What Ethernet twisted pair cable is best with a particular Ethernet switch

tl;dr I want the best supported Ethernet cable for my Ethernet switch. How do I correlate Ethernet cabling speeds measured in hertz with an Ethernet switch "speed" measured in bits-per-second bandwidth?

There are several Ethernet twisted pair cabling standards; "Cat 5e", "Cat 6", "Cat 6A", "Cat 7", etc. each supporting a maximum length and hertz rating. For example, according to Wikipedia "Cat 5e"

The cable standard prescribes performance parameters for frequencies up to 100 MHz

whereas "Cat 6A"

Cat 6A performance is defined for frequencies up to 500 MHz

At the same time, there are many different Ethernet switches available at different standard "speeds" (bandwidths); "100Mb", "1Gb", etc.

For sake of example, this typical 5-Port Gigabit Desktop Switch specification supports

10/100/1000Mbps, Auto-Negotiation, Auto-MDI/MDIX Ports

IEEE 802.3i IEEE 802.3u IEEE 802.3ab IEEE 802.3x IEEE 802.1p

Given some Ethernet switch, what is the method to determine the twisted-pair Ethernet cable that will achieve the largest bandwidth (highest "speed")?

I'm looking for the answers that describe the general thought process or procedure to determine the best match for most typical consumer Ethernet switches and Ethernet cables. However, I'm less interested in blindly throwing money at the "latest and greatest" Ethernet cables, more interested in the technical rationale for choosing.
For sake of simplicity, assume a small office setup with cables no longer than 100 feet.

The Niquist rate says that the maximum rate of independent symbols (f_p) is twice the frequency bandwidth of the channel (B).

f_p ≤ 2B

Hartly's law introduces the idea that the amount of information that can be encoded in each indepenent symbol depends on the number of different levels that the receiver can distinguish and hence the information rate depends on the frequency bandwidth, the amplitude of the signal (A) and how accurately the receiver can distinguish signal levels (ΔV).

R ≤ 2Blog₂(1 + A/ΔV)

The logarithm in there is notable, as it shows us that while increasing the number of levels the receiver can distinguish increases the data rate there are diminishing returns.

The Shannon-Hartly theorem builds on these ideas and the statistical properties of noise and relates the maximum theoretically achievable information rate to the Bandwidth and signal to noise ratio for a channel subject to additive white Gaussian noise and with an encoding scheme that has unlimited context to use for error correction.

C = Blog₂(1 + S/N)

Theoretical maximums and real-world systems are not the same thing. 10BASE-T Ethernet was designed more for simplicity than for bandwidth efficiency and only gets 1 bit per second per Hz for a 10MHz bandwidth. 100BASE-TX uses a muli-level encoding scheme which achieves just over 3 bits per second per Hz for a 31.25 MHz bandwidth. 1000BASE-T uses a more complex encoding scheme which pushes up to 4 bits per second per Hz and it uses all four pairs for transmission in both directions so a 1000 MBps data rate is achieved with only a 62.5 MHz bandwidth.

In other words, thanks to more advanced interface designs, while the data rate increased by a factor of 100, the bandwidth only increased by a factor of 6.25. Unfortunately after 1000BASE-T diminishing returns started to kick in.

10GBASE-T pushed up to 6.25 bits per second per Hz, but with four data lanes that still results in a 400MHz bandwidth. Too much for many existing cable plants. This resulted in the creation of 2.5G and 5G standards based on the modulation scheme used by 10GBASE-T but with lower data rates.

There are also 25GBASE-T and 40GBASE-T standards which take the modulation scheme used by 10GBASE-T and scale it up to higher bandwidths, but I can't seem to find any evidence of anyone actually selling them.

The headline frequency figures for a cable type are only part of the story. A cable doesn't suddenly stop carrying signals at all at a given frequency, the performance just gets gradually worse (more attenuation, more crosstalk, more dispersion). Eventually the signal degradation will reach a point that signals can no longer be reliably received.

Each speed of twisted pair Ethernet was designed for a particular cable type, but there are some sublties.

• 10BASE-T was designed for Cat 3 or better.
• 100BASE-TX was designed for Cat 5 or better.
• 1000BASE-T was designed to run on Cat 5, however it was discovered that the Cat 5 specification did not sufficiently constrain crosstalk, so while it would usually work there was a possibility of problems, Cat 5e was introduced to rectify these deficiencies.
• 10GBASE-T requires Cat6A for the full 100 meter distance, it can work over real world CAT 6 at shorter distances, but similar to the situation with 1000BASE-T over CAT 5 this can't be gauranteed on a "worst case" cable.
• 2.5GBASE-T is designed for Cat5e
• 5GBASE-T is designed for Cat6 and shorter runs of Cat 5e.
• 25GBASE-T and 40GBASE-T are supposed to require Cat 8, but it's not clear to me whether those standards will ever be a thing, or if speeds that high will stick to fiber (and direct attach cables for short runs).

Ethernet did not traditionally take cable parameters into account when determining what speed to link up at, though some manufacturers have added their own extensions to the negotiation process which do. For 2.5 GBASE-T and 5GBASE-T the NbaseT alliance came up with a "downshift" feature to take account of cable quality, but it's not obvious if it's an IEEE standard or not.

Even where negotiation does take account of cable parameters, it's still limited to a fixed set of standard speeds. It's not like Wi-Fi or modern DSL variants which are highly adaptive to channel conditions.

So what to do in practice?

If you think 1G will be enough to last the life of the cabling, then just get at least Cat5e.

