# How is 1000Mbs achieved on 1000BASE-T?

• posted

I'm trying to understand how 1Gbs is achieved on Category 5e cabling using a 1000BASE-T. This Dell document reads:

"1000BaseT physical layer standard provides 1 Gbps Ethernet signal transmission over four pairs of Cat-5 unshielded twisted-pair (UTP) cable. It transmits at 125 Mbaud, the same symbol rate as Fast Ethernet. But by using more sophisticated five-level pulse amplitude modulation (PAM-5) coding along with four wire pairs, it can transmit much more data. To simplify, each wire pair sends and transmits simultaneously, for 250 Mbps per pair (125 Mbaud ¥ 2 bits). Multiplying 250 Mbps by four pairs yields the nominal rate of 1000 Mbps."

So does that mean we get 1000Mbs when in full-duplex only? This link:

states that one can achive 2Gbs in full-duplex. Put simply, assuming a simplex communications scenario, say, just receiving frames, what is the upper limit on 1000Base-T over category 5e cabling?

Thanks for any information,

Mike Mascari

• posted

(snip)

1Gbit/s in each direction at the same time. Most people don't call that 2Gb/s, except those who work in the marketing dept.

It is possible to send data both directions on the same wire pair at the same time. In theory it is easy, in practice complicated echo cancellation circuits are needed.

-- glen

• posted

Okay. So, let me see if I understand correctly:

1. Unlike 10/100BASE-T, which dedicates lines to either transmit or receive, 1000BASE-T can use any pair for either, thus doubling its bandwidth potential. IOW, it can use all four pairs to simultanously transmit data.
2. 4 pairs * 125Mbs = .5Gbps

1. "5 PAM" -> double transmission rate = 1Gbps

If, per pair, one were to simultaneously transmit and receive, one would double the "aggregated bandwidth" or whatever you want to call it to 2Gbps.

Is that right? What does half-duplex vs. full-duplex mean in this context?

Confused,

Mike Mascari

• posted

(snip)

yes

I believe like 100baseTX it uses continuous signaling. When there is no data to transmit it sends codes that indicate that no data is being transmitted.

In that case, it signals a collision in the case that data is received while it is also being transmitted. It would do that even though the wire could transmit data just as easily as idle codes.

-- glen

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