Network Principles (III)

Physical Layer
Definition of ITU in recommendation X.25:
Physical, electrical, functional, and procedural features are used to establish, maintain, and remove physical channels between DTE and DCE.
Here, DTE (data terminal equipment) refers to the data terminal device, which belongs to the userConnected device or WorkstationThey are the source or sink of communication. Such as computers and terminals;
DCE (Data circuit-terminating equipment or data communication equipment) refers to the final connection device or data communication device of the Data circuit. It is a collectively referred to as a network device that provides the network connection point for users, for example, an automatic call response device or a modem.

Physical LayerMechanical PropertiesThe geometric size of the plug and socket, the number of plug or Jack cores, the arrangement mode, and the locking device form are described in detail.
The electrical connections between the DTE and DCE interfaces are as follows:Non-balancing mode, using the non-balancing mode and balancing mode of the differential ReceiverThree.
The electrical characteristics of the physical layer also specify the electrical parameters of the DTE-DCE interface line, such as electrical signal, output impedance of the transmitter and input impedance of the receiver.
The features of the physical layer define the source and function of the interface signal and its relationship with other signals. The interface signal line can be divided into four types by function: data signal line, control signal line, timing signal line and ground line. The signal line name can be usedNumber, combination of letters or abbreviationsThree naming methods.
The physical layer's procedural features define the control steps for data exchange using Switching circuits. The application of these control steps enables the completion of bit stream transmission. In the physical layer, the most important new procedures are EIA RS-449 and X.21.
RS-232CIt is still the most commonly used computer asynchronous communication interface.

EIA RS-232C is one of the most widely used serial physical interface standards promulgated by the American Association of Electronic Industry EIA (Electronic Industry Association) in 1969. RS (rscommended standard) indicates the "Recommendation Standard", 232 indicates the identification number, and the suffix "C" indicates the number of times the recommendation standard has been modified.
The RS-232C standard provides a technical provision that uses a public telephone network as the transmission medium and connects remote devices through a modem.
The RS-232C standard interface can also be used to connect two low-Earth devices directly without a telephone line or a modem. Because the two devices must be paired with DTE and DCE to meet the requirements of the RS-232C standard interface, so in this case, a connection cable using a cross jumper signal line method, make the DTE connected at both ends of the cable through the cable to see the other side like DCE, so as to meet the requirements of RS-232C interface DTE-DCE pair. This connection cable is also called a zero modem.
Specification for Mechanical Properties of RS-232C25 CoresAnd the size of the connector and the arrangement position of the needle or the hole core are described in detail.
Some manufacturers for the RS-232C standard mechanical characteristics made flexible simplification, the use of9 CoresStandard connectors Discard unused signal lines.
The electrical characteristics of RS-232C are defined.The level of the logical "1" is-15 to-5 volts, and the level of the logical "0" is + 5 to + 15 volts., That is, the RS-232C uses +-15 of the negative logic level, between +-5 V for the transitional area is not defined.
The functional characteristics of the RS-232C define 20 signal lines in the 25-core standard connector, of which 2 ground lines, 4 data lines, 11 control lines, 3 timing signal lines, the remaining five wires are used for backup or undefined.

The RS-449 gives a detailed description of the standard connector, because there are many signal lines, using 37-core and 9-core connectors, 37-core connectors define all signals related to the RS-449, the signal of the secondary channel is defined in the 9-core connector.
There are two sub-Standards for the electrical characteristics of RS-449 standard, I .e., standard of balanced RS-422 and non-balanced RS-423.
RS-422 electrical standards are balanced type standards, its transmitters, receivers are balanced transmitter differential receiver, due to the use of completely independent dual-line balanced transmission, crosstalk resistance enhanced. The signal level is defined as the negative logic of +-6 Volts (+-2 volts as the transition region.
The RS-423 electrical standard is a non-balanced standard that uses a single-ended transmitter (I .e. a non-balanced TRANSMITTER) and a differential receiver. The signal level is defined as the negative logic of +-6 Volts (where +-4 volts is the transition region.
The standard of RS-423 electrical characteristics can be considered as a step in the whole transition from RS-232C to RS-449 standard.

