1000BASE-T: Gigabit Ethernetover Category 5 Copper Cabling

T e c h n i c a l P a p e r

Technical Fundamentals

A DIVISION OF

CABLE DESIGN TECHNOLOGIES

11

1000BASE-T: Gigabit Ethernet over Category 5 Copper Cabling

Technical Fundamentals

Contents

Why 1000BASE-T: Gigabit Ethernet over Category 5 Copper Cabling? 2

Exploding Bandwidth Requirements 2

Significant Investment in Ethernet/Fast Ethernet Infrastructure 2

A Simple, Cost-Effective Performance Boost on Existing Category 5 Cabling 3

1000BASE-T Technical Fundamentals 3

Gigabit Ethernet Media Specifications 3

1000BASE-T Key Specifications 5

Detailed 1000BASE-T Cable Specifications 5

1000BASE-T Design 5

Preparing for Deployment over Existing Cabling 5

Testing the Installation of Existing Category 5 Cabling 5

Adjusting Existing Category 5 Cabling to Run 1000BASE-T 6

Preparing for Deployment over New Copper Cabling: Category 5e 7

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

Conclusion 8

Appendix A: Gigabit Ethernet Structure and Support for Different Media 10

Appendix B: Key Performance Parameters for 1000BASE-T Cabling 11

Appendix C: 1000BASE-T Physical Layer Implementation 13

Appendix D: Category 5 Cabling Field Mitigation Procedures 14

1000BASE-T: Gigabit Ethernetover Category 5 Copper Cabling

Technical Fundamentals

By Philippe Ginier-Gillet, 3Com Corporation,

and Christopher T. Di Minico, Cable Design

Technologies Corporation

How can network managers deploy bandwidth-intensive applications over their local area net-works (LANs) when they have tight budgets andmust leverage their existing infrastructure? Thelatest Ethernet technology, 1000BASE-T (Giga-bit Ethernet over Category 5 copper cabling),helps network managers boost their network per-formance in a simple, cost-effective way.

1000BASE-T is Ethernet that providesspeeds of 1000 Mbps over Category 5 coppercabling, the most widely installed LAN cablinginfrastructure. The IEEE Standards Committeeformally ratified 1000BASE-T as an Ethernetstandard in June 1999.

This paper is for network managers whowant a technical understanding of the funda-mentals of 1000BASE-T. It explains how1000BASE-T has been designed by the IEEE802.3ab Task Force to run over Category 5cabling and how to implement 1000BASE-Tover existing Category 5 cable infrastructure andover Enhanced Category 5 (Cat 5e) in new sites.

Why 1000BASE-T: Gigabit Ethernet over

Category 5 Copper Cabling?

1000BASE-T, Gigabit Ethernet over Category5 copper cabling, is an attractive option fornetwork managers for several reasons. Itaddresses the exploding bandwidth require-ments on current networks that are the resultof implementing new applications and theincreasing deployment of switching at theedges of the network. Gigabit Ethernet lever-ages the organization’s existing investment inEthernet and Fast Ethernet infrastructures,and it provides a simple, cost-effective perfor-mance boost while continuing to use thedominant horizontal/floor cabling medium.

Exploding Bandwidth Requirements

New, bandwidth-intensive applications arebeing deployed over Ethernet and Fast Ether-net networks. These applications include thefollowing:• Internet and intranet applications that cre-

ate any-to-any traffic patterns, with serversdistributed across the enterprise and usersaccessing Web sites inside and outside thecorporate network. These applications tendto make traffic patterns and bandwidthrequirements increasingly unpredictable.

• Data warehousing and backup applicationsthat handle gigabytes or terabytes of datadistributed among hundreds of servers andstorage systems.

• Bandwidth-intensive, latency-sensitivegroupware applications such as desktopvideo conferencing or interactive white-boarding.

• Publication, medical imaging, and scientificmodeling applications which produce mul-timedia and graphics files that are explodingin size from megabytes to gigabytes to tera-bytes.

Bandwidth pressures are compounded bythe growing deployment of switching as thedesktop connection of choice. Switching atthe edge tremendously increases the trafficthat must be aggregated at the workgroup,server, and backbone levels.

