Network

Network
 * Open System Interconnection model (OSI model)
 * TCP/IP model

Network Layers
 * Application Layer (Data) : High-level APIs
 * Presentation Layer (Data) : Concerned with the syntax and semantics of the information transmitted.
 * Session Layer (Data) : Provides enhanced services useful in some applications, e.g., remote login, remote file transfer.
 * Transport Layer (Segment TCP / Datagram UDP) : The basic function of this layer is to accept data from the session layer, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other side.
 * Network Layer (Package) : Control the operation of the subnet.
 * Data Link Layer (Frame) : Takes a raw transmission facility from the physical layer and transforms it into a line that appears free of undetected transmission errors to the network layer.

Stop-and-Wait Protocol [Q] What is Piggybacking?
 * Physical Layer (Bit) : Transmitting raw bits over a communication channel.
 * In Stop and Wait Protocol, a sender after sending a frame waits for acknowledgement of that frame and sends the next frame only when acknowledgement of the previous frame has received.
 * [A] Piggybacking is used in bi-directional data transmission in the network layer. The idea is to improve efficiency piggy back acknowledgement (of the received data) on the data frame (to be sent) instead of sending a separate frame.