3
Bryan Chavez, Patrick Cleary, & Kevin Parker
DukeUniversity
PrattSchool of Engineering
Department of Electrical Engineering
Last Modified: April 25, 2005
I. Abstract
Gigabit Ethernet is a powerful and widely-used method for networking. It enables a large number of users to interface at a high-speed on the network at the same time, all without interfering with each other. This particular project focuses on the design and construction of an “optical transceiver module,” which represents the Physical Media Dependent layer of a Gigabit Ethernet. To complete this project, background research will be conducted to determine the specifications needed to be compliant with official IEEE standards. Next, the different optical and electrical components for the construction of the transceiver will be researched and ordered based on performance and cost issues. At the same time, soldering and circuit design skills will be practiced and honed to perfection. Finally, a board layout will be designed, and after the fabrication of the board, the transceiver will be constructed and tested. After the completion of a working transceiver, the results will be analyzed and compared to known transceivers being sold by established companies.
II. Introduction
a. Background Information
Ethernet is a local area network that allows for the transmittal of information over fiber optic cables. Basically, Ethernet works because of a single cable that connects to each and every machine that is part of the network. Each machine can therefore share information with any other machine on the network and is not affected by the addition or subtraction of other machines to or from the network. This ability makes large systems such as a university network possible, because numerous computers from many different locations can all access the network without interfering with each other [1].
The first Ethernet was created by Bob Metcalfe in 1973 at Xerox Corporation’s Palo AltoResearchCenter. Metcalfe developed this invention while attempting to connect a computer to a printer. This original Ethernet was constructed using a coaxial cable and could only transmit information at a rate of 3 Mbps. In 1980, three companies—Xerox, Intel, and Digital Equipment Corporation—all joined forces to create the 10-Mbps Ethernet Version 1.0 (also known as the DIX standard), and the speed and importance of Ethernet began to increase exponentially. Soon after, the IEEE stepped forward and developed an official standard for network technologies. Because it was commissioned in February of 1980, this standard is now called 802.3 standard [1], with the 3 referring to the specific subcommittee of the 802 group that created the standard for a network that was similar to the DIX Ethernet [2] [3].
Improvements continued to be made on the original implementation of Ethernet, and the speed at which it could transmit data increased greatly. In 1998, IEEE developed the Gigabit Ethernet, which could operate at 1 Gbps and is widely used today. In addition to the speed improvements in modern Ethernet, the range over which networks can now extend is staggering. In the early Ethernets developed in the 1970s, the entire network was usually limited to a single building because of restraints in the length of the cables. In comparison, a modern Gigabit Ethernet using multimode fiber has a range of 550 meters, while one using single-mode fiber has a range of 5000 meters [4] [5].
The way in which Ethernet systems handle transmissions is also very interesting. This process is referred to as CSMA/CD, an acronym which stands for carrier-sense multiple access with collision detection. The phrase “multiple access” describes the fact that many devices are connected to a given Ethernet network and that each of them is aware of all transmissions sent by any other device on the network. Since there is only one medium—typically a fiber-optic cable—available for all of these devices to use, Ethernet must contain the ability to prevent one device’s transmission being interfered by other transmissions. The solution, known as “carrier-sense,” involves a protocol in which each device checks the network’s medium to ensure that no other device is transmitting information. If there is a current transmission, then the device will wait until it is finished before beginning its own transmission. At times, two or more devices might attempt to begin a transmission at the exact same instant. When this happens, each device recognizes this fact based on the fact that its own transmission returns with interference from the other transmission, a process known as “collision detection.” When a collision is detected, each transmitting device halts its transmission and then waits a random amount of time before attempting to transmit again. The amount of waiting time must be random so that the two devices do not continue to produce collisions as they attempt to transmit over and over again [3].
