LTE PRACH (1)

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我是靠谱客的博主负责黑猫，最近开发中收集的这篇文章主要介绍LTE PRACH (1)，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

UE开机，通过小区选择和小区重选，驻留到合适的小区后，就进行”Initial EPS Attach”的过程。通过初始EPS附着，UE可以在EPS网络进行分组域服务的注册，同时，建立缺省的EPS承载，进行用户数据的传输。（3GPP 23.401和29.274）。

LTE <wbr>Initial <wbr>Attach <wbr>过程 <wbr>(1)

步骤1:

message c1 : rrcConnectionRequest :

{

criticalExtensions rrcConnectionRequest-r8 :

{

ue-Identity randomValue : '11110111 01001110 00000010 10000110 100 ...'B,

establishmentCause mo-Signalling,

spare '0'B

rrcConnectionReqest是在SRB0上传输的， SRB0一直存在，用来传输映射到CCCH 的RRC信令。

在此消息中， UE-Identity 的目的是为了底层随机接入的竞争消除。它可以是S－TMSI，也可以是UE生成的随机数。在InitialEPSAttach的过程中，UE还没有获得S－TMSI，因此包含了一个40位的随机值。

NAS层通过establishmentCause指明连接的原因。

步骤2：RRCConnectionSetup

message c1 : rrcConnectionSetup :

{

rrc-TransactionIdentifier 0,

criticalExtensions c1 : rrcConnectionSetup-r8 :

{

radioResourceConfigDedicated

{

srb-ToAddModifyList

{

srb-Identity 1,

rlc-Configuration defaultValue : NULL,

logicalChannelConfig defaultValue : NULL

}

                mac_MainConfig

...

}

physicalConfigDedicated

{

。。。

}

通过底层的竞争接入冲突解决机制，UE接收到NodeB的rrcConnectionSetup信令，建立了UE与ENodeB之间的SRB1，NodeB为SRB1配置RLC层和逻辑层信道的属性。ENodeB还可以在此信令中对MAC层和物理层进行配置，如果NodeB没有对此进行配置， 36.331中定义了MAC 层和物理层的缺省值。

UE收到NodeB的rrcConnectionSetup信令后，UE和NodeB之间的SRB1就建立起来了。

步骤3：RRCConnectionSetupComplete

UL-DCCH-Message =

message = c1 = rrcConnectionSetupComplete =

rrc-TransactionIdentifier = 0

criticalExtensions = c1 = rrcConnectionSetupComplete-r8 =

selectedPLMN-Identity = 1

dedicatedInfoNAS = 。。。

在UE接收到RRCConnectionSetup消息后，向NodeB发送一个RRCConnectionSetupComplete消息。其中， selectedPLMN-Identity表示UE选中的PLMN在SIB1中广播的PLMN List中的序号值。

RRCConnectionSetupComplete消息中的dedicatedInfoNAS包含了NAS层的信令，在EUTRAN中UE的初始接入过程中，NAS层信令通常是EMM层的AttachRequest消息和ESM层的PDNConnectivityRequest消息。

