OEO102020 LTE eRAN2.1 Connection Management Feature ISSUE 1.00.pdf

LTE eRAN2.1 Connection Management Feature

Confidential Information of Huawei. No Spreading Without Permission







Connection management in the LTE system involves management of the connections

between the UE, eNodeB, and MME. The connection management is performed in both

control plane and user plane.

Essentially ,the signaling connection is the precondition for all the service procedures, it is

always setup firstly.

The initial signaling connection is without security.



The control plane messages are terminated in eNodeB or MME.The signaling connection

between UE and eNodeB is RRC connection, The initial signaling connection is without

security.After the security establishment, a S1 dedicated connection is setup.

The signaling between UE and MME is called NAS signaling.

Actually, there is no a direct connection between the UE and MME, so the NAS signaling

transmission is based on the RRC connection and S1 interface dedicated signaling

connection .



In the user plane, the services between a UE and a Packet Data Network (PDN)-Gateway

(GW) with the same QoS class are referred to as an Evolved Packet System (EPS) bearer. In

the EPS bearer, the section between the UE and the eNodeB is called RB, and the section

between the eNodeB and the Serving Gateway (S-GW) is S1 bearer. RB and S1 bearer are

collectively referred to as E-RAB.



The procedure for setting up all the connections is shown in the slide:

RA is triggered by a UE when the UE needs to communicate with the network for

purposes such as service request, location update, and paging.

After the RA procedure is complete, the connection between the UE and the MME

in the control plane is started. Connection in the control plane consists of Radio

Resource Control (RRC) signaling connection and dedicated S1 connection.

After the connection in the control plane is complete, the MME causes the eNodeB

to establish E-UTRAN Radio Access Bearer (E-RAB) in the case of service request.

Through RB management, the eNodeB establishes, modifies, and releases the E-

RAB.





Random access is always the first step in the connection setup procedure. It is also useful

in some other scenarios such as handover.

We call random access “RA” for short, RA is performed before a UE begins to

communicate with the network. During RA, a UE requests access to the system, and then

the system responds to the request and allocates a Random Access Channel (RACH).

Through the RA procedure, the UE can obtain the uplink synchronization signals from the

network and request dedicated resources for data transmission.



RA is performed in the following scenarios:

Case 1: initial RRC connection establishment. When a UE is changed from

RRC_IDLE mode to RRC_CONNECTED mode, the UE initiates RA.

Case 2: RRC connection reestablishment. When a radio link fails, the UE needs to

reestablish RRC connection. In this case, the UE initiates RA.

Case 3: handover. When a UE performs handover, the UE initiates RA in the target

cell.

Case 4: downlink data arrival. When an eNodeB needs to transmit downlink data

to a UE in RRC_CONNECTED mode and finds that the UE is in the uplink

synchronization loss state, the eNodeB instructs the UE to initiate RA.

Case 5: uplink data arrival. When a UE in RRC_CONNECTED mode needs to

transmit uplink data to an eNodeB and finds that it is in the uplink synchronization

loss state, the UE initiates RA.



Depending on whether contention is introduced, the RA procedure can be categorized

into the following types:

Contention-based RA: The access preambles are generated by UEs, and there may

be conflicts among the preambles. Therefore, the eNodeB needs to resolve the

contention for UE access. Case 1, case 2, and case 5 are contention-based RA.

Non-contention-based RA: The access preambles are allocated to UEs by the

eNodeB, and each preamble is dedicated to a UE. Therefore, there are no

preamble conflicts. When the dedicated preambles that are allocated by the

eNodeB are used up, non-contention-based RA becomes contention-based RA.

Case 3, case 4 and case 6 are non-contention-based RA.

The RACH is used only for the transmission of RA preambles. The preambles are handled in

the Medium Access Control (MAC) layer. Therefore, no corresponding logical channel is

available for the preambles. The Physical Random Access Channel (PRACH) bears the RACH.

The PRACH has fixed time and frequency resources, which can be obtained from the

common channel configuration parameters in the System Information Block-2 (SIB2).



The procedure is divided into four steps:

UE transmitting an RA preamble,

Upon receiving the preamble, the eNodeB applies for Temporary Cell RNTI (C-RNTI)

and uplink and downlink scheduling resources, then send to the UE.

