Wan ???

Wan Wide Area Network is a computer communication network established in a wide range. A wide range refers to a geographical range that can surpass a city, a country, or even a whole world. Therefore, communication requires a high level of complexity. Wan. In practical applications, a wan can be connected to a LAN, that is, a LAN can be a terminal system of a wan. To organize a wide area network, you must follow a certain network architecture and corresponding protocols to achieve interconnection and collaboration between different systems.

Wan-Main Features

(1) It mainly provides communication-oriented services and supports remote information exchange by users using computers;

(2) wide coverage and long communication distance. factors that need to be considered increase, such as media costs, line redundancy, media bandwidth utilization, and error handling;

(3) The Telecommunications Department or company is responsible for the establishment, management and maintenance, and provides communication-oriented paid services, traffic statistics and billing to the whole society.

Wan-Wan Link

WAN links are divided into two types: one is physical connection and the other is exchange connection.

 

A leased line is a permanent point-to-point service. It is often used to provide core or backbone connections for important companies and schools.

Switching connections can be divided into three types: Circuit Switching, packet switching, and Element Switching.

1)Circuit SwitchingCircuit Switching is a type of switching in the WAN. A dedicated physical circuit can be maintained and terminated for each session through the operator network. Circuit Switching can also provide two transmission modes: datagram and data stream. Circuit Switching is widely used in the network of telecom operators. Its operation process is very similar to that of ordinary telephone calls. Integrated Service Digital Network (ISDN) and DDR are wide area network technologies that adopt circuit switching technology. The circuit switching technology is as follows:

2)Packet ExchangePacket Switching is also a frequently used exchange technology on the Wide Area Network. Through packet switching, network devices can share a point-to-point link and transmit data packets between devices through the carrier network. Packet Switching mainly uses the statistical multiplexing technology to achieve circuit sharing among multiple devices. Frame Relay, smds (Return-type multi-region data service), and X.25 are all WAN technologies using the packet exchange technology.

3)Cell SwitchWhat is "cell "? Divide the digital voice, data, images, and other information into several fixed-length groups, which are called cells. Each cell has a fixed length of 53 bytes, which is divided into two parts: header and information field. The header length is five bytes. In the front of the cell, the content is various control information, mainly the destination address and so on. The length of the information field is 48 bytes, it is behind the letter header, and the content is the information that the user wants to transmit. All kinds of information to be transmitted, including voice, data, and images, must be first divided into tokens. All user-sent cells are pooled into a buffer storage, where they are queued for transmission. Transmission adopts the "Statistics reuse" and "pass without blank" method (see "Statistics reuse ). No matter which user sends the information, it enters the same queue, so that any business can use channel resources as needed, maximize the use of channel resources, and truly achieve business integration.

ATM is a type of cell exchange. Asynchronocis transfer mode (asynchronocis) is also called Asynchronous transfer mode (ATM. The basic feature of ATM is that information transmission, reuse, and exchange are all based on "cells.

Wan-Key Technologies

Key technologies such as SONET, frame relay, X.25, ATM, and PPP can often be found in the wide area network.

ATM: a dedicated connection exchange technology that encapsulates digital information into several units in units of 53 bytes. A single packet unit is not processed with other related packet units at the same time. They are in the queue before being transferred along the communication line. The bandwidth of the ATM network can reach 10 Gbps.

Frame Relay (FR) is a high-speed Packet Exchange data communication service, similar to X.25. Frame Relay is widely used in LAN connection services between LAN, and is also suitable for the rapid development of LAN environments.

Synchronous Optical Network (SONET)/Digital hierarchical network (SDH): an international standard for high-speed communication using optical fiber networks. SONET is able to establish network communication of optical media (OC) level, bandwidth between 51.8 Mbps and 10 Gbps (OC-192), or higher. Digital classification network is a product of Europe and is opposite to SONET.

X.25: The X.25 protocol allows computers on different public networks to communicate at the network layer using intermediate computers.

Peer-to-Peer protocol (PPP): point-to-point connection provides a single Wan communication line from the user's house to the remote network, this line is pre-established and transmitted through media, such as by telephone companies. Point-to-point connection lines are often rented from media companies, so they are also called rental lines. Media companies need to allocate dedicated twisted pair wires and hardware devices to users' point-to-point connections.

Exchange-type multi-region Data Service (SMDS): A broadband technology developed by Bellcore. It is based on the IEEE 802.6 distributed Arrangement Dual-bus (DQDB) technology. SMDS can use optical fiber or copper media. It supports bandwidth of 1.544 Mpbs of DS-1 or 44.736 Mbps of DS-3.

Wan-Access Mode

1. Public switching Telephone Network (PSTN)

Analog dial-up service is a circuit switching service based on standard telephone lines. It is the most common transmission service and is often used as a method for connecting to remote ripple. Typical applications include: the remote connection is interconnected with the local LAN, and remote user dial-up access is used as a backup line for dedicated lines.