Msg

aTTACH_REQUEST

securityHeaderType = '0000'B

protocolDiscriminator = '0111'B

messageType = '01000001'B

nasKeySetId

iei = Omit

tsc = '0'B

nasKeySetId = '111'B

epsAttachType

spare = '0'B

typeValue = '001'B

oldGutiOrImsi

iei = Omit

iel = '0B'O

idDigit1 = '1111'B

oddEvenInd = '0'B

typeOfId = '110'B

otherDigits = '00F11000010112345678'O

ueNetworkCapability

iei = Omit

iel = '02'O

networkCap = 'C0C0'O

esmMessage

iei = Omit

iel = '0005'O

esmPdu = '0201D031D1'O

oldPtmsiSignature = Omit

additionalGuti = Omit

lastVisitedRegisteredTai

iei = '52'O

plmnId = '00F110'O

tac = '0001'O

drxParameter = Omit

msNetworkCapability = Omit

oldLai = Omit

tmsiStatus = Omit

msClassmark2 = Omit

msClassmark3 = Omit

supportedCodecList = Omit

PiggybackedPduList

NAS_UL_Pdu_Type

Msg

pDN_CONNECTIVITY_REQUEST

epsBearerId = '0'H

protocolDiscriminator = '0010'B

procedureTransactionIdentifier = '01'O

messageType = '11010000'B

pdnType

spare = '0'B

typeValue = '011'B

requestType

spare = '0'B

typeValue = '001'B

esmInfoTransferFlag

iei = 'D'H

spare = '000'B

eitValue = '1'B

accessPointName = Omit

protocolConfigurationOptions = Omit

PiggybackedPduList = Omit

在AttachRequest中，oldGutiOrImsi会被MME用来在HSS中查询用户的签约信息。其中<GUTI> = <GUMMEI><M-TMSI>。M－TMSI是32位的在MME内UE的ID。而<GUMMEI> = <MCC><MNC><MME Identifier>

MMGI＝ MME Group ID， MMEC ＝ MME Code。

由于RRCConnectionSetupComplete消息是明文传送的，为了保护IMSI的私密性，应尽量减少IMSI在空口传播，GUTI的使用就是为了这个目的。当然，手机初始附着，由于不存在OldGUTI，还是会发送一次IMSI。如果eNodeB给UE找到的MME不是之前detach那个MME，新的MME将通过old GUTI找到旧的MME（即上次detach时的MME），再发送Identification Request(GTP-C)消息给旧的MME以获得手机的IMSI。这个Identification Request消息包含old GUTI和完整的Attach请求消息。如果(新的)MME仍然无法得到UE的IMSI， MME会发送IdentityRequest消息给UE, 要求UE上报自己的IMSI。

drxParameter指明UE特定的DRX相关参数。UE通过此参数通知EUTRAN自己特定的寻呼周期, 系统广播消息SIB2中的PCCH Config参数中也定义了缺省的寻呼周期,在这种情况下,UE使用两者中的最小值。

lastVisitedRegisteredTai用来帮助MME生成有效的TAI列表，MME将在Attach Accept消息中返回给UE。

ueNetworkCapability包含NAS和AS的安全参数

pdnType 表示PDN连接的IP类型。（IPv4、IPv6或者IPv4/IPv6）

步骤4：Initial UE Message

eNodeB接收到RRCConnectionComplete消息后，根据里面的信息，选定相应的MME，然后通过eNodeB和MME之间的S1-C接口发送Initial UE Message给MME。在此消息中， eNodeB将UE发送的NAS消息转发给MME（36。413），除此之外，在此消息中，还包括如下项：

IE/Group Name	Presence
Message Type	M
eNB UE S1AP ID	M
NAS-PDU	M
TAI	M
E-UTRAN CGI	M
RRC Establishment cause	M
S-TMSI	O
CSG Id	O
GUMMEI	O
Cell Access Mode	O

其中eNB UE S1AP ID 值表示在此eNodeB中UE的S1接口。MME侧将利用此标识来确定UE所对应的S1－C逻辑连接。

TAI值由PLMN IDentitity和TAC组成，唯一表示了UE的Tracking Area。

MME接收到Initial UE Message后，进行网络和UE之间NAS层的安全认证过程。（参见另外的文章）。

NAS层的安全认证成功后，MME会向HSS发送Update Location Request消息，向HSS更新自己的位置信息。同时，MME向HSS请求用户的APN签约信息。包括缺省的APN设置，每个APN对应的PDN类型，缺省的EPS Bearer 的QOS设置等。

此后，MME就可以与SGW，PGW进行信令交互，为缺省的EPS Bearer建立用户面和控制面的GTP Tunnel 了（GTP隧道）。

对于每个PDN的连接，需要建立一个控制面的GTP Tunnel（GTP－C），包括MME和SGW之间的S11接口和SGW和PGW之间的S5接口。对于每个EPS Bearer，需要建立一个用户面的GTP Tunnel（GTP－U），包括eNodeB 和SGW之间的S1－U接口和SGW和PGW之间的S5接口。

在LTE中， GTP－U使用的版本号为1，注册的UDP端口号为2152。GTP－C使用的版本号为2，注册的UDP端口号为2123。

在GTP的头部中，有一个重要的字段，叫做隧道端点标识符（TEID），标识了对端的GTP-U或GTP-C协议中的隧道端点。由GTP隧道的接收端分配本地TEID值，供GTP隧道的发起方使用。通过GTP-C消息在隧道的两个端点间交换TEID（包含在FTEID内）值。通过IP地址，端口号，以及TEID值就可以唯一确定一个GTP的隧道。