The UE transmits uplink scheduled data over the UL-SCH, the data could be “RRC

Connection Request”.

The contention resolution is generated in the RRC layer. Then, the eNodeB

transmits the contention resolution to the UE through the CCCH or DCCH over the

DL-SCH, such as “RRC Setup Complete”.

For contention-based RA, the UE directly uploads an RA preamble if the PRACH

configuration is specified and does not expire. If it expires or is not specified, the UE must

obtain the PRACH configuration.



The UE transmits an RA preamble over the PRACH with the transmit power of PPRACH。

RA preamble is a burst of bits, it includes:

Cyclic Prefix (CP)

The length of a CP is Tcp

preamble sequence

The length of a preamble sequence is Tseq



There are five RA preamble formats, which are used for cells of different radii. LTE FDD

supports preamble formats 0-3, and LTE TDD supports preamble formats 0-4. The

preamble format can be set through the PreambleFmt parameter, and the cell radius can

be set through the CellRadius parameter.



Parameter ID Description

PreambleFmt Indicates the preamble format used in the cell.

CellRadius Indicates the radius of the cell.



Each cell has 64 preamble sequence

The is defined by a cyclic shift of the Zadoff-Chu (ZC) sequence. The logical index of the ZC

sequence is determined by RootSequenceIdx, whose value ranges from 0 to 837. The

logical index of the ZC sequence is cyclic. That is, the logical index 0 is consecutive to 837.

The number of dig

its for cyclic shifts is determined by the eNodeB according to the cell type and cell radius.

The logical index of the ZC sequence and configured cyclic shift value are transmitted in

the PRACH configurations in the SIB2.

the 64 preamble sequences are divided into the random preamble sequence group and

dedicated preamble sequence group. To improve uplink resource allocation, the random

preamble sequence groups are further divided into preamble sequence group A and

preamble sequence group B.

The Contention-based Random Access can choose the random preamble group sequence

only.

When the UE obtains the PRACH configuration, RA preamble sequence group B is selected

if the following conditions are met, or RA preamble sequence group A is selected if any of

the following conditions is not met:

Random preamble sequence group B exists.

The size of the transport block msg3 in the scheduled data transmission is larger

than the threshold of random preamble sequence group A.

The path loss is smaller than the corresponding threshold.

After a random preamble sequence group is selected, a preamble in the group has an even

probability to be selected.

In the system information, The RACH-related parameters consist of the number of random

preamble sequences and proportion of random preamble sequence group A.




RootSequenceIdx Indicates the logical root sequence index, which is used to derive the preamble sequence. Each logical root sequence corresponds to a physical root sequence.



Upon receiving the UE preamble, the eNodeB transmits an RA response over the Downlink

Shared Channel (DL-SCH).

the UE monitors the Physical Dedicated Control Channel (PDCCH) in the Transmission Time

Interval (TTI) until it obtains the required RA response.

The response contains RA-Preamble Identifier, Timing Alignment Information, Initial UL

Grant, and Temporary C-RNTI.

A message on the DL-SCH can carry multiple RA responses to be transmitted to multiple

use.

For the UE ;

If the received RA-Preamble Identifier is consistent with the identifier that the UE

previously sent, the UE infers that the response is successful. Then, the UE

transmits uplink scheduled data.

If the UE does not receive a response within the TTI, or if all received RA responses

contain RA preamble identifiers that do not match the transmitted RA preamble,

the UE infers that the response reception fails. Then, the UE performs RA again if

the number of RA attempts is smaller than the maximum number of attempts. If

the number of RA attempts is not smaller, the RA procedure fails.



With the grant ,the UE transmits uplink scheduled data over the UL-SCH. The scheduled

data could be “RRC Connection Setup Request” message.

The size of the transport block, which is not smaller than 80 bits, is specified in the

preamble. The carried signaling and information in the transport block vary according to

the RA scenario:

Initial RRC connection establishment.The RRC Connection Request message is

transmitted over the CCCH in TM in the RLC layer. NAS UE_ID is carried in the

message, and the message is not segmented.

RRC connection reestablishment. The RRC Connection Reestablishment message is

transmitted in TM in the RLC layer. The message is not segmented, and the NAS message

is not carried in the message.

