WHITE PAPER

1000BASE-T:Gigabit Ethernetover Category 5Copper CablingTechnicalFundamentals

1000BASE-T: GigabitEthernet overCategory 5 CopperCabling

Technical Fundamentals

By 3Com Corporation, and CableDesign Technologies Corporation

How can network managers deploybandwidth-intensive applications overtheir local area networks (LANs) whenthey have tight budgets and must lever-age their existing infrastructure? Thelatest Ethernet technology, 1000BASE-T(Gigabit Ethernet over Category 5 cop-per cabling), helps network managersboost their network performance in asimple, cost-effective way.

1000BASE-T is Ethernet that providesspeeds of 1000 Mbps over Category 5copper cabling, the most widelyinstalled LAN cabling infrastructure.The IEEE Standards Committee for-mally ratified 1000BASE-T as an Eth-ernet standard in June 1999.

This paper is for network managerswho want a technical understanding ofthe fundamentals of 1000BASE-T. Itexplains how 1000BASE-T has beendesigned by the IEEE 802.3ab TaskForce to run over Category 5 cablingand how to implement 1000BASE-Tover existing Category 5 cable infra-structure and over Enhanced Category 5(Cat 5e) in new sites.

Why 1000BASE-T: GigabitEthernet over Category 5Copper Cabling?1000BASE-T, Gigabit Ethernet overCategory 5 copper cabling, is anattractive option for network managersfor several reasons. It addresses theexploding bandwidth requirementson current networks that are theresult of implementing new applica-

tions and the increasing deploymentof switching at the edges of the net-work. Gigabit Ethernet leverages theorganization’s existing investment inEthernet and Fast Ethernet infrastruc-tures, and it provides a simple, cost-effective performance boost whilecontinuing to use the dominant hori-zontal/floor cabling medium.

Exploding BandwidthRequirementsNew, bandwidth-intensive applica-tions are being deployed over Ethernetand Fast Ethernet networks. Theseapplications include the following:

• Internet and intranet applicationsthat create any-to-any traffic pat-terns, with servers distributedacross the enterprise and usersaccessing Web sites inside and out-side the corporate network. Theseapplications tend to make trafficpatterns and bandwidth require-ments increasingly unpredictable.

• Data warehousing and backupapplications that handle gigabytesor terabytes of data distributedamong hundreds of servers andstorage systems.

• Bandwidth-intensive, latency-sensi-tive groupware applications such asdesktop video conferencing orinteractive white-boarding.

• Publication, medical imaging, andscientific modeling applicationswhich produce multimedia andgraphics files that are exploding insize from megabytes to gigabytes totera-bytes.

Bandwidth pressures are compoundedby the growing deployment ofswitching as the desktop connectionof choice. Switching at the edgetremendously increases the trafficthat must be aggregated at the work-group, server, and backbone levels.

1

CONTENTS

Why 1000BASE-T: GigabitEthernet over Category 5 Copper Cabling? . . . . . . . . . . . 1

1000BASE-T TechnicalFundamentals . . . . . . . . . . . . . 2

Preparing for Deployment overExisting Cabling . . . . . . . . . . . 5

Preparing for Deployment overNew Copper Cabling: Category 5e . . . . . . . . . . . . . . 6

Migrating Ethernet/Fast Ethernet Networks Toward High-Speed Networking . . . . . . 7

Conclusion . . . . . . . . . . . . . . . 8

Appendix A: Gigabit EthernetStructure and Support forDifferent Media . . . . . . . . . . . 10

Appendix B: Key PerformanceParameters for 1000BASE-TCabling . . . . . . . . . . . . . . . . . 11

Appendix C: 1000BASE-T Physical Layer Implementation .15

Appendix D: Category 5 Cabling Field MitigationProcedures . . . . . . . . . . . . . . 17

Significant Investment inEthernet/Fast EthernetInfrastructureEthernet is the dominant, ubiquitousLAN technology. More than 85percent of all installed network con-nections were Ethernet at the end of1997, representing more than 118 mil-lion interconnected PCs, worksta-tions, and servers.1

The deployment of Ethernet/Fast Eth-ernet networks involves investment innetwork interface cards (NICs), hubs,and switches, as well as in networkmanagement capabilities, staff train-ing and skills, and cabling infrastruc-ture. In fact cabling infrastructure isthe longest-term networking invest-ment, lasting at least two years andup to 10 years. (On average, almosthalf of the infrastructure is in placefor more than five years.2)

A Simple, Cost-EffectivePerformance Boost on ExistingCategory 5 Cabling 1000BASE-T offers a simple, cost-effective migration of Ethernet/FastEthernet networks toward high-speednetworking, and has the followingbenefits:

• 1000BASE-T scales Ethernet 10/100Mbps performance to 1000 Mbps.Flexible 100/1000 and 10/100/1000connectivity will be offered andwill enable the smooth migration ofexisting 10/100 networks to 1000Mbps–based networks.

