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:
-
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.
-
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 (which
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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