SDLC is a protocol used in IBM’s SNA for data transmission through layer two. It was developed for WAN solutions for IBM mainframe computers and uses byte-based transmissions. SDLC frames contain flags, address, control bytes, data payload, and redundant sequence control. SNA environments using SDLC can support different topologies such as point-to-point, multipoint, loop configuration, and hub green light method.
Synchronous Data Link Control (SDLC) is a protocol that provides data transmission through layer two of what is called the systems network architecture (SNA). SNA was developed by IBM® in the 1970s as a wide area networking (WAN) solution for users of IBM® mainframe computers, networking hardware, and remote terminals. By comparison, SNA is very similar to the open systems interconnection (OSI) model used in Internet Protocol (IP) networking, in which network operations are separated into layers, each responsible for one aspect of network communications. While conceptually similar, the layers of SNA are not compatible with the layers of the OSI model.
In the early days of networked computer communications, telephone companies were not licensed to provide computer processing services, so networks had to be established via privately leased lines. A user leases a line from the telephone company and then sets up his computer hardware to network through that leased line. With such a reliable connection, SNA’s synchronous data link control protocol was able to manage each line and provide a data communication network between users’ computer systems. As a proprietary protocol, SDLC was added to modems and IBM® developed computer systems that made up an SNA environment. Later, IBM® shared the concept of synchronous data link control with standards organizations who then developed the high-level data link control (HDLC) protocol that other hardware vendors started using.
The synchronous data link control protocol was the first of its kind to provide byte-based transmissions responsible for identifying each data frame sent. In SDLC, the data transmission is divided into frames which are streamed over the connection. Each frame contains not only the data being sent, but also a series of bytes which contain information about the address the frame is being sent to, how to arrange all the frames in the correct order, and the system’s ability to recheck the frame for errors that may have occurred during your trip.
The first and last byte of the SDLC frame are called flags, which are essentially the wrapper of the frame, indicating its start and end. The next byte or two make up the address. Control bytes, which can serve multiple purposes depending on the type of frame being transmitted, follow the address and can handle frame sequencing, transmission termination, status checking, polling, and so on. The data payload follows the control bytes and after the data, but before the close flag, there are a couple of bytes used for redundant sequence control.
An SNA environment using synchronous data link control is fairly simple, where each node in the network is identified as either primary or secondary. Primary nodes are most likely a mainframe computer, while secondary nodes are terminals that communicate with the mainframe. However, a network running in SDLC is capable of supporting different types of topologies.
In a point-to-point configuration, there are only two computers communicating with each other: a single primary mainframe and a single secondary terminal. With multipoint, however, the mainframe is responsible for any number of secondary terminals. Another topology is the loop configuration, where the mainframe acts as a kind of primary point in a circle where it passes frames through the loop via only the first or last terminal in the circle. Then there is something called the hub green light method which allocates an outgoing channel to the mainframe and an incoming channel to the terminals.
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