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Contents
1 EPS Architecture ......................................................................................................................... 1-2
1.1 EPS Network Elements ............................................................................................ ..................................... 1-3
1.1.1 User Equipment .................. ................................................................. ................................................ 1-3
1.1.2 Evolved Node B ......................................................................................................... .......................... 1-5
1.1.3 Mobility Management Entity ............................................................................................................... 1-6
1.1.4 Serving Gateway ................................................................................................................... ............... 1-7
1.1.5 Packet Data Network - Gateway .......................................................... ................................................ 1-8
1.2 EPS Interfaces ..................................................................................................................... .......................... 1-9
1.2.1 E-UTRAN Interfaces ..................... ................................................................. ..................................... 1-9
1.2.2 EPC Interfaces ....................... .............................................................. ................................................ 1-9
1.2.3 Additional Network Elements and Interfaces ............................................................. ........................ 1-10
2 EPS Protocols ............................................................................................................................ 2-13
2.1 EPS Signaling.............................................................................................................................................. 2-14
2.2 EPS Protocols ..................................................... ................................................................. ........................ 2-15
2.2.1 NAS Functionality .................................... .............................................................. ........................... 2-15
2.2.2 NAS Concepts and Identities ............................................................... .............................................. 2-16
2.2.3 EMM and ESM .......................................................................................................... ........................ 2-18
2.2.4 NAS States and State Transitions ......................................................... .............................................. 2-20
2.2.5 Uu Interface ................................... ................................................................. ................................... 2-22
2.2.6 Uu Interface - RRC ................................................................... ......................................................... 2-23
2.2.7 Uu Interface - PDCP .......................................................................................................... ................ 2-23
2.2.8 Uu Interface - RLC ................................................................... ......................................................... 2-24
2.2.9 Uu Interface - MAC ............................................................................. .............................................. 2-25
2.2.10 Uu Interface - Physical ............................................................. ........................................................ 2-25
2.2.11 X2 Interface ........................................................ ................................................................. ............. 2-26
2.2.12 X2 Interface - X2 Application Protocol ........................ .............................................................. ..... 2-26
2.2.13 X2 Interface - Stream Control Transmission Protocol ................................................................ ..... 2-27
2.2.14 X2 Interface - GPRS Tunneling Protocol - User ................................................................ .............. 2-27
2.2.15 S1 Interface ........................................................ ................................................................. ............. 2-27
2.2.16 S1 Interface - S1 Application Protocol .......................................................... ................................... 2-28
2.2.17 S1 Interface - SCTP and GTP-U ........................................................ .............................................. 2-28
2.2.18 S11 Interface ................................................................ ................................................................ .... 2-29
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2.2.19 GPRS Tunneling Protocol version 2 - Control ......................................................... ........................ 2-29
2.2.20 S5/S8 Interface .............................................................. ................................................................. .. 2-29
2.2.21 Proxy Mobile IP ............................................................ ................................................................. .. 2-30
2.2.22 S10 Interface ...................................................... ................................................................. ............. 2-30
2.2.23 SGi Interface ...................................................... ................................................................. ............. 2-31
3 LTE/SAE Quality of Service ................................................................................................... 3-32
3.1 EPS Bearer Services and E-UTRA Radio Bearers ...................................................................................... 3-33
3.1.1 QoS in Packet Switched Networks ........................................... ......................................................... 3-33
3.1.2 LTE Bearers .............................................. ................................................................. ........................ 3-33
3.1.3 The Default EPS Bearer ................................................................................................................ ..... 3-35
3.1.4 Dedicated EPS Bearers ........................................ .............................................................. ................ 3-35
3.1.5 EPS QoS Parameters ................................................................. ......................................................... 3-35
3.2 E-UTRA Radio Bearers .................................................................................................................. ............. 3-37
3.2.1 Signaling Radio Bearers ............................................................. ........................................................ 3-37
3.2.2 Data Radio Bearers .............................................................................. .............................................. 3-37
3.2.3 Radio Bearer QoS ................................................................................ .............................................. 3-38
4 Radio Resource Control .......................................................................................................... 4-40
4.1 The RRC Layer ............................................................. ................................................................. ............. 4-41
4.1.2 Services Provided To Upper Layers ................................................................................................... 4-41
4.1.3 Services Expected From Lower Layers .................................... ......................................................... 4-41
4.2 RRC Structure ......................................................................... ............................................................... ..... 4-42
4.3 RRC States ......................................................... ................................................................. ........................ 4-42
4.3.2 Functions .............................................................. ................................................................. ............. 4-43
4.4 RRC Services ..................................................... ................................................................. ........................ 4-45
4.4.1 System Information ......................................................... .............................................................. ..... 4-45
4.4.2 Paging ................................................................. ............................................................... ................ 4-46
4.4.3 RRC Connection Establishment ........................................................... .............................................. 4-47
4.4.4 Initial Security Activation ............................................................................................................. ..... 4-48
4.4.5 RRC Connection Reconfiguration ....................... ................................................................. ............. 4-48
4.4.6 RRC Connection Re-establishment ................................................................. ................................... 4-49
4.4.7 RRC Connection Release .............................................................................................................. ..... 4-50
4.4.8 Radio Link Failure .......................................................................................... ................................... 4-50
4.4.9 Information Transfer ............................................ .............................................................. ................ 4-51
4.4.10 Measurement Configuration ................... ................................................................. ........................ 4-51
4.4.11 Handover Configuration ........................................................... ........................................................ 4-56
4.4.12 Cell Selection ..................................................... ................................................................. ............. 4-56
4.4.13 Cell Reselection ................................................. ................................................................. ............. 4-56
5 Packet Data Convergence Protocol ....................................................................................... 5-58
5.1 PDCP Operation ............................................................ ................................................................. ............. 5-59
5.1.1 Functions .............................................................. ................................................................. ............. 5-59
5.1.2 PDCP Header Compression Profiles .................................................................................................. 5-60
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5.1.3 PDCP Headers ...................................................... ................................................................. ............. 5-61
5.1.4 PDCP ROHC ........................................................ ................................................................. ............. 5-62
5.1.5 PDCP Integrity ..................................................... ................................................................. ............. 5-63
5.1.6 PDCP Ciphering .............................................................. ................................................................. .. 5-64