Target cell access that is contention-based during a handover procedure without a

dedicated RA preamble. The RRC Handover Confirm message and C-RNTI are transmitted

over the DCCH, and if required, Buffer Status Report (BSR) is also carried.

For other scenarios,At least C-RNTI of the UE is transmitted.



The contention resolution is generated in the RRC layer. Then, the eNodeB transmits the

contention resolution to the UE through the CCCH or DCCH over the DL-SCH.

After the UE receives the contention resolution, the contention resolution timer starts. The

UE monitors the PDCCH before the timer expires. In the following situations, the UE infers

that the contention resolution is successful and it notifies the upper layer and stops the :

The UE obtains the C-RNTI when monitoring the PDCCH.

Temporary C-RNTI is obtained when the UE monitors the PDCCH. In addition, the

MAC Packet Data Unit (MAC PDU) is successfully decoded.

If the contention resolution timer expires, the UE infers that the contention resolution fails.

Then, the UE performs RA again if the number of RA attempts is smaller than the

maximum number of attempts.



The non-contention-based RA procedure is as follows:

The eNodeB allocates an RA preamble to the UE in dedicated signaling. The dedicated signaling could be a HO COMMAND message transmitted by the

source eNodeB carries an allocated preamble.

Over the RACH, the UE transmits the dedicated preamble that is allocated.

The eNodeB transmits an RA response over the DL-SCH.

During handover, at least Timing Alignment Information and Initial UL Grant are

contained in the RA response.

Upon downlink data arrival, at least Timing Alignment Information and RA-

Preamble Identifier are contained in the RA response.



In the LTE system, the RACH does not cause any interference to other uplink channels.

Therefore, the LTE system has lower overload control requirement than earlier mobile

communications systems. Generally, the probability of RACH collision is low. If excessive

UEs are admitted on a PRACH, however, UE preamble conflict may occur, and some UEs

fail to access the network. To reduce the conflict probability, backoff control is introduced

in the LTE system to control the time for the UE to retransmit preambles.

The eNodeB notifies the UE of a backoff value through the RA response. If retransmission

is required, the UE selects a value between 0 and the received backoff value as its backoff

time. After the backoff time ends, the UE retransmits the preamble. The backoff control,

however, is not implemented in the following two cases:

During the initial preamble transmission

During the preamble retransmission in non-contention-based RA



Parameter

ID Description

RachAlgoSwitch

RaGrpAdjSwitch: Indicates the switch that is used to enable and disable the group adjustment algorithm. When this switch is set to ON, the algorithm dynamically adjusts the preamble configuration based on the preamble usage. When this switch is set to OFF, the algorithm uses the initial configuration and does not perform dynamic adjustments. RaTrAdjSwitch: Indicates the switch that is used to enable and disable the time-domain resource adjustment algorithm. When this switch is set to ON, the algorithm dynamically adjusts the time-domain resource allocation based on the load on the RACH. When this switch is set to OFF, the algorithm uses the initial configuration and does not perform dynamic adjustments. HoRaSwitch: Indicates the switch that is used to control the random access mode applied during handovers. When this switch is set to ON, the eNodeB instructs UEs to use the non-contention-based random access mode during handovers. When this switch is set to OFF, the eNodeB instructs UEs to use the contention-based random access mode during handovers. UnsyncRaSwitch: Indicates the switch that is used to control the random access mode applied when UEs are out of synchronization in the uplink. When this switch is set to ON, the eNodeB instructs UEs to use the non-contention-based random access mode upon DL data arrival in the case of out-of-synchronization. When this switch is set to OFF, the eNodeB instructs the UE to use the contention-based random access mode upon DL data arrival in the case of out-of-synchronization. MaksIdxSwitch: Indicates the switch that is used to control the reuse of dedicated preambles between UEs. When this switch is set to ON, the eNodeB enables reuse of dedicated preambles between UEs based on the MaskIndex parameter. When this switch is set to OFF, the eNodeB allocates one dedicated preamble to only one user at a given time. BackOffSwitch: Indicates the switch that is used to enable and disable the backoff control algorithm. When this switch is set to ON, the backoff control algorithm is enabled. When this switch is set to OFF, the backoff control algorithm is disabled.