• 1000BASE-T is the most cost-effec-tive high-speed networking tech-nology available now. 1000BASE-Tleverages existing, proven Fast Eth-ernet and V.90/56K modem tech-nologies and will experience thesame cost curve as the Ethernet/Fast Ethernet technologies.1000BASE-T is in fact expected to

be significantly more cost-efficientthan 1000BASE-SX (Fiber Gigabit),which already has the lowest cost-per-data-transmitted per secondamong all LAN technologies (cur-rently less than $1.5 per Mbps).

• 1000BASE-T preserves Ethernetequipment and infrastructureinvestments, including the invest-ment in the installed Category 5cabling infrastructure. There is noneed to undergo the time-consum-ing and high-cost task of replacingcabling located in walls, ceilings, orraised floors.

Leveraging Category 5 copper cablinginfrastructure is of significant impor-tance for two reasons:

• Category 5 is today the dominanthorizontal/floor cabling, providingconnectivity to both desktops andworkgroup aggregators (Figure 1).Fiber is the dominant cabling forconnection of multiple buildings.

• Category 5 is one of the majoroptions for building risers/backbonecabling for connection of differentfloor wiring closets (Figure 2).

1000BASE-T TechnicalFundamentalsGigabit Ethernet MediaSpecificationsGigabit Ethernet cost-effectivelyleverages existing cabling infrastruc-tures. It can be implemented in floor,building, and campus networksbecause it offers a wide range of con-nectivity media and connection dis-tances. Specifically, Gigabit Ethernetis designed to run over four media:

• Single-mode fiber, with connec-tions up to at least 5 kilometers

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 2

ANSI American National StandardsInstitute

BER bit error rate

DSL digital subscriber line

DSP digital signal processing

ELFEXT equal level far-end crosstalk

FEXT far-end crosstalk

GMII Gigabit Media IndependentInterface

IEEE Institute of Electrical andElectronics Engineers

IP Internet Protocol

ISO International Organization forStandardization

LAN local area network

MAC Media Access Control

1 According to industry analyst International Data Corporation (IDC), Framingham, MA.2 From the December 1998 Networking Cabling Market Study by Sage Research, Natick, MA.

• Multimode fiber, with connectionsup to at least 550 meters

• Balanced, shielded copper, withconnections up to at least 25 meters

• Category 5 cabling, with connec-tions up to at least 100 meters

The IEEE 802.3z Gigabit Ethernetstandard approved in June 1998 spec-ified three transceivers to cover threemedia: 1000BASE-LX for the installedbase of single-mode fiber, 1000BASE-SX for the installed base of multimodefiber, and 1000BASE-CX for a bal-anced, shielded copper cable that

could be used for interconnects inequipment rooms. 1000BASE-LXtransceivers can also be used to reachat least 550 meters on multimodefiber.

Another task force, IEEE 802.3ab, hasdefined the physical layer to runGigabit Ethernet over the installedbase of Category 5 cabling. The IEEEStandards Committee approved the1000BASE-T standard in June 1999.Figure 3 summarizes the various Giga-bit Ethernet options and the stan-dards that define them.

3

MLT Multilevel Transmit Signal

MM multimode

NEXT near-end crosstalk

NIC network interface card

OSI Open System Interconnection

PAM Pulse Amplitude Modulation

PCS physical coding sublayer

PMA physical medium attachment

PMD physical medium dependent

SM single-mode

SNR signal-to-noise ratio

STP shielded twisted pair

UTP unshielded twisted pair

Category 5

Thin coax

Category 3

Fiber (MM/SM)

Category 5e

Shielded twisted pair

0% 20% 40% 60% 80%

FIGURE 1. Current Hor izonta l Network Cable Types

Notes: Figure shows the percentage of organizations that have installed the cable type. Multi-ple choices were allowed. Other cabling types (Category 6 STP, Category 4, Category 6 UTP,plastic optical fiber) each scored less than 10 percent.

Source: Sage Research, December 1998, Network Cabling Market Study

Category 5

Multimode fiber

Thin coax

Single-mode fiber

Others

Shielded twisted pair

0% 10% 20% 30% 40% 50%

FIGURE 2. Current Bu i ld ing Backbone Cable Types

Notes: Figure shows the percentage of organizations that have installed the cable type. Multi-ple choices were allowed. Other cabling types (Category 3, Category 5e, Category 6 STP, Cate-gory 4, Category 6 UTP, plastic optical fiber) each scored less than 10 percent.

