The OSI Model

LayGO stacks, such as X.25 or Frame Relay, follow the Open Systems Interconnection (OSI) Reference Model for layered protocols. X.25 is a stable, mature standard which is periodically refined by the ITU (formerly CCITT). Frame Relay is popular in countries where digital networks are in place. Figure 2-2 shows the relationship of protocols between two endpoint stations (or peers) through an X.25 network.

OSI Diagram
Figure 2-2 OSI X.25 peer-to-peer connection through an X.25 network

In the OSI model, the physical layer controls the communications hardware. It is responsible for the physical signals required to deliver an arbitrary stream of bits between two end points. LayGO provides an implementation of the X.21/X.21 bis physical layer protocol. The implementation relies on hardware which supports full-duplex synchronous SDLC framing with checksums, bit-stuffing, and flag and abort handling.

The layer above the physical layer is the data link layer. The task of the data link layer is to establish an end-to-end communication link over which data can be transmitted. LayGO supports LAPB (Link Access Procedure Balanced), LAPF (Link Access Procedure for Frame Mode Bearer Services), and LAPD (Link Access Protocol for ISDN D-channel). LAPB is a data link protocol for point-to-point full-duplex error-free data communications. The protocol handles link establishment and flow control. Data frames which have been lost or damaged are retransmitted, insuring error-free data delivery to the layer above. LAPF is also a data link protocol for point-to-point full-duplex data communications. Unlike LAPB, errors are neither reported nor corrected. Another example of a link layer protocol is LAPM (Link Access Protocol for Modems or V.42).

The layer above the data link layer is the network layer. This is the uppermost layer LayGO currently supports. The X.25 network layer supports multiple virtual circuits through a single data link connection.

Systems incorporating X.25 or Frame Relay can take advantage of popular packet or circuit switching networks in the United States, Europe, Canada, and many other countries, which allow data transmission at rates up to 28.8 kbps over normal phone lines or at higher speeds using leased or ISDN lines. A computer with a link to one of these networks can be made accessible by any computer or terminal which is also connected to the network. This is how information services such as Internet, CompuServe, America On Line support their customer base. The packetizing or framing of data and multiplexing of independent communication channels on a given data or satellite link increases the value of that link while facilitating the charging of users. Networks can also be interconnected, thus increasing the accessibility to information and connection points.