Signaling connection consists of RRC connection on the Uu interface and dedicated S1

connection. Generally, signaling connection is established for the establishment of the

service bearer connection. In certain scenarios, however, signaling connection is used only

for the signaling procedure (such as UE location update), but not for the service bearer

connection.



the UE selects a cause for RRC connection establishment according to NAS procedure, in

which the UE notifies the lower layer of the purpose for the RRC connection. The relations

among NAS procedure, call type, and cause for RRC connection establishment is list below:

NAS Procedure

Cause for RRC Connection

Establishment Call Type

Attach MO-signaling Originating signaling

Tracking area update MO-signaling Originating signaling

Detach MO-signaling Originating signaling

Service request

MO-data (request for radio resources for service bearer)

Originating call

MO-data (request for resources for uplink signaling)

Originating call

MT-access (paging response) Terminating call

Extended service request

MO-data (mobile originating CS fallback)

Originating call

MT-access (mobile terminating CS fallback)

Terminating call

Emergency (mobile originating CS fallback emergency call)

Emergency call



UE_ID is contained in the RRC Connection Request message. If the upper layer provides

the SAE Temporary Mobile Station Identifier (S-TMSI), the UE sends the message

containing the S-TMSI to the eNodeB. If the upper layer does not provide the S-TMSI, the

UE sends the eNodeB the message containing a random value ranging from 0 to 240 - 1. In

the LTE system, the eNodeB does not need to obtain the IMSI information of the UE.

The eNodeB establishes the UE context after receiving the RRC Connection Request

message.

The eNodeB performs the SRB1 resource admission and allocation.

All signaling connections are admitted without any judgment.

If resource allocation fails, the eNodeB responds to the UE with an RRC Connection Reject

message. If resource allocation is successful, the subsequent steps proceed.

The eNodeB responds to the UE with an RRC Connection Setup message over the CCCH.

The message contains detailed information about the SRB1 resource configuration.

The UE performs radio resource configuration after receiving the RRC Connection Setup

message containing the SRB1 resource information, and then the UE sends the eNodeB the

RRC Connection Setup Complete message containing the NAS message.

After the eNodeB receives the RRC Connection Setup Complete message, the RRC

connection is established.




T302

Indicates the length of timer T302.

It refers to the wait time for retransmitting an

RRCConnectionRequest message after the previous request

with a cause other than "MO-Data" and "MO-Signalling"

is rejected.

This timer is started after the UE receives the RRC Connection

Reject message. This timer is stopped when the UE enters the

RRC_CONNECTED mode or performs cell reselection.



Dedicated S1 connection is between the eNodeB and the MME. After receiving the RRC

Connection Setup Complete message, the eNodeB sends the MME the Initial UE Message

containing the NAS message to start dedicated S1 connection establishment.

The dedicated S1 connection establishment procedure is as follows:

After receiving the RRC Connection Setup Complete message, the eNodeB

allocates a dedicated S1APID to the UE. Then, the eNodeB encapsulates the NAS

message that is originally contained in the RRC Connection Setup Complete

message and S1APID in the Initial UE Message before forwarding the Initial UE

Message to the MME.

The MME parses the NAS message contained in the Initial UE Message before

obtaining the cause for the connection establishment. Then, the MME handles the

UE service request based on the cause and allocates a dedicated S1APID to the UE

on S1-MME interface.

The MME sends the eNodeB the Initial Context Setup Request message, which may

contain the common UE context and EPS bearer context.

After receiving the Initial Context Setup Request message, the eNodeB starts the

UE context establishment. At the same time, the eNodeB generates security keys

for service bearer and signaling based on the received security parameters. The

eNodeB performs service admission decision and resource allocation. For the

security keys ,by comparing the eNodeB-supported algorithms with the UE-

supported algorithms, the eNodeB selects a security algorithm supported by both

the eNodeB and the UE and then sends the algorithm to the UE through the

Security Mode Command message.



The eNodeB instructs the UE to start integrity protection and encryption through

the Security Mode Command message. In the process, the UE obtains security keys

based on the algorithms in the message. At the moment of the eNodeB instruction,

downlink encryption has been started.

After receiving the Security Mode Command message from the eNodeB, the UE

selects an encryption algorithm provided by the eNodeB. After the security keys

that are used by both the service bearer and the signaling are successfully

generated, the UE transmits the Security Mode Complete message, which is not

encrypted. After the eNodeB receives the Security Mode Complete message, uplink

encryption is started.