Significant Investment in Ethernet/Fast Ethernet

Infrastructure

Ethernet is the dominant, ubiquitous LANtechnology. More than 85 percent of allinstalled network connections were Ethernetat the end of 1997, representing more than118 million interconnected PCs, worksta-tions, and servers.1

The deployment of Ethernet/Fast Ether-net networks involves investment in networkinterface cards (NICs), hubs, and switches, aswell as in network management capabilities,staff training and skills, and cabling infrastruc-ture. In fact cabling infrastructure is thelongest-term networking investment, lasting at

22

Acronyms andAbbreviations

ANSI

American National

Standards Institute

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 Independent

Interface

IEEE

Institute of Electrical and

Electronics Engineers

IP

Internet Protocol

ISO

International Organization

for Standardization

LAN

local area network

MAC

Media Access Control

1 According to industry analyst International Data Corporation (IDC), Framingham, MA.

least two years and up to 10 years. (On aver-age, almost half of the infrastructure is inplace for more than five years.2 )

A Simple, Cost-Effective Performance Boost on

Existing Category 5 Cabling

1000BASE-T offers a simple, cost-effectivemigration of Ethernet/Fast Ethernet networkstoward high-speed networking, and has thefollowing benefits:• 1000BASE-T scales Ethernet 10/100 Mbps

performance to 1000 Mbps. Flexible100/1000 and 10/100/1000 connectivitywill be offered and will enable the smoothmigration of existing 10/100 networks to1000 Mbps–based networks.

• 1000BASE-T is the most cost-effectivehigh-speed networking technology availablenow. 1000BASE-T leverages existing, provenFast Ethernet and V.90/56K modem tech-nologies and will experience the same costcurve as the Ethernet/Fast Ethernet tech-nologies. 1000BASE-T is in fact expected tobe significantly more cost-efficient than1000BASE-SX (Fiber Gigabit), whichalready has the lowest cost-per-data-trans-mitted per second among all LAN technolo-gies (currently less than $1.5 per Mbps).

• 1000BASE-T preserves Ethernet equipmentand infrastructure investments, includingthe investment in the installed Category 5

cabling infrastructure. There is no need toundergo the time-consuming and high-costtask of replacing cabling located in walls,ceilings, or raised floors.

Leveraging Category 5 copper cablinginfrastructure is of significant importance fortwo reasons:• Category 5 is today the dominant horizon-

tal/floor cabling, providing connectivity toboth desktops and workgroup aggregators(Figure 1). Fiber is the dominant cabling forconnection of multiple buildings.

• Category 5 is one of the major options forbuilding risers/backbone cabling for con-nection of different floor wiring closets (Fig-ure 2 on page 4).

1000BASE-T Technical Fundamentals

Gigabit Ethernet Media Specifications

Gigabit Ethernet cost-effectively leveragesexisting cabling infrastructures. It can beimplemented in floor, building, and campusnetworks because it offers a wide range of con-nectivity media and connection distances.Specifically, Gigabit Ethernet is designed torun over four media:• Single-mode fiber, with connections up to

at least 5 kilometers • Multimode fiber, with connections up to at

least 550 meters

33

2 From the December 1998 Networking Cabling Market Study by Sage Research, Natick, MA.

Category 5

Thin coax

Category 3

Fiber (MM/SM)

Category 5e

Shielded twisted pair

0% 20% 40% 60% 80%

Notes: Figure shows the percentage of organizations that have installed the cable type. Multiple 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

Figure 1. Current Horizontal Network Cable Types

Acronyms andAbbreviations

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

• Balanced, shielded copper, with connectionsup to at least 25 meters

• Category 5 cabling, with connections up toat least 100 meters

The IEEE 802.3z Gigabit Ethernetstandard approved in June 1998 specifiedthree transceivers to cover three media:1000BASE-LX for the installed base of single-mode fiber, 1000BASE-SX for the installedbase of multimode fiber, and 1000BASE-CXfor a balanced, shielded copper cable thatcould be used for interconnects in equipmentrooms. 1000BASE-LX transceivers can also be

used to reach at least 550 meters on multi-mode fiber.

Another task force, IEEE 802.3ab, hasdefined the physical layer to run Gigabit Eth-ernet over the installed base of Category 5cabling. The IEEE Standards Committeeapproved the 1000BASE-T standard in June1999. Figure 3 summarizes the various Giga-bit Ethernet options and the standards thatdefine them.

