X. protocol

X. is the first connection-oriented network and the first public data network. Its data groupings contain 3-byte headers and 128-byte data portions. After 10 years of operation, the the 1980s was replaced by a new network called Frame Relay with no error control, no flow control, and connection-oriented. After the 90 's, there was a connection-oriented ATM network. 1 Basic Introduction X. A is a ITU-T network protocol that uses a telephony or ISDN device as a network hardware device to structure the Wan . Its physical layer, the Data link layer and the network layer (layer 1-3) are all structured according to the OSI architecture model. Internationally, the provider of X. Packet is often referred to as the packet switching network (switched network), especially the state-run telephone companies. Their composite networks, covering the world from the 80 's to the 90 's, are still being used in trading systems. X. The 25 protocol is a protocol recommended by the CCITT(ITU), which defines the connection of the terminal and the computer to the packet switching network . Packet switched networks Select the route to the destination on a network for grouping data. X 25 is a well-implemented packet switching service, traditionally used to connect a remote terminal to a host system . This service is X. the user that is used at the same time provides any point-to-point connection. Signals from multiple users of a network can be selected via the X-channel. 25 interfaces into a packet-switched network , and is distributed to different remote locations. A communication channel called a virtual circuit connects an endpoint site over a network on a predefined path. Although X. The main part of the throughput rate is for error checking overhead, X. The 25 interface supports up to 64Kbps of lines, CCITT the standard in 1992 and increases the rate to 2.048Mbps. X. The 25 packet switching architecture has some advantages and disadvantages. The routing of information packets through a hash network is selected based on the destination address information in the packet header. Users can connect to several different locations, unlike a circuit-oriented network where there is only one dedicated line between any two points. Because the groupings can be transmitted over the shared port of the router , there is a certain distribution delay. While many networks can support overloaded traffic by choosing a route to avoid congested areas, users can still feel slower performance as the number of access networks increases. In contrast, a circuit-oriented network provides a fixed bandwidth between two locations, and it cannot accommodate the transmission requirements exceeding this bandwidth. X. 25 overhead is much higher than frame relay . For example, in X. 25, each node on a grouped transmission path must receive a packet in its entirety, and the error check must be completed before it is sent. The Frame Relay node simply looks at the destination address information in the packet header and forwards the packet immediately, and in some cases, it starts forwarding even before it completely receives a packet. Frame Relay does not require X. The state table in 25 that must exist in each intermediate node for handling management, flow control, and error checking. The endpoint node must check for lost frames and request a re-send. X. 25 is affected by low performance and does not adapt to the requirements of many real-time LANs for LAN applications. However, X. 25 is easy to build, easy to understand, and is being received by remote terminals or computers, and in many cases where the transmission volume is low. X 25 may be the only way to establish a reliable network link in a country where the telephone system network is unreliable. Many countries use X. 25 service. In contrast, it is not impossible to obtain reliable dedicated lines in some countries. in the United States, most telecoms companies and value-added telecommunications (VAC) provide x. 25 services, these companies include at/T, US Sprint, CompuServe, Ameritech, Pacific be1l and other companies. You can also install X by using the user location. 25 Swap equipment and connect these sites with leased lines to create a dedicated x. 25 packet switched networks. X. 25 is proposed before the Open Systems Interconnect (OSI) protocol model, so some are used to explain X. 25 of the special terms are different. This standard defines the protocol in three tiers, which is closely related to the bottom three layers of the OSI protocol stack. HistoryIt was developed by ITU Group VII on the basis of a series of digital network plans , like the research project of the British National Physics Laboratory under Donald Davies, where Donald Davies first proposed a packet Exchange The concept. At the end of the 60 's, an experimental network began to operate, and by the year 1974 a number of networks had been linked to each other in the form of sercnet. SERCnet grew up and changed its name to JANETin 1984, and the network was still running today, only to become a TCP/IP network. Other contributions to the implementation of this standard have also been developed in the 70 's by the countries of France, Canada, Japan and the Arpa Peninsula . A variety of upgrades and additional features make this standard increasingly perfect, and every 4 years the ITU publishes a new technical handbook of different cover colors to describe these changes. structureThe first principle is to create a global packet-switched network based on a bit-error-checking on an analog telephone network. Many of the X. system error rates are very high, so as not to achieve this requirement, so need access protocol lap-b. The X. Zero model essentially establishes a virtual circuit based on connection-oriented, which is provided to the user by the DTE to a virtual connection that looks like a point-to-point link. It was developed in the age of a dumb terminal and needed to be connected to the main computer. Instead of connecting directly to the primary computer-this requires the host computer to have its own modem and phone line, and also requires no local calls for long-distance call requests-the host can establish X. Connect with the network server . So dumb end users can directly dial the connection to the network. Essentially, modems and ports are at one end, and X. is connected at the other end, as defined by the ITU-T x.29 and x.3 standards. After the connection has been established with the pad, the user of the mute terminal informs the pad of a x.121 address similar to the phone number to indicate which host to establish the connection with. Next the pad sends a X. Request to the host, creating a virtual circuit. It is pointed out that X. A virtual circuit is established, and a circuit-switched network is formed, although the data is still transmitted through a packet-switched network . If it is two X x. Communication, of course, you can call the other side directly; no pad. Theoretically, regardless of whether the X. caller and X. Definition parties are on the same transmission, it is not always feasible for a transmission to call each other in the same transport. virtual circuit oriented link in X. History, it was used as a permanent virtual circuit (permanent virtual circuits, PVCs) to make the two main computers accurately linked. These applications are very common, such as in banks, which allow decentralized offices to connect to a central host, which is much cheaper than building actual long-distance telephone connections. The monthly service costs are usually relatively average for X. Its speed gradually increases over time, with a typical value of 48 or kbit/s. The Common X. Network was built in most countries in the 70 's and 80 's, in order to reduce the cost of network services , the user first to connect with the network interface, called "Virtual circuit Switching" (SVCs) or "virtual connection to the public data network", These x. Applications were not used in most places in the 90 's as the advent of the Internet. Many of these systems are directly using X., many of which are privatized applications, but this is the time to X. or the only Internet standard in the world, but X. is still used as the Transport layer . The most basic idea of OSI is to establish a global network standard, but the development of the Internet industry finally adopts the Internet standard. be replaced gradually with the rapid development of a more perfect digital telephony and error correction function for modems , it is no longer practical to discuss X. The result is the emergence of Frame Relay , which is a function of automatic correction with error X. The concept of current virtual circuit is still used in asynchronous transmission mode for congestion control and network taking. 2 composition
Physical Layer it's called X. 21 interfaces, which are defined from the computer/terminal ( Data terminal equipment , DTE) to X. The physical/electrical interface of an attachment node in a packet-switched network . Rs-232-c are usually used for x. 21 interface. The link access layer defines the data transfer as a frame sequence. The protocol used is a balanced link access discipline (Lap-b), which is part of the Advanced Data Link Control (HDLC) protocol. The lap-b is designed for Point-to-point connections. It provides frame structure, error checking, and flow control mechanisms for asynchronous balanced mode sessions. Lap-b provides a way to make sure that a packet has arrived on each link in the network. The packet layer defines a reliable virtual circuit that is switched through a packet switching network. So, X. 25 provides point-to-point data delivery, rather than a point-to-multipoint send. in X. 25, the concept of virtual circuit is very important. A virtual circuit establishes a temporary or permanent "logical" communication channel between two locations across a packet-switched network. Using one circuit ensures that the groupings are arrived in sequence, because they are transmitted in the same way. It provides a reliable way for data to be transmitted over the network. In X. There are two types of virtual circuits in 25:A temporary virtual circuit establishes a call-based virtual circuit and then is removed at the end of the data transfer session. permanent virtual circuit is a fixed virtual circuit designated by the network, like a dedicated line, no need to establish and clear the connection, can directly transfer data. whether switching virtual circuits or permanent virtual circuits, a physical channel is shared by several "virtual" connections. There is at least one physical link between a pair of packet switches, and several virtual circuits can share the physical link. Each virtual circuit has a pair of buffer implementations between adjacent nodes, which are assigned to different virtual circuit codes to show the difference. The process of establishing virtual circuit is the process of allocating buffer and virtual circuit code on the nodes along the line. The virtual Circuit designator in the group is represented by a 12-bit binary number (4-bit group number and 8-bit channel number). In addition to the code 0 for the diagnostic packet reservation, the virtual circuit can be built using the remaining 4,095 code, so theoretically, a DTE up to 4,095 virtual circuits. These virtual circuits multiplex the physical link between the Dte-dce for full-duplex communication. Permanent virtual circuit maintains a fixed connection between two endpoint nodes. X. 25 A communication channel is established between two endpoint sites by using calls to establish a grouping. Once the call is established, the data is grouped between the two sites to transmit the information. Note that because of X. 25 is a connection-oriented service, so the grouping does not require a source address and a destination address. The virtual circuit provides a communication path for the transport packet to reach the destination over the network. However, a number is granted to the packet, which can be identified by the channel that connects the source to the destination. X.2 X. 5 The network is easy to install and maintain. It is charged according to the packet data sent, and in some cases, the connecting time is also considered. Keep in mind that some other services are more suitable for high-speed LAN transmissions (such as frame Relay) or private Connection X. 2. X. The network is just a specification for the interface of a public packet switching network based on virtual circuit Services . It dynamically allocates bandwidth to the traffic flow of the user, and can effectively solve the problem of sudden and large information transmission, and encrypt and effectively control the error of the transmission. the packet switching network is generally only used to require less transmission cost, and the remote transmission rate is not high requirements of the wide area Network usage Environment 3 Status
there is still a global presence, although this percentage has been rapidly declining with the introduction of some second-tier new technologies such as Frame Relay ,ISDN, ATM,ADSL, and Pos. Now only in the Third World countries there are some devices that are still operating reliably, because after all, PDN is probably the most reliable and inexpensive device to connect to the Internet. There is a variant of X. ax.25 is still heavily used in amateur Wireless Packet Communication (wireless packet switching , packet radio), but there have been some calls in recent times to suggest using TCP/IP to replace X. In many parts of the world, Racal Paknet still uses the X. Protocol Standard for secure low-rate wireless transmissions. Paknet is now commonly used as a GPS and POS application.

X. protocol