Service and Support

Chapter 3: Installing and Troubleshooting Cables and Network Boards

 

Objectives:

This chapter discusses network topology problems, including issues about cards, cables and combinations in greater depth than we encountered in the Networking Technologies class. The objectives important to this chapter are on page 3-1:

  1. Describe Network Cabling Types
  2. Configure and Install Network Boards
  3. Support a Token Ring Network
  4. Support an Ethernet Network
  5. Support an FDDI Network
  6. Support an ATM Network
Concepts:

The chapter begins with a discussion of three network cable media types:

  • twisted pair - come in two types:
    • unshielded - UTP does not have an EMI (Electromagnetic Interference) resistant sheath
    • shielded - STP has an EMI resistant sheath
  • coaxial - coax similar to that used for cable TV, but NOT identical
  • fiber optic - glass or plastic channels that conduct light, often red laser light

The graphic on page 3-2 shows a twisted pair of wires. Each UTP cable has two, three,or four such pairs of wires in it. The image on the right shows a cable with four pairs. Each wire is covered with an insulator, and the two wires in the pair complete a circuit. These wires suffer from crosstalk, leakage of signal from one circuit to a nearby circuit, which interferes with the signals that are intended to be on a circuit. The wires in each pair are twisted around each other like the strands in a DNA molecule. The twists help cancel out such leaks.

UTP is available in grades, called CAT 1 (Category 1) through CAT 5 (Category 5). A major difference between the categories is the number of twists per foot in each pair. CAT 1 will have less than 5 twists per foot. CAT 5 will have 25 or more twists per foot, so it is better, and costs more. Note that the better the class of cable, the more bits per second can be passed across it.

Connecting a system with twisted pair wiring is easier than using other cable media. If you make your own cable (as we often do in class) the main thing is to keep to a standard wiring scheme. The terminology used later in this chapter makes no sense to me. The table below is a standard we have used in class:

10/100B-T(X) Patch/Drop Cable
(EIA/TIA 568B spec)
Twisted-Pair Color to RJ-45
Pin Assignments
Pin Color Color Signal Circuit
1 Orange/White
Orange White
TX data + Orange Circuit
2 Orange Orange TX data - Orange Circuit
3 Green/White
Green White
RX data + Green Circuit
4 Blue Blue unused Blue Circuit
5 Blue/White
Blue White
unused Blue Circuit
6 Green Green RX data - Green Circuit
7 Brown/White
Brown White
unused Brown Circuit
8 Brown Brown unused Brown Circuit

The insulation shown in the pictures above should NOT be stripped back on these wires.
If you are making a standard cable (to run from a workstation to a hub) connect both ends as listed above and shown on the right. Insert the wires into the RJ-45 connector, then crimp it with the crimping tool.
If making a crossover cable (to run directly from one NIC to another) swap the orange and green circuits on one end only: put orange/white on 3, orange on 6, green/white on 1, and green on 2. Insert the wires as shown on the right, then crimp.

The factors for UTP:

  • Cost - inexpensive
  • Installation - cheap and easy
  • Capacity - 1 to 100 Megabits per second (Mbps), but 10 Mbps is common (100 Mbps, if fast Ethernet)
  • Attenuation - nothing is perfect, so this is high (poor)
  • Immunity from EMI - also poor. Recommendation: run UTP lines perpendicular to fluorescent lights.

Types of STP (Shielded Twisted Pair) cable are discussed in the chart on page 3-4. This cable is more expensive than unshielded cable, and is less flexible due to the stiff shielding. The shield makes it more EMI resistant than UTP, but not much more.

The factors for STP:

  • Cost - Moderately expensive
  • Installation - harder than UTP, needs special connectors (often IBM-style Token Ring connectors)
  • Capacity - 1 to 500 Megabits per second (Mbps) is possible, but 16 Mbps is common (for Token Ring)
  • Attenuation - high (poor)
  • Immunity from EMI - also poor, but not as bad as UTP.

Coaxial cable is called that because it has two conductors, one wire in center and a conductive sheath around it, that share a "common axis". Most people have seen this style of cable used with cable television.

The wiring standards used for network coax are different from those used for cable TV. You should know the list on page 3-6:

  • 50 ohm cable, available as RG-8 and RG-11. Used in Thick Ethernet, also called "Ether Hose".
  • 50 ohm cable, available as RG-58. Used in Thin Ethernet.
  • 75 ohm cable, available as RG-59. This one is for TV, not networks.
  • 93 ohm cable, available as RG-62. Used in ARCnet. (Actually, you can use almost anything for ARCnet.)