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

44

Category 5

Multimode fiber

Thin coax

Single-mode fiber

Others

Shielded twisted pair

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

Notes: Figure shows the percentage of organizations that have installed the cable type. Multiple choices were allowed. Other

cabling types (Category 3, Category 5e, 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

Figure 2. Current Building Backbone Cable Types

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

Figure 3. Gigabit Ethernet Media Options and Standards

1000BASE-T Key Specifications

The 1000BASE-T standard leverages theexisting cable infrastructure as it is specified tooperate up to 100 meters on Category 5cabling.

The other key specifications of 1000BASE-Tmake it a cost-effective, nondisruptive, andhigh-performing technology. First, it supportsthe Ethernet MAC, and is thus backwardcompatible with 10/100 Mbps Ethernet. Sec-ond, many 1000BASE-T products will support100/1000 auto-negotiation, and 1000BASE-Tcan thus be incrementally deployed in a FastEthernet network. Third, 1000BASE-T is ahigh-performing technology with less thanone erroneous bit in 10 billion transmittedbits (this bit error rate of less than 10–10 is thesame error rate as that of 100BASE-T).

Detailed 1000BASE-T Cable Specifications

1000BASE-T is specified to run over four pairsof Category 5 balanced cabling. The four pairsof Category 5 balanced cabling are specified inANSI/EIA/TIA-568-A (1995). Additionallink performance parameters (return loss andELFEXT) are specified in TIA/EIA-TSB-95.Figure 4 details the standards of reference forthe specification of 1000BASE-T cable perfor-mance parameters. For additional informa-tion, see Appendix B. Category 5 cabling isalso specified in ISO/IEC 11801:1995(“Information Technology: Generic Cablingfor Customer Premises”). The second editionof ISO/IEC 11801:1995 will include theadditional cabling performance parametersspecified to support Gigabit Ethernet.

1000BASE-T Design

1000BASE-T is designed to run over Cate-gory 5 copper cabling. The transmission of 1Gbps is possible thanks to the use of fourtwisted-pair links with 250 Mbps of through-put on each pair (250 Mbps x 4 = 1 Gbps).

1000BASE-T transmits at the same clockrate as 100BASE-T (125 MHz) but uses apowerful signaling and coding/decoding schemethat enables the transmission of double theamount of data as 100BASE-T. Following is acomparison of the two specifications:• 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 is equivalent to100 Mbps, since 100BASE-T uses a 4B/5Bcode—4 bits of data are translated into 5bit-symbols before transmission on the wire; the effective bits throughput is thus125 x 4 / 5 = 100 Mbps.

1000BASE-T cost-effectively leveragesthe design of proven existing Fast Ethernetand V.90/56K modem technologies. Sig-naling and coding/decoding methods alreadyimplemented in 802.3 Fast Ethernet trans-ceivers and in V.90 or 56K modems usingadvanced DSPs are used to implement1000BASE-T. Table 1 on page 6 summarizesthe 100BASE-T technologies and methodsreused by 1000BASE-T. For additional infor-mation, see Appendix C.

Preparing for Deployment over

Existing Cabling

Preparing existing Category 5 copper cablingfor running 1000BASE-T is a straightforwardprocess. The first step is a simple test of theadequacy of the cable installation. In theunlikely event that an existing installationdoes not meet one of the performance para-meters specified by 1000BASE-T, standardcorrective actions can be implemented.

Testing the Installation of Existing

Category 5 Cabling

• Cable testing information is specified in the ANSI/TIA/EIA-TSB-67 standard,

55

TIA/EIA-568-A

NEXTAttenuation

TIA/EIA-TSB-95

TIA/EIA-568-A-1

ELFEXTReturn lossField testing

1000BASE-T

cable specification

NEXTAttenuation

DelayDelay skew

ELFEXTReturn loss

DelayDelay skew

Figure 4. Standards of Reference for 1000BASE-T

Performance Parameters

“Transmission Performance Specificationsfor Field Testing of Twisted-Pair CablingSystem,” which has been used by cablinginstallers since 1995.