6 Radio Link Control and Medium Access Control ............................................................. 6-66
6.1 RLC Functions ........................................ ................................................................. ................................... 6-67
6.1.1 Services Provided to Upper Layers ................................................................. ................................... 6-67
6.1.2 Services Expected from Lower Layers ........................................................... ................................... 6-67
6.1.3 Functions .............................................................. ................................................................. ............. 6-67
6.2 RLC Modes and Formatting ................................................................ ........................................................ 6-68
6.2.1 Transparent Mode ................................................ ................................................................. ............. 6-68
6.2.2 Unacknowledged Mode ....................................... .............................................................. ................ 6-68
6.2.3 Acknowledged Mode ...................................................... .............................................................. ..... 6-69
6.2.4 TMD PDU ............................................................ ................................................................. ............. 6-70
6.2.5 UMD PDU ................................................................................................................. ........................ 6-71
6.2.6 AMD PDU ................................................ ................................................................. ........................ 6-72
6.2.7 RLC Timers .......................................................... ................................................................. ............. 6-75
6.2.8 Configurable Parameters ............................................................ ........................................................ 6-75
6.3 MAC Functions ............................................................. ................................................................. ............. 6-75
6.4 MAC Architecture ......................................................................................... .............................................. 6-76
6.5 MAC Formatting ........................................................... ................................................................. ............. 6-77
6.5.1 MAC Headers ...................................................... ................................................................. ............. 6-77
6.5.2 MAC Subheaders ............................................................ ................................................................. .. 6-78
6.5.3 Random Access Process ............................................................. ........................................................ 6-81
7 X2/S1 Interface and Protocols ................................................................................................ 7-84
7.1 X2AP Functions and Procedures ......................................................... ........................................................ 7-85
7.1.2 Functions of the X2 Application Protocol ....................................................... ................................... 7-85
7.1.3 X2 Elementary Procedures ......................................................... ........................................................ 7-86
7.1.4 Message Formatting ........................................................................................................... ................ 7-87
7.1.5 X2 Basic Mobility Procedures - Handover Preparation ........................................................ ............. 7-88
7.1.6 X2 Load Indication .............................................. .............................................................. ................ 7-92
7.1.7 X2 Resource Status Reporting ............................................................. .............................................. 7-94
7.1.8 X2 Setup ................................................... ................................................................. ........................ 7-95
7.1.9 X2 eNB Configuration .................................................................................................................. ..... 7-96
7.2 S1AP Functions and Procedures ............. ................................................................. ................................... 7-97
7.2.1 S1AP Functions ............................................................... ................................................................. .. 7-98
7.2.2 S1AP Elementary Procedures ................... ................................................................. ........................ 7-99
7.2.3 S1 Setup ..................................................................................................................... ...................... 7-101
7.2.4 eNB and MME Configuration Update ................................................. ............................................ 7-103
7.2.5 NAS Transport .......................................................................... ....................................................... 7-103
7.2.6 Initial Context Setup ............................................ .............................................................. .............. 7-105
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7.2.7 E-RAB Establishment ................................................................................................................... ... 7-107
7.2.8 S1 Handover..................................................................................................................................... 7-109
7.2.9 Path Switch .......................................................... ................................................................. ........... 7-114
7.2.10 Handover Cancel ........................................................... ................................................................. 7-116
7.2.11 Status Transfer ............................................................... ................................................................. 7-117
7.2.12 UE Context Release ................................................................................................ ....................... 7-117
7.2.13 Reset ........................................................ ................................................................. ...................... 7-118
7.2.14 Location Reporting Control ......... ................................................................. ................................. 7-118
7.2.15 Overload ............................................................. ................................................................. ........... 7-119
7.2.16 Direct Information Transfer ................................................................................................ ........... 7-119
7.2.17 Paging ................................................................ ................................................................ ............ 7-120
7.3 User Plane GTP Functions and Procedures .............................................................. ................................. 7-120
7.3.1 GTP Tunnels ........................................................ ................................................................. ........... 7-120
7.3.2 GTPv1-U Header ...................................... ................................................................. ...................... 7-121
7.3.3 Extension Header ............................................................ ................................................................. 7-122
7.3.4 Handling of Sequence Numbers ........................................................... ............................................ 7-123
7.3.5 GTPv1-U Procedures ................................................................ ....................................................... 7-123
7.3.6 Path Management ............................................................ ................................................................. 7-123
7.3.7 UDP header and Port Numbers ....................................................................... ................................. 7-126
8 Mobility in LTE ...................................................................................................................... 8-127
8.1 X2 Handover ...................................................... ................................................................. ...................... 8-128
8.1.1 Handover Phases ............................................................. ................................................................. 8-128
8.1.2 X2 Based Handover with Lossless PDCP ....................................................... ................................. 8-128
8.1.3 Data Forwarding .................................................. ................................................................. ........... 8-132
8.2 S1 Handover ....................................................... ................................................................. ...................... 8-133
8.2.1 Inter MME and S-GW Handover ......................................................... ............................................ 8-133
8.2.2 S1 Status Transfer ........................................................................................... ................................. 8-135
8.3 Inter RAT Handover ................................................................ ................................................................ .. 8-136
8.3.1 E-UTRAN to UTRAN Handover... ................................................................. ................................. 8-136
8.3.2 UTRAN to E-UTRAN Handover.................................................................... ................................. 8-137
9 Glossary ................................................................................................................................... 9-138
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Figures
Figure 1-1 LTE Reference Architecture ....................................... ............................................................... ....... 1-3
Figure 1-2 User Equipment Functional Elements ......................................................... ..................................... 1-4
Figure 1-3 Evolved Node B Functional Elements .............................................................................................. 1-5
Figure 1-4 MME Functional Elements ............................................................................................................... 1-7
Figure 1-5 S-GW Functional Elements ......................................................................... ..................................... 1-8
Figure 1-6 PDN-GW Functional Elements.............. ................................................................. .......................... 1-8
Figure 1-7 E-UTRAN Interfaces ...................................................................................................... .................. 1-9
Figure 1-8 EPC Architecture and Interfaces ....................................................... .............................................. 1-10
Figure 1-9 Additional Network Elements and Interfaces .............................................. ................................... 1-11
Figure 2-1 NAS and AS Control Plane ............................................................... .............................................. 2-14
