Data Link Layer Protocols in OSI Reference Model

There are various protocols in the data link layer, which are as follows:

Synchronous Data Link Control(SDLC) :

SDLC is generally linked layer protocol that is used with Systems Network Architecture (SNA) environment. SNA is proprietary networking architecture of IBM that is developed in 1974. SDLC also supports huge variety of typologies and different types of data links.

Examples include point-to-point links, multipoint links, switched networks, packet networks, etc. It also uses primary station-secondary station model of data communication. On an SDLC data link, all of data and control transmission are simply organized and managed in specific format that is basically known as transmission frame. Transmission frame is also known as SDLC frame or just a frame.

HDLC protocol:

High-Level Data Link Control (HDLC) generally uses term “frame” to indicate and represent an entity of data or a protocol of data unit often transmitted or transferred from one station to another station. Each and every frame on link should begin and end with Flag Sequence Field (F). Each of frames in HDLC includes mainly six fields. It begins with a flag field, an address field, a control field, an information field, an frame check sequence (FCS) field, and an ending flag field. The ending flag field of one frame can serve as beginning flag field of the next frame in multiple-frame transmissions.

The basic frame structure of HDLC protocol is shown below :

PTP Protocol:

Point to Point protocol is basically a Wide Area Network (WAN) protocol that performs or works at layer 2 by simply encapsulating frames for transmission or transferring over different physical links or connections like serial cables, cell phones, fiber optic cable among others, etc. Encapsulation is basically a process in which lower-layer protocol basically receives data from higher layer protocol and then further places this data portion of its frame. In simple words, we can say the encapsulation is process of enclosing one type of packing with help of other types of packet. PPP generally provides encapsulation so that various protocols at the network get supported simultaneously. PPP connections also deliver or transmit packets in sequence and provide full-duplex simultaneous bi-directional operation. PPP usually encapsulates any of the network layer packets in its frame that makes it possible for PPP layer three protocol to become independent and even capable of carrying multiple-layer three packets through a single link or connection. PPP Encapsulation is also required to disambiguate multiprotocol datagrams i.e. removal of ambiguity to make the multiprotocol datagrams clear and easy to be understood. PPP puts data in a frame and transmit it through PPP connection or link. A frame is basically defined as a unit of transmission in Data Link Layer (DLL) of the OSI protocol stack. To form encapsulation, a total of 8-bytes are required. Data is usually transmitted from left to right in frames. General Structure of PPP encapsulation is shown below :