Maintenance of Urban rail

transit communication
transmission system

Teacher： HeKai/khaled
tel:01016705688
1.1 Basic concept of the
transmission system
1.1.1 Basic concepts of transmission system
Definition of transmission system

Speaking of communication, we are familiar with everyone. In ancient times, there were flames,
smoke, drums, waving flags and Shouting, communication tower and other ways to achieve
communication.

With the development of social productive forces, modern communication has been rapidly developed
and widely used, mainly including writing, letters, telegrams, radio, television, intercom,
telephone (mobile telephone / fixed telephone), network, and so on.
1.1.1 Basic concepts of transmission system
Definition of transmission system

It can be seen that news (message) is a reflection of human material or mental state, with
different forms in different periods. People receive messages, concerned about the content
contained in the message that is unknown to be known, namely information (information).
T herefore, the fundamental purpose of the communication is to pass on the information contained
in the message. In essence, communication is a science and technology of information transmission
function. A large number of useful information should be transmitted efficiently without
distortion, and at the same time, useless information and harmful information should be
suppressed as much as possible in the transmission process.
1.1.1 Basic concepts of transmission system
Definition of transmission system

Simply put, communication is the transmission of

information, which can be expressed through language, text,
images, data and other forms. Here, "transmission" can be
considered as a process or mode of information transmission.
The communications discussed in this book are not in broad
sense. ReDefinition of transmission system: the use of
electronic and other technical means, with the help of
electrical signals (including optical signals) to realize
the effective transmission of messages from one place to
another is called communication.
1.1.1 Basic concepts of transmission system
A general model of the communication system

The purpose of communication is to transmit information

and carry out the spatial and temporal transfer of
information. The function of a communication system is to
send information from a source to one or more
destinations.
The collection of device hardware, software, and
transmission media used for communication is called a
communication system（通信系统）. It should be emphasized
that in the past, the definition of the communication
system did not include the software part, but as the
computer entered the communication system, the
communication software became the basic element of the
communication system. From the hardware point of view,
the communication system is mainly composed of five parts:
information source信源, communication host信宿,
transmission medium传输介质 and receiving and sending
equipment收信、发信设备 .
1.1.1 Basic concepts of transmission system
A general model of the communication system

In the communication system, the transmission of a

message is realized by an electrical signal. First,
the message is converted into an electrical signal
and sends the signal into the channel. After the
receiving end, the receiving signal is sent to the
receiving signal and then converted into the original
message.
The functions of each part of the communication
system are briefly described as follows:
1.1.1 Basic concepts of transmission system
information generator

Information source 信源is the place of message generation, and its function is
to convert all kinds of information into original electrical signals.
According to the different types of information, the information sources can be
divided into simulated sources模拟信源 and digital sources数字信源.
Simulated source output continuous analog signal, such as microphone (sound
audio signal), camera (image video signal), Digital source output discrete
digital signal, such as transmitter (keyboard character numeric signal),
computer and other digital terminals, and simulate the source signal after
digital processing can also be converted into digital signal.
Due to the different types and rates of information generated by information
sources, the requirements for the transmission system are also different.
1.1.1 Basic concepts of transmission system
Sending device

The function of the Sending device is to generate a

signal suitable for transmission in the channel, even if
the characteristics of the transmitting signal match the
channel characteristics, has the ability to resist
channel interference, and has sufficient power to meet
the needs of long-distance transmission.
Therefore, the sending device covers a lot of content,
the transformation mode is diverse, in the need to move
the spectrum频谱搬移 of the occasion, modulation调制 is
the most common transformation mode. For the
transmission of digital signals, the transmission device
also includes the information source code信源编码 and
the channel code信道编码.
1.1.1 Basic concepts of transmission system
channel

A channel is a physical medium for transmitting signals, which is used to

transmit signals from a sending device to a receiving end. In a wireless
channel, the channel can be free space. In a wired channel, it can be
cables and optical cables. Both wired and wireless channels have a
variety of physical media. The channel not only provides a transmission
path for the signal, but also generates various interferences and noises
to the signal. The inherent characteristics固有特性 of the channel and
the interference and noise introduced are directly related to the quality
of communication.
1.1.1 Basic concepts of transmission system
Relay equipment or switching equipment

The former refers to the line intermediate equipment, such

as repeater中继器 and EDFA, etc.
The latter refers to the intermediate equipment (such as
the switch交换机) that completes the information exchange
function in the communication system. Its main function is
to complete the logical connection逻辑连接 between
different users according to the requirements of users,
realize the channel of information exchange, and then
complete the exchange function.
1.1.1 Basic concepts of transmission system
receiving device