Figure 2-2 NAS and AS User Plane .............................................................................. ................................... 2-15
Figure 2-3 NAS Protocol stack................................ ................................................................ ......................... 2-16
Figure 2-4 NAS Identities ............................ ................................................................. ................................... 2-17
Figure 2-5 TA and TA List ............................................................. ................................................................. .. 2-18
Figure 2-6 NAS States and State Transtions............................................................................. ........................ 2-21
Figure 2-7 Network Attach ............................................................ ................................................................. .. 2-22
Figure 2-8 Uu Interface Protocols ................................................................................. ................................... 2-23
Figure 2-9 Main RRC Functions .............................................................................................. ........................ 2-23
Figure 2-10 PDCP Functions ......................................................... .............................................................. ..... 2-24
Figure 2-11 RLC Modes and Functions .................................................. ......................................................... 2-25
Figure 2-12 Medium Access Control Functions .................................................................................. ............. 2-25
Figure 2-13 Physical Layer Functions ......................................................................................................... ..... 2-26
Figure 2-14 X2 Interface Protocols .......................................................................................... ........................ 2-26
Figure 2-15 S1 Interface Protocols ........................................................... ........................................................ 2-28
Figure 2-16 S11 Interface Protocols ............................................................................................................ ..... 2-29
Figure 2-17 S5/S8 Interface Protocols ................................................................ .............................................. 2-30
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Figure 2-18 S10 Interface Protocols ......................................................... ........................................................ 2-31
Figure 2-19 SGi Interface Protocols ......................................................... ........................................................ 2-31
Figure 3-1 QoS Packet Scheduling ........................................................... ........................................................ 3-33
Figure 3-2 LTE Bearers ............................................................................................................ ........................ 3-34
Figure 3-3 Service Data Flows ................................................................ ......................................................... 3-34
Figure 3-4 Default and Dedicated EPS Bearers ............................................................ ................................... 3-35
Figure 3-5 Signaling Radio Bearers ................................................................... .............................................. 3-37
Figure 3-6 Data Radio Bearers ................................................................................................ ......................... 3-38
Figure 3-7 E-RAB QoS Parameters to the eNB ............................................................ ................................... 3-38
Figure 3-8 E-UTRA E-RAB QoS ................................................................................................................ ..... 3-39
Figure 4-1 RRC Interaction with Lower Layers ............................................................ ................................... 4-41
Figure 4-2 eNB Structure .............................................................................................. ................................... 4-42
Figure 4-3 RRC States ........................................................ ................................................................. ............. 4-43
Figure 4-4 E-UTRA RRC State Interaction ........................................................ .............................................. 4-44
Figure 4-5 Mobility Procedures between E-UTRA and CDMA2000 ............................................................ .. 4-45
Figure 4-6 MIB and SIB1 Parameters ........................................................................... ................................... 4-45
Figure 4-7 LTE SIBs .................................... ................................................................. ................................... 4-46
Figure 4-8 RRC Paging ................................................................ ................................................................ .... 4-47
Figure 4-9 RRC Connection .......................................................... .............................................................. ..... 4-47
Figure 4-10 RRC Security Mode Command ....................................................................................... ............. 4-48
Figure 4-11 RRC Connection Reconfiguration ............................. ................................................................. .. 4-49
Figure 4-12 RRC Connection Reestablishment ............................................................. ................................... 4-50
Figure 4-13 RRC Connection Release..................... ................................................................. ........................ 4-50
Figure 4-14 Information Transfer ............................................................. ........................................................ 4-51
Figure 4-15 Measurement Configuration ................ ................................................................. ........................ 4-52
Figure 4-16 Measurement Object ............................................................. ........................................................ 4-53
Figure 4-17 Report Configuration ............................................................ ........................................................ 4-53
Figure 4-18 Periodical Reporting ................................................................................................................ ..... 4-54
Figure 4-19 Event Based Trigger (Event A3) ................................................................................................ ... 4-54
Figure 4-20 Event A3 Example ........................................................................................................ ................ 4-56
Figure 5-1 PDCP Functions ........................................................... ................................................................. .. 5-59
Figure 5-2 PDCP Data PDU for SRB ................................................................. .............................................. 5-61
Figure 5-3 User Plane PDCP Data PDU with Long PDCP SN (12 bits) .......................................................... 5-61
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Figure 5-4 User Plane PDCP Data PDU with Short PDCP SN (7 bits) ....................................................... ..... 5-62
Figure 5-5 PDCP Control PDU for PDCP Status Report ................................................................. ................ 5-62
Figure 5-6 PDCP Control PDU for Interspersed ROHC Feedback Packet .............................. ........................ 5-62
Figure 5-7 ROHC Feedback .......................................................... .............................................................. ..... 5-63
Figure 5-8 Derivation of MAC-I .............................................................................................. ........................ 5-64
Figure 5-9 Count Value ........................................................................... ......................................................... 5-64
Figure 5-10 PDCP Ciphering .................................. .............................................................. ........................... 5-64
Figure 6-1 RLC Modes ....................................................... ................................................................. ............. 6-67
Figure 6-2 Transparent Mode RLC .............. ................................................................. ................................... 6-68
Figure 6-3 Unacknowledged Mode RLC ...................................................................... ................................... 6-68
Figure 6-4 Acknowledged Mode RLC ......... ................................................................. ................................... 6-70
Figure 6-5 RLC UMD 5bit SN (No Length Indicators) ................ .............................................................. ..... 6-71
Figure 6-6 RLC UMD 10bit SN (No Length Indicators) .............. .............................................................. ..... 6-71
Figure 6-7 RLC UMD with 2 Length Indicators .............................................................................................. 6-72
Figure 6-8 RLC AMD with no Length Indicators ......................... .............................................................. ..... 6-73
Figure 6-9 RLC AMD with Odd Number of Length Indicators .................................................................. ..... 6-73
Figure 6-10 RLC AMD PDU Segment......... ................................................................. ................................... 6-74
Figure 6-11 AMD Segmentation ........................................ .............................................................. ................ 6-74
Figure 6-12 RLC Status PDU ........................................................ .............................................................. ..... 6-75
Figure 6-13 MAC Structure (UE Side).................................................... ......................................................... 6-76
Figure 6-14 MAC Header .............................................................. ................................................................. .. 6-77
Figure 6-15 MAC Subheaders ................................................................. ......................................................... 6-78
Figure 6-16 Timing Advance Parameter ............................................................. .............................................. 6-79
Figure 6-17 Short BSR and Truncated BSR MAC Control Element .............................................................. .. 6-79
Figure 6-18 Long BSR MAC Control Element ................................................................................................ 6-79
Figure 6-19 Power Control Headroom ........................................................................................................ ..... 6-80
Figure 6-20 Power Headroom Control Element ............................................................ ................................... 6-80
Figure 6-21 Random Access RRC Signaling Procedure .......................................................... ........................ 6-81
Figure 6-22 Random Access Response .............................. ................................................................. ............. 6-82
Figure 6-23 Backoff Indicator ................................................................. ......................................................... 6-83
