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Chapter 6 The Data Communications Interface

Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions Asynchronous Synchronous | William Stallings Data and Computer Communications Chapter 6 The Data Communications Interface Asynchronous and Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions Asynchronous Synchronous * Asynchronous Data transmitted on character at a time 5 to 8 bits Timing only needs maintaining within each character Resync with each character * Asynchronous (diagram) * Asynchronous - Behavior In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard) * Synchronous - Bit Level Block of data transmitted without start or stop bits Clocks must be synchronized Can use separate clock line Good over short distances Subject to impairments Embed clock signal in data Manchester . | William Stallings Data and Computer Communications Chapter 6 The Data Communications Interface Asynchronous and Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions Asynchronous Synchronous * Asynchronous Data transmitted on character at a time 5 to 8 bits Timing only needs maintaining within each character Resync with each character * Asynchronous (diagram) * Asynchronous - Behavior In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard) * Synchronous - Bit Level Block of data transmitted without start or stop bits Clocks must be synchronized Can use separate clock line Good over short distances Subject to impairments Embed clock signal in data Manchester encoding Carrier frequency (analog) * Synchronous - Block Level Need to indicate start and end of block Use preamble and postamble e.g. series of SYN (hex 16) characters e.g. block of 11111111 patterns ending in 11111110 More efficient (lower overhead) than async * Synchronous (diagram) * Line Configuration Topology Physical arrangement of stations on medium Point to point Multi point Computer and terminals, local area network Half duplex Only one station may transmit at a time Requires one data path Full duplex Simultaneous transmission and reception between two stations Requires two data paths (or echo canceling) * Traditional Configurations * Interfacing Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities Need an interface called data circuit terminating equipment (DCE) e.g. modem, NIC DCE transmits bits on medium DCE communicates data and control info with DTE Done over interchange circuits Clear interface standards required *