ITNW 2313 - Networking Hardware
( LAN Hardware/Wiring & Installation )
Prof. Michael P. Harris, CCNA, CCAI

Lesson #5:  Exploring ARCnet Specifications

[Top]ARCnet

ARCnet is an acronym for Attached Resource Computer NETwork, which was founded by the Datapoint Corporation.  Novell used the term RX-Net (2.5 Mbps) to denote its form of this architecture and TRX-Net for the Turbo (25 Mbps) version. ARCnet uses a token-bus  frame/packet passing scheme.

ARCnet operates at 2.5 Mbps throughput, or 25 Mbps for the "Turbo" version and can be connected using (standard IBM SNA)  RG-62 A/U coax cable or unshielded twisted pair (UTP) wiring.  Although ARCnet can support up to 255 nodes (host computers) on a single network, systems of this size are not practical.

Typical ARCnet coax cable specifications:
  • RG-62 A/U, solid conductor, CL2, 90%+ braided shield, PVC jacket, nominal 93 ohm impedance, 15.5 nominal capacitance/ft.

  • RG-62 A/U, solid conductor, CL2P, 90%+ braided shield, plenum jacket, nominall 93 ohm impedance, 12.5 nominal capacitance/ft.

ARCnet was one of the early topologies used in networking but is rarely used as the topology of choice in current LAN environments. ARCnet, however, still is a solid, functional and cost-effective means of networking.


The diagram above shows the components of a typical ARCnet NIC. This card is configured for use with twisted pair (UTP) wiring.  ARCnet cards for use with coax cable have a BNC -Bayonne Neil Councelman connector  (also commonly called British Naval Connector). You will find jumpers or DIP switches for setting the following characteristics:

  • Node number
  • Base I/O address
  • Memory address
  • Interrupt
  • Network timeout

NOTE:  the node number  and network timeout   are characteristics unique to ARCnet.

Each network interface card (NIC) on an ARCnet network is manually assigned a node number. This number-ID is an 8-bit hardware address (insted of a MAC address) that must be unique for each host on the network and be within the range of 1 to 255.

ARCnet manages network access with a token passing bus mechanism. The token (permission to speak on the LAN) is passed from the lowest node number to higher number nodes in ascending order. Lower numbered addresses get the token before the higher numbered addresses.

Network traffic is made more efficient by assigning sequential numbers to nodes using the same order in which they are cabled. Choosing random numbers can create a situation in which a node numbered 23 can be a whole building away from the next number, 46, but in the same room as numbers 112 and 142. the token has to travel in a haphazard manner that is less effective than if you numbered the three workstations in the same office sequentially, 46, 47, and 48, and the workstation in the other building 112. With this configuration, the frame stays within the office before venturing on to other stations.

[Top] ARCnet Board Settings

Depending on the vendor design specifications of your ARCnet card, most base I/O addresses, node addresses, and memory addresses are set by DIP switches. these addresses are set using a binary mode calculation with an on or off setting in the required switch block. Interrupt settings are made by jumper combinations at marked locations on the network interface card (NIC). Many types and brands of ARCnet cards are on the market today. Refer to the documentation for the NIC or call the technical support group for the specific vendor of the component.

Most ARCnet cards require a shared memory address. Many manufacturers use the area of D000:0 to DFFF:0 as the default. Standard DOS memory is limited to the first 640 KB of a PC's memory. However, extended or expanded memory managers can be used to make memory above 640 KB available for use by DOS programs. If memory above the 640 KB line (above hex address A000:0) is used for network boards, however, the memory becomes unavailable for use by a memory manager. this makes it difficult to optimize a PC's upper memory area. If a network card requires a memory area such as D000:0-DFFF:0, the memory available for use by DOS memory managers is reduced by 64 KB. this is usually accomplished with an optional switch setting such as X=D000-DFFF to exclude the shared memory address range.

[Top] ARCnet Cabling

the ARCnet topology uses coax, twisted pair, or fiber optic cabling to connect network devices. An ARCnet network is used primarily with either coax or twisted pair cable. Most ARCnet installations are coax and use RG-62 A/U coax cable terminated with 93 Ohm terminators.  Twisted pair (UTP) installations are newer and use stranded 24 or 26 gauge wire, or solid core 22, 24, or 26 gauge type cable terminated with 100 Ohm terminators. Many ARCnet networks use a mix of both coax and UTP cabling. UTP cable is simple to install and provides a reliable connection to the workstations, whereas coax provides a means to span longer distances.

Typical ARCnet installations are wired as a physical star. ARCnet can run off a linear bus topology using coax or twisted pair as long as the cards specifically support BUS. the most popular star-wired installations of ARCnet run off two types of hubs:

  • Passive hubs  cannot amplify signals. Each hub has four connectors. Because of the characteristics of passive hubs, unused ports must be equipped with a terminator, a connector containing a resistor that matches the ARCnet cabling characteristics. A port on a passive hub can only connect to an active device (an active hub or an NIC). Passive hubs can never be connected to passive hubs.

  • Active hubs  have active electronics that amplify signals and split them to multiple ports. the number of ports on an active hub varies with the manufacturer, but eight is typical. A port on an active hub can be connected to a port on another active device (such as another active hub or an NIC) or to a passive hub.

