Chapter 4 Basic Principles of Wan

Chapter 4 Basic Principles of Wan
Wan and OSI reference model Wan mainly correspond to the physical layer and data link layer of the OSI reference model, that is, the network access layer of the TCP/IP model. The WAN physical layer specifies the physical and electrical properties, mechanical properties, and connection standards of devices, lines, and interfaces that provide services to the WAN. Common standards are as follows: V.24 and V.35 for synchronous and asynchronous modes, G.703 For E1/T1 lines, and E1 for Eurasia, t1 is mostly used in North America; X.21 is used to provide serial communication on synchronous digital lines. It is mainly used for data transmission in Japan and Europe over the wide area network, it must be encapsulated into a data link layer protocol that can be recognized and supported by the WAN. Common data link layer protocol (HDLCPPPLAPB) for Wide Area Network (WAN) Connection Methods Common WAN connection methods include leased line, circuit switching, and group switching. Leased line: the user excludes a permanent, point-to-point, and fixed rate dedicated line, and exclusive bandwidth. Circuit Switching Mode: the connection between user devices is established on demand. When the user needs to send data, the carrier switch connects a physical data transmission path between the caller and the called end. When the user does not send data, the carrier switch disconnects the transmission path. Group switch: This is an exchange method based on the carrier's group switch network. The User device divides the information to be transmitted into a certain-length group (packet) and submits it to the carrier group switch. Each group contains the address ID of the recipient and the sender, the carrier group switch forwards the group to the destination user based on these address IDs. Both the leased line and circuit switching modes belong to the point-to-point mode. Group switching can achieve point-to-point communication. The point-to-point WAN technology introduces the Leased Line connection model. In the Leased Line connection model, the carrier configures a dedicated communication Line for the user through the transmission equipment and transmission lines in the communication network. The serial interfaces of user routers at both ends are connected to CSU/DSU through a local cable of several meters to more than 10 meters, while CSU/DSU is connected to the carrier's transmission network through a connection line of several hundred meters to the nearest kilometer. I usually use v.24, v.35, and other serial cables, and the access line is usually a traditional twisted pair cable. A leased line can be digital, for example, a digital transmission channel that directly uses the carrier's telephone network, or a simulated one. For example, a pair of telephone copper wires are directly used to connect two ends by the carrier. The serial line signal of the router must undergo modulation and conversion from the CSU/DSU device to be transmitted over the leased line. CSU connects the end user to the digital interface device connected to the Bundi digital telephone loop, while DSU adapts the physical interface on the DTE device to the communication network. DSU is also responsible for signal clock and other functions. It is usually mentioned with CSU, called CSU/DSU. the physical interfaces of communication devices can be divided into DCE and DTE: DCE (Data Circuit-terminating Equipment Data Circuit Termination Device): the DCE device provides interfaces for the client to receive network communication services, it also provides clock signals used to synchronize data transmission between the DCE device and the DTE device. DTE (Data Terminal Equipment Data Terminal device): a device that receives a line clock and obtains network communication services. A dte device is usually connected to a transmission line through CSU/DSU and uses the clock signal provided by it. In the leased line model, the line rate is determined by the carrier. Therefore, the CSU/dsu dce device is responsible for sending clock signals to the DTE device and controlling the transmission rate. The user router is usually a DTE device, accept the services provided by the DCE device. In the leased line mode, the user excludes a permanent, point-to-point, and fixed rate dedicated line, and exclusively enjoys its bandwidth. This method is easy to deploy and reliable for communication. It provides a wide range of bandwidth and low transmission latency. However, its resource utilization is low, expensive, and the point-to-point structure is not flexible enough. Circuit Switching connection model the user router connects to CSU/DSU through the serial port cable, and CSU/DSU connects to the carrier's Wan switch through the access line to access the circuit switching network. The most typical circuit switching networks are Public Switched Telephone networks and ISDN (Integrated Service Digital Network, this system uses circuit switching technology to allocate a dedicated voice channel for each call. The voice is transmitted on the PSTN user's return path in analog form, finally, digital signals are transmitted remotely on the carrier's trunk line. The router connects to the PSTN access line through a MODEM-Demodulator (MODEM)-a common telephone line. PSTN is almost everywhere in the office space. It