GSM网络协议详解与实现

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"GSM Network Protocols Terminology and implementation 是一本深入介绍GSM网络协议、术语及实现的书籍，由Gunnar Heine撰写并由Artech House出版社出版。本书是Artech House移动通信图书馆系列的一部分，对全球移动通信系统（GSM）进行了详细解析。" GSM（Global System for Mobile Communications）是一种广泛使用的数字蜂窝通信标准，它在全球范围内为手机通信提供了基础。GSM网络协议是这个系统的核心，它们确保了不同设备之间的高效通信和数据传输。本书深入探讨了以下几个关键知识点： 1. **GSM网络架构**：GSM网络由多个组成部分构成，包括移动台（MS）、基站子系统（BSS）、网络子系统（NSS）以及操作支持子系统（OSS）。每个部分都有其特定的职责，如MS负责用户设备的通信，BSS处理无线接口，NSS处理呼叫控制和用户数据管理，而OSS则负责网络的维护和管理。 2. **无线接口协议**：GSM使用了一系列协议来处理无线链路通信，如Um接口上的LAPDm（逻辑链路控制协议，减小版）用于空中接口的数据传输，以及GPRS（通用分组无线业务）中的GTP（GPRS隧道协议）用于在核心网中建立和管理数据隧道。 3. **呼叫处理**：书中详细介绍了从呼叫建立到结束的整个过程，包括信令流程，如位置更新、鉴权和加密等安全措施。这些过程涉及到SS7（信号系统7）信令，它是GSM网络中用于控制和管理通话的关键协议。 4. **移动性管理**：GSM系统允许用户在不同基站之间无缝切换，这涉及到了小区重选、越区切换等过程，这些过程需要精确的同步和协调。 5. **数据服务**：除了语音通信，GSM也支持数据传输，如SMS（短消息服务）和GPRS，后者提供了连续的分组数据连接，使移动设备能够接入Internet。 6. **安全性**：GSM网络采用了多种安全机制，如SIM卡（subscriber identity module）用于身份验证，A5/1、A5/2等加密算法保护通话内容不被窃听。 7. **协议栈详解**：书中详细阐述了GSM协议栈的各个层次，从物理层到应用层，如MAC（媒体访问控制）、RLC（无线链路控制）、LLC（逻辑链路控制）以及NAS（非接入层）等。 8. **实施与实践**：作者还讨论了实际部署中的挑战和解决方案，提供了理解和应用GSM技术的实用指导。通过这本书，读者可以深入了解GSM网络的工作原理，不仅适合通信工程专业的学生，也对网络运维人员和技术开发者具有很高的参考价值。书中的实例和解释有助于读者将理论知识应用于实际场景，从而更好地理解GSM网络协议的复杂性和重要性。

BSC takes care of all the central functions and the control of the

subsystem, referred to as the base station subsystem (BSS). The BSS

comprises the BSC itself and the connected BTSs.

1.2.2.5 Transcoding Rate and Adaptation Unit

One of the most important aspects of a mobile network is the effec-

tiveness with which it uses the available frequency resources. Effective-

ness addresses how many calls can be made by using a certain

bandwidth, which in turn translates into the necessity to compress

data, at least over the Air-interface. In a GSM system, data compres-

sion is performed in both the MS and the TRAU. From the architec-

ture perspective, the TRAU is part of the BSS. An appropriate

graphical representation of the TRAU is a black box or, more symboli-

cally, a clamp.

1.2.2.6 Mobile Services Switching Center

A large number of BSCs are connected to the MSC via the

A-interface. The MSC is very similar to a regular digital telephone

exchange and is accessed by external networks exactly the same way.

The major tasks of an MSC are the routing of incoming and outgo-

ing calls and the assignment of user channels on the A-interface.

1.2.2.7 Home Location Register

The MSC is only one subcenter of a GSM network. Another subcen-

ter is the HLR, a repository that stores the data of a large number of

subscribers. An HLR can be regarded as a large database that adminis-

ters the data of literally hundreds of thousands of subscribers. Every

PLMN requires at least one HLR.

