Cisco CCNA Sem 1 Chapter 4 Cable Testing, Cabling LAN’s and WAN’s

1. Cisco CCNA Sem 1 Chapter 4Cable Testing, Cabling LAN’s and WAN’s • Terms to understand • Waves – energy traveling form one place to another • Period – time between waves • Frequency – Number of waves in a given time period (measured in waves per second called hertz • Amplitude – Height of wave (for electrical signals, this is volts)

2. Waves • Deliberate disturbance with fixed, predictable duration is called a pulse • Pulses determine value of the data being transmitted • Three types of waves are of interest in networking: • Voltage waves on copper media • Light waves in fiber optic • Alternating electric and magnetic fields in wireless communitcation

3. Sine Waves and Square Waves • Sine waves are graphs of mathematical functions: • Y=5 * Sin(x) • Periodic – repeat at regular intervals • Continuously variable • Analog waves

4. Square Waves • Like analog waves are periodic • Do not vary continuously with time • Represent digital pulses or signals • Describe by Amplitude, Frequency and period

5. Decibels • Decibels are measures of power • dB=10log10(Pfinal/Pref) • dB=20log10(Vfinal/Vref) • dB measures loss or gain of power of a wave. Usually negative • Log10 uses base 10 logarithm • Pfinal is delivered power in watts • Pref is original power in watts • Vfinal is delivered voltage in Volts • Vref is original voltage in Volts

6. Signals in Time and Frequency • Data can be represented by voltage patterns • Voltage patterns can be viewed graphed against time by an oscilloscope • X-axis (domain) represents time • Time-domain analysis • Spectrum analyzer analyzes signals against a frequency as the x-axis. • Frequency-domain analysis

7. Noise in Time and Frequency • Noise – Undesirable signals • Sources of Noise • Nearby cables that carry signals • Radio Frequency Interference (RFI) • Electromagnetic Interference (EMI) • Laser noise at Tx or Rx • Noise that affects all frequencies equally – white noise • Noise that affects only small range of frequencies – narrowband interference

8. Analog and Digital Bandwidth • Analog Bandwidth – refers to frequency range of an analogy electronic system • Range of frequencies transmitted by radio station or electronic amplifier • Units of analog bandwidth is Hz • 3 kHz telephony • 20 kHz for audible signals • 5 kHz for AM radio • 200 kHz for FM

9. Digital Bandwidth • Digital Bandwidth – how much information can flow • Units of measurement are bps • Usually expressed as kbps or mbps

10. Use of analog bandwidth in cable testing • Analog bandwidth is used in cable testing to determine digital bandwidth of copper media • Analog signal Tx on one end, and Rx on other. • Attenuation is calculated • In general, higher analog bandwidth = higher digital bandwidth.

11. Signals and Noise on Networking Media • Noise – any interference on physical media that makes it difficult for receiver to detect signal • Copper media susceptible to several sources of noise • Optical fiber considerably less susceptible • Proper installation of cable and connectors limit noise and attenuation

12. Signals and Noise on Networking Media (Cont’d) • After installation of physical medium, must be tested to meet TIA/EIA 568-B standards • After installation, periodic testing of cables and connectors required in order to insure continued network performance

13. Signaling over Copper and Fiber-Optic Cabling • Bits are represented by voltage changes • Voltage changes are measured against a reference ground. • Voltages are generally at <= 5 volts. • Signals can’t be amplified or extended duration at receiver • As much of the original signal as possible is required to reach receiver

14. 2 types of copper cable • Shielded • Protect against external noise sources • Some types of shielding protect against internal noise sources • Unshielded

15. Coaxial Cable • Coaxial cable- solid copper core surrounded by insulating material, then braided conductive shielding. • Conductive shielding must be properly grounded • Prevents external noise from disrupting signal • Helps keep signal loss down by confining signal to cable • Less noisy than Twisted pair • Bulky, more expensive, must be grounded

16. Twisted pair cable • 2 types • Shielded Twisted Pair (STP) • Screened Twisted Pair (ScTP) • Foil Twisted Pair (FTP) • Outer conductive shield that is grounded • Inner foil shields around each wire pair • More expensive and difficult to install than UTP. Less frequently used • Unshielded Twisted Pair (UTP) • Inexpensive and easy to install

17. Fiber Optic Cable • Tx data by increasing and decreasing light intensity to represent binary 1’s and 0’s • Strength of signal doesn’t diminish over same distance as copper • Not affected by electrical noise • Doesn’t require grounding • Often used between buildings and floors.

