Gigabit Optical Ethernet
Background Paper
Ryan Baldwin
Dave Gewertz
Geoff Sizemore
1/29/02
Gigabit Ethernet
Dr. Robert Metcalf invented the basis for Ethernet in the mid-1970’s at the Xerox Palo Alto
Research Center. The idea was that the office of the future would have a personal computer and
would then need a means of communication with a mainframe and other computers in the office.
The Xerox Alto was to be the first desktop computer in the “office of the future.” The initial design
ran at 3Mbps and was known as “Experimental Ethernet.” The formal specifications were agreed
upon by the three main players in the industry: Xerox, Intel, and DEC. The original, “Experimental
Ethernet”, was expanded to 10-Mbps and standardized by IEEE in a production quality form.
Gigabit Ethernet is designed on standard Ethernet frame format so that its integration
would be as seamless as possible. Ethernet and Fast Ethernet will therefore require no frame
translation to interface with Gigabit Ethernet.
Figure 1. Ethernet Address Fields
When Xerox first designed the Ethernet protocol, a type field was required. In the IEEE 802.3
specifications the type field has been changed to a length field, telling the receiver how many bytes
are in the packet. This leaves the protocol distinction in the data portion of the packet. There are
three main layers that the Logical Link Control (LLC) and the Media Access Control (MAC) must
communicate with in order to interface with the upper layers of the Open System Interconnect
(OSI) stack. The first is the destination service access point (DSAP). This layer is responsible for
providing a unique identifier within the station and the upper layers of the OSI stack. The second is
the source service access point (SSAP), which does the same thing as the DSAP, only with the
source address this time. If every protocol obeyed these OSI stack protocols, then this would be
enough. Since this is not the case, the LCC must carry even more information so that the packet is
interpreted by the end user correctly. The most common protocol that falls into this category is
Internet Protocol, IP. This problem is addressed by implementing a protocol called Subnetwork
Access Protocol, or SNAP. A SNAP is recognized when the DSAP and the SSAP addresses are set
to “0 x AA”. When “0 x AA” is seen, then a 5 byte long SNAP header follows. IEEE assigns what
the first 3 bytes are to be depending on the organization code. The following 2 bytes set the type
value, just like the original Ethernet specifications.
IEEE 802.3z
Recent technologies, such as ATM and FDDI, have commanded a large market share;
pushing Ethernet out of the backbone of the network and confining it to the last hop. ATM and
FDDI technologies delivered higher speeds to keep up with the demands for greater throughput and
higher bandwidth applications. The introduction of Gigabit Ethernet is quickly changing this trend.
The simplicity and reliability of Ethernet is ideal, if it can support the bandwidth needed. IEEE
803.2z is the standardizing body for Gigabit Ethernet. The standardization effort began in mid1995, with a push to have things done as soon as possible. The sense of urgency if felt by designers
that want to design a product that will meet the industry standard. A rough draft was completed in
1997 and then in June of 1998 the technology was standardized by IEEE. The real value to the end
user is that Gigabit Ethernet is built off the Ethernet platform, and it will work seamlessly with
Ethernet and FastEthernet.
Gigabit Ethernet Hardware Overview
In order to better comprehend the hardware technology behind Gigabit Ethernet, the
physical layer must be understood. Gigabit Ethernet technology was created in 1998 with the
convergence of the IEEE 802.3 Ethernet and ANSI X3T11 FiberChannel standards. Figure 2 shows
the features from both standards that were combined in the IEEE 802.3z standard. One advantage
of using the IEEE Ethernet standard is that from the data layer up, Gigabit Ethernet is compatible
with the older and widely deployed Fast Ethernet. The new IEEE 802.3z Gigabit Ethernet standard
also allows use of full and half duplex carrier sense multiple access collision detect (CSMA/CD).
The advantages of Gigabit Ethernet are flexibility in integration with Fast Ethernet devices, as well
as taking advantage of greater bandwidth and lower errors associated with fiber.
To allow configuration flexibility, there are four mediums for Gigabit Ethernet
communication; two purely fiber optic, and two integrated with copper. The 1000BaseSX
standard works with multimode fiber (using short- or long-wave lasers), and the 1000BaseLX works
with single mode fiber (using long-wave lasers). The copper based standards are 1000BaseT (using
CAT5 twisted pair copper) and 1000BaseCX (using shielded 150 ohm copper cable). Our research
will focus on the fiber-based standards, since they provide high bandwidth over a much longer
distance than copper based Gigabit Ethernet (see figure 3).
The quickest and easiest way to integrate Gigabit Ethernet in a fast Ethernet
Figure 3. The four Gigabit Ethernet Standards
environment is to replace Fast Ethernet switches with Gigabit Ethernet switches, and install Gigabit
Ethernet NIC’s (network interface cards) in the servers. Since Gigabit Ethernet is backwards
compatible with Fast Ethernet, these are the only changes in the network necessary that will result in
greater bandwidth availability. Our research will focus on designing a NIC for Gigabit Ethernet that
is both cost effective and robust.
