Lesson 6 Lab Experiment (s)

How 32,256 People Share the Same Optical Fiber

These are the lab experiments for Lesson 6. Please conduct these experiments under direct supervision of an adult including a parent or teacher.

Experiment #1 - Watch My Voice (view your speech)

Experiment #2 - Emulate Interleaving of Conversations

Experiment #3 - Emulate TDM and Packet Transmissions

Experiment #1 - Watch My Voice (view your speech)
Objective: Students get to view their actual voice signal while talking, yelling, singing, whistling, etc.

Required Materials:
1. Smart Phone (IOS or Android)
2. Oscilloscope APP

There are numberous low-cost APPs available for both IOS (iPhone) and Android-based smartphones. The APP used in the examples below is an IOS version of The Eelectronics Engineering Toolkit (EE Toolkit) which is a multi-page set of electrical engineering design tools. The oscilloscope function is on the last page in the APP.

To use the APP, make sure the microphone is ON and select BUFFER Mode. Set the "Time Scale Factor" all the way to left, and adjust "Amplitude Scale Factor" until adequate signal is displayed when talking.

Oscilloscope APP

While observing the screen, have students talk normally, observie the geometry and waveshape of the signals displayed on the screen. Have them speak in low tones (like a bass singer), mid-range and high-pitched words, noting the way the display changes.

The lower-frequency voice components will produce wider, sweeping signals, while high-pitch sounds will show as signals with closer spacing and more acute peaks. Have them whictle a note as well to see they can actually create a sine wave. Have them sing, cough and sneeze, and clap their hands to view signals of all types.

Below are the same photos we used in the Lesson that illustrate voice signals. The key is to allow every student the chance to "see" their own voice signals.


Oscilloscope Trace of Human Voice Saying the Word "Why?"	Oscilloscope Trace of Human Voice Uttering the Letter "S"

Oscilloscope Trace of a Whistled Note

As the students view their voice signals, remind them that everytime they speak into the cell phone, the CODEC in the phone smaples their voice signals. Eight thousand (8,000) times every second, their voice is sliced like a loaf of bread, and each slice is digitized as described in the Lesson.

The point here is that every sound that enters the mouthpiece of the cell phone gets digitized, not just normal voice conversation. The CODEC is pretty dumb in that it does not interpret the sounds, it just codes them to digital as they arrive.

Students are usually spellbound by htis experiment, so have fun with it.

Experiment #2 - Emulate Interleaving of Conversations
Objective - Observe multiple messages transfered through single channel

Required Materials:
1. Four (4) loaves of bread of different types
2. Shoebox with ends removed

This is a simple and fun excersize to illustrate how multiple phone conversations or messages are multiplexed together by a process called interleaving.

To prepare the shoebox for use, cut most of the ends out, retaining only enough to keep the sides from falling flat. This will create a U shapped channel through which we will be passing slices of bread.

Here is the rule to state at the outset of the experiment:
The goal is to pass all four loaves of bread through the shoebox to the other side. The only rule is than all four loaves must be in the shoe box at the same time.

Let them ponder on this for a while to see what they come up with.

Bread Loaf Challange

Here is the correct method:
1. take a slice from Loaf 1 and stand it up in the left end of the shoebox
2. take a slice from Loaf 2 and put it in the shoebox on the left side of the slice from Loaf 1....nudge slice 1 to the right to make room
3. take a slice from Loaf 3 and put it in the shoebox on the left side of the slice from Loaf 2....nudge slice 1 & 2 to the right to make room
4. take a slice from Loaf 4 and put it in the shoebox on the left side of the slice from Loaf 3....nudge slice 1, 2 & 3 to the right to make room
5. now go back and take a second slice from Loaf 1, and put it in the shoebox on the left side of the slice from Loaf 4, nudging all slices to the right to make room
6. repeat taking a slice from each loaf in order until the shoebox is filled with all four bread types alternating 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, etc.

