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How to solve the
communication interface
bottleneck problem
David Young
Director, Datacom Products Division
Marvell Semiconductor, Inc.
Abstract
Gigabit Ethernet
One of the most critical components of many communicationrelated mixed signal ICs is the analog front-end. This paper
describes high precision analog front-ends that are in CMOS
manufacturing processes. These proprietary self-calibration
techniques compensate for the inherent variations of the processes. The proprietary techniques include customized digital
signal processing algorithms. The presentation will show how
to apply the DSP-based mixed signal processing technology to
the high-speed network and dual-port Gigabit Ethernet over
copper products with the option of a Serializer/ Deserializer
(SERDES) function. Additional products include hex/Octal Fast
Ethernet transceivers for network interface cards, routers, repeaters, hubs, and switches.
The Ethernet is high-speed, general-purpose, widely used network that make people share the information and connect to
Internet, it’s a lowest cost solution compared with Token-Ring
and ATM, specially in the LAN. In July 1996 the 802.3z task
force was formed to be responsible for the development of a
Gigabit Ethernet standard. In March 1997 a split was made resulting in the original IEEE802.3z task force and a IEEE802.3ab
task force. The IEEE802.3z standard resulted in a standard (June
1998) that deals with the MAC-layer specifications and the
physical layer specifications for fiber (1000BASE-SX and
1000BASE-LX) and a short copper cable (1000BASE-CX). The
IEEE802.3ab standard only handles the compatibility with the
already installed cabling (UTP CAT5) with cable runs of up to
100 meters (according to the EIA/TIA 586-A spec). The standard have been finished in June 1999 and is only a physical
layer specification called 1000BASE-T. So Gigabit Ethernet
can run over four media: Single-mode fiber, with connection
up to at least 5 kilometers, Multi-mode fiber, with connection
up to at least 550 meters, Balanced and shielded copper, with
connection up to at least 25 meters, CAT5 cabling, with connection up to at least 100 meters. The Gigabit Ethernet media
options and standards is shown in Figure 1.
In the standards, some enhancements had to make for the
CSMA/CD protocol to maintain a 200 meters collision domain
at gigabit rate. The carrier time and Ethernet slot time need to
be extended from their original 64 bytes to 512 bytes. Packets
larger than 512 bytes will not be extended, but packets shorter
than 512 bytes will use the extended time. To prevent the performance lost in networks with a large amount of small packets
a new concept is added called packet burst. This allows devices
to send bursts of small packets to fully utilize the bandwidth.
The Gigabit Media Independent Interface (GMII) connects the
MAC sub-layer to the PHY in the Gigabit Ethernet. It includes
an 8-bit data bus running at 125 MHz, also has clock signals,
carrier indicators and error conditions.
Introduction
With the number of network users increasing and the service
provided by network growing, which include varied multimedia information, such as the audio and video signal, so the requirement to the network bandwidth is rising rapidly. More and
more new subscriber has been utilized, include the ADSL and
Cable MODEM, these device allow high speed data over a single
twisted pair or a cable at the rate above 1Mbps. In other hand,
worldwide there are over 200 million personal computers connected over 10/100M Ethernet. But the bandwidth of network
backbone still limit the transfer speed that users could obtain.
So how to satisfy the demand of users, and ensure enough network bandwidth for the user’s data to be transfer without bottleneck have become a critical problem. 1000M Ethernet is a best
choice to solve the trouble in high speed data communication.
Overview 1000Base-T Ethernet
Figure 1.Gigabit Ethernet media options and standards
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1000BASE-T provides half-duplex (CSMA/CD) and full-duplex 1000Mbps Ethernet service over CAT5 links as defined
by ANSI/TIA/EIA-568-A. Topology rules for 1000BASE-T
are the same as those used for 100BASE-T. CAT5 link lengths
are limited to 100 meters by the ANSI/TIA/EIA-568-A cabling
standard. Only one CSMA/CD repeater will be allowed in a
collision domain. 1000BASE-T also uses the same Auto-Negotiation system employed by Fast Ethernet.
