Synchronous Communications Hardware Hardware that supports sync And

Synchronous Communications Hardware · Hardware that supports sync. And async. Communication is known as USART (Universal Synchronous Asynchronous Receiver/Transmitter). · Intel 8251 A: support Bisync. · Intel 8273 support SDLC. Control Conditions . . . Input (to CPU) Serial output . . . Output (from CPU) USART Serial input

Commkit Synchronous Software • Few PCs are supplied with sync. comm. Hardware. • Commkit can emulate byte-oriented synchronous protocols on asynchronous hardware. - half-duplex, stop-and-wait • The problem: - design a stop-and-wait protocol - design a high-level utility to take a “block” of up to 10 lines of data and transfer it to the remote PC.

A Stop-and-Wait Protocol • Frame format STX Seq Data Frame 0 ETX received ACK Frame 1 NAK Resend Time-out received Frame 1 lost x ACK Frame 1 discarded ACK Frame 0 error CRC

Block Transfer Two foreground processes • On transmitting PC - The foreground process accepts up to 10 lines of text from the user. - When signaled, send it to the stop-and-wait software. - When all lines on the transmitting station have been sent, screen is cleared. - Data is forwarded if ten lines of text are entered or Crtl-Z is pressed. • On receiving PC - When a line is received correctly by the stop-and-wait software, it will be forwarded to the local foreground process for display. - Whenever the number of lines on the screen exceeds 10, the foreground process will clear the screen.

Implementation • High-level Transmission • High-Level Reception • Low-level Transmission • Low-level Reception