LTE基础：LTE⽹络中的IP地址分配
⽹络全IP化，数据均在IP包内传输。当UE连接⽹络，为了和PDN（Packet Data Network）建⽴连接，⼀个PDN地址会分配给UE，同时，会在UE和P-GW之间建⽴默认承载（default bearer），默认承载会保持连接直到UE分离（detach）LTE⽹络。
LTE⽹络为每⼀个⽤户APN建⽴默认承载，为每⼀个APN分配⼀个唯⼀的IP地址，这些IP地址可以是IPv4，IPv6，或者IPv4/IPv6类型。
本⽂将介绍当UE附着⽹络时，LTE如何为⽤户分配IP地址？
IP地址分配的类型
当UE initial Attach LTE⽹络，它请求⼀个PDN连接，为此，P-GW会分配⼀个IP地址给UE，并在默认承载建⽴的同时将IP地址传送给UE。有了这个IP地址，UE才能使⽤通过PDN提供的服务。
P-GW分配两种类型IP地址：动态和静态IP地址。动态IP地址下，UE每次接⼊⽹络，系统会⾃动分配⼀个IP地址给UE。静态IP地址下，每⼀次会分配⼀指定的IP地址。
动态IP分配
⽹络⾸先在P-GW提供⼀个IP池（IP pool），当UE initial Attach LTE⽹络，PGW会动态分配⼀个IP地址给UE。
静态IP地址分配
彩色卡通⽹络会分配⼀个永久的IP地址给UE，⽤户的静态IP信息存储在HSS（Home Subscriber Server，⽤户归属地服务器），当UE initial Attach⽹络，P-GW从HSS获取UE静态IP地址，并转发给UE。
动态IP地址分配
流程如下（点击放⼤看）：
P-GW IP分配
在P-GW，会提供⼀个包含IP地址的IP pool，以及DNS服务器IP地址。
1. [UE ->MME] Requesting for PDN（internet）Connectivity
UE发送 PDN Connectivity Request （PDN type=IPv4，PCO=DNS Server IPv4 Address Request）消息到MME，请求⼀个UE的IPv4地址和DNS 服务器IP地址（通过PCO域）。PDN Connectivity Request是⼀个ESM消息，该消息包含在Attach Request的ESM Message container内。
2~3. [MME->S-GW->P-GW] Requesting for Session Creation
MME从HSS获取⽤户属性（subscription profile），发送 Create Session Request（IMSI，PDN Type=IPv4，PDN
Adress=0.0.0.0，PCO=DNS Server IPv4 Address Request）消息到P-GW。因为这是动态IP地址分配，⽤户信息并不包含IP地址。在Create Session Request消息中，PDN地址域设置为0.0.0.0。
4. [P-GW] 分配PDN地址和DNS服务器地址
P-GW检查PDN类型和PDN地址（0.0.0.0），知道需要分配IPv4地址，它从IPv4 pool⾥选择⼀个IP 地址（⽐如：UE IP=1.1.1.5）分配给UE。同时相应分配DNS服务器地址。
5~6. [MME<-S-GW<-P-GW] Responding to Create Session Request
作为对2~3步骤请求的响应，P-GW发送Create Session Respon 消息给MME。这⼀消息包含了在PDN地址域的UE IP地址（由本地P-GW动态分配）和在PCO域的DNS服务器地址。
7. [UE<-MME] Requesting for Activation of Default Bearer Context
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七一大会MME发送UE Activate Default EPS Bearer Context Request (PDN Type=IPv4，PDN Address=UE IP （1.1.1.5），PCO={Primary DNS IP，Secondary DNS IP}）消息以激活默认承载内容。这⼀EMS消息包含DNS服务器IP地址和UE IP地址，当传送时嵌⼊Attach Accept 消息内。
8. [UE] Obtaining Dynamic IP Address for using PDN rvice
UE获得动态IP地址（1.1.1.5）和DNS 服务器IP地址（Primary DNS IP=10.1.1.1，Secondary DNS IP=10.1.1.2）。默认承载在UE和P-GW间建⽴。UE现在可以连接PDN（internet），可以通过⾃⼰的动态IP地址使⽤internet服务。
静态IP地址分配
流程如下（点击放⼤）：
HSS
组成英文HSS提供每个⽤户的⽤户属性（subscription profile），这些属性信息包括⽤于PDN连接的PDN类型和PDN地址。
P-GW
P-GW已设置好DNS服务器IP地址
当⽤户开机，UE开始initial Attach LTE⽹络。
1. [UE->MME] Requesting for PDN(Internet) Connectivity
UE发送 PDN Connectivity Request （PDN type=IPv4，PCO=DNS Server IPv4 Address Request）消息到MME，请求⼀个UE的IPv4地址和DNS 服务器IP地址（通过PCO域）。
2. [MME->HSS] Requesting the LTE Network for Registration
MME发送Update Location Request 消息通知HSS，MME1下有UE请求注册LTE⽹络。
3. [MME<-HSS] Forwarding Subscription Profile
HSS确认UE在MME1注册后，通过Update Location Answer（IMSI，PDN Type=IPv4，PDN Address = Static UE IP(1.1.1.1)）消息发送UE ⽤户属性到MME1 。这⼀⽤户属性中包含了分配给UE的静态IP地址。
书香门第是什么意思4~5. [MME->S-GW->P-GW] Requesting for Session Creation
当MME从HSS接收到⽤户属性后，MME知道了UE的静态IP地址(1.1.1.1)。MME准备Create Session Request（IMSI，PDN
Type=IPv4，PDN Adress=Static UE IP(1.1.1.1)，PCO=DNS Server IPv4 Address Request）消息，并发送到P-GW。这⼀消息包含了在PDN域的静态IP地址。
6~7. [MME<-S-GW<-P-GW] Responding to Create Session Request
作为对4~5步骤请求的响应，P-GW发送Create Session Respon （IMSI，PDN Type=IPv4，PDN Adress=Static UE
IP(1.1.1.1)，PCO={Primary DNS IP，Secondary DNS IP}）消息给MME。这⼀消息包含在PDN地址域的静态UE IP地址和在PCO域的DNS 服务器IP地址。
8. [UE<-MME] Requesting for Activation of Default Bearer Context
MME发送UE Activate Default EPS Bearer Context Request (PDN Type=IPv4，PDN Address=Static UE IP(1.1.1.1)，PCO={Primary DNS IP，Secondary DNS IP}）消息以激活默认承载内容。这⼀EMS消息包含DNS服务器IP地址和UE IP地址(1.1.1.1)，当传送时嵌⼊Attach Accept消息内。
9. [UE] Obtaining Dynamic IP Address for using PDN rvice
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UE获得静态IP地址（1.1.1.1）和DNS 服务器IP地址（Primary DNS IP=10.1.1.1，Secondary DNS IP=10.1.1.2）。默认承载在UE和P-GW间建⽴。UE现在可以连接PDN（internet），可以通过⾃⼰的静态IP地址使⽤internet服务。
当⽤户完成有效⽹络注册后，PDN地址（IP地址）和默认承载会⼀直保留给⽤户，即使⽤户不在使⽤Internet服务，这叫为⽤户提
供“always-on IP connectivity”。
The high-level network architecture of LTE is comprid of following three main components:
The Ur Equipment (UE).