The number associated with each RG specification tells you the relative size of the center conductor. Smaller numbers mean thicker wires. The coaxial line is essentially a single bus, going from one station to the next. At each end of the line, the cable has to have a terminator on it. At one end, it also has to be grounded. Workstations can be connected two ways. If using thin Ethernet, T-connectors are used. If using thick Ethernet, vampire taps are used. They are called vampire taps because little teeth bite into the cable when you screw a clamp down. Note that the vampire tap just provides a place to tap into the cable. The workstation also needs a patch cable to connect its network card to the tap.

The factors for Coax:

  • Cost - Relatively low to Moderately expensive (depending on thickness of the cable)
  • Installation - simple to install, hard to modify
  • Capacity - high rates are possible, but 10 Mbps is common
  • Attenuation - high, but less than twisted pair
  • Immunity from EMI - moderate

Fiber optic can be glass or plastic, and is meant to conduct light instead of electricity. The conductor is called a waveguide, and is covered with cladding, a material to reflect the signal back into the center of the conductor. A common configuration is depicted on page 3-8. Loose configuration has a liquid filler between the outer sheath and the conductor. Tight configuration has wire or kevlar fibers around the conductor to add strength to the cable.

Fiber optic comes in three modes:

  • single mode conducts a single signal
  • multi-mode graded conducts many signals simultaneously.
  • multi-mode stepped also conducts many signals simultaneously.

You may want to know that 62.5 micron core with 125 micron cladding, multi-mode, is the most common type used. Fiber optic is much harder to install and splice than electrical conductors. Fiber optic media require two connectors for each station, a line in and a line out.

The factors for fiber optic:

  • Cost - Expensive, mostly for installation
  • Installation - difficult
  • Capacity - 100 Mbps at up to 20 kilometers per segment
  • Attenuation - very low
  • Immunity from EMI - immune. This is light, not electricity.

All network cable has a fire rating, as discussed on page 3-9. Four levels are listed:

  • Restricted Cable - least fire resistance, probably polyvinyl chloride (PVC) insulation; gives off toxic fumes when it burns, must be in separate conduit
  • General Purpose Cable - does not require separate conduit, but cannot be used in risers or plenums
  • Riser Cable - may be used in vertical shafts in buildings
  • Plenum Cable - best fire resistance, probably Teflon Fluorinated Ethylene Propylene (TFEP) insulated, may be used in air conduits. (A plenum is defined as the space between the actual physical ceiling and the drop ceiling of any floor in a building.)

The next portion of the chapter concerns network boards. It begins on page 3-12. Three types of boards are listed:

  • Manually configured - settings are generally made on the board
  • Software configured - settings are made through applications or boot files
  • Plug-and-play (PNP) - easy to use IF:
    1. the BIOS is PNP compliant
    2. the card is PNP compliant
    3. the operating system is PNP compliant

On page 3-13, there are five steps to installing a board. Note that none of the three types uses all five steps.

Step Manual Software PNP
1. Choose board to match cabling and architecture. yes yes yes
2. Identify a working configuration for the board. Yes yes no
3. Configure jumpers or DIP switches on board. Yes no no
4. Install board. Yes yes yes
5. Configure board with software. no yes no

Step 1: Choosing the Board to match:

  • LAN topology - a board such as a Network Interface Card (NIC) must be chosen to match the LAN topology being used. As noted on page 3-12, you need to match the kind of LAN (Ethernet, token ring, etc.) as well as the kind of cables (UTP, coax, etc.) you use.
  • PC bus - six major bus architectures are listed (others will emerge in time):
    • ISA - the oldest standard listed, Industry Standard Architecture. 16 bits wide, this is a bottleneck in NetWare, which is a 32 bit system. These slots can generally use 16 or 8 bit boards (8 bit boards do not use the slot extension) unless the 8 bit board has a skirt (a support that drops to the system board). Boards can be 4.8 or 4.2 inches tall.
    • MCA - Micro Channel Architecture, a proprietary IBM bus. Short slots are 16 bit, long ones are 32 bit.
    • EISA - Extended Industry Standard Architecture. The bus expects 32 bit boards, but some older ISA 16 and 8 bit boards may be used, if there are no conflicts. Note the two rows of contacts on an EISA card. If such a card is plugged into an ISA slot, only the first row of contacts will touch the slot. Conflicts are expected. Boards are 5 inches high.
    • VESA Local Bus (Local Bus) - the bus speed on this bus is theoretically the same as the system clock. The standard is set by VESA (Video Electronics Standards Association), so it is only for video cards.
    • PCI - Peripheral Component Interconnect supports bus mastering (CPU can offload some tasks to the card), 32 bit cards, yields a peak bandwidth of 132 Mbps at 33 MHz, can scale to 64 bit systems
    • PCMCIA - now just called "PC Card", these are shaped like credit cards, and are used mainly with laptops

