SDLC Types and Topologies

Synchronous Data Link Control (SDLC) protocol was developed by International Business Machine (IBM) in mid-1970s for its use in environments of Systems Network Architecture (SNA). SDLC is first link-layer protocol that is based in synchronous, bit-oriented operation. Data Link Layer generally provides error-free transmission of data among Network Addressable Units (NAUs) within given network of communication via this SDLC protocol. There are generally two types of links :

  • I/O Channel Communication link –
    These links are basically hard-wired among all co-located nodes. They also usually operate at almost high rate of data and generally error-free.

  • Non-I/O Channel Communication Link –
    These links generally use some form of data communications equipment to transmit data among several nodes. SDLC protocol is basically required to ensure error-free performance over all these links.

SDLC was initially used in Wide Area Network (WANs) that basically use leased line simply to connect mainframe SNA hosts and remote terminals. SDLC is also equivalent to layer 2 of Open System Interconnection (OSI) model of network communication. It is primary protocol that is very essential in entire layering and data transfer process.

SDLC also supports various types of architectures as given below :

  • Normal Disconnected Mode (NDM) and Normal Response Mode (NRM)
  • Two-way alternate (half-duplex) data flow
  • Secondary station point-to-point, multipoint, and multi-multipoint configurations
  • Primary station point-to-point and multipoint configurations
  • Modulo 8 transmit-and-receive sequence counts
  • Nonextended (single-byte) station address

Types of network nodes in SDLC :
SDLC generally identifies and describes two types of Network nodes as given below :



  1. Primary Node –
    The primary node is generally responsible for handling all secondary nodes and also responsible for controlling all operations and links. Data is processed only through this node. This node polls secondaries in predetermined order and then secondaries can then send if they have any data to be sent. This node also sets up and tears down links and manages link while it is operational.

  2. Secondary Node –
    A secondary node is generally responsible for sending all of data that is being received to primary node. This node is controlled by primary node i.e., secondaries will only transmit data to primary only if primary allows and grants permission to do so.

Basic Configurations followed by SDLC nodes :
SDLC primary and secondary node generally get connected in four different basic configurations as given below :

  1. Point-to-Point –
    As the name suggests, in Point-to-Point configuration, there are only two nodes. One node is primary and other one is secondary node.

  2. Multipoint –
    As the name suggests, in Multipoint configuration, there are multiple nodes. One node is primary and all of other are secondary nodes. It is also known as multidrop configuration. In this, secondaries are polled separately in predefined sequence.

  3. Loop –
    As the name suggests, in Loop configuration, primary node is connected to first secondary node and last secondary node in loop. In simple words, primary node is connected to secondary nodes and has two nodes on either side. In this, intermediate secondary nodes transmit data through one another as they give responses to primary requests.

  4. Hub go-ahead –
    This configuration generally involves inbound and outbound channels. The primary node simply requires outbound channel to transmit data with secondary node whereas secondary node simply requires inbound channel to transmit data with primary node.

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