I always have seen Ethernet Standards written in the form "low value/middle value/max value", and always wondered why.

Won't a router just reproduce a lower speeds anything below its maximum if it's configured as such, therefore making "/" unnecessary?

  • 2
    Many fiber SFPs for instance only support exactly one transmission speed. If you want to switch a fiber line from 1000 single-mode to 100 single-mode, you have to remove the fast SFP and insert a slower one - on both ends.
    – Alexander
    Feb 24, 2016 at 8:08
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5 Answers 5


They include this because not all ports are able to run at multiple speeds or certain speeds.

Running at only one speed was probably most common when 100BASE-TX first came out and a number of switches had fixed 100BASE-TX ports as uplink ports with 10BASE-T ports for providing access. However, it is common for many GBIC/SFP based ports to only run at a certain designated speed (for example 100 Mbps or 1000 Mbps).

Moving forward, you will start seeing support for some lower speeds getting dropped. Cisco is no longer providing support for 10BASE-T with their new "multi-Gig" ports. I am not sure if this is a design choice by Cisco or also part of the IEEE 802.3bz amendment that is still in the works.

You also will find most (AFAIK all, but not 100% certain) ports on the Cisco Nexus platform will no longer support 10BASE-T and are designated as 100/1000 ports (or 1000/10000 ports which don't support 100BASE-TX).

And it isn't only Cisco. Other vendors providing 10G capability often limit the ports to 1/10G or 100/1000/10000 speeds. As examples, HP has several products that follow the dual-speed model and Extreme tends to use the tri-speed model for their 10G copper ports.

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    I wonder what would happen if such a port was attached to 1Gb/s network interface which drops to lower speeds when listening for WOL packets. I guess it would mean you could not use WOL in such a configuration.
    – kasperd
    Feb 24, 2016 at 10:39
  • @kasperd Indeed it would mean that. No need to wonder... Aug 31, 2017 at 2:14

Good question. To answer it fully would involve a pretty deep look at Ethernet Wiring. But I'll try to explain it in simpler language.

All three speeds (10, 100, 1000) run over the same physical wiring: Unshielded Twisted Pair (UTP). UTP is made up of 4 pairs of wires (8 total wires) -- each pair is twisted around each other. Each pair of wires work together to send signals across to the other end.

What enables the same physical cabling to carry bits across the wire at such vastly different speeds are the different ways each of the 4 pairs of wires are used. We'll have to discuss three separate ideas: Standards and Wire Usage, Bits Transmitted, and Frequency.

Standards and Wire Usage

For example, 100BASE-TX is the predominant standard that governs transmission rates of 100mbps over UTP. It does this by dedicating one pair of wires for transmission, and the other pair of wires for receiving -- the remaining two pairs are unused.

1000BASE-T is the predominant standard that governs transmission rates of 1000mbps over UTP. It does this by using all four wire pairs, at the same time, in both directions. Each pair is responsible for carrying about 250Mbps of traffic at a time, providing for a total throughput of 1000Mbps, or 1Gbps.

Bits Transmitted

One of the other differences between 100BASE-TX and 1000BASE-T is that each transmit bits across the wire differently.

Basically, at any given time a certain voltage signal on the wire represents a certain value. In 100BASE-TX, there are only two possible values that can exist: a value of 0 or a value of 1. You could say that at any given instance, a single bit can be transferred across the wire.

In 1000BASE-T, there are four possible voltage values that can exist on the wire: 00 01 10 11. Or, said differently, each instance of signal being applied to UTP in the 1000BASE-T standard transfers two bits at a time.


100BASE-TX calls for a frequency of 100Mhz, which means each a signal can be applied and read by the other side 100 million times per second. This ends up being the speed at which each pair of wires can transmit either a 1 or a 0. This is what gets 100BASE-TX its 100Mbps speed, because each 'instance' of signal being applied equates to a single bit being transmitted. And since there are two pairs being used (one in each direction), that equates to 100Mbps in one direction, and 100Mbps in the other, or a total of 100Mbps full duplex.