In other words, slices from each loaf were interleaved, allowing all 4 loaves to be in the box at the same time as they pass through. Keep adding slices until the very first slice arrives at the right side of the shoebox.

As the excersize continues, take the first slice out the right side of the shoebox (is from Loaf 1) and place on the right top. Take the second sllice from the shoebox (is from Loaf 2), placing it in the second spot from top. Take the next slice (is from Loaf 3), placing it in the third spot from top. Take the next slice from the right side of shoebox, placing it on the fourth spot from the top. We are now reassembling the loaves on the right side of the box.

Bread Loaves Pass Shoebox

Continue until all the loaves on the left are now on the right.

Bread Loaves Reassembled on Right

Conclusion:
By the process of interleaving, it was possible to pass all four loaves of bread through the shoe box at the same time.

The slices of bread represent samples of the digitized human voice. As described in this Lesson, 8,000 times per second, the CODEC in your cell phone slices up your voice, assigning a number to each slice. As thousands of other voices are transported across an optical fiber, the voices are interleaved, and reassembled at the far end of the conversation.

Experiment #3 - Emulate TDM and Packet Transmissions

The basic premise of this experiment is to provide a practical example of the way thousands of text messages and tweets are transported across communications networks at the same time. We will use pingpong balls to create a word or phrase by writing letters on them, then arranging them into egg cartons as four (4) sets of messages to be transmitted.

The goal is to move the pingpong ball messages from the transmit side (egg carton with pingpong balls) to the receive side (an empty egg carton). The two methods used show the difference between the very rigid TDM transmission method and packet transmission technology. One is orderly and precise, the other is chaos and anarchy, but both produce the same results....the messages are delivered.

This experiment is actually the best way to demonstrate the principles in the Lesson, so we hope the message is not lost in the presentation.

The experiment consists of two parts:

1. Emulate Time Division Multiplexing Transmission
2. Emulate Packet-based Transmission

Items required for this Experiment:
4 each "one-dozen" egg containers
1 each cardboard tube (1/2 inch diameter by approx 3 - 5 feet in length)
24 each standard white ping pong balls
1 each black permanant marker pen (or any color desired)
2 each empty shoebox bottoms
optional 6 each orange ping pong balls (or any color other than white)

Preparation of materials:

1. If using posterboard or flexible material for tube, make sure inner edges to not inhibit pingpong balls from moving through the tube. If this occurs, enlarge the tube diameter slightly. CAUTION- do not make the tube so larrge that ping pong balls can pass each other as they move through the tube. Once inserted into the end of the tube, ping pong balls should remain in the order in which they were "transmitted" into the tube.

2. Remove the lid and other fastener nubs on the top of all egg cartons, leaving only the bottom section with egg holders, then fasten two sections together to form a 4 x 6 matrix as shown below:

3. Use pingpong balls and a black permanent marker, create phrases to simlate four (4) different conversations (email, tweets, phone calls, etc.). These must be very short words with room for spaces as shown in the example below. The orange balls represent spaces:

Emulating Time Division Multiplexing Transmission

Once the tube is constructed and egg cartons assembled for each end of the simulation circuit and messages are created, put the carton which contains the phrases at the right-hand end of the tube; this is our Transmit end of the circuit. Place the empty carton on the left-hand end of the tube; this is the Receive end of the circuit.

Two factors are critical to the success of this experiment:
1. Transmit the ping pong balls in exact order as follows (have someone at the receiving end hold a hand over the tube to block any balls from exiting the tube):
    A. Insert the first letter of Row 1 into the tube (letter H)
    B. Insert the first letter of Row 2 into the tube (letter N)
    C. Insert the first letter of Row 3 into the tube (letter W)
    D. Insert the first letter of Row 4 into the tube (letter I)
    E. Insert the second letter of Row 1 into the tube (letter E)
    F. Insert the second letter of Row 2 into the tube (letter O)
    G. Insert the second letter of Row 3 into the tube (letter H)
    H. Insert the second letter of Row 4 into the tube (letter Apostrophe)
    I. Insert the third letter of Row 1 into the tube (letter L)
    J. Insert the third letter of Row 2 into the tube (orange for space)
    K. Insert remaining balls in the order as above until all are "transmitted"

2. It is critical that the person at the receiving end of the tube hold their hand over the end so that no balls escape until they are ready to be received one-at-a-time. Allow the "transmitter" to put a dozen or so ping pong balls into the tube before you begin receiving them. However, once you begin, do not drop ANY balls out of sequence, keep them in the exact order as you fill the egg carton (receiver).

