Synchronous Data Link Control (SDLC) is a transmission protocol used to synchronously transfer code-transparent, serial-by-bit data over a communications channel.
This data link layer protocol is Layer 2 of the Open Systems Interconnection (OSI) network communication model. It ensures that data arrives successfully from one network point to the next and flows at the right pace.
Developed by IBM in the 1970s, SDLC is a more efficient replacement for the older Binary Synchronous Communications protocol, first introduced in 1964. It also replaced other inefficient, character-oriented synchronous protocols, like Digital Data Communications Message Protocol.
SDLC is the Layer 2 protocol for IBM's Systems Network Architecture (SNA). It is commonly used in IBM terminal-to-computer communications and in wide area networks, where leased lines are used to connect mainframe SNA hosts and remote terminals.
SDLC uses commands to control data transfer over a communications line. Transmission exchanges can be full-duplex or half-duplex over switched or nonswitched lines. SDLC can be used in remote and local sites.
In SDLC, data is sent as a synchronous bit stream. This stream is divided into frames. Each frame contains addressing and control information, as well as the data payload.
SDLC uses a primary/secondary architecture. In the primary station/secondary station model of communication, one station is designated as primary, while the remaining stations are secondary.
The primary station role is to do the following:
Each secondary station has its own address. Data is processed only through the primary node. The secondary nodes send data received by the primary node.
In IBM's SNA, all the functions can be defined and slotted into layers, where each layer has its own functionality. Using SDLC, data is transferred error-free from the data link control layer to the network addressable units.
SDLC simplifies the data control process into a basic pattern. Data moves from the higher layers to the lower layers and eventually reaches the communication port. SDLC is the primary protocol and is vital in the entire layering and data transfer process.
SDLC operates independently on each communications link. It can also mix full-duplex primary stations and half-duplex secondary stations on four-wire circuits to help lower the cost of dedicated facilities.
Other features also contribute to the SDLC framework and functionality, including the following:
SDLC supports numerous connection configurations, topologies and data links:
Ring topologies are also possible in SDLC. Here, a primary node controls a ring of secondary nodes and is itself part of this ring.
High-level Data Link Control (HDLC) is a modified version of SDLC. It was updated by the International Organization for Standardization in the late 1970s. HDLC refers to a set of communications protocols used to transmit data between nodes. The protocols are bit-oriented and applicable to both multipoint and point-to-point communications.
Like SDLC, HDLC protocols are also data link layer protocols for synchronous communications. Data fields in both are framed by control information at the beginning and end. They also contain additional addressing information, making them suitable for use with communication networks.
However, HDLC differs from SDLC in several ways. It contains more control information than SDLC, and it adds error correction and Flow control capabilities to SDLC, as well as two additional modes:
HDLC also offers additional features:
While SDLC and Normal Response Mode are efficient protocols for closed private networks with dedicated lines, other modes of HDLC serve X.25 and frame relay protocols that manage packets on shared-line switched networks, like those used by the internet.
There are other protocols derived from SDLC:
SDLC is still a commonly encountered and prevalent data link protocol in today's mainframe environment.
25 Aug 2021