移动网络演进：从GSM到4G Pro与5G详解

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《从GSM到LTE-高级-pro与5G：移动网络与移动宽带入门》第三版是一本深入介绍移动通信发展历程的权威书籍，由Martin Sauter撰写。本书首次出版于2017年，由John Wiley & Sons Ltd出版，旨在为读者提供从早期的第二代（2G）GSM系统到当前先进的第四代（4G）长期演进（LTE）技术，以及最新的第五代（5G）移动网络的全面概述。在第一部分，作者回顾了GSM的发展历程，强调其在移动通信领域的基础地位，包括频分多址（FDMA）和时分多址（TDMA）技术的应用。随后，书中的第二部分深入解析了从GSM向LTE演进的过程，讲解了长期演进技术（LTE）的核心原理，如OFDM（正交频分复用）、MIMO（多输入多输出）以及eNodeB（ Evolved Node B）的作用，展示了4G网络如何通过更高的数据速率、更低的延迟和更大的容量提升用户体验。进入第三部分，作者将焦点转向5G，讨论了5G技术的突破性进展，如新空口（New Radio, NR）标准、毫米波通信、大规模天线阵列（Massive MIMO）以及切片技术等。5G不仅追求更高的数据速度，还着重于连接密度、低功耗广域网（LPWAN）和物联网（IoT）的兼容性，以支持未来智能化和自动化社会的需求。书中还涵盖了网络架构的演变，从核心网（Core Network, CN）的演进，如EPC（Evolved Packet Core）到5G的NGC（Next Generation Core），以及接入网（Access Network）从GSM的基站到LTE的eNodeB，再到5G的gNB（gNodeB）。此外，章节中会探讨频谱管理、无线资源分配、网络优化和安全性等内容，这些都是确保高效、安全通信的关键要素。《From GSM to LTE-Advanced Pro and 5G》第三版不仅适合通信工程专业的学生和研究者，也对电信运营商、设备制造商和技术开发者具有参考价值，帮助他们理解和应对移动网络技术的快速变革。通过这本书，读者可以掌握从传统到现代移动通信系统的全面理解，以及如何适应并推动未来5G时代的到来。

From GSM to LTE-Advanced Pro and 5G

1.1.1 Classic Circuit Switching

The GSM mobile telecommunication network has been designed as a circuit‐switched

network in a similar way to fixed‐line phone networks. At the beginning of a call, the net-

work establishes a direct connection between two parties, which is then used exclusively for this

conversation. As shown in Figure1.1, the switching center uses a switching matrix to con-

nect any originating party to any destination party. Once the connection has been estab-

lished, the conversation is then transparently transmitted via the switching matrix between

the two parties. The switching center only becomes active again to clear the connection in

the switching matrix if one of the parties wants to end the call. This approach is identical in

both mobile and fixed‐line networks. Early fixed‐line telecommunication networks were

designed only for voice communication, for which an analog connection between the par-

ties was established. In the mid‐1980s, analog technology was superseded by digital tech-

nology in the switching center. This meant that calls were no longer sent over an analog

line from the originator to the terminator. Instead, the switching center digitized the analog

signal that it received from the subscribers, which were directly attached to it, and for-

warded the digitized signal to the terminating switching center. There, the digital signal

was again converted back to an analog signal, which was then sent over the copper cable to

the terminating party. In some countries, ISDN (Integrated Services Digital Network) lines

were quite popular. With this system, the transmission became fully digital and the conver-

sion back to an analog audio signal was done directly in the phone.

GSM reused much of the fixed‐line technology that was already available at the time the

standards were created. Thus, existing technologies such as switching centers and long‐

distance communication equipment were used. The main development for GSM, as

shown in Figure1.2, was the means to wirelessly connect the subscribers to the network.

In fixed‐line networks, subscriber connectivity is very simple as only two dedicated wires

are necessary per user. In a GSM network, however, the subscribers are mobile and can

change their location at any time. Thus, it is not possible to use the same input and output

in the switching matrix for a user for each call as is the case in fixed‐line networks.

As a mobile network consists of many switching centers, with each covering a certain

geographical area, it is not even possible to predict in advance which switching center a

Figure1.1 Switching matrix in a

switching center.

Global System for Mobile Communications (GSM)

call should be forwarded to for a certain subscriber. This means that the software for

subscriber management and routing of calls of fixed‐line networks cannot be used for

GSM. Instead of a static call‐routing mechanism, a flexible mobility management archi-

tecture in the core network became necessary, which needed to be aware of the current

location of the subscriber and thus able to route calls to them at any time.

