Ethernet goes for Gigabit speeds

Dave Wilson looks at how the Ethernet standard is evolving from 10Mbit/sec to 1Gbit/sec transfer rate

As the sophistication of CAD solids modelling packages has increased, so too has the demand for PCs and workstations with greater speed and memory capacity. But as file sizes have burgeoned and concurrent engineering has boomed, fast networking has become equally crucial.

10Mbit/sec Ethernet, which became the defacto standard for local area networking, was followed by Fast Ethernet, boasting speeds of 100Mbit/sec. But the computer business never stands still, and on the horizon is a faster Ethernet yet: a 1000Mbit/sec or Gigabit Ethernet that looks set to gain IEEE standard status this year.

Propelling the Gigabit Ethernet standard forward is a group of predominantly US companies collectively known as the Gigabit Ethernet alliance. They comprise over 100 well known vendors in the networking field: both large systems vendors and chip makers such as IBM, Siemens, 3Com and Rockwell Semiconductor.

The new Gigabit Ethernet standard itself will be 100% backwards compatible with its earlier Ethernet predecessors. That’s because Gigabit Ethernet employs an enhanced CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol, same frame format and same frame size as its predecessors. It will be transparent to most of the network, supporting existing applications, network operating systems and network protocols, such as IP, IPX and AppleTalk. In contrast, other high speed technologies use fundamentally different frame formats. High speed ATM (Asynchronous Transfer Mode), for example, implements a fixed length data cell. When connecting Ethernet and Fast Ethernet to ATM, a switch or router must be used to translate each ATM cell to an Ethernet frame and vice versa.

The Gigabit Ethernet standard itself makes use of a physical signalling technology employed by another high speed networking standard called Fiberchannel to support a Gigabit per second data rate. The specification defines the use of laser-based optical components to propagate data over multi mode fibre. Lasers were specified because they function at the required baud rates and longer distances required for Gigabit Ethernet. So the first set of Gigabit Ethernet standards will support both multi mode and single mode fibre optic cable as well as short-haul copper, or coax. Standards for long haul copper, on the other hand, are expected to appear in early 1999.

As for maximum lengths of Gigabit Ethernet links, the standards committee’s initial goals were for 500m over multi mode fibre (1000 Base SX), 3000m over single mode fibre (1000 Base LX), 25-100m with Category 5 UTP (1000 Base T) and up to 25m with coax (1000 Base CX). At a recent meeting of the standards group, however, the distance for 1000 Base LX single mode fibre was increased to 5000m and that for the 1000 Base SX increased to 550m.

In the definition of the standard, the IEEE 802.3z task force identified a certain condition known as differential mode delay, or DMD. This phenomenon exists for any high speed technology using a coherent laser source for transmission over MM fibre. The resulting characteristics create an additional element of jitter which can limit the reach of Gigabit Ethernet over MM fibre.

DMD is an effect where a single laser light pulse excites a few modes equally within a MM fibre. These modes then follow two or more different paths. These paths may be of different lengths and may have different transmission delays.

With DMD, a distinct pulse propagating down the fibre no longer remains a distinct pulse, or, in extreme cases, may become two independent pulses. A string of pulses will tend to interfere with each other such that the data cannot be recovered reliably.

The solution to the problem requires what is known as `conditioning a laser launch’. The objective is to scramble the modes of light such that the power is distributed more equally in nearly all modes and not concentrated in just a few modes.

Many Gigabit Ethernet products, such as those from Alteon, 3Com and Cabletron, have already hit the market. Some companies are already using Gigabit Ethernet equipment too. Banco Santander S.A., Spain’s largest banking group, is using a Gigabit Ethernet “backbone” network at its UK headquarters in London based on 3Com’s end-to-end 10/100/1000 Mbps migration system.

The current proposed additions to the standard to resolve the DMD issue will not necessarily mean that early products will not conform to the additional specifications. Early implementers of the Gigabit Ethernet standard, however, may be faced with some uncertainty when dealing with pre-standard products.

Helping with interoperability issues is the Gigabit Ethernet InterOperability Test Consortium, an independent group, hosted by the University of New Hampshire Interoperability Laboratory (IOL). The Consortium will help customers keep track of the standards as well as the products that are verified as compliant.

Of course, widescale implementation of Gigabit Ethernet LANs is still a way off. And while vendors ready their Gigabit Ethernet offerings, other technologies will vie for position as the defacto standard. Most notably, there’s ATM, offering as it does certain advantages that Gigabit Ethernet does not – most notably, a guarantee of bandwidth for the duration of an application request as well as a scaleable topology that can accommodate any range of speeds. Which ever way it goes, one thing is certain: designers will all benefit from these newer, faster networking solutions.

Rochester New Hampshire-based Cabletron Systems’ 9G421/9-02 SmartSwitch modules are designed with an interface that is built to the IEEE P802.3z draft specification. They provide fibre optic interfaces for both multimode and single mode connectivity.

{{TABLE 1Gigabit Ethernet 1000BASE Nomenclature

1000 BASE SX 850nm multi mode fibre1000 BASE LX 1300nm multi mode and single mode fibre1000 BASE CX Short-haul copper (“twinax” STP)1000 BASE T Long-haul copper over UTP}}

{{TABLE 2Gigabit Ethernet Comparison Summary

Ethernet Fast Ethernet Gigabit 10 BASE T 100 BASE T Ethernet Goals

Data rate 10Mbps 100Mbps 1Gbps

Category 5 UTP 100m (min) 100m 25-100m

STP/Coax 500m 100m 25m

Multi mode fibre 2km 412m (hd) 550m 2km (fd)

Single mode fibre 25km 20km 5km}}

In February this year, San Jose based-Alteon Networks launched its 10/100/1000 Ethernet server switch. Called the ACESwitch 180, it offers systems integrators the ability to upgrade to Gigabit Ethernet from Fast Ethernet.