Timothy
Parker Consulting Incorporated
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Gigabit Ethernet Remember when we used to think 10Base-T Ethernet was fast?
Although 10Base-T never reached its theoretical limit of 10Mbps, most
networks using twisted pair wiring could wring out 4Mbps or so of sustained
network traffic. Then, we ran out of patience with 10Base-T and went to
100Base-T. Instead of 100Mbps (the
theoretical limit), most Fast Ethernet twisted pair 100Base-T networks could
obtain between 40 and 60Mbps, which would suffice for a long time.
Well, not really. The simple fact is network people are never happy with the
speed of their networks, and while we are locked in to Ethernet and TCP/IP we do
want to get even more speed from the network.
Hence gigabit Ethernet. Gigabit Ethernet has been around for a while, but adoption
of the network standard has been slow primarily because of the costs involved in
upgrading existing equipment. Lately,
though, market saturation and dropping prices have helped make gigabit Ethernet
economically viable for many networks. In this article we take a look at what gigabit Ethernet is,
what it can do for you, and how you can upgrade or install a network to support
gigabit speeds. We’ll look at
some real-world numbers from a production network, and let you decide if the
performance boost and costs are justified. What is gigabit Ethernet? Gigabit Ethernet runs over any Ethernet media, including
twisted pair. Naturally, the speed over a twisted pair network isn’t as fast
as over a thick Ethernet or fiber optic cables, but the advantage to twisted
pair is that most of our networks have lots of it already strung, ready to
handle gigabit speeds. Gigabit
Ethernet over twisted pair is called 1000Base-T, which makes sense since
10Base-T (10Mbps over twisted pair) and 100Base-T (100Mbps over twisted pair)
are common terms. The standards for gigabit Ethernet are defined in a supplement
to the IEEE 802.3ab standard, and grow out of the existing Ethernet systems
although with a lot of changes to handle the higher speeds and increased
collisions of packets. If you’ve worked with networks in the past, you might
think it would be difficult to get 1000Mbps speeds over a lowly twisted pair
network cable, and you would be correct. In
order to achieve anything remotely like gigabit speeds over twisted pair, some
tricks have to be employed. The
first trick is to combine standard Ethernet signaling techniques with an
encoding method that was originally designed for the seldom-used 100Base-T2 and
100Base-T4 standards. (The
100Base-T2 -- not 100Base-T -- system used two pairs of twisted pair cable with
a complex encoding method for the signals to be sent. 100Base-T4 -- seldom
encountered in the real world -- used
a method of sending and receiving signals simultaneously over the same pairs of
wires.) The 100Base-T4 technique
for sending and receiving data simultaneously over pairs of wires was adopted by
1000Base-T. Four pairs of Cat 5
cable are used in each 1000Base-T cable (as opposed to only two pairs – four
wires – in 10 and 100Base-T). Finally, the popular 100-BaseTX Fast Ethernet
system signaling standard was adopted for 1000Base-T. Thus, 1000Base-T is really
an amalgam of the best techniques from several other Ethernet standards.
It is also quite different from the 10Base-T and 100Base-T systems we
know. All of the signaling and encoding processes as well as the
much higher transmit speeds means that the cable used for twisted pair Gigabit
Ethernet are sensitive. Cat 5
cables must be used, although Cat 5e is a better choice since it has better
signaling carrying capabilities. There are a number of cables that exceed Cat 5
specifications, and they can all be used for gigabit Ethernet, too.
If you have a network comprised of existing Cat 5 twisted pair, it can be
used with Gigabit Ethernet right away, although you may not achieve optimum
throughput depending on the cable quality, length of run, external source of
interference, and so on. Forget about building homemade cables for 1000Base-T:
the cable strand twists have to be maintained all the way to the connector body,
and it’s very difficult to construct a cable manually that can meet Cat 5
specifications. The recommended
maximum cable length for 1000Base-T is 100 meters (280 feet), and there can be
no more than two transceivers in the segment from NIC to hub. Network components The gigabit Ethernet network components are much the same
as existing Ethernet components, but carry the technology to handle the higher
speeds of gigabit Ethernet. For the
most part, existing network interface cards (NICs), hubs and switches, and other
switching devices all will not work with gigabit Ethernet unless specifically
rated for that purpose. A 1000Base-T network card looks the same externally as a 10
or 100Mbps interface card. There is
usually a single 8-pin RJ45 connector on the back plane with a number of status
LEDs to show line conditions. Internally,
the card is quite different. That’s because both 10 and 100Mbps Ethernet allow for
exposed connectors and transceivers, as well as external transceiver cabling but
that’s not allowed with 1000Base-T. Instead,
the transceiver is built into the interface card, which makes the card much more
expensive. The same is true of hubs
and switches supporting 1000Base-T where a transceiver is used internally for
each cable. Most 1000Base-T network
cards for PCs use autonegotiation to allow the card to adjust to the fastest
supported speed, 10, 100, or 1000Mbps. Gigabit
Ethernet over twisted pair does support half-duplex mode, so the potential is
there for double speed connections. There is a special configuration of gigabit Ethernet for
fiber optic cables, but the costs of fiber optic restrict this network type to
those with lots of money and a specific need to avoid twisted pair (usually for
shielding purposes). Handling fiber
optic cables requires some skill and practice, especially considering the cost
of mistakes. Installing gigabit Ethernet Installing a Gigabit Ethernet twisted pair network is the
same as any other twisted pair network, except for the care taken with the
cables. The network interface cards
install in the same manner as any other network card, and the hubs or switches
are the same except for the new ratings. To test gigabit Ethernet we equipped our test lab with a
dozen Windows 92/2000 machines connected to four servers: two Linux (RedHat 6.2
and Mandrake 7.1), Sun Solaris, and SCO UnixWare 7. We then used scripts to
transfer files back and forth between all fourteen machines using FTP, ran
simultaneous X sessions from the servers on the Windows machines (using
Hummingbird exceed), and scripted HTTP access from all four servers running
Apache web servers. We monitored
network traffic speed and collisions using in-line network monitoring devices. From the user’s point of view, gigabit Ethernet is
amazing. Transfers from servers are
so fast it’s hard to see any lag from request to completion.
In many cases, the hard drives couldn’t keep up with the file
transfers. On our test network using twisted pair cables, we managed a peak
speed of 450Mbps, with average sustained traffic volumes of about 120Mbps.
This is a far cry from the theoretical maximum of 1000Mbps, but still a
considerable improvement over the 30Mbps of Fast Ethernet! As mentioned at the start of this article, the costs for gigabit Ethernet are dropping rapidly. When purchasing NICs and hubs in bulk, you can equip a medium network with gigabit equipment for approximately $100 per node. For small networks, the investment won’t pay off unless you are constantly suffering network congestion. For larger networks, congestion is a routine occurrence and gigabit Ethernet can solve the problem easily. As prices continue to fall, it will not be uncommon to see many networks move to gigabit speeds. The users benefit. |
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tparker@tpci.com with
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