• The additional test parameters of return lossand ELFEXT for 1000BASE-T are specifiedin the ANSI/TIA/EIA-TSB-95 Bulletin,“The Additional Transmission PerformanceGuidelines for 100 Ohm 4-Pair Category 5Cabling.”

• These additional tests are incorporated intothe current versions of cable test tools (for alist of them, refer to the Gigabit EthernetAlliance Web site, http://www.gigabit-ethernet.org/technology/whitepapers/gige_0399/copper.html#installed). Field testing isperformed by connecting the two handhelddevices—one at each end of the cablingunder test (see Figure 18 in Appendix D)—with a field test cord, and then activating

the 1000BASE-T test function. A pass orfail will be indicated for the 1000BASE-Ttest and for the specific test parametersunder test. Many field testers include diag-nostic functions to help identify the cause offailures. Corrective actions are provided inAppendix D.

Adjusting Existing Category 5 Cabling to

Run 1000BASE-T

In the unlikely event that an existing Cate-gory 5 installation does not meet one of the performance parameters specified by1000BASE-T, corrective actions are defined ina simple field procedure detailed in theANSI/TIA/EIA-TSB-95. Three types of cor-rective measures can be applied:• Use of high-performance Category 5e patch

cables (see following section for definitionof Category 5e)

66

Table 1. 100BASE-T Technologies Used in 1000BASE-T

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

A Category 6 cabling standard, specified to250 MHz, is under development in the ANSI/TIA/EIA TR-42.7.1 Copper Cabling SystemsWorking Group and in the InternationalStandard Committee ISO/IEC/SC25/WG3.Network managers and cable system plannersmay want a cabling infrastructure thatprovides greater bandwidth or “headroom” toaccommodate future high-speed technologies.

A Category 7 cabling standard, specified to600 MHz, is under development in theInternational Standard Committee ISO/IEC/SC25. Category 7 cable is constructed with

individually shielded pairs with an additionalshield over the pairs. Category 7 cablingrequires termination to a shielded connector.The Category 7 standard is still in the earlystages of development.

1000BASE-T will operate on the cabling specifiedin the current draft 5 of ANSI/TIA/EIA-Category 6(estimated release, late 2000) and the currentdraft of the Category 6 specifications proposedfor the second edition of ISO/IEC 11801:1995(estimated release, late 1999) and Category 7(estimated release, late 2000).

Other Cabling Categories

• Reduction in the number of connectorsused in the link

• Reconnection of some connectors in the linkAppendix D describes the corrective

actions. In most cases in which an installationis not initially compliant, it is not necessary toperform all the corrective actions.

Preparing for Deployment over New Copper

Cabling: Category 5e

The Gigabit Ethernet Alliance recommendsthat all new cable installations designed for1000BASE-T deployment should be specifiedas Category 5e (enhanced Category 5). Cate-gory 5e cabling is manufactured to meet all1000BASE-T transmission performance parame-ters. When field testing for Category 5e cablingperformance is performed, 1000BASE-T–spe-cific field testing is not required.

The Category 5e specification includestransmission parameters that are only informa-tive recommendations for Category 5. (Theseparameters are the measures for return loss andELFEXT, described in Appendix B.) Cablemanufacturers such as Mohawk/CDT (http:

//www.mohawk-cdt.com) with its MegaLAN

cable can be held accountable for the transmis-sion specifications of Category 5e. Category 5ealso provides a further enhanced margin overthe worst-case 1000BASE-T link requirements.

The 1000BASE-T standard specifiesoperation over the installed base of Category 5cabling. 1000BASE-T will also run overCategory 6 and Category 7 copper cablingsystems (see box, “Other Cabling Categories,”on page 6).

Migrating Ethernet/Fast Ethernet Networks

Toward High-Speed Networking

1000BASE-T allows a simple performanceboost to support exploding bandwidth require-ments on today’s networks. 1000BASE-T is bestsuited for unclogging network bottlenecksthat occur in three main areas: • Workgroup aggregation• Connections to high-speed servers• Desktop connections

The following scenario describes a typicalmigration of an Ethernet/Fast Ethernet net-work to Gigabit Ethernet. As shown in Figure 5,

77

Fast Ethernet switch

Ethernet users

Power users

(Fast Ethernet

users)

Ethernet

Fast Ethernet

Figure 5. Ethernet/Fast Ethernet Network Before Migration to Gigabit Ethernet

the initial building backbone is 10/100 MbpsEthernet/Fast Ethernet. Several Ethernet orFast Ethernet segments are aggregated into a10/100 Mbps switch, which in turn has sev-eral 10/100 Mbps Ethernet/Fast Ethernetserver connections. Some users have dedicated10/100 switched connections to their end sta-tions. In this configuration, users are startingto experience slow response times and powerusers are experiencing bottlenecks.

