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.

[edit] 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

NEXT Noise Near End Cross Talk (NEXT) originates from other wires in the same cable. Cross talk is avoided by a network technician using proper installation procedures including: Strict adherence to RJ-45 termination procedures (Chapter 5); Using high quality twisted pair cabling
EMI/RFI Noise EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference) attack the quality of electrical signals on the cable. Sources of EMI/RFI include: Fluorescent lighting (EMI) Electrical motors (EMI) Radio systems (RFI) Two ways to prevent EMI/RFI Noise: Through shielding the wires in the cable with a metal braid or foil. (Increases cost and diameter of the cable) Through cancellation the wires are twisted together in pairs to provide self-shielding within the network media.
Canceling EMI/RFI Noise UTP Cat 5 has eight wires twisted into four pairs. In each pair, one wire is sending data and the other is receiving. As the electrons flow down the wire, they create a small, circular magnetic field around the wire. Since the two wires are close together, their opposing magnetic fields cancel each other. They also cancel out outside magnetic fields (EMI/RFI). Twisting of the wires enhances cancellation

Timing Problems 

Dispersion—similar to attenuation; is the broadening of a signal as it travels down the media. Jitter—caused by unsynchronized clocking signals between source and destination. This means bits will arrive later or earlier than expected. Latency—is the delay of a network signal caused by:

Time it takes a bit to travel to its destination Devices the bit travels through

Collisions 

Collisions occur in broadcast topologies where devices share access to the network media. A collision happens when two devices attempt to communicate on the shared-medium at the same time. Collisions destroy data requiring the source to retransmit. The prevention of collisions will be discussed in more detail later in the semester.

Encoding is the process of converting binary data into a form that can travel on a physical communications link. For our purposes, you only need to know the two types of encoding schemes most commonly used:

• Manchester
• NRZ (non-return to zero)

[edit] Standards -Cable Testing

The TIA/EIA-568-B standard specifies 10 test that a copper cable must pass if it is to be used for modern, high-speed Ehternet LANs .

The primary test parameters that must be avefified for a cable link to meet TIA/EIA-568-B standards are as follows:

• Wire Map
• Insertion Loss
• Near-end crosstalk (NEXT)
• Power sum near-end crosstalk(PSNEXT)\\
• Equal-level far-end crosstalk (ELFEXT)
• Power sum equal-level far-end crosstalk (PSELFEXT)
• Return Loss
• Propagation Delay
• Cable Length
• Delay Skew

The Ethernet standard specifies that each of the pins on an RJ-45 connector has a particular purpose. A NIC transmit signals on pin 1 & 2 and it receives on pin 3 & 6. The wires in UTP cable must be connected to the proper pins at each end of a cable.

TIA/EIA-A

TIA/EIA-568-B

MODULAR JACK PINOUT
1	TD+	Transmit data, positive going
2	TD-	Transmit data, negative going
3	RD+	Received data, positive going
4	UNUSED
5	UNUSED
6	RD-	Received data, negative going
8	UNUSED
9	UNUSED