED. In addition to that the procedure also establishes the default SAE bearer for the UE and thus allocates the required IP addresses for the subscriber in the external packet data network. 1. The UE connects to the serving cell and the associated eNB. The UE sends the ATTACH REQUEST message (NAS) including IMSI/ old GUTI, old TAI, old GUMMEI and old ECGI. The eNB selects an available MME and forwards the message to it. 2. The first task of the MME is to identify and authenticate the subscriber. Thus it contacts the HSS (in case IMSI is used for identification) or the old MME (in case the UE is identified via old GUTI) with IDENTIFICATION REQUEST (GTP-C). The response should contain the IMSI (when contacting old MME) and some authentication vectors for the subscriber. ( Flowchart s hows direct contact with HS S ). 3. Using the authentication vectors from the old MME/HSS the new MME can start an authentication procedure (NAS). The authentication mechanism is the same as in 3G. 4. After a successful authentication the new MME can begin to update the HSS and download the subscription data from there. This is achieved via Diameter procedures UPDATE LOCATION and INSERT SUBSCRIBER DATA. During this process the HSS will also force the old MME to clear the stored data about the subscriber using the Diameter operation CANCEL LOCATION .

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5. Based on the subscription data the new MME must decide whether a default bearer has to be created or not. The default access point name (default APN) assists the MME in selection of an appropriate SAE GW. To this serving gateway the CREATE DEFAULT BEARER REQUEST message (GTP-C) is sent to. The SAE GW will now create the S5/S8 tunnel. This is done with the same message, but sent to the PDN GW. 6. When the EPC resources for the default bearer are prepared, the new MME can give the ATTACH ACCEPT message to eNB. The S1-AP message which will contain it is the Initial Context Setup request and it will also hold the tunnel endpoint identifier allocated by the Serving GW for S1-U interface. The eNB creates the radio bearer for the default SAE bearer and returns ATTACH COMPLETE to the MME. The S1-AP message this one is in will hold the TEID allocated by the eNB for S1-U interface. Via an UPDATE BEARER procedure the MME will give this parameter to the Serving GW. 7. Now the default SAE bearer is complete and the UE is in state EMM_REGISTERED and ECM_CONNECTED.

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1. The eNB can send the message S1 RELEASE REQUEST (S1-AP) to the MME to request the release of all EUTRAN resources for a UE. The message can for instance be triggered by detection of a too long inactivity period. 2. When the MME gets a trigger to release the UE from EUTRAN, it will release the S1 tunnels allocated for the SAE bearers of the UE. This is done by sending an UPDATE BEARER REQUEST message (GTP-C) to the Serving GW. In the message the indication of the release of the S1 resources is contained. 3. In parallel to the previous step the MME will send the S1-AP message S1 RELEASE COMMAND to the eNB. It will trigger the release of the UE on the air interface with message RRC CONNECTION RELEASE (RRC). This will bring the UE to RRC_IDLE state and with that also to ECM_IDLE state. The UE acknowledges with RRC CONNECTION RELEASE ACK .

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The transition to EMM_DEREGISTERED state is achieved by the NAS detach procedure. The Detach procedure allows: −

the UE to inform the network that it does not want to access the EPS any longer

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the network to inform the UE that it does not have access to the EPS any longer

The UE is detached either explicitly or implicitly: −

Explicit detach: The network or the UE explicitly requests detach and signal with each other

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Implicit detach: The network detaches the UE, without notifying the UE. This is typically the case when the network presumes that it is not able to communicate with the UE, e.g. due to radio conditions.

The procedure consists of the DETACH REQUEST / DETACH ACCEPT procedure between UE and MME and the DELETE BEARER procedure between MME and Serving GW and PDN GW. Furthermore at the end the S1 RELEASE procedure between MME and eNB deletes all radio resources.

