3G接入网信令详解Ver2.1

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"3G_RAN_Signalling_2.1 - 一本关于3G接入网信令的手册，适合通信初学者" 3G接入网的信令是移动通信领域中的重要组成部分，对于理解和掌握3G网络的操作至关重要。该手册"3G_RAN_Signalling_2.1"由Orbitage提供，旨在为学习者提供详尽的3G无线接入网（RAN）信令知识。手册内容覆盖了从基本概念到复杂的信令流程，对于通信行业的初学者来说，是一份宝贵的参考资料。 1. **UMTS网络架构** (1.1章节) UMTS（通用移动通信系统）网络架构包括多个组件，如UE（用户设备）、UTRAN（UMTS陆地无线接入网）、RNC（无线网络控制器）以及核心网络。这一部分会介绍这些组件的职责和它们之间的交互关系。 2. **UMTS的FDD和TDD模式** (1.2章节) UMTS支持频分双工（FDD）和时分双工（TDD）两种模式。FDD使用不同的频率进行上行和下行链路传输，而TDD在同一频率上通过时间分配来实现上下行链路的切换。 3. **UMTS承载模型** (1.3章节) 承载模型定义了数据如何在不同层次的通道上传输，包括公共和专用承载，以及它们在满足不同服务质量（QoS）需求时的角色。 4. **UMTS QoS类** (1.4章节) UMTS定义了多种QoS类，以满足不同类型服务（如语音、视频、数据等）的需求，包括延迟、吞吐量、丢包率等关键性能指标。 5. **无线资源管理** (1.5章节) - **准入控制** (1.5.1): 确保网络资源的有效利用，控制新连接的建立。 - **调度器** (1.5.2): 分配无线资源给不同用户，确保公平性和效率。 - **负载控制** (1.5.3): 监控网络状态，动态调整资源分配以应对变化的用户需求。 - **切换控制** (1.5.4): 保证用户在移动过程中保持服务连续性。 - **功率控制** (1.5.5): 调整UE和基站的发射功率，减少干扰并提高覆盖范围。 6. **UTRAN信道** (1.6章节) - **逻辑信道** (1.6.1): 定义了信息类型，如控制信息和用户数据的信道。 - **下行链路传输和物理信道** (1.6.2): 描述了数据如何在物理层上实际传输，包括信道编码、调制等技术。该手册还包含了即将出版的书籍《3G网络融合技术》的部分内容，由Jeffrey Bannister、Paul Mather和Sebastian Coope合著，可在Wiley Europe网站获取更多信息。这份手册不仅提供了理论知识，而且可能包含实际操作案例和解释，有助于读者深入理解3G RAN的信令过程及其在网络性能优化中的作用。

3G RAN Signalling

Page 16 of 16 Ver 2.1 © Orbitage 2003

Figure 5. The HLR/VLR/EIR are generalised and referred to as the HLR Subsystem

(HSS).

RAN

Packet Core

Backbone

HSS

Uu Iu

SGSN

RNC

PSTN/

ISDN/

CSPDN

WBTS

IMS

GGSN

IP Network

Figure 5 – UMTS Release 5 architecture

Now the last remnants of traditional telecommunications switching are removed, leaving

a network operating completely on the IP protocol, and generalised for the transport of

many service types. Real-time services are supported through the introduction of a new

network domain, the IP Multimedia Subsystem (IMS).

Currently the 3GPP are working on Release 6, which purports to cover all aspects not

addressed in frozen releases. Some call UMTS Release 6 4G and it includes such

issues as interworking of hot spot radio access technologies such as wireless LAN.

1.2 UMTS FDD and TDD

Like any CDMA system, UMTS needs a wide frequency band in which to operate to

effectively spread signals. The defining characteristic of the system is the chip rate,

where a chip is the width of one symbol of the CDMA code. UMTS uses a chip rate of

3.84Mchips/s and this converts to a required spectrum carrier of 5MHz wide. Since this

is wider than the 1.25MHz needed for the existing cdmaOne system, the UMTS air

interface is termed ‘wideband’ CDMA.

There are actually two radio technologies under the UMTS umbrella: UMTS FDD and

TDD. FDD stands for Frequency Division Duplex, and like GSM, separates traffic in the

uplink and downlink by placing them at different frequency channels. Therefore an

operator must have a pair of frequencies allocated to allow them to run a network,

hence the term ‘paired spectrum’. TDD or Time Division Duplex requires only one

frequency channel, and uplink and downlink traffic are separated by sending them at

different times. The ITU-T spectrum usage, as shown in Figure 6, for FDD is 1920-

1980MHz for uplink traffic, and 2110-2170MHz for downlink. The minimum allocation an

operator needs is two paired 5MHz channels, one for uplink and one for downlink, at a

separation of 190MHz. However, to provide comprehensive coverage and services, it is

recommended that an operator be given three channels. Considering the spectrum

allocation, there are 12 paired channels available, and many countries have now

completed the licencing process for this spectrum, allocating between two and four

channels per licence. This has tended to work out a costly process for operators, since

the regulatory authorities in some countries, notably in Europe, have auctioned these

licences to the highest bidder. This has resulted in spectrum fees as high as tens of

billions of dollars in some countries.

