Even though subset of High-Level Data Link control (HDLC), Synchronous Data Link Control (SDLC) was initially created by IBM before HDLC. SDLC was first link-layer protocol that is totally based upon synchronous, bit-oriented operation. SDLC is basically precursor to HDLC. SDLC basically performs various functions such as :
- It is used to transfer data or message unit across links.
- It also manages link-level flow.
- It also manages error recovery and error correction procedures for errors that might occur during transmission.
Physical links supported by SDLC : There are various types of physical links that are highly supported by SDLC as given below :
- Data Channels – These are used to transmit or transfer bits in parallel manager, due to these channel-attached nodes should be located very close to one another.
- SDLC Links – This is most common and widely used method to connect remote nodes together.
- X.25 Interface – This interface remote nodes might also get connected over packet-switched network with help of protocol known as QLLC Qualified logical link control (QLLC) simply to carry SDLC frames.
Derivative Protocols : There are number of protocols that are highly popular and have been derived from SDLC protocol. These protocols are standardized by numerous standards bodies. Some of important derivative protocols that are simply based on SDLC are given below :
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Link Access Procedure, Balanced (LAPB) – LAPB is used to implement Data Link Layer (DLL) as described and defined in X.25 protocol suite. LAPB is best and popular known protocol for its presence in X.25 protocol stack. It is reliable synchronous serial protocol basically used to manage packet framing among DTE () and DCE stations.
It is bit-oriented protocol that is being derived from HDLC that is used to ensure that all of data frames are error-free and are in correct or right sequence. It shares same format of frames, types of frames, and functions of different fields similar to SDLC and HDLC. As the name suggests, LAPB is balanced protocol that usually operates in ABM (Asynchronous Balanced Mode). -
High-Level Data Link Control (HDLC) – HDLC is basically standard that is being adopted by International Standards Organizations (ISO). It is bit-oriented code-transparent SDLC protocol that is developed by ISO. HDLC and SDLC both share same format of frame. HDLC fields also provide functionality similar to those in SDLC.
HDLC generally provides both services i.e. connection-oriented and connectionless. It generally supports three different types of data transfer modes that are NRM (Normal Response Mode), ABM (Asynchronous Balanced Mode), and Asynchronous Response Mode (ARM). But is does not support loop and hub go-ahead configurations. It is also used by Cisco Systems routers for some serial communication over leased lines as an alternative way to Point-to-Point Protocol (PPP). -
Logical Link Control (LLC) – LLC was basically developed by IEEE 802.2. It is generally required to provide HDLC style services on LAN (Local Area Network). It is highly popular data link protocol for LANs. LLC is subset of HDLC. It uses Asynchronous Balanced Mode (ABM) subclass of HDLC. LLC usually provides flow control and ARQ (Automatic Repeat Request) error management techniques. There are basically three versions of LLC as given below :
- (i). Physical Services Header (PSH)
- (ii). Qualified Logical Link Control (QLLC)
- (iii). Enhanced Logical Link Control (ELLC)
- Qualified Logical Link Control (QLLC) – QLLC is an IBM-defined DLL protocol that simply allows SNA (Systems Network Architecture) data to be transmitted across X.25 networks. It also provides data-link control capabilities that are very much required to transmit SNA data. QLLC and X.25 together replace SDLC in SNA protocol. QLLC is version of LLC.
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