CCNA 1 Module 4 Cable Testing

2 Waves Frequency: Frequency: –the number of waves per second Period: Period: –the amount of time that it takes to complete 1 cycle, in seconds Amplitude: Amplitude: –the height of the wave, measured in volts

3 Analog signal – sine wave Varies continuously with time

4 Analog signal – sine wave Phase = 1 Phase =.5 Phase = 0

5 Digital signal – square wave Maintains one value and then suddenly changes to a different value maintains one value and then suddenly changes to a different value

6 Signals Bits (0s and 1s) need to be transmitted from one host to another Bits (0s and 1s) need to be transmitted from one host to another Each bit is placed on the cable as a signal or pulse Each bit is placed on the cable as a signal or pulse On copper cable the pulses are electrical signals of different voltage levels On copper cable the pulses are electrical signals of different voltage levels The simplest signalling schemes have only two voltage levels The simplest signalling schemes have only two voltage levels

7 A pulse

8 Fourier Synthesis of a Square Wave

9 Noise Noise is unwanted additional electrical signals on a cable Noise is unwanted additional electrical signals on a cable The extra signals are added to the data signals and distort them The extra signals are added to the data signals and distort them Original signal Noise Resulting signal

10 Possible sources of noise: Possible sources of noise: –Nearby cables which carry data signals –Radio frequency interference (RFI), which is noise from other signals being transmitted nearby –Electromagnetic interference (EMI), which is noise from nearby sources such as motors and lights –Laser noise at the transmitter or receiver of an optical signal Noise

11 Bandwidth Two ways of considering bandwidth in communications systems are analog and digital. Two ways of considering bandwidth in communications systems are analog and digital. –Analog bandwidth describes the range of frequencies transmitted ( hertz; Hz) –Digital bandwidth measures how much information can flow from one place to another in a given amount of time (bps)

12 Signals and Noise Bits are represented by either no voltage (0) or +3 to +6 Volts (1). 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. A Signal Reference Ground attached close to a computer’s digital circuits establishes the baseline for no voltage. The reference level is called a signal ground The reference level is called a signal ground Bits must arrive at the destination undistorted in order to be properly interpreted. Bits must arrive at the destination undistorted in order to be properly interpreted.

13 Shielded vs Unshielded Cable Shielding material protects the data signal from external sources of noise and from noise generated by electrical signals within the cable Shielding material protects the data signal from external sources of noise and from noise generated by electrical signals within the cable Benefits of shielding Benefits of shielding –protects the inner conductor from external electrical noise –keeps the transmitted signal confined to the cable –protects each wire pair from noise generated by the other pairs Shielded cable examples: coaxial, STP, ScTP Shielded cable examples: coaxial, STP, ScTP Unshielded cable: UTP Unshielded cable: UTP

14 Shielded cable helps insulate the signal from external noise. Shielded cable helps insulate the signal from external noise. Shielding makes the cable thicker and therefore more difficult to install. Shielding makes the cable thicker and therefore more difficult to install. Twisted pair cable uses twisting of pairs of wires to cancel signals from adjacent wires. Twisted pair cable uses twisting of pairs of wires to cancel signals from adjacent wires. Shielded vs Unshielded Cable

15 Optical Cable Fiber optic cable: Fiber optic cable: –transmits data signals by increasing and decreasing the intensity of light. –needs no insulation as it is not affected by electrical noise. –does not need to be grounded. –is often used between buildings and between floors within the building.

16 What six things can distort a bit? Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing problems Timing problems Collisions Collisions

17 Propagation The travelling of bits down a wire The travelling of bits down a wire –Devices (eg printer) at receiving end cannot handle the speed, and data is lost –Solve with buffers on the device or a message to slow down the transmission rate

18 Bits are distorted by: Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing problems Timing problems Collisions Collisions

19 Attenuation Loss of signal strength as bits move down a wire (dB) Loss of signal strength as bits move down a wire (dB) –1s look like 0s, and the message is unreadable –Test by using the highest frequencies possible with the cable. –Solved by using repeaters, hubs to amplify the signal

20 Impedance is caused: Impedance is caused: –Exceeding the maximum recommended cable length Use repeaters or don’t exceed the 100m Use repeaters or don’t exceed the 100m –by defective connectors –when signal energy is lost, when it leaks through the insulation of the cable –by the resistance of the copper cable converting some of the electrical energy of the signal to heat Impedance

21 Bits are distorted by: Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing problems Timing problems Collisions Collisions