has the advantage of low installation cost, wide distribution and easy deployment. The disadvantage is that the maximum bandwidth is only 56 Kbps, and the signal is vulnerable to interference. ISDN: A digital communication network connected by dialing. ISDN transmits user data through a dedicated B channel through an independent D channel. Isdn bri provides 2B + D channels, with each B channel having a rate of 64 Kbps and a maximum rate of 128 Kbps. ISDN T1 Pri provides 23B + D, while ISDN E1 PRI provides 30B + D channels. The router connects to the ISDN network through an independent or built-in TA (Terminal Adaptor Terminal adapter. ISDN has the advantages of fast connection, reliable transmission, and high bandwidth. ISDN calls are slightly higher than normal calls, but its dual-B channel enables it to support two independent applications at the same time. It is a suitable network access method for individuals or small offices. In the circuit switching mode, the connection between user devices is established as needed. When the user needs to send data, the carrier's Wan switch connects a physical data transmission path between the caller and the called. When the user does not send data, a wan switch disconnects the transmission path. The circuit switching method is suitable for temporary and low-bandwidth communication, which can reduce the cost. The disadvantage is that the connection delay is large and the bandwidth is usually small. Common interfaces and cables
The visible part usually includes the serial port, serial port cable, CSU/DSU, access cable and connector. Vrouters support many WAN interface types, including asynchronous serial port, AUX interface, AM interface, FCM interface, same asynchronous serial port, isdn bri interface, CE1/PRI interface, CT1/PRI interface, S13. CT3 interface, ATM interface, etc.. Serial Port is the most basic and commonly used one. A router is usually connected to a WAN through a serial port to receive WAN services. Serial ports can work in two ways: asynchronous and synchronous. Some serial ports support both asynchronous and synchronous modes. Synchronous serial ports can work in the DTE and DCE modes. Generally, synchronous serial ports are in the DTE mode. Asynchronous serial ports can work in Protocol mode and stream mode. When the asynchronous serial port is external to Moderm or isdn ta, it can be used as a dialing interface. In Protocol mode, the link layer protocol can be PPP. The serial port of the router is connected to CSU/DSU through the serial port cable. One end of the serial cable matches the serial port of the router, and the other end matches the CSU/DSU interface. Common Serial cable standards include V.24, V.35, X.21, RS-232, RS-449, RS-530, etc. Depending on the physical interface, cables are also divided into DTE and DCE. The router uses the DTE cable to connect to CSU/DSU. The device can automatically detect the external cable type of the synchronous serial port and select the electrical characteristics. Generally, no manual configuration is required. CSU/DSU is connected to the carrier network through an access cable. The end of this cable is usually shielded or unshielded twisted pair wires, and the CSU/DSU is usually inserted as a RJ-11 or RJ-45 connector. Common router serial cable: V.24 V.35 cable ISDN BRIX.21RS-449RS-530 link layer protocol in point-to-point connection, link Layer Protocols are usually used between end systems. Common link layer protocols include HDLC, PPP, SLIP, SDLC, etc. The wide area network technology is introduced in the packet switching mode, the user router connects to the carrier group switch through the access line. The carrier group switch network is responsible for creating point-to-point Virtual circuits (Virtual Circuit VC) on demand or permanently ). Each user router can use a physical interface to connect to multiple peer routers through multiple virtual circuits. The User device divides the information to be transmitted into a certain-length group (packet) and submits it to the carrier group switch. Each group contains the address ID of the recipient and the sender, the carrier group switch forwards the packets to the target user device through virtual circuits based on these address identifiers. The user's access line uses the same connection method as the synchronized leased line, and works in the same way as the point-to-point synchronous leased line. The physical layer you see is no different from the point-to-point synchronization mode. The user router can be considered to be connected to the group switch through a synchronous leased line. This method features flexible structure and convenient migration, and the cost is lower than that of the leased line. The disadvantage is that the configuration is complicated and the transmission latency is large. Common group switches include Frame Relay and Asynchronous Transfer Mode Asynchronous transmission ). The typical technology used in group switching includes X.25, frame relay, and ATMX.25. The built-in error correction, traffic control, and packet loss retransmission mechanisms make it highly reliable and suitable for long-distance high-noise lines. However, the secondary effect is slow speed, low throughput, and high latency. The maximum rate of early X.25 was only a limited 64 Kbps, making it very limited to provide services. Frame Relay ATM