Ethernet - Computer Busses - page 443

Gigabit Ethernet is an excellent challenger for network backbones as it interconnects
10/100BASE-T switches, and also provides high-bandwidth to high-performance servers.
Initial aims were:

• Half/full-duplex operation at 1000 Mbps.
• Standard 802.3 Ethernet frame format. Gigabit Ethernet uses the same variable-length
frame (64-1514-byte packets), and thus allows for easy upgrades.
• Standard CSMA/CD access method.
• Compatibility with existing 10BASE-T and 100BASE-T technologies.
• Development of an optional gigabit media independent interface (GMII).

The compatibility with existing 10/100BASE standards make the upgrading to Gigabit
Ethernet much easier, and considerably less risky than changing to other networking types,
such as FDDI and ATM. It will happily interconnect with, and autosense, existing slower
rated Ethernet devices. Figure 26.23 illustrates the functional elements of Gigabit Ethernet.
Its main characteristics are:

• Full-duplex communication . As defined by the IEEE 802.3x specification, two nodes
connected via a full-duplex, switched path can simultaneously send and receive frames.
Gigabit Ethernet supports new full-duplex operating modes for switch-to-switch and
switch-to-end-station connections, and half-duplex operating modes for shared connec-
tions using repeaters and the CSMA/CD access method.
• Standard flow control . Gigabit Ethernet uses standard Ethernet flow control to avoid
congestion and overloading. When operating in half-duplex mode, gigabit Ethernet
adopts the same fundamental CSMA/CD access method to resolve contention for the
shared media.
• Enhanced CSMA/CD method . This maintains a 200 m collision diameter at gigabit
speeds. Without this, small Ethernet packets could complete their transmission before
the transmitting node could sense a collision, thereby violating the CSMA/CD method.
To resolve this issue, both the minimum CSMA/CD carrier time and the Ethernet slot
time (the time, measured in bits, required for a node to detect a collision) have been ex-
tended from 64 bytes (which is 51.2 µ s for 10BASE and 5.12 µ s for 100BASE) to 512

s for 1000BASE). The minimum frame length is still 64 bytes.
Thus, frames smaller than 512 bytes have a new carrier extension field following the
CRC field. Packets larger than 512 bytes are not extended.
• Packet bursting . The slot time changes affect the small-packet performance, but this
has been offset by a new enhancement to the CSMA/CD algorithm, called packet burst-
ing. This allows servers, switches and other devices to send bursts of small packets in
order to fully utilize the bandwidth.

bytes (which is 4.1 µ

Devices operating in full-duplex mode (such as switches and buffered distributors) are not
subject to the carrier extension, slot time extension or packet bursting changes. Full-duplex
devices use the regular Ethernet 96-bit interframe gap (IFG) and 64-byte minimum frame
size.