The eNodeB sends the UE the RRC Connection Reconfiguration message on which

encryption and integrity protection is performed. This is used for the establishment

of the SRB2 and DRB.

The UE establishes corresponding resources according to the RRC Connection

Reconfiguration message. After the resources are successfully established, the UE

responds to the eNodeB with an RRC Connection Reconfiguration Complete

message.

The eNodeB sends the MME the feedback through the Initial Context Setup

Response message, which indicates that the bearer is successfully established.



RRC connection Reestablishment involves the SRB1 reestablishment and security

reactivation. A UE in RRC_CONNECTED mode, whose security is activated, can initiate RRC

connection reestablishment for resuming RRC connection. RRC connection reestablishment

can be triggered in the case of a handover failure, RRC reconfiguration failure, radio link

failure, or integrity check failure. If security in the AS is not activated, a UE cannot initiate

RRC connection reestablishment.

A UE may be admitted for RRC connection reestablishment by a cell only if the UE requests

RRC connection reestablishment in the cell that is served by an eNodeB having the UE

context information. If the eNodeB does not have the UE context information, the UE will

be rejected on RRC connection reestablishment. After the RRC connection is successfully

reestablished, the SRB1 can be resumed, whereas other bearers cannot be resumed.

When handling the RRC connection reestablishment, the eNodeB reconfigures the SRB1,

resumes data transmission on the service bearer, and reactivates the security in the AS

without modifying the security algorithm.



The RRC connection reestablishment procedure is as follows:

The UE sends the RRC Connection Reestablishment Request message. The cause for the

RRC connection reestablishment and cell information contained in the message varies

according to the scenario.

The cause for the RRC connection reestablishment triggered by a reconfiguration

failure is reconfigurationFailure. C-RNTI and physCellId in the cause are the

information of the serving cell.

The cause for the RRC connection reestablishment triggered by a handover failure

is handoverFailure. C-RNTI and physCellId in the cause are the information of the

source cell.

The cause for the RRC connection reestablishment triggered by a radio link failure

is otherFailure. C-RNTI and physCellId in the cause are the information of the

serving cell.

If the UE fails to be verified, the eNodeB rejects the RRC connection reestablishment

request of the UE.

Otherwise ,Over the CCCH, the eNodeB sends the UE the RRC Connection Reestablishment

message, which contains the information of the allocated resources. After receiving the

RRC Connection Reestablishment message, the UE reconfigures radio resources according

to the instructions in the message and then starts encryption and integrity protection.

The UE sends the eNodeB the RRC Connection Reestablishment Complete message.



Signaling link release involves the release of the dedicated S1 connection and RRC

connection. RRC connection release consists of the release of the signaling link and all the

radio bearers between the UE and the eNodeB. Signaling link release can be initiated by

the MME or eNodeB. If service between the UE and the MME in the NAS is complete or a

UE decides to stop the service, the MME sends the eNodeB a signaling link release

command. If an exception is detected, the eNodeB sends a signaling link release request to

the MME.



This is a signaling link release procedure after eNodeB detecting an exception：

The eNodeB sends the MME a UE Context Release Request message. Then, the

eNodeB must wait until the MME sends a UE Context Release Command message.

The eNodeB releases transmission resources and triggers the release of the RRC

connection over the Uu interface.

The eNodeB sends the UE an RRC Connection Release message to release the

resources over the Uu interface. In this case, the eNodeB does not need to wait for

the response from the UE.

The eNodeB releases the radio resources in the system.

The eNodeB sends the MME the UE Context Release Complete message, indicating

that the resource release is complete.

After sending the UE Context Release Complete message, the eNodeB releases the

corresponding UE context. In this case, the UE is changed from RRC_CONNECTED

mode to RRC_IDLE mode.

This typical procedure is applied after UeInactiveTimer expires. The eNodeB monitors

data transmission and reception of a UE, After the timer expires, the eNodeB sends the

MME the signaling link release request if the UE does not receive or send any data.

The MME can release the dedicated S1 connection of a UE to release all the S1 resources

for all the service bearers.




