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Ethernet Overview |
Gigabit LAN Standards Gigabit Ethernet Leads the Way Over Fiber
Specifications
Bus-Standard Ethernet (Coax)
10BASE5
Twisted-Pair Ethernet (Unshielded Twisted Pair):
10BASE=T, UTP
Bus-Thin Net Ethernet (Coax):
10BASE2
Star-Fiberoptic Ethernet
FOIRL or 10BASEN-FL
There are two versions of Ethernet over fiberoptic cable, meeting the older FOIRL (Fiberoptic Inter-Repeater Link) and the more recent 10BASE-FL standards.
Ethernet is the most widely used network topology. You can choose between bus and star topologies, and coaxial, twisted-pair, or fiber optic cabling. But with the right connective equipment, multiple Ethernet-based LANs (local area networks) can be linked together no matter which topology and/or cabling system they use. In fact, with the right equipment and software, even Token Ring, Apple Talk, and wireless LANs can be connected to Ethernet.
The access method Ethernet uses is CSMA/CD (Carrier Sense Multiple Access with Collision Detection). In this method, multiple workstation access a transmission medium (Multiple Access) by listening until no signals are detected (Carrier Sense). Then they transmit and check to see if more than one signal is present (Collision Detection). Each station attempts to transmit when it "believes" the network is free. If there is a collision, each station attempts to retransmit after a preset delay, which is different for each workstation.
Collision detection is an essential part of the CSMA/CD access method. Each transmitting workstation needs to be able to detect that simultaneous (and therefore data-corrupting) transmission has taken place. If a collision is detected, a "jam" signal is propagated to all nodes. Each station that detects the Collision will wait some period of time and then try again.
The two Possible topologies for Ethernet are bus and star. The bus is the simplest (and the traditional) topology. Standard Ethernet (10BASE5) and Thin Ethernet (1OBASE2), both based on coaxial cable systems, use the bus.
In this one-cable LAN, all workstations are connected in succession (a "bus" arrangement) on a single cable. All transmissions go to all the connected workstations. Each workstation then selects those transmissions it should receive, based on the address information contained in the transmission.
In a star topology, all attached workstations are wired directly to a central hub, which establishes, maintains, and breaks connections between them (in the event of an error). The advantage of a star topology is that it is easy to isolate a problem node. The disadvantage is that if the hub fails, the entire system is compromised.
Twisted-Pair Ethernet (10BASE-T), based on unshielded twisted pair, and Fiberoptic Ethernet (FOIRL and 10BASE-FL), based on fiberoptic cable, use the star.
Ethernet Topologies
Switched Ethernet
Switched Ethernet relies on centralized multiport Switches to provide physical link between multiple LAN segments. Inside each intelligent S" high-speed circuitry supports wire-speed virtual connections between all the segments, for maximum bandwidth allocation on demand. Adding new segments to a switch increases the aggregate network speed while reducing overall congestion, so Switched Ethernet provides superior configuration flexibility. It also gives you an excellent migration path from 10- to 1 00-Mbps Ethernet, since both segments can often operate via the same Switch.
Benefits of Switched Ethernet
It is a cost-effective technique for increasing the overall network throughput and reducing congestion on a 10-Mbps network. Other than the addition of the switching hub, the Ethernet network remains the same the same network interface cards, the same client software, the same LAN cabling.
100BASE-T (IEEE 802.3u)
100BASE-T retains the familiar CSMA/CD media access technique used in 1 0-Mbps Ethernet networks. It also supports a broad range of cabling options: two standards for twisted pair, one for fiber. 100BASE-TX supports 2-pair Category 5 UTP or Type 1 STP cable. 100BASE-T4 uses 4-pair Category 3 or 4 cable. And 100BASE-FX allows fiber optic links via duplex multimode fiber cable.
Benefits of 100BASE-T
It retains CSMA/CD so existing network management systems don't need to be rewritten. It can easily be integrated into existing 10-Mbps Ethernet LANs so your previous investment is saved.
100VG (IEEE 802.12)
100VG uses an encoding scheme called Quartet Signaling to transmit data simultaneously over all four pairs in the network cable, so it achieves a full tenfold increase in transmission speeds over 1 OBASE-T. It also replaces the CSMA/CD media access control protocol with Demand Priority to optimize network operation and eliminate the overhead of packet collisions and recovery. Demand Priority works like this: The hub directs all transmissions, acknowledging higher-priority packet requests before normal-priority requests. This effectively guarantees bandwidth to time-sensitive applications like voice, video, and multimedia applications.