We will be using the Intel Pro/1000 XF Server adapter in our test bed. This adapter uses the
1000BaseSX standard with multimode 62.5 - 50um fiber. It fits into the PCI slot of the server’s
motherboard, which is important because the server bus must be able to handle 1 Gb/second data
rates. Table 1 shows throughput of several computer bus types, and demonstrates why Gigabit
Ethernet NIC’s must use a PCI bus.
Table 1. Data Throughput for different server bus types
Bus Type
Throughput
ISA
64 Mb/s
EISA
264 Mb/s
MCA
320 Mb/s
PCI (32 Bit, 33 MHz)
1056 Mb/s
PCI (32 Bit, 66 MHz)
4224 Mb/s
The most expensive component of the NIC is the transceivers. The transceiver contains the
transmitting laser, receiver, optical subassemblies, electrical subassemblies, and the packaging
housing. The 1000BaseSX standard uses shorter wavelength transmitters (770-860nm) than
1000BaseLX (1270-1355nm). Shorter wavelength transmitters are less expensive than higher
wavelength transmitters, but the signals can’t travel as far (500m vs. 3km). We will be looking at the
Agilent HFBR-53D5 family of 850nm VCSEL (vertical cavity emitting lasers). Picolight is another
manufacturer of low cost 850 nm transceivers, and Sumitomo Electric Industries is a producer of
photodiode receivers that offer yet another alternative.
To improve on error rates, we will continue the work of past groups in converting the
transceiver’s single ended input into a differential input. The single ended input has a high error rate
because it uses a fixed reference to the input signal. Differential inputs will reduce errors because the
receiving and transmitting signals will use inverted signals as reference points, thus eliminating
errors. We will use Maxim evaluation boards to test Maxim amplifiers and the Agilent VCSEL in
creating a lower cost, lower error NIC with differential inputs.
Fiberoptics in the LAN
Before the advent of Gigabit Ethernet, copper Ethernet cable could be trusted to provide
the necessary performance standards to meet the requirements of the application. Data rates have
reached a point where copper has become an unsatisfactory medium for transmission. As a result,
fiberoptics, or the transmission of data using light waves, is starting to become the method of choice
for end-user communication needs. These needs are centered on the fact that computer networking
has become almost a necessity in the today’s world. Technology has limited how widespread
fiberoptics have been deployed into current computers networks. Originally, fiber was only used in
the long haul usually between wide area networks and metropolitan area networks. It was during
that time that 80% of the bandwidth existed on the backbone of the Internet. The expansion of
local area networks (LAN) coupled with the ever-growing bandwidth expectations has necessitated a
faster mode of communication, i.e. Gigabit Ethernet.
Fiber vs. Copper
Most LAN systems currently rely on unshielded twisted-pair (UTP) technology. Engineers
are more familiar with the phrase Category 5, the most widely used standard of UTP cable. The
distance restrictions of UTP cables are the driving force behind the use of fiber optic cables in the
LAN. Table 2 shows a comparison between the current Ethernet technologies and their relative
distance restrictions. As one can see, the current limit on copper is 25 meters for Gigabit Ethernet.
New technologies discussed in the previous section are beginning to broaden the capabilities of the
UTP cable, but fiber is still the predominant leader in terms of performance. Unfortunately distance
restrictions are not the only thing hampering copper cabling. Fiber is much thinner and lighter than
copper cable, and fiber cable is immune to the electromagnetic noise that is created by most pieces
of electronic equipment; copper is not. Fiber does have its drawbacks. It is expensive to
manufacture and difficult to maintain because of the fragile nature of the cable.
Table 2. Fiber Vs. Copper (UTP)
Ultimately the cost of fiber comes down to the connector, which can vary in design. ST, SC,
and FDDI connectors are the most prominent types. ST connectors are for fiber to the desk, while
SC is usually used for long haul fiber. FDDI connectors work with a Fiber Data Distributed
Interface, a token ring technology operating at 100 Mb/s at distances up to 2000 meters. The
expensive part, a precision ceramic ferrule, in each connector aligns the laser and protects the fiber.
(Fiber optic Tech)
Making A Fiberoptics Link
Making a fiber optic link involves three main components: input, output, and
optoelectronics. The input, or the receiver is implemented with a photodetector. A photodetector
detects light waves of a certain frequency and then passes the digital electrical information to the
optoelectronics of the circuit. The output (transmit) of any fiber optic link is made up of a laser
diode and driver. Together with the optoelectronics, they convert an AC or DC signal into a light
wave that can be transmitted over fiber optic cable.
Much like information over a copper cable, data over fiber consists of digital bits of
information; each bit being a ‘1’ or ‘0’. These bits are put together into packages called packets.
Packets make up the majority of network traffic. In order to transmit packets over fiber, the DC
signals of 5 volts and 0 volts must first be converted into light waves. These operations are handled
by the laser diode. Once an electrical current hits the driver, it is passed to the diode. Depending
on the state of the bit, either a pulse is emitted or no pulse is emitted. These pulses are detected at
the other end by the photodetector and the bits are passed into the remaining portion of the circuit
to be analyzed. Although this is a very basic understanding of the process, we intend to go into
more detail for the design portion of our research. This further discussion will involve filters,
amplifiers, error correction, and possible problems that we may encounter when designing the
receiver portion of the project.
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