Hands-on Activity: Acting Out Phone Call Routing: Hello, Are You Listening?

Contributed by: Making the Connection—Women in Engineering Programs and Advocates Network (WEPAN)

Three photos show people talking on cell phones, one with a tall truss-style tower behind him that has drum- and dish-like attachments.
How does a phone call find you?
copyright
Copyright © 2004 Microsoft Corporation, One Microsoft Way, Redmond, WA 98052-6399 USA. All rights reserved.

Summary

Students gain a basic understanding of the engineering components behind telecommunications, in particular, the way telephone communication works to link one phone to another for conventional landline and cellular telephones. During this entire-class activity, students simulate how phone calls are connected by acting out a variety of searches for both local and long-distance calls. Students end up with a good understanding of how phone calls are transmitted from callers to recipients.

Engineering Connection

Since the invention of the telephone, human communication devices have increased in their variety, complexity and pervasiveness in our lives. Engineers are big players in these changes, and the solution for call routing has existed for decades. By developing their own strategies and understanding of what type of information is required to get calls from one location to another, students develop a more thorough understanding of how this type of communication operates. The activity also illustrates how engineers approach solutions to systems that require the transfer of large amounts of information.

Learning Objectives

After this activity, students should be able to:

  • Understand technology as a system with inputs and outputs.
  • Make decisions related to advantages and disadvantages of products and processes.
  • Gain knowledge by using resources, such as people, references and the internet.
  • Use mathematical scaling.
  • Consider environmental impact of designs.

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Middle School Lesson

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Systems have parts or components that work together to accomplish a goal. (Grades K - 2) Details... View more aligned curriculum... Do you agree with this alignment?
  • Technology enables people to communicate by sending and receiving information over a distance. (Grades K - 2) Details... View more aligned curriculum... Do you agree with this alignment?
  • Information and communication systems allow information to be transferred from human to human, human to machine, and machine to human. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Identify and explain the components of a communication system, i.e., source, encoder, transmitter, receiver, decoder, storage, retrieval, and destination. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Explain how the various components (source, encoder, transmitter, receiver, decoder, destination, storage, and retrieval) and processes of a communication system function. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
Suggest an alignment not listed above

Materials List

For Part 1, the class needs:

  • 2 sheets of paper that are two different colors, on which to copy Making the Connection Worksheets A and C doubled-sided on one piece, and copy Worksheets A and D double-sided on the different color piece
  • scissors

For Part 2, the class needs:

  • 2 sheets of paper that are two different colors, on which to copy Making the Connection Worksheets B and C doubled-sided on one piece, and copy Worksheets B and D double-sided on the different color piece
  • masking tape, to mark a line on the floor, ~20-30 meters
  • yarn or string, ~85-125-meters (the amount needed will vary depending on your class size and how many student cell towers you assign; this estimate is based on needing 6-10 10-meter lengths for cell tower circles, a 4-meter length for the traveling student, and no more than a 20-meter length for the stationery landline)
  • scissors
  • paper and pencils, for student sketching
  • (optional to print out) Part 2 Cell Phone Activity Setup Sketch

Introduction/Motivation

Think of the hundreds of homes in your town. How would you determine an effective way for people to place calls, not only to each other, but also to homes in other communities and states?

People who make calls want to make sure that when they place a call it gets through to the destination without any problems. How do you determine what is effective? Is it fast connections? Is it as many connections as possible? Is it a combination of both?

Let's experiment and find out.

Vocabulary/Definitions

cell phone: An instrument that uses two frequencies, one for receiving and one for transmitting, to send voice messages.

cell site: A single tower's coverage area.

central processing: A switching center for long distance calls.

conjunction: A special switching site between cell towers and landline phones.

landline: Telephones connected to a conventional wiring system. Many home phones are landlines.

local call: A call to a place within the same town, city or neighboring community.

local switching center: A site at which incoming calls are routed to the correct number.

long-distance call: A call to a place outside your local call area.

telephone: An instrument that sends and receives voice messages and data.

Procedure

Background for Part 1: Call Routing in Conventional Telephone Communications — When you lift a telephone handset, it connects to a local phone company's switching center and sends a dial tone over the line to let you know that the line is open and good.

When you dial a number that you want to call, the local switching center compares the dialed town exchange (first three of a seven-digit number, for example, 723-xxxx) to its own three-digit exchange. If the exchanges are the same, the local switch opens a direct connection to the number you are calling and makes the receiving phone ring. If the local exchange is different that the exchange of your local switch, the local switch center opens a connection to the external switching center in another town that has the same local exchange you are dialing. Once the switching center has received the information, it opens the connection to the number you are dialing.

