Physical Issues

Cable Issues

Using the correct fiber optics

Fiber mismatching

Core and cladding sizes are relatively standard

  • Fiber and frequencies must match equipment
  • Signal errors will be seen on the interface

There will be writing on the cable about size and type.

Cable Categories

Cable construction is standardized

  • Telecommunication Industry Association (TIA)

TIA sets the minimum physical cable parameters

  • Cables meeting the standard are assigned a category (cat)
  • Insertion loss, near end crosstalk, far end crosstalk, etc.

IEEE networking standards refer to the TIA cable categories

  • 1000BASE-T minimum cable is category 5
  • 10GBASE-T minimum cable is category 6 and 6A

Using the right cable

Speed/bandwidth

  • Theoretical maximum data rate
  • Usually measured in bits per second
  • The size of the pipe

Throughput

  • Amount of data transferred in a given timeframe
  • Usually measured in bits per second
  • How much water is flowing through the pipe

Distance

  • Know the maximum distance
  • Varies based on copper, fiber, repeaters, etc.

The right cable category

Validate the cable

  • Use best practices during installation
  • Tester matches to the closest cable category

Cable should meet the minimum requirements

  • Physical errors will increase error counts
  • Signal attenuation (transmission loss), loss of signal, CRC errors

Unshielded and shielded cable

UTP (Unshielded Twisted Pair)

  • No additional metal shielding
  • The most common twisted pair cabling

STP (Shielded Twisted Pair)

  • Additional shielding protects against interference
  • Shield each pair and/or the overall cable
  • Requires the cable to be grounded

Crosstalk (XT)

Signal on one circuit affects another circuit

  • In a bad way

Leaking of signal

  • You can sometimes hear the leak

Measure XT with cable testers

  • Some training may be required

Crosstalk metrics

Near End Crosstalk (NEXT)

  • Interference measured at the transmitting end
  • The near end

Far End Crosstalk (FEXT)

  • Interference measured at away from the transmitter

Alien Crosstalk (AXT)

  • Interference from other cables

Attenuation to Crosstalk Ratio (ACR)

  • Difference between insertion loss and NEXT
  • Signal-to-Noise Ratio (SNR)

Troubleshooting Crosstalk

Almost always a wiring issue

  • Check your crimp

Maintain your twists

  • The twist helps to avoid crosstalk

Category 6A increases cable diameter

  • Increased distance between pairs

Test your installation

  • Solve problems before they are problems

Avoiding EMI and Interference

Electromagnetic interference

Cable handling

  • No twisting — don’t pull or stretch
  • Watch your bend radius
  • Don’t use staples, watch your cable ties

EMI and interference with copper cables

  • Avoid power cords, fluorescent lights, electrical systems, and fiber prevention components

Test after installation

  • You can find most of your problems before use

Attenuation

Usually gradual

  • Signal strength diminishes over distance
  • Loss of signal intensity as signal moves through a medium

Happens across all mediums

  • Electrical signals through copper
  • Light through fiber
  • Radio waves through the air

Troubleshooting termination

Cables can foul up a perfectly good plan

  • Test your cables prior to implementation

Many connectors look alike

  • Do you have a good cable mapping device?

Get a good cable person

  • It’s an art (REALLY!)

Improper Termination

Near and far pins in cables aren’t where they are supposed to be

  • Pin 1 goes to pin 1, pin 2 to pin 2, etc.

Performance or connectivity issues

  • May drop from 1 Gbit/sec to 100 Mbit/sec
  • May not connect at all

Reversing transmit and receive

Wiring mistake

  • Cable ends
  • Punch downs

Easy to find with a wire map

  • 1-3, 2-6, 3-1, 6-2
  • Simple to identify

Some network interfaces will automatically correct (Auto-MDIX)

  • Don’t rely on this functionality

Interface Issues

Monitoring the interface

Often your first sign of trouble

  • The local problems are easy to attack

Can sometimes indicate a bigger issue

  • Problem with a switch or congestion in the network

View in the OS

  • Interface details

Monitor with SNMP

  • Remote monitoring of all devices
  • Most metrics are in MIB-II
  • Proprietary MIB may be available

Interface Monitoring

Link status

  • Link up, or link down?
  • May be a problem on the other end of the cable

Utilization

  • Per-interface network usage
  • Run bandwidth tests to view throughput

Error rate

  • Problems with the signal
  • CRC errors, runts, giants, drops

The Ethernet Frame

Counting the Errors

CRC (Cyclic Redundancy Check) error detecting

  • Add a frame check sequence to an Ethernet frame
  • Receive the frame, recalculate the CRC, and compare to the original
  • Non-matching CRC is an error

Runts — Frames that are less than 64 bytes

  • May be a result of a collision

Giants — Frames that are more than 1518 bytes

  • Or more than the configured maximum frame size

Drops

  • Frames not transmitted or received due to contention

#show interfaces f0/1

Error disabled

Some problems should be stopped in their tracks

  • They go on and on

Disable the interface to fix the symptom

  • This does not fix the problem

Many reasons

  • Link flapping (up/down), port security violations, duplex mismatch, etc.

Must be administratively re-enabled

  • Intervention is required

Port status

Administratively down

  • The device admin has “turned off” an interface
  • This was intentional
  • Does not work again until administratively enabled

Suspended

  • The configuration is not compatible with the current connection
  • This is similar in function to “error disabled”, but occurs immediately
  • Set LACP (Link Aggregation Control Protocol) on one side, but not the other

Hardware Issues

Power over Ethernet (PoE)

Power provided on an Ethernet cable

  • One wire for both network and electricity
  • Phones, cameras, wireless access points
  • Useful in difficult-to-power areas

Power provided at the switch

  • Built-in power — Endspans
  • In-line power injector — Midspans

PoE, PoE+, PoE++

PoE

  • The original PoE specification
  • 15.4 watts DC power, 350 mA max current

PoE+

  • 25.5 watts DC power, 600 mA max current

PoE++

  • 51 W (type 3), 600 mA max current
  • 71.3 W (type 4), 960 mA max current
  • PoE with 10GBASE-T

Compare the device with the switch support

  • PoE+ won’t power a PoE++ device

PoE switch

Power over Ethernet interfaces

  • Commonly marked on the switch or interfaces

Check switch for total PoE power supported

  • “Up to 600 watts”
  • Calculate the device requirements for the power budget

Single mode vs. multimode

Transceivers have to match the fiber

  • Single mode transceiver connects to single mode fiber

Transceiver needs to match the wavelength

  • 850 nm, 1310 nm, etc.

Use the correct transceivers and optical fiber

  • Check the entire link

Signal loss

  • Dropped frames, missing frames

Transceiver mismatch

May look the same, but work on different wavelengths

Transceiver signal strength

Devices must receive enough signal

  • Can’t work if the signal isn’t strong enough

Each device has a sensitivity level

  • Some devices can “hear” better than others

Calculate the power budget

  • Determine transmitter power (often measured in dBm)
  • Calculate signal loss based on distance, connectors, splices, etc.
  • Subtract signal loss from the transmitter power
  • Compare to minimum receive power