The receiving device amplifies and reversely transforms the signal (such as
decoding译码, demodulation解调, etc.) to correctly restore the original
electrical signal from the damaged received signal.
For multiplexed signals多路复用信号, the receiving device also includes
demultiplexing to achieve the correct branching function. In addition, it must
minimize the impact of noise and interference during the transmission process.
1.1.1 Basic concepts of transmission system
recipient

The recipient, referred to as the destination, is the

place where the message is transmitted. Its function
is opposite to that of the source, that is, to
restore the original electrical signal into the
corresponding message, such as a telephone restores
the electrical signal sent by the other party into
sound.
1.1.1 Basic concepts of transmission system
noise source

The noise is mainly derived from the channel. In the process of

information transmission, due to the existence of various external or
internal factors外界或内部因素, produce a variety of interference,
collectively referred to as noise. Noise is usually random and in
various forms, and in practical engineering applications工程应用,
noise interference cannot be completely eliminated.
1.1.1 Basic concepts of transmission system

Question：
From the hardware point of view, the communication system is mainly composed of five parts: ___________信
源, _________信宿, __________传输介质 and ______________收信、发信设备 .

information source、communication host、

transmission medium、receiving and sending
equipment
1.1.1 Basic concepts of transmission system
Communication classification

According to the different types of channel transmission

signals, the communication system can be divided into two
categories: analog communication system and digital
communication system.模拟通信系统和数字通信系统
1.1.1 Basic concepts of transmission system
Simulating Communication System Modelsl

In an electrical communication system, the transmission of a message is a load on a

certain parameter of the electrical signal (e. g., the amplitude幅度, frequency频率,
or phase相位 of a continuous wave; the amplitude幅度, width宽度, or position位置 of
the pulse wave脉冲波). According to the different value methods of the electrical
signal parameters, the signals can be divided into two types: analog signal and
digital signal. To judge whether a signal is analog signal or digital signal, the
key is to determine which physical quantity the signal parameter carrying the
message is, and then judge according to the value state of the physical quantity.
1.1.1 Basic concepts of transmission system
analog signal

The signal parameter carrying the message has a

continuous infinite 连续无限value. As can be
seen from the figure, the magnitude of the
signal changes with time, so it can be
determined that the signal parameter carrying
the message is the magnitude of the signal.
Whether the speech signal or the sampling signal,
the value state of the amplitude is continuous
and infinite. The difference is that the former
is a time-continuous signal and the latter is a
time-discrete signal. the speech signal the sampling signal
1.1.1 Basic concepts of transmission system
digit signal

The signal parameter carrying the message has limited

discretization. In Figure (a), the physical parameter
carrying the message is the amplitude of the
rectangular wave, and the amplitude changes in only
two states; in Figure (b), the physical parameter
carrying the message is the phase of the sine wave,
and the phase changes in only 0 and π. Therefore,
they are all digital signals, and they are all
temporally continuous signals.
1.1.1 Basic concepts of transmission system
digit signal

Generally, communication systems are divided into analog communication

systems and digital communication systems according to whether analog
signals or digital signals are transmitted in the channel. Analog
communication systems are communication systems that use analog signals
to transmit messages, and the corresponding analog communication
systems are designed according to the transmission characteristics of
analog signals.
1.1.1 Basic concepts of transmission system
Digital communication system model

Digital communication system (Digital Communication System, DCS) is a

communication system that uses digital signals to transmit information.
The digital communication system can be further subdivided into digital
frequency band transmission communication system, digital baseband
transmission communication system and analog signal digital transmission
communication system. Explain them separately below.
1.1.1 Basic concepts of transmission system
Digital frequency band transmission and communication system

A digital communication system containing a modulator /

demodulator is usually called a digital band transmission
communication system数字频带传输通信系统
1.1.1 Basic concepts of transmission system
Digital baseband transmission and communication system

Corresponding to the frequency band transmission system,

we refer to the digital communication system without the
modulator / demodulator as the digital baseband
transmission communication system数字基带传输通信系统
1.1.1 Basic concepts of transmission system
Analog signal digital transmission and communication system

In the digital communication system discussed above, the signals output by the
information source are all digital baseband signals. In fact, most of the
signals (such as voice signals) are analog signals with continuous changes. In
order to realize the transmission of the analog signal in the digital
communication system, the analog signal must be digitized at the sending end,
that is, A / D conversion: the opposite conversion at the receiving end, that
is, D / A conversion.
上面讨论的数字通信系统中，信源输出的信号均为数字基带信号。实际上，在日常生活中大部
分信号（如语音信号）为连续变化的模拟信号。要实现模拟信号在数字通信系统中传输，则必
须在发送端将模拟信号数字化，即 A/D转换：在接收端需进行相反的转换，即 D/A转换。
1.1.2, the concept of transmission