22 Reflection Reflected energy moves back towards signal, distorting it as bits run into each other. Reflected energy moves back towards signal, distorting it as bits run into each other. –Caused by impedance mismatch (NIC and media) –Jitter is created as the reflected signal bounces back and forth. –Solve by correct wiring

23 Bits are distorted by: Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing problems Timing problems Collisions Collisions

24 Noise Unwanted additions to a signal Unwanted additions to a signal Results in 1s turned into 0s and 0s into 1s. Too much noise can destroy the message. Results in 1s turned into 0s and 0s into 1s. Too much noise can destroy the message. It is important to keep the signal to noise ratio as high as possible (little noise) It is important to keep the signal to noise ratio as high as possible (little noise) It is not possible to avoid noise altogether It is not possible to avoid noise altogether

25 Four kinds Four kinds –Thermal –Reference ground noise –EMI/RFI –Crosstalk Noise

26 Thermal noise is everywhere but isn’t a problem. Thermal noise is everywhere but isn’t a problem. Reference ground noise is AC line noise, fixed by rewiring. Reference ground noise is AC line noise, fixed by rewiring. Electromagnetic interference (EMI) Electromagnetic interference (EMI) Radio Frequency interference (RFI) Radio Frequency interference (RFI) External Noise

27 EMI/RFI EMI and RFI attack the quality of electrical signals on the cable. EMI and RFI attack the quality of electrical signals on the cable. Sources of EMI/RFI include: Sources of EMI/RFI include: –EMI – fluorescent lights; electric motors –RFI – Radio systems

28 EMI/RFI noise Source computer sends out a digital signal. Source computer sends out a digital signal. EMI noise occurs along the path. EMI noise occurs along the path. The extra signal is added to the data signals and distorts it. The extra signal is added to the data signals and distorts it. Original signal Noise Resulting signal

29 Two ways to prevent EMI/RFI Noise: Two ways to prevent EMI/RFI Noise: –shielding the wires in the cable with a metal braid or foil. (Increases cost and diameter of the cable) –cancellation – the wires are twisted together in pairs providing self-shielding within the network media. EMI/RFI noise

30 EMI/RFI noise UTP Cat 5 has eight wires twisted into four pairs. UTP Cat 5 has eight wires twisted into four pairs. In each pair, one wire is sending data and the other is receiving. In each pair, one wire is sending data and the other is receiving. Electrons flowing down the wire create a small, circular magnetic field around the wire. Electrons flowing down the wire create a small, circular magnetic field around the wire.

31 EMI/RFI noise Since the two wires are close together, their opposing magnetic fields cancel each other. Since the two wires are close together, their opposing magnetic fields cancel each other. They also cancel out outside magnetic fields (EMI/RFI). They also cancel out outside magnetic fields (EMI/RFI). Twisting of the wires enhances cancellation Twisting of the wires enhances cancellation

32 Noise on Copper Media

33 Internal Noise - Crosstalk 1. A voltage signal passes along a wire 2. It generates energy that radiates out, like a radio signal 3. Other nearby wires can pick up the signal 4. This signal adds a small voltage to the signal on the nearby wires 5. This effect is called crosstalk 6. Crosstalk is cancelled by the twisting of pairs of wires.

34 Testing for Crosstalk The cable testing instrument puts a signal on one wire pair in a cable. The cable testing instrument puts a signal on one wire pair in a cable. It then measures, in dB the amplitude of the crosstalk signals. It then measures, in dB the amplitude of the crosstalk signals. The lower (smaller) the negative number, the more noise there is. The lower (smaller) the negative number, the more noise there is. –-30dB is a CLEARER signal than -10dB. Each pair should be measured from each other pair in a UTP link, and from both ends of the link. Each pair should be measured from each other pair in a UTP link, and from both ends of the link.

35 Types of Crosstalk - NEXT Near-end Crosstalk Near-end Crosstalk NEXT is the test signal measured from the same end of the link (cable). NEXT is the test signal measured from the same end of the link (cable). Radiated EM energy

36 Types of Crosstalk - FEXT Far-end Crosstalk Far-end Crosstalk FEXT occurs further away from the source, like at the other end of the cable. FEXT occurs further away from the source, like at the other end of the cable. Due to attenuation, FEXT creates less noise than NEXT. Due to attenuation, FEXT creates less noise than NEXT. Generates weak FEXT at other pairs

