Gigabit Ethernet Optoelectronic Link

“GIGABIT ETHERNET OPTOELECTRONIC LINK”

GROUP 7 DESIGN TEAM

Casey Brister

Thee Kong

Charles A. Lewis II

Muhammad Qayyum

My Tran

Bounsysavanh Vongsamphanh

ECE4006-B

Capstone Design Class

MAJOR DESIGN PROJECT

Instructors: Dr. Jokerst & Dr. Brooke

Paper#1

THURSDAY, October 24, 2002

Georgia Institute of Technology

School of Electrical and Computer Engineering

ATLANTA, GEORGIA

Abstract3
Design Team Organization4
Background4
Testing theory9
Procedure (for provided Pre-fab board)11

VI.Results (for provided Pre-fab board)12

VII.Conclusions (for provided Pre-fab board15

VIII.Designing a Gigabit Ethernet Receiver and Transmitter16

A.Introduction16

B. Component Analysis and Selection16

C. Receiver Board (RX design)24

D. Transmit Board #1 (TX1 design)26

E. Transmit Board #2 (TX2 design)28

Outlook / Paper2 preview30

Reference32

APPENDIX A34

APPENDIX B35

I. ABSTRACT

This paper provides a general introduction to Gigabit Ethernet theory and serves as a comprehensive summery summary of the work to date of the ECE4006-B G7 major design team. That includes the theory, design, build, testing, and analysis of a Gigabit Ethernet optoelectronic (OE) link. The OE device will include both optical transmit and receive structures that transfer data at 1.25Gb/sec and operate within the specifications stated in the IEEE 802.3z standard. Results will be tested using 8B/10B encoded random data inputs. Performance assessment will be based upon bit error rate (BER) of the data, eye diagram clarity, and other parameters.

II. Design Team Organization

In order to ensure efficient and timely completion of this project, the Group 7 design team established an internal operating structure. Charles A. Lewis II was appointed design team leader. Then the team was divided into Transmit (TX) and Receive (RX) specialization teams. The group them conducted extensive independent research on Gigabit Ethernet and related topics, with each member paying additional attention to there assigned area of specialization. A management plan was then developed as a production guide for the project. Done in Gantt chart format, it is regularly updated as serves as the most recent version of that management plan is shown in APPENDIX B.

III. Background

The purpose of this project is to design a gigabit optoelectronic data communication link. This system would be capable of transferring the data at a rate of 1 gigabit per second with low errors. It is important to understand the importance and mechanics of gigabit Ethernet before getting into design details. All of us know that most of the networks are LAN connections and among those connections above 80% are Ethernet based.

Gigabit Ethernet is the new dimension in the world of Ethernet. Ethernet is an extremely successful technology for the fact that it is simplewithout much complexity, yet and that produces reliable and low- cost,low-maintenance networks. Gigabit Ethernet incorporatesadd furtherall of the qualities oftoan already stable system, plus gives the advantage of increased speed. At 1000 Mbps, Gigabit Ethernet is 100 times faster than Ethernet, and 10 times faster than Fast Ethernet. and that is fast data rates and convenient scalability.

However, with faster data rates the probability of errors increases as well. So, the issue of delivering data without corruption is an important issue when it is being delivered at faster data rates. The adaptability of Ethernet structure was used to avoid this problem. With the development of IEEE 802.3 Media Access Controller (MAC) the problem of the deliverance of uncorrupted data was taken care of.

The three major components of thepurpose of this project is toproject are design, build, and test a Gigabit Ethernet optoelectronic (OE) link:. That link will include both a

tTransmitter (TX) and

Optoelectronics (OE)

rReceiver (RX).

In order to discuss the details of the design, it is important to study the background information about the IEEE standard 802.3z, and the key components of the TX and RX. As it is quite clear that Ggigabit Ethernet is explicitly defined in IEEE Standard 802.3z. Within the standard 8B/10B is listed as the block coding scheme to be used for Gigabit Ethernet data transfer. (See IEEE Standard 802.3z 1.4.24 and 36.2.4) 8B/10B is the scheme used by the gigabit Ethernet so that the data can be interpreted correctly at the receiver. It is a DC balancedoctetorientated dataencodingformat. In this encoding scheme, eight bits of data are sent along with two extra bits. The transmission rate of these ten bits is 1 GP/s. The extra bits are needed for control information like the start of the packet, end of the packet, and idle. If invalid bits are received then a transmission error occurs. This encoding scheme is useful because it makes bit synchronization easy and it insures that incoming bit stream has frequent transitions for performing clock recovery. (The 8B/10B encoding scheme is more completely explained in the Testing Theory section of this paper.)

