GSM短信基础解析：编码与解码概览

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本文档深入介绍了GSM短消息服务（Short Message Service, SMS）的基本概念和技术细节。GSM是一种全球广泛采用的移动通信标准，而短信作为其主要的文本通信方式，在早期移动通信中占据了核心地位。在本篇文章中，作者Gwenae和Alcatel（一家来自法国的电信设备制造商）提供了对SMS编码和解码技术的入门级解读。首先，文章从移动消息技术与服务的概述开始，明确了SMS、电子邮件（Email）和多媒体消息服务（Multimedia Messaging Service, MMS）之间的区别和各自的应用场景。短信以其简洁高效的特点，尤其适合在一言简意赅的信息传递中发挥作用，比如日常交流、新闻通知或商业广告。在GSM网络中，短信的发送过程涉及到编码和解码两个关键步骤。编码是将用户输入的文字转换成适于无线传输的数字信号，通常使用的是七位ASCII码，每条短信限制在160个字符以内，包括网络标识符和结束标志。解码则是接收端将接收到的数字信号还原回可读的文本形式。此外，文章可能还会探讨了GSM协议中的关键技术，如寻呼系统（ Paging System）和电路交换与分组交换（Circuit Switched vs Packet Switched）在短信传输中的作用。短信的可靠性、延迟和费用也是讨论的重要内容，因为这些特性影响着用户对服务的接受度和应用范围。为了确保数据的安全性和隐私，文中可能会提到加密技术如何保护短信内容不被未经授权的第三方获取。同时，随着技术的发展，短信服务也经历了从简单文本到支持多媒体内容（如图片、音频等）的扩展，尽管MMS在一定程度上替代了纯粹的短信功能，但短信因其简单易用和普遍性仍保持着一定的市场地位。总结来说，本篇文档为读者揭示了GSM SMS的基本原理和操作机制，从技术层面分析了短信的编码、解码流程，以及其在移动通信生态系统中的角色。对于想要理解移动通信历史和短信技术演变的专业人士或是初学者，这是一份宝贵的参考资料。

Foreword

The advent of Cellular Mobile Communications will no doubt go down in history as one of

the most signiﬁcant ‘inventions’ that took place in the 20th century and has had a vast impact

on how we conduct our business and social lives.

The main attraction for GSM in the late 1980s was that it would provide mobility for

speech communications. Some spare unused capacity in the signalling channel was identiﬁed

and so in the background, the GSM standards body invented a simple ‘two way pager like’

messaging service called the Short Message Service (SMS). SMS had problems of latency

because of its ‘store and forwar d’ characteristics and also a severe bandwidth problem of 160

characters. Disparaging comments emerged such as ‘Why would you want to send a text

message to somebody rather than talk to them?’

Little did the proponents of such ‘messages of gloom’ or the inventors of SMS realize the

impact that SMS would have on GSM customers and network operators’ revenue streams

within a decade.

Today, hundreds of millions of short messages are sent every day and SMS is the biggest

revenue generator for most network operators other than speech.

The growth of SMS trafﬁc has been slowing over the past two years and has been showing

signs of reaching a plateau within the next year or two. This may be attributed to the fact that

SMS is beginning to look like an old technology because, with the advent of 3GPP, the focus

has shifted to a far more sophist icated messaging service called the Multimedia Messaging

Service (MMS).

Network operators recognized the need to encourage further growth in SM S whilst await-

ing MMS and one mechanism to address this requirement, and also provide customers with a

new SMS experience, was the invention of the Enhanced Messaging Service (EMS) by the

3GPP standards body during the past 12 months. This has not de-focussed the attention on

MMS and will hopefully provide SMS users with a new experience and an introduction to

MMS.

The invention of MMS has brought new challenges for its inventors. For the ﬁrst time, the

Internet and the mobile telecommunications environments must be merged. This will require

a change in the cultural thinking of the traditional user s of these two very different worlds. For

example, the Internet does not seem to know any bounds of bandwidth or storage capacity and

the general perception is that the use of the Internet is ‘free’. By contrast, the mobile tele-

communications environment is limited in terms of bandwidth, storage capacity and the cost

for using it is certainl y not ‘free’ and is unlikely to be.

Despite such differences, MMS has been identiﬁed as being the most important service for

3GPP. It has no option but to succeed in order to satisfy the public expectations for mobile

access to the Internet and for the communication of various message media types already in

widespread use on the Internet.

Preface

Communications in general, and messaging in particular, have always been of key impor-

tance for the organization of human societies. First messaging systems can be traced back to

early civilizations such as the American Indians. Amerindians have been known to commu-

nicate using smoke signals. With this method, the exchange of com plex messages could be

carried out by agreeing on a common set of smoke signals between communicating tribes.

Later, the exchange of hand-written letters allowed a more reliable messaging service where

message carriers were foot or horse couriers or even ‘carrier pigeons’. In the modern world,

messaging has beneﬁted from advanc es in high technology. With the development of mobile

communications networks, mobil e messaging has become a very popular and reliable way for

communicating with almost anybody, at anytime and from almost anywhere. All message

services in the telec ommunication wor ld are based on a store-and-forward paradigm where

messages are kept temporarily if users are not able to immediately retrieve them.