MME分配相应的缺省EPS Bearer ID（即EBI），构造S11接口（控制面）上GTP－C Tunnel的MME端标识MME F－TEID （注意，此信令中只有S11上的控制面TEID，而不包含S1-U的用户面FTEID，S1－U的控制面终结在eNodeB和SGW之间，eNodeB的FTEID-U 在后面的Modify Bearer Request消息中发送），向GW发送Create Session Request消息。

在Create Session Request中，主要包含如下一些主要内容

（1）用户的身份标识，如IMSI， MSISDN, MEI, ULI (User Location Information)等

（2）用户接入网的一些信息，（E－UTRAN， UTRAN等）

（3）服务网络的信息，包括MCC, MNC等。

（4） GTP－C Tunnel的信息，包括MME F－TEID

（5） S5/S8 Interface 的信息，包括协议类型（GTP－C），PDN 的地址（包含在PDN F－TEID内）。（另外一个可能的协议类型就是PMIPV6）。

（6） PDN的类型（IPV4，IPV6或者IPV4、IPV6），APN，

（7）将要建立的Default EPS Bearer的相关信息，包括EBI （EPS Bearer ID）, QoS， APN－AMBR等，以及用于切换时的Indication Header等。

Create Session Request

Flags: 72

010. .... = Version: 2

.... 1... = T: 1

Message Type: Create Session Request (32)

Message Length: 201

Tunnel Endpoint Identifier: 0 （SGW的TEID值，由于此时并没有建立GTP－C，因而取值为零）

Sequence Number: 7660

Spare: 45056

International Mobile Subscriber Identity (IMSI)

。。。

RAT Type :

IE Type: RAT Type (82)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

RAT Type: EUTRAN (6)

Fully Qualified Tunnel Endpoint Identifier (F-TEID) :

IE Type: Fully Qualified Tunnel Endpoint Identifier (F-TEID) (87)

IE Length: 9

000. .... = CR flag: 0

.... 0000 = Instance: 0

1... .... = V4 (True-IPV4 address field Exists,False-Doesn't Exist in F-TEID): True

.0.. .... = V6 (True-IPV6 address field Exists,False-Doesn't Exist in F-TEID): False

...0 1010 = Interface Type: S11 MME GTP-C interface (10)

TEID/GRE Key: 3300033 （TEID值是由接收端分配而由发送端使用）

F-TEID IPv4: 30.0.1.1 (30.0.1.1)

Fully Qualified Tunnel Endpoint Identifier (F-TEID) :

IE Type: Fully Qualified Tunnel Endpoint Identifier (F-TEID) (87)

IE Length: 9

000. .... = CR flag: 0

.... 0001 = Instance: 1

1... .... = V4 (True-IPV4 address field Exists,False-Doesn't Exist in F-TEID): True

.0.. .... = V6 (True-IPV6 address field Exists,False-Doesn't Exist in F-TEID): False

...0 0111 = Interface Type: S5/S8 PGW GTP-C interface (7)

TEID/GRE Key: 0

F-TEID IPv4: 20.0.0.1 (20.0.0.1)

PDN Type :

IE Type: PDN Type (99)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... .001 = PDN Type: IPv4 (1)

Selection Mode :

IE Type: Selection Mode (128)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... ..00 = Selection Mode: MS or network provided APN, subscribed verified (0)

PDN Address Allocation (PAA) :

IE Type: PDN Address Allocation (PAA) (79)

IE Length: 5

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... .001 = PDN Type: IPv4 (1)

PDN IPv4: 0.0.0.0 (0.0.0.0)表示需要PGW分配IPV4 Address

Indication :

IE Type: Indication (77)

