Gigabit Ethernet primer

Networked professional A/V applications need bandwidth and lots of it. It is no surprise, therefore, that we are always pushing the limits of networking
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Networked professional A/V applications need bandwidth and lots of it. It is no surprise, therefore, that we are always pushing the limits of networking technology. Gigabit Ethernet (GigE) is becoming common, and this month, we will look at some of the details behind this technology.

The GigE standard

GigE is standardized by the Institute of Electrical and Electronic Engineers (IEEE). It is described in a single standard, IEEE std. 802.3-2005. Before 2005, GigE was described in two separate standards.

The first standard, 802.3z, described the upper layers (media access control and coding) of GigE as well as three physical interfaces — a shielded copper wire interface (not compatible with existing 10Base-T and 100Base-T) and two fiber interfaces. This standard was completed in 1995.

The second standard, 802.3ab, defined the coding and physical layer for 1000Base-T, which is compatible with 10Base-T and 100Base-T. Clearly, there was a lot of demand for this standard, which was completed in 1998. Some specifications for these standards are shown in Table 1.

Gigabit interface converter

1000Base-X fiber technology introduced the gigabit interface converter (GBIC). The GBIC is an optical-to-electrical interface that plugs into the back of a switch or device. Different GBICs support different types of fiber, laser frequency and fiber modes. If you want to purchase fiber-based equipment, you can decide on a particular fiber technology today. Later, if your requirements change, you can repurpose the switch by only replacing the GBIC. This could be useful if you purchased an expensive switch for short-haul applications but later want to use it over a long-haul fiber network.

Encoding and duplexing

1000Base-T requires more complicated signal coding than previous Ethernet implementations. Five signal levels can be represented on each of the four pairs in a Cat 5e cable. Signals are coded using phase amplitude modulation. Thus, the coding is known as four dimension phase amplitude modulation (4-D PAM-5).

As with fast Ethernet, GigE over UTP supports half and full duplex modes. The full duplex mode allows the simultaneous transmission of 250Mb/s on each of four pairs in both directions. (See Figure 1.)

Duplexing gives 2Gb of overall bandwidth across the link, 1Gb in each direction. It does something else as well; it eliminates the requirement for any collision detection and handling circuitry. Because it is OK to talk in both directions at the same time, there is no such thing as a collision. This increases effective throughput across the link.

Fiber vs. UTP

One of the decisions you will face sooner or later is whether you will stick with UTP copper wiring or change to fiber. Of course, UTP provides backward compatibility with existing 10Base-T and 100Base-T installations. But it appears that the end of the UTP road is coming with 10GigE. We will not be able to use UTP forever.

Fiber has a number of advantages. It travels farther, you can get more fibers in a limited space, and it appears that some fiber types will be compatible with speeds beyond 10GigE. But fiber is harder to work with, it is less forgiving of physical abuse, and it works much better if you use support hardware that is specifically made for fiber-optic cable, both in racks and at the desktop.

There are several fiber types and operating modes engineers will need to become familiar with, especially in the 10GigE standard. That said, fiber has one important characteristic, especially for the broadcaster: Fiber provides electrical isolation between interconnected devices. This can be valuable in situations where lightning or high RF fields make copper an undesirable interconnection medium.

Cat 5 cable compatibility

There is some confusion within the broadcast community about whether you can use your existing Cat 5 cable with 1000Base-T.

Cat 5 cable manufactured prior to 1995 may not meet the technical requirements for GigE. This can result in unreliable operation. Cable manufactured in 1995 may work, depending on whether it meets the revised Cat 5 specifications that were standardized in that year. Cat 5 cable manufactured after 1995 should work just fine. Cat 5e (the “e” stands for extended) will work fine regardless of the year it was manufactured, and of course, cables with a higher Cat designation will also work fine. If in doubt, you should contact the cable manufacturer.

Backward compatibility

There are several important things to know about 1000Base-T and backward compatibility. First, the cabling and topology limitations for 1000Base-T are identical to those of 10Base-T and 100Base-T.

Second, you should know that almost all 1000Base-T cards will automatically sense the capabilities of cards they are connected to. Specifically, a 1000Base-T card will match the speed and duplexing capabilities of the other card. In other words, a 1000Base-T card will interoperate with a slower 100Base-T or even 10Base-T card just fine, but at the slower rate.

Third, 1000Base-T uses all of the pairs in a UTP cable. If you are using existing wiring that previously was used for 10Base-T or 100Base-T and you cannot get things to work, check to be sure that all pairs are correctly wired. This will not be a problem for wires that are terminated in RJ-45 connectors, but I have seen patch panels where installers failed to punch down all of the pairs because they thought they would never be used.


It seems appropriate to mention a few things about 10GigE in this column. Our drive for higher speed networking is insatiable. Work is well under way on a new standard that will deliver 10Gb/s connectivity. For those of you who are interested, the relevant standard is 802.3-2005. 10GigE not only provides high speeds, but also it travels long distances as well — up to 40km over fiber-optic cable. One important thing to know is that UTP is not currently supported in this standard and probably never will be.

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Table 1. 802.3ab specification standards. Note that the figures in this table are simplified. There are more combinations of cable type, laser frequency and operating modes than are shown here. Media type Maximum distance 1000Base-CX Shielded copper 25m 1000Base-T Unshielded twisted pair Cat 5e 100m 1000Base-LX 62.5µm multi-mode fiber 550m 50.0µm multi-mode fiber 550m 10.0µm single-mode fiber 5000m 1000Base-SX 62.5µm multi-mode fiber 275m 50.0µm multi-mode fiber 550m