Benefits of 100VG
It uses a transmission frequency very similar to traditional Ethernet, and works on any conventional cabling system (Category 3, 4, or 5 UTP, Type 1 STP, and fiber optics) and uses the same connectors. In addition, 100VG may soon support Token-Ring networks-a potential advantage over its rival standard 100BASE-T.
ATM
Asynchronous Transfer Mode (ATM) is a cell-based fast-packet communication technique that supports data-transfer rates ranging from sub-Tl speeds (less than 1.544 Mbps) up to 10 Gbps. Like other packet-switching services (Frame Relay, SMDS), ATM achieves its high speeds in part by transmitting data in fixed-size cells, and dispensing with error-correction protocols. Instead, it relies on the inherent integrity of digital lines to ensure data integrity.
Benefits of ATM
Networks are extremely versatile. An ATM network can be treated as a single network, whether it connects points in a building or across the country. Its fixed-length cell-relay operation, the signaling technology of the future, offers more predictable performance than variable-length frames. And it can be integrated into an existing network as needed, without having to upgrade the entire LAN.
The Advantages of 100BASE-T
100Base-T (IEEE 802.3u) |
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| Variations of This Standard | 100BASE-TX |
| Supported Cable Type | 100BASE-TX:
5 (2-Pair) IBM Category Type 1 (2-Pair) 100BASE-T4: Category 3 or 4 (4-Pair) 100BASE-FX: Duplex Multimode or Single-Mode Fiber |
| Maximum Cable
Segments (HUB-TO-NODE) |
100BASE-TX or
T4: Category 3, 4, or 5-1 00 m IBM Type 1-100 m 100BASE-FX: MultimodeFiber-2km,Single-Mode-l0km (HUB-TO-NODE) 100BASE-TX or T4: Category 3, 4, or 5-100 m IBM Type 1-100 m 100BASE-FX: MultimodeFiber-2km,Single-Mode-l0km |
| Best Applications | Backbone
utilizing Ethernet switches to provide increased
throughput Small to medium workgroups using applications (i.e.-. CAD, CAM) which output huge data files. |
By Erica Roberts
Gigabit LANs at
a Glance
Gigabit LAN Standards, It Takes Two to Tangle
Do 1-Gbit/s versions of 100Base-T and 100VG mean double
trouble for net managers?
Chalk it up to dèjà vu all over again. In 1993, when the IEEE
couldn't make the call on a single 100-Mbit/s LAN transport, it
"decided" on two: 100Base-T and 100VG-AnyLAN. Now the
standards body seems to think that playing doubles is a smart
bet: It's getting ready to define 1-Gbit/s versions of both
high-speed LAN specs.
At first glance, two new gigabit standards look like a good deal.
Both standards have the horsepower needed to turbo-charge
overtired LANs. Both run over copper and fiber (naturally enough,
since they're both based on parts of the ANSI Fibre Channel
spec). Both make it possible to build so-called scalable
Ethernets. And both are relatively cheap. Vendors are already
hinting at prices of $1,500 per node, even though products aren't
slated to ship until next year.
Too bad it all could add up to double trouble for net managers.
Two standards are bound to lead to compatibility questions.
What's more, neither spec guarantees delivery of time-sensitive
voice or video. And for all the big talk about big bandwidth, it
may not be possible to run either standard over copper without
severely cutting speed or limiting the distance between nodes.
Given these shortcomings, "it's unlikely that gigabit
Ethernet will ever become a dominant desktop technology,"
says Paul Sherer, vice president of technology development at
3Com Corp. (Santa Clara, Calif.). Nevertheless, it could make its
mark on the backbone--as a fat fire hose for async data.
But does the world really need two 1-Gbit/s Ethernet standards?
(Did it really need two 100-Mbit/s LAN specs?) At this point,
100Base-T and 100VG vendors have too much invested in their
technologies to let an opportunity like this slip by.
They better move fast. ATM is an obvious--and formidable--rival
for the backbone. Prices for ATM (asynchronous transfer mode) hardware are coming down even as we speak, and gigabit ATM gear
is on the way. So even if the IEEE manages to push through both
very-high-speed specs in record time, ATM could still leave them
in the dust.
DIVIDE AND STANDARDIZE
The 802.12 working group has already received an IEEE Project
Authorization Request (PAR) to define a gigabit version of 100VG.