Because analog telephone lines are optimized for voice frequency (30-4000Hz), the speed of data transmission over analog lines is limited to 33.4 kbit/s, in addition, the quality of analog telephone lines is good or bad. The communication quality of analog telephone lines in many places cannot be guaranteed, and the existence of line noise will directly affect the data transmission rate.

2. Integrated Service Digital Network (ISDN)

The Integrated Service Digital Network (ISDN) provides end-to-end digital communication lines. Currently, ISDN has two types of interface standards: Basic Rate Interface (BRI) and basic group Rate Interface (PRI ). The Basic Rate Interface (BR) provides 2B + D digital channels. Two B channels (each B channel is 64 kbit/s) are bearer channels for data transmission between the two ends, channel D (16 kbit/s) is a control channel used to transmit call control protocol packets between users and ISDN exchange nodes. One group has two rate standards. One is equivalent to the transmission rate of the E1 line, which is 31 B channels, and the other corresponds to the transmission rate of the T1 line, which is 24 B channels, one of the B channels is used as the signaling transmission channel, which is equivalent to the D channel of BRI. The connection between the ISDN User end and the ISDN switching node also uses a common Stranded Wire. Therefore, when you want to change the analog telephone line to an Integrated Service Digital Network (ISDN) line, you do not need to re-lay the user line.

Although both the analog dialing service and the ISDN service belong to the Circuit Switching Service, there is a big difference between the two. Because ISDN provides a digital channel between the end and end, the transmission rate is 2 × 64kbit/s (BRI), and voice, data, and image information can be transmitted through digital channels. Because digital signals are transmitted, shaping and regeneration of signals will not introduce noise, which will make the transmission quality of ISDN lines much higher than that of ordinary analog telephone lines.

Because of the high-speed, high-reliability, fast call connection, and the same user line as the analog dialing service, more and more ISDN lines are used by users to connect to remote ripple.

3. X.25

X.25 is currently the most widely used protocol standard and has been used as the interface standard between the user network and the group switching network for many years. The Group switching network dynamically allocates bandwidth to user-transmitted information flows, effectively solving the transmission problem of abrupt and large information flows, the group exchange network can encrypt the transmitted information and effectively control errors. Although various error detection and mutual validation responses waste some bandwidth and increase packet transmission latency, for physical transmission lines with poor early reliability, it is an effective way to improve the reliability of message transmission. However, as optical fibers become more and more widely used as transmission media, the probability of transmission errors is getting smaller and smaller. In this case, repeated error control at the link layer and network layer not only seems redundant, but also wastes bandwidth, increase message transmission latency.

Because the X.25 packet switching network was low in the early days. The physical link with a high error rate is developed, and its features are no longer suitable for the current high-speed remote connection requirements. Therefore, it is generally only used to require less transmission costs, however, the remote transmission rate is not high in the WAN use environment.

Wan-services provided

In terms of layers, the top layer of the Wan is the network layer. Services provided by the network layer for hosts connected to the network can be divided into two categories: datagram service and virtual circuit service.
1. datagram Service

 

The datagram Service (connectionless) refers to the group (that is, the datagram) sent by the host at any time by the network ). The Network selects routes for each group independently. The network only tries its best to deliver the group to the target host, but there is no commitment to the source host. The datagram service is unreliable and cannot guarantee the service quality.

2. virtual circuit Service

A virtual circuit is a logical circuit that enables reliable communication between two network devices. Virtual circuits have two different forms: switched virtual circuits (SVC) and permanent virtual circuits (PVC ).

SVC is a virtual circuit dynamically established as needed. When data transfer ends, the circuit is automatically terminated. The communication process on SVC consists of three phases: Circuit creation, data transmission, and circuit termination. In the circuit creation phase, a virtual circuit is established between devices of both parties. In the data transmission phase, data is transmitted between devices through a virtual circuit; in the circuit termination phase, virtual circuits established between communication devices are withdrawn. SVC is mainly applicable to non-regular data transmission networks because SVC requires more network bandwidth during circuit creation and termination. But SVC costs less than permanent virtual circuits.

PVC is a permanent virtual circuit with only one mode of data transmission. PVC can be used in a network environment with frequent data transmission. This is because PVC does not need to use extra bandwidth to create or terminate the circuit, so the bandwidth usage is higher. However, the cost of permanent virtual circuits is high.

Wan-routing selection mechanism

The route table in the vswitch. The so-called routing means that when the group arrives at the Node switch, the Node switch should select the path with the least time or the path with the lowest cost, and then forward the group. Each node switch stores a route table. The Node switch searches for the route table to determine the route for the group.