Source: Sage Research, December 1998, Network Cabling Market Study

For more information on how GigabitEthernet is defined to support differ-ent media, see Appendix A.

1000BASE-T Key SpecificationsThe 1000BASE-T standard leveragesthe existing cable infrastructure as itis specified to operate up to 100meters on Category 5 cabling.

The other key specifications of1000BASE-T make it a cost-effective,nondisruptive, and high-performingtechnology. First, it supports theEthernet MAC, and is thus backwardcompatible with 10/100 Mbps Ether-net. Second, many 1000BASE-T prod-ucts will support 100/1000 auto-negotiation, and 1000BASE-T canthus be incrementally deployed in aFast Ethernet network. Third,1000BASE-T is a high-performingtechnology with less than one erro-neous bit in 10 billion transmittedbits (this bit error rate of less than10–10 is the same error rate as that of100BASE-T).

Detailed 1000BASE-T CableSpecifications1000BASE-T is specified to run overfour pairs of Category 5 balancedcabling. The four pairs of Category 5

balanced cabling are specified inANSI/EIA/TIA-568-A (1995). Addi-tional link performance parameters(return loss and ELFEXT) are specifiedin TIA/EIA-TSB-95. Figure 4 detailsthe standards of reference for thespecification of 1000BASE-T cableperformance parameters. For addi-tional information, see Appendix B.Category 5 cabling is also specified inISO/IEC 11801:1995 (“InformationTechnology: Generic Cabling for Cus-tomer Premises”). The second editionof ISO/IEC 11801:1995 will includethe additional cabling performance

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 4

802.3 Media Access Control (MAC) media

Gigabit Media Independent Interface (GMII)

1000BASE-X 8B/10Bencoder/decoder

PMA (SERDES)

1000BASE-TTrellis encoder/Viterbi decoder

1000BASE-CXshieldedcopper

1000BASE-LXlong-haul

fiber

1000BASE-SXshort-haul

fiber

1000BASE-TCategory 5

copper

IEEE 802.3z IEEE 802.3ab

GMII

PMA

F IGURE 3. Gigabi t E thernet Media Opt ions and Standards

TIA/EIA-568-A

NEXTAttenuation

TIA/EIA-TSB-95

TIA/EIA-568-A-1

ELFEXTReturn lossField testing

1000BASE-Tcable specification

NEXTAttenuation

DelayDelay skew

ELFEXTReturn loss

DelayDelay skew

F IGURE 4. Standards ofReference for 1000BASE-TPerformance Parameters

parameters specified to support Giga-bit Ethernet.

1000BASE-T Design1000BASE-T is designed to run overCategory 5 copper cabling. The trans-mission of 1 Gbps is possible thanksto the use of four twisted-pair linkswith 250 Mbps of throughput on eachpair (250 Mbps x 4 = 1 Gbps).

1000BASE-T transmits at the sameclock rate as 100BASE-T (125 MHz)but uses a powerful signaling andcoding/decoding scheme that enablesthe transmission of double theamount of data as 100BASE-T.Following is a comparison of the twospecifications:

• 1000BASE-T: 125 MHz x 2 bits =250 Mbps

• 100BASE-TX: 125 MHz x 1 bit-symbol = 125 Mbit-symbol/s

Note: 125 Mbit-symbol/s isequivalent to 100 Mbps, since100BASE-T uses a 4B/5B code—4bits of data are translated into 5 bit-symbols before transmission on the wire; the effective bits throughputis thus 125 x 4 / 5 = 100 Mbps.

1000BASE-T cost-effectively leveragesthe design of proven existing FastEthernet and V.90/56K modem tech-nologies. Sig-naling and coding/decoding methods already imple-mented in 802.3 Fast Ethernet trans-ceivers and in V.90 or 56K modemsusing advanced DSPs are used toimplement 1000BASE-T. Table 1 sum-marizes the 100BASE-T technologies

and methods reused by 1000BASE-T.For additional information, seeAppendix C.

Preparing for Deploymentover Existing CablingPreparing existing Category 5 coppercabling for running 1000BASE-T is astraightforward process. The first stepis a simple test of the adequacy of thecable installation. In the unlikelyevent that an existing installationdoes not meet one of the performanceparameters specified by 1000BASE-T,standard corrective actions can beimplemented.

Testing the Installation ofExisting Category 5 Cabling• Cable testing information is specified

in the ANSI/TIA/EIA-TSB-67 stan-dard, “Transmission PerformanceSpecifications for Field Testing ofTwisted-Pair Cabling System,”which has been used by cablinginstallers since 1995.