IE Length: 2

000. .... = CR flag: 0

.... 0000 = Instance: 0

0... .... = DAF (Dual Address Bearer Flag): False

.0.. .... = DTF (Direct Tunnel Flag): False

..0. .... = HI (Handover Indication): False

...0 .... = DFI (Direct Forwarding Indication): False

.... 0... = OI (Operation Indication): False

.... .0.. = ISRSI (Idle mode Signalling Reduction Supported Indication): False

.... ..0. = ISRAI (Idle mode Signalling Reduction Activation Indication): False

.... ...0 = SGWCI (SGW Change Indication): False

.... 0... = PT (Protocol Type): False

.... .0.. = TDI (Teardown Indication): False

.... ..0. = SI (Scope Indication): False

.... ...0 = MSV (MS Validated): False

Access Point Name (APN) :

IE Type: Access Point Name (APN) (71)

IE Length: 18

000. .... = CR flag: 0

.... 0000 = Instance: 0

APN (Access Point Name): apn-1.example.com

APN Restriction :

IE Type: APN Restriction (127)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

APN Restriction: 0

Aggregate Maximum Bit Rate (AMBR) :

IE Type: Aggregate Maximum Bit Rate (AMBR) (72)

IE Length: 8

000. .... = CR flag: 0

.... 0000 = Instance: 0

AMBR Uplink (Aggregate Maximum Bit Rate for Uplink): 655360000

AMBR Downlink(Aggregate Maximum Bit Rate for Downlink): 655360000

Bearer Context : [Grouped IE]

IE Type: Bearer Context (93)

IE Length: 31

000. .... = CR flag: 0

.... 0000 = Instance: 0

EPS Bearer ID (EBI) :

IE Type: EPS Bearer ID (EBI) (73)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... 0101 = EPS Bearer ID (EBI): 5

Bearer Level Quality of Service (Bearer QoS) :

IE Type: Bearer Level Quality of Service (Bearer QoS) (80)

IE Length: 22

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... ...1 = PVI (Pre-emption Vulnerability): True

..00 00.. = PL (Priority Level): 0

.0.. .... = PCI (Pre-emption Capability): False

Label (QCI): 9

Maximum Bit Rate For Uplink: 65535000

Maximum Bit Rate For Downlink: 65535000

Guaranteed Bit Rate For Uplink: 0

Guaranteed Bit Rate For Downlink: 0

接收到MME发送的Create Session Request消息后，SGW会为S5接口上的GTP Tunnel创建SGW侧的标识，以供PGW侧发来的下行GTP Tunnel使用。由于S5接口上既包含有UE用户面的数据，也包含有控制面的数据。因此，需要建立GTP－C和GTP－U的Tunnel，SGW需要创建SGW GTP－C的FTEID 和SGW GTP－U的FTEID。

SGW向PGW发送Create Session Request消息，包含上述 TEID信息和接收到MME的Create Session Request中的部分信息。

PGW为UE分配相应的IP地址，建立UE到PDN之间的路由。返回Create Session Response 给SGW。Create Session Response的内容包括有：分配的PDN Address，以及PGW TEID－C和PGW TEID－U等。这样SGW和PGW之间的EPS Bearer就建立起来了。SGW分配SGW TEID-C 和SGW TEID－U，并将他们包含在返回给MME的Create Session Response中。Create Session Response中的GTP Header的TEID值取为SGW在Create Session Request中报上来的SGW FTEID－C。

GPRS Tunneling Protocol V2

Create Session Response

Flags: 72

010. .... = Version: 2

.... 1... = T: 1

Message Type: Create Session Response (33)

Message Length: 126

Tunnel Endpoint Identifier: 3300033

Sequence Number: 7660

Spare: 45056

Cause :

IE Type: Cause (2)

IE Length: 2

000. .... = CR flag: 0

.... 0000 = Instance: 0

Cause: Request accepted (16)

.... ...0 = Cause Source (CS: True-Error originated by remote node, False-Error originated by Node sending the Message): False

PDN Address Allocation (PAA) :

IE Type: PDN Address Allocation (PAA) (79)

IE Length: 5

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... .001 = PDN Type: IPv4 (1)

PDN IPv4: 40.0.0.1 (40.0.0.1)为UE分配的IPV4地址

Fully Qualified Tunnel Endpoint Identifier (F-TEID) :

IE Type: Fully Qualified Tunnel Endpoint Identifier (F-TEID) (87)

IE Length: 9

000. .... = CR flag: 0

.... 0000 = Instance: 0

1... .... = V4 (True-IPV4 address field Exists,False-Doesn't Exist in F-TEID): True