It was also necessary to be able to flexibly change the routing of an ongoing call as a

subscriber can roam freely and thus might leave the coverage area of the radio transmit-

ter of the network over which the call was established. While there was a big difference

between the software of a fixed switching center and a Mobile Switching Center (MSC),

the hardware as well as the lower layers of the software which are responsible, for exam-

ple, for the handling of the switching matrix were mostly identical. Therefore, most

telecommunication equipment vendors like Ericsson, Nokia Solutions and Networks,

Huawei and Alcatel‐Lucent offered their switching center hardware both for fixed‐line

and mobile networks. Only the software in the switching center determined whether

the hardware was used in a fixed or mobile network (see Figure1.2).

1.1.2 Virtual Circuit Switching over IP

While in the 1990s voice calls were the dominating form of communication, this has

significantly changed today with the rise of the Internet. While voice calls remain

important, other forms of communication such as e‐mail, instant messaging (IM), social

networks (e.g. Facebook), blogs, wikis and many more play an even bigger role. All these

services share the Internet Protocol (IP) as a transport protocol and globally connect

people via the Internet.

While circuit switching establishes an exclusive channel between two parties, the

Internet is based on transferring individual data packets. A link with a high bandwidth

is used to transfer the packets of many users. By using the destination address contained

in each packet, each network node that the packet traverses decides over which outgo-

ing link to forward the packet. Further details can be found in Chapter2.

Owing to the rise of the Internet and IP‐based applications, network operators thus

had to maintain two separate networks: a circuit‐switched network for voice calls and a

packet‐switched network for Internet‐based services.

As the simultaneous operation of two different networks is very inefficient and costly,

most network operators have, in the meantime, replaced the switching matrix in the

Switching and signaling

software

Fixed line subscriber and

call control

Operating system of the

switching center

Mobile subscriber management

Mobility management

Call control

Switching and signaling

software

Operating system of the

switching center

Figure1.2 Necessary software changes to adapt a fixed‐line switching center for a wireless network.

From GSM to LTE-Advanced Pro and 5G

MSC with a device referred to as a media gateway. This allows them to virtualize circuit

switching and to transfer voice calls over IP packets. The physical presence of a circuit‐

switched infrastructure is thus no longer necessary and the network operator can

concentrate on maintaining and expanding a single IP‐based network. This approach

has been standardized under the name ‘Bearer‐Independent Core Network’ (BICN).

The basic operation of GSM is not changed by this virtualization. The main differences

can be found in the lower protocol levels for call signaling and voice call transmission.

This will be looked at in more detail in the remainder of this chapter.

The trend toward IP‐based communication can also be observed in the GSM radio

network especially when a radio base station site supports GSM, UMTS and LTE

simultaneously. Typically, connectivity is then established over a single IP‐based link.

The air interface between the mobile devices and the network is not affected by the

transition from circuit to packet switching. For mobile devices, whether the network

uses classic or virtual circuit switching is therefore completely transparent.

1.2 Standards

As many network infrastructure manufacturers compete globally for orders from

telecommunication network operators, standardization of interfaces and procedures is

necessary. Without standards, which are defined by the International Telecommunication

Union (ITU), it would not be possible to make phone calls internationally and network

operators would be bound to the supplier they initially select for the delivery of their

network components. One of the most important ITU standards, discussed in Section1.4,

is the Signaling System Number 7 (SS‐7), which is used for call routing. Many ITU stand-

ards, however, only represent the lowest common denominator as most countries have

specified their own national extensions. In practice, this incurs a high cost for software

development for each country as a different set of extensions needs to be implemented

in order for a vendor to be able to sell its equipment. Furthermore, the interconnection

of networks of different countries is complicated by this.

GSM, for the first time, set a common standard for Europe for wireless networks. Due

to its success it was later adopted around the globe. This is the main reason why sub-

scribers can roam in GSM networks across the world that have roaming agreements

with each other. The common standard also substantially reduces research and devel-

opment costs as hardware and software can now be sold worldwide with only minor

adaptations for the local market. The European Telecommunication Standards Institute

(ETSI), which is also responsible for a number of other standards, was the main body

responsible for the creation of the GSM standard. The ETSI GSM standards are com-

posed of a substantial number of standards documents, each of which is called a techni-

cal specification (TS) and describes a particular part of the system. In the following

chapters, many of these specifications are referenced and can thus be used for further

information about a specific topic. Due to the global success of GSM, the 3

Generation

Partnership Project (3GPP) was later founded as a global organization and ETSI became

one of the regional standardization bodies of the project. Today, 3GPP is responsible for

maintaining and further developing the GSM, UMTS, LTE and 5G standards. All docu-

ments are freely available on the Internet at http://www.etsi.org [1] or at http://

www.3gpp.org [2].