• The additional test parameters ofreturn loss and ELFEXT for1000BASE-T are specified in theANSI/TIA/EIA-TSB-95 Bulletin,“The Additional Transmission Per-formance Guidelines for 100 Ohm4-Pair Category 5 Cabling.”

• These additional tests are incorpo-rated into the current versions ofcable test tools (for a list of them,refer to the Gigabit Ethernet AllianceWeb site, http://www.gigabit-ethernet.org/technology/whitepapers/gige_0399/copper.html#installed).

5

TECHNOLOGY/METHOD 1000BASE-T 100BASE-TX 100BASE-T2

Multi-level signaling Five-level PAM Five-level PAM

Symbol clock rate 125 MHz 125 MHz

Transmit spectrum MLT-3–like MLT-3

Digital signal processing Yes Available Yes

Transmission Bidirectional Bidirectional

TABLE 1. 100BASE-T Technolog ies Used in 1000BASE-T

Field testing is performed by con-necting the two handheld devices—one at each end of the cablingunder test (see Figure 18 in Appen-dix D)—with a field test cord, andthen activating the 1000BASE-Ttest function. A pass or fail will beindicated for the 1000BASE-T testand for the specific test parametersunder test. Many field testersinclude diagnostic functions to helpidentify the cause of failures. Cor-rective actions are provided inAppendix D.

Adjusting Existing Category 5Cabling to Run 1000BASE-T In the unlikely event that an existingCategory 5 installation does not meetone of the performance parametersspecified by 1000BASE-T, correctiveactions are defined in a simple fieldprocedure detailed in the ANSI/TIA/EIA-TSB-95. Three types of correctivemeasures can be applied:

• Use of high-performance Category5e patch cables (see following sec-tion for definition of Category 5e)

• Reduction in the number of con-nectors used in the link

• Reconnection of some connectors inthe link

Appendix D describes the correctiveactions. In most cases in which aninstallation is not initially compliant,it is not necessary to perform all thecorrective actions.

Preparing for Deploymentover New Copper Cabling:Category 5eThe Gigabit Ethernet Alliance recom-mends that all new cable installationsdesigned for 1000BASE-T deploymentshould be specified as Category 5e(enhanced Category 5). Category 5ecabling is manufactured to meet all1000BASE-T transmission perfor-mance parameters. When field testingfor Category 5e cabling performanceis performed, 1000BASE-T–specificfield testing is not required.

The Category 5e specificationincludes transmission parameters thatare only informative recommendations

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 6

A Category 6 cabling standard, specified to 250 MHz, is under develop-ment in the ANSI/ TIA/EIA TR-42.7.1 Copper Cabling Systems WorkingGroup and in the International Standard Committee ISO/IEC/SC25/WG3.Network managers and cable system planners may want a cabling infra-structure that provides greater bandwidth or “headroom” to accommo-date future high-speed technologies.

A Category 7 cabling standard, specified to 600 MHz, is under develop-ment in the International Standard Committee ISO/IEC/ SC25. Category 7cable is constructed with individually shielded pairs with an additionalshield over the pairs. Category 7 cabling requires termination to ashielded connector. The Category 7 standard is still in the early stages ofdevelopment.

1000BASE-T will operate on the cabling specified in the current draft 5of ANSI/TIA/EIA-Category 6 and the current draft of the Category 6specifications proposed for the second edition of ISO/IEC 11801:1995,and Category 7 (estimated release, 2001).

OTHER CABL ING CATEGORIES

for Category 5. (These parameters arethe measures for return loss andELFEXT, described in Appendix B.)Cable manufacturers such as Mohawk/CDT (http: //www.mohawk-cdt.com)with its MegaLAN cable can be heldaccountable for the transmission spec-ifications of Category 5e. Category 5ealso provides a further enhanced mar-gin over the worst-case 1000BASE-Tlink requirements.

The 1000BASE-T standard specifiesoperation over the installed base ofCategory 5 cabling. 1000BASE-T willalso run over Category 6 and Category7 copper cabling systems (see box,“Other Cabling Categories,” on page 6).