18. Attenuation and Insertion Loss on Copper Media • Attenuation – decrease in signal amplitude over length of link • Long cable lengths and high frequencies lead to greater attenuation • Attenuation measured by cable tester using highest frequencies that cable is rated to support • Attenuation expressed in dB using negative numbers • Smaller negative dB values indicate better link performance

19. Factors leading to attenuation • Resistance of copper cable converts energy of signal to heat • Signal lost when leaks through insulation of cable • Impedance caused by defective connectors

20. Impedance • Measurement of resistance of cable to AC current in ohms (Ω) • CAT 5 normal is 100 Ω • Improper connector installation creates a different impedance than cable • Impedance discontinuity or Impedance mismatch • Causes attenuation because part of signal is reflected back to Tx (similar to an echo). • Multiple discontinuities compound problem. As echo reverberates through cable, Rx can’t accurately detect signal values. • Effect is called Jitter • Combination of Attenuation and Impedance discontinuities called Insertion Loss

21. Source of Noise on Copper Media • Noise – any electrical energy on Tx cable that makes it hard for Rx to interpret data • TIA/EIA-568-B requires testing for variety of noise.

22. Types of Noise • Crosstalk – Tx of signals from one wire pair to nearby pairs • Wires act like radio antennas generating similar signals • Cause interference with data on adjacent wires • Can come from separate nearby cables • Comes from other cables called alien crosstalk • More destructive at higher Tx frequencies • Cable testing applies signal to one pair of wires and measures amplitude of unwanted signals induced in other pair of wires • Occurs when wire pairs untwisted

23. Three types of Crosstalk • Near-end crosstalk (NEXT) • Far-end crosstalk (FEXT) • Power sum near-end crosstalk (PSNEXT)

24. NEXT • Computed as ratio in voltage amplitude between test signal and crosstalk signal when measured from same end of the link • Expressed in negative dB values • Low negative values indicate more noise • Cable testers don’t show negative sign • 30 (really -30) dB is better than 10 (-10) dB • Needs to be measured every pair to every pair

25. FEXT • Far-end crosstalk • Less noise than NEXT because of attenuation • Noise is still sent back to Tx, but is significantly less because of attenuation • Not as significant as NEXT

26. PSNEXT – Power sum near-end crosstalk • Measures cumulative effect of NEXT from all wire pairs • Combined affect from multiple simultaneous transmission can degrade signal • TIA/EIA-568-B now requires PSNEXT test • 1000BASE-T receive data simultaneously from multiple pairs in same direction. PSNEXT is important test

27. Cable Testing Standards • Primary tests to meet TIA/EIA-568-B • Wire map • Insertion loss • Near-end cross talk – 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

28. Wire map • Assures no Open or Short circuits in cable • Open circuit – wire not attached correctly at a connection • Short circuit – two wires connected to each other • Also assures wires attached to correct pins on both sides • Reversed pair fault: Correct on one side, reversed on other • Split-pair: 2 wires from different wire-pairs are connected to wrong pins on both ends of the cable • Transposed pair: wire pair is connected to completely different pins at both ends or two different color codes used on punch-down blocks (T568A and T568B)

29. Other Test Parameters • Crosstalk • NEXT • ELFEXT: Equal-level far-end crosstalk • Measure FEXT • Pair-to-pair ELFEXT expressed in dB as difference between measured FEXT and insertion loss • Important test in 1000BASE-T networks • PSELFEXT • Combined effect of ELFEXT from all wire pairs • Return loss • Measured in dB from return signals due to impedance. Not loss in signal, but in signal jitter.

30. Time-Based parameters • Propagation delay – time it takes for signal to travel along cable being tested. • Depends on length, twist rate, electrical properties • Delays measured in hundreths of nanoseconds. • Basis of cable length measurements based on Time Domain Reflectometry (TDR) • Can also identify distance to wiring faults • Delay difference between pairs of wires is called Delay Skew • Critical in 1000BASE-T networks

31. Testing Fiber-Optic Cables • Subject to optical equivalent of impedance discontinuities • Portion of light reflected back along path resulting in less light at receiver • Improperly installed connectors main cause of impedance discontinuities • Amount of acceptable light loss is called optical link loss budget • Fiber test instrument measure light loss, and can indicate where optical discontinuities exsist. • After faults are corrected, cable must be retested

32. New Cable Standard • June 20, 2002 ANSI/TIA/EIA-568-B.2.1 – CAT 6 standard • Standard sets tests for certification • CAT 6 same as CAT 5 but higher standards • Lower levels of crosstalk and return loss • Capable of supporting frequencies of 250 MHz

33. LAN Physical Layer (Layer 1 OSI) Ethernet 1000BASE-CX 100BASE-T4 1000BASE-T 100BASE-FX 100BASE-TX 10BASE-F 10BASE-T 10BASE2 10BASE5

