by Dinesh Thakur Category: Routing

The SDLC or the Synchronous Data Link Control was first developed by IBM in 1975. It was modified by ISO and was named HDLC. Thus SDLC is a subset of HDLC and includes several minor additional features.

The frame format of SDLC is shown in fig.

                               Frame Format of SDLC

• The flag sequence of 8 bits 01111110 marks the beginning and ending of the frame.
• The address field and control field serve the same purpose as discussed in HDLC and are of 8 bit each.
• The information field or user data field carries the data and is of variable length.
• ECF stands for error checking field and is of 16 bits. It is used for error control.

It is basically a linked layer protocol which can be used with systems network architecture or the SNA environment. In this system all the functions in a network can be defined and slotted into layers. There are different types of layers and each of these layers has its own functionality which is some what similar to the OSI layers.
For example the data is transferred without errors form the Data Link Control layer to the Network Addressable Units or the NAU’s using the SDLC or the synchronous data link control protocol. There are physical control layers and logical layers.

Data progresses from the higher layers to the lower layers and is eventually transferred to the communication ports. Here the basic ingredient is the SDLC which is important in the entire layering and data transfer process as it is the primary protocol that is being used.


SDLC Protocols and Topologies


The SDLC protocol supports a vast variety of topologies and types of data links. Like for example point to point links, multi links, switched networks and packet networks and many more. The SDLC protocol simplifies the data control process into a basic pattern.

There are two types of networks identified by the SDLC, the primary node and the secondary node.

The primary node is responsible for handling all the secondary nodes and controls all the links. The secondary nodes actually send all the data that is received to the primary node. Data is processed only through the primary node.

However there are some basic configurations that are followed by these nodes like

Point to Point in which there are only two nodes, one is the primary and the other is the secondary.

Multipoint where there is one primary node and the remaining are all secondary nodes.

Loop nodes where the first node which is the primary is connected to the secondary nodes and has two nodes on either side.


SDLC Features


There are many features that contribute to the functionality of the SDLC frame work.


• Flag: The flag feature is used by the SDLC format to initiate and terminate the errors by keeping them under a check.

• Address: The SDLC address feature helps identify if the frames are being received from the primary or the secondary because it contains information of group address, specific address, broadcast address and so on.

• Controls: There are three different formats followed by the controls feature, Information Frame, Supervisory Frame and Unnumbered Frame.

• Information Frame Controls: Information Frame controls the sequence in which the frames are being sent and number the frames. By doing this it can keep a tab on which was received first and which needs to be processed first. The primary and the secondary communicate with each other to check on the immediate responses.

• Supervisory Frame: Supervisor Frame provides information about the control; however it does not contain an information field. It also acknowledges and sends receipts when it receives an information frame.

• Unnumbered Frames: Unnumbered Framesare not in sequence and contain no numbers. These frames can be used to initialize the secondary frames.

• The Data feature contains information of the path and exchange, identification and so on. It basically condenses all the information related to the data.



The SDLC can be used in a remote site or a local site. For example it can be used to connect a token ring network to the SNA or the systems network architecture backbone using a 56kbps line based on SDLC.

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About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.C.A, MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.