Some time ago, I was elected Chief Technical Editor of the new IEEE Gigabit Ethernet specification. Let me tell you, it's been a wild ride.To begin with, the whole premise of local area networking at 1 billion bits per sounded, at first, almost farcical. As I dug deeper into the project, however, it soon became apparent that these people were serious, that a PC on your desk may actually need a gigabit LAN in the near future, and that manufacturers who believe otherwise had better get out of the way!
What convinced me? Look at these numbers:
- Most personal computers now come equipped with a PCI bus. The PCI bus is 32 bits wide and runs at 33 MHz. In actual use, the burst transfer rate of this bus is very close to 1 Gbit/s.
- Forty percent of all major LAN customers say they want Gigabit Ethernet right now.
- One-hundred and thirteen vendors of networking equipment have joined the Gigabit Ethernet Alliance, an organization dedicated to promoting the development of Gigabit Ethernet products and standards. Many of these vendors are former advocates of ATM or Fibre Channel.
- There are presently 190 registered voters in the IEEE 802.3 (Ethernet) standards committee, the biggest enrollment ever.
- Ethernet physical-level interface components are currently shipping at a rate of more than 30,000,000 annually.
What this all means is that there will soon be some new physical layer transmission chips available. They will be mass-produced at very low cost levels, and you might want to take a look at them if you are planning any kind of campus-wide, high-speed data transmission project.
The standard provides for a number of physical layer transmission interfaces, including various flavors of twinax, fiber, and category-5 Unshielded Twisted Pair (UTP) wiring (category-5 UTP is a specialized variety of building wiring used for data, LAN, and phone connections).
The twinax and fiber transmission interfaces are fairly straightforward. They each use 8b10b serial line coding, which adds two bits to each octet for the purpose of enforcing a minimum number of clock transitions and maintaining dc balance. The dc balance property helps the coded signal pass through transformers and other ac-coupled circuits. The 8b10b-coded interfaces provide a raw data link transfer capability of 1.000 Gbit/s, with a coded line rate of 1.250 Gbit/s. They each use their respective media in the obvious way, with no requirements for adaptive equalization, AGC, or other fancy receiver features.
Gigabit Ethernet applications will be supported by new versions of the fibre channel 10-bit serializer chip, which have been tweaked for operation at a coded line rate of 1.250 Gbit/s. The serializer chip controls both transmitted and received data. On its transmit side, the chip accepts data from a 10-bit bus at 125 MHz, and serializes it to a rate of 1.250 Gbit/s. The serializer circuit includes a 10x clock multiplier that internally synthesizes the 1.250 GHz output clock from the 125 MHz transmit bus clock.
The receive side of the serializer includes a PLL clock recovery circuit and a deserializing circuit that captures the incoming serial data in a 10-bit, 125 MHz output register. The deserializing function recognizes and acquires word synchronization by looking for special patterns hidden within the data stream. The 10-bit output register is, therefore, always properly word-aligned. This is a handy part, reminiscent of the AMD TAXI serializer, but quite a bit faster.
Most of the serializer chips being designed for Gigabit Ethernet can directly drive 150-Ω balanced cabling. No intervening external amplifiers will be required.
For fiber applications, the serializer must be coupled to a fiber-optic transmit/receive components.
The fiber transceivers used for Gigabit Ethernet will initially be the same parts used for some Fibre Channel applications, although over time the vendors may begin to specialize their components. The system is designed to permit use of either 850-nm (SWL) optical transceivers or 1300-nm (LWL) optical transceivers; however, the transmit and receive wavelengths used at each end of a link must match. Depending on the choice of wavelength and fiber type, the supported distances will range from 300 m to 10 km or more.
The Unshielded Twisted Pair (UTP) transceiver is going to be particularly interesting. It's probably one of the most aggressive data communications standards ever. This transceiver is going to make use of advanced digital signal processing techniques similar to those used in 28 Kbit/s telephone modems, but running 35,000 times faster. It will work on four pairs of category-5 UTP, at distances up to 100 m. If you want to send a lot of data, and you want to use category-5 UTP, this will be the transceiver to pick.
It's exciting to be involved in a new standard. It's also a lot of work. If you would like to tune in to what is going on, and maybe help plan the next great standard, check out the IEEE Standards Association.