37 Power Sum Near-end Crosstalk Power Sum Near-end Crosstalk PSNEXT is the combined effect of NEXT on one pair of wires from the other 3 pairs of wires in the cable. PSNEXT is the combined effect of NEXT on one pair of wires from the other 3 pairs of wires in the cable. Cables need to be tested to measure the amount of PSNEXT. Cables need to be tested to measure the amount of PSNEXT. It is particularly important when all the wires in a cable are being used for transmission, as in Gigabit Ethernet. It is particularly important when all the wires in a cable are being used for transmission, as in Gigabit Ethernet. Types of Crosstalk - PSNEXT

38 Bits are distorted by: Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing problems Timing problems Collisions Collisions

39 Timing Problems Dispersion – similar to attenuation; is the broadening of a signal as it travels down the media. Dispersion – similar to attenuation; is the broadening of a signal as it travels down the media. Jitter – the reflected signal strikes the first discontinuity, and some of the signal rebounds in the original direction, creating multiple echo effects. The echoes strike the receiver at different intervals. This is called jitter and can result in data errors. Jitter – the reflected signal strikes the first discontinuity, and some of the signal rebounds in the original direction, creating multiple echo effects. The echoes strike the receiver at different intervals. This is called jitter and can result in data errors. Latency – is the delay of a network signal caused by: Latency – is the delay of a network signal caused by: –The time it takes a bit to travel to its destination –Devices the bit travels through

40 Bits are distorted by: Propagation Propagation Attenuation Attenuation Reflection Reflection Noise Noise Timing problems Timing problems Collisions Collisions

41 Collisions Collisions occur in broadcast topologies where devices share access to the network media. 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. 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. Collisions destroy data requiring the source to retransmit. The prevention of collisions will be discussed in more detail later in the semester. The prevention of collisions will be discussed in more detail later in the semester.

42 Cable Testing Standards TIA/EIA-568-B standard applies to copper cable in a LAN. TIA/EIA-568-B standard applies to copper cable in a LAN. Each pair of pins on an RJ45 connector have a specific purpose. Each pair of pins on an RJ45 connector have a specific purpose. –A NIC transmits signals on pins 1 and 2, and receives signals on pins 3 and 6. –The other four pins are not used except in gigabit ethernet.

43 TIA/EIA-568-B Standard – the ten cable tests 1.Wire map 2.Insertion loss 3.Near-end crosstalk (NEXT) 4.Power sum near-end crosstalk (PSNEXT) 5.Equal-level far-end crosstalk (ELFEXT) 6.Power sum equal-level far-end crosstalk (PSELFEXT) 7.Return loss 8.Propagation delay 9.Cable length 10.Delay skew

44 Wire Map ensures that no open or short circuits exist on the cable (Good Wire Map) ensures that no open or short circuits exist on the cable (Good Wire Map) an ‘Open’ circuit occurs if a wire does not attach properly at the connector. an ‘Open’ circuit occurs if a wire does not attach properly at the connector. an ‘Short’ circuit occurs if two wires are connected to each other. an ‘Short’ circuit occurs if two wires are connected to each other.

45 Wire Map and Wiring Faults The wire map verifies that all eight wires are connected to the correct pins on both ends of the cable The wire map verifies that all eight wires are connected to the correct pins on both ends of the cable The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector. The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector.

46 Wire Map and Wiring Faults A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Less cancellation means more crosstalk. A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Less cancellation means more crosstalk. A transposed pair fault occurs when two wire pairs are correct at one end but reversed at the other end. A transposed pair fault occurs when two wire pairs are correct at one end but reversed at the other end.

47 –Wires inside the cable are twisted –Signals actually travel farther than the physical length of the cable TDR measurement: TDR measurement: –it sends a pulse signal down a wire pair and measures the amount of time required for the pulse to return on the same wire pair TDR test TDR test –determine length –identify the distance to wiring faults such as shorts &opens –When the pulse encounters an open, short, or poor connection, all or part of the pulse energy is reflected back to the tester Time Domain Reflectometry-TDR

48 A New Standard On June 20, 2002, the Category 6 (or Cat 6) addition to the TIA-568 standard was published On June 20, 2002, the Category 6 (or Cat 6) addition to the TIA-568 standard was published The official title of the standard is ANSI/TIA/EIA-568-B.2-1 The official title of the standard is ANSI/TIA/EIA-568-B.2-1 Cables certified as Cat 6 cable must pass all 10 tests Cables certified as Cat 6 cable must pass all 10 tests Cat 6 cable must pass the tests with higher scores to be certified. Cat 6 cable must pass the tests with higher scores to be certified.

49 Good Luck on the Exam!!