NETWORK TROUBLE-SHOOTING: 2

In LAN's we deal with voltages (electrical signals) which are generally very small (less than 5volts peak to peak). The differences between no signal and signal can be less than 1 volt.

In fact, noise voltages can even be interpreted as signals. This is one reason why the cabling media must be protected from noise sources and be free from induced noise voltages.

Because a LAN comprises many components, its often difficult to decide where to start finding the problem. However, there are a few basic rules we can learn which will point us in the right direction, with the aim of minimizing downtime.

Information is vital for troubleshooting. You need access to

• cable layouts
• cable type, location of terminators, splices, repeaters etc
• workstation location
• network interface card types, settings
• workstation information (screen type, memory, software configurations)
• knowledge of basic network topologies (star, ring, bus)
• knowledge of basic troubleshooting procedures (good/bad measurements)
• past history (recent changes, is fault repeatable)

Cabling
Cable generates 90% of all LAN faults. Needless to say, cabling at installation time should be certified in writing, along with test results detailing the measurements taken. The major problems associated with cable are,

• shorts and open circuits
• extraneous voltages, noise, ground loops
• excessive cable lengths (lets add just one more workstation)
• poor termination's, connectors
• wrong type of cable
• incorrectly installed to begin with

There are restrictions on cable types, distances and number of workstations attached.

Thin Ethernet (coax)

• 30 T connectors per cable segment
• 300 meters per cable segment
• minimum distance between each T connector is 1 meter
• RG-58AU cable (50 ohm)
• 50 ohm terminator each end of the cable, ONLY ONE of which is earthed
• no T connector shall be earthed
• each T connector plugs directly into a network card installed in a PC

Ethernet systems use a BUS topology. This describes the manner in which the various hardware equipment of the LAN is interconnected.

• each workstation is attached to a single cable thread
• each end of the cable is terminated in 50 ohms
• connections to PC's are made via T connectors
• signals are received by all workstations

However, bus systems do have drawbacks,

• a cable fault will take down the entire LAN
• a faulty workstation will take down the entire LAN
• there are limits on the number of workstations
• there are limits on the length of cable
• workstations must be a minimum distance apart

Bus Topology DONT'S

• mix different cable types
• mix 75 ohm and 50 ohm T connectors or terminators
• solder connectors
• run cable near power cables, or air conditioning units, or lighting
• earth the cable in more than one place
• mix different ethernet network card types
• exceed cable distances or workstation limits
• bend or trample the cable
• run spurs or drop cables from a T connector

Bus Topology DO'S

• use a single cable type (RG58AU)
• one type of network card
• 50 ohm T connectors from a single supplier
• ground the cable at one end only
• keep the cable away from noise sources
• observe cable and workstation limits

LAN TEST INSTRUMENTS
The basic instruments for fault finding are

• multimeter (resistance, continuity and voltage) suitable when the network is NOT ALIVE
• time domain reflectometer TDR
• visual inspection
• software diagnostics

Where do you start?

• is the entire LAN down, all users affected
  • scan cable, measure resistance, noise
  • check to see if server running, LOAD MONITOR, TRACK ON
• some users affected
  • check to see if PC can still run DOS
  • run NIC diagnostics
  • any recent changes

Any other problems are generally configuration faults, particularly in the way in which the network has been configured or managed. These are problems like

• inability to run certain applications
• can't access certain files or directories

Often problems are sporadic and intermittent in nature. Faults can be linked to external activity (the cable deriving noise voltages via lift motors etc). With intermittent faults, it is important to obtain information about

• which users noticed it first
• what were they doing when it failed
• has this happened before
• is it repeatable
• is there any difference between this and the last time

The trouble-shooter must know the hardware and software configurations of the various workstations and servers on the network. These details should be filled out and kept in folders. Included should be cable layouts, access points, cable distances, workstation locations etc. Remember that INFORMATION is the key. The more you know about the system, the easier your job will be.

We always recommend to any new Network Manager that the first thing they do is fully document the entire LAN, both software and hardware.

CATEGORIZING LAN PROBLEMS

• HARDWARE
  • Cabling, Connectors, Cable, MAU's
  • Workstations, keyboards, base units, monitors, drives memory, PSU, NIC and peripherals, mice etc
  • Servers
  • Bridges/Routers
  • Printers
  • Power Supply
• SOFTWARE
  • Lan NOS and workstation shell
  • Application Software
  • Incorrect Configurations
  • Access rights, drive mappings
  • Data Integrity, backups

HANDLING LAN PROBLEMS

• IMPLEMENT A LAYERED APPROACH
  • Help Desk
    • first level of repair
    • interacts with user to determine nature of problem
    • uses a standard battery of questions
    • logs details of problem
    • initiates repair by solving problem or passing on
    • Tracks problems passed on to experts
    • Does not require high degree of expertise
    • Requires access to database of problems and solutions
    • information of users system required
  • Front Line Experts - Hardware and Software
  • On-Call Experts - Cablers, In-house Consultants
  • Installers/Trouble-shooters/LAN Authorized Agents

HELP DESK, TYPlCAL QUESTlONS TO ASK

• Location of users machine. contact name and number
• Machine configuration, memory, drivers, monitor type etc
• DOS Version, Network NOS type and revision number
• What happened, time and date, can it be repeated?
• Are other network users affected?
• Can computer run DOS as standalone
• What were the error messages displayed?
• Do other users experience the same problem?
• Were there any recent changes to the computer or network
• What solutions have you tried to solve the problem? (reset, power-off/on), another machine, user account)
• Is this something new or have you tried this before?

CABLING PROBLEMS ARE GENERALLY

• connectors and terminators
• crimps, splices, bends
• noise, voltages and ground loops
• shorts and open circuits
• exceeding cable and workstation limits

BASIC TROUBLESHOOTING HARDWARE TOOLS TO USE

• ohm-meter
  • tests resistance of cable, presence of terminators, open and short circuits
  • limited use on live network
• time domain reflectometer
  • tests for cable imperfections, shorts and open circuits, impedance mismatches
• volt-meter
  • tests for voltage, -1.7v on ethernet means streaming NIC

CABLE TROUBLESHOOTING STRATEGY

• divide and conquer
  split the cable segment in half, and take a measurement on both halves. This should isolate the fault into one half. Repeat the process till the fault is found.

© Copyright Brian Brown, 1988-2000. All rights reserved.