Cisco CCNA Sem 1 Chapter 4 Cable Testing, Cabling LANs and WANs Terms to understand Waves – energy traveling form one place to another Period – time between.
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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)
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:
1. Voltage waves on copper media2. Light waves in fiber optic3. Alternating electric and magnetic fields in
wireless communitcation
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
-6
-4
-2
0
2
4
6
1 3 5 7 9 11 13 15 17 19
Series1
Square Waves
Like analog waves are periodic
Do not vary continuously with time
Represent digital pulses or signals
Describe by Amplitude, Frequency and period
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
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
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
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
Digital Bandwidth
Digital Bandwidth – how much information can flow Units of measurement are bps
Usually expressed as kbps or mbps
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.
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
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
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
2 types of copper cable
Shielded Protect against external noise sources Some types of shielding protect against
internal noise sources Unshielded
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
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
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.
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
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
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
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.
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
Three types of Crosstalk
Near-end crosstalk (NEXT) Far-end crosstalk (FEXT) Power sum near-end crosstalk
(PSNEXT)
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
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
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
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
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)
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.
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
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
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
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
Ethernet Media Connector Requirements
Media Max. Segment Length
Topology Connector
10BASE2 50-ohm Coax (Thinnet)
185 m Bus British Naval Connector (BNC)
10BASE5 50 Ω Coax (Thicknet)
500 m Bus Attachment unit interface (AUI)
10BASE-T CAT 3,4,5 UTP, 2 pair
100 m Star RJ-45
100BASE-TX CAT 5 UTP, 2 pair 100 m Star RJ-45
100BASE-FX 62.5/125 multimode fiber
400 m Star Duplex media interface connector MIC, ST, SC
1000BASE-CX STP 25 m Star RJ-45
1000BASE-T CAT 5 UTP, 4 pair 100 m Star RJ-45
1000BASE-SX 62.5/50 micro multimode fiber
275 m for 62.5 μ; 550 m for 50μ
Star SC
1000BASE-LX 62.5/50 micro multimode fiber; 9 μ single mode fiber
440 m for 62.5 μ;550 m for 50 μ3 -10 km on single mode fiber
Star SC
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)
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.
Pinouts – Straight Through cable
RJ-45 Pin Label RJ-45 Pin Label
1 RD+ 1 TD+
2 RD- 2 TD-
3 TD+ 3 RD-
4 NC 4 NC
5 NC 5 NC
6 TD- 6 RD-
7 NC 7 NC
8 NC 8 NC
Pinouts – Crossover cable
RJ-45 Pin Label RJ-45 Pin Label
1 RD+ 1 TD+
2 RD- 2 TD-
3 TD+ 3 RD-
4 NC 4 NC
5 NC 5 NC
6 TD- 6 RD-
7 NC 7 NC
8 NC 8 NC
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
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
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
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
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
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
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
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
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
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
Client/Server Networks
Specialized computers respond to Client requests Easy to Scale Better security Introduces single point of failure to
system Require additional hardware and
specialized software = increased cost
Cabling the WAN
WAN cabling standards are different than LAN
WAN Services provide different services and connection methods Serial connections Integrated Services Digital Network Basic Rate
Interface (ISDN BRI) Digital Subscriber Line (DSL) Cable Console connections
WAN Physical Layer
Physical layer requirements depend on speed, distance, and actual service utilized Serial connections support dedicated leased lines
that use Point-to-Point Protocol (PPP) or Frame Relay.
Speed 2400 bps to T1(1.544Mbps) ISDN – utilizes dial-on-demand services or dial
backup ISDN BRI – 2 64-kbps bearer channels (B channels)
for data and 1 16-kbps delta channel for control (D channel)
Typically uses PPP protocol for B Channels DSL/Cable services to businesses and homes
DSL can achieve T1/E1 speeds
WAN Serial Connections
Physical connections depend on equipment, and services
Serial connectors used to connect end-user devices and service providers
V.35 is most common Ports on Cisco routers use Cisco’s
proprietary 60 pin “Smart serial” Connector.
Routers and Serial Connections
After determining cable type, need to determine if Date Terminal Equipment (DTE) or Data Communications Equipment (DCE) is required.
DTE is endpoint of users device on WAN DCE used to convert data from DTE to
form that can be used on WAN link If connecting to service provider or device
that performs signal clocking (CSU/DSU) the router is a DTE and requires DTE Serial cable. Most typical case Sometimes routers will be DCE
Routers and Ports
Routers can have either fixed or modular ports. Type of port affects syntax used to configure port Fixed ports use the syntax: port type
and port number Serial 0
Modular ports use the syntax: port type slot number/port number
Serial 1/0
Routers and ISDN BRI connections
2 type of interfaces BRI S/T
If service provider uses an NT1 device then an S/T connection is required
BRI U If customer needs
to provide NT1 device, then U connection is used
Pin Signal
1 Unused
2 Unused
3 Tx+
4 Rx+
5 Rx-
6 Tx-
7 Unused
8 Unused
Routers and DSL Connections
DSL – modem technology inexpensive high speed transmission over existing phone lines
Uses RJ-11 connectors
Pin Signal
1 Unused
2 Unused
3 Tx
4 Rx
5 Unused
6 Unused
Routers and cable connections
Coaxial cable carries signal (same as television) Radio grade (RG-59) RG-6 – recommended F connector
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