It really boils down to needing to support legacy devices and cabling. Cisco has a pretty good document, [Ethernet Technologies][1], which explains a lot in depth. Ethernet has been around for a very long time. It was commercialized in 1981 at 10 Mbps. At first, it was pretty expensive. I remember ethernet cards costing $750 at a time when that was a lot of money. It took many years and a lot of argument for it to reach 100 Mbps, and a little while for the current 100BASE-TX standard to emerge as the winner. At the time, it was pretty expensive compared to 10BASE-T, so it was relegated to data centers and high-speed needs for a long time. The ports were built with backward compatibility with 10BASE-T because that was the most prevalent out there, and you may need to connect with that from a switch, router, etc. The price of 100 Mbps finally came down when 1000Base-T was released. Again, it was quite expensive compared to 100Base-TX, and it remained in data centers an big servers for a long time before the price came down. And it needed to be compatible with the more prevalent slower speeds. 10GBASE-T is now in the premium position, used primarily in data centers and on servers. It, too will eventually become mainstream, built into most new equipment, but it will need to maintain backward compatibility for many years. 40 and 100 Gbps Ethernet currently have a niche in the market, and market forces will determine which, and which standard of each, becomes the most prevalent. As they become more common, prices for 10GBASE-T will drop. The R&D has already been done for the slower speeds, and ethernet is basically built on a chip now, so it really doesn't cost any more to offer the slower speeds since there are still legacy devices around which need the slower speeds. The Cisco document in the link above was written a long time ago, but the principles are still valid, and the first question is basically the same thing you are asking. > Review Questions > > Q - Shouldn't all 10Base-T networks just be upgraded to 100 Mbps? Why > or why not? > > A - Not necessarily-if the current 10Base-T network is repeater-based, > replacing the repeaters with 10/100 nonsaturating switches would > result in an automatic n times increase in the average available > bandwidth for each end station. > > Q - Which 100Base version(s) are recommended? Why? > > A - 100Base-TX is recommended if the horizontal wiring is Category5 or > better UTP. If the horizontal cabling is Category 3, 100BaseT4 can be > used, but it may be difficult to acquire (some reports indicate that > because 100Base-TX was available more than a year before T4, it > captured as much as 95 percent of the market). 100Base-T2 is not > available. > > Q - Which 1000Base version(s) are recommended? Where would they be > used? > > A - 1000Base-T, is recommended if the horizontal cabling is Category 5 > or better UTP. 1000Base-SX can be used if the horizontal cabling is > multimode optical fiber, as well as for some multimode backbones. > 1000Base-LX can be used for either single-mode or multimode optical > fiber (see Table 7-5). 1000Base-CX can be used for short-haul > equipment-room jumpers up to 25 meters. > > Q - What cable types should be used for new networks? For upgrading > existing networks? Why? > > A - New or replacement UTP links may be Category 5E or better to allow > for data rate growth to 1000 Mbps. Multimode fiber may be used as > indicated in Table 7-5 for 1000Base-SX, or as noted in the paragraph > following Table 7-5 for 1000Base-LX. (These fibers will also provide > future support for shorter distances [between 100 and 300 meters, > depending on the wavelength] at 10,000 Mbps.) To be truly future-proof > and to ensure that you will be able to operate longer-distance > backbones, choose single-mode fiber. > > Q - How do you know when a network needs to be upgraded? Where do you > start? > > A - There are several ways: > > - Your users will tell you (but often only after they have crossed the frustration threshold). > - Your network management system should be capable of indicating the load characteristics for each DCE port. > - Your organization is considering adding new applications (such as multimedia) that will require more communication bandwidth. > - Your organization is growing, and there are not sufficient DCE ports in the right locations to accommodate the additional users. > > After you have determined the need, you can consider the options. > Remember that the network elements with the longest useful life (the > link media, followed by the network servers and network switches) can > also be the most expensive to replace. Choose with an eye to future > growth, and consider reusing these elements wherever possible. [1]: http://docwiki.cisco.com/wiki/Ethernet_Technologies