Eli5 why computer networking cables (ie cat5e) need four pairs of wires when the dsl that feeds my home only uses a single pair?

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Eli5 why computer networking cables (ie cat5e) need four pairs of wires when the dsl that feeds my home only uses a single pair?

In: Technology

You can only squeeze so much data down a pair of wires. Even top end DSL only does about 150mbps and that’s just in one direction. 4 pairs of wires can currently do 10 gigabits birectionally.

Your DSL is relatively slow, most DSL tops out at 10s of Mbps

100 Mbps ethernet uses two pairs of wires (one pair for talking, the other for receiving), while 1000 Mbps ethernet uses all four pairs (two talking and two receiving). Four twisted pairs in CAT5e supports up to 5 Gbps, about 50x more than the two wires in your DSL line with just 4x the number of wires.

CAT5 and CAT6 have 4 twisted pairs because it greatly reduces noise and allows for really high speed communication so you don’t have to upgrade all your cabling every time standards enable higher bandwidth.

Note that 100 Mbps Ethernet (the 100BASE-TX of “Fast Ethernet”) only uses two of the four pairs; one for transmit and one for receive. It’s only when we stepped up to Gigabit Ethernet (1000BASE-T) that we started using all four pairs. In addition, Gigabit Ethernet uses each pair for both transmit and receive simultaneously so it has four transmit lanes and four receive lanes available compared to only one for Fast Ethernet. That’s how most of the factor-of-ten speed increase was achieved.

100 Mbps Ethernet is still widely used by things like home routers.

It’s true there were things like 100BASE-T4 which used four low-quality pairs for 100 Mbps but this was much rarer than the two-pair 100BASE-TX which used to be synonymous with “Ethernet” for many people.

So why did we run four pairs everywhere when we only needed two. Mostly future-proofing. It was also possible to run a phone line on one pair and Ethernet on another other two, all down a single cable.