Figure 7-1 X2 Control and User Plane ............................................................................................................. 7-85
Figure 7-2 X2 Handover Request ............................................................. ........................................................ 7-89
Figure 7-3 X2 Handover Preparation Failure ...................................................................................... ............. 7-90
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Figure 7-4 X2 SN Status Transfer ............................................................ ........................................................ 7-91
Figure 7-5 X2 UE Context Release .......................................................................................... ........................ 7-92
Figure 7-6 X2 Handover Cancel ............................................................... ........................................................ 7-92
Figure 7-7 X2 Load Indication .................................................................................................................... ..... 7-93
Figure 7-8 X2 Uplink Interference .............................................................................................................. ..... 7-93
Figure 7-9 Downlink RNTP .................................................................................................. ........................... 7-93
Figure 7-10 X2 Resource Status Request .................................................................................................... ..... 7-94
Figure 7-11 X2 Resource Status Update............................................................. .............................................. 7-95
Figure 7-12 X2 Setup Request ..................... ................................................................. ................................... 7-96
Figure 7-13 eNB Configuration Update .............................................................................................. ............. 7-97
Figure 7-14 S1 Control and User Plane .............................................................. .............................................. 7-98
Figure 7-15 S1 Setup Request .......................................................................................................... .............. 7-102
Figure 7-16 S1 Setup Response ................................................................ ...................................................... 7-102
Figure 7-17 S1 Initial UE Message .......................................................................................... ...................... 7-104
Figure 7-18 S1 Downlink and Uplink NAS Transport .................................................. ................................. 7-104
Figure 7-19 S1 Initial Context Setup Request ............................................................... ................................. 7-106
Figure 7-20 Initial Context Setup Response ................................................................................................ ... 7-107
Figure 7-21 S1 E-RAB Setup Request ............................................................... ............................................ 7-108
Figure 7-22 S1 E-RAB Setup Response ......................................................................................................... 7-108
Figure 7-23 E-RAB Release Indication .......................................................................................................... 7-109
Figure 7-24 Requirement for S1 Handover Procedures............................................................ ...................... 7-110
Figure 7-25 S1 Handover Required .......................................................... ...................................................... 7-111
Figure 7-26 S1 Handover Command ....................................................... ....................................................... 7-112
Figure 7-27 S1 Handover Request ............... ................................................................. ................................. 7-113
Figure 7-28 Handover Request Acknowledge ................................................................................................ 7-114
Figure 7-29 Handover Notify ........................................................................................................... .............. 7-114
Figure 7-30 S1 Path Switch Request ................................................................................................ .............. 7-115
Figure 7-31 Path Switch Request Acknowledge ................................................................................. ........... 7-116
Figure 7-32 Handover Cancel ....................................................... .............................................................. ... 7-116
Figure 7-33 UE Context Release .............................................................. ...................................................... 7-117
Figure 7-34 UE Context Release Request ........................................................................................... ........... 7-118
Figure 7-35 S1 Reset .......................................................... ................................................................. ........... 7-118
Figure 7-36 S1 Trace Start ............................................................. ......................................错误 未定义书签
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Figure 7-37 Location Report Control ............................................................................................................. 7-119
Figure 7-38 Overload Start ............................................................ ................................................................. 7-119
Figure 7-39 Paging .................................................................................. ....................................................... 7-120
Figure 7-40 GTP Tunnel ..................................................... ................................................................. ........... 7-121
Figure 7-41 GTPv1-U Header ................................................................. ....................................................... 7-121
Figure 7-42 GTP Extension Header .............................................. .............................................................. ... 7-122
Figure 7-43 GTP Echo Procedure.................................................. .............................................................. ... 7-124
Figure 7-44 Supported Extension Headers Notification ........................................................... ...................... 7-125
Figure 7-45 End Marker Procedure .................................................................................................. .............. 7-125
Figure 8-1 Handover Phases .......................................................... .............................................................. ... 8-128
Figure 8-2 X2 Based Handover with Lossless PDCP.................... .............................................................. ... 8-129
Figure 8-3 Mobility Control Information .................................................................................................... ... 8-130
Figure 8-4 X2AP SN Status Transfer ............................................................................ ................................. 8-131
Figure 8-5 S1 Based Inter MME/S-GW Handover ............................................ ............................................ 8-133
Figure 8-6 S1 Based Inter MME/S-GW Handover Continued ............................................................ ........... 8-134
Figure 8-7 S1 Based Inter MME/S-GW Handover Continued ............................................................ ........... 8-135
Figure 8-8 E-UTRAN to UTRAN Handover ................................ ................................................................ . 8-136
Figure 8-9 E-UTRAN to UTRAN Handover Continued .......................................................... ...................... 8-137
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Tables
Table 1-1 UE Categories ..................................................... ................................................................. ............... 1-4
Table 2-1 NAS EMM and ESM Procedures ....................................................... .............................................. 2-19
Table 3-1 QCI Attributes .............................. ................................................................. ................................... 3-36
Table 5-1 Supported Header Compression Protocols and Profiles ...................................................... ............. 5-60
Table 6-1 RLC PDU Formats .................................. .............................................................. ........................... 6-70
Table 6-2 FI Field Interpretation ............................................................... ........................................................ 6-72
Table 6-3 LCID Coding for DL-SCH ....................................................................................... ........................ 6-77
Table 6-4 LCID Coding for UL-SCH ....................................................................................... ........................ 6-78
Table 6-5 Power Headroom Report Mapping ................................................................ ................................... 6-80
Table 6-6 Uplink Grant ....................................................... ................................................................. ............. 6-82
Table 7-1 Mapping between X2AP Functions and X2AP EPs ..... ............................................................... ..... 7-86
Table 7-2 Class 1 Elementary Procedures ............................................... ......................................................... 7-86
Table 7-3 S1AP Class 1 Elementary Procedures ................ ................................................................. ............. 7-99
Table 7-4 S1AP Class 2 Elementary Procedures ................ ................................................................. ........... 7-100
Table 7-5 Messages in GTP-U ................................................................. ....................................................... 7-123
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1EPS Architecture
About This Chapter
The following table lists the contents of this chapter.
Section
1.1 EPS Network Elements
1.2 EPS Interfaces
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1.1 EPS Network Elements
The term EPS (Evolved Packet System) relates to the Evolved 3GPP Packet SwitchedDomain. In contrast to the 2G and 3G networks defined by the 3GPP, LTE can be simplydivided into a flat IP based bearer network and a service enabling network. The former can be
further subdivided into the E-UTRAN (Evolved - Universal Terrestrial Radio Access Network)
and the EPC (Evolved Packet Core) where as support for service delivery lies in the IMS (IPMultimedia Subsystem). This reference architecture can be seen in Figure 1-1
Figure 1-1 LTE Reference Architecture
UEeNB
E-UTRAN EPC
S-GW
MME
PDN-GW
IMS
HSS Video ASCSCF
eNB
Whilst UMTS is based upon WCDMA technology, the 3GPP developed new specifications
for the LTE air interface based upon OFDMA (Orthogonal Frequency Division MultipleAccess) in the downlink and SC-FDMA (Single Carrier - Frequency Division Multiple Access)in the uplink. This new air interface is termed the E-UTRA (Evolved - Universal Terrestrial
Radio Access).
1.1.1 User Equipment
Like that of UMTS, the mobile device in LTE is termed the UE (User Equipment) and is
comprised of two distinct elements; the USIM (Universal Subscriber Identity Module) and the
ME (Mobile Equipment).