Migrating Ethernet/FastEthernet Networks TowardHigh-Speed Networking 1000BASE-T allows a simple perfor-mance boost to support exploding

bandwidth requirements on today’snetworks. 1000BASE-T is best suitedfor unclogging network bottlenecksthat occur in three main areas:

• Workgroup aggregation

• Connections to high-speed servers

• Desktop connections

The following scenario describes atypical migration of an Ethernet/FastEthernet network to Gigabit Ethernet.As shown in Figure 5, the initialbuilding backbone is 10/100 MbpsEthernet/Fast Ethernet. Several Ether-net or Fast Ethernet segments areaggregated into a 10/100 Mbpsswitch, which in turn has several10/100 Mbps Ethernet/Fast Ethernetserver connections. Some users havededicated 10/100 switched connec-tions to their end stations. In thisconfiguration, users are starting to

7

Fast Ethernet switch

Ethernet users

Power users

(Fast Ethernet

users)

Ethernet

Fast Ethernet

F IGURE 5. Ethernet /Fast Ethernet Network BeforeMigrat ion to Gigabi t E thernet

experience slow response times andpower users are experiencing bottle-necks.

The first upgrade phase is imple-mented in three areas (Figure 6):

• Upgrading the backbone with a100/1000 Mbps Fast Ethernet/Giga-bit Ethernet switch

• Upgrading the workgroup switchesthat support power users or largeworkgroups with Gigabit Ethernetdownlink modules

• Implementing 100/1000 Mbps FastEthernet/Gigabit Ethernet NICs inkey servers

As a result of these measures, thespeed of the backbone increases ten-fold to accommodate the overallincrease in network bandwidthdemand while the investment in exist-

ing workgroup switches, end-stationNICs, and existing cabling is pre-served.

The second migration phase is theupgrading of power users to 100/1000Mbps Fast Ethernet/Gigabit EthernetNICs (Figure 7). Fast Ethernet and,over time, Gigabit Ethernet to thedesktop are now supported, givingpower users full access to theresources of the network.

Conclusion1000BASE-T, Gigabit Ethernet overCategory 5 copper cabling, helps net-work managers boost their networkperformance in a simple, cost-effec-tive way while enabling migration oftoday’s Ethernet/Fast Ethernet net-works toward high-speed network-ing. Following is a summary ofGigabit Ethernet characteristics:

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 8

Gigabit Ethernet switch

Gigabit Ethernet

Ethernet/Fast

Ethernet users

Power users

(Fast Ethernet

users)

Ethernet

Fast Ethernet

F IGURE 6. F i r s t Phase of Gigabi t E thernet Migrat ion

• 1000BASE-T is Ethernet, providingspeeds of 1000 Mbps.

• 1000BASE-T is designed to runover Category 5 copper cabling, themost widely installed LAN cablinginfrastructure.

• 1000BASE-T leverages the design ofproven, cost-effective existing FastEthernet and modem technologies.

• 1000BASE-T can be progressivelydeployed in a Fast Ethernet net-work since 100/1000 auto-negotia-tion will be supported in many1000BASE-T products.

9

Gigabit Ethernet switch

Gigabit Ethernet

Ethernet/Fast

Ethernet users

Power users

(Gigabit Ethernet

users)

10/100 Ethernet

F IGURE 7. Second Phase of Gigabi t E thernet Migrat ion

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 10

Gigabit Ethernet specifies technology covering the bottom two layers ofthe OSI model:

• The data link layer, which controls access to the physical medium oftransmission

• The physical layer, which controls the actual transmission over thephysical medium

Gigabit Ethernet implements data link layer functionality by supportingthe Ethernet Media Access Control (MAC) sublayer. The MAC sublayertransforms data sent by the upper layers of communication, into Ether-net frames and determines how data is scheduled, transmitted, andreceived. The Gigabit Ethernet MAC is the Ethernet/Fast Ethernet MAC,which ensures backward compatibility between Ethernet/Fast Ethernetand Gigabit Ethernet frames.

Frames are sent or received by the MAC layer through the Gigabit MediaIndependent Interface (GMII). Because the GMII is designed to enableGigabit Ethernet MAC devices to hook up in a standard way to any ofthe physical layers defined by the Gigabit Ethernet standards, the IEEE802.3ab committee was able to concentrate its effort on designing aphysical layer for Gigabit Ethernet over Category 5 copper. The IEEE802.3ab Task Force specified 1000BASE-T at the same time the IEEE802.3z Task Force designed and standardized the overall Gigabit Ethernetstandard and physical implementation over fiber and shielded copper.

The physical layer defines the electrical signaling, link states, clockingrequirements, data encoding, and circuitry needed for data transmissionand reception. There are several sublayers to perform these functionalities:

• The physical coding sublayer (PCS): Codes/decodes the data transmit-ted by the GMII to a form suitable for transmission over the physicalmedium.

• The physical medium attachment (PMA) sublayer: generates andreceives the signal to and from the wire.

• The physical medium dependent (PMD) sublayer: Provides physicalconnections to the wire.

Table 2 details the specifications for Gigabit Ethernet media, the associ-ated cabling specifications, and the minimum range certified by the IEEE.

