Computer Network Technology and Application

Chapter 7 Fundamentals of Network Technology (2)
1. Frequency Division Multiplexing
Frequency Division Multiplexing (FDM) is to take different carrier frequencies for multiple signals and allocate a certain bandwidth so that they each form a channel (Channel).These channels each occupy a frequency band of the transmission line and do not overlap each other.In this multiplexing method, each user only occupies a part of the bandwidth of the line, but can transmit at the same time all the time.

2. Time division multiplexing
Time Division Multiplexing (TDM) is to divide a physical channel into several time slices according to time, and allocate them to multiple signals in turn.Each time slice is occupied by a signal.In this way, multiple signals can be transmitted on one physical channel by using the time crossing of each signal.

Time-division multiplexing TDM is not limited to the transmission of digital signals, but can also cross-transmit analog signals at the same time.

3. Wavelength division multiplexing
Wavelength Division Multiplexing (WDM) refers to a technology that uses different wavelengths to simultaneously transmit multiple optical signals on one optical fiber.

The principle of WDM is basically the same as FDM, the difference is that WDM is applied to the optical wave transmission process on the fiber channel, while FDM is applied to electrical analog transmission. The WDM optical fiber system has high reliability, and the carrier frequency of each WDM optical fiber channel is a million times that of the FDM carrier frequency.

WDM generally uses a wavelength division multiplexer and a demultiplexer (also called a multiplexer/demultiplexer), which are placed at both ends of the optical fiber to realize the coupling and separation of different light waves.

The wavelength division multiplexer is a technology that connects multiple different single-wavelength optical fiber signals on the terminal equipment to a single optical fiber channel, and supports the transmission of 2 to 4 wavelengths on each optical fiber channel.A wavelength division multiplexer decomposes a multi-wavelength data stream into multiple single-wavelength data streams.

2.1.7 Data Exchange Technology

Large-scale networks generally use point-to-point channels, and point-to-point channels use the store-and-forward method to transmit data, that is to say, the data communication from the source node to the destination node needs to be forwarded through several intermediate nodes.The technology used for this data forwarding is the data exchange technology.

There are three main types of data switching technologies: circuit switching, message switching, and packet switching.

1. Circuit Switching

The concept of switching originated from the telephone system.When the user dials, the switch (Telephone Switch) in the telephone system establishes an actual physical line between the caller's phone and the receiver's phone (this physical line may include twisted pair, coaxial cable, optical fiber or various media including wireless circuits, or bandwidth obtained through multiplexing).It establishes the call, after which the phones on both ends own the dedicated line until the call ends.The exchange referred to here is embodied within the telephone exchange.

2. Message Switching

The data transmission unit of the message exchange mode is a message, and its length is unlimited and variable.

When a station wants to send a message, it attaches a destination address to the message; the network node sends the message to the next node according to the destination address information on the message, and then from this node to the other node , until it is forwarded to the destination node.After each node receives the entire message and checks it is correct, it temporarily stores the message, then uses the routing information to find out the address of the next node, and then transmits the entire message to the next node.

This technique above is called store-and-forward.There is no prior physical circuit established between the source and destination.

3. Packet switching
Packet Switching technology is an improvement of packet switching technology.

Packet switching is also called packet switching technology. It first divides the data to be transmitted into data blocks of a certain length, and each part is called a packet.After each packet is marked, it uses dynamic multiplexing technology on a physical line and transmits it at the same time.Temporarily store the data from the source in the memory of the switch, and then forward it to the following nodes.After each packet arrives at the destination, remove the packet header and reassemble each data field into a complete message in order.

Packet switching decomposes longer messages into a series of message packets, and uses the "store-and-forward" switching method to communicate in groups.

(1) virtual circuit
In virtual circuit packet switching, in order to carry out data transmission, a logical path must be established between the source node and the destination node of the network.In addition to containing data, each packet also contains a virtual circuit identifier, indicating the logical connection (path) used to transmit this packet.

Each node on the pre-established logical path knows where to send these packets, no routing is required.During the transmission of the packet, the logic circuit is fixed.

Finally, a station ends the connection with a clear request packet.

There are two types of virtual circuits: switched virtual circuits (SVC) and permanent virtual circuits (PVC).

(2) Datagram
Each datagram itself carries sufficient address information.When a node receives a datagram, it finds a suitable way out according to the address information in the datagram and the routing information stored by the node, and sends the datagram to the next node.

Since the paths taken by each datagram are not necessarily the same, there is no guarantee that each datagram will arrive at the destination in order, and some datagrams may even be lost halfway.

During the entire transmission process, no virtual circuit needs to be established, but routing must be made for each datagram.

2.1.8 Channel Specifications

The performance of the channel directly affects the performance of the network. To evaluate the quality of a channel requires a series of evaluation indicators.In section 2.1.2, the signal types transmitted by channels have been introduced, and channels can be divided into analog channels and digital channels.

An analog channel refers to a channel for transmitting analog signals, and a digital channel refers to a channel for transmitting digital signals.To measure the quality and performance of a channel, the following channel technical indicators are currently mainly referred to.

1. Data and signal transmission rate
(1) Data transmission rate
The number of bits of binary data transmitted per second, the unit is bit/second, or bps, which can also be written as b/s.

(2) Signal transmission rate
The number of symbols transmitted through the channel per unit time is the number of times the modulated analog signal changes per second, and the unit is Baud. This rate is also called the baud rate.

【例21】　采用四相调制方式，即N=4，且T=833×10-6秒，则S=1／T· log2N=1／（833×10-6）·log24=2400（bps）

B=1／T=1／（833×10-6）=1200（Baud）

2. Channel capacity
Channel capacity represents the maximum data transmission rate of a channel, which is a limit parameter, and the unit is bit/second (bps).The difference between channel capacity and data transmission rate is: the former indicates the maximum data transmission rate of the channel, which is the limit of the channel's data transmission capacity; the latter is the actual data transmission rate.

