From Mycomputer Notes

Revision as of 19:05, 28 February 2006

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

1. Buffers the arriving bits into memory for later processing.
2. 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. Attenuation is prevented by

1. Not exceeding a medium’s distance requirement (100 meters for Cat 5 cable)
2. By using repeaters that “amplify” the signal

Reflection 

Reflection refers to reflected energy resulting from an impedance mismatch between the NIC and network media. Impedance is the resistance to the flow of current in a circuit provided by the insulating material. When impedance is mismatched, the digital signal can “bounce back” (reflect) causing it to be distorted as bits run into each other

Noise 

Noise is unavoidable and too much can corrupt a bit turning a binary 1 into a binary 0 and vice versa, thus destroying the message. There is 5 kinds of noise

• • NEXT A
  • Thermal Noise
  • Impulse/Reference
  • Ground Noise
  • EMI/RFI
  • NEXT B

Our signaling is usually strong enough to override the effects of thermal noise. Reference Ground Noise can usually only be solved by an electrical contractor. There is only 2 types of noise we can control

1. NEXT (Near End Cross Talk) whether at the source (A) or the destination (B)
2. EMI/RFI