Now changes in the physical layer in accordance with the requirements of gigabit Ethernet will be discussed. In order to confirm the transfer rate of 1 GP/s, the physical layer uses Fiberre Channel modifications. The Fiberre Channel uses long wave lengths to transfer data over a fiber optic cable. These wavelengths known as1000Base-X standard are given below. The 1000BASE-X family contains 1000Base-SX, 1000Base-LX, and 1000Base-CX. 1000Base-SX, for Short wavelength 850 nm laser on multi-mode fiber, isthe one that will be used for this design project.

Figure 1 - Gigabit Ethernet physical Layers.

(Fiber Channel is discussed further in the Testing Theory section of this paper. For a complete explanation of Fiber Channel, please refer to ANSI X3.230 Standard.)

The MAC layer for the gigabit Ethernet has all the capabilities used for other Ethernet technologies. In addition to old capabilities, the two additions are carrier extension and frame bursting which will be discussed later.

In the IEEE standard 802.3z, MAC layer can perform operations in half and full duplex modes. In case of half duplex mode, a channel can transmit and receive data but not at the same time. However, in full duplex mode, packets can be received and sent at the same time. In full duplex mode, flow control is used, however, in half duplex mode, CSMA/CD access method is employed. In this method, a frame is only sent when the transmission medium is clear. The network listens for a collision of the frames. Now, if the transmission had been clear for a while, then both end of the network might transmit frames at the same time and that would result in a collision. CSMA/CD detects these collisions and discards the damaged frames, and the frames are retransmitted after the transmission medium is clear.

CSMA/CD is effective in the network but it has a timing problem because of the frame length. This problem is solved by employing carrier extension mechanism. . The carrier extended frame is shown in Figure 1.

Figure 2 - Figure 1: Carrier-extended frame.

Carrier extension technique increases the frame length from 64 bytes to 512 bytes. If a frame is sent into the transmission medium with a length less than 512 bytes then non-data extension bytes are added. CSMA/CD access method is further improved with the introduction of frame bursting mechanism. This mechanism has the ability to send multiple frames of small sizes so that bandwidth does not go to waste. Figure 2 gives an overall picture as to how this method works.

Figure 23 -:Illustration of CSMA/CD method

In short, the purpose of the IEEE standard 802.3z is to provide essential specifications for the Media Access Control (MAC) and Physical Layer of the gigabit Ethernet. It also provides half duplex and full duplex operations. The standard 802.3z depends on 802.3 Ethernet frame format and Carrier Sense Multiple Access / Collision Detection (CSMA/CD) access method. As this standard is compatible with 10MPS and 100MPS technologies, so this provides a less complex and more stable technological advancement.

The Group optoelectronic link will have two active components.They are a Vertical Cavity Surface Emitting Laser (VCSEL) on the transmit side and on the receive side it would be a photodiode (PD).

Vertical Cavity Surface Emitting Laser (VCSEL) is a key component in the TX design. The differential input coming into an integrated circuit and comes out as current which drives the VCSEL to produce a laser. This laser enters an optic fiber leading up to the photodetector on the RX side.

VCSELs are composed of several layers of mirrors that are produced by semi-conductors with varying compositions. These mirrors reflect a narrow range of wavelengths back into the cavity which in return caused the light emission of just one wavelength. Figure 3 demonstrates this description.

Figure 4- Structure of a VCSEL. / Figure 5. Images of the two types of VCSELS to be used in this project. (On the left, an “unconnectorized” VCSEL, and on the right an SC Connectorized VCSEL.)

VCSELs are used because they give high performance and are cost effective. They also tend to have other advantages including low threshold currents, low temperature sensitivity, high transmission speed with low power consumption, and easy alignment and production packaging due to same geometry as the photodetectors (PDs) used. (Photodetectors are explained below).

VCSELs can be connectorized or unconnectorized. Connectorized components are pre-aligned and factory-tested to ensure minimal insertion-loss. The outside connectorized end will generally have a protective shroud that prevents damage to the connectors during installation.

