The WAN is a cross-regional data communication network that uses equipment provided by telecom operators as an information transmission platform. Compared with the OSI reference model, the WAN technology is mainly located at the bottom three levels, namely the physical layer, the data link layer and the network layer.
First, PSTN (dial-up Internet)The PSTN provides an analog proprietary channel that is connected between several telephone exchanges. When two host or router devices need to connect through the PSTN, they must be used on the network access side (ie, the user loop side) at both ends.
The PSTN modem (Modem) implements analog-to-digital, digital-to-analog conversion of signals. From the perspective of the OSI seven-layer model, PSTN can be seen as a simple extension of the physical layer, without providing users with services such as flow control and error control. Moreover, since PSTN is a circuit-switched way, a path can be used by devices at both ends of the path from setup to release, even if there is no data to transfer between them. Therefore, this circuit switching method cannot achieve full utilization of network bandwidth. Example of Network Interconnection Through PSTN The following figure is an example of a network interconnection that connects two LANs through the PSTN. In these two local area networks, there is one router, each router has a serial port connected to the Modem, and the Modem is connected to the PSTN, thereby realizing the interconnection of the two LANs.
ISDN is a network that evolved from IDN to provide end-to-end digital connectivity to support a range of services, including voice and non-voice services, providing users with a versatile standard interface to access the network. The integration of communication services is to provide a variety of services such as telephone, fax, viewable text and data communication by using a subscriber line.
In addition to being used to make calls, the Integrated Services Digital Network can also provide services such as video telephony, data communication, and conference television to integrate services such as telephony, fax, data, and images into a unified digital network. Transmission and processing, which is the origin of the name "Integrated Services Digital Network".
Since ISDN has a much wider range of access than ADSL and LAN access, ISDN seems to be the only solution for high-speed Internet access without broadband access. After all, 128kbps is much faster than dial-up. ISDN charges the same time as the phone, so for some users who have less time on the Internet (such as users under 20 hours per month), it is much cheaper than using ADSL. In addition, since the ISDN line is a digital line, it is much better to use it to make calls (including VoIP) than regular phones.
It transmits voice and data at a higher rate and quality over ordinary copper cables. ISDN is a popular form of telephone network in Europe. The GSM mobile phone standard can also transmit data based on ISDN. Because ISDN is a fully digital circuit, it provides stable data services and connection speeds, unlike analog lines. It is easier to carry out more digital information services on the digital line that are not analogous or difficult to prove quality. For example, in addition to the basic calling features, video, image and data services are also available. ISDN requires a fully digital network to carry digital signals (only 0 and 1), the biggest difference from ordinary analog phones is here.
In addition, ISDN also refers to the use of this technology to establish a protocol group for maintaining and disconnecting circuit switching or an abbreviation for isosorbidedinitrate isosorbide dinitrate.
(1) Achieve high reliability and high quality communication. Since the channel between the terminal and the terminal is completely digitized, the noise, crosstalk, and signal fading distortion are very small due to the increase in the distance and the number of links, so the communication quality is high.
(2) Easy to use. The information channel and the signal channel are separated. 8 terminals can be connected to a subscriber line of about 2B+D, and 3 stations can work simultaneously.
(3) Low cost.
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