Transmission as the name implies, the meaning of transmission. The

purpose of communication is to transfer information from one location to
another, and the transmission is the bridge and link between the two
points. The transmission has one-way transmission (such as broadcast)
and two-way transmission (such as call). If you want to communicate
between multiple points, it is necessary to build a multi-point to
multi-point complex transmission network. The modern transmission
network is often called the information superhighway, which provides
transmission channels for various service networks.
The transmission network is a network used as the transmission channel,
just like the highway, which is composed of various transmission lines
and transmission equipment, where the transmission line completes the
signal transmission and the transmission equipment completes the signal
processing.
1.1.2, the concept of transmission
Classification of transmissions

Transmission is usually divided into wired

01

01 transmission and wireless transmission by

transmission medium.

02

Cred transmission refers to the transmission medium for cable,

03
02
optical cable, waveguide and other forms of communication, its
characteristic is that the medium can be visible, touch such as
cable communication, optical cable communication.

Wireless transmission refers to a form of communication in

03
which the transmission medium cannot be seen and touched (such
as electromagnetic waves), such as microwave communication,
mobile communication, satellite communication, etc.
1.1.2, the concept of transmission
The status and role of the transmission network in the communication network

Lets first understand what the communication network is used for.

Communication mainly refers to the process in which people transmit and
exchange information. There are directions for information exchange, such
as A and B talking on the phone, including information transmission in two
directions from A to B and B to A.
If A and B are communicating through the communication network, the
communication network will complete the communication task in three stages
from A to B.
First, the information of A is packaged at the A end, and then the
packaged information is transmitted to the location of B through the
communication network. Finally, the information is received at the B end
to remove the package and restore the information from A. This process is
similar to the process of delivering the messenger. A puts the letter into
the envelope is the first step, the post office sorts the letter and then
loads the mail car to the destination.
1.1.2, the concept of transmission
The status and role of the transmission network in the communication network

According to the two processes listed above, the terminal equipment such as
telephone, mobile phone and computer connected to the network of the communication
network realizes the "envelope" function, and the equipment such as switches and
routers is equivalent to the letter sorting system of the post office. The
transmission network is equivalent to a mail train, or a mail transport system.
Figuratively speaking, the role of the transmission network in the communication
network is the porter, carrying the information from A to B. Is the role of a porter
very important? People often ignore its existence, users often see the envelope, the
post office wants to move to the sorting speed and accuracy of the sorting system,
few people care about which car or what car the letter was sent. It can be said that
the porter is a behind-the-scenes hero, information can be sent to depends on the
porter can go to the destination, when to rely on the porters working time and
efficiency. The technology of porters is constantly innovating, and can "cross
mountains and rivers", communication can cross these geographical barriers.
The location of the transmission network in the communication network is shown in
Fig.
1.1.2, the concept of transmission
The status and role of the transmission network in the communication network
1.1.2, the concept of transmission
The composition of the transmission network

The transmission network contains two types of entities: transmission equipment and various
cables. Transmission network maintenance work is therefore divided into two aspects:
equipment maintenance and line maintenance设备维护和线路维护.
There are two main kinds of transmission lines: cable and optical cable.电缆和光缆
Transmission equipment includes: SDH equipment, WDM equipment, PTN equipment, OTN equipment,
etc. In the continuous process of technological innovation of transmission network, the types
and standards of transmission equipment are also developing and changing, but there is no
kind of equipment in the dominant position of "unifying the world". It can be said that each
has its own use.
SDH (synchronous digital system) transmission network consists of different types of NE
equipment through optical cable routes. These NEs can realize the functions of synchronous
reuse, cross connection, network fault self-healing and so on of SDH system. The basic
network elements of the SDH transmission network include terminal multiplexer (TM), partition
multiplexer (ADM), regeneration repeater (REG) and digital cross connection device (DXC),
which will be introduced in the following sections.
1.1.2, the concept of transmission
The composition of the transmission network