 

A route table usually does not specify the complete path of a route, but indicates the next node to a destination and the cost of arriving there. For example, in the network, the cost is related to the connection of adjacent nodes. If you want to find the most economical route, that is, the total cost of connecting adjacent nodes in the route is the minimum. For example, there are multiple routes from A to F, but the most economical is from A to B (cost 2), B to E (cost 3 ), and e to F (cost 2), the total cost is 7. Some route tables of nodes A, B, and E are displayed. The table of node A indicates that all information sent to Node B, E, or F should be directly transmitted to B. Then, the table of Node B indicates the next node in the most economical route. Similarly, any information sent to C or D should be sent to node C first, and then the route table of C indicates the next step.

For example, assume that an application in node A needs to transmit data to node F. First, the logic of a looks for entries with the purpose of F in its route table. This entry lists that node B is a successor and the network protocol sends data to Node B. Then, B's logic checks its route table and looks for entries corresponding to the destination F. The 3rd entries in the table indicate that the data should be sent to node E. Finally, the route table of node e indicates that F is the next node in the path.

Wan-Example

1. X.25 Network

 

Psdn-Packet Switched Data Network (psdn-packet) is a communication network that exchanges data with a group (packet) as the basic data unit. It uses the Packet Exchange (Packet Exchange) transmission technology and lease lines. It is a public data network for Packet Exchange. Because the public group exchange data network uses the X.25 protocol standard, it is also called the X.25 network.

In 1970s, many European countries began to develop public data networks (networks that can be used by anyone who needs network services ). The problems they face are different from those in the United States. In the United States, public data networks can be developed in most regions as long as existing telephone lines are rented. In Europe, because of the inherent communication system that spans national borders, this is not easy to achieve. Therefore, European countries abandoned the development of independent incompatible standards, but developed unified standards with the support of I t u. The result is a public data network service interface called the X Series protocol. Common protocols include X.25, X.3, X.28, and x.29.

2. Frame Relay Network

Frame relay is a new public data exchange network that emerged in 1980s and has just emerged in recent years. It is a fast group exchange technology developed by X.25. Similar to X.25, Frame Relay uses group exchange technology to implement peer-to-peer communication. However, there are also some differences between them. The main difference is that the X.25 protocol includes the Low Layer 3 protocol, and the frame relay protocol only includes the physical layer and the data link layer protocol.

In terms of design, the difference between Frame Relay and X.25 is that frame relay focuses on fast transmission, while X.25 emphasizes high reliability. Therefore, the transmitted data is verified within the X.25 network, it also has an error handling mechanism, while Frame Relay does not. Therefore, frame relay transmission speed is fast (64 kbps-2.048mbps ).

Frame Relay Technology leverages the latest high-quality, high-speed, and higher-performance communication link technologies (such as optical fiber communication ). The error detection is still implemented on the basis of each hop, but the error recovery process is moved to the edge of the network. Various intelligent functions, such as traffic control and error recovery, are implemented by the terminal system connected to the frame relay network.

3. ATM (asynchronous transmission mode) Network

It can use the fixed data packet size to increase from 25 ~ 622 M B p s transmission rate.

A fixed-size packet of at m is also called a cell. It consists of 4 8 bytes of data and 5 bytes of header information. By using a fixed-size data packet, at m provides an predictable communication mode and can better control bandwidth usage. At m uses a virtual circuit. It can use a dedicated virtual circuit (p v c) or an switched virtual circuit (s v c ).

S v c is a logical point-to-point connection. It depends on the at m switch to select the optimal path between the sender and receiver. The at m switch establishes this connection before transmitting at m data over the network. On the contrary, Ethernet transmits data first, and the router and switch are offline to determine how to guide data transmission.

At m relies on "clean" digital transmission media, such as optical fiber, to achieve high transmission rates. However, it can also be connected to other types of media, such as copper axis cables or twisted pair wires, and other systems that use methods such as Ethernet or frame relay transmission.

Data that requires strict time delay, such as video, audio, images, and other ultra-large files, is very suitable for at m technology. At m's high-quality services, load balancing capabilities, transmission rates, and interoperability may make it an ideal remote communication method. Like other emerging technologies, the disadvantage of at m is that it is too costly and lacks well-defined standards.

4 SONET (synchronous optical fiber network) Network

Bell Communications lab developed the s o n e t in the S and S to connect different telephone systems around the world. If X. 2 5 is the originator of Wan transmission technology, s o n e t can only be regarded as a new generation. SONET defines a signal hierarchy, similar to T media, but extends to higher bandwidth. It can be provided from 64 Kbps ~ Gbps data transmission rate, which uses the same T d M technology as t medium.

Because s o n e t standardizes optical fiber transmission, it can be directly compatible with different standards in different countries, it has become the best choice for connecting Wan between North America, Europe and Asia. Internationally, s o n e t is what everyone knows about s d h (synchronous data layer, Synchronous Digital hierarchy ). S o n e t has good interoperability with T media, I s d N and at M technologies, making it a long distance (or even within the same country) A good choice for connecting Wan and LAN.