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X-25, Benefits of Packet Switched Networks
 PSNs, packet-switched networks, provide remote offices with either permanent or switched connections that feature high-levels of throughput (typically up to DS1).
 An important advantage of PSNs is that they offer customers a way to share facilities with other customers, thereby reducing the cost of WAN service.
 Paths through the PSN are called virtual circuits (VCs). A virtual circuit is a logical path, not a physical one.
 Virtual circuits make it possible for a remote site to maintain connections to multiple sites over the same physical interface.
 A site can send data directly to several other remote sites via different virtual circuits in the carrier network. This requires that the customer mark, or tag, each unit of data in some way so the provider's WAN switch can determine which destination to route the traffic through the cloud (refer to the figure).
 In frame relay networks, the VC information is called a data page link control identifier (DLCI) and is included in the frame header. In X.25 networks, the VC information is called the logical channel identifier (LCI) and is included in the packet header.
 PSNs allow providers to charge their customers on the basis of the number of packets transmitted. Because the customer can "pay as they go," PSNs can provide optimal cost-effectiveness.
 X.25 was one of the earliest packet switched technologies and the first to be deployed worldwide. In fact, since X.25 is still frequently used in developing countries and for legacy equipment, X.25 continues to be the world's most common packet-switched technology, and can be found in virtually every region that supports data communications.
X.25
 X.25 is a standard that defines the connection between a terminal and a PSN. In other words, X.25 is an interface specification. It does not specify the characteristics of the PSN itself. Despite this, the networking industry commonly uses the term X.25 to refer to the entire suite of X.25 protocols.
 Developed in the early 1970s, X.25 was designed to transmit and receive data between alphanumeric "dumb" terminals through analog telephone lines. X.25 enabled these terminals to remotely access applications on mainframes or minicomputers. Later, X.25's capability was expanded to support a variety of networking protocols, including TCP/IP, Novell IPX, and AppleTalk.
 The X.25 suite of protocols includes Packet Layer Protocol (PLP), Link Access Procedure, Balanced (LAPB), and various physical-layer serial interfaces (e.g., X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, and G.703). The figure maps the key X.25 protocols to the layers of the OSI reference model.
 Both PLP and LAPB include mechanisms for error checking, flow control, and reliability. By including these mechanisms at both Layer 2 and Layer 3, X.25 provides a high level of reliability.
 If a network is built on unreliable circuits, error checking at the hardware level (the Data page link layer) can handle transmission errors more efficiently than processes in software (the Network layer and above).
 X.25 is "over-engineered" when implemented over modern WAN links. Newer technologies, such as Frame Relay, take advantage of lower error rates by providing a stripped-down, unreliable data link. Such technologies rely on error detection and correction in the upper layers (typically the Transport layer).
X.25 Network Devices
X.25 network devices fall into three general categories:
 Data terminal equipment (DTE).
 Data circuit-terminating equipment (DCE).
 Packet switching exchange (PSE).
 X.25 Network Devices
 DTE devices are end systems that communicate across the X.25 network. They are usually terminals, routers, or network hosts, and are located on the premises of individual subscribers.
 DCE devices are communications devices such as modems and packet switches. They provide the interface between DTE devices and a PSE. X.25 DCEs are typically located in the carrier's facilities.
 PSEs are switches that compose the bulk of the carrier's network. They transfer data from one DTE device to another through the X.25 PSN. Figure illustrates the relationships between the three types of X.25 network devices.
Packet Assembler/disassembler (PAD
 The packet assembler/disassembler (PAD) is a device commonly found in X.25 networks. PADs are used when a DTE device, such as a character-mode terminal, is too simple to implement the full X.25 functionality.
 The PAD is located between a DTE device and a DCE device, and it performs three primary functions:
 buffering.
 packet assembly.
 packet disassembly.
 The PAD buffers data sent to or from the DTE device. It also assembles outgoing data into packets and forwards them to the DCE device. This operation includes adding an X.25 header.
 Finally, the PAD disassembles incoming packets before forwarding the data to the DTE. This includes removing the X.25 header.
 Some ITU-T recommendations defining the PAD are as follows:
 X.3 - Specifies the parameters for terminal-handling functions (e.g., baud rate, flow control, character echoing, and other functions) for a connection to an X.25 host. The X.3 parameters are similar in function to Telnet options or attention (AT) command set for modems.
 X.28 - Specifies the user interface for locally controlling a PAD. X.28 identifies the keystrokes that you would enter at a terminal to set up the PAD, similar to the AT command set for modems.
 X.29 - Specifies a protocol for setting the X.3 parameters via a network connection. When a connection is established, the destination host can request that the PAD or terminal change its parameters by using the X.29 protocol. A PAD cannot tell the destination host to change its X.3 parameters, but it can communicate that its own parameters were changed.
 X.75 - Specifies the gateway between the clouds. It defines the signaling system between two PDNs. X.75 is essentially a Network-to-Network Interface (NNI).
 When discussing X.25, the term virtual circuit (VC) is used interchangeably with the following terms: logical channel identifier (LCI), virtual circuit number (VCN), logical channel number (LCN), and virtual channel identifier (VCI).