The first upgrade phase is implementedin three areas (Figure 6):• Upgrading the backbone with a 100/1000

Mbps Fast Ethernet/Gigabit Ethernet switch• Upgrading the workgroup switches that

support power users or large workgroupswith Gigabit Ethernet downlink modules

• Implementing 100/1000 Mbps Fast Ether-net/Gigabit Ethernet NICs in key servers

As a result of these measures, the speed ofthe backbone increases tenfold to accommodatethe overall increase in network bandwidth

demand while the investment in existingworkgroup switches, end-station NICs, andexisting cabling is preserved.

The second migration phase is theupgrading of power users to 100/1000 MbpsFast Ethernet/Gigabit Ethernet NICs (Figure 7).Fast Ethernet and, over time, Gigabit Ethernetto the desktop are now supported, givingpower users full access to the resources of thenetwork.

Conclusion

1000BASE-T, Gigabit Ethernet over Category5 copper cabling, helps network managersboost their network performance in a simple,cost-effective way while enabling migration oftoday’s Ethernet/Fast Ethernet networkstoward high-speed networking. Following is asummary of Gigabit Ethernet characteristics:• 1000BASE-T is Ethernet, providing speeds

of 1000 Mbps.

Gigabit Ethernet switch

Gigabit Ethernet

Ethernet/Fast

Ethernet users

Power users

(Fast Ethernet

users)

Ethernet

Fast Ethernet

Figure 6. First Phase of Gigabit Ethernet Migration

88

• 1000BASE-T is designed to run over Cate-gory 5 copper cabling, the most widelyinstalled LAN cabling infrastructure.

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

• 1000BASE-T can be progressively deployedin a Fast Ethernet network since 100/1000auto-negotiation will be supported in many1000BASE-T products.

Gigabit Ethernet switch

Gigabit Ethernet

Ethernet/Fast

Ethernet users

Power users

(Gigabit Ethernet

users)

10/100 Ethernet

Figure 7. Second Phase of Gigabit Ethernet Migration

99

Gigabit Ethernet specifies technology coveringthe bottom two layers of the OSI model:• The data link layer, which controls access to

the physical medium of transmission • The physical layer, which controls the actual

transmission over the physical mediumGigabit Ethernet implements data link

layer functionality by supporting the EthernetMedia Access Control (MAC) sublayer. TheMAC sublayer transforms data sent by theupper layers of communication, into Ethernetframes and determines how data is scheduled,transmitted, and received. The Gigabit Ether-net MAC is the Ethernet/Fast Ethernet MAC ,which ensures backward compatibilitybetween Ethernet/Fast Ethernet and GigabitEthernet frames.

Frames are sent or received by the MAClayer through the Gigabit Media IndependentInterface (GMII). Because the GMII isdesigned to enable Gigabit Ethernet MACdevices to hook up in a standard way to any ofthe physical layers defined by the Gigabit Eth-ernet standards, the IEEE 802.3ab committeewas able to concentrate its effort on designinga physical layer for Gigabit Ethernet over Cat-

egory 5 copper. The IEEE 802.3ab Task Forcespecified 1000BASE-T at the same time theIEEE 802.3z Task Force designed and stan-dardized the overall Gigabit Ethernet standardand physical implementation over fiber andshielded copper.

The physical layer defines the electricalsignaling, link states, clocking requirements,data encoding, and circuitry needed for datatransmission and reception. There are severalsublayers to perform these functionalities:• The physical coding sublayer (PCS):

Codes/decodes the data transmitted by theGMII to a form suitable for transmissionover the physical medium.

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

• The physical medium dependent (PMD)sublayer: Provides physical connections tothe wire.

Table 2 details the specifications for Giga-bit Ethernet media, the associated cablingspecifications, and the minimum range certi-fied by the IEEE.