UeInactiveTimer

Indicates the time threshold that specifies when an idle UE should be disconnected from the network. The eNodeB monitors whether UEs are receiving or sending data. When a UE has neither received nor sent data for a duration exceeding this threshold, the eNodeB releases the radio resources of the UE. When this parameter is set to 0, the eNodeB does not monitor the data transmitting or receiving state of the UE.





The initial data radio bearer is setup with SRB2 in the initial UE context procedure after

encryption and integrity protection completion .

This data radio bearer is for the default EPS bearer.



After the UE context is established, the DRB establishment can be triggered by the E-RAB

Setup Request message sent from the MME. drb-ToAddModList which is originally

contained in the Radio Resource Config Dedicated message is contained in the RRC

Connection Reconfiguration message. According to the instructions contained in the

message, the UE establishes a corresponding PDCP entity and configures the related

security parameters, establishes an RLC entity and configures related parameters, and

establishes a DTCH and configures the logical channel.



The DRB modification is triggered by the MME through the E-RAB Modify Request

message. According to the instructions in the RRC Connection Reconfiguration message,

the UE reconfigures the corresponding PDCP entity, corresponding RLC entity, and DTCH.



The DRB is normally released altogether with the signaling connection.

In some cases, the DRB may be released by the E-RAB Release Command message sent

from the MME. During the DRB release, drb-ToReleaseList that is originally contained in

the Radio Resource Config Dedicated message is contained in the RRC Connection

Reconfiguration message. Upon receiving the message, the UE releases all the

corresponding DRB resources.

Parameter description



MO Parameter ID Description

CellAlgoSwi

tch

RachAlgoSwitch RACHAdjSwitch: Indicates the switch that is used to enable and disable the

RACH resource adjustment algorithm, which adaptively adjusts the RACH

resources in the cell based on the access type and the number of accesses.

If this switch is set to ON, the RACH resource adjustment algorithm is

enabled.

If this switch is set to OFF, the RACH resource adjustment algorithm is

disabled.

HoRaSwitch: Indicates the switch that is used to control the random access

mode applied during handovers.

If this switch is set to ON, the eNodeB instructs UEs to use the non-

contention-based random access mode during handovers.

If this switch is set to OFF, the eNodeB instructs UEs to use the contention-

based random access mode during handovers.

UnsyncRaSwitch: Indicates the switch that is used to control the random

access mode applied when UEs are out of synchronization in the uplink.

If this switch is set to ON, the eNodeB instructs UEs to use the non-

contention-based random access mode upon DL data arrival in the case of

out-of-synchronization.

If this switch is set to OFF, the eNodeB instructs the UE to use the

contention-based random access mode upon DL data arrival in the case of

out-of-synchronization.

MaksIdxSwitch: Indicates whether the dedicated preamble is reused among

UEs.

If this switch is set to ON, the eNodeB enables reuse of a dedicated preamble

among UEs based on the MaskIndex parameter.

If this switch is set to OFF, the eNodeB allocates a dedicated preamble to

only one UE at a time.

BackOffSwitch: Indicates the switch that is used to enable and disable the

backoff control algorithm.

If this switch is set to ON, the backoff control algorithm is enabled.

If this switch is set to OFF, the backoff control algorithm is disabled.

Cell RootSequenceIdx Indicates the index of the logical root sequence, which is used to derive the

preamble sequence. Each logical root sequence corresponds to a physical

root sequence.

Cell PreambleFmt Indicates the preamble format used in the cell.

Cell CellRadius Indicates the radius of the cell.

RRCConnSt

ateTimer

T302 Indicates the length of timer T302.

T302 specifies the wait time for retransmitting an RRCConnectionRequest

message after the previous request with a cause other than "MO-Data" and

"MO-Signalling" is rejected.

This timer is started after the UE receives the RRCConnectionReject message.

This timer is stopped when the UE enters the RRC_CONNECTED mode or

performs cell reselection.

RRCConnSt

ateTimer

UeInactiveTimer Indicates the length of the UE inactivity timer. The eNodeB monitors whether

UEs are receiving or sending data. When a UE has neither received nor sent

data for a duration exceeding this period of time, the eNodeB releases the

radio resources for the UE. If this parameter is set to 0, the UE inactivity

timer is not used.



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OEO102020 LTE eRAN2.1 Connection Management Feature ISSUE 1.00.pdf