3. Nyquist criterion
In 1924, Nyquist (Nyquist) derived the Nyquist criterion, which is used to calculate the limit transmission speed of the communication system.The relationship between the Nyquist (Nyquist) symbol rate limit value B and the channel bandwidth (frequency range) W under no noise is as follows:
B=2W(Baud)=2Wlog2N(bps)

In the formula: W is the bandwidth of the channel, that is, the difference between the upper and lower limit frequency of channel transmission, in Hz; N is the number of discrete values ​​taken by one symbol; the bandwidth of the channel is proportional to B.

【例22】　普通电话线路带宽约3kHz，则码元速率极限值为B=2×W=2×3k＝6kBaud；若码元的离散值个数N=8，那么最大数据传输速率C=2×3k×log28=18kbps。

4. Shannon formula
In 1948, Shannon (Shannon) studied the channel capacity formula with noise.

【Example 23】 Given that the signal-to-noise ratio is 30dB and the bandwidth is 6kHz, find the maximum data transmission rate of the channel.

因为10lg（S／N）=30，推出S／N=1030／10=1000
所以C=6klog2（1＋1000）≈30k（bps）

5. BER

The bit error rate (Pe) is the ratio of errors in the transmission of binary data bits, and it is an indicator to measure the transmission reliability of the data communication system under normal working conditions.Bit error rate formula:

Pe=Ne/N
In the formula: Ne is the number of error bits; N is the total number of data bits transmitted.

【Example 24】 It is known that the information rate of a binary digital communication system is 3000b/s, and a total of 10 error symbols have occurred within 18 minutes at the receiving end. Try to find the bit error rate of the system.

依题意得出系统误码率=18／（3000×10×60）=1×10-5
2.2 Transmission medium
The transmission medium is the medium for data transmission, there must be a transmission medium between the computer and the transmission equipment, and between the transmission equipment and the transmission equipment.Transmission media are usually divided into wired media and wireless media.The wired medium confines the signal within a physical conductor, such as twisted pair, coaxial cable, and optical fiber; the wireless medium cannot confine the signal within a certain spatial range.

2.2.1 Wired media
1. Twisted pair

Twisted pair (TP) is currently the most widely used and relatively cheap transmission medium.

(1) Physical structure of twisted pair

Twisted pair is composed of two mutually insulated copper wires. The typical diameter of the wire is 1mm. The two wires are twisted together to reduce electrical interference to adjacent wire pairs.

A cable made up of several twisted pairs is called a twisted pair cable.Twisted pairs can be placed side by side in the protective sleeve.

(2) Features of twisted pair

Twisted pair cables can transmit both analog and digital signals.When digital signals are transmitted over twisted-pair wires, the data transfer rate is related to the length of the cable.From a few meters to a few kilometers, the data transmission rate can reach 1000Mbps to 10Gbps, and the short-distance transmission can even reach more than 10Gbps.There are the following two types of twisted pair cables.

①UTP（非屏蔽双绞线）：其特点是误码率为10-5～10-6，速率为1～155Mbps，100m／段。

②STP（屏蔽双绞线）：其特点是误码率为10-6～10-8，速率超过500Mbps，100m／段。

The technology and standards of twisted pair are relatively mature, the price is relatively low, and the installation of twisted pair cable is relatively easy.

The biggest disadvantage of twisted-pair cables is that they are sensitive to electromagnetic interference, and twisted-pair cables cannot support very high-speed data transmission.

(3) Application of twisted pair

At present, twisted pair cables are widely used in telephone systems and LAN indoor wiring.The signal transmitted in the twisted pair does not need to be amplified within a few kilometers, but an amplifier must be used when the transmission distance is relatively long.In the experiments in this chapter, twisted-pair cables will be made.

(4) The standard of twisted pair

There are two main standards for twisted pair cables:
① TIA (Telecommunication Industry Branch) of EIA (Electronics Industry Association), also known as EIA/TIA. EIA is responsible for the "Cat" (ie "Category") series of unshielded twisted pair (Unshielded Twisted Pair, UTP) standards.Among them, Cat 1 is suitable for telephone and low-speed data communication; Cat 2 is suitable for ISDN and T1/E1, supporting data communication up to 16MHz; Cat 3 is suitable for 10Base T or 100Mbps 100Base T4, supporting data communication up to 20MHz; Cat 5 is suitable 100Base TX and 100Base T100 at 4Mbps, supporting data communication up to 100MHz.

②IBM, IBM is responsible for the "Type" series of shielded twisted pair standards, such as IBM's Type 1, Type 2, etc.

2. Coaxial cable
The common coaxial cable (Coaxial Cable) is a TV antenna. The difference from the coaxial cable used in the network lies in the resistance.

The outer conductor is a circular empty tube braided by metal wire, and the inner conductor is a circular metal core wire.

The baseband coaxial cable with a resistance of 50Ω is used in local area networks (it has gradually withdrawn from the market), and the other is a broadband coaxial cable with an impedance of 75Ω, which is widely used in TV antennas.

3. Optical fiber
The optical transmission system consists of three parts: optical fiber transmission medium, light source and detector.

(1) The optical fiber transmission medium is ultra-fine glass or fused silica fiber, the appearance of light pulse means "1", and the absence of light pulse means "0".

(2) The light source is a light emitting diode (Light Emitting Diode, LED) or a laser diode.Both types of diodes emit light pulses when energized.

(3) The detector is a photodiode which, when exposed to light, generates an electrical pulse.

Installing an LED or laser diode at one end of the fiber and a photodiode at the other end constitutes a unidirectional data transmission system.

(End of this chapter)