APPENDIX A: GIGABIT ETHERNET STRUCTURE ANDSUPPORT FOR DIFFERENT MEDIA

11

TABLE 2. Gigabi t E thernet Spec i f i cat ions

MM = multimode

SM = single-mode

* 3Com certifies 1000BASE-LX Gigabit Interface Converters (GBICs) to distances of up to10,000 meters.

GIGABIT ETHERNET

TRANSCEIVERS

1000BASE-LX

1000BASE-SX

1000BASE-CX

1000BASE-T

FIBER TYPE

62.5 µm MM50 µm MM50 µm MM10 µm SM

62.5 µm MM62.5 µm MM50 µm MM50 µm MM

N/A

N/A

MODAL

BANDWIDTH

(MHZ*KM)

500400500N/A

160200400500

N/A

N/A

MINIMUM RANGE

SPECIFIED BY IEEE

(METERS)

2–5502–5502–5502–5000*

2–2202–2752–5002–550

25

100

Performance Parameters Specific to 1000BASE-T CablingThe additional cabling performance parameters of return loss and far-end crosstalk (FEXT) specified for 1000BASE-T and not specified for10BASE-T and 100BASE-TX are related to differences in the signalingimplementation. 10BASE-T and 100BASE-TX signaling is unidirectional—signals are transmitted in one direction on a single wire pair. In contrast,Gigabit Ethernet is bidirectional—signals are transmitted simultaneouslyin both directions on the same wire pair; that is, both the transmit andreceive pair occupy the same wire pair (Figure 8). 1000BASE-T uses bidi-rectional signaling on four wire pairs. Bidirectional data transmission ona single pair is enabled by devices called hybrids. The hybrid stops thelocal transmitted signals from being mixed with the local received signals.

F IGURE 8. Unid i rect iona l and B id i rect iona l Transmiss ion

APPENDIX B: KEY PERFORMANCE PARAMETERS FOR1000BASE-T CABL ING

T

Unidirectional transmission

R

TR

T

Bidirectional transmission

Hybrid Hybrid

R

TR

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 12

Bidirectional transmission on the same wire results in echo (Figure 9).Echo is the combined effect of the cabling return loss and the hybridfunction.

F IGURE 9. Echo in B id i rect iona l Transmiss ion Systems

Return loss is a measure of the reflected energy caused by impedancemismatches in the cabling system. Echo is countered by echo cancella-tion. Echo cancellation is proven in established phone technologies.

FEXT is the noise induced by a transmitter at the near-end into a far-endreceiver due to unwanted signal coupling (Figure 10). FEXT can be a fac-tor in multi-pair, bidirectional signaling such as 1000BASE-T. It is coun-tered by the use of cancellation.

Equal level far-end crosstalk (ELFEXT) is defined as the measure of theunwanted signal coupling from a transmitter at the near end into aneighboring pair measured at the far end relative to the received signallevel measured on that same pair (Figure 10).

F IGURE 10. Far-End Crossta lk (FEXT) and Equa l -Leve l Far-End Crossta lk (ELFEXT)

Performance Parameters for 10BASE-T, 100BASE-TX, and1000BASE-T CablingThe transmit signal is subject to impairments introduced by the cablingand external noise sources (Figure 11). In order for the receiver to oper-ate reliably, the impairments to the transmit signal need to be controlled.

Echo Echo Echo

T

Hybrid Hybrid

R

TR

FEXT

T

Hybrid Hybrid

R

T

R

T

Hybrid Hybrid

R

T

R

Vxtalk

Vin1

VxtalkFEXT =

Vin1

Vin2

Vout2

Vout2

VxtalkELFEXT =

13

The signal-to-noise ratio (SNR), the ratio between the impairments (typi-cally referred to as noise) and the transmit signal, is maintained in orderto achieve an acceptable bit error rate (BER).

F IGURE 11. Impai rments on the Transmit S igna l

The following key cabling performance parameters characterize the sig-nal impairments:

• Attenuation is a reduction in signal power due to cabling losses,expressed in decibels (Figure 12). Attenuation is a function of fre-quency and is proportional to the cable length; that is, at a given fre-quency, the attenuation of 50 meters is one-half of the attenuation of100 meters. The effect of attenuation is countered at the receiver bythe equalization of the signal, which compensates for the cablinglosses. Ethernet, Fast Ethernet, and Gigabit Ethernet signals are subjectto cabling attenuation.

