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Cat 5 cable can handle Gigabit Ethernet

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The move from 10M bit/sec or 100M bit/sec Ethernet to Gigabit Ethernet is frequently perceived as requiring a costly upgrade from copper cable to fiber-optic cable.

However, that's not necessarily true. In most cases in which Fast Ethernet is run in an enterprise on copper, the existing Category 5 cable can also provide reliable support for Gigabit Ethernet.


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The IEEE 1000Base-T specification supports the use of Cat 5, or enhanced Cat 5 cable, for successful gigabit transmission. The jump from 100M bit/sec to gigabit over existing cable is accomplished by a number of signaling changes that take additional advantage of the cable already installed in most enterprise networks.

Cat 5 cable is typically unshielded twisted pair, containing four twisted wire pairs. Fast Ethernet (100Base-T) and 10Base-T use only two of these pairs, leaving two pairs unused. Gigabit Ethernet (1000Base-T) uses all four pairs.

Similar to full-duplex Fast Ethernet, 1000Base-T transmits and receives simultaneously. The difference is that 1000Base-T uses four transmit/receive pairs, each pair operating at 250M bit/sec.

In some respects, running Gigabit Ethernet on Cat 5 cabling is easier than wiring for 10/100M bit/sec Ethernet. The 1000Base-T specification provides for the automatic negotiation of link characteristics, including automatic crossed cable correction. Automatic negotiation enables successful cable connections between 1000Base-T network interface cards (NIC), hubs, switches or other devices that may operate at half-duplex when the ports are initialized.

The intelligence built into many 1000Base-T interfaces can also negotiate the proper speed to use if attached to a 100M bit/sec port by mistake. The gigabit port will operate at the highest common denominator speed thereby preventing any damage to either device interface. Using 1000Base-T NICs on servers and associated switching devices in the network allows a company to continue using its Cat 5 cable infrastructure, while providing an effective high-speed connection.

The move from 10M bit/sec to 100M bit/sec seemed somewhat miraculous when it was announced years ago, even though it also meant an upgrade to higher quality cable. It was worth the expense.

The transition from 100M bit/sec to gigabit data transfer rates is even more remarkable today. Simultaneous transmit and receive, improved data encoding methods and exceptional filtering technologies have made this move possible. What may be even more amazing is the fact that today's tenfold improvement has been achieved using existing cable technologies. Enhanced Cat 5 cable and a still-unspecified Cat 6 cable promise to further improve the quality of the gigabit signal.

The next question, based on the history of computer technologies, is: "Can we look forward to 2G bit/sec, or perhaps even 10-Gigabit Ethernet over Cat 5 (or higher) cable?" Engineers like challenges like this, but in the case of unshielded twisted pair cabling, the answer is probably not.

The amount of computing power required to encode and decode the data is likely to be cost-prohibitive, and it could take longer to perform the encoding, filtering and error correction, degrading the viability of the interface

In the event that an increase far beyond Gigabit Ethernet becomes possible, the cost of implementing such a technology may push it beyond the cost of the more appropriate optical interface solutions.

The move to 10-Gigabit Ethernet will require fiber optics for many reasons. First, optical technologies are already proven well beyond 10G bit/sec. Second, optical technology advancements in dense wave division multiplexing optics offer the potential for significant increases in data rates beyond those currently in use. Third, the cost of optical components today is very competitive with copper cabling. Fourth, long distance is a problem for fiber and copper operating at 1G bit/sec. The 1000Base-T specification only supports 100 meters. The IEEE committee now working on 10G bit/sec standards is expecting 10G bit/sec communications on multimode fiber to be limited to only 85 meters.

Net result? The move to optical cable is inevitable as network bandwidth continues to grow. But for the next few years, the use of existing Cat 5 cable for Gigabit Ethernet will be a viable alternative for many of today's enterprise computing needs.



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Kuciel is president and chief operating officer of SysKonnect. He can be reached at jkuciel@syskonnect.com.

The Copper Alternative
New switch interfaces allow you to connect devices at Gigabit Ethernet speeds using ordinary Cat 5 cable.
Network World, 03/20/00.

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