Gigabit Ethernet over Copper
By Sailesh K. Rao and Juan M. Jover
Here is a summary of the standards time table for the Gigabit Ethernet over copper standard, 1000BASE-T, the challenges inherent in transmitting 1,000
Mbps over four pairs of Category 5 unshielded twisted pair cabling (UTP-5), and the use of simple DSP techniques to create robust systems.
Some people think that it is impossible to transmit 1,000 Mbps in a LAN over standard UTP-5 copper cable over distances of 100 m.
Figure 1
shows to scale the magnitude of this challenge.
To determine whether Gigabit Ethernet over copper is possible, we need to examine the standards-making process, the technical challenges, the
proposed solutions, and whether these solutions are based on time-tested principles and products.
Gigabit Ethernet over copper is called in the standards process 1000BASE-T, and it is currently under development by the IEEE 802.3ab Task Force. This work represents the joint effort of more than eighty-five individuals (working for about forty-four different organizations). This joint effort means that the proposals are carefully scrutinized, analyzed, debated, and simulated to ensure they can be built and
can interoperate with existing Ethernet standards. Many of the individuals bring extensive experience to the process, as they are veterans of many other LAN standards, including the pioneering work on Ethernet. The consensus in the task force is that there is a solid draft for a standard, which can be implemented soon.
The challenge to increase the speed on copper cable by 10 may appear daunting.
Figure 1
shows that there is a relationship between cable and DSP. First,
the industry was able to multiply by 10 the speed of Ethernet by going from Category 3 cable to Category 5 (which yielded the 100BASE-Tx standard). Then, by using advanced DSP techniques, they were able to keep the same speed of 100 Mbps on Category 3 cable (this is the 100BASE-T2 standard). Now the industry is able to go to 1,000 Mbps by using DSP and going back to Category 5 cable. Note that 1000BASE-T uses four pairs of cable instead of the two pairs used by 100BASE-T2.
The rest of the article
explains the status of the standard, the challenges for 1000BASE-T, the solutions proposed to make it a reality, and how these solutions are based on time-tested principles.
Status of the standard
1000BASE-T is being standardized under the auspices of the IEEE. The requirements for the 1000BASE-T physical layer (PHY) are outlined in the sidebar on page 22. These requirements were used to drive the definition of the transmission and coding system upon which the current 1000BASE-T draft is based.
The task force has just completed the seventh draft (called Draft D3.0) of 1000BASE-T. This draft was presented to the plenary of the task force members in July for a vote as well as for request for a vote by the working group (IEEE 802.3). (At the time this article was written, the outcome of the vote was unknown, but was expected to be favorable.)
Figure 2
outlines the history of 1000BASE-T and the most likely future timeline. All the levels of approval for the standard
should be completed between March and June 1999, and you should see commercial products next year.
The challenges
There are eight main challenges to be considered for transmitting gigabit speeds over Category 5 UTP cable:
•
Attenuation
. The attenuation of a signal over UTP-5 cables for a 100 m link is considerable and has the following characteristics:
• It increases significantly with frequency.
• It increases with temperature (typically 0.3% per degree
above 20°C).
• It is affected by magnetics.
•
Channel return loss (echo)
. In addition to attenuation, there is echo, which is defined as the ratio of the reflected signal from the cable to the transmit signal. The echo is a consequence of 1000BASE-T transmitting in full duplex on each of the four pairs of cable and is also due to the imperfections inherent in the coupling of the cable and the magnetics.
The worst-case return loss (echo) was the subject of intense debate in the
cabling groups recently, since this parameter is of great importance for 1000BASE-T. The worst-case return loss is now specified in International Standards Organization (ISO)/ International Electrotechnical Commission (IEC) 11801 to be 15 dB for frequencies less than 20 MHz and (15-10log(f/20)) for frequencies greater than 20 MHz.
•
Near-end crosstalk (NEXT)
. NEXT arises from the coupling of transmit signals on adjacent wire pairs onto the received signal. NEXT energy in UTP-5 cabling
increases significantly with frequency. Worst-case NEXT is also specified in ISO/IEC 11801.
•
Far-end crosstalk (FEXT).
FEXT arises from the coupling of received signals on adjacent wire pairs onto the received signal. This problem is mostly due to the connectors and is stronger for short cables, where the effect of connectors is more important.
•
Differential delay
. A further challenge to the 1000BASE-T system designer arises from the need to align the received signals over the four
pairs of UTP-5 wires. This differential delay arises due to differences in the length of each pair (twist ratios), manufacturing variability of the insulation of the wire pairs, and, in some recent UTP-5 installations, differences in insulation materials used in the wire pairs.