The ME supports a number of functional entities including:
RR (Radio Resource) - this supports both the Control Plane and User Plane and in sodoing, is responsible for all low level protocols including RRC (Radio Resource Control),
PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), MAC (Medium
Access Control) and the PHY (Physical) Layer.
EMM (EPS Mobility Management) - is a Control Plane entity which manages themobility management states the UE can exist in; LTE Idle, LTE Active and LTE
Detached. Transactions within these states include procedures such as TAU (TrackingArea Update) and handovers.
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ESM (EPS Session Management) - is a Control Plane activity which manages theactivation, modification and deactivation of EPS bearer contexts. These can either be
default EPS bearer contexts or dedicated EPS bearer contexts.
Figure 1-2 User Equipment Functional Elements
UE
EPS Mobility & EPS
Session Management
IP Adaptation
FunctionRadio Resource
Control
Plane
User
Plane
EPS Session Management
Bearer Activation
Bearer ModificationBearer Deactivation
Radio Resource
RRC, PDCP, RLC, MAC &
PHY Layer Protocols
EPS Mobility Management
Registration
Tracking Area UpdateHandover
In terms of the Physical Layer, the capabilities of the UE may be defined in terms of thefrequencies and data rates supported. Devices may also be capable of supporting adaptive
modulation including QPSK (Quadrature Phase Shift Keying), 16QAM (16 Quadrature
Amplitude Modulation) and 64QAM (Quadrature Amplitude Modulation).
In terms of the radio spectrum, the UE is able to support several scalable channels including;1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz whilst operating in FDD (Frequency
Division Duplex) and/or TDD (Time Division Duplex). Furthermore, the UE may also
support advanced antenna features such as MIMO (Multiple Input Multiple Output).
Table 1-1 UE Categories
UE Category MaximumDownlinkData Rate
Number ofDownlinkData Streams
MaximumUplinkData Rate
Support forUplink64QAM
1 10.3Mbit/s 1 5.2Mbit/s No
2 51.0Mbit/s 2 25.5Mbit/s No
3 102.0Mbit/s 2 51.0Mbit/s No
4 150.8Mbit/s 2 51.0Mbit/s No
5 302.8Mbit/s 4 75.4Mbit/s Yes
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1.1.2 Evolved Node B
In addition to the new air interface, a new base station has also been specified by the 3GPP
and is referred to as an eNB (Evolved Node B). These, along with their associated interfacesform the E-UTRAN and in so doing, are responsible for:
RRM (Radio Resource Management) - this involves the allocation to the UE of the physical resources on the uplink and downlink, access control and mobility control.
Date Compression - is performed in both the eNB and the UE in order to maximize the
amount of user data that can be transferred on the allocated resource. This process isundertaken by PDCP.
Data Protection - is performed at the eNB and the UE in order to encrypt and integrity
protect RRC signaling and encrypt user data on the air interface.
Routing - this involves the forwarding of Control Plane signaling to the MME and User
Plane traffic to the S-GW (Serving - Gateway).
Packet Classification and QoS Policy Enforcement - this involves the “marking” of
uplink packets based upon subscription information or local service provider policy. QoS(Quality of Service) policy enforcement is then responsible for ensuring such policy isenforced at the network edge.
Figure 1-3 Evolved Node B Functional Elements
eNB
Radio Resource
Management
DataCompression
Data ProtectionRouting
Packet
Classification
and QoS Policy
Enforcement
Security in LTE is not solely limited to encryption and integrity protection of information passing across
the air interface but instead, NAS encryption and integrity protection between the UE and MME alsotakes place. In addition, IPSec may also be used to protect user data within both the E-UTRAN and
EPC.
eNB Identities
In addition to the UE identities already discussed, there are a number of specific identitiesassociated with the eNB. These include:
TAI (Tracking Area Identity) - is a logical group of neighboring cells defined by the
service provider in which UEs in LTE Idle mode are able to move within withoutneeding to update the network. As such, it is similar to a RAI (Routing Area Identity)
used in 2G and 3G packet switched networks.
ECGI (E-UTRAN Cell Global Identifier) - is comprised of the MCC, MNC and ECI(Evolved Cell Identity), the later being coded by each service provider.
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Femto Cells
In order to improve both network coverage and capacity, the 3GPP have developed a new typeof base station to operate within the home or small business environment. Termed the HeNB(Home Evolved Node B), this network element forms part of the E-UTRAN and in so doing
supports the standard E-UTRAN interfaces. However, it must be stated that HeNBs do notsupport the X2 interface.
The architecture may include an HeNB-GW (Home Evolved Node B - Gateway) whichresides between the HeNB in the E-UTRAN and the MME / S-GW in the EPC in order toscale and support large numbers of base station deployments.
1.1.3 Mobility Management Entity
The MME is the Control Plane entity within the EPC and as such is responsible for the
following functions:
NAS Signaling and Security - this incorporates both EMM (EPS Mobility Management)
and ESM (EPS Session Management) and thus includes procedures such as TrackingArea Updates and EPS Bearer Management. The MME is also responsible for NASsecurity.
S-GW and PDN-GW Selection - upon receipt of a request from the UE to allocate a
bearer resource, the MME will select the most appropriate S-GW and PDN-GW. Thisselection criterion is based on the location of the UE in addition to current load
conditions within the network.
Tracking Area List Management and Paging - whilst in the LTE Idle state, the UE istracked by the MME to the granularity of a Tracking Area. Whilst UEs remain within the
Tracking Areas provided to them in the form of a Tracking Area List, there is norequirement for them to notify the MME. The MME is also responsible for initiating the
paging procedure. Inter MME Mobility - if a handover involves changing the point of attachment within the
EPC, it may be necessary to involve an inter MME handover. In this situation, the
serving MME will select a target MME with which to conduct this process.
Authentication - this involves interworking with the subscriber’s HSS (Home SubscriberServer) in order to obtain AAA (Access Authorization and Accounting) information with
which to authenticate the subscriber. Like that of other 3GPP system, authentication is based on AKA (Authentication and Key Agreement).
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Figure 1-4 MME Functional Elements
NAS Signaling
and Security
S-GW andPDN-GW
Selection
Tracking Area ListManagement and
Paging
Inter MME
Mobility
Authentication
MME
1.1.4 Serving Gateway
The S-GW terminates the S1-U Interface from the E-UTRAN and in so doing, provides the
following functions:
Mobility Anchor - for inter eNB handovers, the S-GW acts as an anchor point for theUser Plane. Furthermore, it also acts as an anchor for inter 3GPP handovers to legacy
networks - GPRS and UMTS.
Downlink Packet Buffering - when traffic arrives for a UE at the S-GW, it may need to be buffered in order to allow time for the MME to page the UE and for it to enter the
LTE Active state.
Packet Routing and Forwarding - traffic must be routed to the correct eNB on thedownlink and the specified PDN-GW on the uplink.