If you think you may want 10G now or in the future then you may consider Cat 6 or 6A, the trouble is though that actually achieving the Cat 6 and 6A standards can be challenging, there is lots of crap out there that doesn't meet the standards it claims and field terminations require a lot of care to meet the 6A specification. Cat6a also seems to be substantially more expensive than Cat5e.

I don't think there is any point installing Cat 7 or Cat 8 at this stage, if you think you may need more than 10G at some point in the future, either install single mode fiber now or install ducts/conduits so that you can install the cable you need when you need it.

In short:

• 100BASE-TX (100 Mbit/s) and 1000BASE-T (1 Gbit/s) require category 5 ("100 MHz").
• 10GBASE-T (10 Gbit/s) requires Category 6A ("500 MHz").
• "Smartrate" 2.5GBASE-T and 5GBASE-T work with Category 5, 6 ("250 MHz"), or 6A with varying reach.

To add to the confusion, there's an old Category 5 that was augmented to Category 5e in 2001. It's just called Category 5 today (or "Cat 5"). Many old Category 5 cables should be fine but if you want to rely on them you need a Category 5e or a newer cable.

With the exception of "Smartrate" ports, Ethernet does not "train" the cable. If it doesn't perform as required the link doesn't work (reliably). There is no fallback to lower speeds!

You can find all these IEEE 802.3 standards here (free after registration).

Edit: As @MSalters has correctly pointed out, there is a fallback, but only for 1000BASE-T ports where not all four pairs can link. It falls back to two-pair 100BASE-TX - that's not a given but Broadcom's proprietary Ethernet@Wirespeed and it's not a fallback for cable quality.

I understand your confusion.

The source of that is that some standards originate from the fields of electrical engineering and others from signal processing and network engineering, each with their own conventions and history.

In layman’s terms: The lowest common denominator in the series of network switch on one end , cable in the middle, network card or switch on the other hand governs what will be maximum possible network speed in that segment.
The connector on the cable must match the port of the device it connects to of course, RJ45 is common.
Then potentially environmental aspects come into play as well (are there for example many different cables in the same run and are shielded varieties necessary? Flexible cabling or solid strands for more rigid , structural cabling?).

Imagine a 10/100/1’000/10’0000 mbit/s port on the switch but a NIC with only a 10/100 mbit/s port
==> the cable won’t need to support more then 100 mbit/s before maximum speed is reached.
Almost any working cable will suffice for that.
CAT5 will do the job.
CAT6e will also do the job but although supporting much higher network speeds that potential still won’t increase the speed of this setup which is limited to 100 mbit by the NIC.

Don’t overthink it too much though. Gold plated connectors, silver coated copper strands , exotic dark matter or whatever audiophiles like to use for speaker cabling, network cables with those things really won’t make a difference compared to normal network cables with the same rating.

Anything that is of a specification suitable for link speed, link length, and the environment it is used in, and meets that specification(!), should be absolutely fine.

There are basically, practically only four "states" on a wired ethernet connection:

• It's working flawlessly, and everything is in the specification (cable types, length...). All bandwidth the switch and connected equipment allow can be used (however, a port being 1000baseT does NOT always mean the device with the port can unconditionally handle/supply data at that speed!)

• It's still working flawlessly, on equipment (including cable plant) that is marginal/out of specification. Still, all bandwidth is available. The slightest change (rerouting cables, degrading cables, changing switch vendor or model, using different patch panels or patch cables or adapters, environmental changes, light interference) can upset this!

• There are frequent upsets (frame drops, renegotiation, etc, will be registered in rx/tx error counters on the switch) to the connection due to equipment either being out of specification (includes cables used with unsuitable bundling, length, bend radii, etc)/marginal/defective, or being operated in an environment that provides electromagnetic or other environmental interference beyond what the equipment is suitable for (eg in an industrial environment). Can also manifest in the link being automatically downgraded to 100baseT but working stable (can happen eg if certain wires/contact in the cable or plug are broken completely).

• Connection is dead.

Best chances at getting a cable that is in spec and in the right environment, or certainly satisfactory:

• Buy brands that you would commonly find in the catalog of a supplier that caters to professional office or datacenter environments, or make from parts from reputable vendors (eg Hirose plugs, Draka cable)

• Avoid anything that is specified as "CCA cable". Aluminum belongs in the cable shield of foil shielded cables, nowhere else.

• If you want to cut corners, do so on the links that are shortest and have the least plugged-into-another sections.

• Be aware there are maximum length recommendations for a given pair of "link speed" and "CAT rating"

• Factor in the environment. Tight bends and lightweight needed, maybe don't use thick PIMF. Cables bundled together over tens of meters, or tied to metal structures, or crossing rooms with industrial equipment - get something that is shielded well and also rated for any severe temperature/outdoor operation/chemicals/....

• Consider a higher CAT rating to future-proof fixed cable plant. That also holds somewhat true of patch cables in controlled environments (datacenters etc). Forget about office last-metre stuff, it wears out and/or gets grungy eventually, you will want to renew it when you upgrade.

• Consider DAC/Twinax cables for short hauls involving SFP switches, they are sometimes cheaper than a pair of SFPs.

• Consider fiber for long hauls that connect building wings or similar. If using copper, consider getting dedicated surge protectors.

• Be aware of building codes re: cable materials you can and cannot use in plenums, risers, etc.

If you actually want to VERIFY cable plant being to specification, there are dedicated instruments for sale or rent, usually from electrical test equipment vendors, usually costing four figures and up.