Place the incoming ping pong balls into the empty egg carton in the exact order they are recvied as follows:

    A. Insert the first ping pong ball into the left-hand column of Row 1 (letter H)
    B. Insert the second ping pong ball into the left-hand column of Row 2 (letter N)
    C. Insert the third ping pong ball into the left-hand column of Row 3 (letter W)
    D. Insert the fourth ping pong ball into the left-hand column of Row 4 (letter I)
    E. Insert the fifth ping pong ball into the second column of Row 1 (letter E)
    F. Insert the sixth ping pong ball into the second column of Row 2 (letter O)
    G. Insert the seventh ping pong ball into the second column of Row 3 (letter H)
    H. Insert the eighth ping pong ball into the second column of Row 4 (letter Apostrophe)
    I. Insert the nineth ping pong ball into the third column of Row 1 (letter L)
    J. Insert the tenth ping pong ball into the third column of Row 2 (orange for space)
    K Insert remaining balls in the order as above until all are "received"

If all ping pong balls were transmitted and received correctly, the received egg carton should be a mirror image of the what the transmit egg carton looked like prior to transmission. NOTE- this experiment is more fun when the receiving team is unaware of the content of the messages. Once all balls transmitted and received, have them read you the received messages to see if they match what was transmitted?

The process demonstrated above is the Time Division Muliplexing (TDM) method that has been used in communications for approximately 50 years. Like slicing a loaf of bread, TDM utilizes a process called "interleaving", rapidly transmitting small slices of multiple emails, phone calls, etc. You will note that all four messages are now in the tube at the same time as slices of each are interleaved together, yet still seperated from each other in time (the time it takes to pass through the tube).

Repeat the experiment above a few times, allowing students the opprotunity to both transmit and receive. After several successful "transmissions", drop a ping pong ball at the transmit end occasionally, observing how it affects the received messages?

It should be obvious that the TDM method of transmission has one glaring potential failure point: if there are errors in the transmission elements or path, the received messages are unintelligible. It is critical that great care is taken at both ends of the circuit so that the receiver is synchronized with the transmitter. If the transmitter and receiver are not synchronized, occasional "bits" are lost, resulting in the need to retransmit information.

Emulating Packet-Based Transmission

In this experiment we will reuse all items from the preceding Time Domain Multiplexing Transmission excercize. However, for packet-based transmission, we will modify the ping pong balls, labeling each ball with a Row/Column address as follows:

Just above the letter on each ball, assign it the Row/Column address that cooresponds to the location it resides in the egg carton. There are four rows counting top to bottom and 6 columns counting left to right. For example, Row 1 Column A is the letter H, so we will add a "1a" just above the letter H, Row 1 Column B is the letter E, so we'll add a "1b" just above the letter E. Complete this for all 24 ping pong balls, including the orange balls that represent spaces.

Now dump all the ping pong balls into a shoebox.

"Transmit" the balls from the shoebox randomly into the tube. At the receiving end, rather than covering the tube with a hand, let all the ping pong balls just drop out of the tube into another cardboard box. As the ping pong balls begin to arrive in random order, grab a ball from the receiving shoe box and place it in the approriate Row/Column in the egg carton based on the address on the ball until all ping pong balls are received as shown below:

When all the ping pong balls have been received and "routed" to their proper destination (correct location in the egg carton), the receive egg carton should be a mirror image of the messages thet were in the transmit egg carton before transmission.

