Synchronous Data Link Control (SDLC)

SDLC is a data link control protocol developed by IBM for use on serial WAN links in SNA environments. Developed in the 1970s, SDLC was the basis for the HDLC protocol later standardized by the ISO. Unlike HDLC, in which connected systems operate as equal peers, SDLC follows a hierarchical communications structure made up of primary and secondary stations.

On an SDLC network, a primary station controls the communication process, and determines when a secondary station can send data through a polling mechanism. For example, imagine an SNA network with one central host (a mainframe) and many terminals at remote locations. In this case, the mainframe would act as the primary station, and the terminals as secondaries. As the primary, the mainframe will poll secondary devices, one at a time, to see whether they have data to send. A secondary can only send data when permitted by the primary. If a secondary does have data to send, it will wait to be recognized by the primary, and then send as much data as permitted before control is passed back to the primary.

The figure below illustrates an SNA network with SDLC links between a host and two remote locations.

On an SDLC network, the primary station will poll secondary stations one at a time to see whether they have data to transmit. Secondaries can only send data if polled by a primary.

While the vast majority of networks no longer run the SDLC protocols, a number of other protocols in use on networks today were ultimately derived from the foundation it provided. Derivative protocols of SDLC include:

High Level Data Link Control (HDLC). This protocol, looked at earlier in the chapter, is the default serial interface encapsulation method on Cisco routers.

Link Access Procedure, Balanced (LAPB). A variation of the HDLC protocol that handles framing, error and flow control mechanisms on X.25 networks.

IEEE 802.2 (Logical Link Control). Works with popular LAN protocols like 802.3 (Ethernet) and 802.5 (Token Ring) to provide connectionless and connection-oriented services at the Data Link layer.

Qualified Logical Link Control (QLLC). This protocol provide services at the Data Link layer to allow SNA traffic to be transported over X.25 networks.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.