LAN of hope and glory…..

LAN of hope and glory…..The transition to Fast Ethernet local area networks may be happening but there is still uncertainty in the market. Richard Wilson reports It is three years since National Semiconductor introduced the first seriously integrated transceiver chipset for the 100Mbit/s local area network (LAN) protocol, known as Fast Ethernet. National might claim it kick-started the evolution of PC local area networks (LANs) from the 10Mbit/s data rate of traditional Ethernet networks to the 100Mbit/s rate systems, more attuned to the networking needs of the 1990s. Despite the probability that there are still more 10Mbit/s LANs than 100Mbit/s LANs in operation around the world, the transition to the higher data rate is definitely taking place. But the combination of a notoriously problematic signal modulation technology and aggressive competition between silicon suppliers means that the transition may not be an easy ride for many users and suppliers. Intel is one transceiver supplier that may take issue with National’s claims over market leadership, but what is less debatable is that today’s 100Mbit/s Fast Ethernet LAN chipset market is one of the most price competitive in the whole semiconductor market. One estimate suggests that Fast Ethernet transceivers conforming to the 100Base-Tx (two-wire cabling) LAN standard must now meet price targets of as little as $5 per LAN port connection. The transceiver function, the capability to transmit and receive data on the PC’s LAN interface card, incorporates digital coding and decoding of the data, scrambling, line equalisation, clock recovery and D/A conversion for transmission down the cable. Whereas three years ago a handful of devices were needed, today designers can take their choice of single chip transceivers from any one of 10 suppliers. These are not simple devices to design and produce. The capability to drive the 100Mbit/s data rate over 100m of twisted-pair cabling using the modulation scheme defined by the Fast Ethernet protocol standard is not easily achieved in the highly integrated device required by a price sensitive market. In the interests of keeping costs down some big name networks suppliers, notably Cisco, implemented their own 100Mbit/s transceiver designs in Asics. Most leading Asic suppliers, like Lucent Technologies, Fujitsu, LSI Logic and NEC, have 100Base-Tx cores in their libraries. Short product life-times also mean that some LANcontroller designs never get beyond the FPGAstage, which is good news to Altera and Xilinx. However, there is a growing standard chipset market for Fast Ethernet and in the last few months some of the leading suppliers have introduced new single chip transceivers. National Semiconductor, which claims an installed base of 20 million first generation devices, introduced late last year a single-chip physical layer and transceiver device, the DP83843 Phyter that supports 10/100Mbit/s data rates on two-wire category 5 twisted pair and fibre cabling. Communications chipset specialist, Broadcom soon followed with its first single-chip 100Base-Tx repeater, the BCM5205, with four category 5 twisted pair transceivers. Just last month, Intel introduced its latest single-chip transceiver, the 82559, that also incorporates for the first time a number of network management functions on-chip. Intel also claims to have reduced board space and power requirements by around 75 per cent on previous designs, and states its target to now put 100Mbit/s transceivers on PC motherboards as early as next year. Lucent Technologies has also recently added a low power 3V multi-channel Fast Ethernet transceiver to its Fastcat family. This device is based on a proprietary digital line filtering technology developed by a Californian start-up called Enable Semiconductor,and which Lucent licensed last year. Enable, which is represented in the UK by Focus Electronic Distrsibution, will act as an alternative supplier for the device. Lucent claims to have shipped four million 100Mbit/s ports based on its 5V transceiver design. Other transceiver suppliers include Mitel Semiconductor, Level One, PMC-Sierra and AMD, which last week announced its latest 100Mbit/s physical layer device, a single port device that connects to category 5 cable for 100Mbit/s LANs and cheaper category 3 cable for basic 10Mbit/s LANs. The distinction is an important one. All 100Mbit/s transceivers on the market will also operate at the lower 10Mbit/s Ethernet rate, which is just as well for many of the higher speed devices are still only operating on 10Mbit/s Ethernet LANs. While network managers are upgrading the interface cards with 100Mbit/s capability, many are not yet using the higher data rate. While one reason may be a lack of network demand for 100Mbit/s data rates, the true reason may be that managers are reluctant to risk upgrading their LANs which consist of literally miles of cabling of probably uncertain type and quality. While 10Mbit/s will work on basic unscreened category 3 twisted pair cabling, 100Mbit/s requires higher grade unscreened category 5 (cat 5) twisted pair cabling. While cat 5 is the norm in LANs being built today, established networks built up over many years will contain a mix of cats 5 and 3 cable types. Try to run a 100Mbit/s data rate over cat 3 cabling and the noise and signal distortion on the line will inevitably mean the transceivers switch back to the slower data rate as they are designed to do. So it is not the appearance of 100Mbit/s transceivers that will necessarily lead to higher speed LANs, but the proliferation of the higher quality cat 5 cabling in corporate networks. Three years on there is still much uncertainty in the Fast Ethernet chipset market. What is controlling the move from 10 to 100Mbit/s data rates – clever silicon or dodgey cabling? One consequence is that some suppliers, notably Hewlett-Packard, Cypress Semiconductor and Vitesse Semiconductor, have tried to regain the silicon initiative by moving quickly on, not to just 100Mbit/s, but to Gigabit Ethernet LAN technology. Even Intel announced its first Gigabit Ethernet part just last month. The attraction of Gigabit Ethernet to the chip suppliers is that the cabling becomes less of an issue. Gigabit data rates will require optical fibre cabling it t hey are to work over sensible distances in LAN backbones. Vitesse already has a 1.25Gbit/s Ethernet transceiver, based largely on an existing fibre channel device. One transceiver can support both fibre protocols. Could Gigabit Ethernet turn out to be the “dream” LAN technology for chip suppliers, after they have grappled for a few years with the twisted-pair cabling problems in 100Mbit/s Fast Ethernet market? LAN technology at the limit 10Mbit/s Ethernet may work on little better than a piece of damp string, but 100Mbit/s Fast Ethernet 100Base-Tx technology only works on unscreened category 5 two-wire twisted pair cabling. There is precious little performance margin for error. The 100Base-Tx standard’s use of a three level signal modulation scheme, known as MLT3, to achieve 100m transmission lengths has left the transceiver designers with little or no performance margin. The level of concern over whether first generation 100Base-Tx transceivers can cope with the signal distortion even on cat 5 cable, has led firms like Broadcom, Lucent Technologies and Enable Semiconductor to use sophisticated digital line equalisation and baseline compensation circuits in their second generation transceivers. Enable for one believes this is a necessary development for true 100Mbit/s LANs, as a company spokesman put it: “Just watch what happens to those other transceivers when they try to drive 100Mbit/s over anything but perfect cable.”


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