Guide to Using Fast Ethernet

Networking Guide. Guide to Using Fast Ethernet

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The demand for higher transmission speeds for near-term network growth has been realized by the new Fast Ethernet specification (IEEE 802.3u) known as 100BASE-T. This new LAN standard has raised the Ethernet speed limit from 10 Megabits per second to 100 Megabits per second with only minimal changes to the existing cable structure. The building blocks of today’s networks call out for a mixture of legacy 10BASE-T Ethernet networks and Fast Ethernet.

Typically 10Mbps networks utilize Ethernet switches (like the Lantronix LSW8) to improve the overall efficiency of the Ethernet network. Between Ethernet switches, Fast Ethernet repeaters are used to connect a group of switches together at the higher 100 Mbps rate. Shared servers can also be connected to the Fast Ethernet repeaters to ensure the avoidance of bottlenecks at the server. Many client/server networks suffer from too many clients trying to access the same server which creates a bottleneck where the server attaches to the LAN. Fast Ethernet, in combination with switched Ethernet, creates the perfect cost-effective solution for avoiding slow networks. When integrating 100BASE-T into a 10BASE-T network, the only thing that has changed in the wiring is that the corporate premise distributed wiring system must now include Category 5 rated twisted pair cable in these areas running 100BASE-T.

The Fast Ethernet specification calls for three types of transmission schemes over various wire media. The first is 100BASE-TX, which, from a cabling perspective, is very similar to 10BASE-T. It uses Category 5-rated twisted pair copper cable to connect the various hubs, switches and end-nodes together. It uses an RJ45 jack just like 10BASE-T and the wiring at the connector is identical. That is why 100BASE-TX is the most popular form of the Fast Ethernet specification. The second variation is 100Base-FX which is used primarily to connect hubs and switches together either between wiring closets or between buildings. 100BASE-FX uses multimode fiber-optic cable to transport Fast Ethernet traffic. The third variation of the Fast Ethernet specification is 100BASE-T4. This scheme incorporates the use of two more pairs of wiring to allow Fast Ethernet to operate over Category 3-rated cables or above.

While this tutorial focuses on the 100BASE-T family of Fast Ethernet standards, an alternative Fast Ethernet protocol called 100VG-AnyLAN was developed by Hewlett Packard Co. and later modified by the IEEE 802.12 committee. This version of 100 Megabit Ethernet supports a token passing style of architecture rather than the collision strategy of the 10BASE-T and 100BASE-T standards. The 100VG-AnyLAN standard involves a different wiring scheme and the use of different network management tools. For these reasons it has not gained the same popularity as the 100BASE-T Fast Ethernet standards.

Rules of the Road

The basic building block for the Fast Ethernet LAN is the Fast Ethernet repeater. The two types of Fast Ethernet repeaters offered on the market today are:

  1. Class I Repeater: The Class 1 repeater operates by translating line signals on the incoming port to a digital signal. This allows the translation between different types of Fast Ethernet like 100BASE-TX and 100BASE-FX. A Class I repeater intoduces delays when performing this conversion such that only one repeater can be put in a single Fast Ethernet LAN segment.
  2. Class II Repeater: The Class II repeater immediately repeats the signal on an incoming port to all the ports on the repeater. Typically Class II repeaters, like the Lantronix LFR8, have all ports of the same Fast Ethernet type (100BASE-TX). Very little delay is introduced by this quick movement of data across the repeater; thus two Class II repeaters are allowed per Fast Ethernet segment.

Network Managers have become accustomed to the 150 meter distance limitation of 10BASE-T Ethernet. At the higher operating speeds, Fast Ethernet is limited to 100 meters over Category 5-rated cable. The EIA/TIA cabling standard recommends using no more than 90 meters between the equipment in the wiring closet and the wall connector. This allows another 10 meters for patch cables between the wall and the desktop computer.

In contrast, a Fast Ethernet network using the 100BASE-FX standard is designed to allow LAN segments up to 412 meters in length. Even though fiber-optic cable can actually transmit data greater distances (i.e. 2 Kilometers in FDDI), the 412 meter limit for Fast Ethernet was created to allow for the round trip times of packet transmission. Typical 100BASE-FX cable specifications calls for multimode fiber-optic cable with a 62.5 micron fiber-optic core and a 125 micron cladding around the outside. This is the most popular fiber optic cable type used by many of the LAN standards today. Connectors for 100BASE-FX Fast Ethernet are typically ST connectors (whick look like Ethernet BNC connectors). Many Fast Ethernet vendors will be migrating to the newer SC connectors used for ATM over fiber. A rough implementation guideline to use when determining the maximum distances in a Fast Ethernet network is the equation:

400 - (r x 95)

where r is the number of repeater. The network manager needs to take into account the distance between the repeaters and the distance between each node from the repeater. For example look at Figure 1 that uses two LFR8 repeaters connected to two Fast Ethernet switches and a few servers.

Figure 1: Fast Ethernet Distance Calculations With Two Repeaters

Maximum Distance Between End nodes:
400-(rx95) where r = 2 (for 2 repeaters)
400-(2x95) = 400-190 = 210 feet, thus
A + B + C = 210 Feet

There is another variation of Ethernet called full-duplex Ethernet. All of the LSW8 switched Ethernet ports (both 10 Mbps Ethernet ports and 100 Mbps ports) can support full-duplex Ethernet. Simply put, by adding another pair of wires and removing collision detection, the connection speed is doubled. In the terms of Fast Ethernet, essentially 200 Mbps of throughput is the theoretical maximum per full-duplex Fast Ethernet connection. This type of connection is limited to a node-to-node connection and is typically used to link two Ethernet switches together. The LFR8 Fast Ethernet repeater does not support full-duplex connections.

Maintaining a Quality Network

The Category 5 cable specification is rated up to 100 MHz and meets the requirement for high speed LAN technologies like Fast Ethernet. The EIA/TIA (Electronics industry Association/ Telecommunications Industry Association) formed this cable standard which describes performance the LAN manager can expect from a strand of twisted pair copper cable. Along with this specification, the committee formed the EIA/TIA-568 standard named the “Commercial Building Telecommunications Cabling Standard” to help network managers install a cabling system that would operate using common LAN types (like Fast Ethernet). The specification defines Near End Crosstalk (NEXT) and attenuation limits between connectors in a wall plate to the equipment in the closet. Cable analyzers can then be used to ensure accordance with this specification and thus guarantee a functional Fast Ethernet network. The basic strategy of cabling Fast Ethernet systems is to minimize the retransmission of packets caused by high bit-error rates. This ratio is calculated using NEXT, ambient noise, and attenuation of the cable. A cable is rated for Category 5, but also punch down blocks and connectors.

Fast Ethernet Migration

Most network managers will migrate from 10BASE-T or other Ethernet 10 Mbps variations to higher bandwidth networks. Typically this starts at the workgroup with the replacement of hubs with Ethernet switches. These switches still operate at 10 Mbps speeds but help isolate and reduce overall traffic by localizing the bulk of the data flow to its own segment. From here, full- duplex connections can connect two Ethernet switches together, or connect the switch to a file server. Eventually, Fast Ethernet ports on these switches will be used to provide even greater bandwidth between the workgroups at 100 Mbps speeds. Equipment like the LFR8 Fast Ethernet repeater will be used in common areas to group Ethernet switches together with server farms into large 100 Megabit pipes. This is the cost effective method of growing most networks within the average enterprise. The LFR8 Fast Ethernet repeater (besides all of Lantronix standard Ethernet hubs and switches) are the perfect complement to a LAN growth strategy.


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