Fast Ethernet Information and Recommendations
Table of Contents
Introduction to Fast Ethernet Technology
Fast Ethernet is a new higher-speed implementation of the familiar 10 Mbps
(Megabit per second) Ethernet. A necessary addition to modern networking,
Fast Ethernet provides a fairly economical bandwidth increase for
today's high-speed servers and high-end workstations. While Network 21's
use of switched 10BaseT will in itself provide improved bandwidth, design
of the Network 21 infrastructure includes an upgrade path that can be
utilized for certain implementations of Fast Ethernet.
Cabling requirements for Fast Ethernet differ from those required for the
well-known 10BaseT standard in use today. In many cases, particular
Fast Ethernet requirements are not compatible with existing wiring
facilities on campus.
Fast Ethernet Standards
The numerous types of Fast Ethernet include 100BaseTx, 100BaseFx,
100BaseT4, and 100BaseVG. Each differs from the others and is unable to
interoperate beyond 10 Mbps for the twisted-pair implementations.
Two separate approaches were proposed during the development of Fast
Ethernet. The first implements Fast Ethernet over two pairs of
twisted pair cabling as does 10BaseT with the requirement that these
pairs must be Category 5. The second approach implements high bandwidth by
distributing signals across more pairs of cabling (this can be done with
four pairs of Category 3 cable). The various Fast Ethernet cabling
requirements are shown below:
Standard
|
Cable Type
|
Pairs/Strands Needed
|
100BaseT4
|
Category 3, 4, or 5 UTP
|
4 pairs
|
100BaseVG UTP
|
Category 3, 4, or 5 UTP
|
4 pairs
|
100BaseVG STP
|
150 Ohm STP or IBM Type 1
|
2 pairs
|
100BaseTx
|
Category 5 UTP
|
2 pairs
|
100BaseFx (1)
|
62.5/125 MM fiber
|
2 strands
|
(1) 100BaseFx is not generally used to the desktop.
Much of the existing cabling on campus was installed in 1986 before
Category 3 standards, yet this cable meets Category 3 standards when
tested. This large installed base of Category 3-like cabling makes
100BaseT4 and 100BaseVG seem like good candidates for campus use.
However, the Category 3 cabling installed in 1986 has only three pairs
making it unusable for these standards. 100BaseTx uses two pairs of
Category 5 cabling, which is also incompatible with the Category 3
wiring installed in 1986. A new or recently installed Category 5 NAM
is required to use any of these Fast Ethernet standards.
100BaseTx has been chosen for use with the Network 21 Project because it
uses the fewest Category 5 cable pairs and is the most widely
implemented version of Fast Ethernet. Most of the common adapters
available today support 100BaseTx; however, it is still a good idea to
check the technical specifications for each particular card.
Recommendations for Appropriate Use of Fast Ethernet
Network 21's switched 10 Mbps connections to the desktop will provide a
significant performance improvement over shared 10 Mbps connections in use
on campus today. In performance tests comparing switched 10 Mbps with
shared 100 Mbps networks, the switched 10 Mbps networks performed about
as well as the shared 100 Mbps networks under general conditions.
For most single-user microcomputer workstations, Fast Ethernet is not the
most appropriate solution from a performance and cost point of view. Even
high-end machines cannot sustain a full 100 Mbps transfer. For example,
using 100BaseTx to transfer files in a RAM-based filesystem using FTP
between two Sun SPARCstation 20s with dual CPUs produced a transfer rate
of 5 MByte per second or about 40% of available bandwidth. Even though
this is beyond the bandwidth provided with dedicated 10 Mbps, it
demonstrates that most computers are not yet ready to push the limits of
Fast Ethernet.
For most network devices, the switched 10 Mbps Ethernet to the desktop
should be a vast improvement over current connections and provide for
enough growth for one or two years. Most departments should try the
dedicated 10 Mbps connections before deciding to bear the cost of
upgrading to Fast Ethernet. Like those of all technologies, the costs
associated with Fast Ethernet will be reduced over time.
Under certain situations, Fast Ethernet can improve on the 10 Mbps
connections provided by the Network 21. In some cases, the added expense
associated with upgrading the Network 21 infrastructure for Fast Ethernet
can be justified. A machine that meets the following criteria could be a
candidate for Fast Ethernet:
- High bandwidth file server
- Server supporting more than 20 workstations
- Unix NFS server supporting more than 5 workstations;
- Visualization or multimedia workstation (ie. GIS, CAD/CAM, imaging) which accesses files across the network;
- Video or multimedia server;
- Workstation clustering (ie. VMS clusters).
The following machines are capable of using Fast Ethernet effectively:
- Pentium with a 100BaseTx PCI network card
- Unix (or other high-end) workstation with high bandwidth network card
- Other machines with high speed busses capable of containing a high
bandwidth network card.
Those departments wishing to keep their options open for a simple upgrade
path to Fast Ethernet may purchase 10/100 cards for a
nominally higher cost. 10/100 cards have the capability to connect to both
10BaseT and 100BaseTx Ethernet ports with minimal changes to the
workstation. Department servers are wise choices for installation of 10/100
cards.
Since some computer busses are not capable of utilizing the increased
100 Mbps bandwidth, Fast Ethernet cards should be installed only in
machines with high-speed local busses (or the equivalent). By installing
10/100 cards in new machines and as convenient in existing machines,
departments may prevent future downtime. However, convenience must be
balanced with the anticipated lower price and higher performance of the
next generation of 10/100 cards.
Benefits of Fast Ethernet
- Fast Ethernet provides higher-speed networking for machines that
require it
- 100BaseTx is an economical way to provide higher performance networking
capabilities.
- Fast Ethernet uses the same low-level Ethernet techniques, allowing
economical implementation and easier interoperability with existing
Ethernet.
Limitations of Fast Ethernet
- Fast Ethernet costs more than similar 10 Mbps Ethernet (100BaseTx
vs 10BaseT, 100BaseFx vs 10BaseFL).
- Many network cards and/or software device drivers cannot yet utilize Fast
Ethernet, resulting in lower than expected performance.
- Most existing workstations cannot sustain 100 Mbps transfer rate.
- Fast Ethernet allows only two repeaters between workstation and IDF,
as opposed to four for standard 10BaseT.
Summary
For machines and applications which require additional bandwidth and have
the hardware to support Fast Ethernet (100BaseTx in particular), it is a
cost-effective means of obtaining extra performance. However, currently
very few machines, cards, and applications require or can fully utilize
Fast Ethernet.
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- Editing corrections to: Clare Homan
Last modified 10/29/98