Georgia Institute of Technology

School of Electrical and Computer Engineering

ECE 4006 Senior Design

Gigabit Ethernet for the Masses

Talal M. JaafarIbrahima B. Sow

Mohammad F. Zaaman

Supervisor: Dr. Martin Brooke

Introduction

At no other time in the history of human civilization has the importance of

information gathering, processing, and distribution been so critical. People are constantly

checking their e-mail, stock quotes, sports scores and their bank accounts online. With

the advancement of fiber optic communication networks, people have been able to access

information at speeds and accuracy that were unimaginable only a few years ago.

However as improvement to such existing networks continues unperturbed the number of

beneficiaries from such technologies is not increasing at the same rate. The fundamental

drawback is the cost involved in accessing these technologies. Gigabit Ethernet

technology is a prime example. Implemented as a major improvement over its

predecessors the Ethernet and Fast Ethernet, the Gigabit Ethernet (which has since been

experimentally succeeded by the 10-Gigabit Ethernet) is used by few to none in the

general public.

Local area networks (LAN) were invented in the mid-1970’s. The immediate

future development of LANs seems centered on enhancing what exists today; making the

current LANs faster, and more reliable rather than inventing something that is completely

new. LAN technologies operate at the Data Link layer (layer 2 of the OSI Reference

Model), and Ethernet is just one of many different LAN types.

Ethernet was invented in 1973 (by Dr. Metcalfe), and it was conceived as a

passive coaxial cable medium (a physical and logical bus topology) with a contention-

based medium access protocol. The data transmission is done through broadcasting, and

all the stations that are on the network will be listening to the message. By contention-

based medium we mean that only one station may transmit at a time, so a protocol is

needed to ensure that all stations will fairly share access to the wire. The original

protocol envisioned by Metcalfe was standardized by some companies namely Digital

Equipments, Intel, and Xerox and it is known as “DIX” Ethernet.

What is Ethernet?

In the early 1980s, the IEEE created a formal standardization framework to

encompass all existing and yet-to-be-invented LAN technologies. Ethernet is

synonymous with the IEEE 802.3 for a 1-persistent CSMA/CD (Carrier Sense Multiple

Access with Collision Detection) LAN. CSMA/CD is the protocol that enables the

medium (i.e. the wire) to be shared fairly among all stations. “Carrier sense” means that

every station that is about to transmit data has to sense the cable to determine if it is being

used by another station. If the cable is busy, then the station will wait until the cable goes

idle and then transmits its frame immediately. Another improvement in the 802.3

protocol is that the stations will abort their transmissions as soon they detect a collision.

In other words, when two stations sense the cable to be free and start transmission at the

same time, they should stop transmitting their frames as soon as a collision is detected

since both frames will be garbled. The DIX Ethernet was used to connect more than 100

workstations on a 1 km cable. Since the system was successful, it was standardized by as

IEEE 802.3 with a data rate of 10 Mbit/s. Initially there were two types of coaxial cables

used. These were known as Thick Ethernet (10Base5) and Thin Ethernet (10Base2).

Later, the main type of cable used was the unshielded twisted pair (UTP) copper cable.

What is Fast Ehternet?

As computer technology advanced, the demand for faster and more reliable

methods of communications has been increasing. The 10 Mbit/s DIX Ethernet did not

provide enough bandwidth to fulfill the users’ demands. That led to the standardization

of the Fast Ethernet in the form of IEEE 802.3u in 1995. The Fast Ethernet technology

was defined for 100 Mbit/s which has ten times more bandwidth and has improved

features such as full-duplex operation. The IEEE 802.3 networks used to operate only in

half-duplex mode, whereas the IEEE 802.3u networks allowed its stations to operate in

full-duplex mode, which enabled them to send and receive data simultaneously.

What is Gigabit Ethernet?

Due to the unpredictable growth rate of the computer industry, there has been a

major demand for higher bandwidth communication channels. More than 120 companies

united in the Gigabit Ethernet Alliance to promote a new standard of Ethernet. The new

IEEE 802.3z was achieved in June 1998. The design objectives of the new standard were

the following:

Offers high bandwidth of 1,000 Mbps.

Uses the IEEE 802.3 Ethernet frame format, with the addition of carrier extension

field.

Employs the same half-duplex and full-duplex MAC operation schemes as the

predecessors.

Addresses backward-compatibility with 10 Mbps and 100 Mbps Ethernet

technologies.

Supports all existing network protocols.

The physical layer of the Gigabit Ethernet was adopted from Fibre Channel, a

technology for connecting workstations, supercomputers, storage systems and peripheral

devices. Also, the Ethernet standard 802.3z (1000Base-X) supports transmission via

multi-mode and single-mode optical fibers as well as twinax cable. The transmission

over twisted pair cabling was set up in an independent standard known as IEEE 802.3ab

(1000Base-T). The four physical media types that the Gigabit Ethernet is based on are

illustrated in Figure 1.

Figure 1. Gigabit Ethernet physical Layers.

How does Gigabit Ethernet work?

The Fibre Channel technology uses long wavelength lasers to transmit data over a

fiber optic cable. The IEEE 802.3z standard (also known as 1000Base-X) has defined

three physical layers: 1000Base-CX (short-haul copper), 1000Base-LX (long-wavelength

optics), and 1000Base-SX (short-wavelength optics.) In addition, the IEEE 802.3ab

created a physical layer that allows the work over four pairs of cat5 UTP copper.

