Gigabit Ethernet: Is the Time Right?
Using gigabit Ethernet is a lot like having the fastest car in the world. Just as most current highways can’t accommodate a car that operates that fast, a substantial portion of installed Ethernet devices are incapable of leveraging gigabit Ethernet speed, which is 10 times faster than the well-established fast Ethernet technology. As a result, adoption of gigabit Ethernet has been slow, because some users do not see a clear reason to upgrade.
However, this situation is beginning to change. Some Ethernet environments, such as large-scale newspaper printing plants, demand high bandwidth and users are converting to gigabit Ethernet. Also, as speed becomes an issue in other industrial application areas, more users are considering, and then converting to, gigabit Ethernet. And, as more applications are built requiring gigabit Ethernet, the industry will gradually transition and make gigabit Ethernet the established standard.
Today, however, there are several challenges to consider when implementing gigabit Ethernet:
Interconnection problems. How do you choose the right equipment? What is the best way to connect gigabit Ethernet?
Installation and reliability. Is gigabit Ethernet difficult to install and will it produce interference problems?
High-bandwidth challenge. Many applications don’t currently require extra speed, making it difficult to justify the expense of converting to gigabit Ethernet. However, a growing number of critical applications can benefit from it.
What is gigabit Ethernet?
First developed from research by Xerox Corp. in the 1970s, Ethernet was implemented in the 1980s and has since grown to dominate the networking market. The advent of a fast Ethernet protocol increased data transmission speed from 10 to 100 megabits per second (Mbps). Gigabit Ethernet, a family of networking protocols, was developed to increase speed to 1,000 Mbps. The initial standard for gigabit Ethernet was issued by IEEE in 1998 as 802.3z for fiber optic connections and in 1999 as 802.3ab for UTP copper cabling. Both standards are more commonly referred today as 1000BASE-X.
|This diagram illustrates the two IEEE gigabit Ethernet structures in both the MAC (media access controller) and PHY (physical interface transceiver) layers.|
Fast Ethernet has been a very effective technology, is well accepted, and nearly all networking devices and systems can use it. Gigabit Ethernet uses fast Ethernet as a base, but there are several key differences. For example, standard Ethernet Cat 5 cables have eight wires (four pairs), but in basic Ethernet 10baseT (10Mbps) and fast Ethernet 100Base T (100 Mbps), only four (two pairs) of these wires are used, one wire pair transmits data and a second, separate pair receives it.
In 10Mbps Ethernet, one bit of data is encoded into one transmitted symbol, using only two voltage levels; Fast Ethernet uses a 4B/5B MLT-3 encoding scheme with 3 voltage levels. 1000Base-T, however, codes two bits of data into one transmitted PAM5 symbol and uses all eight wires (four pairs) to bi-directionally transmit and receive data. Although gigabit Ethernet runs at the same symbol rate as fast Ethernet (125 Mbaud), it achieves 1,000 Mbps speed by transmitting eight bits at a time, using 5 voltage levels and all four available wire pairs to simultaneously transmit and receive data.
It is evident that increased data rates and advanced multi-level coding schemes put greater demands on the quality of the copper cabling. This results from higher frequencies and signal-to-noise ratio. While the fast Ethernet standard was written for Cat 5 cabling, users may want to consider upgrading to newer Cat 5e or Cat 6a cable to avoid interference problems. Cat 5e cable is sufficient for gigabit Ethernet where as Cat 6a offers the most protection from internal interference and is also suitable for future 10 gigabit Ethernet applications. However, a general decision has to be made whether to use a shielded twisted pair (STP) or unshielded twisted pair (UTP).
STP cabling has advantages with regard to crosstalk between pairs in the same cable and immunity against external interference, but is more costly than unshielded copper cable and less commonly used in the United States. UTP cable is cheaper and easier to install but will definitely reach its limits with the new Cat 6a standard at 500 MHz.
