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CECS 474 Computer Network Interoperability
CHAPTER 5
Local Asynchronous Communication (RS-232)
Computers are digital, thus they use binary digits (bits) to represent data. Defn : Communication is said to be asynchronous if the sender and receiver do not need to coordinate before the data can be sent. Advantage: A sender can wait arbitrarily long between transmissions; the sender can transmit whenever the data is ready to send.
Using Electric Current to Send Bits
Example : Digital Encoding Medium: Copper Wire Energy Form: Electric Current Encoding: Negative voltage encodes 1 Positive voltage encodes 0
Fig 5.1 A waveform diagram. The X-axis corresponds to time. The Y- axis corresponds to voltage
The RS-232C Standard
RS-232-C is the most widely accepted way to transfer characters across copper wires between a computer and a device such as a modem, keyboard, or terminal. RS-232 defines serial, asynchronous communication. RS-232 specifies the physical connection as well as the electrical details: Specified by the EIA Voltage is +15 or - Cable limited to ~50 feet Latest EIA standard is RS-422 (ITU standard is V.24) It specifies the transfer of characters (usually 7-bit) Example use: connection to a keyboard or mouse via the serial port on a PC
Asynchronous Communication
Sender and receiver must agree on Number of bits per character Duration of each bit Receiver Does not know when a character will arrive May wait forever To ensure meaningful exchange send: Start bit before each character One or more stop bits after each character
Baud Rate, Framing and Errors
The duration of a bit in RS-232 is determined by the baud rate. Defn : The baud rate of transmission hardware is the number of changes in the signal per second that the hardware generates. Examples : Typical baud rates 9.6 Kbaud 14.4 Kbaud 28.8 Kbaud For RS-232 (it is a very simple scheme), the baud rate is exactly equal to the number of bits per second. 28.8 Kbaud = 28.8 kbits per second The duration of a bit = 1/(baud rate) To make RS-232 more general, manufacturers design each piece of hardware to operate at a variety of baud rates. Sender and receiver must agree on the baud rate Receiver samples the signal to verify agreement Disagreement results in a framing error
Full Duplex Asynchronous Communication
Note : All electric circuits require a minimum of two wires The current flows out on one The current flows back on the other (often called the ground ) When RS-232 is used with twisted pair wiring, one wire carries the signal and the other is ground. (For coax, the signal travels down the center conductor and the ground returns on the shield.) Defn : Transfer in which information is sent one direction is called half duplex or simplex transmission. Defn : Transfer in which information can be sent simultaneously in two direction is called full duplex transmission. Full Duplex requires that each side have a transmitter and a receiver.
Limitations of Real Hardware
Because of the limitations of electrical transmission (e.g., energy dissipates as it travels, resistance, and interference distortion occur), RS-232 must handle minor distortions. RS-232 will : • Take multiple samples per bit
- Tolerate less than full voltage Unfortunately, this means that we can not use electrical current for long-distance transmissions.
Performance Issues -- Review
Recall: Propagation Delay Determined by physics Time required for a signal to travel across the medium Bandwidth Electrical property of a physical transmission system Maximum times per second the signal can change Fundamental Performance Measures: Delay The amount of time required for a bit of data to travel from one end to the other Usually, the same as the propagation delay in the underlying hardware Throughput The number of bits per second that can be transmitted Related to the underlying hardware bandwidth
The Bad News
Physics tells us that real systems emit and absorb energy (e.g., thermal ) Engineers call unwanted energy noise In Nyquist's Theorem, a noise-free system is assumed. Nyquist's Theorem only works in theory. We turn to Shannon's theorem to correct for noise.
Digital Throughput vs. Bandwidth (Again)
The real relationship between digital throughput and bandwidth is given by: Shannon's Theorem: C = B log 2 (1 + S/N) where: C is the effective channel capacity in bits per second B is the hardware bandwidth S is the average power (signal) N is the noise S/N is the signal-to-noise ratio Example: Conventional Telephone System Engineered for voice Bandwidth is 3000 Hz Signal-to-noise ratio is approximately 1000 Effective capacity = 3000 log 2 (1 + 1000) = ~ 30,000 bps Conclusion : Dialup modems have little hope of exceeding 28.8 Kbps