1.2.2.8 Visitor Location Register

The VLR was devised so that the HLR would not be overloaded with

inquiries on data about its subscribers. Like the HLR, a VLR contains

subscriber data, but only part of the data in the HLR and only

while the particular subscriber roams in the area for which the VLR

is responsible. When the subscriber moves out of the VLR area,

the HLR requests removal of the data related to a subscriber from the

VLR. The geographic area of the VLR consists of the total area cov-

ered by those BTSs that are related to the MSCs for which the VLR

provides its services.

6 GSM Networks: Protocols, Terminology, and Implementation

TRAU

MSC

HLR

VLR

1.2.2.9 Equipment Identity Register

The theft of GSM mobile telephones seems attractive, since the iden-

tities of subscribers and their mobile equipment are separate. Stolen

equipment can be reused simply by using any valid SIM. Barring of a

subscriber by the operator does not bar the mobile equipment. To

prevent that kind of misuse, every GSM terminal equipment contains

a unique identifier, the international mobile equipment identity

(IMEI). It lies within the realm of responsibilities of a network opera-

tor to equip the PLNM with an additional database, the EIR, in which

stolen equipment is registered and so can be used to bar fraudulent

calls and even, theoretically, to track down a thief (by analyzing the

related SIM data).

1.3 The Focus of This Book

This book describes briefly the GSM subsystems, their structure, and their

tasks. However, the focus of this book lies not on the GSM network elements

themselves but on the interfaces between them.

Among others, the following issues will be addressed:

•

What signaling standards and what protocols are used to serve connec-

tion requests by mobile subscribers?

•

How are the various interfaces utilized?

•

What happens in case of errors?

•

Although GSM uses available signaling standards, where are the GSM

specific adaptations?

One has to remember that most of the signaling is necessary to support

the mobility of a subscriber. All messages of the area mobility management

(MM) and radio resource management (RR), in particular, serve only that

purpose. Only a fraction of the exchanged messages are used for the connec-

tion setup as such, and those are all the messages that are related to call

control (CC).

A presentation of the Open System Interconnection (OSI) Reference

Model is mandatory in a book in which the focus is on signaling.

Another focus of the text is on the application of the various protocols for

error analysis. Which error indication is sent by the system and when? How is

such an indication interpreted? What are the potential sources of errors?

Introduction 7

EIR

A word on coding of parameters and messages should be added here:

Coding of message types and other essential parameters are always included.

However, because this book has no intention of being a copy of the GSM Rec-

ommendations, it deliberately refrains from providing a complete list of all

parameters of all interfaces.

The value of protocol test equipment for error analysis and routine testing

is indisputably high, but what help do programs for automatic analysis provide?

Those questions will be answered as well.

A large part of this book is taken up by a glossary, which provides descrip-

tions of all abbreviations, terms, and processes that a reader may confront dur-

ing work on GSM.

1.4 Signaling

The main focus of this book is on the signaling between the various network

elements of GSM. The questions arise of what signaling actually constitutes

and what it is used for. Although we do not want to go back to the basics of

telecommunications to answer those questions, a number of basic explanations

do seem necessary.

1.4.1 What is Signaling?

Signaling is the language of telecommunications that machines and computers

use to communicate with each other. In particular, the signals that a user enters

need to be converted to a format that is appropriate for machines and then

transmitted to a remote entity. The signals (e.g., the identity of a called party)

are not part of the communication as such, that is, they are not a payload or a

revenue-earning entity.

Signaling is comparable to the pilots and the flight attendants on an

airplane. The crew members are no “payload,” but they are necessary to carry

the payload. Another, perhaps more appropriate, illustration is to consider the

now almost extinct telephone operator, whose function it was to carry out the

signaling function and switching of a telecommunications system by connect-

ing cables between the appropriate incoming and outgoing lines.

