The Wide Area Network (WAN) is a cross-regional data communication network that uses infrastructure provided by telecom operators as the underlying platform for information transmission. Compared to the OSI reference model, WAN technologies primarily operate at the lower three layers: the physical layer, data link layer, and network layer. These layers are responsible for the actual transmission of data across long distances, ensuring connectivity between different geographical locations.
**First, PSTN (Dial-Up Internet)**
PSTN, or Public Switched Telephone Network, provides an analog private channel that connects multiple telephone exchanges. When two devices, such as hosts or routers, need to communicate through the PSTN, they must both have a Modem on the user side. The Modem is responsible for converting analog signals to digital and vice versa. From the perspective of the OSI model, PSTN functions as an extension of the physical layer, without offering features like flow control or error correction. Additionally, since PSTN uses circuit switching, a dedicated path is established between the two ends, even if no data is being transferred. This can lead to inefficient use of bandwidth, as the connection remains active throughout the session.
An example of network interconnection via PSTN involves connecting two local area networks (LANs). Each LAN has a router with a serial port connected to a Modem, which in turn connects to the PSTN. This setup enables communication between the two LANs over the PSTN.
**Second, ISDN (Integrated Services Digital Network)**
ISDN evolved from the Integrated Digital Network (IDN) to offer end-to-end digital connectivity, supporting a wide range of services including voice and non-voice communications. It provides users with a versatile interface to access the network, integrating various services like telephone, fax, and data communication over a single line.
ISDN not only supports traditional phone calls but also offers advanced features such as video conferencing, data transfer, and image transmission. By combining these services into one digital network, ISDN is named "Integrated Services Digital Network." Its digital nature allows for higher quality and faster transmission of voice and data over standard copper wires.
Compared to ADSL and LAN access, ISDN offers broader coverage and was once considered the best option for high-speed internet access before broadband became widespread. At 128 kbps, it's significantly faster than dial-up, and it charges based on usage time, making it more cost-effective for users who spend less than 20 hours online per month. Additionally, ISDN lines are ideal for VoIP and other digital communication services due to their stability and clarity.
ISDN is particularly popular in Europe and has been used in mobile standards like GSM for data transmission. As a fully digital network, it ensures stable performance and is better suited for handling complex digital services compared to analog lines. For instance, it supports not just basic calling but also video, images, and data transmission.
In addition to its role in telecommunications, ISDN can also refer to a set of protocols used for managing and terminating circuit-switched connections. It also stands for isosorbide dinitrate, a medication used in medicine.
Key advantages of ISDN include:
1. **High reliability and quality:** Since the communication channel is fully digital, there is minimal noise, crosstalk, and signal distortion, resulting in superior call quality.
2. **Ease of use:** The network separates the information and signaling channels, allowing up to eight devices to be connected simultaneously on a single line.
3. **Cost-effective:** ISDN offers efficient use of network resources, making it a practical solution for businesses and individuals seeking reliable connectivity.
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