Ethernet uses \$\pm0.85\;\mathrm{V}\$ signaling so a signal will only be able to "activate" one diode pair. So any transmission only has a chance to reach the two immediate neighbors (no current will flow more than 1 "hop" round the loop).
Now you can notice the trick which will work only with 3 devices: both of computer A TX lines' have RX lines from computers B and C in immediate neighborhood and it's own RX lines on the far side of the loop. You really don't want to hear what you are sending since it would trigger the collision detection algorithm.
Two interesting points:
The diodes will heavily attenuate the signal so it probably won't work over longer distances.
You cannot use resistors since they would linearly attenuate the signal and after going all the way around the loop it would finally reach your own RX line. It would be attenuated but the receiver circuit is very sensitive so it would still be able to detect is as a collision. You need a nonlinear element (like a diode) that provides a sharp cutoff.
PS. It is a really clever circuit. I cannot imagine inventing something like that on my own. :)
There are actually three terms you want to know about
Bandwidth
Bandwidth is measured in Hz. It describes the frequency band that a communication channel is able to transmit with low loss.
Typically we talk about a 3-dB bandwidth, meaning the range of frequencies a channel can transmit with less than 3 dB of loss. For a baseband system, the bandwidth extends from 0 Hz to a frequency B which we call the bandwidth. For a modulated system if the carrier is at f0, then the transmission band would be from \$f_0 - B/2\$ to \$f_0 + B/2\$.
Also, outside of information theory, the term bandwidth may be used more broadly as a synonym for bit rate, or for data processing capacity, but when the units are Hz, we know we're talking about the analog bandwidth of a signal path of some kind.
Baud
You didn't ask about this, but its also important to keep this separated in your mind from the other two terms. Baud is the number of symbols transferred per second on the channel.
Bit rate
Bit rate indicates the amount of information transferred on a channel, and is measured in bits per second or bps. Bit rate is different from baud if more than one bit is transferred per symbol. For example, in a 4-level amplitude modulation scheme, each symbol can encode 2 bits of information. Alternately, for example when an error-correcting code is used, the bit rate can be less than the baud rate, as a larger number of symbols are used to convey a smaller number of bits of independent information.
The Shannon Theorem shows how bit rate is limited by bandwidth and the signal-to-noise ratio of the channel:
\$C = B\ \log_2(1 + \mathrm{SNR})\$
where C is the capacity (maximum bit rate of the channel), B is the bandwidth of the channel, and SNR is the signal to noise ratio.
Best Answer
To elaborate on @Majenko's answer, both SGMII and 1000Base-x are dual 1000Mbps SERDES pairs (one in each direction), at least until the 1000Base-X signals reach the optical transceiver. The main difference is in the auto-negotiation capabilities.
In 1000Base-X, auto-negotiation is limited to flow-control (and duplex, which is not really used since it's always full-duplex).
In SGMII, auto-negotiation also allows the PHY to indicate to the MAC the post-PHY link speed. Even though the MAC-to-PHY SGMII link is always 1000Mbps, it supports 10, 100 and 1000Mbps past the PHY and the MAC need to know this to space out the bits properly (e. g. if the external link is 100Mbps, each bit on the SGMII link is sent 10 times).