The ITU 2.4 Recommendations for interface standards between DTE-DCE are 100 series, 200 series two. The 100 series interface is used as an interface between DTE and DCE (such as a modem) without automatic call devices. In the DCE (for example, network controller) of an automatic call device, interfaces between the DTE and the automatic call device are completed in the 200 Series Interface Standard. If the system uses manual calls, you do not need to set the 200 series interfaces.
The mechanical characteristics of the 100 series interfaces are defined in two standards. When the transmission rate is BPS ~ Standard 25-core connector is used for BPS; Standard 34-core connector is used for 48 kbps transmission rate.
The 200 series interfaces use 25-core standard connectors.
The standard electrical characteristics of the 100 series interfaces are recommended in V.28 and V.35. When the transmission rate is 200bps ~ 100 bps, v.28v recommended; when the transmission rate reaches 48 kbps, series in addition to the control signal still use V.28 recommendations, data lines and timing lines adopt V.35 recommendations. The electrical characteristics of the 200 series standard interfaces are recommended for V.28.

One of the design goals of X.21 is to reduce the number of signal lines. Its Mechanical Properties use a 15-core standard connector to replace the familiar 25-core connector, and eight interface lines are defined.

X.21 and X.21 bis define physical circuits for three types of services:Lease circuit (dedicated line) services, direct call service and device address call service. The lease circuit is designed as a continuous connection between two terminals. Direct call services like "hotlines" allows users to directly connect to the specified target at any time; A call from a device address is like a "dial-up" phone. You must specify a target for each connection.

The selection of transmission media depends on the following factors:The structure of the network topology, the actual communication capacity, reliability requirements, and affordable price range.

Data Information is transmitted through communication between computer components, between computer and various external devices, and between computer and computer. There are two basic communication methods,Serial mode and parallel mode.
Generally, the parallel mode is used for short-range communication, and the serial mode is used for long-distance communication.

Parallel Communication: The sending device transmits eight data bits to the receiving device through eight data lines, and can append one data check bit. The receiving device can receive the data at the same time and can be directly used without any changes. Data communication within a computer is usually performed in parallel. Parallel data transmission lines are also called bus.

Serial Communication Mode: When transmitting data in a serial mode, the 8-bit parallel data is first converted into a serial mode by a transmission device in a computer with an 8-bit bus, then, the device arrives at the receiving station through the transmission line, and re-converts the data from the serial mode to the parallel mode at the receiving end for the receiver to use.
The direction structure of serial communication: There are three kinds of direction structures for serial data communication:Work order, half duplex and full duplex.
In particular, there are three methods for using the call status and frequency in mobile communication:Single-work, half-duplex, and duplex. Because in wireless modeSingle Frequency and Dual Frequency.

We hope that a channel can transmit multiple signals at the same time. This is the so-called multiplexing technology. Multiplexing technology can be used to combine multiple signals for transmission over one physical channel, greatly saving cable installation and maintenance costs during distance transmission.
Frequency Division Multiplexing (OFDM) and time division multiplexing (TDM) are two of the most common Multiplexing technologies.
For optical fiber channels, a variant that does not use frequency division multiplexing is wavelength division multiplexing (WDM ).

Synchronous group transmissionProcess:
In a group synchronization communication system, the transmitted information is divided into several "groups ". The so-called "group" is generally measured in characters,Each character is prefixed with the start position and end position to form a character sequence.
During data transmission, characters can appear in bit streams sequentially, and the interval between characters is arbitrary, that is, asynchronous timing is used between characters, however, each bit in the character is transmitted at a fixed clock frequency.In data communication, Group Synchronization is often called "asynchronous transmission ". The asynchronous timing between characters and the synchronous timing between characters in bits are the characteristics of group synchronization, that is, asynchronous transmission.
In this transmission mode, each character is separated by the Start and Stop bits. Therefore, it is also called "start-stop" transmission.

Digital signal conversionProcess availabilityIncluding sampling, quantization, and encodingThree steps.
The first step is to sample and produce the analog signal value at the sampling frequency fs;
The second step is quantization to change the continuous analog signal to the discrete value on the timeline, that is, the grading process. The sampled value is "rounded up" to produce a discontinuous value.
The third step is encoding. discrete values are compiled into binary numbers of a certain number of digits.