It is notable that, like TDM, Packet-Based transmission also utilizes a process called "interleaving", rapidly transmitting small slices of multiple emails, phone calls, etc. You will note that all four messages are now in the tube at the same time as slices of each are interleaved together but in random order (in TDM they were in exact order), yet still seperated from each other in time (the time it takes to pass through the network).

Repeat the experiment above a few times, allowing students the opportunity to both transmit and receive. After several successful "transmissions", drop a ping pong ball at the transmit end occasionally, observing how it affects the received messages? With Packet-Based transmission, even dropping occasional packets still allow the messages to be deciphered (figuring out what the missing letter should have been).

The process demonstrated above is the Packet-Based method that has been used in various forms and speeds in communications for approximately 45 years. Like TDM, Packet Transmission also slices messages like a loaf of bread, but rather than relying on a fixed method of exact end-to-end transmission, samples are assigned packet addresses and transmitted into a network of packet switches and routers that resemble a spider web network configuration.

In packet-based networks, switches and routers are located at every point where lines intersect, forming dozens or hundreds of potential routes from any point in the network to any other point, regardless of distance. Whereas the endpoints on a TDM network are largely fixed, a packet-based network provides many avenues for packets to travel so that if any segment fails, the packets automatically route around another way. In a TDM network, a single failure often means no messages can go through until the fault has been repaired.

Another interesting concept of packet-based networks is that, due to network congestion, temorary blocked routes or cut cables, as packets are transmitted each packet may actually travel a seperate route to the destination. For example, one packet may be sent along a faster route than another packet, so even though packets are transmitted in exact order at the originating end, they often arrive at the opposite end in random order. This is very different from a TDM network where bits of messages MUST arrive in exact order.

Once all the packets have arrived, the receiving equipment (iPhone, PC, TV, etc.) arranges the packets into the proper order so the messages are a true copy of what was originally transmitted.

Conclusions

After completing the experiments above, you may be wondering why anyone would still use TDM technology in a network? After all, aren't Packet-Based networks much more efficient and fault-tolerant? The answer is that TDM and Packet-Based networks must both provide reliable end-to-end pathways for two types of services:

1. Delay-Sensitive Services (voice phone calls, streaming video and music)
2. Non-Delay-Sensitive Services (email, twitter, text messaging, stored video/DVR, photos)

The original TDM network was designed to faithfully transmit voice, video and audio signals. These signals cannot be delayed and all "bits" of the messages must arrive in exact order without errors, or TV screens may have dropouts or freeze, audio "clicks" or voice dropouts interfere with phone calls, and in the day of FAX machines and dial-up modems, connections can drop.

Trying to send packets over a TDM network was like stuffing envelopes into hundreds of railroad cars on a long train.....doable, but not very efficient. Likewise, trying to send voice calls or streaming audio/video over a packet-based network requires the entire network between endpoints to "act" like a TDM network, as all transmitted bits of the voice call must arrive without delay and in exact order for the call to be intelligible.

So, we see there are advantages to both types of networks. Up until around the year 2000, most networks were predominantly TDM. However, by that time, most communcications were non-voice (emails, texts, streaming video, tweets, etc.), so Packet-Based networks began growing in popularity. Today, a majority of all network traffic is non-voice. Even though millions of phone calls take place every day, the network capacity for voice is very small as compared with the requirements for video, file transfers, email, tweets, texts, stored video, etc.), so all newer networks are Packet-Based. While several types of Packet-Based networks are in use, the most popular are Ethernet-based. Ethernet is the transmission medium used in your home and business, whether wired or wireless. Employing Ethernet at very high speeds in the network allows simple on ramp/off ramp of your data packets without the need for complicated conversions.

This concludes the experiments for Chapter 6- How 32,000 People Share the Same Optical Fiber

Don't forget you can order a sample fiber from a real telephone cable for use in some experiments by clicking the ORDER FIBER button on the top right of the Home page. Prices range from $.50 to $1.00 per fiber. Great way to earn extra credits in your next Technical Presentation, Science Fair or Merit Badge!