Chapter 14 Cyganski Book

Monica Stoica

Info on the chapter• The information revolution began in the mid-19th century

with the first transmission of pulses of electricity on a wire. Before the turn of the 20th century we had learned to transmit information with electrical pulses, flashes of light, and beams of radio energy; every means that we use today. Yet despite living in the age of information transmission technology, most users have only an outsider's view of it.

• In this part we will examine the origin of the limits of bandwidth of transmission systems. We will get to the root of the question: if bandwidth is so necessary to information infrastructure, what's stopping us from just turning it up?

Chapter Info• It will turn out that the only way that the Internet

can subsume the telephone system, television system, etc., is to first greatly expand the bandwidth of the links that interconnect its components. We will take a close look in this part at fiber-optic technology which is the transmission revolution behind the modern information revolution.

• We will also examine the nature of radio transmission, because at the same time that the Internet is becoming the telephone system, the cell phone is becoming the universal telephone handset, computer interface,Web browser and general information companion.

Chapter Info• Finally, we will take an in-depth look at the

only alternatives to moving information in digital cables that can compete for total bandwidth: moving information in boxes packed in aircraft and trucks! The optical technologies that are at the base of the Internet revolution have had considerable application in the storage of information in physical media such as the CD and the DVD.

Latency and Information Rate• Latency is another word for delay; specifically, it is

the time delay involved in the movement of a message from one location to another. It is the time that it takes to move 1 bit from source (origin) to sink (destination).

• Information rate is the number of bits that can be sent at the source or received at the sink per second.

• We can have communication channels that suffer huge latencies (seconds, or even minutes) while moving data at a rate of billions of bits per second, because in most systems there is no need to wait for a bit to be delivered applicant sink before inserting another for delivery at the source.

Real Time Data Transmissions• real-time transmission means that the data is available soon

enough after its creation to be of as much use as it would be if no processing or transmission delays had been involved.

• For example, if a human voice is delayed by 0.1 second, the average listener will never notice the difference in a conversation as compared to undelayed speech. This audio could be said to be delivered in real time.

• On the other hand, if the delay is increased to one half second, we wouldn't only notice an annoying delay between statements and reactions, but we would also find ourselves tripping on each other's interruptions!

Real Time Transmissions• For other applications, acceptable time delays

differ.

• Professional photographers often use ``slave'' flash units to enhance their lighting. The slave flash has a photoelectric sensor that detects the flash from the photographer's hand-held camera.The slave unit then flashes its light soon enough so that it contributes to the light seen by the camera during the open shutter time. To do this, the slave flash must light within less than 1/200th of a second,or within 5 milliseconds.

Real Time

• Traffic engineering involves changing the electronic signs on highways and streets to route traffic so as to minimize traffic tie-ups.

• To do this, traffic engineers require real-time data regarding the occupancy of streets and the flow of traffic. Sensors planted in the streets can count cars and relay information every minute over low-speed(low-cost) special-purpose telephone connections to centrally located computers, where it can be processed and displayed within a couple of minutes.

• This information arrives sufficiently quickly to appear to be immediate reactions to traffic conditionsas perceived by the drivers.

For other applications, acceptable time delays differ.

• As these examples illustrate, the definition of real time is quite application-specific.

• The delay that is allowed within the context of a use of the information system determines a maximum delay time, or latency, that we allowed in that application.

• While the impact of latency is quite specific to the application,the source of latency is often outside our control!

Einstein• There is an ultimate limit as to how small a delay

can be achieved in the movement of data. This irrevocable limit is due to the finite speed of light and the now well-verified theory by Albert Einstein that no information can be relayed at speeds that exceed the speed of light.

• That the movement of information has a speed limit may seem rather difficult to believe. One may readily imagine how poorly received the idea was when originally presented by Einstein.

• Does the speed of light actually impact practical human communications? It certainly does. Following are a few examples.

Geosynchronous Satellites• Prior to the use of fiber-optic cable as the means of choice

for connecting remote locations on Earth for telephone communications,the geosynchronous satellite was a common means for telephone information transmission.

• A geosynchronous satellite orbits the Earth at a height of 22,300 miles. At this height the time for one orbit of the Earth is 24 hours.

• Because the earth itself also rotates once every 24 hours, if the satellite is rotating in the same direction as the Earth, the satellite appears to stay still over the same spot on Earth.

• Thus, a geosynchronous satellite is always in the same place with respect to antennas on the ground, a very useful attribute for simple, low-cost and continuous communications coverage.

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• Now at this distance from Earth, due to the travel time of a radio signal moving at the speed of light, there is a delay from one side of the connection to the other of 270 milliseconds.Thus, the delay between when you say something and when you hear the other person's response is 540 milliseconds--over half a second.

• Anyone who has ever had a conversation over a satellite link can attest applicant annoying quality of this delay time.

Inter planetary Communications• The moon is 240,000 miles from Earth. The finite speed of

light introduces a round-trip delay time of 2.6 seconds in any conversation with an astronaut. Is it any wonder that these conversations sound very unnatural?

• We have placed robots on the surface of Mars that are controlled from Earth. The manipulators of these robots, however, have quite a difficult job working the robotic arms to pick up rocks even though they have a ``live'' video feed from the robot.Now, the distance between Mars and Earth is constantly changing because both are in elliptically shaped orbits with different orbit times (the martial and terrestrial years).

