CSIT 220 (Blum)1 RS-232 (Local Asynchronous Communication) Based on Chapter 5 in Computer Networks...

CSIT 220 (Blum)1 RS-232 (Local Asynchronous Communication) Based on Chapter 5 in Computer Networks and Internets by D. Comer

CSIT 220 (Blum)2 Human-Computer Communication Computers are not designed to perform some particular large-scale task, instead they are designed to perform a set of small-small tasks. Programming a computer consists of instructing it to perform the small-scale tasks in some particular order. We must communicate our program to the computer. At a higher level we communicate which programs we want executed. This level of communication is not between two computers but between a computer and its peripheral devices and ultimately between computer and user.

CSIT 220 (Blum)3 Categorization I: Directionality One scheme for categorizing communication systems is based on the direction(s) in which information flows, i.e. one way or two way. Simplex Half-duplex Full-duplex

CSIT 220 (Blum)4 Simplex Information flows in only one direction. There are separate transmitters and receivers. Example: radio The radio station has the transmitter. The listener only receives the signal that is, your radio is a receiver.

CSIT 220 (Blum)5 Half-duplex Information flows in both directions, but only one direction at a time. Each unit has a transmitter and receiver, but it is either in a transmitting or receiving mode. Example: walkie-talkie only one person can talk at a time

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7 Full-duplex Information flows in two directions simultaneously. The transmitter and receiver are both continuously operational. Example: telephone Each person can talk and listen all of the time.

CSIT 220 (Blum)8 Modem: full or half Many modems can work in either mode (half- or full-duplex). A switch determines which. In half-duplex mode, there is a local echo, each character transmitted is also displayed locally In full-duplex mode, there is a remote echo, each character is displayed only after it was transmitted and returned by the distant device (so you know it got there intact).

CSIT 220 (Blum)9 Categorization II: Timing Another scheme for categorizing communication systems is based on the timing between the transmission of characters or bytes. Asynchronous Synchronous Isochronous

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CSIT 220 (Blum)11 Synchronous definitions When we talked about satellites, we used definition #4. (Recall geosynchronous.) For this lecture we will focus on definition #5 concerning whether characters are transmitted as a group or individually. You might use definition #1 if you were comparing a chat session (synchronous, i.e. requires that people logged on at same time) to a discussion database (asynchronous, i.e. people can be logged on at different times).

CSIT 220 (Blum)12 Asynchronous (definition #4) One character (byte of data) at a time. There is an unknown length of time between the sending of one character and the next; the data flow is intermittent. ABC Different times between character transmissions

CSIT 220 (Blum)13 Synchronous (definition #4) Data is transmitted in large groups of characters (frame). Within the frame, character transmissions are sent at regular intervals. So one character immediately follows the next and so on. ABCDEFGHIJK

CSIT 220 (Blum)14 Asynchronous vs. Synchronous Asynchronous is less restrictive than synchronous Synchronous requires more from the hardware and a greater degree of coordination between transmitter and receiver. Asynchronous is slower/less efficient than synchronous The percentage of the signal that is not actual data is higher in asynchronous.

CSIT 220 (Blum)15 Asynchronous Example The connection between the keyboard and the computer is often asynchronous. The information is transmitted a character (keystroke) at a time. The information arrives slowly and intermittently. (Its possible to buffer the keyboard input before transmitting it to the computer.)

CSIT 220 (Blum)16 Beginning, middle and end Synchronous and asynchronous both have Beginning: Im sending you data Middle: the actual data End: Im done Overhead: the percentage of information that is not actual data Overhead = Beg+End 100% Beg + Mid + End

CSIT 220 (Blum)17 Overhead example Let us assume that we are sending a byte (8 bits) of data, that there is no parity (error detection) bit, that there is one bit to indicate the start of the transmission, and that there is one bit to indicate the end of the transmission. Then the overhead is: Overhead =1 + 1 100% = 20% 1 + 8 + 1

CSIT 220 (Blum)18 Isochronous Character transmissions are sent at regular intervals but some of them may be empty. So the length of time between the sending of one character and the next is some multiple of the time it takes to send one character. ABC More restrictive than asynchronous, less restrictive than synchronous.

CSIT 220 (Blum)19 Speed and accuracy The desire is for the information to be conveyed as quickly and as accurately as possible. These two factors compete: Factors which improve the speed, such as representing more levels (states), can reduce the accuracy. Factors which improve accuracy, such as adding error correcting codes, can slow down transmission (since extra bits are sent).

CSIT 220 (Blum)20 Noise Unpredictable energy (waves) that permeate the environment. They can never be completely eliminated. The noise waves add to the waves carrying a signal, changing it. Enough noise will corrupt the information being carried. Occurs in digital and analog signals alike, but the information in a digital signal is more robust against the effect of noise.

