Cable testing
From Mycomputer Notes
In this chapter will discribe all the issues related to testing of the media used by the Physical Layer. In order for a LAN or WAN to properly work the physical layer medium must meet the industry standards. The Standard specified the data rate used to trasmit over Ethernet(10, 100, 1000 or 10,000 Mbps) In this chapter we will encounter the term signal, and it refers to the data signal that moves from the transmitter to the receiver. We know based on previous chapter that what travel as a signal is an electrical current. As the signal travels thorgh the physical media this signals [attenuates] however, the receiver must still be able to clearly determine the state of each bit (1 or 0) of data.
Analog signals have a continuously varying voltage-versus-time graph Digital signals have a square wave with instant transitions from low to high voltage states (0 to 1). Networks Use Digital Signaling Bits are represented by either no voltage (0) or +3 to +6 Volts (1). A Signal Reference Ground attached close to a computer’s digital circuits establishes the baseline for no voltage. Bits must arrive at the destination undistorted in order to be properly interpreted.
What six things can distort a bit?
- Propagation
- Attenuation
- Reflection
- Noise
- Timing Problems
- Collisions
- Propagation
- Propagation means travel. A bit takes at least a small amount of time to travel (propagate) down the wire from the source to the destination. If the receiving device cannot handle the speed of the arriving bits, data will be lost. To avoid data loss, computer have two mechanism
- Buffers the arriving bits into memory for later processing.
- Sending a message to the source to slow down the speed of propagation.
- Attenuation
- Attenuation is the loss of signal strength. The signal degrades or losses amplitude as it travels (propagates) along the medium. Loss of amplitude means that the receiving device can no longer distinguish a 1 bit from a 0 bit.
