Gigabit 1000BASE-T & 1000BASE-X
Media Options for Gigabit Networks
a whitepaper by Ken Johnson, Transition Networks, Inc.
Once thought to be improbable, if not impossible, Gigabit speeds are
now a reality over twisted pair copper cabling. Many of the large network
hardware manufactures have been quick to adopt this new technology,
and most are now offering products with Gigabit UTP interfaces. Gigabit
speeds have been available over fiber optic media for veral years,
while Gigabit over copper media is a relatively new phenomenon. With
the thirst for network bandwidth increasing exponentially, moving to
Gigabit in network backbone connections is not as much a choice, as it
is an eventuality. Although the need for greater bandwidth is becoming
less of a question, the type of interface you choo (copper vs. fiber) is.
Background
司马台In the Fall of 2000, the IEEE 802.3ab task force began working on a
口译技巧scheme which, by design, would allow data to be delivered at Gigabit
speeds over much of the existing installed bad of Category 5 UTP
cabling. This group propod using an 8B/10B (8 bit ur data
converted to 10 bit symbols) encoding/decoding scheme which would
rve to push the center frequency transmission below the 100MHz
threshold required for category 5 copper cables. The 802.3ab task force
had the foresight to ba Gigabit on existing proven specifications, like
FibreChannel (ANSI X3T11), as well as published Ethernet standards
(IEEE 802.3). This ensured that the standard could be developed quickly
and that it would provide for compatibility with existing Ethernet and Fast
Ethernet devices. The IEEE, under 802.3ab, approved the standard for
Gigabit transmission over UTP in the Fall of 2000; now known as
"1000BASE-T.”
Gigabit over fiber optic media borrowed heavily from the existing
FibreChannel standards. Again, the IEEE developed this standard via a
task force which published its work in 1998 under IEEE 802.3z
(1000BASE-X). Approved and proven, the standard for Gigabit over fiber
describes high-speed transmission of data over SX (850nm short
wavelength fiber), LX (1310nm long wavelength fiber), as well as CX富都酒店
(Gigabit over twinaxial copper cabling). Fiber optic media, becau of its
inherently high bandwidth carrying capability, emed to be the most炒藕丁的家常做法
logical choice for transmitting higher speed protocols. However, the
higher cost of fiber optic interfaces (vs. copper interfaces) and the large
number of networks with installed Category 5 cabling made development
of a copper solution for Gigabit necessary.
Using copper or fiber interfaces on your network hardware is a choice that热感冒症状
requires some thought bad on your unique requirements. Copper
interfaces allow you to add Gigabit to your network at a lower cost than
fiber, and theoretically, allow you to deploy it over your existing Category
5 cabling plant. However, there are technical issues with Gigabit
transmission over UTP that you should understand before you decide
which media type you will u in your Gigabit network connection.
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Applications for Gigabit Gigabit speed has become necessary in network backbones as a result of ever-increasing thirst for bandwidth. Applications for Gigabit include:•Aggregation of bandwidth; such as backbone connections for Fast Ethernet switches •High-speed data transfers between clients and rver farms •Accommodating very high bandwidth urs in CAD and image editing applications Currently, the most common application for Gigabit is aggregation of bandwidth for backbone connections between Fast Ethernet devices (most often switches). This Gigabit connection is often accomplished by using a modular device that can be installed in switches, which is available from all of the major switch manufacturers, known as a Gbic.Gbics are relatively inexpensive and can facilitate most Gigabit backbone connections of this type; provided they do not exceed the maximum distance allowed by the media and fiber optic transceiver ud.With increasing numbers of urs demanding more frequent access to storage devices and rvers, requirements for higher speed con