Not surprisingly, the push is headed up by Hewlett-Packard Co.
(HP, Palo Alto, Calif.), the driving force behind the original
AnyLAN. Joining HP are Compaq Computer Corp. (Houston), Texas
Instruments Inc. (Dallas), and the Semiconductor Division of
Motorola Inc. (Austin, Texas). The spec should receive "full
and final approval" by the summer of 1997, according to
Patricia Thaler, chair of the 802.12 working group and principal
engineer for LAN architecture and standards with Hewlett-Packard
Co. Roseville Networks Division (Roseville, Calif.).
Like its 100-Mbit/s predecessor, the gigabit version of 100VG
will handle both Ethernet and token ring frames. It also will
boast several new features, including burst mode and redundant
links between repeaters. The plan now is to define transmissions
of 500 Mbit/s and 1 Gbit/s, with 4 Gbit/s possibly coming in the
future. The group also is working on a full-duplex version of the
spec.
The 802.3 working group is still waiting to receive its PAR for
the gigabit version of 100Base-T. A task force that includes Bay
Networks Inc. (Santa Clara, Calif.), Cisco Systems Inc. (San
Jose, Calif.), Packet Engines Inc. (Union City, Calif.), and 3Com
Corp. (Santa Clara, Calif.) is working on a proposal that will be
presented at the plenary meeting of the 802 LAN/MAN Standards
Committee this month.
"Assuming we get through the politics of the plenary
meeting, we could have a standard out as early as 1998,"
says Brian MacLeod, director of business development for Packet
Engines.
The gigabit 100Base-T group is looking to keep things simple. It
will retain the minimum and maximum frame sizes defined by the
802.3 spec and accommodate full-duplex communications for
point-to-point switched connections. It also may incorporate new
developments like flow control as they are added to 100Base-T.
SHARE TACTICS
Both specs will retain the MAC protocols employed by their
100-Mbit/s forerunners. Thus, gigabit 100Base-T will use CSMA/CD
(carrier-sense multiple access with collision detection), while
gigabit 100VG will go with the more recently developed
demand-priority mechanism.
CSMA/CD and demand priority are both shared-media schemes, which
means sending stations contend for bandwidth. CSMA/CD uses a
back-off algorithm to prevent more than one device from sending
information at a time. Demand priority relies on a round-robin
polling sequence to give each network node the opportunity to
transmit.
Shared-media schemes suffer from a simple shortcoming: As the
number of nodes on the network rises, the bandwidth available to
each drops. To make sure that the new gigabit networks don't wind
up suffering from bandwidth starvation, vendors say they'll
deliver switched versions of their technologies.
RECYCLED FIBRE
For all the apparent differences between the two gigabit specs,
there's one underlying similarity: Both rely on Fibre Channel's
physical (PHY) layer as their transmission technology. For
gigabit 100Base-T, that means mapping the 802.3 MAC layer to the
PHY, so Ethernet packets can be carried via Fibre Channel
encoding. For gigabit 100VG, both the 802.3 and the 802.5 (token
ring) MAC layers must be mapped.
"The Fibre Channel PHY is a good piece of engineering,"
comments Thaler. "Why reinvent it?" Recycling Fibre
Channel in this way reduces time to market. What's more, it
should help bring down prices. Gigabit gear will use Fibre
Channel transceivers; as silicon shipments climb, chip prices
should plummet. It's still very early in the game, but gigabit
over copper could cost between $1,500 and $2,000 per switched
connection (which includes the price of the adapter and the
switch port). Fiber is expected to cost $3,000 to $4,000.
Fibre Channel aficionados view the gigabit vendors' recycling
plans as a tacit acknowledgment of the merits of their
technology. They also argue that if a little bit of Fibre Channel
is good, all of it would be even better. Scot Ruple, director of
product marketing for Emulex Corp. (Costa Mesa, Calif.), expects
to see Fibre Channel take off for storage and backbone
applications: "Gigabit Ethernet is our chance to slip
through the back door and onto the LAN." Thaler disagrees,
saying Fibre Channel's MAC layer would add too much overhead to
LAN applications.
THE CABLING CATCH
Fiber Channel's PHY layer defines transmissions over fiber and
copper. Fiber is more expensive, but it shouldn't have any
trouble handling gigabit data. Multimode fiber should be able to
carry gigabit transmissions to 500 meters; with single-mode
fiber, that distance should reach 2 kilometers.