Lawful Interception - this incorporates the monitoring of VoIP (Voice over IP) and other packet services.
GTP/PMIP Support - if PMIP (Proxy Mobile IP) is used on the S5/S8 Interfaces, theS-GW must support MAG (Mobile Access Gateway) functionality. Furthermore, support
for GTP/PMIP chaining may also be required.
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Figure 1-5 S-GW Functional Elements
Mobility Anchor
Downlink
Packet
Buffering
Packet Routing
and Forwarding
Lawful
Interception
GTP/PMIP
Support
S-GW
1.1.5 Packet Data Network - GatewayThe PDN-GW is the network element which terminates the SGi Interface towards the PDN(Packet Data Network). If a UE is accessing multiple PDNs, there may be a requirement for
multiple PDN-GWs to be involved. Functions associated with the PDN-GW include:
Packet Filtering - this incorporates the deep packet inspection of IP datagrams arriving
from the PDN in order to determine which TFT (Traffic Flow Template) they are to beassociated with.
Lawful Interception - as with the S-GW, the PDN-GW may also monitor traffic as it
passes across it.
IP Address Allocation - IP addresses may be allocated to the UE by the PDN-GW. This is
included as part of the initial bearer establishment phase or when UEs roam betweendifferent access technologies.
Transport Level Packet Marking - this involves the marking of uplink and downlink
packets with the appropriate tag e.g. DSCP (Differentiated Services Code Point) basedon the QCI (QoS Class Identifier) of the associated EPS bearer.
Accounting - through interaction with a PCRF (Policy Rules and Charging Function), thePDN-GW will monitor traffic volumes and types.
Figure 1-6 PDN-GW Functional Elements
Packet Filtering
Lawful
Interception
IP Address Allocation
Transport
Level Packet
Marking
Accounting
PDN-GW
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1.2 EPS Interfaces
1.2.1 E-UTRAN Interfaces
As with all 3GPP technologies, it is the actual interfaces which are defined in terms of the protocols they support and the associated signaling messages and user traffic that traversethem. Figure 1-7 illustrates the main interfaces in the E-UTRAN.
Figure 1-7 E-UTRAN Interfaces
E-UTRAN EPC
X2
Uu
S-GW
MMEeNB
eNB
S1-MME
S1-MMES1-U
S1-U
Uu Interface
The Uu Interface supports both a Control Plane and a User plane and spans the link betweenthe UE and the eNB / HeNB. The principle Control Plane protocol is RRC (Radio Resource
Control) while the User Plane is designed to carry IP datagrams.
X2 Interface
The X2 interface interconnects two eNBs and in so doing supports both a Control Plane and
User Plane. The principle Control Plane protocol is X2AP (X2 Application Protocol).
S1 Interface
The S1 interface can be subdivided into the S1-MME interface supporting Control Plane
signaling between the eNB and the MME and the S1-U Interface supporting User Plane traffic between the eNB and the S-GW. The principle Control Plane protocol is S1AP (S1
Application Protocol).
1.2.2 EPC Interfaces
Figure 1-8 illustrates the fundamental architecture of the EPC and in so doing identifies the
key interfaces which exist between the network elements. It should be stated however that
there exists additional interfaces which link the EPC with the IMS and legacy 3GPP / Non3GPP architectures.
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Figure 1-8 EPC Architecture and Interfaces
EPC
S1-MME
S1-U
S11
S5/S8
S-GW
SGi
PDN-GW
MME
MME
S10
1.2.3 Additional Network Elements and Interfaces
In addition to the network elements, interfaces and associated protocols discussed so far, the
EPC connects with numerous other nodes and networks. These are illustrated in Figure 1-9.
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Figure 1-9 Additional Network Elements and Interfaces
EPC
S11
S5/S8
S10
MME
MME
SGSN
RNC
PCRF
HSS
EIR
S6a
S101
S2a
Gx
ePDG
S2bS103
S12
S4
S3
S13
CDMA
2000
CDMA
2000
TrustedNon 3GPPIP Access
UntrustedNon 3GPPIP Access
PDN-GW
Wn
S-GW
These include, but are not limited to the:
HSS (Home Subscriber Server) - this can be considered a “master” database within thePLMN. Although logically it is considered as one entity, the HSS in practice is made up
of several physical databases depending upon subscriber numbers and redundancyrequirements. The HSS holds variables and identities for the support, establishment and
maintenance of calls and sessions made by subscribers. It is connected to the MME via
the S6a Interface which uses the protocol Diameter.
PCRF (Policy and Charging Rules Function) - this supports functionality for policycontrol through the PDF (Policy Decision Function) and charging control through the
CRF (Charging Rules Function). As such, it provides bearer network control in terms ofQoS and the allocation of the associated charging vectors. The PCRF downloads this
information over the Gx Interface using the Diameter protocol.
ePDG (evolved Packet Data Gateway) - which is used when connecting to Untrusted
Non 3GPP IP Access networks. It provides functionality to allocate IP addresses inaddition to encapsulating / de-encapsulating IPSec (IP Security) and PMIP tunnels. Itconnects to the PDN-GW via the S2b Interface.
RNC (Radio Network Controller) - which forms part of the 3GPPs UTRAN (UniversalTerrestrial Radio Access Network), the RNC connects to the S-GW to support the
tunneling of User Plane traffic using GTP-U. The interface linking these network
elements is the S12 Interface.
SGSN (Serving GPRS Support Node) - this forms part of the 3GPPs 2G and 3G packetswitched core domain. It connects to both the MME and S-GW in order to support
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packet switched mobility and uses the GTPv2-C and GTP-U protocols respectively. TheSGSN connects to the MME via the S3 Interface and the S-GW via the S4 Interface.
EIR (Equipment Identity Register) - this database enables service providers to validate a
particular IMEI (International Mobile Equipment Identity) against stored lists. It
connects to the MME via the S13 Interface and uses the Diameter protocol for messagetransfer.
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2 EPS ProtocolsAbout This Chapter
The following table lists the contents of this chapter.
Section
2.1 EPS Signaling
2.2 EPS Protocols
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2.1 EPS Signaling
The connectivity between the UE and the EPS can be split into a Control Plane and a UserPlane. Both of these can further split into the NAS (Non Access Stratum) and AS (AccessStratum). The Access Stratum consist of the protocols and signaling involved with the
E-UTRAN, i.e. maintain both the air interface and S1 interfaces. In contrast, the Non Access
Stratum, as its name suggests, is not part of the Access Stratum and is defined as higher layersignaling and traffic (IP datagrams).
Control Plane
Figure 2-1 illustrates the concept of NAS and AS signaling, i.e. the Control Plane. It is worth
noting that the NAS signaling is effectively transparent to the E-UTRAN. Access Stratumsignaling provides a mechanism to deliver NAS signaling, as well as the lower layer signaling
required to setup, maintain and manage the connections. The X2 interfaces are also part ofthis methodology and as such it also is part of Access Stratum signaling.