nections have become a necessity.The increasing complexity of graphics ud in engineering CAD software as well as software ud by graphic artists, will require that the "power urs" have access to a bigger and faster pipe.Issues in Half-Duplex Networks Gigabit achieves 1Mbps throughput by effectively transmitting 250 Mbps of data over each of four wire pairs simultaneously in both directions;where the cumulative result is a 1 Gbps duplex connection. The Gigabit 1000BASE-T standard was written to accommodate both full-duplex and half-duplex operation (Shared Ethernet regulated under CSMA/CD rules).Full-duplex is clearly the preferred architecture, as there are some inherent problems with running Gigabit in a shared architecture over copper, in terms of distance and throughput.In shared Ethernet, an increa in speed typically equates to a decrea in distance, becau of the method in which Ethernet deals with collisions. Ethernet devices "listen" for an opportunity to transmit on a shared wire pair. If a device detects that no other devices are transmitting,it deems it safe to nd its data. Collisions occur if two devices on the same network attempt to transmit at the same time. The collisions, if not too frequent, are perfectly normal and easily dealt with by the protocol (under the provision of CSMA/CD - Carrier Sen Multiple Access/Collision Detection - part of the IEEE 802.3 standard).In Ethernet, the smallest packet size allowed is 64 bytes (8 bits per byte = 512 bits). The purpo of establishing a minimum packet size was to ensure that a station could detect collisions at the furthest point of the network, all
owing the CSMS/CD portion of the protocol to deal with it appropriately (referred to as the 512 bit-time rule). As speed increas by factors of 10 (10 Mbps to 100 Mbps to 1 Gbps), the distance that you 2With increasing numbers of
urs demanding more frequent
access to storage devices and
rvers, requirements for higher
speed connections have become a necessity.
can transmit and still properly detect collisions is decread by a factor of
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10. Conquently, at Gigabit speeds in a shared Ethernet environment,
you are limited to about 20 meters over UTP.
The Gigabit standard address this distance limitation issue by a
method known as "carrier extension." Carrier extension effectively
increas the packet size to 512 bytes (4096 bits), by adding "extension
symbols" to increa the size of the packet to a size that can be detected
by all devices on a Gigabit link up to 100 meters away. The end device
then strips this additional data or "extension symbols" off when it is
received. The problem is that increasing the packet size (adding 448
bytes of extension symbols) means that you have actually decread the
throughput to about 100 Mbps Fast Ethernet speed. (Sending larger
amounts of data down a larger pipe nets you no significant gain.)
To deal with this reduction in throughput, a method known as "packet
bursting" is ud in conjunction with carrier extension. Packet bursting
improves the efficiency of carrier extension by decreasing the inter-packet
gap when multiple packets are transmitted. (Reducing the amount of data
you nd down a larger pipe nets you a nominal gain.) However, even
when both methods are ud, throughput in half-duplex Gigabit remains
hindered and never achieves full 1 Gbps speed. The bottom line is that
half-duplex is possible but not recommended in Gigabit environments.
Carrier detection and packet bursting are not required in a full-duplex
Gigabit environment.
Cabling Considerations
Theoretically, IEEE 802.3ab intended to make u of much of the existing
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Category 5 cabling by enabling 1000BASE-T to operate at the 100 MHz
rating of CAT 5 UTP. The cabling system ud to support 1000BASE-T
requires four pairs of Category 5 balanced cabling with nominal
impedance of 100 ohms as required in the TIA/EIA 568-A standard. The
demands placed on a Category 5 cabling plant to support Gigabit speed
may surpass the ability of much of the installed ba of Category 5 cable
to support it reliably. To make certain that a given Category 5 cabling
plant is able to support 1000BASE-T, IEEE developed additional
requirements.
In addition to the requirements stated in EIA/TIA 568A for Category 5
cabling, additional requirements were added (Annex 40A) with further
requirements for the 1000BASE-T installations. The transmission
parameters in Annex 40A call out additional requirements for inrtion
loss, delay, characteristic impedance, return loss, and most importantly,
NEXT/ELFEXT (Near-end cross talk/equal level far-end cross talk). Cross-
talk is simply electrical interference to each of the individual transmitters
caud by noi from the other three transmitters on a gment (NEXT) or
interference to each receiver caud by the three adjacent transmitters
(ELFEXT). Effectively, much of the installed Category 5 UTP, becau it
was installed prior to the publication of Annex 40A, and therefore, not
tested to meet its requirements, may not support 1000BASE-T. To provide
a safety margin, some network hardware manufacturers recommend that
Category 5e cabling be ud in 1000BASE-T installations. Category 5e
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