Gigabit 1000BASE-T &1000BASE-X__Media Options for

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 several 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 choose (copper vs. fiber) is.

Background

In the Fall of 2000, the IEEE 802.3ab task force began working on a

scheme which, by design, would allow data to be delivered at Gigabit

speeds over much of the existing installed based of Category 5 UTP

cabling. This group proposed using an 8B/10B (8 bit user data

converted to 10 bit symbols) encoding/decoding scheme which would

serve 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 base 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

(Gigabit over twinaxial copper cabling). Fiber optic media, because of its

inherently high bandwidth carrying capability, seemed to be the most

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

requires some thought based 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 use 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 server farms •Accommodating very high bandwidth users 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 used.With increasing numbers of users demanding more frequent access to storage devices and servers, requirements for higher speed connections have become a necessity.The increasing complexity of graphics used in engineering CAD software as well as software used by graphic artists, will require that these "power users" 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 increase in speed typically equates to a decrease in distance, because 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 send its data. Collisions occur if two devices on the same network attempt to transmit at the same time. These collisions, if not too frequent, are perfectly normal and easily dealt with by the protocol (under the provision of CSMA/CD - Carrier Sense Multiple Access/Collision Detection - part of the IEEE 802.3 standard).In Ethernet, the smallest packet size allowed is bytes (8 bits per byte = 512 bits). The purpose of establishing a minimum packet size was to ensure that a station could detect collisions at the furthest point of the network, allowing the CSMS/CD portion of the protocol to deal with it appropriately (referred to as the 512 bit-time rule). As speed increases by factors of 10 (10 Mbps to 100 Mbps to 1 Gbps), the distance that you 2With increasing numbers of

users demanding more frequent

access to storage devices and

servers, requirements for higher

speed connections have become a necessity.

10. Consequently, at Gigabit speeds in a shared Ethernet environment,

you are limited to about 20 meters over UTP.

The Gigabit standard addresses this distance limitation issue by a

method known as "carrier extension." Carrier extension effectively

increases the packet size to 512 bytes (4096 bits), by adding "extension

symbols" to increase 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 decreased 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 used 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 send down a larger pipe nets you a nominal gain.) However, even

when both methods are used, 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 use of much of the existing

Category 5 cabling by enabling 1000BASE-T to operate at the 100 MHz

rating of CAT 5 UTP. The cabling system used 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 base 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 insertion

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

caused by noise from the other three transmitters on a segment (NEXT) or

interference to each receiver caused by the three adjacent transmitters

(ELFEXT). Effectively, much of the installed Category 5 UTP, because 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 used in 1000BASE-T installations. Category 5e

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6more Gigabit ports than you require for your application, and at a time

when the cost of the technology is at a premium. (As the technology

matures, prices will eventually decrease.)

There is always the option of purchasing a Gigabit device that offers a

slide-in-module option for adding either copper or fiber modules. These

devices typically offer a single port for an uplink module that can be

populated with a fiber Gbic or copper UTP card. The intent is, most often,

to use this as a backbone connection, and is most commonly populated

with a fiber Gbic slide-in-card. This is a good choice if all you require is a

single Gigabit backbone connection. However, there are two issues that

may require additional consideration. Although you have the flexibility of

"modularity" with the slide-in-card, the remaining ports will likely be of a

"fixed" configuration. Should you need to change the type of media on

any of these fixed ports you, will need to employ an external device, such

as a media converter, or purchase a new network device altogether. You

need to also consider that not all Gigabit network devices offer a modular

uplink port, and if they do, it is usually at a premium.

There is yet another option that offers the flexibility of being able to add

any port configuration (copper or fiber) you require where and when you

need it. This device can allow you to take advantage of the lower cost of

purchasing a switch with copper Gigabit interfaces and add Gigabit fiber

links only where you need them. The device is known as a "Media

Converter," and can be used to change one media type (copper or fiber)

to another media type (copper or fiber) to facilitate the transition between

two disparate media types. There are media converters that will allow you

to convert 1000BASE-T to 1000BASE-SX/LX or to convert 1000BASE-SX

multimode to 1000BASE-LX single mode port-by-port as the need arises.

Converters can be added to a Gigabit link without disturbing the

communication over other links on the device. Media converters are also

relatively inexpensive and can be managed via an SNMP GUI interface.

Long Haul Options - Extending the Reach of your LAN

An option that is currently unique to media converters is the concept of

"Long Haul" for extending fiber connections up to 65 kilometers away.

Longer distances can be achieved across a Gigabit fiber link by

increasing the launch power and receive sensitivity of the fiber optic

transceivers used. These long haul options are currently not available

from major switch manufacturers. There are several applications for

"Long Haul" Gigabit devices.

• MANs - Metropolitan Area Networks

• Large Campus LANs

There is a growing popularity of MANs (metropolitan area networks)

which allow native LAN protocols to be transmitted between remote

facilities over leased or privately owned single mode fiber. Entities that

have remote facilities can effectively make a LAN backbone connection

between buildings that are as far as 65 kilometers away from each other.

In recent years, companies such as telcos, public utilities, etc., have

been installing vast amounts of single mode fiber optic cable in their

right-of-ways. Much of this fiber (as much as 90%) is currently "dark"

(not currently being used). Companies with excess fiber are, in many

cases, offering access to their fiber through a lease arrangement.

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Summary

The IEEE 802.3z and 802.3ab task forces have developed standards that

have enabled network hardware manufacturers to produce products with

transmission speeds of 1 Gigabit per second. Most recently, they have

developed standards that allow Gigabit speed over UTP copper cabling.

Although some issues do exist with regard to cost (cabling infrastructure)

and throughput (half-duplex), 1000BASE-T is well on its way to

becoming a viable standard. Fiber-based solutions for Gigabit, although

relatively expensive, offer network managers the additional reach to

connect virtually any two network devices together. In most cases, a need

will exist for both the cost benefit of copper and the distance benefit of

fiber in a Gigabit-enabled network. Various options exist to facilitate the

co-existance of copper and fiber in a Gigabit network. Conversion

technology offers an easy to implement, low-cost option for connecting

disparate interfaces with long haul options not available from major

switch manufacturers.

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