•
Emitted electromagnetic radiation
. The amount of emissions that a transmitter can radiate above 30 MHz is regulated in the US by FCC Class A and B and in Europe by CISPR22 A and B. 1000BASE-T resolved this issue by
selecting the modulation rate and the transmitter filter characteristic so that the power spectral density of the transmitted signals remains below that for 100BASE-Tx.
•
Susceptibility to radiated energy
. A transmission system operating over unshielded cable must be capable of withstanding radiated energy from other sources, which include AM, CB, and short-wave radio, as well as continuous wave sources.
•
Impulse noise
. A further design objective is to have as much tolerance as
possible to impulse noise. This type of noise can be generated by analog ringers, power line transmissions, electrical fast transients, electrostatic discharge (ESD), and other sources.
Design approach
The challenges just listed were addressed in the 1000BASE-T draft standard by using a number of DSP techniques.
•
Full-duplex transmission and multilevel signaling.
Full-duplex transmission consists of transmitting and receiving signals simultaneously in both directions of the
four pairs of UTP-5 cable. In multilevel signaling, a transmitted symbol represents more than one information bit. Both techniques allow for the reduction of the signal bandwidth and improvement of the spectral efficiency.
•
Pulse shaping.
Pulse shaping is achieved with a combination of analog and digital filtering elements used at the transmitter, the receiver, or both. With this technique, the spectral characteristics of the transmitted signals can be matched to the channel to maximize the
signal-to-noise ratio (SNR). Also, the high-frequency and low-frequency signal components can be modified to reduce electromagnetic radiation, crosstalk, and interference from high-frequency external noise, and to mitigate interference from low-frequency disturbances.
•
Equalization
. Signal equalization is used at the receiver to compensate for signal distortion introduced by the communication channel.
•
Scrambling.
Scrambling is used to create uncorrelated data symbols, which is
required for proper operation of the adaptive receiver functions. In addition, scrambling avoids the presence of spectral lines in the transmit signal spectrum.
•
Forward-error correction (FEC) coding
. FEC is a well-known technique borrowed from voice-band modems and other communication systems to significantly improve the noise susceptibility of a system.
•
Echo/NEXT/FEXT cancelation.
The most significant impairment for 1000BASE-T is the presence of Echo, NEXT, and FEXT.
Fortunately, since the source of these impairments is known to the receiver (transmitted symbol sequence, received symbol sequence), it is possible to use simple DSP techniques to create robust systems.
Block diagram
The 1000BASE-T PHY employs full-duplex baseband transmission over four wire pairs. The aggregate data rate of 1,000 Mbps is achieved by transmission at a modulation rate of 125 Mbps over each wire pair in each direction simultaneously. A 1000BASE-T transceiver has four identical transmit
sections and four identical receive sections (
Figure 3
). Full-duplex operation is achieved by using hybrid elements to separate the two directions of transmission. A more detailed diagram is shown for transmitter A and for receiver A.
The transmit signal can be obtained by the cascade of the digital transmit filter, the DAC, and the analog transmit filter (
Figure 3
). The sole purpose of the digital transmit filters is to force the
transmit spectrum of 1000BASE-T to coincide with that of 100BASE-Tx for compatibility and emissions purposes.
Five-level pulse amplitude modulation (PAM-5) is employed for transmission over each wire pair. The modulation rate is 125 megabaud, which matches the Gigabit medium independent interface (GMII) clock rate of 125 MHz. The corresponding symbol period is 8 ns, nominal. This specification permits the use of Category 5 cable. The amplitude of the modulated pulse at the line input is 2V peak-to-peak,
which provides 500mV spacing between symbol levels. The symbols to be transmitted are selected from a four-dimensional (4D) code group of five level symbols. Each symbol is selected from the set {-2, -1, 0, +1, +2}. The 4D code group is a quartet of symbols. The 4D encoding and decoding functions are performed at either end of the receiver and transmitter sections.
The inherent redundancy in the 4D code groups allows the use of FEC coding of the transmitted symbols. This technique improves the
performance of the 1000BASE-T receiver by 6 dB.
As shown in Receiver-A of
Figure 3
, the signal at the input of the receiver is adjusted in amplitude by a variable gain amplifier (VGA), then filtered by an analog receive filter, and converted into digital form by an A/D converter (ADC). The digital signal is then processed through a linear feed-forward equalizer (FFE), which effectively cancels out any precursor intersymbol interference (ISI) in the received signal. To achieve
reliable communications in a full-duplex transmission system, at least four additional signals must be canceled. Since ideal separation of the transmit and receive paths at the hybrid cannot be achieved in practice, cancellation of the echo signal from TRANSMITTER_A to RECEIVER_A is required. Furthermore, since the signal at the input of RECEIVER_A will be corrupted by NEXT due to coupling of the signal transmitted by TRANSMITTER_B, TRANSMITTER_C and TRANSMITTER_D, cancellation of these self NEXT signals must
also be performed.