Data Exchange Technology

During data transmission, a dedicated physical connection line between the source node and the target node is established between the source node and the target node until the data transmission ends. If the channel capacity between two adjacent nodes is large, multiple circuits can be reused between the two adjacent nodes. To use circuit switching technology to complete data transmission, you must go through the following three steps: Circuit establishment, data transmission, and circuit demolition:
Advantages:
Data transmission is reliable and fast, so data is not lost and the original sequence is maintained.
Disadvantages:
In some cases, the channel capacity is wasted when the circuit is idle. In addition, when the duration of the data transmission phase is not long, the time used for circuit establishment and demolition is not worth the candle. Therefore, it is suitable for situations where the system requires high-quality mass data transmission. The billing method of this communication mode is generally calculated based on the reserved bandwidth, distance, and time.

Packet Exchange Technology
The unit of data transmission in the message exchange mode is the message, and the message is the data block to be sent at one time by the site. Its length is not limited to variable. Message exchange does not require the establishment of a dedicated path between two sites. The transmission mode is "Storage-forwarding.
Compared with circuit switching, packet switching has the following advantages:
1. High circuit utilization efficiency.
2. In a circuit switching network, new calls cannot be accepted when the traffic increases. In the Packet Exchange Network, messages can still be received when the traffic is large, but the transmission delay will increase.
3. The packet switching system can send a single packet to multiple destinations, which is difficult for the circuit switching network.
4. The packet switching network can convert the speed and code.
Disadvantages:
It cannot meet the requirements of real-time or interactive communication, and the delay of packets passing through the network is long and variable. This method cannot be used for inter-language connection, nor is it suitable for connection from an interactive terminal to a computer.
Sometimes when a node receives too much data without storage space or cannot forward it in time, it has to discard the packet, and the sent packets do not arrive at the destination in order.

Group exchange
Limited-length grouping reduces the storage capability required by each nodeGroups can be stored in the memory instead of on the hard disk, improving the switching speed. Group exchange is applicable to interactive communication, such as communication between a terminal and a host. The specific process of group switching can be divided into two types: virtual circuit group switching and datagram grouping.
The main feature of the virtual circuit technology is to establish a path between the station and the station before data transmission. Note that this is not to say that it looks like a circuit switch to find a specific path, grouping on each node still needs to buffer, and queuing for output on the line.

Virtual circuit and DatagramMake a comparison:
Virtual circuit group switching is suitable for long-time data exchange between two ends. Especially when the data transmitted each time in an interactive session is very short, the additional overhead of address information is not required for each group. It provides a more reliable communication function to ensure that each group arrives correctly and maintains the original order. You can also control the traffic of two data endpoints. When the receiver cannot receive data, it can notify the sender to suspend the sending group.
The virtual circuit has a weakness. When a node or link fails, all virtual circuits that pass through the fault point will be destroyed immediately.
The datagram group exchange eliminates the call establishment stage. It transfers a small number of groups more easily and flexibly than the virtual circuit. In the datagram mode, the group can reach the destination without bypassing the fault zone, so the impact of the fault is much smaller than that of the virtual circuit. However, the datagram does not guarantee that the packets arrive in order, and the loss of data is not immediately known.

Summary The main features of one of the three exchange technologies:
1. circuit switching. A dedicated path must be set before data transmission starts. Before online release, the route is completely occupied by one user. For burst communication, the circuit switching efficiency is not high.
2. Packet Exchange. The message is transmitted from the source point to the destination using the "Storage-forwarding" method. During packet transmission, only one channel is used at a time. In the exchange node, data needs to be buffered and stored, and packets need to be queued. Therefore, message exchange cannot meet the requirements of real-time communication.
3. group exchange. The exchange mode is similar to the packet exchange mode, but the packet is divided into groups for transmission, and the maximum length of the group is specified. In the packet group exchange, the destination needs to re-assemble the packet. In the virtual circuit group exchange, a virtual circuit must be set through the virtual call before data transmission.Group Exchange TechnologyIt is the most widely used exchange technology in computer networks.