One of my first jobs was designing cables for the ultrasound industry, where I worked with 38 to 42 AWG copper conductor in cables containing anywhere from 96 to 528 coaxes. I’ve even designed a coax .007″ in diameter for a catheter application that used a 46 AWG (.00157″ nominal) center conductor, so I have a special (odd) affinity for cable design.
Rather than spend a lot of time on the basics of Ethernet cable, I’m focusing on other aspects of the cable design that many people take for granted, such as why copper is used, the benefits of twisted pairs for signal propagation, and what makes a cable Cat 5 versus Cat 6.
A Brief Look at Ethernet Cable Construction
There’s a lot of variations of Ethernet cable out there: shielded, unshielded, 4 pair, 25 pair, etc. For blogging’s sake, I’m directing my attention to unshielded 4 pair.
At its most basic, Ethernet cable consists of 4 twisted pairs covered in an outer sheath. Some cables may include a spline or star filler to help keep the cable round. Other options include rip cords (for stripping the sheath) and drain wires. Available in both stranded and solid conductor forms, permanent wiring (the wiring inside the wall that connects a wall socket to a central patch panel) is solid core, while patch cables are typically stranded. Although it’s fairly durable, never bend the cable tighter than four times the outside diameter of the cable – the cable will develop hotspots and performance will degrade.
Most network cables are insulated with polyethylene or PVC, which are both halogen-based. In a fire, halogen-containing plastics release hydrogen chloride, a poisonous gas that forms hydrochloric acid when it comes in contact with water. Halogen-free cables, sometimes called low smoke zero halogen (LSZH or LSOH), do not produce a dangerous gas/acid combination or toxic smoke when exposed to flame. Low smoke zero halogen sheathing is becoming popular and, in some cases, a system requirement. Other benefits of halogen-free cable include:
- Weight: LSOH is often lighter, so overall cable network weight can be reduced
- Environmental Impact: The fewer toxic chemicals used in construction mean less impact downstream
Some cables are “UV-rated” or “UV-stable” meaning they can be exposed to outdoor UV radiation without significant destruction/degradation. Non-UV stable jackets, when subjected to direct sunlight and other substances, will leach out its plasticizers, often making the cable jacket feel slightly greasy.
10BASE-T and 100BASE-TX only require two pairs to operate, located on pins 1 plus 2 and pins 3 plus 6. Since 10BASE-T and 100BASE-TX need only two pairs and Cat 5/5e cable has four pairs, it is possible (but not standards compliant) to use the unused pairs (pins 4–5, 7–8) for PoE. However, 1000BASE-T requires all four pairs to operate.
Why Copper Is Used in Cables
Copper is an ideal conductor for building wiring, but not because it’s the most conductive metal. Silver is actually the best conductor, with an electrical conductivity 106% of that of annealed copper. However, the high cost of silver combined with its low tensile strength limits its use to special applications, such as joint plating and sliding contact surfaces.
Pound for pound (literally speaking), copper’s not even as good a conductor as aluminum. When you compare the cubic densities of copper (559 lb./cu. ft.) and aluminum (169 lb./cu. ft.), and factor in that aluminum has 56% the conductivity of copper, the result is that one pound of aluminum has the same electrical capability as nearly two pounds of copper. At first blush this sounds very promising, but the decreased conductivity of aluminum means that for the same current carrying capacity, an aluminum conductor’s cross section will be ~50% larger than that of copper, making copper better suited for applications where space is a premium. For applications where conductor thickness is an advantage, such as aerial electric power transmission cables, copper is rarely used.
Copper has a number of other benefits:
- Tensile Strength: Copper’s high strength (~2x that of aluminum) resists stretching, neck-down, creep, nicks and breaks, which reduces field failures and service interruptions.
- Ductility: Copper’s ductility makes it easy to draw down to diameters with very close tolerances. This doesn’t conflict with tensile strength, per se–copper is just less susceptible to breaking under tensile load than other conductors.
- Thermal Expansion: Copper has a low coefficient of thermal expansion. By comparison, aluminum expands about 30% more than copper. This higher degree of expansion, along with aluminum’s lower ductility, can cause electrical problems when bolted connections are improperly installed.
- Corrosion Resistance: Copper is fairly resistant to moisture, industrial pollution, and other corrosives. A significant differentiating factor is that oxides or other chemical compounds that do form on copper are conductive. Therefore, copper connections and terminations will not overheat from corrosion due to increased conductor resistance.
- Pliability: Usually the stronger a metal is, the less pliable it is. This is not the case with copper. A unique combination of high strength and high ductility makes copper ideal for wiring systems. At junction boxes and at terminations, for example, copper can be bent, twisted, and pulled without stretching or breaking. As pliable as it is, though, it’s typical for larger cables to use stranded copper (versus solid). Stranding improves cable life by making it more flexible and has nearly the same conductivity as a single-strand (solid) conductor. Solid wire is typically reserved for smaller diameters.
The Benefits of Twisted Pair Cable Construction
Invented by Alexander Graham Bell in 1881 for the telegraph market, twisted pairs were designed to reduce or minimize crosstalk between long cable lengths running in parallel. Known as differential mode transmission, the two wires carry equal and opposite signals, and the destination detects the difference between the two. Interference signals tend to couple to both wires equally, producing a common-mode signal which is cancelled at the receiver when the difference signal is taken.
This method starts to fail, though, when the source of interference is close to the signal wires; the closer wire will couple with the noise more strongly and the common-mode rejection of the receiver will fail to eliminate it. In a situation like Ethernet cables, one pair can induce crosstalk in another and it becomes additive along the length of the cable. By twisting the wires at different rates, the crosstalk effect is negated. At each half twist, the wire nearest to the noise source is swapped. Providing the interfere source remains relatively uniform over the distance of a single twist, the induced noise will remain common-mode.
The twist rate (also called “pitch of the twist” and usually defined in twists per meter) makes up part of the specification for Ethernet cable. By altering the length of each twist, crosstalk is reduced.
Cable category certification is primarily concerned with the frequency ranges (100MBit/s, 1GBit/s, 10GBit/s) that can be successfully transmitted through a cable while meeting established guidelines for certain transmission characteristics (propagation delay, insertion loss, and near-end crosstalk to name a few).
Cat 3– 16MHz (obsolete)
Cat 5/5e*– 100MHz (standard for Fast Ethernet)
Cat 6– 250MHz (standard for Gigabit Ethernet)
Cat 6a– 500MHz
*The category 5e specification tightened some crosstalk specifications and added some crosstalk specifications not present in the original. The bandwidth of Cat 5 and 5e is the same– 100 MHz.
One subtle physical difference between network cables is the nominal conductor gauge that’s used: Cat 5/5e cables typically use 24-26 AWG wire, while Cat6/6a cables tend to use slightly larger conductors of 22-24 AWG, making them a little stiffer and heavier. As a result of Cat 6’s higher rated operating frequency (and related susceptibility to EMI), the specifications for Cat 6 crosstalk and system noise are more stringent than those for Cat 5/5e.
Another item worth mentioning is the maximum length of cable run. When used for 10/100/1000BASE-T, the maximum allowed length of a Cat 6 cable is 100 meters (90 meters of cabling between the patch panel and the wall jack, plus 10 meters of cable between the jack and attached device). When used for 10GBASE-T though, Cat 6 cable’s maximum length is 55 meters or less, depending on crosstalk. Cat 6a does not have this limitation and can transmit 10GBASE-T up to 100 meters without issue.
We’ll take a look at the anatomy of other signal conductors in future blog posts.