After echo and self NEXT cancellation, the signal is fed to a sequential decision device where 5-level symbol decisions are made. The estimated symbols are processed by decision feedback equalizers (DFE), potentially over multiple surviving paths in the sequential decision-making circuit (Viterbi decoder). Decision-feedback equalization provides better performance than linear equalization in the presence of additional disturbances such as narrowband interferers. In the sequential
decision device, it is also necessary to mutually whiten the incoming soft symbol values from the four receivers in order to mitigate the effect of FEXT in the system. Finally, quartets of estimated 5-level symbols are fed into the descrambler/decoder to generate data, idle, or control signals at the GMII.
Requirements for 1000BASE-T.
INTERFACE
- Compliant with 802.3z GMII
- Compliant with 802.3z MAC
- Compliant with auto-negotiation (802.3 clause 28)
- Compliant with 802.3z repeater configuration bit budget
PHY SIGNALING
- Provides 1000 Mbps data transfer rate at the GMII interface
- Allows implementation to meet FCC Class A and preferably Class B, and CISPR
022 A and B (also known as EN 55022 A and B) specifications for radiated
emissions
- Allows implementation to meet EN 55024 and EN 50082-1 specifications for noise
susceptibility.
- Operates on four
pairs ISO/IEC 11801 Category 5 balanced cabling over 100-m
link segments.
- Capable of simultaneous symmetrical transmit and receive at speed. The
standard must specify simultaneous transmit and receive operation, though
implementations may or may not provide it.
- Provide non-data code points.
- Provides spectral compatibility with 100BASE-Tx for ease of 100/1000
implementations, especially for shared magnetics.
- Meets 802 FRD, with the exception of Section 5.6.3, Hamming Distance
Requirement.
- Meets BER at the GMII Interface.
IMPLEMENTATION
- Able to be implemented with low cost and complexity, preferably in a single VLSI chip.
- Allow for flexibility in implementations.
- Implementable with on-chip analog filters.
- Able to operate with available analog and digital technologies.
- Enable robust dc recovery.
|
Synchronous transmission
1000BASE-T is based on synchronous transmission to facilitate the
cancelation of Echo/NEXT/FEXT interferences at the receivers. To achieve synchronous transmission between the two PHYs at the ends of a link, a master-slave clocking relationship is established by the PHYs. The master-slave relationship between two stations sharing a link segment is established during auto-negotiation. The master PHY uses an external clock to determine the timing of transmitter and receiver operations. This master clock is also provided to the other stations in the network. The slave PHY
recovers the clock from the received signal and uses it to determine the timing of transmitter operations. In a typical network, the PHY at the repeater will become the master and the PHY at the data terminal equipment (DTE) will become the slave.
The master-slave relationship essentially establishes a 1000BASE-T connection as a closed-loop system that spans the link segment. This is similar to the arrangement found in 100BASE-T2, ISDN, high-rate digital subscriber line (HDSL), and other applications, but
this is different from that found in 100BASE-Tx, 100BASE-T4, and 10BASE-T. In a master-slave arrangement, the slave PHY has to perform its receiver functions correctly in order for the master to establish proper operation of its receiver. In contrast, in 100BASE-Tx, for example, each PHY can independently establish proper receiver operation.
Making progress
Indeed, transmission at Gigabit speeds over UTP-5 is achievable. The draft of the IEEE standard is progressing, thanks to the commitment of
multiple companies. The proposed standard is based on techniques that have been simulated, analyzed, and debated by multiple individuals working for several vendors. Finally, Gigabit Ethernet over copper is a technology that will be available for volume deployment in 1999.
Sailesh Rao received his Ph.D. in electrical engineering from Stanford University for his work on Digital Signal Processing. He is the father of three standards: ATM 155Mbps over Cat-3 cable, 100Base-T2 Ethernet, and the new
1000Base-T. He was employed at Bell Laboratories before founding Silicon Design Experts, Inc., which he sold two years ago to Level One Communications, Inc. He can be reached at srao@level1.com.
Juan M. Jover is currently with Level One Communications. He received a Fulbright Fellowship in 1980, an MS in engineering management in 1984, and a PhD in electrical engineering in 1985 from Stanford University. He can be reached at jjover@level1.com.
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