4 Gigabit Ethernet

Not to be outdone by ATM or Fibre Channel, Ethernet aficionados are pushing Ethernet to even higher speeds. Ethernet originally ran at 10 Mbps. The 100Base-T and 100VG-AnyLAN Ethernet standards run at 100 Mbps. The Gigabit Ethernet Alliance is an industry consortium that has developed the specifications for Gigabit Ethernet, working closely with the Institute for Electrical and Electronic Engineers (IEEE) which formed a working group, 802.3z to develop the Gigabit Ethernet standard. In June 1998, the IEEE 802.3z working group approved the Gigabit Ethernet standard.

Gigabit Ethernet uses the same 802.3 frame format, and access protocol (Carrier Sense, Multiple Access with Collision Detection, CSMA/CD) to remain backwards compatible with standard 10 Mbps and 100 Mbps Ethernet. The packet format is variable length (64-1514 byte) packets, unlike ATM, which uses a fixed length cell.

The Gigabit Ethernet CSMA/CD algorithm has been improved so operation at gigabit speeds is possible. Otherwise, minimum sized Ethernet packets could complete transmission before the sending stations sees a collision, which violates the CSMA/CD rules. Basically, the CSMA/CD extensions have increased the carrier time, from 64 bytes to 512 bytes. For frames smaller than 512 bytes, the carrier signal is extended to assure that only one small frame at a time can be on the shared wire. Like 100 Mbps Ethernet switches, both full and half-duplex operation is supported. In non-switched devices, (e.g. hubs) the media is shared, so half-duplex mode must be used. Full-duplex devices will continue to use the regular 96-bit interframe gap and 64-byte minimum packet size.

The 802.3z working group has drawn heavily on the Fibre Channel specification FC-0 and FC-1 for the physical layer. This permits the same integrated circuits to be utilized for lower cost and quicker time to market. Initial products will use fiber for connectivity. Work is still in progress to define support for twisted pair cabling.

Typical Ethernet networks have throughput of approximately 70% of the wire speed. Early simulations of Gigabit Ethernet show improvement over 100 Mbps Ethernet by an order of magnitude [24]. Other advantages as well as disadvantages of the Gigabit Ethernet specifications are listed below.

Advantages:

• same frame format as 10/100 Mbps standards
• full duplex with switching devices (802.x standard)
• backwards compatible in shared mode using carrier extension
• extension of a well understood, well characterized network approach.
• LAN oriented
• wide range of hardware and software available
• Highly modular hardware approach.

Disadvantages:

• Products are not yet available
• Gateway device necessary to interface to wide area link protocol
• Ethernet does not support time-sensitive data

Because of the large installed base, costs are expected to be lower than competing technologies such as ATM or Fibre Channel. The primary disadvantages are due to the immaturity of the technology, though that will of course change with time.