Figure 2-1 NAS and AS Control Plane
UE
E-UTRAN EPC
S-GW
MME
PDN-GWeNB
Non AccessStratum
Signaling
Access
StratumSignaling
User Plane
The User Plane focuses on the delivery of IP datagrams to and from the EPC, namely the
S-GW and PDN-GW. Figure 2-2 illustrates this concept.
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Figure 2-2 NAS and AS User Plane
UE
E-UTRAN EPC
S-GW
MME
PDN-GWeNB
Non AccessStratum IP
Datagrams
AccessStratum
Transport
In the case of the User Plane the higher layer NAS is an IP datagram. This effectively is
delivered between the UE and the PDN-GW, with the eNB and S-GW acting as lower layer
relaying devices.
2.2 EPS Protocols
2.2.1 NAS Functionality
The Non Access Stratum (NAS) protocols are used for signaling exchange between the UEand the Mobility Management Entity (MME).
NAS sits on top of RRC layer in the UE and S1AP of the MME. All NAS messages are
carried by RRC and SIAP messages in radio interface and S1-MME interface respectively.
The NAS signaling is identified as EPS Mobility Management (EMM) and EPS SessionManagement (ESM).
The EMM Protocol signaling is related to UE mobility and security procedures. The ESM
protocol handles signaling related to the default and dedicated user plane bearers.
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Figure 2-3 NAS Protocol stack
RLC
MAC
PHY
PDCP
RRC
eNBUE
Uu
NAS
Non Access Stratum
RLC
MAC
PHY
PDCP
RRC
IP
L1/L2
SCTP
S1AP
IP
L1/L2
SCTP
S1AP
NAS
MME
S1
2.2.2 NAS Concepts and Identities
Tracking Area
The NAS layer makes use of Tracking Area (TA) for mobility management. The Tracking
Area is the same concept as GSM/UMTS concept of Routing Area, it is an operator definedgroup of cells. The MME is aware of the location of an attached UE at the TA or TA list level
through the TA Update procedure. The TA is typically the area within which the UE is pagedfor incoming calls.
Tracking Area Identity (TAI) is a cell configuration.
TAI = MCC + MNC + TAC, where
TAI : Tracking area identity
MCC : Mobile Country Code
MNC : Mobile Network Code
TAC : Tracking Area Code
As an operator option, there may also be MME Pool (MME Group) areas defined. An MMEPool Area is defined as an area within which a UE may be served without need to change theserving MME. An MME Pool Area is served by one or more MMEs in parallel. MME poolAreas are a collection of complete Tracking Areas. MME Pool Areas may overlap each other,
as seen in Figure 2-4
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Figure 2-4 NAS Identities
MME 1
T A 3
T A 4
T A 1
T A 2
T A 5
T A 6
T A 7
M
ME 2
MME
1
MME 1
MME 2
MME G r o u
pA
MME G
r o u pB
MME G r o u p C
P L MN
G U T I
S - T M S I
U E C on t e x t
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GUTI
The EPC uses the IMSI number as the permanent user identifier (or rather, USIM identifier).As in the legacy core Network a temporary identifier is also used, for subscriber identityconfidentiality reasons, in place of the IMSI whenever possible. The temporary identifier in
the EPS is called the Globally Unique Temporary Identity (GUTI).
The use of the GUTI is very similar to the use of the legacy TMSI (CS domain) and PTIMSI
(PS domain) numbers. There is a difference however: the GUTI explicitly links with theMME pool Area concept.
GUTI = MCC + MNC + MMEGI + MMEC + M-TMSI, where
MMEGI: MME Group Identifier (16 bit)
MMEC: MME Code (8 bit)
M-TIMSI : M- Temporary Mobile Subscriber Identity(32 bit)
The GUTI is allocated when the UE performs initial registration (Attach) with an MME. The
GUTI is then typically changed whenever the UE performs some EMM procedure, such as TAupdate. The S-TMSI is a shortened version of the GUTI that uniquely identifies the user with
an MME Group. The S-TMIS ,rather than the complete GUTI, is used within most NASmessages.
Tracking Area List
In order to avoid frequent TA updating, the MME may order the UE to keep multiple TAswhich is called TA List of a UE.
UE only updates the TA information to MME when it moves to a TA not in its TA list.
Figure 2-5 TA and TA List
TA 1
Cell 1 Cell 2 Cell 3 Cell 4
TA2
Cell 5 Cell 6 Cell 7 Cell 8
UE1 UE 2
List (TA1, TA2) List ( TA2)
2.2.3 EMM and ESM
The NAS signaling between the UE and the EPC is identified as EMM or ESM. Table 2-1
illustrates the main EMM and ESM signaling procedures.
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Table 2-1 NAS EMM and ESM Procedures
EMM Procedures ESM Procedures
Attach Default EPS Bearer Context Activation
Detach Dedicated EPS Bearer Context Activation
Tracking Area Update EPS Bearer Context Modification
Service Request EPS Bearer Context Deactivation
Extended Service Request UE Requested PDN Connectivity
GUTI Reallocation UE Requested PDN Disconnect
Authentication UE Requested Bearer Resource Allocation
Identification UE Requested Bearer Resource Modification
Security Mode Control ESM Information Request
EMM Status ESM Status
EMM Information
NAS Transport
Paging
EMM Procedures
The key EMM procedures include:
Attach - this is used by the UE to attach to an EPC (Evolved Packet Core) for packetservices in the EPS (Evolved Packet System). Note that it can be also used to attach to
non-EPS services.
Detach - this is used by the UE to detach from EPS services. In addition, it can also be
used for other procedures such as disconnecting from non-EPS services.
Tracking Area Updating - this procedure is always initiated by the UE and is used for the
various purposes. The most common include normal and periodic tracking area updating.
Service Request - this is used by the UE to get connected and establish the radio and S1
bearers when uplink user data or signaling is to be sent.
Extended Service Request - this is used by the UE to initiate a Circuit Switched fallback
call or respond to a mobile terminated Circuit Switched fallback request from thenetwork.
GUTI Reallocation - this is used to allocate a GUTI (Globally Unique TemporaryIdentifier) and optionally to provide a new TAI (Tracking Area Identity) list to a particular UE.
Authentication - this is used for AKA (Authentication and Key Agreement) between the
user and the network.
Identification - this is used by the network to request a particular UE to provide specificidentification parameters, e.g. the IMSI (International Mobile Subscriber Identity) or the
IMEI (International Mobile Equipment Identity).
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Security Mode Control - this is used to take an EPS security context into use, andinitialize and start NAS signaling security between the UE and the MME with the
corresponding NAS keys and security algorithms.
EMM Status - this is sent by the UE or by the network at any time to report certain error
conditions. EMM Information - this allows the network to provide information to the UE.
