Ethernet - Computer Busses - page 446

switches require to be upgraded at the same time; this allows for gradual migration.
• Switch-to-server links - involves upgrading the connection between a switch and the
server to 1Gbps. The server requires an upgraded gigabit Ethernet interface card.
• Switched fast Ethernet backbone - involves upgrading a fast Ethernet backbone switch
to a 100/1000BASE switch. It thus supports both 100BASE and 1000BASE switching,
using existing cabling.
• Shared FDDI backbone - involves replacing FDDI attachments on the ring with gigabit
Ethernet switches or repeaters. Gigabit Ethernet uses the existing fibre-optic cable, and
provides a greatly increased segment bandwidth.
• Upgrade NICs on nodes to 1 Gbps. It is unlikely that users will require 1 Gbps connec-
tions, but this facility is possible.

26.13.6 1000BASE-T

One of the greatest challenges of gigabit Ethernet is to use existing Cat-5 cables, as this will
allow fast upgrades. Two critical parameters, which are negligible at 10BASE speeds, are:

• Return loss - defines the amount of signal energy that is reflected back towards the
transmitter due to impedance mismatches in the link (typically from connector and cable
bends).
• Far-end crosstalk - noise that is leaked from another cable pair.
The 1000BASE-T task force estimates that less than 10% of the existing Cat-5 cable were
improperly installed (as defined in ANSI/TIA/EIA568-A in 1995) and might not support
1000BASE-T (or even, 100BASE-TX). 100BASE-T uses two pairs, one for transmit and one
for receive, and transmits at a symbol rate of 125 Mbaud with a 3-level code. 1000BASE-T
uses:

• All four pairs with a symbol rate of 125 Mbaud (symbols/per second). One symbol con-
tains two bits of information.
• Each transmitted pulse uses a five-level PAM (pulse amplitude modulation) line code,
which allows two bits to be transmitted at a time.
• Simultaneous send and receive on each pair. Each connection uses a hybrid circuit to
split the send and receive signals.
• Pulse shaping matches the characteristics of the transmitted signal to the channel so that
the signal-to-noise ratio is minimised. It effectively reduces low frequency terms (which
contain little data information, can cause distortion and cannot be passed over the trans-
former-coupled hybrid circuit), reduces high frequency terms (which increases crosstalk)
and rejects any external high-frequency noise. It is thought that the transmitted signal
spectrum for 1000BASE will be similar to 100BASE.
• 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 FEC bit uses the
fifth level of the five-level PAM.

A five-level code (-2, -1, 0, +1, +2) allows two bits to be sent at a time, if all four pairs are
used then eight bits are sent at a time. If each pair transmits at a rate of 125Mbaud (sym-
bols/sec), the resulting bit rate will be 1 Gbps.