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The transition to EMM_DEREGISTERED state is achieved by the NAS detach procedure. The Detach procedure allows: −

the UE to inform the network that it does not want to access the EPS any longer

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the network to inform the UE that it does not have access to the EPS any longer

The UE is detached either explicitly or implicitly: −

Explicit detach: The network or the UE explicitly requests detach and signal with each other

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Implicit detach: The network detaches the UE, without notifying the UE. This is typically the case when the network presumes that it is not able to communicate with the UE, e.g. due to radio conditions.

The procedure consists of the DETACH REQUEST / DETACH ACCEPT procedure between UE and MME and the DELETE BEARER procedure between MME and Serving GW and PDN GW. Furthermore at the end the S1 RELEASE procedure between MME and eNB deletes all radio resources.

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•

From time to time a UE must switch from ECM_Idle to ECM_connected

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The reasons for this might be UL data is available, UL signaling is pending (e.g. tracking area update, detach) or a paging from the network was received.

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The UE sends the NAS message SERVICE REQUEST towards the MME encapsulated in an RRC message to the eNodeB. If there are multiple MME connected to the eNB it is the task of the eNB to select the right MME (the one the UE is registered with) from S-TMSI/GUTI and TAI. The service type parameter indicates the above mentioned reason for the service request.

2.

The eNodeB forwards NAS message to MME. NAS message is encapsulated in an S1-AP: Initial UE Message (NAS message, TAI+ECGI of the serving cell, S-TMSI, CSG ID, CSG access Mode).

3.

NAS authentication procedures may be performed.

4.

The MME sends S1-AP Initial Context Setup Request (Serving GW address, S1-TEID(s) (UL), EPS Bearer QoS(s), Security Context, MME signaling Connection Id, Handover Restriction List,…) message to the eNodeB. This step activates the radio and S1 bearers for all the act ive EPS Bearers. The eNodeB stores the Security Context, MME signaling Connection Id, EPS Bearer QoS(s) and S1-TEID(s) in the UE RAN context.

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The eNodeB performs the radio bearer establishment procedure. The user plane security is established at this step.When the user plane radio bearers are setup the Service Request is completed and EPS bearer state is synchronized between the UE and the network

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The uplink data from the UE can now be forwarded by eNodeB to the Serving GW. The eNodeB sends the uplink data to the Serving GW address and TEID provided in the st ep 4. The Serving GW forwards the uplink data to the PDN GW .

7.

The eNodeB sends an S1-AP message Initial Context Setup Complete (eNodeB address, List of accepted EPS bearers, List of rejected EPS bearers, S1 TEID(s) (DL)) to the MME.

8.

The MME sends a Modify Bearer Request message (eNodeB address, S1 TEID(s) (DL) for the accepted EPS bearers, Delay Downlink Packet Notification Request, RAT Type) to the Serving GW. The Serving GW is now able to t ransmit downlink data towards the UE.

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The Serving GW sends a Modify Bearer Response to the MME.

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1. When the Serving GW receives a downlink data packet for a UE known as not user plane connected (i.e. the S-GW context data indicates no downlink user plane TEID), it buffers the downlink data packet and identifies which MME is serving that UE. 2. The Serving GW sends a Downlink Data Notification message to the MME for which it has control plane connectivity for the given UE. The MME respond to the S-GW with a Downlink Data Notification Ack message. If the Serving GW receives additional downlink data packets for this UE, the Serving GW buffers these downlink data packets and the Serving GW does not send a new Downlink Data Notification. 3. The MME sends a Paging message (NAS ID for paging, TAI(s), UE identity based DRX index, Paging DRX length, list of CSG IDs for paging) to each eNodeB belonging to the tracking area(s) in which the UE is. 4. If eNodeBs receive paging messages from the MME, the UE is paged by the eNodeBs. Steps 3-4 are omitted if the MME already has a signaling connection over S1-MME towards the UE. 5. When UE is in the ECM-IDLE state, upon reception of paging indication in EUTRAN access, the UE initiates the UE triggered Service Request procedure.

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A Tracking Area Update takes place if: −

UE detects it has entered a new Tracking Area that is not in the list of TAIs that the UE registered with the network;

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the periodic Tracking Area update timer has expired;

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1.