If you dial a long-distance number, it always starts with a 1, followed by the area code and seven-digit number. When the local switching center "sees" the 1, it passes the call to the nearest switching center for your long-distance carrier.

The long-distance switching center then finds the best path to route the call to get to the correct area code and local switching center. Once the call gets to the correct receiving local switching center, a direct connection is made to the receiving phone.

As you go through the activity with the students, lead them through the process by asking the questions provided in the activity. While they will not be designing and building a physical device or object, they are developing a suggested approach for the solution and should use the outlined process. Encourage them to be creative with their solutions.

Background for Part 2: Cellular Telephone Communications — Cell phones are sophisticated radios transmitting information (calls) by radio waves through the air. Every cell phone carrier breaks up geographic areas into "cells." Each cell site has a base station with a transceiver and antenna. The range for each base station ranges from 2-10 miles (3-16 km) from the antenna. Each carrier in an area runs a central office called the mobile telephone switching office (MTSO).

To receive a call, the MTSO gets the call first and tries to find your phone in each cell of the region until your phone responds. When it finds your phone, it connects you to the base station of that cell so you can talk and listen. As you move to the edge of the cell, you cell's base station notes that your signal strength is diminishing and another base station detects that the signal is increasing in the new cell. The two base stations coordinate themselves through the MTSO and your phone gets a signal to change frequencies. This is called a "handoff." The cell phone user is unaware that any of this is occurring.

If the call is between a cell phone and a landline, the MTSO makes the connection through a local conjunction. The conjunction is a special switching site between the cell tower bases and the copper wires used for connecting calls through the switching centers.

Before the Activity

For Part 1:

  • Make a double-sided copy of Making the Connection Worksheet A and C.
  • On a different color piece of paper, make a double-sided copy of Worksheet A and Worksheet D.
  • Cut each page along the dotted-line marked sections. Use as many phone numbers as the number of students.

For Part 2:

  • Identify a larger open space for the activity, such as a hallway or classroom. Use masking tape to mark a "highway" that winds across the floor of the space. Refer to the attached Part 2 Cell Phone Activity Setup Sketch.
  • Make a double-sided copy of Making the Connection Worksheet B and C.
  • On a different color piece of paper, make a double-sided copy of Worksheet B and Worksheet D.
  • Cut each page along the dotted-line marked sections. Use as many phone numbers as the number of students.
  • Have string/yarn and scissors ready to cut pieces.

With the Students: Part 1: Call Routing in Conventional Telephone Communications

Routing Local Calls

  1. Have students spread out across the room. Give each one a home number and a call number, making sure that the phone numbers are different.
  2. Have students think about what might happen when you make a phone call. What path does the phone call follow? Have them propose efficient solutions and explain how it works. Have the students demonstrate their ideas to see if they work.
  3. Select one student to "make a call." Have the remaining students place their phone numbers face down so that the word "Home #" faces up. The calling student takes his/her Call # and goes around trying to find the matching Home #. To do this, s/he approaches students and asks them to show her their Home #s. When s/he finds the matching one, the call is connected. Try to make the connection as quickly as possible.

Routing Long-Distance Calls

  1. Distribute one Home # and Call # to each student.
  2. What have we learned so far? Engage students to discuss how they want to set up the calling network, given their experiences with the local calling activity. Do they want to just try to find numbers? Do they want to use exchange managers? Should the exchange managers be local, state or both?
  3. Have students create and test their system for how quickly the calls are placed.
  4. Discuss the solution tested. Do you have other approaches that you would like to try?
  5. Finish with a discussion about how the students acted as engineers.