The above description, however, illustrates a very basic Ethernet network, and numerous modifications have been necessary to allow the system to be useful with a large number of users, such as a college campus. The process of “segmentation” is one method that allows many devices to be connected to the same network without interfering with each other too often. In segmentation, a single Ethernet medium is split into multiple segments, so that multiple transmissions can be made at the same time. To enable all of the devices on the same network to continue to interact with each other, “bridges” exist to connect the different segments together. These bridges transmit and receive information just like other devices on the network, and they follow all of the rules of collision detection and carrier-sense. However, the only information that a bridge transmits is the echo of a transmission from an adjacent segment [3].
b. Project Overview
A Gigabit Ethernet contains many layers of abstraction, but the only layer that was constructed in this project and addressed in this paper is the Physical Media Dependent (PMD) Layer of the Physical Layer. Specifically, the scope of this project involves the design and creation of an “optical transceiver module” that is IEEE 802.3z standard compliant for an optical Gigabit Ethernet. The optical transceiver module can be broken into two components: the transmitter (Tx) and receiver (Rx). In the transmitter, a digital signal is input to the Maxim 3287 chip, which is called the laser driver. The laser driver converts the digital logic to a current which must be large enough to drive an optical transmission device called a VCSEL. The VCSEL transmits an optical signal to a corresponding optical receiver device which is called a ROSA. These two optical devices are connected by a fiber-optic cable which also attenuates the signal. The ROSA actually contains two devices: an optical component called a photodiode (PD), which converts the optical signal from the Tx portion of the transceiver to an electrical current signal, and an electrical component called a trans-impedance amplifier (TIA), which converts the electrical current signal from the PD to a differential voltage. The differential output from the ROSA is then sent to another amplifier, which is called the limiting amplifier and is implemented using the Maxim 3264 chip. This limiting amplifier transforms the differential output from the ROSA to a digital signal—identical to the one input to the laser driver chip—which becomes the output of the transmitter [6].
To accomplish this task, a number of preliminary steps have been taken to ensure that each group was properly prepared for the construction and implementation of the transceiver. Firstly, a number of presentations were covered by the instructors over the first half of the semester, serving as introductions to the various optical concepts important to the project. At the same time, the group both organized itself into a management structure to appropriately divide the work of the project and also projected the time-table of the different phases of the project so that a guide could exist over the course of the semester. Next, the transceiver link was planned both electrically and optically and was incorporated into the “optical link budget” to ensure that electrical and optical signals within the transceiver circuit are within the constraints of the circuit devices. In order to practice soldering and to gain more familiarity with the physical components of the project, a test transmitter board was constructed and evaluated using only electrical components. With the knowledge gained from the presentations and from the hands-on work, the transceiver link was then planned. This process included the research and ordering of the proper parts, the design of the transmitter and receiver circuits, and the layout of the circuits and optical devices on a PCB board [7].
After the aforementioned preparation, ordering of supplies, and circuit design process, the group was ready to construct and test the entire optical transceiver module, and the remaining portion of the semester was utilized to accomplish this task. The group was provided with a $350 budget for the completion of the project. The funds in this budget were for the components to be used in the transceiver and for the fabrication of the PCB board, as well as any shipping costs incurred in the process.
c. Optical Background
The two optical devices that were incorporated in this project are the Vertical Cavity Surface Emitting Laser (VCSEL) and the photo-detector or photodiode (PD), which is included in the Receiver Optical Sub-Assembly (ROSA).
A VCSEL is made up of a double heterostructure compound semiconductor that is forward biased. Carriers generated by the forward bias are injected into the active region of the semiconductor, producing light at varying wavelengths. Mirrors set at different distances from the cavity of the VCSEL reflect the light into the cavity, combining all of the light into one wavelength that can be output as a circular symmetric laser through a fiber-optic cable. Lasers are utilized in Ethernet applications rather than LEDs for a number of reasons. When lasers are powered with an input current greater than a threshold current, they experience an exponential increase in optical power output, rather than the linear relationship between input current and output power seen in LEDs. In addition, lasers are much more efficient in transmitting light to a fiber because of its divergence angle of 6°, compared to the 90° divergence angle of LEDs [8].
Photodiodes are typically made up of a P-i-N compound semiconductor. The semiconductor is doped as a double heterostructure and is operated in reverse bias. The entering light from a fiber-optic cable passes through the i portion of the semiconductor, which is the smallest region and serves as the absorbing area of the photodiode. An electric field caused by the reverse bias then forces the free carriers to the output of the photodiode, thereby creating a current. In order to ensure that a photodiode will produce the desired performance, the device is designed so that a high efficiency, or responsivity, and a low capacitance exist [9].
d. Organizational Management
As suggested by the instructors, the major tasks of the project were divided so that one member of the group assumed the primary responsibility for ensuring that a particular component of the project was accomplished. In addition, another member of the group was assigned to each task in a secondary role, both to assist the lead team member and to assume responsibility in case of emergency. In no way, however, was this structure meant to burden one team member with the entire responsibility of a particular task, and an important goal of the management structure was that each team member would be able to participate in each phase of the project. The assignment of roles is specified below in the following table.