.0.. .... = V6 (True-IPV6 address field Exists,False-Doesn't Exist in F-TEID): False

...0 1011 = Interface Type: S11/S4 SGW GTP-C interface (11)

TEID/GRE Key: 1

F-TEID IPv4: 30.0.2.1 (30.0.2.1)

Fully Qualified Tunnel Endpoint Identifier (F-TEID) :

IE Type: Fully Qualified Tunnel Endpoint Identifier (F-TEID) (87)

IE Length: 9

000. .... = CR flag: 0

.... 0001 = Instance: 1

1... .... = V4 (True-IPV4 address field Exists,False-Doesn't Exist in F-TEID): True

.0.. .... = V6 (True-IPV6 address field Exists,False-Doesn't Exist in F-TEID): False

...0 0111 = Interface Type: S5/S8 PGW GTP-C interface (7)

TEID/GRE Key: 1

F-TEID IPv4: 20.0.0.1 (20.0.0.1)

APN Restriction : （具体内容请参见29.274）

IE Type: APN Restriction (127)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

APN Restriction: 0

Bearer Context : [Grouped IE]

IE Type: Bearer Context (93)

IE Length: 63

000. .... = CR flag: 0

.... 0000 = Instance: 0

EPS Bearer ID (EBI) :

IE Type: EPS Bearer ID (EBI) (73)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... 0101 = EPS Bearer ID (EBI): 5

Fully Qualified Tunnel Endpoint Identifier (F-TEID) :

IE Type: Fully Qualified Tunnel Endpoint Identifier (F-TEID) (87)

IE Length: 9

000. .... = CR flag: 0

.... 0000 = Instance: 0

1... .... = V4 (True-IPV4 address field Exists,False-Doesn't Exist in F-TEID): True

.0.. .... = V6 (True-IPV6 address field Exists,False-Doesn't Exist in F-TEID): False

...0 0001 = Interface Type: S1-U SGW GTP-U interface (1)

TEID/GRE Key: 33

F-TEID IPv4: 30.0.2.1 (30.0.2.1)

Fully Qualified Tunnel Endpoint Identifier (F-TEID) :

IE Type: Fully Qualified Tunnel Endpoint Identifier (F-TEID) (87)

IE Length: 9

000. .... = CR flag: 0

.... 0001 = Instance: 1

1... .... = V4 (True-IPV4 address field Exists,False-Doesn't Exist in F-TEID): True

.0.. .... = V6 (True-IPV6 address field Exists,False-Doesn't Exist in F-TEID): False

...0 0101 = Interface Type: S5/S8 PGW GTP-U interface (5)

TEID/GRE Key: 33

F-TEID IPv4: 20.0.0.1 (20.0.0.1)

Cause :

IE Type: Cause (2)

IE Length: 2

000. .... = CR flag: 0

.... 0000 = Instance: 0

Cause: Request accepted (16)

.... ...0 = Cause Source (CS: True-Error originated by remote node, False-Error originated by Node sending the Message): False

Bearer Level Quality of Service (Bearer QoS) :

IE Type: Bearer Level Quality of Service (Bearer QoS) (80)

IE Length: 22

000. .... = CR flag: 0

.... 0000 = Instance: 0

.... ...0 = PVI (Pre-emption Vulnerability): False

..00 00.. = PL (Priority Level): 0

.0.. .... = PCI (Pre-emption Capability): False

Label (QCI): 9

Maximum Bit Rate For Uplink: 65535000

Maximum Bit Rate For Downlink: 65535000

Guaranteed Bit Rate For Uplink: 0

Guaranteed Bit Rate For Downlink: 0

Recovery (Restart Counter) :

IE Type: Recovery (Restart Counter) (3)

IE Length: 1

000. .... = CR flag: 0

.... 0000 = Instance: 0

Restart Counter: 0

MME接收到SGW发送的Create Session Response后，在相应的TAI中为UE注册。并且构造NAS层的相应消息，包括EMM层的Attach Accept消息和ESM层的Activate Default EPS Bearer Context 消息。相应的TAI列表也返回给eNodeB，MME还为UE分配相应的GUTI。MME将上述信息通过InitalContext Setup Request消息返回给eNodeB。SGW的上行GTP－U的TEID值也包含在InitialContextSetupRequeset消息中。