1100

Table 2. Gigabit Ethernet Specifications

MM = multimode

SM = single-mode

* 3Com certifies 1000BASE-LX Gigabit Interface Converters (GBICs) to distances of up to 10,000 meters.

Appendix A:Gigabit Ethernet Structure and Support for Different Media

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

1111

Appendix B:Key Performance Parameters for 1000BASE-T Cabling

Performance Parameters Specific to 1000BASE-T

Cabling

The additional cabling performance parametersof return loss and far-end crosstalk (FEXT)specified for 1000BASE-T and not specifiedfor 10BASE-T and 100BASE-TX are relatedto differences in the signaling implementation.10BASE-T and 100BASE-TX signaling isunidirectional—signals are transmitted in onedirection on a single wire pair. In contrast,Gigabit Ethernet is bidirectional—signals aretransmitted simultaneously in both directionson the same wire pair; that is, both the transmitand receive pair occupy the same wire pair(Figure 8). 1000BASE-T uses bidirectionalsignaling on four wire pairs. Bidirectional datatransmission on a single pair is enabled bydevices called hybrids. The hybrid stops thelocal transmitted signals from being mixedwith the local received signals.

T

Unidirectional transmission

R

TR

T

Bidirectional transmission

Hybrid Hybrid

R

TR

Figure 8. Unidirectional and Bidirectional

Transmission

Bidirectional transmission on the samewire results in echo (Figure 9). Echo is thecombined effect of the cabling return loss andthe hybrid function.

Echo Echo Echo

T

Hybrid Hybrid

R

TR

Figure 9. Echo in Bidirectional Transmission

Systems

Return loss is a measure of the reflectedenergy caused by impedance mismatches inthe cabling system. Echo is countered by echocancellation. Echo cancellation is proven inestablished phone technologies.

FEXT is the noise induced by a transmit-ter at the near-end into a far-end receiver dueto unwanted signal coupling (Figure 10).FEXT can be a factor in multi-pair, bidirec-tional signaling such as 1000BASE-T. It iscountered by the use of cancellation.

Equal level far-end crosstalk (ELFEXT) isdefined as the measure of the unwanted signalcoupling from a transmitter at the near endinto a neighboring pair measured at the farend relative to the received signal level mea-sured on that same pair (Figure 10).

FEXT

T

Hybrid Hybrid

R

T

R

T

Hybrid Hybrid

R

T

R

Vxtalk

Vin1

VxtalkFEXT =

Vin1

Vin2

Vout2

Vout2

VxtalkELFEXT =

Figure 10. Far-End Crosstalk (FEXT) and Equal-

Level Far-End Crosstalk (ELFEXT)

Performance Parameters for 10BASE-T,

100BASE-TX, and 1000BASE-T Cabling

The transmit signal is subject to impairmentsintroduced by the cabling and external noisesources (Figure 11 on page 12). In order forthe receiver to operate reliably, the impair-ments to the transmit signal need to be con-trolled. The signal-to-noise ratio (SNR), theratio between the impairments (typicallyreferred to as noise) and the transmit signal, is

cellation to 1000BASE-T is an enhancementto Ethernet/Fast Ethernet because it pro-vides additional immunity to noise.

• Delay is the time it takes for a signal totravel through a medium compared to thespeed of light. Delay skew is the difference indelay between pairs (Figure 14). The delayskew specification ensures that the transmitsignals divided across the four pairs can bereassembled in Ethernet/Fast Ethernet andGigabit Ethernet.

1122

T

Attenuated signal

R

Figure 12. Attenuation

Datasource T

Noise

R

Figure 11. Impairments on the Transmit Signal

• Near-end crosstalk (NEXT) is the noiseinduced by a transmitter to a neighboringreceiver due to unwanted signal coupling(Figure 13). NEXT can impair both Ether-net/Fast Ethernet and Gigabit Ethernet. It iscountered by the use of NEXT cancellation.Gigabit Ethernet technology employs theuse of NEXT cancellation, while Fast Ether-net does not. The addition of NEXT can-

T T

T

Hybrid Hybrid

R

T

R R

RT

Hybrid Hybrid

R

T

RDelay

Delayskew

Figure 14. Delay and Delay Skew

T

NEXT NEXT

R

TR

Figure 13. Near-End Crosstalk (NEXT)

