> | Lab 4.2.5 Communications Circuit Estimated time: 50 min. Objectives: Design, build, and test a simple, complete, fast, and reliable communication system, using common materials. Background: For reliable communications to take place on a network many things must be defined ahead of time including the physical method of signaling and the meaning of each signal or series of signals. With this lab, you will create a very simple physical network and agree on some basic rules for communication in order to send and receive data. This will be a digital network based on the American Standard Code for Information Interchange (ASCII). It will be somewhat similar to the old telegraph Morse code based systems where the only means of communicating over long distance was by sending a series of dots and dashes as electrical signals over wires between locations. While the technology used will be much simpler than real systems, many of the key concepts of data communications between computers will arise. This lab will also help to clarify the functions of the layers of the OSI model. Tools / Preparation: Prior to starting the lab, the teacher or lab assistant should have several multimeters available (one for each team of students) and various items for construction of a simple communication network. Work in teams of 2 to 4. The following resources will be required. Be sure to review the purpose of each of the required items listed below since it will help in designing your network.
> Network construction item Required | Purpose
> Fluke 12B multimeter (or equivalent) | For testing communication connections
> 20' - Cat 5 UTP cable | For the physical communications lines (the cabling medium)
> ASCII chart | To help with coding and interpretation of signals (If you do not have a hardcopy of the 7-bit ASCII code chart, search for the Internet for the words "ACSII chart" and you will find several listed)
> Light switch | To activate the signaling device in order to create the digital on/off (binary) signals
> Light bulbs (6v) with bulb bases (as an alternative use LEDs with current-limiting resistors) | To act as the signaling device
> 6-Volt lantern battery | To power the signaling device
> Wire cutters/stripper | To adjust the length and prepare the ends of the communication lines
Lab Goals: Your group must design, build, and test a communications circuit with another team. You must communicate as much data as possible, quickly and with as few errors as possible. Spoken, written, or miscellaneous non-verbal communication of any kind is not allowed - only communication over the wire. You will agree as a team on the physical connections and on the coding you will use. One of the main goals is to send a message to the other team and have them interpret what you intended without them knowing ahead of time what your message was. Keep the OSI model in mind as you design your system. Layer 1 issues - You must connect 2 pairs of wire in order to have communication in both directions (half or full duplex.) Layer 2 issues - You must communicate some sort of frame start and stop sequence. This is a sequence of bits that is different than the character and number bits you will be transmitting. Layer 3 issues - You must invent an addressing scheme (for hosts and networks) if it is more than point-to-point communication. Layer 4 issues - You must include some form of control to regulate quality of service (e.g. error correction, acknowledgment, windowing, flow controls, etc.). Layer 5 issues - You must implement some way of synchronizing or pausing long conversations. Layer 6 issues - You must use some means of data representation (i.e., ASCII encoded as optical bits). Layer 7 issues - You must be able to communicate an idea supplied by your instructor or come up with a message on your own. Reflection Questions: What issues arose, as you tried to build your communications system, that you think apply to data communications between computers? Analyze your communications system in terms of the OSI layers.