1000BASE-T calls for the use of 125Mhz frequency, which means a signal can be read off the (pair of) wires 125 million times per second. Since each instance of a signal being applied in 1000BASE-T sends two bits across the wire, this means each pair can transfer 250 million bits across the wire per second, or 250Mbps. Since there are four pairs being used, this grants a total of 1000Mbps, or 1gbps speed.


These are some of the methods that a single physical cabling specification (UTP) is able to transmit data at such vast different speeds (10Mbps, 100Mbps, 1000Mbps -- or 10/100/1000). Think of it from the terms of technology evolution -- engineers found new ways to transmit data over the same wire. As such, the single wire can do multiple speeds at the same time. A single UTP wire can do 10, 100, or 1000 million bits per second, hence the wires and interfaces are labeled 10/100/1000.

I'm afraid I don't know the specifics of how 10BASE-T transferred bits across the wire, so I can't describe how that works like I did the other two standards

  • 1
    "All three speeds (10, 100, 1000) run over the same physical wiring" - not true. 10 and 100 run over 2 pairs while 1000 runs over 4.
    – Agent_L
    Feb 24, 2016 at 9:03
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    @Agent_L The same physical UTP cable, which includes all four pairs. The rest of the post describes the different usage of the individual wire pairs between the two major standards.
    – Eddie
    Feb 24, 2016 at 16:36
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    1000BASE-T ... each pair can transfer 250 million bits across the wire per second, or 250Mbps. ... four pairs ... grants a total of 1000Mbps - That doesn't sound right. I thought 1000BASE-T was full duplex, which mean 1000Mbps in each direction was possible at the same time?
    – Zoredache
    Feb 24, 2016 at 18:34
  • 3
    @Zoredache In 1000BASE-T, each pair of wires does 250Mbps in both directions at the same time. Which means total you have 1000Mbps full duplex across the UTP cable (as you expected). This is possible because of what is known as Echo Cancellation.
    – Eddie
    Feb 25, 2016 at 1:13
  • 3
    Very good explanation that includes the reasoning. It is not different network speeds - it is different standards down to a wiring level.
    – TomTom
    Feb 25, 2016 at 9:20

It really boils down to needing to support legacy devices and cabling. Cisco has a pretty good document, Ethernet Technologies, 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.

  • I think $750 for a network card is still a lot of money. :) For reference, it's about $2300 in today's money.
    – MichaelS
    Feb 25, 2016 at 6:24
  • $750 for anything consumer electronics is a lot of money. However, I might pay $750 for K.I.T.T.
    – Daniel
    Feb 25, 2016 at 19:55

Pure Ethernet hubs used to only run one speed - you'd not be able to connect a 10 mbit device (or other hub, switch, router at that speed) to a 100mbit hub. That was extremely inconvenient, so dual-speed hubs (basically a two port switch) got built. Those were the breakthrough for 100mbit Ethernet in home/SOHO markets. Same goes of course for switches with dualspeed ports later. The marketing term to differentiate from the old single-speed stuff was '10/100' and it was so heavily marketed you can't even imagine.


Ports are labeled "10/100/1000" to show their compatibility - a port like this will link with another port at the highest mutual speed. This is a good way to upgrade your network as you go. Many started with 100 Mbit/s, or even 10 Mbit/s, and by replacing devices over time, these begin to link at higher speeds.

However, the PHY standards are only single speed: 10BASE-T, 100BASE-TX, 1000BASE-T, 10GBASE-SR, ... There's no mandatory compatibility for a 100BASE-TX port to also support 10BASE-T, but it is optional and extremely common. So, a port with multiple speeds is compatible with several different PHYs.

Perfect downward compatibility often ends with 10G ports which very commonly only also support 1G and no slower speed.

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