One of the greatest flexibilities of ARCnet is that you can integrate connections from active hubs to a linear bus connection as long as you terminate at the last connection point.

A maximum time limit of 31 microseconds is allotted for an ARCnet signal frame. This is also called a network timeout. Signals on an ARCnet can travel up to 20,000 feet during the 31 microsecond  default time-out period. You can sometimes extend the range of an ARCnet by increasing the time out value. However, 20,000 feet is the distance at which ARCnet signals begin to seriously degrade. Extending the network beyond that distance can result in unreliable or failed communication. Therefore, the time-out parameter and cabling distance recommendations should be increased only with great caution.

The maximum cable distances between individual components in an ARCnet network are dependent on how the components are connected (see the table at the end of this lesson).

In cabling ARCnet networks with coax cable, you must follow several rules:
  • Never connect a passive hub to another passive hub directly.
  • Passive hubs should never be used to connect two active hubs.
  • Passive hubs are only used to connect an active hub and a node.
  • Unused connectors on active hubs do not need to be terminated.
  • Unused connectors on passive hubs must be terminated using a 93 Ohm terminator.

The figure above shows an ARCnet configuration using active and passive hubs.  Active hubs are required to extend the network for long distances and to configure networks that have more than four nodes.  Passive hubs are used as an economical means of splitting a port on an active hub to support three devices.

[Top] ARCnet Trouble shooting

Common sources of problems on ARCnet networks are as follows:

  • Duplicate addresses. No more than one node can have a given node address on the same network. If two or more nodes share an address, one of the two workstations will either lose its network connection or will not be able to find a network.

  • Missing terminators

    NOTE: Missing terminators may not present visible problems on a small network. Missing terminators cause data retransmits on smaller systems, eventually appearing as transmit time out errors or network errors.
  • Using a terminator with an incorrect rating. Coax uses 93 Ohm; UTP must use 100 Ohm terminators.

    NOTE: A terminator's value in ohms depends on the impedance of the cable. the cable's impedance and the terminator's value should always match.
  • Failed NICs

  • Failed active hubs (or a port on that hub)

  • Cable lengths that exceed specifications (see table). Twisted pair, cabled in a bus rather than a star, cannot have more than ten NICs per segment. this number varies with different manufacturers.

    NOTE: ARCnet UTP installed in a bus configuration is generally used only in very small networks of six nodes or less. this configuration has the major drawback of halting the network if a single cable is disconnected. In an ARCnet bus configuration, the network must be brought down to make any changes or service to the ARCnet cards.
  • Coax connector not built and/or crimped correctly. Twist-on connectors are responsible for more intermittent errors on a network than most other failures because of their design.

    NOTE: Twist-on coax connectors became popular in the IBM 3XXX systems. these systems used RG-62 coax cable and operated at 1.5 Mbps throughput. the twist-on connectors are not recommended for use on any modern LAN cable system because of the higher data rates employed.

Maximum ARCnet Cable Distances
Maximum
Distance
From To
2,000 feet

2,000 feet

100 feet

Not supported

100 feet

2,000 feet

20,000 feet
Network node

Active Hub

Active Hub

Passive Hub

Network node

Network node

Farthest node
Active hub

Active hub

Passive hub

Passive hub

Passive hub

Network node

Farthest node


The primary characteristics of ARCnet are:

  • ARCnet operates at 2.5 Mbps throughput using a token-passing-bus scheme.

  • ARCnet is cabled with RG-62 A/U coax with 93 Ohm terminators or the newer UTP with 100 Ohm terminators.

  • the maximum time it takes for the ARCnet signal to travel the length of the network is 31 microseconds.

  • the maximum distance an ARCnet signal can travel between the two nodes farthest away from each other is 20,000 feet.

  • the absolute maximum number of ARCnet nodes that can occupy a given network segment is 255. An ARCnet segment consists of all cabling and nodes that share a given network address.

[Top] Application:

We will be building a typical ARCnet coax cable for our hands-on lab project for this lesson.

From your LAN-Wire materials kit locate the:

  • 6' length of RG-62 A/U coax cable
  • RG-62 A/U crimp-on BNC connectors (Qty. 3)

Fabrication Instructiions:

  1. Set the Coax wire stripper to the RG-59 setting. (RG-59 & RG-62 are the same diameter)

  2. the dual-set blades of the coax wire stripper will strip the end of the coax cable with a precise two-stage cut. Strip the ends of your RG-62 cable using a rotary motion with the coax stripper.

  3. Crimp the center pin of the BNC connector onto the end of the center conductor.

  4. Slide the crimp ferrule over the coax cable.

  5. Slip the body of the BNC connector under the braided shield as far as it will go.

  6. Slide the crimp ferrule over the braided shield up to the BNC body shoulder.

  7. Use the ratchet crimping tool to evenly and completely crimp the ferrule to the BNC body.

  8. Repeat for the other end of the cable.

  9. Use a digital multimeter to test your cable for shorts. (any Ohm reading other than infinity)

  10. Using a BNC butt-splice and terminating resistor, also test for a perfect 93 Ohm reading.



This page is maintained by:   Prof. Michael P. Harris, CCNA, CCAI

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 Last modified:  22-Jul-2012
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