Step 2: Identifying Configuration Information
You may need to know the following:

  • IRQ for the card - a list of common IRQ assignments is on page 3-19
  • DMA Support - used or not; if used, what DMA channel
  • I/O Address - port address for the service the card provides
  • Memory Address - a part of RAM that the card wants to "share"

An IRQ is a hardware interrupt. It is a number from 0 through 7 on XT class machines, 0 through 15 on AT (or better) class machines, that gives a device two things: the right to interrupt the processor and request service NOW, and a place in the pecking order of such assignments. The number represents an actual wire leading to the CPU. 0 is always assigned to the system timer, and it has the highest priority. Other common assignments are on page 3-22. In general, never assign the same interrupt number to two devices that could conceivably need attention at the same time. If possible, do not share interrupts at all.

DMA channels give a device direct access to memory. An XT class machine had channels 0 through 3, later machines have 0 through 7. Windows NT systems DO NOT want you to use this at all!

I/O Addresses are compared to a mail stop, a place in memory that the processor will check periodically. This sort of addressing is used for devices that do not need immediate attention from the CPU, whose requests can wait for a cycle or two. Again, devices should not share port addresses, unless confusion and disaster are desired. You should be able to read a table like the one on page 3-24 to determine that address conflicts do or do not exist.

Base memory is sometimes required by a card or device. Essentially, this is RAM that is assigned for the use of the device, not for communication with the CPU. Note in the chart on page 3-25 that a commonly vacant section of memory is the range from CA000 to DFFFF. Picture a nice country inn in Wales. It has a vacancy. We can stay there. It's in Cardiff? CA...DFFF?

Step 3: Configure Jumpers or DIP Switches:
On page 3-26, we enter the world of jumpers. A jumper is just a connector that sits across two pins on a board to complete a circuit. The trick is knowing which pins to bridge for what to happen. The information in the SupportSource CD will show you which pins to bridge with jumpers to achieve certain effects on certain boards. Note that a jumper can also be called a shunt.

On page 3-27, you see one possible configuration for DIP switches. Dual In-line Package (DIP) switches are used the same way jumpers and pins are used, to configure features on boards. For example, you must configure the addresses of ARCnet cards using DIP switches. The actual switches may be rockers, levers, or other designs. They may be numbered from left to right, right to left, top to bottom, bottom to top, or not at all. You may have more than one bank of DIP switches for different purposes, on mother or daughter boards.

Step 4: Install the Board
The text advises us to be careful installing boards, that proper screws are provided with them and they should be used. As advised above, don't do anything to change the intended electrical conductivity of devices.

Step 5: Configure the Board with Software
Next, we are given examples of boards that come with software utilities for configuration. This is fairly common when not in a Plug and Play environment. The utilities used may resemble the menu system you encounter when installing a Novell server. The book also mentions the CMOS settings for machines. A machine often has information stored here about drives, memory, boot order, ports and other intimate details. If you have ever had a machine lose these settings, you already understand that you should have a paper copy of them stored in a safe place.

Token Ring troubleshooting begins on page 3-32.

The most common form for a token ring is a star-wired ring. It can use unpowered (passive) concentrators called MSAUs (Multi-Station Access Units) or powered (active) concentrators called CAUs (Controlled Access Units).

Data packets are passed from station to station in one direction, each station acting as a repeater. A token is attached to messages. A token is a 3 byte frame.

Advantages to token rings, from page 3-33:

  • Performs well under high loads.
  • Good for LAN connections to mainframes.
  • Built-in troubleshooting: beaconing and autoreconfiguration.
  • Can use UTP.
  • Can use ring-wrap,

Disadvantages to token rings, from page 3-33:

  • Expensive.
  • Administrator must have considerable expertise.

The way token ring works is discussed. The token is a frame, an envelope that is attached to a message being passed. When it is received by the intended node, that node reverses two bits in the the token, and passes the package on around the ring. When the sender sees that the two bits of the token have been reversed, this is taken as an acknowledgment of receipt. The sender then releases the token and passes a new token to the next node in the ring, making it that node's turn to transmit.