The UE sends TRACKING AREA UPDATE REQUEST with its current GUTI or IMSI, old TAI and EPS Bearer Status information to the eNB. This one has to forward the message to a MME. If the old MME cannot be selected, then a new MME must be chosen by the eNB.

2. The new MME must first of all get the identity (IMSI) of the subscriber and authenticate him/her. Therefore the new MME contacts the old one via GTP-C CONTEXT REQUEST . The CONTEXT RESPONSE contains IMSI, authentication vectors, but also all information about the currently active SAE bearers of this user. 3. With one of the authentication vectors the new MME can start authentication. 4. After a successful authentication the new MME analyzes if a Serving GW change is needed 5. New MME informs the old one that it is ready to take control over the UE (Context Acknowledge message). The old MME will now start a timer and wait for the cancellation of the subscriber record. 6. In parallel to the previous step the new MME sends GTP-C CREATE BEARER REQUEST to the Serving GW it has selected. The message will trigger the setup of new S1 tunnels and trigger an update towards PDN GW. This will change the traffic path from PDN GW to new Serving GW to new eNB.

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7. Also simultaneously with the previous steps the MME will update the HSS. During this the HSS will cancel the subscriber record in the old MME. The old MME will of course also delete the old tunnels in the old Serving GW. 8. At the end the UE gets a NAS message TRACKING AREA UPDATE ACCEPT . In it a new GUTI and new tracking area (or tracking area list) will be contained. The UE has to acknowledge with TRACKING AREA UPDATE COMPLETE .

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1. The external data network triggers the request for a new IP connectivity bearer (SAE bearer) via the PCRF connected to the PDN gateway that owns the default SAE bearer of this user. This is sent in form of a Policy and Charging Control (PCC) decision (QoS policy) from PCRF to PDN GW. 2. The PDN GW first of all uses GTP-C CREATE DEDICATED BEARER REQUEST to setup the tunnel between PDN GW and Serving GW. 3. The Serving GW allocates the resources for the S5/S8 tunnel and forwards an associated request to the MME for the S1 tunnel. 4. If the UE is currently ECM_IDLE it must be paged. Thus the MME sends PAGING messages of S1-AP protocol to all eNB that own cell’s of the UE’s current tracking area (or tracking areas). If the UE receives such a paging it will respond with the SERVICE REQUEST procedure. in the following the default SAE bearer will be re-established.

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5. The UE NAS layer builds a Session Management Response including EPS Bearer Identity. The UE then sends a Direct Transfer (Session Management Response) message to the MME. 6. Upon reception of the Bearer Setup Response message and the Session Management Response message in step 5, the MME acknowledges the bearer activation to the Serving GW by sending a Create Bearer Response (EPS Bearer Identity, S1-TEID) message. 7. The Serving GW acknowledges the bearer activation to the PDN GW by sending a Create Bearer Response (EPS Bearer Identity, S5/S8-TEID) message. 8. If the dedicated bearer activation procedure was triggered by a PCC Decision Provision message from the PCRF, the PDN GW indicates to the PCRF whether the requested PCC decision (QoS policy) could be enforced or not, allowing the completion of the PCRF-Initiated Session Modification procedure.

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DATA FORWARDING Downlink

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source eNB forwards all downlink RLC SDUs that have not been acknowledged by the UE to the target eNB

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target eNB re-transmits and prioritize all downlink RLC SDUs forwarded by the source eNB as soon as it obtains them

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reordering and duplication avoidance in the UE

Uplink

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source eNB forwards all successfully received uplink RLC SDUs to the EPC

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UE re-transmits the uplink RLC SDUs that have not been successfully received by the source eNB

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reordering and duplication avoidance in EPC