1000BASE-T is a good way to improve network performance by allowing the customer to deploy Gigabit Ethernet
over the existing network infrastructure. No costly hardware
upgrades, or expensive technical expertise is required. But
1000BASE-T can provides speed of 1000 Mb/s-10 times the
speed of Fast Ethernet- over CAT5 unshielded twisted pair
copper cabling, it’s the most affordable and widely installed
LAN cabling infrastructure.
The challenge 1000Base-T Ethernet
Transmitting The 1000 Mb/s data stream over four pairs of CAT5
twisted pair cables presents several design challenges to the
transmission due to signal attenuation, echo, return loss, and
crosstalk of cables, as well as electromagnetic emissions.
Attenuation is the signal loss of the cabling from the transmitter to the receiver. Attenuation increases with frequency, so
designers are challenged to use the lowest possible frequency
range that is consistent with the required data rate.
Echo is a by-product of dual-duplex operation, where both
the transmitting and receiving signal occupy the same wire pair.
The residual transmitting signal caused by the trans-hybrid loss
and the cabling return loss combine to produce an unwanted
signal referred to as echo.
Return loss is a measure of the amount of power reflected
due to cabling impedance mismatch.
Crosstalk is the unwanted signal coupled between near wire
pairs. Since 1000BASE-T will use all four wire pairs, each pair
is affected by crosstalk from the near three pairs. Crosstalk is
characterized in reference to the transmitter. Near-end crosstalk
(NEXT) is crosstalk that appears at the output of a wire pair at
the transmitter end of the cable and far-end crosstalk (FEXT) is
crosstalk that appears at the output of a wire pair at the far end
of the cable from the transmitter. Equal level far-end crosstalk
(ELFEXT) is FEXT with the cable attenuation removed to provide equal-level comparisons, i.e., crosstalk and receiving signals voltages are compared at the end of the cabling opposite
the transmitter.
Basic tehnology of 1000Base-T
The characteristics and the constraints of CAT5 cabling
described above create a lot of problems in implement of the
1000Mbps physical layer. But digital communications techniques developed in recent years can be used to design the new
transceivers. 1000BASE-T takes advantage of several of these
techniques to transform the desired bit rate into an acceptable
baud rate over 4-pair CAT5 cabling.
In a 1000Base-T system, Dual-duplex transmission consists
of transmitting and receiving data simultaneously in both directions on each of the four wire pairs, minimizing the symbol
rate on each wire pair by one half, as compared to unidirectional transmission and reception. Hybrid circuits are used to
enable bi-directional transmission over single wire pairs by
filtering out the transmit signal at the receiver.
Some special technology have been utilized in 1000BaseT, which include transmitting over existing 4-pair CAT5 cable
to keep symbol rate at or below 125 Mbaud, PAM-5 coding
to increase the amount of information sent with each symbol,
4D 8-state Trellis Forward Error Correction coding to offset
the impact of noise and crosstalk , pulse shaping techniques to
condition the transmitted spectrum, state-of-the-art DSP signal
equalization techniques to manage the problems of noise, echo
and crosstalk interferences, and to ensure a bit error rate.
Hybrid networks with good trans-hybrid loss minimize the
amount of transmitted signal that is coupled into the receiver,
but still cannot remove all of the transmitted signal. The residual transmitted signal from the hybrid and the return loss of
the cable require that canceller have to be added to each wire
pair to remove the transmitted echo signal.
5-level PAM provides better bandwidth utilization than binary signaling, where each transmitted symbol represents just
one bit (0 or 1.). In 5-level PAM, each transmitted symbol represents one of five different levels (-2, -1, 0, + 1, +2). Since
each symbol can represent two bits of information (four levels
used to present two bits, plus an extra fifth level used in the
Forward Error Correction coding), the symbol rate, and therefore also the signal bandwidth, are reduced by a factor of two.
The costs of multilevel signaling include the need for a higher
signal-to-noise ratio for a given error rate, the use of multibit D/A and A/D converters and the need for better receiver
equalization.