A common question about token rings can be illustrated with the graphic on page 3-35. Note the connections between the two MSAUs in the diagram. To connect MSAUs together, it is necessary to run the cable from the RI (Ring In) port of one to the RO (Ring Out) port of the other. As two cables have been used, the RI and RO ports of both MSAUs have been used, creating a logical ring. Node are attached to the MSAUs with UTP cables. If any cable to a node breaks, the ring survives, because of the lines between the MSAUs.

The various cable types used in token ring are discussed on page 3-38. These are referred to as Type 1 through Type 9 (as distinguished from the Cat 1 through Cat 5 of Ethernet).

The concept of beaconing is illustrated on page 3-41. In this example, there is a break in the cable between nodes 1 and 2. Each node in a ring expects to receive signals from its NAUN (Nearest Active Upstream Neighbor). Since the cable is broken, node 2 is not receiving from node 1. Node 2 begins beaconing, sending a message to the ring that there is a problem. The message includes its address, the NAUN's address, and the type of problem.

On page 3-41, the book also discusses autoreconfiguration. Token ring technology is somewhat self-correcting. If a board detects a problem, it will remove itself from the ring, run diagnostics and try to rejoin the ring. This is autoreconfiguration. If it fails, the administrator must take action to put the node back in the ring.

Troubleshooting tips are on page 3-42:

  • Check for internal errors reported by boards.
  • Keep up-to-date documentation of the cabling.
  • Sometimes, it is good to disconnect the cables from the MSAUs, reset the ports and connect again.
  • Addresses for token ring cards are burned into ROM, like MAC addresses. They can be changed, but take care not to use one already in use or outside allowable values.
  • Boards can have resource conflicts.
  • The data rate for a token ring board must match the data rate for the network, else a beacon error is generated.
  • Three parameters may be changed for boards, using device drivers: address, shared RAM location and Early Release. They must be set for each board in a machine.
  • Try to use MSAUs from the same vendor, to avoid impedance errors.

This chapter continues with a discussion of Ethernet:

  • Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
  • Ethernet is Contention based, meaning all nodes contend (compete) for access
  • Ethernet is inexpensive and commonly runs at 10 or 100 Mbps
  • It is a proven (stable) technology and supports multiple wiring types
  • It supports many applications
  • It is easy to install
  • High loads can degrade performance
  • A linear bus can make it hard to isolate problems

A description of how Ethernet works starts on page 3-53. Review this material and be familiar with the ideas of contention, packets, datagrams, and collision. You should be aware that Ethernet uses a logical bus, and Collision Detection results in retransmission of packets.

Five common implementations of Ethernet appear in a chart on page 3-54. This information was presented in the Net Tech class as well:

  • 10BASE2 - 10 Mbps, baseband transmission, T-connectors, thin coaxial cable, limited to about 200 yards per segment (Actually 185 meters, or 607 feet, but those numbers are not mnemonic)
  • 10BASE5 - 10 Mbps, baseband transmission, Vampire taps, thick coaxial cable, limited to 500 meters per segment
  • 10BASE-T - 10 Mbps, baseband, RJ-45 connectors, Unshielded Twisted Pair (UTP) cable
  • 100BASE-T - 100 Mbps, baseband, RJ-45 connectors, Cat 5 Unshielded Twisted Pair (UTP) cable
  • 10BASE-F - 10 Mbps, baseband, Fiber Optic cable, book tells us little except that it is a standard

Refer to the diagrams for each type in the discussions of each type. You need to know what these diagrams should look like, and be able to recognize errors in them.

Regarding 10BASE2:

  • Called Thin Ethernet
  • Follows the 5-4-3 Rule: you can have up to 5 segments, connected by 4 repeaters, and 3 of the segments can be populated (have nodes on them).
  • Devices must be at least half a meter apart.
  • Segments cannot exceed 185 meters (about 200 yards) in length.
  • Entire network cannot exceed 925 meters in length.
  • Uses a 50-ohm coax bus, and each end must be terminated with a 50-ohm resistor. One end of the segment must also be grounded. NOTE: in the diagram on page 3-4, one end of EACH segment is grounded. A test question may show one or more grounds missing.
  • Proper cable will be marked "10BASE2". RG-58A/U and RG-58C/U cable are similar and may be used. RG-58U may not be used (DO NOT use it).
  • The transceiver should be on the NIC. Repeaters are needed if mixing with Fiber Optic. If using repeaters, turn off the Signal Quality Error test (SQE) on NICs.

