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Stressing 1 GbE Receivers on the
Physical Layer
Application Note
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Introduction
1000Base-T Ethernet, and the technical challenges
it creates for design and test
The standard for the 1000Base-T Ethernet (IEEE 802.3ab) describes data rates
of 1 Gb/s over a star topology, over up to 100 meters of Category 5 (or better),
unshielded, balanced, twisted-pair, copper cable.
To make the 1000BASE-T physical layer standard as compatible as possible
with existing standards, it reuses aspects of the existing IEEE 100BASE-TX,
100BASE-T4, and 100BASE-T2 standards. In particular, the 125 Mbaud baseband
signaling from 100BASE-TX, the use of all four twisted pairs simultaneously for
full duplex transmission from 100BASE-T4, and the 5-level pulse amplitude
modulation (PAM5) from 100BASE-T2. This enables dual-speed 100/1000 Ethernet
physical layer implementations to reuse existing infrastructures. Cherry-picking
elements from existing standards also helped reduce the time taken to develop
the newer standard.
Baseband signaling at a modulation rate of 125 Mbaud over each twisted pair
matches the GMII clock rate of 125 MHz, as used for 100BASE-TX, and results
in a symbol period of 8 ns. Each twisted pair also uses ive-level (-2, -1, 0, +1,
+2), Pulse Amplitude Modulation (PAM5) to transmit 2-bits per symbol. The
resulting data rate of 250 Mb/s in one direction over each twisted pair achieves
a combined data rate of 1000 Mb/s. Furthermore, by using canceller symbols
and magnetic hybrids (for echo cancellation), 1000BASE-T can optionally
transmit and receive simultaneously on each twisted pair, to achieve full-duplex
transmission at 2000 Mb/s.
03 | Keysight | Stressing 1 GbE Receivers on the Physical Layer - Application Note
The design implications for 1000BASE-T receivers
Gigabit Ethernet promises a cost effective migration to 1000BASE-T for installations
that are already running 10/100Base-T over Category 5 cabling. However, because
these installations were originally only speciied to 10Base-T or 100Base-T, this is also
a potential weakness. Inadequate cabling can introduce distortions, attenuation, noise
(including cross-talk and echoes that have not been properly removed by the hybrids),
and propagation delay, and delay skew between the different twisted pairs.
Distortion Attenuation Noise Delay
TDX RDX
T R
Hybrid Hybrid
Crosstalk
R T
Figure 1: Effects of attenuation, distortion, noise and delays on transmission
04 | Keysight | Stressing 1 GbE Receivers on the Physical Layer - Application Note
Noise sources
Further noise environment speciications
The noise with which a receiver has to contend comes from a
Return loss
number of sources.