1000Base-SX uses short-wavelength laser transmitters (770-860 nm) running over

multi-mode fiber, and relatively short distances. In contrast, 1000Base-LX uses long-

1 2 3 4

wavelength laser transmitters (1270-1335 nm). 1000Base-LX can run over multi-mode

fiber and short distances, or over single-mode fiber and great distances.

Gigabit Ethernet Specifications

The supported (coverage) distance is directly proportional to the type of fiber, as

well as the wavelength used, as shown in Table 1.

Table 1. Gigabit Ethernet over Fiber.

As shown above, the maximum traveling distance over a multi-mode fiber is

550m for the 1000Base-SX, and 1000Base-LX. Also, the 1000Base-LX allows a

traveling range up to 5000m over a single-mode fiber. At present short-wavelength are

cheaper than long-wavelength, and since 1000Base-SX goes nearly as far as the

1000Base-LX (500m comparing to 550m), it is likely that most people would use the

1000Base-SX especially for LANs.

The copper based physical layer in the 1000Base-X family is 1000Base-CX.

1000Base-CX operates over two pairs of 150 shielded twisted pair cable. It was

designed for sever-to-switch interconnection in relatively small rooms. Thus, the

distance was limited to 25m. Another copper based physical layer is the 1000Base-T,

which is being developed with a goal of 1000 Mbps operation over 100m category 5

unshielded twisted pair infrastructure. 1000Base-T will use all four pairs of the RJ-45

connector and squeeze five times the bandwidth out of each pair. As term of use,

1000Base-CX might be used for attaching servers to the Gigabit Ethernet infrastructure

over very short distances, and 1000Base-T will replace 1000Base-SX if it is cheaper.

Goals and Challenges

The purpose of this project is to successfully construct a test-bed to perform

experiments and determine methods to implement Gigabit Ethernet more affordably for

the general consumer. The various components of a Gigabit Ethernet device will be

individually isolated for investigation. It is our ultimate goal to reconstruct a fully

functional prototype at the lowest possible cost with optimal performance. Initial cost

estimations suggest that the principal source of cost involved with such a communication

system is the optical transceiver of the Ethernet card. Thus, the immediate objective is to

isolate the existing transceiver module on an existing Ethernet board and mount it onto an

external evaluation board for analysis and experimentation.

The evaluation board chosen utilizes differential inputs. The group that worked on

this project in a previous semester opted for an evaluation board that utilized single-

ended inputs. Differential inputs add some complexity to the circuits, but they do offer

better noise immunity and signal transmission characteristics. The complexity added by

the differential inputs is in the form of additional components for the transmission line

terminations and additional connectors and cables.

Accomplishing the goals discussed previously has to be done in several parts.

The first step is the procurement of the components that will be used to reconstruct the

transceiver off of the Ethernet board.

This first step involves testing the old modules in order to verify that the

optoelectronic components work. The modules were tested in Dr Brooke’s laboratory

with his help. The modules that worked were kept for future use on the transceiver

boards. The components that are not already in the laboratory from the previous project

will be ordered through on-line merchants. The main optoelectronic module is an Agilent

HFBR 53D5 fiber-optic transceiver. This chip is driven by the Agilent HDMP 1636A

SERDES chip. The circuit below shows the board that will be built. However, it only

shows the components that will be on the external board.

Figure 2. Transceiver without the driving circuitry.

Since the driver circuitry of the transceiver would be rather difficult and time

consuming to rebuild, it will be left on the original Intel Ethernet card. Therefore, the

module that will be built will only contain the optical transceiver with the proper line

terminations for the time being. There are future plans to also move the driver circuitry

off of the Ethernet board for further experimentation. Once the modules were tested, they

had to be unsoldered from the previous single ended input boards. Due to the relatively

limited soldering experience of the group members, Mr. Edgar Jones helped familiarize

the group members with the soldering and unsoldering equipment and techniques.

Once the transceiver boards are built, they will have to be connected back to the

Ethernet card that will reside in a host PC. These connections will use coaxial cables that

have been chosen to handle speeds of up to 3GHz since the whole system will work at 1

gigabit per second using NRZ signaling. On one end, the cables will attach to the

transceiver boards using SMA connectors. The use of SMA connectors will ensure the

signals are properly transmitted without having to worry about line terminations at the

junctions since SMA connectors provide connections that are better suited for the speeds

involved and their speed rating is much higher than that of BNC connectors for instance.

The other end of the cables will be directly soldered on the Intel Ethernet card. Soldering

the coaxial cables on the PCB of the Ethernet card will be challenging; therefore, extra

care has to be taken to ensure that the soldering is done properly. Another challenge

facing the group is soldering the surface mount components on the transceiver board.

These components are very small and require a great deal of skill to be soldered properly

and neatly. The team will also have to be mindful of properly terminating the

transmission lines and properly grounding the board itself as well as the incoming coaxial

lines. In the circuit in Figure 2 above, Vcc will be provided using off-the-shelf 5V power

supplies.

Bibliography

1. Tanenbaum, Andrew S., Computer Networks. New Jersey: Prentice Hall. 1996

2. “Gigabit_Ehternet”, August 9, 1999. http://www.syskonnect.com/syskonnect/technology/Gigabit_Ethernet.PDF

3. “Gigabit Ethernet”, December 2, 2000. http://www.comm.toronto.edu/~karen/projects/21.GbEthernet-802.3/overview.html

4. “Ethernet Web”, January 31, 2000. http://www.ots.utexas.edu/ethernet