F IGURE 12. Attenuat ion

• Near-end crosstalk (NEXT) is the noise induced by a transmitter to aneighboring receiver due to unwanted signal coupling (Figure 13).NEXT can impair both Ethernet/Fast Ethernet and Gigabit Ethernet. It iscountered by the use of NEXT cancellation. Gigabit Ethernet technol-ogy employs the use of NEXT cancellation, while Fast Ethernet doesnot. The addition of NEXT cancellation to 1000BASE-T is an enhance-ment to Ethernet/Fast Ethernet because it provides additional immu-nity to noise.

F IGURE 13. Near-End Crossta lk (NEXT)

Datasource T

Noise

R

T

Attenuated signal

R

T

NEXT NEXT

R

TR

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 14

• Delay is the time it takes for a signal to travel through a medium com-pared to the speed of light. Delay skew is the difference in delaybetween pairs (Figure 14). The delay skew specification ensures thatthe transmit signals divided across the four pairs can be reassembled inEthernet/Fast Ethernet and Gigabit Ethernet.

F IGURE 14. De lay and De lay Skew

Note on performance margins: It is important to realize that 1000BASE-Thas been designed to operate under worst-case conditions at the maxi-mum distance. That is, 1000BASE-T has been designed to work whenthe performance characteristics of each and all of the components in thelink (cable and connecting hardware) are worst case. It is highly unlikelyin a real network that the installed infrastructure would contain multipleor all worst-case components. Additionally, the attenuation of the cablescales by distance; that is, 50 meters of cable has half the attenuation of100 meters. Therefore, most installed cabling and the average installationwill enjoy a significant performance margin or “headroom.”

SummaryAll Ethernet twisted-pair techn ologies are subject to signal impairment.But in the case of 1000BASE-T, these disturbances are cancelled. Forexample, 1000BASE-T echo cancellation uses established, proven tech-nologies leveraged from telecommunications. In addition, 1000BASE-Tcrosstalk cancellation uses digital signal processing (DSP) technology thathas been used by many advanced modems and digital subscriber line(DSL) devices. Table 3 summarizes the signal impairments and correctiveactions implemented in 1000BASE-T.

TABLE 3. 1000BASE-T S igna l Impai rments and Correct iveAct ions

SIGNAL DISTURBANCE SIGNAL INTEGRITY RESTORATION

Attenuation Adaptive equalizers

NEXT NEXT cancellers

FEXT FEXT cancellers

Return Loss Echo cancellers

T T

T

Hybrid Hybrid

R

T

R R

RT

Hybrid Hybrid

R

T

RDelay

Delayskew

15

As described earlier, the 1000BASE-T physical layer is composed of vari-ous sublayers.

From the MAC layer, frames of 8 bits are transmitted to the physical cod-ing sublayer (PCS) through the Gigabit Media Independent Interface(GMII). To encode eight GMII bits, 26 = 256 codes are needed. A two-level signal used on each of the four pairs of transmission would enablethe coding of 24 = 16 data codes. Similarly a three-level signal wouldgive 34 = 81 codes. A five-level signal gives 54 = 625 potential codes andhas been chosen by the IEEE for implementation. The specific five-levelsignal used by 1000BASE-T is Pulse Amplitude Modulation 5 (PAM-5),which was already implemented in 100BASE-T2 (Fast Ethernet over twopairs of Category 3).

By comparison, 100BASE-T uses three-level signaling (MLT-3). Figure 15shows the eye patterns of 100BASE-T and 1000BASE-T signaling. Theeye pattern illustrated was produced by a modulated random-datawaveform, with each symbol period tracing from left to right and start-ing in the same place on the left. The figure shows that 1000BASE-T

F IGURE 15. Eye Pat tern of 100BASE-T Versus 1000BASE-TS igna l ing

APPENDIX C: 1000BASE-T PHYSICAL LAYERIMPLEMENTATION

–1 –2 –0 2

–0.8–1

–0.6–0.4–0.2

00.20.40.60.8

1

4 6 8 10 12

Eye pattern of MLT-3 signaling

0.5 volt

–1 –2 –0 2

–0.8–1

–0.6–0.4–0.2

00.20.40.60.8

1

4 6 8 10 12

Eye pattern of PAM-5 signaling

Time (in ns)

Time (in ns)

1.0 volt

1000BASE-T: GIGABIT ETHERNET OVER CATEGORY 5 COPPER CABL ING 16

provides closer consecutive levels of signals and hence a greater sensitiv-ity to transmission distortions— that is, it has a reduced signal-to-noisemargin compared to 100BASE-T.

But the reduced noise margin lost at the level of PAM-5 is recoveredthanks to the use of convolution coding. Convolution coding imple-mented by 1000BASE-T (called Trellis coding) allows error detection andcorrection by the receiver (through Viterbi decoding). These are estab-lished, proven technologies defined and used in modems for more than10 years. In comparison, 100BASE-TX uses block coding (4B5B coding,four bits coded by five symbols). Block coding uses simple codes that donot offer error detection or correction.