The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
The Evolved Packet Core (EPC).
The evolved packet core communicates with packet data networks in the outside world such as the internet, private corporate networks or the IP multimedia subsystem. The interfaces between the different parts of the system are denoted Uu, S1 and SGi as shown below:
The Ur Equipment (UE)
The internal architecture of the ur equipment for LTE is identical to the one ud by UMTS and GSM which is actually a Mobile Equipment (ME). The mobile equipment comprid of the following important modules:
Mobile Termination (MT) : This handles all the communication functions.
Terminal Equipment (TE) : This terminates the data streams.
Universal Integrated Circuit Card (UICC) : This is also known as the SIM card for LTE equipments. It runs an application known as the Universal Subscriber Identity Module (USIM).
A USIM stores ur-specific data very similar to 3G SIM card. This keeps information about the ur'
s phone number, home network identity and curity keys etc.
The E-UTRAN (The access network)
The architecture of evolved UMTS Terrestrial Radio Access Network (E-UTRAN) has been illustrated below.
The E-UTRAN handles the radio communications between the mobile and the evolved packet core a
nd just has one component, the evolved ba stations, called eNodeB or eNB. Each eNB is a ba station that controls the mobiles in one or more cells. The ba station that is communicating with a mobile is known as its rving eNB.
LTE Mobile communicates with just one ba station and one cell at a time and there are following two main functions supported by eNB:
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The eBN nds and receives radio transmissions to all the mobiles using the analogue and digital signal processing functions of the LTE air interface.
The eNB controls the low-level operation of all its mobiles, by nding them signalling messages such as handover commands.
中式沙发图片大全Each eBN connects with the EPC by means of the S1 interface and it can also be connected to nearby ba stations by the X2 interface, which is mainly ud for signalling and packet forwarding during handover.
A home eN
B (HeNB) is a ba station that has been purchad by a ur to provide femtocell coverage within t
he home. A home eNB belongs to a clod subscriber group (CSG) and can only be accesd by mobiles with a USIM that also belongs to the clod subscriber group.
The Evolved Packet Core (EPC) (The core network)
The architecture of Evolved Packet Core (EPC) has been illustrated below. There are few more components which have not been shown in the diagram to keep it simple. The components are like the Earthquake and Tsunami Warning System (ETWS), the Equipment Identity Register (EIR) and Policy Control and Charging Rules Function (PCRF).
Below is a brief description of each of the components shown in the above architecture:
The Home Subscriber Server (HSS) component has been carried forward from UMTS and GSM and is a central
databa that contains information about all the network operator's subscribers.
The Packet Data Network (PDN) Gateway (P-GW) communicates with the outside world ie. packet data networks PDN, using SGi interface. Each packet data network is identified by an access point name (APN). The PDN gateway has the same role as the GPRS support node (GGSN) and the rving GPRS support node (SGSN) with UMTS and GSM.
The rving gateway (S-GW) acts as a router, and forwards data between the ba station and the PDN gateway.
The mobility management entity (MME) controls the high-level operation of the mobile by means of signalling messages and Home Subscriber Server (HSS).
The Policy Control and Charging Rules Function (PCRF) is a component which is not shown in the above diagram but it is responsible for policy control decision-making, as well as for controlling the fl
ow-bad charging functionalities in the Policy Control Enforcement Function (PCEF), which resides in the P-GW.
The interface between the rving and PDN gateways is known as S5/S8. This has two slightly different implementations, namely S5 if the two devices are in the same network, and S8 if they are in different networks.
Functional split between the E-UTRAN and the EPC
Following diagram shows the functional split between the E-UTRAN and the EPC for an LTE network:
2G/3G Versus LTE
Following table compares various important Network Elements & Signaling protocols ud in 2G/3G abd LTE.
2G/3G LTE
GERAN and UTRAN E-UTRAN SGSN/PDSN-FA S-GW GGSN/PDSN-HA PDN-GW HLR/AAA HSS
VLR MME
SS7-MAP/ANSI-41/RADIUS Diameter DiameterGTPc-v0 and v1GTPc-v2 MIP PMIP