Transport of NAS messages - this is to carry SMS (Short Message Service) messages inan encapsulated form between the MME and the UE.
Paging - this is used by the network to request the establishment of a NAS signaling
connection to the UE. Is also includes the Circuit Switched Service Notification
ESM Procedures
The key ESM procedures include:
Default EPS Bearer Context Activation - this is used to establish a default EPS bearer
context between the UE and the EPC.
Dedicated EPS Bearer Context Activation - this is to establish an EPS bearer context
with specific QoS (Quality of Service) and TFT (Traffic Flow Template) between the UE
and the EPC. The dedicated EPS bearer context activation procedure is initiated by thenetwork, but may be requested by the UE by means of the UE requested bearer resource
allocation procedure.
EPS Bearer Context Modification - this is used to modify an EPS bearer context with a
specific QoS and TFT.
EPS Bearer Context Deactivation - this is used to deactivate an EPS bearer context or
disconnect from a PDN by deactivating all EPS bearer contexts to the PDN.
UE Requested PDN Connectivity - this is used by the UE to request the setup of a
default EPS bearer to a PDN.
UE Requested PDN Disconnect - this is used by the UE to request disconnection from
one PDN. The UE can initiate this procedure to disconnect from any PDN as long as it isconnected to at least one other PDN.
UE Requested Bearer Resource Allocation - this is used by the UE to request anallocation of bearer resources for a traffic flow aggregate.
UE Requested Bearer Resource Modification - this is used by the UE to request a
modification or release of bearer resources for a traffic flow aggregate or modification ofa traffic flow aggregate by replacing a packet filter.
ESM Information Request - this is used by the network to retrieve ESM information, i.e. protocol configuration options, APN (Access Point Name), or both from the UE during
the attach procedure.
ESM Status - this is used to report at any time certain error conditions detected uponreceipt of ESM protocol data.
2.2.4 NAS States and State Transitions
There are separate protocol state machines for the EMM protocol and ESM protocol.
EMM protocol state machine relates to whether the UE is properly registered in the networkor not and whether there exists an active NAS Signaling Connection between the UE and
MME or not. The ESM protocol state machine deals exclusively with the existence or not of
EPS bearers.
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EMM protocol state machine contains two sets of states: EMM states and ECM states(EPSConnection Management). The UE is either “EMM REGISTERED” OR “EMM
DEREGISTERED”, i.e. attached or not. The ECM states are only relevant in the EMM
REGISTERED state and reflect whether there is an active NAS Signaling Connectionestablished (ECM Connected) or not (ECM Idle).
The NAS signaling Connection is required for any exchange of NAS message with the
exception of the very messages that triggers the establishment of the NAS SignalingConnection itself (e.g. Attach Request or Paging).
Figure 2-6 NAS States and State Transitions
ESM ACTIVE
PDN Contents:
IP Adress, APN, QoS Paramters
S5 IP address & TEIDS11 IP address & TEID(S1-U IP address & TEID)
Data Transfer Possible when ECMconnected
One Default BearerZero, one or more Dedicated Bearer
EMM REGISTERED
MME context:IMSI, GUTI, Talist
IP address, Security association
ECM IDLENo NAS Signaling ConnectionTracking Area Updates
ECM CONNECTEDNAS Signaling Connection
Data transfer possible
NAS ConnectionRelease
NAS ConnectionEstablishment
ESM INACTIVENo PDN context
EMM DEREGISTEREDNo MME context
EPS BearerEstablishment
Last EPS BearerReleased
Attach Detach
The ESM states are quite straightforward: when at least one (default) bearer is established the
UE is in the “ESM Active” state, otherwise it is in the “ESM Inactive” state. The ESMsignaling needed to establish a bearer requires that the UE is properly registered in the
network. It therefore naturally follows that the UE must be in the EMM registered statewhenever it is ESM Active.
It also follows that there must be a NAS Signaling connection present during the ESM
signaling phase when a bearer is being established, i.e. the UE is then ECM connected.However, there is no requirement to keep the NAS Signaling Connection active for the
lifetime of the EPS bearer. Hence the UE may very well be ECM Idle while being ESM
Active. This makes sense, since the UE may be attached for days, weeks or even months onthe end.
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The NAS states (MME related states) are aligned with the RRC states (eNodeB related states).A UE in RRC Idle state is, from the MMEs point of view, in the NAS state ECM Idle. Paging
or a request from higher layers to transmit uplink data or signaling will cause a transition from
RRC Idle to RRC Connected, causing also a transition from ECM Idle to ECM Connected.This is not shown in Figure 2-6.
Figure 2-7 Network Attach
eNBUERRC CONNECTION
AUTHENTICATION
MME
EMM: EPS ATTACH REQUESTESM: PDN CONNECTIVITY REQUEST
SECURITY MODE
EMM: EPS ATTACH ACCEPT
ESM: ACTIVE DEFAULT EPSBEARER CONTEXT REQUEST
EMM: EPS ATTACH COMPLETE
ESM: ACTIVE DEFAULT EPSBEARER CONTEXT ACCEPT
Enter state
ECM_CONNECTED
Enter state
EMM_REGISTEREDESM_ACTIVE
2.2.5 Uu Interface
The Uu Interface supports both a Control Plane and a User plane and spans the link between
the UE and the eNB / HeNB. The principle Control Plane protocol is RRC in the AccessStratum and EMM (EPS Mobility Management)/ ESM (EPS Session Management) in the
Non Access Stratum. In contrast, the User Plane is designed to carry IP datagrams. However, both Control and User Planes utilize the services of the lower layers, namely PDCP (PacketData Convergence Protocol), RLC (Radio Link Control) and MAC (Medium Access Control),
as well as the PHY (Physical Layer).
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Figure 2-8 Uu Interface Protocols
RLC
MAC
PHY
PDCP
RRC
RLC
MAC
PHY
PDCP
IP
Control Plane eNBUE
Uu
User Plane
EMM and ESM Non Access Stratum
Access Stratum
2.2.6 Uu Interface - RRC
The main air interface control protocol is RRC (Radio Resource Control). For RRC messages
to be transferred between the UE and the eNB it uses the services of PDCP, RLC, MAC andPHY. Figure 2-9 identifies the main RRC functions. In summary, RRC handles all the
signaling between the UE and the E-UTRAN, with signaling between the UE and Core Network, i.e. NAS (Non Access Stratum) signaling, being carried by dedicated RRC
messages. When carrying NAS signaling, RRC does not alter the information but instead, provides the delivery mechanism.
Figure 2-9 Main RRC Functions
eNB
RLC
MAC
PHY
PDCP
RRC
NAS Signaling
System InformationPLMN and Cell Selection
Admission Control
Security Management
Cell ReselectionMeasurement Reports
Handovers and Mobility