Figure2:Dual-duplex transmission uses hybrids
Forward Error Correction (FEC) provides a second level of
coding that helps to recover the transmitted symbols in the presence of high noise and crosstalk. The 4-Dimensional 8-State
Trellis Forward Error Correction encoding facilitates recovering the transmitted symbols in the presence of high noise and
crosstalk. This technique improves the signal-to-noise ratio at
the “slicer” the decision element in the Analog-to-Digital (A/
D) converter in the receiver.
Pulse shaping matches the spectral characteristics of the
transmitted signals to those of the channel in order to maximize the signal-to-noise ratio. This is achieved with a combination of analog and digital filtering elements used at the transmitter, at the receiver, or both. Pulse shaping is used to minimize the transmitted signal energy at frequencies where distortion and disturbances are significant, to reduce both low and
high frequency signal components, and to reject high-frequency
external noise components. Through pulse shaping the
1000BASE-T transmitted signal spectrum will be essentially
identical to the 100BASE-TX spectrum.
Signal equalization is used to compensate for signal distortion introduced by the communication channel. Linear digital
equalization is usually provided by a finite impulse response
(FIR) filter. But Non-linear equalization is usually provided by
a decision-feedback equalizer (DFE) and often provides better
signal equalization than linear equalization, especially when
the transmission medium introduces strong signal attenuation
within specific frequency regions. Unlike a linear equalizer,
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a DFE does not modify the noise. Therefore, using a DFE does
not enhance high-frequency noise. However, using a DFE to
equalize the channel is prone to error propagation, where a single
error causes mis-equalization and further errors. One solution
to this problem is to use multiple DFEs.
Scrambling is used to randomize the sequence of transmitted symbols and avoid the presence of spectral lines in the transmitted signal spectrum. Scrambling also creates essentially
uncorrelated data symbols, which improves operation of the
adaptive receiver functions.
Implement of 1000Base-T
1. Alaska Gigabit Ethernet-over-copper transceiver
To solve the 1000Mbps data transfer over the haul copper. A
new technology has been introduced, the Alaska Gigabit
Ethernet-over-copper transceiver, which is an advanced Gigabit physical layer (PHY) device with very low power dissipation. This transceiver is the world’s first Gigabit PHY fabricated using 0.18 micron CMOS process technology. It’s designed to enable enterprise switch/router system manufacturers for the first time to develop higher port density Gigabit
switches- 2 to 3 times more ports than is possible using existing Gigabit-over-copper transceivers.
This technique incorporates all three Ethernet speeds- 10,
100 and 1000Mbps into a single chip solution with standardbased auto-negotiation to assure compatibility with all existing
Ethernet networks, and employ IEEE 802.3 compliant Gigabit
Media Independent Interface (GMII) and Media Independent
Interface (MII), as well as the de facto standard 10-bit Interface
(TBI), allowing for a direct connection to existing MAC/
Switches. In this device, the Auto-MDI/MDIX function is incorporated at all three speeds. The MDI/MDIX feature offers
users the benefit of end-to-end wiring tolerance and correction
without the need for external crossover cable. According to standard of 1000BASE-T, transmitting and receiving data simultaneously on all four pairs of cable, the Alaska chip achieves 2
gigabits per second data throughput.
To implement digital adaptive equalization, echo cancellation, cross-talk cancellation, digital timing recovery, line driver
support, encoders, and decoders, Alaska Gigabit Ethernet transceiver uses state-of-the-art DSP architecture, advanced mixedsignal processing and high speed digital circuit technology. The
using of mixed-signal and DSP design techniques result in high
differential/integral linearity, high power supply noise rejection and low error rates, and the high precision analog-to-digital converters result in robust performance in noisy environments with the added feature of low power dissipation. With
robust power management techniques, the device achieves ultra low power dissipation of only 1.8 watts. hence, this allows
networking system suppliers to build high density port count
switches that do not require large heat sinks or cooling fans.
For A 24-port Gigabit switch card using this transceiver, the
power dissipation can be reduced about more than 100 watts,
significantly the new technique reduce cooling requirements of
the network room and thus increase system reliability.