WDM (wave division multiplexing) optical network, due to the emergence of multimedia
communication, the development of the Internet and the increase of mobile phone users, the
capacity demand for transmission network is increasing, for the "optical fiber depletion"
phenomenon, how to use the existing optical fiber communication system to achieve the
maximum expansion, has become a concern. The results show that a single fiber can load
dozens or even hundreds of different wavelengths, resulting in the WDM technology.
Usually, the multiplexing with large optical channel interval (even on different Windows
of optical fiber) is called optical wave division multiplexing (WDM), and then the WDM
with small channel interval in the same window is called dense wave division multiplexing
(DWDM). With the progress of science and technology, modern technology has been able to
realize the multiplexing of wavelength interval of nanoscale, or even the reuse of
wavelength interval of a few tenths, but the technical requirements of devices are more
strict. Therefore, the 1270nm to 1610nm wavelength interval of 20nm is called coarse wave
division multiplexing (CWDM).
At present, the optical transmission network construction based on WDM is one of the
fastest growing fields in the communication industry.
1.1.2, the concept of transmission
The hierarchy of the transmission network

In order to facilitate the network planning, construction, operation and maintenance, the structure of the
transmission network is divided into three layers: inter-provincial backbone network, provincial backbone
network and local network, which realize different functions.
The inter-provincial backbone network has also been used as a first-level backbone network, which is
responsible for providing transmission channels between provinces. Each province has one or two inter-
provincial exports, which are connected by an interprovincial backbone network. Inter-provincial exports are
generally municipalities directly under the Central Government, provincial capitals or cities under separate
state planning. The construction of the inter-provincial backbone network belongs to the management and
implementation project of the headquarters, and the headquarters directly participates in the daily
maintenance work.
The provincial backbone network, also known as the secondary backbone network, is responsible for providing
the transmission channels between cities and cities within the province. The construction of the provincial
backbone network belongs to the implementation project in the charge of the province, and the headquarters
manages the work of the provincial backbone network through the data reported by the branch company.
If the local network is responsible for providing transmission channels between switches, base stations,
central equipment rooms and service access points within a municipal jurisdiction area, it can be considered
as capillaries and play the role of service convergence. The local network can be divided into convergence
layer and access layer.
1.1.2, the concept of transmission
The hierarchy of the transmission network

Backbone network refers to the network established between

the main nodes, with the backbone network in both the local
network and the metropolitan area network.
At present, there are many technical means of transmission
network, each has its own use, the content of this book
involves the transmission network only uses a wide range of
SDH and DWDM technology in the backbone network, they are
only the mainstream technology of backbone transmission, do
not represent the transmission of all, please pay attention
to readers, to avoid overgeneralization.
At present, Chinas SDH transmission network is divided into
four levels: provincial trunk line level, provincial trunk
line level, relay network level and user access network
level.
1.1.3 Transmission system model
Transmission system model

Users requirements for communication systems are diverse. For example, the
communication distance can be far and near, the information transmitted can be
voice, image or data, each relay segment can be divided or inserted information,
can be directly transferred, the road noise is large and small, and so on. Thus,
it is difficult to set a unified quality standard for different communication
lines.
In order to compare the performance of various communication equipment and
communication lines, a assumed communication line can be specified in advance,
and the communication equipment is installed on this line, and the transmission
quality of the line can be investigated. This communication line is usually
called the assumed reference channel HRP (Hypothetical Reference Path).
1.1.3 Transmission system model
Overall structure of the transmission lines

The line connecting the telecommunication bureau and the user

terminal equipment is called the user line, according to the
transmission signal, the user line can be an optical cable or cable.
1.1.3 Transmission system model
Overall structure of the cable transmission user line

Realize the construction of the whole cable network equipment model from the
wiring frame to the user cable box. That is, the end user through the user
introduction line —— cable box (or wire box) —— cable —— handover
box —— trunk wire (or feeder cable) —— distribution frame into the
bureau.
1.1.3 Transmission system model
distributing frame

Total remittance of inner and outer lines, connecting

outer core line and switch (connection function); set
up security unit to protect internal exchange
equipment and personal safety (security function);
have test plug hole for check and repair obstacles
and alignment, test (test function); and establish
jumper for line expansion, cutover and user change
(maintenance function).
1.1.3 Transmission system model
distribution box

The handover box is a typical wiring equipment in the user line network.
Its main function is to use the jumper to connect the main cable
(refers to the cable connecting the telecom end bureau and the handover
box) and the wiring cable (refers to the cable after the handover box),
so that the main line and the distribution line pair connect
arbitrarily through the jumper, so as to achieve the purpose of
flexible dispatching line pair.
1.1.3 Transmission system model
joint box

Branch box is a kind of wire equipment with security

device, installed on the dividing point or wiring point of
the cable network, used to communicate the core wire of
the wiring cable and the user terminal equipment (such as
telephone, etc.). The so-called "dividing point" and
"wiring point" refer to the points used by the
distribution of the dividing equipment to the nearest user.
Therefore, it is a kind of line terminal equipment
installed at the branch point or terminal of the line
network, through it, and then it can be connected to the
user with a soft leather wire.
1.1.3 Transmission system model
adapter junction box