CSIT 220 (Blum)21 External and internal noise Noise is sometimes divided into two categories: internal and external Internal: generated within the communication devices External: generated outside of the communication devices Wired communication is less susceptible to external noise since the channel is somewhat isolated from its environment. In wireless communication, on the other hand, the channel is the environment.

CSIT 220 (Blum)22 Minimizing noise One way to reduce noise is to reduce a signals bandwidth as much as possible. The smaller the range of frequencies making up the signal, the smaller the amount of noise. But reducing the bandwidth reduces the rate of information flowing. Digitizing helps!

CSIT 220 (Blum)23 Digital signals 0 1 1 0 1 0 0 0 These signals are different if they are analog signals, but they might be the same if they are digital signals.

CSIT 220 (Blum)24 The competition The competition between speed and accuracy can be seen in the theorems of Nyquist and Shannon-(Hartley). Nyquists theorem tells one how to send information faster. But Shannons theorem tells how that speed is ultimately limited by noise.

CSIT 220 (Blum)25 A bit on logarithms Both theorems involve a logarithm. Recall that a bit (binary digit, a 0 or 1) is the smallest unit of digital information. The relationship between the number of bits and the number of states is logarithmic.

CSIT 220 (Blum)26 A bit on logarithms (Cont.) One bit Two states (0, 1) Two bits Four states (00, 01, 10, 11) Three bits Eight states (000, 001, 010, 011, 100, 101, 110, 111) Number of States = 2 (Number of bits) Exponential Number of bits = log 2 (Number of States) Logarithmic

CSIT 220 (Blum)27 A bit on logarithms (Cont.) Log 2 (1) = 0 Log 2 (2) = 1 Log 2 (4) = 2 Log 2 (8) = 3 Log 2 (16) = 4 Log 2 (32) = 5 Log 2 (64) = 6 Log 2 (128) = 7 Log 2 (256) = 8 Log 2 (512) = 9 Log 2 (1024) = 10 Log 2 (2048) = 11 Log 2 (4096) = 12 Log 2 (8192) = 13 Log 2 (16384) = 14 Etc.

CSIT 220 (Blum)28 States Bits So if you know the number of states, you take the logarithm (base 2) to determine the number of bits required to represent those states. The number of bits required is a measurement of the information contained therein.

CSIT 220 (Blum)29 Nyquists theorem D = (2) B Log 2 K Where D is the maximum data rate in bits per second (bps). Where B is the bandwidth. Where K is the number of states. This maximum is a goal for communications engineers, but it is rarely achieved.

CSIT 220 (Blum)30 The problem with higher frequencies D = (2) B Log 2 K Recall the bandwidth can be increased by working at higher frequencies. So why not just work at high frequencies to increase the rate of data transmission? Physics says that higher frequency means higher energy Not only do higher frequency waves (like ultra- violet or x-rays) take more energy to generate, but they can also cause more damage (like cancer).

CSIT 220 (Blum)31 K= 2 two bits per cycle 1 0 0 1 0 1 0 0 One cycle 2 possible amplitudes

CSIT 220 (Blum)32 K= 4 four bits per cycle 11 01 00 01 10 01 4 possible amplitudes.

CSIT 220 (Blum)33 More levels less room for error

CSIT 220 (Blum)34 More levels less room for error D = (2) B Log 2 K Note that an error of approximately 0.2 would probably not cause one to confuse the states represented in the first case, but it could lead to confusion of states (i.e. data corruption) in the second case. This is how Shannons theorem places a limit on Nyquists.

CSIT 220 (Blum)35 K=2 with some noise

CSIT 220 (Blum)36 K=4 with some noise

CSIT 220 (Blum)37 Shannons theorem C = B Log 2 (1 + S/N) Where C is the channel capacity in bits per second. Where B is the bandwidth. Where S is the signal power. And where N is the noise power. The ratio S/N is known as the signal-to- noise ratio.

CSIT 220 (Blum)38 Signal-to-noise ratio In communications (digital or analog), the signal-to-noise ratio, denoted S/N or SNR, measures a signals strength (power) relative to the background noise.

CSIT 220 (Blum)39 Decibels Sometimes the signal-to-noise ratio is reported in decibels (S/N) in decibels = 10 Log 10 (S/N) When using Shannons theorem we do not want S/N expressed in decibels In some cases, one can produce a larger signal, but that requires more energy and hence costs more money. Lowering the temperature sometimes helps since some noise is thermal.

CSIT 220 (Blum)40 Local Communication Local communication is the exchange of information between two things which are close here it typically refers to exchange between a computer and one of its peripheral devices. Data from a keyboard or mouse does not flow at steady rate but rather is intermittent. Thus asynchronous communication is appropriate.