Global System for Mobile Communications (GSM)

1.3 Transmission Speeds

The smallest transmission speed unit in a classic circuit‐switched telecommunication

network is the digital signal level 0 (DS0) channel. It has a fixed transmission speed of

64 kbit/s. Such a channel can be used to transfer voice or data, and thus it is usually not

called a speech channel but simply referred to as a user data channel.

The reference unit of a telecommunication network is an E‐1 connection in Europe

and a T‐1 connection in the United States, which use either a twisted pair or coaxial

copper cable. The gross datarate is 2.048 Mbit/s for an E‐1 connection and 1.544 Mbit/s

for a T‐1. An E‐1 is divided into 32 timeslots of 64 kbit/s each, as shown in Figure1.3

while a T‐1 is divided into 24 timeslots of 64 kbit/s each. One of the timeslots is used for

synchronization, which means that 31 timeslots for an E‐1 or 23 timeslots for a T‐1,

respectively, can be used to transfer data. In practice, only 29 or 30 timeslots are used

for user data transmission while the rest (usually one or two) are used for SS‐7 signaling

data (see Figure1.3). More about SS‐7 can be found in Section1.4.

A single E‐1 connection with 31 DS0s is not enough to connect two switching centers

with each other. An alternative is an E‐3 connection over twisted pair or coaxial cables.

An E‐3 connection is defined at a speed of 34.368 Mbit/s, which corresponds to

512 DS0s.

For higher transmission speeds and for long distances, optical systems that use the

synchronous transfer mode (STM) standard are used. Table1.1 shows some datarates

and the number of 64 kbit/s DS0 channels that are transmitted per pair of fibers.

Table1.1 STM transmission speeds andnumber ofDS0s.

STM level Speed (Mbit/s)

Approximate number

of DS0 connections

STM‐1 155.52 2300

STM‐4 622.08 9500

STM‐16 2488.32 37,000

STM‐64 9953.28 148,279

Synchronization

31 timeslots with 8 bits (1B) each for user data o

SS-7 signaling

Repetition interval: 8000 Hz

Speed: 32 timeslots × 8 Bit × 8000 1/s=2.048 Mbit/s

Figure1.3 Timeslot architecture of an E‐1 connection.

From GSM to LTE-Advanced Pro and 5G

For virtual circuit switching over IP, optical Ethernet links are often used between

network nodes at the same location. Transmission speeds of 1 Gbit/s or more are

used on these links. Unlike the circuit‐switched technology described above,

Ethernet is the de facto standard for IP‐based communication over fiber and copper

cables and is widely used. As a consequence, network equipment can be built much

more cheaply.

1.4 The Signaling System Number 7

For establishing, maintaining and clearing a connection, signaling information needs to

be exchanged between the end user and network devices. In the fixed‐line network,

analog phones signal their connection request when the receiver is lifted off the hook

and a phone number dialed that is sent to the network either via pulses (pulse dialing)

or via tone dialing, which is called dual tone multifrequency (DTMF) dialing. With

fixed‐line ISDN phones and GSM mobile phones, the signaling is done via a separate

dedicated signaling channel, and information such as the destination phone number is

sent as messages.

If several components in the network are involved in the call establishment, for exam-

ple, if originating and terminating parties are not connected to the same switching

center, it is also necessary that the different nodes in the network exchange information

with each other. This signaling is transparent for the user, and a protocol called the SS‐7

is used for this purpose. SS‐7 is also used in GSM networks and the standard has been

enhanced by ETSI to fulfill the special requirements of mobile networks, for example,

subscriber mobility management.

The SS‐7 standard defines three basic types of network nodes:

● Service Switching Points (SSPs) are switching centers that are more generally referred

to as network elements and that are able to establish, transport or forward voice and

data connections.

● Service Control Points (SCPs) are databases and application software that can influ-

ence the establishment of a connection. In a GSM network, SCPs can be used, for

example, for storing the current location of a subscriber. During call establishment to

a mobile subscriber, the switching centers query the database for the current location

of the subscriber to be able to forward the call. More about this procedure can be

found in Section1.6.3 about the Home Location Register (HLR).

● Signaling Transfer Points (STPs) are responsible for the forwarding of signaling mes-

sages between SSPs and SCPs as not all network nodes have a dedicated link to all

other nodes of the network. The principal functionality of an STP can be compared

to an IP router in the Internet, which also forwards packets to different branches of

the network. Unlike IP routers, however, STPs only forward signaling messages that

are necessary for establishing, maintaining and clearing a call. The calls themselves

are directly carried on dedicated links between the SSPs.

Figure1.4 shows the general structure of an SS‐7 circuit‐switched telecommunication

network and the way the nodes described above are interconnected with each other.

The SS‐7 protocol stack is also used in virtual circuit‐switched networks for com-

munication between the network nodes. Instead of dedicated signaling timeslots on an

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