Regarding 10BASE5:

  • Called Thick Ethernet, or EtherHose
  • Cable is thick and hard to use, marked every 2.5 meters for taps.
  • Requires vampire taps for the bus (trunk) cable, drop cables to connect vampire taps to the nodes. There is no minimum length for drop cables, but they cannot exceed 50 meters.
  • The vampire taps are transceivers and must be at least two and a half a meters apart. Cable will be marked at this interval.
  • Segments cannot exceed 500 meters in length. Another limit is 100 nodes per segment.
  • Entire network cannot exceed 2.5 kilometers in length.
  • You can connect to a thin Ethernet LAN with special boards.

Regarding 10BASE-T:
There is more information in your book about 10BASE-T because it is the most commonly used topology.

  • Uses UTP wiring and concentrators (hubs). This is a logical bus, but a physical star, also called a star-wired bus.
  • The length of the cable from a node to the concentrator must be a minimum of .6 meters and a maximum of 100 meters. This applies to connecting concentrators to each other as well.
  • Cable must be UTP, not STP.
  • Can have up to 512 nodes on a segment, 1024 nodes on a network.
  • The 5-4-3 Rule changes to a 5-4 Rule: there is no difference between populated and unpopulated segments. For 10BASE-T, any two workstations can be separated by up to 5 segments connected by 4 concentrators.
  • Cat 3 cable may be used, but Cat 5 is preferred.

10BASE-F, is a version of Ethernet using fiber optic cable. The topology is similar to 10BASE-T, but 62.5/125 micron fiber-optic cable is used. Note that two fiber-optic lines connect each transceiver to a concentrator in the picture on page 3-62. Each two-line circuit between a transceiver and a concentrator is considered a separate segment.

Three varieties of 100BASE-T are mentioned:

  • 100BASE-TX - uses two pairs of Cat 5 UTP or two pairs of Type 1 Shielded
  • 100BASE-T4 - uses four pairs of Cat 3, 4 or 5
  • 100BASE-FX - does not use UTP at all, uses two-strand fiber-optic

Notes regarding 100BASE-T:

  • For UTP, the segment length limit is 100 meters
  • For fiber-optic, the segment length limit is 412 meters
  • Cable bundles cannot be used.
  • Coax cannot be used.
  • Tricky one: the wires used must be twisted in their respective pairs, and cannot be untwisted until within a half inch of the connector. This is a tight fit.

Troubleshooting Ethernet:

  • Check physical connections.
  • Check segment length and spacing rules.
  • Try to isolate and locate faulty hardware like jabbering (chattering) network cards.
  • Check connecting devices and terminators.
  • Check resistance on terminators.
  • Check cable itself.
  • Check for bad transceivers (NICs).
  • Check frame types on the workstations and the servers. Make sure you know what is used on this particular LAN.
  • Check cards with diagnostic software.
  • Check jumper and port settings.
  • Clean connector contacts. DO NOT use your fingers or rubber erasers.
  • Test for conflicts. Some boards use interrupts 3 or 4, which are also COM port interrupts.

FDDI is a fiber-optic topology. It uses two rings that can move data in opposite directions. Typically, however, data move on one ring, while backups and services take place on the other. This is a timed token rotation system, as explained on page 3-83. The difference is that tokens move on to the next station at negotiated intervals.

Advantages to FDDI, from page 3-83:

  • 100 Mbps
  • long cables are supported (up to 200 Km)
  • built-in network management
  • fair access through negotiation
  • increased reliability of fiber optic, resistant to eavesdropping
  • non-electric, maintains ground isolation between buildings
  • cable cost comparable to UTP

Disadvantages to FDDI:

  • concentrators and boards are expensive
  • substantial expertise needed to install and maintain

FDDI systems can survive a broken cable by wrapping, using the other ring to wrap back around the break.

Troubleshooting tips for FDDI, from page 3-85:

  • Multi-mode fiber can be used for distances up to 2 Km, single mode fiber is required for longer distances.
  • You can use a flashlight to check for a complete break, but should use an Optical Time Domain Reflectometer (OTDR) to test cables.
  • Loss of power greater than 11 decibels is serious.
  • Clean connections with lint-free cloth and alcohol.
  • Plastic cable is limited to 50 meters in length and 10 Mbps.
  • Use Packet Burst to overcome unavoidable delays (up to 4 ms).
  • Use source routing where possible.

ATM, Asynchronous Transfer Mode, is another technology you may see. It can run over UTP or fiber, from 51 to 622 Mbps. It is intended for backbones and high end applications. Be aware that ATM uses 53-byte packets called cells. This packet size can cause error messages on routers that have not been configured to accept it.