In fact, the use of Trellis coding and Viterbi decoding makes 1000BASE-Teven more resilient to external noise than 100BASE-T, since 1000BASE-Ttransmits uncorrelated symbols in the transmitted symbol stream; no cor-relation is allowed between symbol streams traveling in both directionson any pair combination, and no correlation is allowed between symbolstreams on each pair. External noise pickup is generally correlated (com-mon) to each pair. External noise can be cancelled statistically, providingimprovements in noise immunity that are not available in 100BASE-TX.

Summary1000BASE-T is designed to run reliably on the same Category 5 cablingas Fast Ethernet because it implements powerful digital signaling andcoding/decoding methods that maintain the integrity of the signal whentransmitted over Category 5 cabling. Figure 16 shows the architecture ofa 1000BASE-T transceiver, which implements the various technologiesdetailed earlier.

F IGURE 16. Arch i tecture of a 1000BASE-T Transce iver

Scramblingand Trellis

coding

GMII Tx[7:0]

GMII Rx[7:0]

Scrambler

Descrambler

Echocanceller

NEXT canceller

NEXT canceller

NEXT canceller

Tx filter

Tx filter

Tx filter

Tx filter

PAM-5symbol

mapping

Viterbidecoder

D/A

D/A

D/A

D/A

Hybrid

Hybrid

Hybrid

Hybrid

A/D

Equalizer

+

FEXT canceller

FEXT canceller

FEXT canceller

17

ANSI/TIA/EIA-TSB-95 (1998) defines corrective actions that can be takento improve return loss and ELFEXT performance. These corrections applyto the different elements of the Category 5 cabling systems as specifiedby ANSI/TIA/EIA-568-A (Figure 17).

The cross-connect is a facility (generally a type of patch panel) thatallows a connection between the horizontal cable and the equip-mentcable using a patch cord or jumper. Cross-connections are not generallyused for high-speed data communication cabling solutions. 1000BASE-Trecommends an interconnect cabling solution as illustrated in Figure 18.The interconnect predominates in data communication cabling. Theinterconnection (patch panel) provides for the direct connection of thehorizontal cable to the equipment cable.

The following five steps are to reduce the maximal configuration to theminimal configurations. They reference the elements in Figures 17 and 18.

1 Replace the patch cord with a cord constructed from a Category 5epatch cable, a patch cable designed to comply with the return loss andELFEXT parameters.

2 Reconfigure the cross-connect as an interconnect. 3 Replace the transition point or consolidation point connector with a

Category 5e transition point or consolidation point connector. 4 Replace the work area outlet connector with a Category 5e work area

outlet connector.5 Replace the interconnect with a Category 5e interconnect.

A retest for compliance is recommended after each option is implemented.In practice, some flexibility in the order of implementation of theseoptions is possible. For example, it may be more convenient to substitutea new patch cord as a first step while the tester is at that location.

F IGURE 17. Max imum: Category 5 Cabl ing Systems as perANSI /T IA/E IA-568-A

F IGURE 18. Category 5 Cabl ing Systems as perANSI /T IA/E IA-568-A with Trans i t ion Po int

APPENDIX D: CATEGORY 5 CABL ING F IELD MIT IGATIONPROCEDURES

Work area

Work area cable Equipment cable

OutletOutlet

Telecom closet

Channel

Cross-connect

Transition pointor consolidationpoint connector

Work area Transition pointor consolidationpoint connector

Work area cable Equipment cable

OutletOutlet

Telecom closet

Channel

Inter-connect

3Com Corporation, Corporate Headquarters, 5400 Bayfront Plaza, Santa Clara, CA 95052-8145

To learn more about 3Com solutions, visit www.3com.com. 3Com Corporation is publicly traded on Nasdaq underthe symbol COMS.

The information contained in this document represents the current view of 3Com Corporation on the issues discussed asof the date of publication. Because 3Com must respond to changing market conditions, this paper should not be inter-preted to be a commitment on the part of 3Com, and 3Com cannot guarantee the accuracy of any information presentedafter the date of publication. This document is for informational purposes only; 3Com makes no warranties, express orimplied, in this document.

Copyright © 2000 3Com Corporation. All rights reserved. 3Com is a registered trademark of 3Com Corporation. The3Com logo is a trademark of 3Com Corporation. MegaLAN is a trademark of Mohawk/Cable Design Technologies. Allother company and product names may be trademarks of their respective companies.

503047-002 1/01