X.25 - Linux

At one time, one of the data communication methods in common use was SNA, used widely withIBM mainframe computers. However, around the same time, a need emerged for a communication protocol that would make use of the spare bandwidth in the PSTN. This protocol was called X.25. X.25 is an International Telecommunications Union (ITU) specification. It was originally limited to 2 MB per second. In recent times, X.25 is not used very much in the PSTN. However because of its robust nature, it is still in wide use today over slow but unreliable terrestrial radio connections. In addition, it sometimes is used at higher speeds than originally specified for certain specialized applications. X.25 is important to include in our discussion of foundational data communication protocols. It is the first example of a truly layered protocol in common use in the public network outside of a particular vendor’s control. X.25 actually incorporates the first three layers of the OSI model, the PHY, link layer, and network layer. It was designed for low-quality physical transmission lines so it has error control and correction at both the link and network layers. This redundant error checking is really no longer needed for reliable modern optical or even electrical transmission lines. Currently, X.25 is only used to provide reliable data delivery for low-quality wireless links for certain specialized aviation and military communications applications.

X.25 was the first protocol to use the concept of a virtual circuit (VC) for data transmission. The purpose of VCs is to connect customer equipment in one location across the common carrier’s PSTN to customer equipment at another location by using negotiated connections that are analogous to voice circuits. VCs can be multiplexed by combining many separate streams of data packets into a single stream over the public network. The X.25 protocol defines a service access point where the common carrier’s equipment and the customer equipment meet. X.25 hides the internal function of the PSTN from the customer’s equipment. The carrier’s equipment is calleddata communications equipment(DCE), and the customer’s equipment is calleddata terminalequipment(DTE).

The top layer or network layer of X.25 is called thepacket layer. The job of this layer is to manage either connections of switched virtual circuits (SVCs) or permanent virtual circuits (PVCs). The packet layer is responsible for establishing and breaking down these connections, and this connection negotiation is also termedcall managementorsignaling. It is interesting to point out that the termssignalingandcall managementcome from voice telephony. The packet layer supports two types of packets, control packets and data packets. The control packets are used for call management and the data packets contain user payload data. X.25 also provides an individual packet delivery service. This service consists of transmitting individual user datagrams without first doing connection negotiation to establish a connection or circuit. As implied earlier, X.25 provides for support of PVCs, which are VCs that are provisioned rather than set up dynamically with a signaling protocol. Refer to Figure for an illustration of the layers of X.25.



The middle layer in the X.25 protocol stack is the equivalent of Layer 2, the link layer. It incorporates a protocol called Link Access Protocol Balanced (LAPB). LAPB is another variant of HDLC. The types of HDLC are described in more detail in Section In the X.25 protocol stack, the link layer provides both a connection-oriented service and a user datagram service. Even though X.25 contains its own network layer—when X.25 or for that matter, other WAN protocol stacks are interfaced to TCP/IP—the entire stack sits under TCP/IP’s network layer and appears as a Layer 2 interface.

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