Daisy-Chain Ethernet

How Farallon Computing and Tut Systems make twisted-pair Ethernet wiring as flexible as PhoneNet

Stan Miastkowski

Conventional wisdom says that you have two types of 10-Mbps Ethernet network wiring choices, depending on the topology you want to use. For bus, or daisy-chain, topology, there's thick RG-8 (10Base-5) coaxial cable or the more-common thin RG-58 (10Base-2) coaxial cable. For star topology, you use 10Base-2 UTP (unshielded twisted pair) cable.

But two companies are challenging that wisdom with technologies that offer daisy-chain Ethernet using inexpensive and easier-to-install copper wire instead of coaxial cable. Farallon Computing's (Alameda, CA) EtherWave does the job with two pairs of standard Level 3 or Level 5 UTP. Tut Systems ' (Pleasant Hill, CA) Silver Streak stretches the technology further by putting full 10-Mbps Ethernet on a single pair of wires of virtually any type, including the nontwisted "silver satin" flat phone wire that's used to connect telephones and modems to telephone jacks.

Your network wiring is the physical layer, the first and essential layer of the seven-layer ISO OSI (Open Systems Interconnection) LAN model. Coaxial cable has been used for daisy-chain bus topology for the simple reason that its robust electrical characteristics let it easily meet the signal integrity and compatibility standards of daisy-chain connections. But the IEEE 802.3 Ethernet standard doesn't specify that coaxial cable must be used for daisy-chain networks.

Integrating Technology

Farallon's EtherWave integrates Level 3 UTP in a daisy-chain topology by combining 10Base-T hub and transceiver technologies into a single proprietary ASIC (application-specific IC) that's designed to handle the precise n anosecond timing needed for Ethernet. The ASIC is included on each network node, either in an ISA, NuBus, or PDS/LC NIC (network interface card) or in external connection boxes for AUI (attachment unit interface)/AAUI-equipped PCs, Mac PB adapters, LocalTalk network printers, or the Apple Newton.

EtherWave devices behave like nonreclocking repeaters, which allow users to add nodes to a 10Base-T network without impacting hop count limitations. Normal star topology 10Base-T networks are limited to four hubs (hops) between nodes, but EtherWave does away with this hop limitation. EtherWave also handles up to eight nodes per segment, with a total maximum segment length of 330 feet. However, for network expansion, each eight-node segment can be connected to any standard 10Base-T hub. For example, a 12-port hub can handle a total of 96 EtherWave-equipped nodes.

EtherWave includes auto-termination, which eliminates additional termination resistors at each end of the daisy chain (required with coaxial ca bling). This proprietary technology senses impedance changes in the cable and instantly terminates or passes through the network signal. This scheme also allows shutting down or hot-swapping individual network nodes without shutting down the network, largely eliminating a major disadvantage of coaxial-based daisy-chain topology. In addition, EtherWave's ASICs incorporate Auto Crossover, sensing which wires are connected in the UTP and how they should be used. Auto Crossover eliminates keeping track of whether your 10Base-T cables are straight-through or crossover.

Analog Worlds

While Tut's Silver Streak offers nearly all the above advantages of daisy-chain topology without coaxial cable (except for the lack of internal auto-termination), Silver Streak's essential difference is its ability to use nearly any type of wire. In the digital world of computers, it's important to remember that LAN wiring carries analog signals (with the exception of fiber optics, there is no such thing a s a digital cable). In fact, the essential part of every network is a transceiver, which translates digital data into RF analog signals for outgoing data and vice versa for incoming signals. Twisted-pair and coaxial networks have their transceivers integrated into the NICs; thick coaxial wiring uses external transceivers "tapped" into the cable run.

One of the major realities of cable--any type of cable--is that it distorts signals. And because distortion increases with transmission speed (frequency in the analog world), pumping a 10-Mbps Ethernet signal through thin wiring is a technical challenge.

There are five distinct types of distortion: attenuation, intersymbol interference, NEXT (near-end cross talk), noise pickup, and radiation. Each causes problems that slow data transmission. Interactions among these types of distortion result in a morass of problems that must be overcome to put full 10-Mbps Ethernet on standard flat telephone cable. Silver Streak uses a variety of approaches to solve these problems.

Attenuation

When it comes to high-frequency signals, speed kills. RF signals ride on the surface of a copper wire in what's called the skin effect. The thinner the wire, the smaller the circumference, the less surface area, and the greater the amount of signal attenuation. In addition, attenuation increases as the square root of frequency (e.g., quadrupling the frequency doubles the attenuation, cutting the signal level in half). Standard 10Base-T Ethernet using Level 3 or Level 5 (both 24-gauge) UTP is limited to 330-foot segments, because beyond that distance, the signal is attenuated to the point where it no longer meets 802.3 standards. The attenuation problem would seem, at first glance, to be sufficient reason why you normally can't use the 18-gauge wire in standard silver satin telephone cord for Ethernet. But if you control the other aspects of distortion, especially noise (see "Intersymbol Interference"), flat telephone wire becomes usable.