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1. The source eNB configures the UE measurement procedures with MEASUREMENT CONTROL 2. UE is triggered to send MEASUREMENT REPORT to the source eNB. It can be event triggered or periodic. 3. Source eNB makes handover decision based on UE report + load and service information. 4. When the source (current serving) eNB decides to start a handover of an UE to a neighbor cell in a new (target) eNB it will contact this target eNB. This is done via the X2-AP message HANDOVER REQUEST . The message will contain the target cell for the UE, the current serving MME and SAE GW. It is task of the target eNB to allocate virtual capacity in the target cell via its admission control function. 5. If this is done the target eNB returns part of the handover message for the UE within the X2-AP message HANDOVER REQUEST ACKNOWLEDGE . In this message also a data forwarding tunnel (TEID from target eNB) is indicated. It allows the source eNB to forward still buffered or still arriving downlink packets to the target eNB. 6. The source eNB can now give the HANDOVER COMMAND (RRC) to the UE. The command contains the configuration for the UE in the new cell and possibly already an UL/DL resource allocation. The UE will detach from the old cell and synchronize itself to the new cell. In the mean time the source eNB can start downlink packet forwarding via X2 interface.

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7. UE performs the final synchronization to target eNB and accesses the cell via RACH procedure (DL pre-synchronization is obtained during cell identification and measurements). 8. Target eNB gives the uplink allocation and timing advance information. 9. Once synchronization between UE and the new cell is achieved, the UE confirms the handover with RRC message HANDOVER CONFIRM . This will trigger a HANDOVER COMPLETE message of S1-AP to be sent to the MME. It simply informs the MME that now a new eNB is responsible for the UE. Thus this message will contain the IP addresses and TEIDs of the target eNB for the S1 tunnels.Additionally it contains the TAI and the target cell ECGI. 10. The MME’s task is to send this information via GTP-C UPDATE BEARER REQUEST to the Serving GW. This will switch the traffic path now completely from Serving GW to target eNB. 11. Serving Gateway switches the downlink data path to the target side. 12. Serving Gateway sends an UPDATE BEARER RESPONSE message to MME. 13. MME confirms the Handover Execution with the HANDOVER COMPLETE ACK message. 14. By sending RELEASE RESOURCE the target eNB informs success of handover to source eNB and triggers the release of resources. 15. Upon reception of the RELEASE RESOURCE message, the source eNB can release radio and C-plane related resources associated to the UE context.

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For further information, please refer to 3GPP TS 33.401 and TS 33.102 (SAE Security Architecture)

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The generation of keys is triggered by Authentication and Key Agreement (AKA) procedures.

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In LTE the MME acts as the Access Security Management Entity (ASME). This is the access network entity that receives top level keys from the HSS.

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UMTS AKA is capable of agreeing two keys, CK and IK , on the USIM and in the AuC. For LTE these keys never leave the HSS. Instead they are used to derive KASME , which is transferred from the HSS to the MME as part of the Authentication Vector .

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The keys used for UP, NAS and AS protection shall be dependent on the algorithm with which they are used.

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The keys used for UP, NAS and RRC (AS) protection shall be dependent on the algorithm with which they are used.

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KEY GENERATION PROCEDURE 1. When a UE initially attaches to the network the MME will authenticate the subscriber using UMTS4 AKA . This triggers generation of security keys by the UE and HSS. At this point the UE and HSS know the PLMN ID which is used in the generation of KASME . 2. The UE and HSS generate CK and IK from K and the RAND value used in UMTS-AKA. 3. The UE and HSS derive KASME from CK , IK and PLMN-ID. 4. The HSS transfers KASME to the MME as part of the Authentication Vector used in EPS AKA. 5. Once the UE has successfully been authenticated the MME and UE generate the keys for NAS signaling security - KNAS int and KNAS enc 6. The MME and UE generate the KeNB key from KASME and the eNB-ID. 7. The MME transfers KeNB to the eNB across the S1-MME. This key is transferred as part of the Initial Context Setup Request message to the eNB. 8. The eNB and UE generate the keys used for protection of RRC signaling (KeNB RRC-int and KeNB RRC-enc ) and U-plane traffic (KeNB UP-enc ), using KeNB.

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04_TM51154EN04GLA2_LTE Air Interface Overview

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