X.25Is a person who uses the phone orISDNThe device is structured as a network hardware deviceWANOfITU-TNetwork protocol.itsEntity layer，data link layerandnetwork layer(Level 1-3) are all based onOSI modelTo structure.Internationally, X.25 providers usually call X.25Packet switched network(Packet switched network), especially those state-owned telephone companies.Their composite networks covered the world from the 1980s to the 1990s and are still used in trading systems today.
10. 25 was developed by ITU Group VII according to a series of digital network plans, such asDonald DaviesLed by the National Physical Laboratory of the United Kingdom,Donald DaviesFirst proposedpacket switchingThe concept of.At the end of the 1960s, an experimental network began to operate. By 1974, a series of networks hadSERCnetThe forms of are linked to each other.SERCnet continued to grow and was renamed in 1984JANETThe network is still running today, but it has become aTCP/IPNetwork.Other contributions to the implementation of this standard are the ARPA plan jointly developed by France, Canada, Japan and Scandinavia since the 1970s.Various upgrades and additional functions make this standard increasingly perfect. Every four years, ITU will publish a new technical manual with different cover colors to describe these changes.
structure
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10. The first principle of 25 is to create a global packet switching network over an analog telephone network based on bit error checking.Many X.25 systems have high bit error rates, which can not meet this requirement, so access procedures are requiredLAP-B。10. The 25 model is essentially a connection oriented virtual circuit that provides users with virtual connections that look like point-to-point links through DTE.[1]
10. 25 is in aDummy terminalThe era of development, need to connect to the main computer.Instead of connecting directly to the host computer - this requires the host computer to have its ownModemAnd telephone lines, and there needs to be no local call to make long-distance call requests - the host can establish an X.25 connection with the network server.In this way, dumb end users can directly dial up to connect to the network.In essence, the modem and port are at one end, and X.25 is connected at the other end, which is defined by ITU-T X.29 and X.3 standards.
After the connection with the PAD is established, the user of the dumb terminal notifies the PAD of a phone numberX.121Address to indicate which host to connect with.Next, the PAD sends an X.25 request to the host to establish a virtual circuit.It is pointed out that X.25 has established a virtual circuit, thus forming a circuit switching network, although in fact data is still transmitted through the packet switching network.If two X.25s communicate, you can call the other party directly;No PAD.In theory, it does not matter whether the X.25 caller and the X.25 definer are on the same transmission. In fact, it is not always feasible for one transmission to call other transmissions.
constitute
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physical layer
It is called X.21 interface and defines the slave computer/terminal（Data terminal equipment, DTE) to X.25Packet switched networkThe physical/electrical interface of the attachment node in.RS － 232 － C is usually used for X.21 interface.
The link access layer defines data transmission like frame sequence.The protocol used is balancedLink Access Procedure(LAP － B), which isAdvanced Data Link Control(HDLC) protocol.LAP-B is designed toPoint to pointconnect.It provides frame structure, error checking and flow control mechanism for asynchronous balanced mode sessions.LAP-B provides a way to ensure that a packet has arrived at each link of the network.
Packet layer
Define reliable virtual circuits through packet switched networks.In this way, X.25 provides point-to-point data transmission instead of one to many transmission.
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 crossing the packet switching network.Using one circuit can ensure that packets arrive in order, because they are all transmitted along the same path.It provides a reliable way for data transmission on the network.
There are two types of virtual circuits in X.25:
Temporary virtual circuit: the call based virtual circuit will be established and then removed at the end of the data transmission session.
Permanent virtual circuit: It is a fixed virtual circuit designated by the network. Like a special line, it can directly transmit data without establishing and clearing connections
Whether it is a switched virtual circuit or a permanent virtual circuit, several "virtual" connections share a physical channel. 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 is realized by a pair of buffers between adjacent nodes,These buffers are assigned to different virtual circuit codes to distinguish them. The process of establishing a virtual circuit is to assign buffers and virtual circuit codes to nodes along the line
12 bits for virtual circuit code in the groupBinary digitRepresent (4-digit group number and 8-digit channel number). In addition to code 0 reserved for diagnosis group, the remaining 4095 codes can be used to create virtual circuits. Therefore, theoretically, a DTE can create up to 4095 virtual circuits.These virtual circuits multiplex the physical links between DTE-DCEFull duplex communication。
Connection oriented virtual circuit
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In the history of X.25, it was used asPermanent virtual circuit(Permanent virtual circuits, PVCs).These applications are very common, such as in the bank, so that decentralized offices can be connected to a central host, which is much cheaper than establishing an actual long-distance telephone connection.10. The monthly service charge of 25% is usually relatively average.Its speed gradually increases with time, and the typical value is 48 or 96 kbit/s.The public X.25 network was built in the 1970s and 1980s in most countries. In order to reduce the cost of network services, users first need to connect to the network interface, called“Virtual circuit switching”(SVCs) or "virtual connection to the public data network", these X.25 applications were not used in most places with the emergence of the Internet in the 1990s.
Many systems directly use X.25, many of which are private applications. However, this is when X.25 was the only network standard in the world, butX.400E-mail system X.25 is still used as the transport layer.The basic idea of OSI is to establish a global network standard. However, the development of the Internet industry has finally adopted the Internet standard.
Gradually replaced
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With more perfect digital telephone service and error correction functionModemThe rapid development of X.25 is no longer of practical significance.The result isFrame relayFrame relay is X.25 with automatic error correction function.At present, the concept of virtual circuit is stillAsynchronous transmission modeIt is used in for congestion control and network reuse.
present situation
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By 2011, as optical fiber is more and more widely used as a transmission medium, the probability of transmission errors is getting smaller and smaller. In this case, repeatedly implementing error control at the link layer and network layer not only appears redundant, but also wastes bandwidth and increases the delay of message transmission.
Since X.25 packet switching network is developed on the basis of early low-speed and high error rate physical links, its characteristics can no longer meet the requirements of high-speed remote connection, so it is generally only used in WAN environments that require less transmission costs and low remote transmission rates.Although it has been gradually replaced by the network with better performance, this famous standard is promotingPacket switched networkHas made great contributions to the development of.[2]
Today, X.25 is still used all over the world, although this proportion has been increased with someThe second floorNew technologies such asFrame relay，ISDN，ATM，ADSLThe introduction of POS is declining rapidly.Now only inThird World CountriesSome devices 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.25 calledAX.25Still at amateurWireless packet communication(Wirelesspacket switching, packet radio), but there have been some calls for it recentlyTCP/IPTo replace X.25.RACAL Paknet is still used in many parts of the world10. 25 ProtocolThe standard is used for secure low rate wireless transmission.Paknet is now commonly used as a GPS and POS application.