The dividing box is a cable division of the device without

security device, its connection function is exactly the
same as the dividing box. There is a layer of wiring
terminal board made of transparent Plexiglass, and the wire
box is divided into inner and outer parts. When wiring, the
tail cable is connected with the terminal column in the
inner layer of the terminal board, and the outer layer is
connected with the leather wire. Using full chromatography
full plastic cable, it is easy to see the color of the
inner core line from the outer layer, bringing convenience
to the maintenance and construction.
1.1.3 Transmission system model
Overall structure of optical cable transmission user lines

Realize the construction of the whole optical cable network equipment model
from the ODF (optical distribution frame) to the optical terminal box. That is,
the end user through the optical terminal box- -user introduction line- -fiber
box (or fiber box) - -distribution optical cable- -optical handover box- -
trunk optical cable (or feeder optical cable) - -ODF.
1.1.3 Transmission system model
Composition of the optical fiber-optic transmission system

In the whole system and transmission process of communication transmission, the optical
transmission system is the most basic and important part, and it is also a system with the rapid
development of new technology. In the future technology development, the optical communication
system will play a major role in both the communication access network system, man or long-
distance wide area network.
The optical transmitter converts the electrical signal to be transmitted into the optical signal,
the optical receiver converts the optical signal into the original electrical signal, and the
optical fiber transmission part sends the optical signal emitted by the optical transmitter to
the optical receiver.
In the transmitting device, the photoelectric conversion device converts the digital pulse
electrical signal into the optical signal (E / O conversion), and sends it to the optical fiber
for transmission. At the receiving end, an optical signal detection device is provided that
converts the received optical signal into a digital pulse signal (O / E transformation). In the
way of transmission, when the distance is far, the optical relay equipment is used to transmit
the communication signal after regeneration processing.
1.1.3 Transmission system model
Composition of the digital optical fiber optic transmission system

The basic structure of the digital signal optical fiber transmission system is divided into the
following four parts: analog / digital signal conversion part (digital terminal), electrical /
optical signal conversion part (optical terminal), transmission optical cable, and optical
signal regeneration repeater.

The main function of the digital terminal is to convert various kinds of the user signals into
digital signals, and form a certain digital transmission structure (usually 2M PCM frame
structure) coding signal (usually HDB 3 code, etc.) through reuse equipment, and then send the
digital signal flow to the optical terminal.

The optical terminal machine transforms the digital signal sent by the digital terminal
machine again, mainly in the form of ordinary binary code (NRZ or RZ code), into the optical
fiber for long-distance transmission, to the receiving end of the opposite transformation.
1.1.3 Transmission system model
Composition of the digital fiber-optic transmission system

The optical terminal machine is mainly composed of optical transmission system, optical receiving
system, signal processing and auxiliary circuit.
In the optical transmission part, photoelectric conversion device is the core device of optical
transmission circuit. At present, there are mainly semiconductor laser (LD) and light emitting diode
(LED), which are responsible for converting digital pulse electrical signals into optical signals (E /
O conversion).
1.1.3 Transmission system model
Composition of the digital fiber-optic transmission system

In the optical reception part, the core optical detection device mainly includes
photodiode (PIN) and avalanche diode (APD), which transforms the received optical
signal into digital pulse electrical signal, that is, the optical signal into
electrical signal (O / E conversion).
The signal processing system is mainly to convert the digital pulse signal such as HDB
3 code sent by the digital terminal machine into the ordinary binary digital signal
encoded by NRZ or RZ, so that it can adapt to the needs of the signal conversion of
optical transmission.
Auxiliary circuit mainly includes alarm, official business, monitoring and interval
communication, and so on.
The function of optical regeneration repeater is to restore the optical pulse signal
again to achieve the purpose of extending the transmission distance.
1.1.3 Transmission system model
Composition of the digital optical fiber-optic transmission system

The signals transmitted by the optical transmission

equipment are mainly the voice service signal with
"synchronous digital series (SDH)" as the technical carrier
and the broadband Internet communication digital signal
characterized by high-speed IP / TCP and Ethernet data
information package. Among them, SDH optical transmission
system mainly uses terminal multiplexer (TM), partition
multiplexer (ADM) and digital cross connection device (DXC)
to build optical transmission network, while high-speed
Internet data information flow often uses optical fiber
transceiver as the common point-to-point optical
transmission equipment.
thanks