In general, interference is a more challenging problem with gigabit Ethernet than with fast Ethernet. This factor should be carefully considered when choosing an architecture for a particularly noisy environment. One option to avoid this issue is by using fiber optic cabling, which will work for both fast Ethernet and gigabit Ethernet systems.
There are four different physical layer standards for gigabit Ethernet using optical fiber and several sub-standards based on the distance the cable will run. It’s also important to know that fiber optic is not the universal remedy that some people might believe. Using older fiber variants sufficient for fast Ethernet will restrict lengths for gigabit Ethernet because of internal dispersion effects.
Whether you use fiber or copper, take note that installation practices can also greatly influence media performance. Treating Cat 5e cable as if it were a power feed to a motor control center will definitely hamper network reliability. If correctly implemented, however, gigabit Ethernet should not present interference problems and will operate as effectively as fast Ethernet—and faster.
It is also important to make sure switches used in a gigabit Ethernet network are designed to handle the higher speeds. They should support speeds of 10, 100 and 1,000 Mbps on all ports, but some switches only support all of those speeds on two ports with the remaining ports supporting only 10 and 100 Mbps. Note that some switches only support speeds of 1,000 Mbps, whidh is bound to land you with backward compatibility issues on all devices in your facility incapable of anything other than speeds of 10 and 100 Mbps.
A good practice is to ensure that each switch used on a gigabit Ethernet system be rated for 2,000 Mbps multiplied by the number of ports on the switch. This ensures the switch is capable of bi-directionally transmitting data at up to 1,000 Mbps while receiving data at up to 1,000 Mbps.
Connectors are another issue. For harsh, industrial applications, RJ45 connectors alone cannot be used. Additional housing is required to protect the connector. A better option for harsh applications would be the M-12 connector, but it traditionally is offered with only four pins. New M-12 connectors with eight pins are required for gigabit Ethernet applications in harsh environments.
Gigabit Ethernet is compatible with most computers and can improve the performance of high-traffic industrial equipment networks. For example, if more than 20 devices are connected, gigabit Ethernet eliminates data congestion problems by reducing, by a factor of 10, the time needed for a data packet to go through a switch. This can be a critical difference for fast, real-time applications.
|Standard||Standard first released||Data rate||Topology||Cable type||Connector technology||Max. cable segment length (meters)|
|Source : Control Engineering with data from OvisLink Corp.|
|10BaseT||802.3i-1990||10Mbps||Star||Two pairs of 100-ohm Cat 3 or better UTP||RJ45 modular jack (8 pins)||100||100|
|100Base-TX||802.3u-1995||100Mbps||Star||Two pairs of 100-ohm Cat 5 UTP||RJ45 8-pin UTP 9-pin D-type STP||100||100|
|100Base-FX||802.3u-1995||100Mbps||Star||Two optical fibers||Duplex SC connector ST connector & FDDI MIC connector||412||2000|
|1000Base-LX||802.3z-1998||1 Gbs||Star||LWL (1300 nm) over||Duplex SC connector|
|1000Base-SX||802.3z-1998||1 Gbs||Star||SWL (850nm) over||Duplex SC connector|
|1000Base-T||802.3ab-1999||1 Gbs||Star||Four pairs of 100-ohm Cat 5 or better cable||8-pin RJ45 connector||100||100|
Related links and ONLINE extra on industrial Ethernet
More from the October 2008 Control Engineering supplement on Engineering-Drive Ethernet:
Gigabit Ethernet: Is the Time Right?
Industrial Ethernet applications in real time with SERCOS III, EtherCAT
-SERCOS III Ethernet protocol has replaced the traditional network for direct control of I/O connections, monitoring subsystems, and other field bus devices on Rovema packaging machines. The open, standardized SERCOS III interface gives Rovema a universal drive and automation interface compatible with drives and devices from many manufacturers.
-Popcorn bags open better with EtherCAT, Beckhoff Automation:
|Ralf Kaptur is product manager for Molex Automation and Electrical Products Division. For more information, visit editorial director.|