But net managers who expect to see those sorts of speeds and
distances over copper are going to be disappointed. It's a
question of physics. Electromagnetic radiation increases in
proportion to the speed at which data is carried and the length
of the cable. In order to stay within the EMI limits set by the
FCC for UTP (unshielded twisted pair), net managers must reduce
data rates or shorten cabling runs (or both).
That leads to two immediate questions: How slow? How short?
Proponents of gigabit 100Base-T say they can hit 1 Gbit/s over
UTP by locating the switch within 50 meters of the end-station.
And that's an optimistic estimate. Some vendors think things
could get even tighter. Remember, gigabit 100Base-T is a CSMA/CD
scheme. As the speed of a CSMA/CD network increases, so does the
likelihood of collisions. In effect, this means that the length
of the cabling runs are inversely proportional to the bit rate.
That's why some gigabit 100Base-T backers are saying link
distances will likely be held to 25 meters on Category 5 UTP.
Even with these reduced runs, getting 1 Gbit/s over UTP is going
to be a good trick. The Fibre Channel spec is silent when it
comes to unshielded cabling. It does, however, call for a top
speed of 100 Mbit/s over shielded twisted pair.
Packet Engine's MacLeod has an answer: "Technology has moved
ahead since the Fibre Channel specs were ratified." He's
hopeful that new connectors and transceivers will make it
possible to push Ethernet frames at higher speeds over longer
distances on Category 5.
Where does all this leave gigabit 100VG? Its backers say they can
hit 500 Mbit/s over 100-meter UTP runs. But to do so, they're
developing a new physical layer and transmission scheme expressly
for four-pair UTP. They'll use the Fibre Channel PHY strictly for
fiber. HP recently submitted a new physical layer to the 802.12
working group.
BACKBONE BONUS
Questions about copper are one of the reasons that vendors are
talking up gigabit 100Base-T and 100VG as high-speed fiber links
for point-to-point applications. On the backbone, the gigabit
standards are being pitched as an alternative to FDDI or ATM.
"As fast Ethernet picks up momentum over the next few years,
we're going to need bigger pipes to servers and for interswitch
connections," says Peter Tarrant, vice president of product
management at Bay Networks.
What's more, since both gigabit specs field conventional Ethernet
frames, they give net managers a way to build
"scalable" networks. These configurations run at
different speeds in different spots but use the same frame format
end to end. For instance, a company could run 10-Mbit/s Ethernet
to the desktop, 100-Mbit/s 100Base-T between departments, and
gigabit 100Base-T on the backbone--all without having to install
pricey, performance-impairing equipment that converts between one
format and another. This one-frame-fits-all approach isn't
possible with FDDI or Fibre Channel.
THE ATM ARGUMENT
Backers of the new gigabit LANs also argue that their standards
are out ahead of ATM when it comes to price and performance. They
point out that a 1-Gbit/s fiber connection costs $3,000 to $4,000
per switch port (including adapter). A 622-Mbit/s ATM connection,
in contrast, today costs $15,000 per port (not counting the
adapter).
But this is a good case of the figures not telling the entire
story. Only one LAN vendor offers 622-Mbit/s ATM. Prices are sure
to fall as competition works its market magic.
"By the time gigabit Ethernet arrives many of the ATM issues
will be resolved," says Esmeralda Silva, LAN analyst with
International Data Corp. (Framingham, Mass.). "ATM prices
are already down. I see them dropping even more over the next few
quarters, with bigger savings in 1997."
And ATM has a very strong argument in its favor--advanced
multimedia capabilities. Gigabit 100Base-T does nothing to
guarantee the delivery of time-sensitive traffic. Gigabit 100VG
lets net managers assign two priority levels to traffic. But if
the network gets busy there's still a good chance that voice and
video won't arrive on time.
HOLD THE LINE
Given all of the outstanding issues, it's not surprising that
some net managers are taking a wait-and-see attitude. "With
gigabit 100Base-T, it's going to be very interesting to see what
the actual performance is," comments John Scoggin,
supervisor of network operation with Delmarva Power and Light
(Newark, Del.). He's been testing gigabit Fiber Channel but
notes, "We're not getting anywhere close to a gigabit over
it."
Scoggin knows that it's easy to play a waiting game when he
doesn't need the throughput. But net managers won't be able to
put off their decisions forever. "Right now I can't imagine
what I'd need gigabit Ethernet for. But 10 years ago I couldn't imagine networks running faster than T1."
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