With the Students: Part 2: Cellular Telephone Communications

  1. Provide a larger open working space for this part of this activity, such as a hallway or classroom, giving students room to move around. Mark a "road" with masking tape that winds across the floor to represent a major highway crossing your state, or a smaller road that goes between local towns in your area. Refer to the Part 2 Cell Phone Activity Setup Sketch.
  2. Assign student roles. One student remains stationary and is the person placing the call from a conventional phone. Another student travels along the road with a cellular phone. The remaining students represent cell towers (or, if space is limited, choose 6-10 students to represent cell towers), one being a special tower that connects conventional (landline) telephones to cellular telephones.
  3. Cut for each cell tower student a 10-meter length of yarn. Cut for the traveling student a 4-meter length of yarn. The stationary caller gets a piece of yarn long enough to reach the special tower.
  4. Ask the student "engineers":
  • Where would be best to place the cellular phone towers.
  • Are some traffic routes used more than others?
  • Propose your locations and be ready to explain why you selected them.
  • Are your placements possible?
  • What is the range of each tower?
  • What happens if you are driving a fairly long distance and pass several towers?
  • Will your phone call continue to have a strong signal?
  • What other information do you need to know?
  1. Have the cell tower students arrange themselves along the road. Have them each make a circle out of the yarn and stand at the center of it. The circle represents the area in which the cell tower can transmit and receive calls. To help students strategize, have them sketch out their solution(s) on paper first and then place themselves according to the sketch. (Math inclusion: Provide a scaled map of the area and have students determine the scaled representation of coverage area for their cell towers.)
  2. After the cell towers are placed, the traveling student moves along the road, holding one end of the 4-meter yarn. When inside a tower's coverage area (inside a marked circle) that cell tower holds the other end of the 4-meter yarn, symbolizing the communication between the cell phone and the tower.
  3. As the traveling phone moves, the yarn end is passed from tower to tower. If a place is found where the traveler is outside all cell tower circles, one end of the yarn is free, meaning no connection for the call exists.
  4. After students have created a complete path for the cell phone user, stop and mark off a section on the road that is designated as preservation land. No cell towers can be built in this area. Have the cell towers re-position themselves to try to create as much coverage for travelers as possible.
  5. Conclude with a class discussion on how the communication network would change if it were one cell phone calling another. If time permits, have students demonstrate how this system would work.

Attachments

Investigating Questions

  • Do you think each phone call checks every other phone until it finds the correct one? (Answer: No, the local switch center connects it to the one right number.)
  • How long do you think it takes to find the right phone? (Answer: It typically takes less than 1/100 of a second for the local switching center to receive a call and make a connection to the correct number.)
  • Would it make sense to have one central processing place for all phone calls? (Answer: No, because it would have to look at all incoming calls and make each connection one at a time, which would take a long time.)
  • How do you think the tower keeps track of all the different phones for which it is picking up signals? (Answer: The MSTO tracks all the cell phones that are on, even if they are not being used. Each phone has its own identification number that is transmitted while the phone is on.)
  • How do you think a connection is made between a cell phone and a conventional (or landline) phone? (Answer: The cell tower transmits the call to the MSTO, which then sends the call through a local conjunction to the copper wires connecting land-based phones.)

Assessment

Pre-Activity Open Discussion: Ask the class and solicit answers from students:

  • How would you determine an effective way for people to place calls, not only to each other, but also to homes and businesses in other communities and states?

Activity Embedded Assessment: Observe student participation during the activity. Clarify concepts as needed.

Post-Activity Class Discussion: Conclude with a class discussion (and acting out, if time) to explore how engineers would change the network to accommodate cell phones calling each other. During this discussion, gauge students' depth of understanding of the call routing factors to be considered.

Activity Extensions

Research how many switching centers some long-distance carriers have and where they are located. Show them on a national map.

Build your own telephone network. Supplies: two telephones, one 9-volt battery, a 300-ohm resistor, and telephone wire. Follow the "Creating Your Own Telephone Network" activity description at: http://communication.howstuffworks.com/telephone4.htm.

Assign one or two students to represent more powerful transmission cell towers. Give them each 15-meter long sections of yarn to define their transmission areas, and re-do the activity.

Research the location of cell site towers in your community and surrounding area. Make a map that shows each tower and its coverage area. If you were assigned to expand the coverage area, where would you put the new towers?

Activity Scaling

  • For more advanced students, have them conduct one or more of the activity extensions.
  • For more advanced students, add a math component. Provide a scaled map of your area and have students determine the scaled representation of coverage area for their cell towers.

References

Brain, Marshall. "How Telephones Work." Updated April 1, 2000. HowStuffWorks.com. Accessed August 26, 2010. http://communication.howstuffworks.com/telephone.htm

Brain, Marshall, Jeff Tyson and Julia Layton. "How Cell Phones Work." Updated November 14, 2000. HowStuffWorks.com. Accessed August 26, 2010. http://www.howstuffworks.com/cell-phone.htm

Farley, Tom, and Mark van der Hoek. Cellular Telephone Basics. Posted January 1, 2006. Privateline Telecommunications Expertise. Accessed August 26, 2010. http://www.privateline.com/mt_cellbasics/index.html

"Telephone." Updated June 11, 2009. Wikipedia, The Free Encyclopedia. Accessed August 26, 2010. http://en.wikipedia.org/w/index.php?title=Telephone&oldid=295832290

Contributors

Martha Cyr

Copyright

© 2013 by Regents of the University of Colorado; original © 2001 WEPAN/Worcester Polytechnic Institute

Supporting Program

Making the Connection—Women in Engineering Programs and Advocates Network (WEPAN)

Acknowledgements

Project funded by Lucent Technologies Foundation.

Last modified: February 17, 2018

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