2. Alaska Gigabit (PHY) transceiver with the SERDES
function
Alaska Gigabit (PHY) transceiver with the SERDES function
integrated on-chip, allowing for higher port count Gigabit
switches and enabling 1000BASE-T GBICs and single-chip
copper to fiber optic media conversion.
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A new technique, serializer/deserializer (SERDES) function, is integrated in the Gigabit (PHY) transceiver. As an alternative interface between the PHY and the Switch/MAC, the
SERDES interface reduces the I/O (Input/Output) pin count
from today’s implementations requiring as many as 24 pins per
port, to only 4 pins per port, and will allow manufacturers, for
the first time, to develop higher performance and higher port
count networking solutions. As we know, the GMII (Gigabit
Media Independent Interface) or the TBI (10-Bit Interface) defines the interface between the Gigabit PHY and the Gigabit
MAC. The GMII interface consists of 24 I/O pins. Sixteen pins
are used for data transfer, which consists of two 8-bit parallel
buses running at 125 MHz, for transmitting and receiving operations. The remaining GMII I/O pins include several control
I/O pins and data clocks.
The Alaska plus SERDES interface reduces the PHY/MAC
interface to only 4 pins, while maintaining all the functionality
of the GMII interface. This reduction in I/O provides significant benefits to manufacturers of high density Gigabit switches.
For example, a 24-port Gigabit switch using the Alaska plus
SERDES interface would reduce the interface I/O count by 480
pins (24 ports multiplied by 20 pins) on both the PHY and MAC
sides. Reducing I/O count translates to reduced system cost.
Additionally, the SERDES interface simplifies the layout of
the switch (fewer traces) and enhances the performance and
reliability of the system.
To improve the system performance, the SERDES interface of the Alaska device embeds the clock with the serial data.
Thus, the differential transmit signal, which runs at 1.25GHz,
has built-in clocking information. The receive portion of the
Alaska plus SERDES receives differential data at the 2-pin input and separates the 1.25GHz serial clock from the serial data.
By embedding the clock with the data, the Alaska plus SERDES
device eliminates PCB skew issues and simplifies the design.
Existing implementations use separate PCB traces for data and
clock potentially creating signal skew and timing problems.
In addition, the Alaska plus SERDES PHY create a new
function, Gigabit Ethernet- a 1000BASE-T Gigabit Interface
Converter (GBIC). The GBIC is a hot-swappable, plug-andplay, single-port module used in today’s 1000BASE-SX and
1000BASE-LX Gigabit over fiber applications. The Gigabit
Ethernet over copper GBIC requires three critical features of
the 1000BASE-T PHY-low power dissipation, small package
outline and a 4-pin 1.25 GHz SERDES. Alaska plus SERDES
device meets these requirements, enabling the availability of
1000BASE-T GBIC modules.
In other hand, Alaska plus SERDES PHY can implement
fiber optic Gigabit Ethernet media conversion. The Alaska plus
SERDES device is the first to provide a single-chip solution
for Gigabit Ethernet media conversion. The Gigabit Ethernet
media converter offers bi-directional conversion between Gigabit fiber and Gigabit copper networks. With the built-in 1.25
GHz SERDES, the device interfaces directly to standard Gigabit fiber-optic modules- 850nm wavelength optics for the
1000BASE-SX standard or 1300nm wavelength optics for the
1000BASE-LX standard. The Alaska plus SERDES PHY will
offer a significant cost reduction for the Gigabit media converter market, as the device implements the function in a single
chip, as opposed to the 2 to 3 chips currently required.
Conclusion
Existing network is difficult to satisfy the requirement of user
that need more and more high speed running over the network.
It is a trend to expand the bandwidth to 1000Gigabit for eliminating the bottleneck in communication interface. According
to the discussion above, Alaska Gigabit (PHY) transceiver with
the SERDES function can be regard as a good solution over the
barrier.
Author’s contact details
David Young
Marvell Semiconductor, Inc.
Phone: (1-408) 222 2500
Fax: (1-408) 328 0120
E-mail: dyoung@marvell.com
Web site: www.marvell.com
Presentation Materials
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