Intersymbol Interference

After passing through the NIC's transceiver, digital data travels through network wiring as a series of analog DC pulses (each pulse representing a bit of digital data). The nature of DC pulses is that they have (when viewed on an oscilloscope) a fast rise time but a long decay (sometimes called a tail) because of the memory inherent in copper wire. At Ethernet speeds, this can cause pulses to interfere with each other, giving rise to errors, retransmissions, and slow network throughput. To combat intersymbol interference, a factor in all copper-wired networks, Silver Streak uses an equalizer circuit. Essentially a simple analog filter circuit that consists of two high-speed Schottky diodes connected back to back, it effectively cuts off the tail of each DC pulse, allowing full-speed Ethernet transfers without the pulses getting in each other's way.

NEXT

Unless wire pairs are shielded (which they aren't in UT P), there is always some crossover of signals between the two pairs of wires used in 10Base-T wiring (this is comparable to what happens when you hear another conversation in the background while you are talking on the telephone). Wire pairs that touch each other couple capacitively, and fast-rise time pulses (i.e., Ethernet signals) travel easily through capacitors. This causes noise, collisions, and slow network throughput. The twists in twisted pair minimize cross talk by minimizing the capacitive coupling between wire pairs for two reasons: The distance between pairs is random, and individual conductors are not in constant contact with each other. In Level 3 and Level 5 UTP, this causes null effects that essentially cancel out the cross talk. In standard silver satin phone wire, where pairs are not twisted, NEXT would be unacceptable at Ethernet speeds. However, NEXT is eliminated when only a single pair of wires is used for concurrent transmitting and receiving, instead of the dual pair arrangement used in standard 10Base-T setups. Silver Streak uses this single-pair method.

Putting both signals onto a single wire pair at the same time is trivial. All that's needed is a simple hybrid circuit--essentially the same thing found in any telephone set--letting you talk and listen at the same time. So why isn't a single pair with hybrid circuit used in UTP to eliminate NEXT? Mainly, it's because NEXT is a minor problem compared with noise.

Noise

This is by far the biggest and most difficult problem in copper-wired networks. By definition, electrical noise is any unwanted disturbance that interferes with the signal. Any wire is a receiving antenna, and every office environment is rife with noise sources that adversely affect a network. Fluorescent lights, AC power lines, and the RF fields generated by PCs can interfere with network wiring. There are more-serious problems, too. The normal voltage carried in Ethernet wiring is approximately 2 VDC. However, a static shock caused by walking across a carpeted floor can put a 6000-V spike on network wiring. Ringing signals on analog telephone systems use 180 VDC, and network and phone signals are often run together, using the extra pairs in UTP.

The key to Silver Streak's ability to use virtually any wire is a proprietary balun (short for balanced to unbalanced). All networks use baluns, which are transformers that connect the NIC transceiver to the physical wiring. All baluns attenuate noise, and the baluns used in most network hardware attenuate at about a 100-to-1 ratio (40 decibels). Unfortunately, this isn't enough to attenuate serious--yet common--noise sources below the point where they cease to interfere with network transmissions. Tut's proprietary balun is unique, rejecting noise by a ratio of 10,000 to 1 (80 dB). It attenuates virtually all noise, even the most serious, below the point where it affects the network. The balun and lack of noise is also the reason that a Silver Streak segment can extend up to a len gth of 800 feet with a maximum of 30 nodes per segment. Like Farallon's EtherWave, the total number of nodes in a Silver Streak installation can be extended by using standard 10Base-T hubs.

Radiation

This is the other side of the noise coin, because all wires carrying signals are transmitting antennas, which radiate RF energy that can interfere with televisions, radios, and other electronic equipment. But properly done noise reduction also minimizes radiation from wiring, and Silver Streak's balun keeps radiation to a minimum. In fact, Silver Streak is the only network system that meets the stringent FCC Class B standards for consumer-level equipment. All other networking systems meet the less-stringent FCC Class A standards for commercial equipment and applications.

Wiring Choices

Farallon's EtherWave and Tut's Silver Streak open up new horizons for installing new networks and for expanding existing installations without major investments in wiring. Although the details of how each implements daisy-chain Ethernet with copper wiring are different and largely proprietary, the crucial fact is that both systems are 100 percent compatible with all 802.3 Ethernet specifications (10Base-2, 10Base-5, and 10Base-T).

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COMPANY INFORMATION

EtherWave

Farallon Computing, Inc.
2470 Mariner Square Loop
Alameda, CA 94501
(510) 814-5100
fax: (510) 814-5020


Silver Streak

Tut Systems, Inc.
2446 Estand Way
Pleasant Hill, CA 94523
(510) 682-6510
fax: (510) 682-4125

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Stan Miastkowski is a BYTE consulting editor who has been writing about networking and communications technology for over 16 years. He is the coauthor of Windows for Workgroups Bible (Addison-Wesley, 1993). You can contact him on MCI Mail at 530-9979 or on the Internet or BIX at stanm@bix.com .