3G RAN Signalling

Page 17 of 17 Ver 2.1 © Orbitage 2003

FDD-UL TDDTDD FDD-DL

1980 2010 2025 2110 217019201900 MHz

Figure 6 – UMTS frequency allocation

The Time Division Duplex (TDD) system, which needs only one 5MHz band in which to

operate, often referred to as unpaired spectrum. The differences between UMTS FDD

and TDD are only evident at the lower layers, particularly on the radio interface. At

higher layers, the bulk of the operation of the two systems is the same. As the name

suggests, the TDD system separates uplink and downlink traffic by placing them in

different time slots. As will be seen later, UMTS uses a 10ms frame structure which is

divided into 15 equal timeslots. TDD can allocate these to be either uplink or downlink,

with one or more breakpoints between the two in a frame defined. In this way, it is well

suited to packet traffic, since this allows great flexibility in dynamically dimensioning for

asymmetry in traffic flow.

The TDD system should not really be considered as an independent network, but rather

as a supplement for an FDD system to provide hotspot coverage at higher data rates. It

is rather unsuitable for large scale deployment due to interference between sites, since

a BTS may be trying to detect a weak signal from a UE, which is blocked out by a

relatively strong signal at the same frequency from a nearby BTS. TDD is ideal for

indoor coverage over small areas.

Since FDD is the main access technology being developed currently, the explanations

presented here will focus purely on this system.

1.3 UMTS Bearer Model

The procedures of a mobile device connecting to a UMTS network can be split into two

areas: the access stratum (AS) and the non-access stratum (NAS). The access stratum

involves all the layers and subsystems that offer general services to the non-access

stratum. In UMTS, the access stratum consists of all of the elements in the radio access

network, including the underlying ATM transport network, and the various mechanisms

such as those to provide reliable information exchange. All of the non-access stratum

functions are those between the mobile device and the core network, for example,

mobility management. Figure 7 shows the architecture model. The AS interacts with the

NAS through the use of service access points (SAPs).

3G RAN Signalling

Page 18 of 18 Ver 2.1 © Orbitage 2003

User

Equipment

Radio Access

Network

Core

Network

Access Stratum

Non Access Stratum

Radio

Protocols

Radio

Protocols

Uu Iu

SAPSAP

Figure 7 – UMTS general architecture

The UMTS radio access network (UTRAN) provides this separation of NAS and AS

functions, and allows for AS functions to be fully controlled and implemented within the

UTRAN. The two major UTRAN interfaces are the Uu, which is the interface between

the mobile device, or User Equipment (UE) and the UTRAN, and the Iu, which is the

interface between the UTRAN and the core network. Both of these interfaces can be

divided into control and user planes each with appropriate protocol functions.

A Bearer Service is a link between two points, which is defined by a certain set of

characteristics. In the case of UMTS, the bearer service is delivered using radio access

bearers.

A Radio access bearer (RAB) is defined as the service that the access stratum (i.e.

UTRAN) provides to the non-access stratum for transfer of user data between the User

Equipment and Core Network. A RAB can consist of a number of subflows, which are

data streams to the core network within the RAB that have different QoS characteristics,

such as different reliabilities. A common example of this is different classes of bits with

different bit error rates can be realised as different RAB subflows. RAB subflows are

established and released at the time the RAB is established and released, and are

delivered together over the same transport bearer.

A Radio Link is defined as a logical association between a single User Equipment (UE)

and a single UTRAN access point, such as an RNC. It is physically comprised of one or

more radio bearers and should not be confused with radio access bearer..

Looking within the UTRAN, the general architecture model is as shown in Figure 8

below. Now shown are the Node B or Base Station (BTS) and Radio Network Controller

(RNC) components, and their respective internal interfaces. The UTRAN is subdivided

into blocks referred to as Radio Network Subsystems (RNS), where each RNS consists

of one controlling RNC (CRNC) and all the BTSs under its control. Unique to UMTS is

the interface between RNSs, the Iur interface, which plays a key role in handover

procedures. The interface between the BTS and RNC is the Iub interface.

3G RAN Signalling

This distinction is because the BTS to RNC link is a point-to-point connection in that a

BTS or RNC will only communicate with the RNC or BTS directly connected to it, and

will not require communication beyond that element to another network element.

For each user connection to the core network, there is only one RNC, which maintains

the link between the UE and core network domain, as highlighted in Figure 10. This

RNC is referred to as the serving RNC or SRNC. That SRNC plus the BTSs under its

control is then referred to as the SRNS. This is a logical definition with reference to that

UE only. In an RNS, the RNC that controls a BTS is known as the controlling RNC or

CRNC. This is with reference to the BTS, cells under its control and all the common and

shared channels within.

As the UE moves, it may perform a soft or hard handover to another cell. In the case of

a soft handover, the SRNC will activate the new connection to the new BTS. Should the

new BTS be under the control of another RNC, the SRNC will also alert this new RNC to

activate a connection along the Iur interface. The UE now has two links, one directly to

the SRNC, and the second, through the new RNC along the Iur interface. In this case,

this new RNC is logically referred to as a drift RNC or DRNC, see Figure 10. It is not

involved in any processing of the call and merely relays it to the SRNC for connection to

the core. In summary, SRNC and DRNC are usually associated with the UE and the

CRNC is associated with the BTS. Since these are logical functions it is normal practice

that a single RNC is capable of dealing with all these functions.

A situation may arise where a UE is connected to a BTS for which the SRNC is not the

CRNC for that BTS. In that situation, the network may invoke the Serving RNC

Relocation procedure to move the core network connection. This process is described in

Section 3.

Core

Network

DRNS

DRNC

BTS

Iub

Iur

SRNS

SRNC

BTS

Iub

Figure 10 – Serving and drift RNC for soft handover

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