For this project VCSELs will be assessed by divergence angle, threshold current, and slope efficiency. Beam divergence angle is defined as the light intensity full width at the 1/e2 intensity level. Threshold current is the minimum current that turns the VCSEL on. Slope efficiency is the ratio of the output power (micro watts) to input current (mA).

Photodetectors be selected based upon their sensitivity at the required wavelength, responsivity, divergence angle, (preferably large), speed, and capacitance (preferably low).

Responsivity of a photodetector is the ratio of current (A) produced to the input power (W). Photodetectors are also available as either connectorized or unconnectorized.

Table 1 - Gigabit Ethernet over Fiber.

ABSTRACT

This paper provides a general introduction to Gigabit Ethernet theory and serves as a comprehensive summary of the work to date of the ECE4006-B G7 major design team. That includes the theory, design, build, testing, and analysis of a Gigabit Ethernet optoelectronic (OE) link. The OE device will include both transmit and receive structures that transfer data at 1.25Gb/sec and operate within the specifications stated in the IEEE 802.3z standard. Results will be tested using 8B/10B encoded random data inputs. Performance assessment will be based upon bit error rate (BER) of the data, eye diagram clarity, and other parameters.

I. Introduction and Background Information

The purpose of this project is to design a gigabit optoelectronic data communication link. This system would be capable of transferring the data at gigabit rate with low errors. It is important to understand the mechanics of gigabit Ethernet before getting into design details. Most networks today are local area network (LAN) connections and among those connections above 80% are Ethernet based. Gigabit Ethernet is the new dimension in the world of Ethernet. Ethernet is an extremely successful technology for the fact that it is simple without much complexity and that produces reliable and low cost maintenance networks. Gigabit Ethernet add further qualities to this already stable system and that is fast data rates and convenient scalability.

The three major components of the project are: Transmitter (TX), Optoelectronics (OE), and Receiver (RX). In order to discuss the details of the design, it is important to study the background information about the IEEE standard 802.3z, and the key components of the TX and RX. As it is quite clear that gigabit Ethernet is about transferring data at fast rates, so, it is important to choose an appropriate encoding scheme. 8B/10B is the scheme used by the gigabit Ethernet so that the data can be interpreted correctly at the receiver. In this encoding scheme, eight bits of data are sent along with two extra bits. The transmission rate of these ten bits is 1 GP/s. The extra bits are needed for control information like the start of the packet, end of the packet, and idle. If invalid bits are received then a transmission error occurs. This encoding scheme is useful because it makes bit synchronization easy and it insures that incoming bit stream has frequent transitions for performing clock recovery.

1000Base-SX: Short wavelength 850 nm laser on multi-mode fiber.

1000Base-LX: Long wavelength 1300 nm laser on single-mode and multi mode fiber.

1000Base-CX: wavelength of 800nm on shielded copper cable.

Figure 1. Gigabit Ethernet physical Layers.

Figure 1: Carrier-extended frame

Figure 2: Illustration of CSMA/CD method

Figure 3: Structure of a VCSEl.

VCSELs are used because they give high performance and are cost effective as devices are completed and tested at wafer level. Other advantages include low threshold currents, same geometry as a photodetector that provides easy alignment and packaging, low temperature sensitivity, and high transmission speed with low power consumption. VCSELs are categorized in two types connectorized and unconnectorized. A connectorized VCSEL has an attachment interface that allows the fiber cable to be pre-aligned with the outgoing laser. For unconnectorized VCSEL, alignment tolerance must be performed to maximize the power of the light going into the fiber cable.

In order to better understand the characteristics of the VCSELs, it is important to understand the meaning of terms like divergence angle, threshold current, and slope efficiency. Beam divergence angle is defined as the light intensity full width at the 1/e2 intensity level. Threshold current is the minimum current that turns the VCSEL on. Slope efficiency is the ratio of the output power (microwatts) to input current (mA). The quality of a VCSEL is determined based on the characteristics described before.

Photodetector (PD) is an important component on the receiver side. The light coming from the VCSEL through the fiber optic enters the photodetector which converts that light into current. This current enters a trans-impedance amplifier which amplifies that current and converts the current into a voltage. The limiting amplifier increases the voltage and performs an analog to digital conversion of the signal. Photodetectors have to meet certain criteria; they have to be sensitive at the required wavelength, have high quantum efficiency, have a fast response time, and have low noise interference.