1.4.2 How is Signaling Performed?

When calls were set up manually, signaling consisted mostly of direct current

impulses, which allowed a central office to determine the dialed digits. Some

8 GSM Networks: Protocols, Terminology, and Implementation

readers may still remember the rotary telephone sets, in which the impulses

were created mechanically by the spin of the rotor. The situation changed com-

pletely with the entry of computer technology into telecommunications. The

microchip utilized by telecommunications opens today, at the end of the

twentieth century, a multitude of new signaling functionality, which were

unthinkable even 20 years ago. Computers are the backbone of modern tele-

communications systems.

This new technology makes mobile communications possible in the first

place. The signaling requirements of modern mobile systems are so vast that the

former technology would not have been able to manage them. Computeriza-

tion, however, did not change much of the principle. As in the old days, electri-

cal or optical signals are sent, over an appropriate medium (typically serially)

and interpreted by the receiver. What did change is the speed of the transmis-

sion. The progress in this area has been exponential.

The smallest unit of a signal is called a bit and can, for example, be repre-

sented by an electric voltage, which a receiver can measure during a specified

period of time. If the receiver measures the voltage as “low” over the specified

time period, it interprets the value as a 0. If the voltage is “high,” the receiver

interprets the value as a 1. It does not matter which level represents which

value, so long as both the sender and the receiver agree on which is which.

A sequence of bits allows the coding and sending of complex messages,

which, in turn, allows a process to be controlled or information to be conveyed.

The result is a bit stream, as shown in Figure 1.3.

Pulse code modulation (PCM) is the worldwide process for transmission

of digital signals. PCM is used to transmit both signaling data and payload.

PCM is categorized into hierarchies, depending on the transmission rate. The

PCM link of 2 megabits per second (Mbps) (one that is referred to frequently

in this book) is only one variant of many. By utilizing a time-division

Introduction 9

bit value = 0

U(low) U(high)<

bit value = 1

U(high) U(low)>

time

hex

1110000 0011

}

Figure 1.3 Decoding of a bit stream.

multiplexing technique, such a 2-Mbps PCM link can, among others, be parti-

tioned into 32 independent channels, each capable of carrying 64 kilobits per

second (Kbps).

Another aspect of the change that the digital technology has enabled

reveals its advantage only after a second look. Almost all signaling standards,

like Signaling System Number 7 (SS7) and Link Access Protocol for the

D-channel (LAPD) separate the traffic channel from the signaling or control

channel. This is referred to as outband signaling, in contrast to inband signal-

ing. In the case of inband signaling, all the control information is carried within

the traffic channel. Although outband signaling requires the reservation of

a traffic channel, it makes a more efficient use of resources overall. The

reason for that lies in the reduced occupation time of the traffic channel,

which is not needed during call setup. Both call setup and call release can be

carried out for many connections via one control channel, since signaling

data use the resources more economically. One 64-Kbps time slot out of a

2-Mbps PCM link typically is used for signaling data; a call setup consumes

about 1 to 2 Kbps.

1.4.3 What is Signaling Used For?

The main task of signaling is still to set up and to clear a connection between

end users or machines. Today, constantly new applications are added. Among

them are automated database accesses, in which telecommunications systems

call each other and which are fairly transparent to a caller, or the wide area of

supplementary services, of which only call forwarding is mentioned here as an

example. The glossary provides a list of all GSM supplementary services.

1.5 Representation of Messages

When working with protocol test equipment and in practical work, message

names usually are abbreviated. Most GSM and ITU Telecommunication

Standardization Sector (ITU-T) Recommendations list the well-defined

abbreviations and acronyms, which this book also uses to a large extent. The

complete message names and explanations can be found in the respective

chapters.

Since a picture often expresses more than a thousand words, this book

contains a large number of figures and protocol listings. The various messages

illustrated in the figures show parameters, which are formatted per interface

and are presented as shown in Figures 1.4(a) through 1.4(e).

10 GSM Networks: Protocols, Terminology, and Implementation

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