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Ethernet Topologies
Technology Overviews

Ethernet Topologies

Designing your Ethernet LAN with Black Box.



Key features of Ethernet topologies:
  • Fast, reliable throughput speed-10 Mbps.
  • Accurate transmission-CSMA/CD access method.
  • Easy compatibility-more LAN components match Ethernet standards than any other.
  • Maximum flexibility-two topologies (bus or star) and five kinds of cable (standard or Thin coax; unshielded twisted pair; FOIRL or 10BASE-FL fibre optic).

Overview:
Ethernet is the most widely used network topology. You can choose between bus and star topologies, as well as coax, twisted-pair, or fibre optic cabling. And with the right connective equipment, multiple Ethernet-based LANs can be linked together. In fact, with the right equipment and software, even Token Ring, AppleTalkĀ®, and wireless LANs can be connected to Ethernet. Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection).

In this method, multiple workstations access a transmission medium (multiple access) by listening until no signals are detected (carrier sense). Then they transmit and check to see if more than one signal is present (collision detection). Each station attempts to transmit when it "believes" the network is free. If there is a collision, each station attempts to retransmit after a preset delay, which is different for each workstation.
When a collision is detected, a "jam" signal is propagated to all nodes. Each station that detects the collision will wait some period of time and then try again.


The two possible topologies for Ethernet are bus and star.
The bus is the simplest (and the traditional) topology. Standard Ethernet (10BASE5) and Thin Ethernet (10BASE2), both based on coax cable systems, use the bus. In this one-cable LAN, all workstations are connected in succession (a "bus" arrangement) on a single cable. All transmissions go to all the connected workstations.

Each workstation then selects the transmissions it should receive based on the address information contained in the transmission.


In a star topology, all attached workstations are wired directly to a central hub that establishes, maintains, and breaks connections between them (in the event of an error). The advantage of a star topology is that it's easy to isolate a problem node. The disadvantage is that if the hub fails, the entire system is compromised. Twisted-Pair Ethernet (10BASE-T), based on unshielded twisted pair, and Fibre Optic Ethernet (FOIRL and 10BASE-FL), based on fibre optic cable, use the star.


Typical applications.
Use a bus topology for a large network with many users and longer segments. With repeaters or media converters, you can easily interconnect to other networks with different topologies.


Use a star topology when you want to use twisted-pair cabling (10BASE-T Ethernet) for a multiple-building campus setup (you might already have twisted pair-telephone wire-installed on your premises). Use a star topology for your fibre optic links.

Technically speaking...choose your cabling system.


Standard Ethernet (Coax): 10BASE5

  • The maximum length of a segment is 500 metres.
  • A maximum of 2 IRL (InterRepeater Links) is allowed between devices; the maximum length of cable is 2.5 km.
  • Devices attach to the backbone via transceivers.
  • The minimum distance between transceivers is 2.5 metres.

  • The maximum length of a transceiver cable is 50 metres.
  • Up to 100 transceiver connections can be attached to a single segment.
  • Only transceivers without SQE ("heartbeat") test enabled should be used with repeaters.
  • Both ends of each segment should be terminated with a 50-ohm resistor.
  • One end of each segment should be grounded to earth.


ThinNet Ethernet (Coax): 10BASE2

  • The maximum length of a segment is 185 metres.
  • A maximum of 2 IRL (InterRepeater Links) is allowed between devices; the maximum length of cable is 925 metres.
  • Typically, devices use Ethernet network interface cards (NICs) with built-in BNC transceivers, so connections can be made directly to the ThinNet cable.

  • Devices are connected to the cable with T-connectors, which must be plugged directly into the card. No cable is allowed between the T and the card. Workstations are daisychained with an "in-and-out" cabling system.
  • The minimum distance between T-connectors is 0.5 metres.
  • If the interface card does not have its own built-in BNC transceiver, a BNC transceiver and transceiver cable are required. The maximum length of a transceiver cable is 50 metres.

  • Up to 30 connections can be attached to a single segment.
  • Both ends of each segment should be terminated with a 50-ohm resistor.
  • One end of each segment should be grounded to earth.


Twisted-Pair Ethernet (Unshielded Twisted Pair): 10BASE-T, UTP

  • There are two versions of Ethernet over unshielded twisted pair: 10BASE-T (the standard) and its predecessor UTP.
  • 10BASE-T and UTP segments can coexist on the same network via a transceiver and transceiver cable or converter when each hub is attached to a common segment.

  • The cable used is 22 to 26 AWG unshielded twisted pair (standard telephone wire), at least Category 2 with two twists per foot. Category 3 or 4 is preferred. Category 5 supports 100BASE-T (Fast Ethernet) and is recommended for all new installations.
  • Workstations are connected to a central concentrator ("hub") in a star configuration. Concentrators can be attached to a fibre optic or coax network and can be daisychained to form larger networks.

  • A hub usually also has an AUI port for standard Ethernet connections.
  • The maximum segment distance from concentrator to node is 100 metres.
  • The maximum number of devices per segment is two: the hub port and the 10BASE-T or UTP device.
  • Ethernet network interface cards (NICs) are available with built-in 10BASE-T transceivers.

  • Devices with standard AUI ports may be attached with a twisted-pair transceiver.
  • Twisted pair is the most economical cable type, since it may already be installed, and it is the easiest to work with. But it's not recommended for installations with a great deal of EMI/RFI interference (for example, in industrial environments).


Fibre Optic Ethernet: FOIRL or 10BASE-FL

  • There are two versions of Ethernet over fibre optic cable, one for the older FOIRL (Fibre Optic InterRepeater Link) and one for the more recent 10BASE-FL standards.
  • FOIRL and 10BASE-FL fibre optic Ethernet differ only in how far each will transmit (the maximum length of a segment). For FOIRL it is 1 km; for 10BASE-FL it is 2 km.
  • The maximum number of devices per segment is two: the hub port and the 10BASE-FL device.

  • Fibre optic cable provides the best signal quality as well as the greatest point-to-point distance and is completely free of EMI/RFI interference.
  • Fibre optic cable runs point to point only-it cannot be tapped or daisychained. A fibre optic hub or multiport repeater is required to carry the signal to multiple devices (for FOIRL, a FOIRL multiport repeater and transceivers).

  • Since fibre optic cable does not carry electrical charges, there are no electrical cable problems. And it's immune to electronic eavesdropping. When outdoor-quality fibre optic cable is used to link buildings, grounding problems and voltage spikes are eliminated.


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