Note on performance margins: It is importantto realize that 1000BASE-T has been designedto operate under worst-case conditions at themaximum distance. That is, 1000BASE-T hasbeen designed to work when the performancecharacteristics of each and all of the compo-nents in the link (cable and connecting hard-ware) are worst case. It is highly unlikely in areal network that the installed infrastructurewould contain multiple or all worst-case com-ponents. Additionally, the attenuation of thecable scales by distance; that is, 50 meters ofcable has half the attenuation of 100 meters.Therefore, most installed cabling and the aver-age installation will enjoy a significant perfor-mance margin or “headroom.”

Summary

All Ethernet twisted-pair technologies are sub-ject to signal impairment. But in the case of1000BASE-T, these disturbances are cancelled.For example, 1000BASE-T echo cancellationuses established, proven technologies leveraged

maintained in order to achieve an acceptablebit error rate (BER).

The following key cabling performanceparameters characterize the signal 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 cablelength; that is, at a given frequency, theattenuation of 50 meters is one-half of theattenuation of 100 meters. The effect ofattenuation is countered at the receiver bythe equalization of the signal, which com-pensates for the cabling losses. Ethernet,Fast Ethernet, and Gigabit Ethernet signalsare subject to cabling attenuation.

1133

Table 3. 1000BASE-T Signal Impairments and Corrective Actions

Signal Disturbance Signal Integrity Restoration

Attenuation Adaptive equalizers

NEXT NEXT cancellers

FEXT FEXT cancellers

Return Loss Echo cancellers

Appendix C:1000BASE-T Physical Layer Implementation

As described earlier, the 1000BASE-T physicallayer is composed of various sublayers.

From the MAC layer, frames of 8 bits aretransmitted to the physical coding sublayer(PCS) through the Gigabit Media Indepen-dent Interface (GMII). To encode eight GMIIbits, 26 = 256 codes are needed. A two-levelsignal used on each of the four pairs of trans-mission would enable the coding of 24 = 16data codes. Similarly a three-level signal wouldgive 34 = 81 codes. A five-level signal gives 54

= 625 potential codes and has been chosen bythe IEEE for implementation. The specific

five-level signal used by 1000BASE-T is PulseAmplitude Modulation 5 (PAM-5), which wasalready implemented in 100BASE-T2 (FastEthernet over two pairs of Category 3).

By comparison, 100BASE-T uses three-level signaling (MLT-3). Figure 15 shows theeye patterns of 100BASE-T and 1000BASE-Tsignaling. The eye pattern illustrated was pro-duced by a modulated random-data wave-form, with each symbol period tracing fromleft to right and starting in the same place onthe left. The figure shows that 1000BASE-Tprovides closer consecutive levels of signals

–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

Figure 15. Eye Pattern of 100BASE-T Versus 1000BASE-T Signaling

from telecommunications. In addition,1000BASE-T crosstalk cancellation uses digitalsignal processing (DSP) technology that hasbeen used by many advanced modems and

digital subscriber line (DSL) devices. Table 3summarizes the signal impairments and cor-rective actions implemented in 1000BASE-T.

and hence a greater sensitivity to transmissiondistortions— that is, it has a reduced signal-to-noise margin compared to 100BASE-T.

But the reduced noise margin lost at thelevel of PAM-5 is recovered thanks to the useof convolution coding. Convolution codingimplemented by 1000BASE-T (called Trelliscoding) allows error detection and correctionby the receiver (through Viterbi decoding).These are established, proven technologiesdefined and used in modems for more than 10years. In comparison, 100BASE-TX uses blockcoding (4B5B coding, four bits coded by fivesymbols). Block coding uses simple codes thatdo not offer error detection or correction.

In fact, the use of Trellis coding andViterbi decoding makes 1000BASE-T evenmore resilient to external noise than100BASE-T, since 1000BASE-T transmitsuncorrelated symbols in the transmitted sym-

bol stream; no correlation is allowed betweensymbol streams traveling in both directions onany pair combination, and no correlation isallowed between symbol streams on each pair.External noise pickup is generally correlated(common) to each pair. External noise can becancelled statistically, providing improvementsin noise immunity that are not available in100BASE-TX.