Task / Task Leader / Primary SupportCoordination and Budget / Pat / Kevin
Optical Link Design and Budget / Kevin / Bryan
Board Construction / Soldering / Bryan / Pat
Part Research / Ordering / Pat / Kevin
Testing / Kevin / Bryan
Design / Layout / Bryan / Pat
Website / Kevin / -
e. Time Management
To ensure that the group did not fall behind schedule in the creation of the Gigabit Ethernet, a GANTT chart was created to project the timeline of the different tasks that must be accomplished throughout the semester. This timeline was based primarily off the final GANTT charts from groups in previous semesters of this project. To construct the initial GANTT chart, the process was conducted end-to-beginning. Based on the deadline and the amount of time for previous groups to build and troubleshoot their transceivers, it was determined that the first board fabrication should be ordered in mid-March. Because this date coincided with Spring Break, the week before spring break was designated the ordering deadline for the PCB board. This decision was made to ensure that enough time was left at the end of the semester for the completion of the project, even with unexpected problems. The tasks to be completed before the first board fabrication were then projected accordingly, so that an approximately equal amount of work would be accomplished each week.
The final GANTT chart is shown below. The light blue bars indicate the projected range of completion for each phase of the project, and the darker blue bars indicate the progress on that particular task. As shown below, the only tasks which are incomplete at the conclusion of the semester are the ones involving the second, optional, board fabrication.
Looking back at the initial creation of the GANTT chart, it appears that the projected ranges of time for the completion of various tasks were chosen with overall success. Because the testing and troubleshooting of the first board fabrication extended past its projected deadline, a second board fabrication would have been difficult to accomplish in the remaining time left in the semester, but since the optional fabrication was never ordered (discussed later in this paper), the overall timeline was a success.
III. Identification and Selection of Parts
The initial venture into the VCSEL and photodiode search provided a myriad of results, some encouraging and others not. The most challenging aspect of the VCSEL search was sifting through options which appeared to be a perfect match for the purposes of a Gigabit Ethernet, but were instead slightly different from the desired part. Photodiodes, however, proved to be much more difficult to find, and the use of a ROSA as a replacement was eventually decided upon.
Several manufacturers were eliminated early in the process of determining the best VCSEL for this particular project. Luxnet Corporation and Kyosemi were removed from consideration because of the potential difficulties that might occur when ordering from overseas companies, especially ones which were unreliable when responding to questions. Oepic appeared to be promising because of its performance and because of the company’s location in the United States, but was only available at speeds of 10 Gb/s, which would be inefficient and potentially problematic in terms of noise for a Gigabit Ethernet. One of the best manufacturers identified was Roithner Lasertechnik, whose TTR-D1 VCSEL had excellent performance. This company was decided against, however, because of price and overseas shipping concerns. Finally, Advanced Optical Components was chosen as the best source of VCSELs. Its 1.25 Gb/s VCSEL’s “four corners” analysis did not meet the current requirements of the optical link budget, but its 2.5 Gb/s VCSEL (HFE419x-541) produced a “four corners” analysis that was adequate. In addition, the VCSEL’s excellent price ($14.50 per part) and the company’s location in the United States made this particular VCSEL the logical choice. After ordering from Advanced Optical Components, Emcore Corporation offered to send twenty free samples of their LC-TOSA. While it would have been ideal to learn about this before the ordering deadline, the free VCSELs were very much appreciated.
Photodiodes were much more difficult to find, and after analyzing the different possibilities, it was determined as a class to focus on ROSAs instead of photodiodes. Because Advanced Optical Components had already been chosen as a VCSEL provider, it made sense to investigate their ROSA, as well. It was quickly determined that the company’s HFD3180-103 ROSA was the best option because of its price ($10.00 per part), its availability, and the convenience of ordering all optical parts from one company.
The passive components were considerably easier to identify and obtain. The majority of the parts—including the ferrite inductors, two types of potentiometers, four types of resistors, the capacitors, and the power jacks—were ordered from Digi-Key. SMA connectors were ordered from Jameco, and the PCB boards were fabricated by Express PCB.