34. LAN Physical Layer Symbols • Token Ring • FDDI Ring • Ethernet Line • Serial Line Token Ring FDDI

35. Ethernet technologies in campus LAN • Fast Ethernet and Gigabit Ethernet • User level for good performance • Clients or servers with high bandwidth • Link between user-level and network devices • Connecting to Enterprise level servers • Switches and Backbone

36. Ethernet Media Connector Requirements

37. Connection Media • RJ-45 – A connector used for finishing twisted-pair wire • AUI – Attachment Unit Interface • An interface for connecting NIC that may not match media connecting to it • GBIC – Gigabit Interface Converter • Used at interface between Ethernet and fiber-optic systems • GBIC transceiver converts electrical currents to optical signals • Short wavelength (1000BASE-SX) • Long wavelength (1000BASE-LX/LH) • Extended distance (1000BASE-ZX)

38. UTP Implementation • Wires in the cable must be connected to correct pins in terminator • Straight-through cable: maintains pin connection all the way through cable (i.e. pin 1 to pin 1, pin 2 to pin 2, etc) • Crossover cable: critical pair of wires is crossed over in order to make sure Rx-Tx pairing.

39. Pinouts – Straight Through cable

40. Pinouts – Crossover cable

41. Using cables • Straight through • Switch to router • Switch to PC or server • Hub to PC or server • Crossover • Switch to switch • Switch to Hub • Hub to Hub • Router to router • PC to PC • Router to PC

42. LAN Connection Devices • Repeaters • Regenerate and retime signals at bit level to allow greater distances • Four repeater rule (5-4-3 rule) • 5 network segments connected end-to-end by 4 repeaters with only 3 segments with hosts on them • Primarily used in Bus topology networks, not with switches and extended star topologies • Hubs – Repeaters on steroids • Active – Requires power to regenerate and amplify signal • Changes Bus topology to Star topology • All devices attached to Hub hear all traffic – single collision domain

43. LAN Connection Devices (Cont’d) • Bridges – used to break up large LAN to smaller segments • Decreases traffic on a single LAN and extends geographical area • Layer 2 (Datalink) • Makes intelligent decisions about how to pass on a frames • Frame is examined for destination MAC address • Address on same segment as source MAC, blocks frame from going to other segment – filtering • Address on different segment, Bridge forwards to correct segment • Address unknown, Bridge sends frame to all segments - flooding

44. Switches • Multiport Bridge (Layer 2) • Like Bridges, Switches build forwarding tables based on MAC address for decision making • More sophisticated than Bridge • Improve network performance • Often replace shared Hubs • Two basic functions • Switching data frames • Maintenance of switching operations • Operate at higher speeds than bridges • Support other functionality (VLAN’s) • Provide collision free environment

45. Wireless Networking Media • Utilize radio frequency (RF), laser, infrared (IR) or satellite/microwave to carry signals. • Requires Transmitters (Tx) and Receivers (Rx) • Most common techonologies RF and IR • IR – Must be line of sight and signal easily obstructed • RF – limited range and single frequency easily monitored by others

46. Security in Wireless Environment • Radio waves radiate in all directions • Must protect waveform from eavesdropping • Waveform of wireless bridges concentrate in single beam. Must be in the path of the beam in order to intercept data stream • Encryption is required to assure security

47. WEP • Main Goals • Deny access to unauthorized users • Prevent decoding of captured WLAN traffic • Same key needs to be used by encrypting and decrypting endpoints • Not extremely robust security – should be supplemented with firewalls or VPN

48. 802.1X/EAP – Extensible Authentication Protocol • Centralized authentication and dynamic key distribution • Standard for port-based network access control • Allows client adapters that support different authentication types to communicate with back-end servers • Cisco’s LEAP uses mutual authentication: Both user and access point must be authenticated before allowed on to network • Centralized authentication and key distribution • Large-scale WLAN deployment

49. NIC’s and Interfaces • PC board that fits into expansion slot on motherboard • Provides connectivity for host to network medium • Operates at Layer 1 and Layer 2 of OSI model • Considered Layer 2 because every NIC has a Media Access Control (MAC) address. • Layer 1 because only looks at bit and not higher level protocols • Transceiver built-in

50. Workstation and Server Relationships • Computer issuing a request is Client • Computer responding is Server • Peer-to-Peer network • Computers act as equal partners (peers) • Referred to as workgroups • Each computer acts as both client and server at different times • Individual users control own resources • Easy to install • Works well with small number of hosts <=10 • Do not scale well • Security can be a problem