Mars to Earth• The average (over many orbits) of the distance

between Mars and Earth at the point in any year when they are closest to each other in their orbits is 50 million miles.

• The round-trip time between them at the speed of light is, in this case, 9 minutes. Not exactly real time for almost any purpose! This explains why it was so difficult to jockey the Mars Rover around rocks and to free it from occasional traps. The controller on Earth needs to wait at least 9 minutes before the result of a command is evident.

To parallel or not to parallel• It is not uncommon to hear on the nightly news about some

new success by certain groups in cracking ever more complicated, ever more supposedly secure encryption systems. Quite often, we hear that the key was the use of thousands of computers scattered throughout the country, cooperating in a massive brute-force effort. It makes sense that if a lock has a huge number of combinations, then huge numbers of people can cooperate in finding the right key by simultaneously trying different keys on many identical copies of the lock.

• We say that such a problem has a parallelizable solution. That means that we can solve the problem N times faster by having N people(or computers as the case may be) working in parallel to solve it.

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• Unfortunately, many interesting and important problems do not have parallelizable solutions.

• Consider, for example, looking up a word in a dictionary. If the word has 10 letters, it does not help in any way for 10 people to be using 10 dictionaries in this effort, because the letters must be searched in order. Each would accomplish the entire task in the same amount of time as a single searcher.

• Some problems are piecewise parallelizable; that is, a step in the solution process can be parallelized. But, all the results obtained from the N participants must still be brought together at one place before the next step can be taken.

• Many real problems fall into this category, and some improvements in solution time can be obtained by dividing the workload among many computers. Unfortunately,the limitations introduced by the speed of light may again intervene to make the parallel solution slower than the one-computer solution.

Speed of light• In the previous image we see the relationships among

several locations in the United States and the time it takes to move information between them at the speed of light.

• Consider a problem in which a single step may be parallelized, and suppose that one of the parallel elements of that step takes 1 millisecond to solve.

• Then, a single computer can solve five such elements in 5 milliseconds, doing them one at a time, or serially.

• If we were to distribute the step to four other locations to get a hand, we would find that the total time to a complete solution is at least 20 milliseconds now, the time for the farthest result to make its way back to the home system.

One fast computer• Of course, we have just taken light-speed travel

time into account in this illustration.

• On the real Internet, our information packet may have to wait in line with others to use the communications channel, incurring further delay.

• Hence, even in the heyday of the Internet, it is still true that often the fastest way to solve a problem is with one very fast computer.

Inside a computer• The speed-of-light limitation applies to all

information transmission, regardless of the means or medium.

• Thus, the movement of data inside a computer between its brain (the central processing unit, or CPU) and the memory, display, and disk storage devices is also subject to light-speed induced delays.

• The actual speed at which information travels along the wires on circuit cards in a computer is actually less, equal to about one half the speed of light.

Inside a computer• A fundamental action in the operation of a computer is the

retrieval of information from the memory for use in the CPU.

• You can think of a computer as being the electronic equivalent of a postal clerk in front of a wall of addressed (street names and numbers for example) pigeon holes, with an index card in each pigeon hole that can be either read or written upon.

• The useful action of the computer is obtained by a programmer arranging for the cards in the pigeon holes to contain simple directions for the clerk to follow and data (information) that will be used to generate still other information that will be written back onto some of the cards.

CPU• Now, the act of obtaining a piece of information

from memory by the CPU is a two-step operation that we liken to the postal example, as follows.• 1. The CPU must first send a message to the

memory that indicates the numerical address of the information to be read--the postal clerk extends a hand to the pigeonhole that has the address of the information that is needed.

• 2. The memory device responds by sending the information contained in the addressed memory slot--the postal clerk withdraws his hand from the slot with the card that was in it.

Speed of postal clerck• Obviously the speed of our postal clerk is determined by the

distance that her hand must travel and the speed with which she can move it. So it is with our personal computer (PC). Here the speed limit is the speed of light (or less), and the distance is the distance between the CPU and memory components inside the computer.

• Now, the typical distance from a CPU to a memory device (measured along the entire path of the wire) is 1 foot. The speed of light is approximately 1 foot per billionth of a second (one billionth of a second is referred to as a nanosecond, or ns). Hence, at half the speed of light, the round-trip time for the above transaction is about four billionths of a second.

Real electronics• Thus, if no other delays were involved, the fastest that this

operation could be repeated over this distance would be at a rate of 250 million instructions (that is,operations) per second, also referred to as 250 MIPs.

• Real electronics, however, take time to operate. That is, the memory chips cannot respond in zero time, and it also takes time for the CPU and memory to recognize the presence of new information and to issue it.

• Thus, the 4 ns delay above is just part of the total time for these interactions, and real computers cannot achieve speeds even near 250 MIPs when these distances and delays are involved.

Cache memory• Current PC technology incorporates two so-called cache

levels. • In addition to fast, power hungry Level 2 cache memory

situated near the PC, are even smaller amount of so-called Level 1 cache is located on the silicon chip with the CPU.

• This space on the chip is costly to include, because chip production costs grow very rapidly with increased chip size.

• However, this addition of only a small amount more of cache memory means that at a relatively low cost, the speed of light apparently can be beaten, because the most needed information sits so much closer to where it is needed than in the main memory chips.