Summary

1000BASE-T is designed to run reliably onthe same Category 5 cabling as Fast Ethernetbecause it implements powerful digital signalingand coding/decoding methods that maintainthe integrity of the signal when transmittedover Category 5 cabling. Figure 16 shows thearchitecture of a 1000BASE-T transceiver,which implements the various technologiesdetailed earlier.

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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

Figure 16. Architecture of a 1000BASE-T Transceiver

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Appendix D:Category 5 Cabling Field Mitigation Procedures

ANSI/TIA/EIA-TSB-95 (1998) defines cor-rective actions that can be taken to improvereturn loss and ELFEXT performance. Thesecorrections apply to the different elements ofthe Category 5 cabling systems as specified byANSI/TIA/EIA-568-A (Figure 17).

The cross-connect is a facility (generally atype of patch panel) that allows a connectionbetween the horizontal cable and the equip-ment cable using a patch cord or jumper.Cross-connections are not generally used forhigh-speed data communication cabling solu-tions. 1000BASE-T recommends an intercon-nect cabling solution as illustrated in Figure18. The interconnect predominates in datacommunication cabling. The interconnection(patch panel) provides for the direct connec-tion of the horizontal cable to the equipmentcable.

The following five steps are to reduce themaximal configuration to the minimal config-urations. They reference the elements in Fig-ures 17 and 18.

1 Replace the patch cord with a cord con-structed from a Category 5e patch cable, apatch cable designed to comply with thereturn loss and ELFEXT parameters.

2 Reconfigure the cross-connect as an inter-connect.

3 Replace the transition point or consolidationpoint connector with a Category 5e transi-tion point or consolidation point connector.

4 Replace the work area outlet connector witha Category 5e work area outlet connector.

5 Replace the interconnect with a Category 5einterconnect.

A retest for compliance is recommendedafter each option is implemented. In practice,some flexibility in the order of implementa-tion of these options is possible. For example,it may be more convenient to substitute a newpatch cord as a first step while the tester is atthat location.

Work area

Work area cable Equipment cable

OutletOutlet

Telecom closet

Channel

Cross-connect

Transition point

or consolidation

point connector

Work area Transition point

or consolidation

point connector

Work area cable Equipment cable

OutletOutlet

Telecom closet

Channel

Inter-connect

Figure 17. Maximum: Category 5 Cabling Systems as per ANSI/TIA/EIA-568-A

Figure 18. Category 5 Cabling Systems as per ANSI/TIA/EIA-568-A with Transition Point

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© 1999 3Com Corporation. All rights reserved. 3Com and the 3Com logo are registered trademarks and More connected. is a trademark of 3Com Corporation. MegaLAN is a trademark of Mohawk/CableDesign Technologies. Other product and brand names may be trademarks or registered trademarks of their respective owners. All specifications are subject to change without notice.

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About 3Com Corporation

With more than 200 million customers worldwide, 3Com Corporation connects more people inmore ways to information than any other networking company. 3Com delivers innovative infor-mation access products and network system solutions to large, medium, and small enterprises;carriers and network service providers; PC OEMs; and consumers. 3Com. More connected.™

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About Mohawk/CDT

As a worldwide leader in innovative connectivity technology, Cable Design Technologies (CDT)is a recognized resource in the design, development, and manufacturing of both fiber optic andcopper cables, structured wiring systems, and cable management solutions. Mohawk/CDT, adivision of CDT, is the industry leader in the manufacture of high-performance network cablingsolutions. To learn more about CDT and Mohawk/CDT, visit our Web sites at www.cdtc.comand www.mohawk-cdt.com, respectively.

Referred to as Mohawk Accredited Contractors, Mohawk/CDT has a network of certified con-tractors accredited through a formal training process including the design, installation, and fieldtest verification of infrastructure cabling. In cooperation with 3Com, Mohawk/CDT offers aservice to verify installed Category 5 cabling for 1000BASE-T operation. For more information,e-mail MAC@mohawk-cdt.com.

Mohawk/CDT

A Division of Cable DesignTechnologies9 Mohawk DriveLeominster, MA 01453Phone: 1 800 422 9961Fax: 1 978 537 4358World Wide Web:

www.mohawk-cdt.com