CSIT 220 (Blum)41 Fig. 5-1 A possible approach: bit-at-a-time

CSIT 220 (Blum)42 Pros and Cons of the bit-at-a-time approach +15 represents 0; -15 represents 1; and 0 corresponds to not transmitting a bit. Pro: very lenient, not a lot of rules Con: wastes a lot of time, theres an extra state (0 V) that does not correspond to any information in the signal and one must spend time in this state to distinguish for example between two consecutive 1s. And while keyboard data is intermittent, it does tend to come in bytes instead of bits.

CSIT 220 (Blum)43 Fig. 5.2 Framing a group of bits: a byte- plus-at-a-time

CSIT 220 (Blum)44 Protocol In order to improve the speed on the bit-at-a-time approach, We want to reduce the number of states to two We want to eliminate the time between bits within a group of bit that make up one character. We need rules! We need physical rules so one device does fry the other. We need logical rules so the devices can agree on the information they are transmitting.

CSIT 220 (Blum)45 Rules In order to distinguish between two consecutive 1s, we use timing, each bit lasts a set amount of time If the signal is at 15V for (roughly) twice this interval, that corresponds to two consecutive 1s. Since the signal is asynchronous, we also need to distinguish between no signal and a signal of all 1s.

CSIT 220 (Blum)46 Frame All 1s is distinguished from no signal in that all 1s will a frame. In ordinary language, a frame is an open border or case for enclosing a picture, mirror, etc. We frame the information, i.e. place it within a border that indicates where it begins and ends.

CSIT 220 (Blum)47 RS-232 Short for recommended standard-232C Approved by the Electronic Industries Association (EIA) for connecting serial devices. In 1987, the EIA released a new version with the name to EIA-232-D. In 1991, the EIA teamed up with Telecommunications Industry association (TIA) and issued yet another version that goes by EIA/TIA-232-E. But still commonly called RS-232.

CSIT 220 (Blum)48 Newer standards While EIA-232 remains the most common standard for serial communication, the EIA has developed successors called RS-422 and RS-423. These newer standards are backward compatible so that devices adhering to the old standard (RS-232) can be used in a new RS-422 port.

CSIT 220 (Blum)49 Serial Port Most personal computers have an RS232 serial port. Serial refers to sending data bit-by-bit (as opposed to parallel, in which several bits are sent at once). A (physical) port is a computer interface to which one can connect a peripheral device (e.g. mouse, keyboard, modem, etc.)

CSIT 220 (Blum)50 Serial ports

CSIT 220 (Blum)51 Some terminology A PC has an EIA-232 port for connecting to a modem or other device. The modem (or other device) adheres to the EIA- 232 standards (specifications). One calls the device connecting to the port a Data Communications Equipment (DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).

CSIT 220 (Blum)52 Connectors The EIA-232 standard supports two types of connectors a 25-pin D-type connector (DB-25) a 9-pin D-type connector (DB-9). The type of serial communications used by PCs requires only 9 pins so either type of connector will work equally well.

CSIT 220 (Blum)53 Why so many pins? If we are sending information a bit at a time, dont we need just one wire? (Dont forget about the need for a return or ground so lets make that: dont we need just two wires?) True, two wires are used to transmit information in one direction, but the other wires are used to control the transmission.

CSIT 220 (Blum)54 Control pins The serial connector has pins. Those that are not explicitly for data are known as control pins. The control pins are used to negotiate the exchange of information. Is the transmitting device ready? Is the receiving device ready? Etc.

CSIT 220 (Blum)55 USB the new standard in town The development of serial port standards was important in the development of computers. Peripheral device developers knew that if they adhered to the standards, users could add their device to the computer with little trouble. Over the last few years Universal Serial Bus (USB) ports and standards have taken over. Our purpose here is to introduce some ideas about communication and standards. The USB standards are somewhat more complicated, so the serial port standards while they are becoming outmoded suit our purpose.

CSIT 220 (Blum)56 Some stray terminology Throughput: (like bandwidth, but typically only used in reference to digital signals) The rate at which data is transmitted or processed processed. Data transfer rates for disk drives and networks are measured in terms of throughput. Typically, throughputs are measured in kbps, Mbps and Gbps. Bps bits per second

CSIT 220 (Blum)57 Some stray terminology (Cont.) Baud: the number of signaling elements that occur each second. The term is named after J.M.E. Baudot, the inventor of the Baudot telegraph code. Since it is possible to encode more than one bit in each electrical change, baud rates and bit rates need not be equal.

CSIT 220 (Blum)58 Other References http://www.webopedia.com http://www.whatis.com http://www.m-w.com/ http://computer.howstuffworks.com/serial- port.htm/printable

CSIT 220 (Blum)1 RS-232 (Local Asynchronous Communication) Based on Chapter 5 in Computer Networks...

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