Notes On Chapter Five -- Local Asynchronous Communication (RS-232)

• 5.1 Introduction
  
  
  • How can one send bits through short distances over an electrical conductor?
    
    
  • How should we measure the performance of communication channels?
    
    
• 5.2 The Need For Asynchronous Communication
  
  
  • In asynchronous (async) communication, the sender sends whenever it is ready. There is no need to synchronize with the receiver first. The signal does not contain information that tells where individual bits begin or end.
    
    
  
• 5.3 Using Electric Current To Send Bits
  
  
  • There are many possible ways to do this. The simplest idea is perhaps just to put a negative voltage on the line to represent 1, and a positive voltage to represent 0.
    
    
• 5.4 Standards for Communication
  
  
  • One must have standards to agree on symbols, voltages to be used to represent symbols, how long pulses last, how close together they can be, etc.
    
    
  • RS-232 is a widely used standard for sending characters asynchronously over a wire by transmitting bits one after another (serially).
    
    
  • According to RS-232, the line is held low (to -15 volts) when idle. There is a certain duration of time T for sending a bit. When starting to send a character, the line is held high (at +15 volts) for time T. This allows the receiver to detect that a character will be coming. Next each bit of the character is impressed on the line for time T each, with no spacing in between bits. Low means "1" and high means "0." After the last bit of the character has been sent the sender brings the line low for at least time T, after which it may send another character when it wishes.
    
    
  • Using a timer, the receiver samples each sent bit several times and computes an average value of the sampled voltage, and "decides" the value of each bit -- "O" or "1".
    
    
  
• 5.5 Baud Rate, Framing, And Errors
  
  
  • The time T is something the sender and receiver must agree upon. T determines the baud rate -- how often the voltage can change in a second, and how fast information can be sent. The actual measurements used most often are baud rates like 19,200 bits per second. Typically RS-232 hardware can be configured to operate in some range of baud rates. Often a mismatch between sender and receiver baud rates will be detected by the receiver and reported as a framing error. The receiver detects framing errors by noticing that multiple samples of a bit do not agree, or by noticing that the stop bit does not arrive when it should. The break key on a computer keyboard causes a framing error.
    
    
  • Technically baud is not the same as bits per second. (They amount to the same thing in the case of RS-232.) If the voltage can change to more than two distinct levels, then more than one bit can be encoded in a voltage change. In that case the baud rate is different than the bit rate.
    
    
  • For example if there are eight different voltage levels to which the line can change, then each voltage change communicates one of eight values. This is equivalent to transmitting 3 bits of information because there are 8 patterns possible in a set of 3 bits. So in this case if the baud rate is R changes per second then the bit rate is 3R bits per second.
    
    
  • Keep in mind, however, that this discussion ignores the "overhead" of sending start and stop bits, parity bits, and so on.
    
    
  
• 5.6 Half And Full Duplex Asynchronous Communication
  
  
  • RS-232 actually is pretty complex and describes the roles of 25 wires, most of which are used for specialized control functions, and are not often needed.
    
    
  • Under RS-232, data can flow between two devices in both directions simultaneously -- full duplex communication. There is a wire for each direction of communication, plus a ground wire that is shared.
    
    
  • "Clear to Send" is one of the control line functions. This is used by the receiver to tell the sender to keep sending.
    
    
  • Computers and modems use different pin numbers for sending and receiving data. A computer sends on 2 and receives on 3. A modem sends on 3 and receives on 2. With this set up, you can connect a computer to a modem without crossing the 2-3 wires. Of course, it means you *do* have to cross them if you are connecting two computers.
    
    
  
• 5.7 Limitations Of Real Hardware
  
  
  • Real hardware does not transmit perfect square waves. This is the reason that several samples are taken of each bit value in the RS-232 standard. Also care is taken to gather samples in the middle of the time interval during which the bit is supposed to be transmitted, since the "edge" values are more likely to be erroneous.
    
    
• 5.8 Hardware Bandwidth And The Transmission Of Bits
  
  
  • There are limits to how fast voltage levels can change on a wire, and limits to how many distinct voltage levels can be sensed by hardware. Therefore there is a limit to how many bits per second can be sent down a wire. The same principle applies to any communication channel.
    
    
  • We measure the data rate limit of a channel indirectly, by using bandwidth -- the highest frequency at which an oscillating signal can be sent down the channel. Bandwidth is measure in Hertz (cycles per second).
    
    
  • Basically the Nyquist sampling theorem states that the highest possible data rate would come about by detecting one voltage change for each half-cycle of the bandwidth frequency. Therefore the max baud rate of a channel is 2B where B is the Bandwidth. If K voltage levels are possible then each voltage level transmits log₂K bits. Therefore the Nyquist bit rate limit is
    
    2Blog₂K
    
    
• 5.9 The Effect Of Noise On Communication
  
  
  • Important work done by Claude Shannon shows that max data rate is further limited by the effect of the presence of interference and other "noise." According to Shannon's Theorem
    
    C = B(log₂(1+(S/N))
    
    where C is the effective max on channel capacity in bits per second, B is the "hardware" bandwidth, S is the average signal power, and N is the average noise power.
    
    S/N is the so-called signal-to-noise ratio. Actually, the quantity engineers use is equal to ten times the common logarithm of S/N. So for example if the power of the signal is 1000 times the power of the noise then log₁₀(S/N) is 3 and so the "decibel" level is 10*3 = 30 decibels. Also, since log₂(1001) is barely more than log₂(1000) == about 10, the value of C in this case would be around 10B. That's a bit rate of ten times the "hardware" bandwidth.
    
    
  • In fact, a typical telephone line actually does have a signal-to-noise ratio of about 30 dB. Also the hardware bandwidth of a telephone line is about 3000 Hz. Therefore Shannon's Theorem tells us that we can't transmit data at a rate of more than about 30,000 bits per second over a typical phone line.
    
    
• 5.10 Significance For Data Networking
  
  
  • Nyquist's theorem is an incentive to find clever ways to encode more voltage changes per unit time, thus increasing the theoretical bit rate limit.
    
    
  • Shannon's theorem lets us know the limitations stemming from the signal-to-noise characteristic of the channel.
    
    
• 5.11 Summary
  
  
• See photos of RS-232 equipment.