The Short Message Service (SMS) has proved to be a very popular messaging serv ice,

supported by most GSM, TDMA and CDMA mobile networks. An entire chapter of this book

describes the Short Message Service introduced by the European Telecommunications Stan-

dards Institute (ETSI) for GSM and GPRS networks. The 3rd Generation Partnership Project

(3GPP) is now the organization responsible for maintaining SMS technical speciﬁcations. In

its simplest form, the Short Message Service allows users to exchange short messages

composed of a limited amount of text. In more advanced SMS extensions, short messages

can be concatenated in order to increase the amount of data that can be exchanged between

mobile users. The ﬁrst SMS message is believed to have been sent in December 1992 from a

personal computer to a European mobile phone network. In 2001, an estimated 102.9 billion

SMS messages were exchanged worldwide. Gartner Dataquest, one of the industry’s major

research agencies, expects the number of SMS messages to grow to 146 billion in 2002 and to

peak at around 168 billion in 2003 before declining.

The Enhanced Messaging Service (EMS), an application-level extension of SMS, super-

sedes limited SMS features by allowing elements such as pictures, animations, text format-

ting instructions and melodies to be inserted in short or concatenated messages.

With SMS , the most common usage scenario is the exchange of a short text message

between two mobile users. This person-to-person scenario is also applicable in the EMS

case. Furthermore, the ability to create content-rich EMS messages introduces new business

opportunities. For instance, EMS allows content providers to generate revenue by pushing

compelling content to selected mobile devices. In this book, this usage scenario is referred to

as the machine-to-person scenario.

In the machine-to-person scenario, the messaging service is usually perceived in two

different ways by the user. On the one hand, the content provider can generate messages

composed of elements such as text, pictures, animations and melodies (weather forecasts,

newsupdates,etc.).Thisscenarioisadirectextensionoftheperson-to-personscenariowhere

theuserreadsthemessageandthenreplies,forwardsordeletesit.Ontheotherhand,speciﬁc

messagingservicesalsoenablethecustomizationofmobiledevicesaccordingtouser

requirements.Thisisknownasthedownloadservicewherethemobilehandsetreceivesa

downloadmessagecontainingelementssuchasmelodiesoranimations.Inthissituation,the

userdoesnotreadthemessageinthenormalway.Instead,elementsextractedfromthe

downloadmessagearestoredinthemobiledeviceandcanbeusedasringtonesor

switch-on/off

animations,forinstance.

Recently,severalstandardizationorganizationsinitiatedtheworkonthedevelopmentof

theMultimediaMessagingService(MMS).TheMultimediaMessagingServicedeﬁnesa

frameworkfortherealizationofservicesenablingtheexchangeofmultimediamessages.The

MultimediaMessagingServiceencompassestheidentiﬁcationanddeﬁnitionofalarge

numberofhigh-levelmultimediafeatureswhichsofarhaveonlybeenprovidedbyﬁxed

messagingsystemssuchaselectronicmail.MMSfeaturesincludetheexchangeofmulti-

mediamessageschoreographedas‘slideshows’(similartoMicrosoftPowerpointpresenta-

tions).Aslideshowisconstructedasaseriesofslides,eachslidebeingcomposedoftext,

audio,imagesand/orvideoorganizedoverapredeﬁnedgraphicallayout.Thedeployment

andoperationofMMSrequiressigniﬁcantnetworkresourcesintermsofequipmentand

airtime.Consequently,advancednetworktechnologies,suchasGPRSandUMTS,aredesir-

ableforMMS.

ThedevelopmentofSMSandEMSwaslargelydrivenbytheavailabilityofunderlying

technologies.UnlikeSMSandEMS,high-levelservicerequirementsforMMShavebeen

identiﬁedﬁrstandappropriatetechnicalrealizationshavethenbeendevelopedaccordingly.

ForMMS,the3GPPhasconcentratedonidentiﬁcationofhigh-levelservicerequirementsand

architecturalaspects.Additionally,the3GPPhasprovidedtechnicalrealizationsforseveral

interfacesallowingcommunicationsbetweenelementsoftheMMSarchitecture.Tocomple-

mentthe3GPPwork,theWAPForumhasdeﬁnedtechnicalrealizationsforMMSonthe

basisofWAPandInternettransporttechnologies.MMStechnicalspeciﬁcationshave

reachedafairlymaturestageandMMScommercialsolutionsareappearingonthemarket.

BusinessanalystshavealreadyidentiﬁedthehighpotentialofMMSandthishasledto

signiﬁcantinvestmentsintheservicefromnetworkoperators,devicemanufacturersand

serviceproviders.

Chapter1introducesthebasicconceptsofmobiletechnologiesandservices.Itincludesan

overviewofthethreegenerationsofmobilenetworktechnologiesalongwithadescriptionof

supportedservices.Chapter2demystiﬁestheworkingproceduresofselectedstandardization

developmentorganizations.Forthispurpose,Chapter2presentsthestandardization

processesofrelevantorganizationssuchasthe3GPP,theWAPForum,theIETFandthe

W3C.Howtoidentifyandretrievethenecessarytechnicalspeciﬁcationsproducedbythese

organizationsisofparticularinterest.Chapter3providesanin-depthdescriptionoftheShort

MessageService(SMS).Referencematerialsareprovidedforanyengineerwillingto

developSMS-basedapplications.Chapters4and5describetwoapplication-levelextensions

ofSMS:thebasicEnhancedMessagingService(EMS)andtheextendedEMS.Chapter6

providesacomprehensivedescriptionoftheMultimediaMessagingService(MMS).Finally,

Chapter7providesanintroductiontoothermessagingservicesandtechnologiesrelevantto

theworldofmobilecommunications.

MobileMessagingTechnologiesandServicesxviii

Switch-onandswitch-offanimationsareshortanimationsdisplayedwhenthemobiledeviceisswitchedonoroff.

剩余391页未读，继续阅读

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