User`s guide

Signamax 065-7931 16-Port 10/100/1000BaseT/TX WebSmart Switch

An Ethernet MAC transmits frames in half-duplex and full-duplex modes. In

the half-duplex operation mode, the MAC can either transmit or receive a frame at a

given moment, but cannot do both jobs at the same time.

As the transmission of a MAC frame with the half-duplex operation exists

only in the same collision domain, the carrier signal needs to spend time to travel to

reach the targeted device. For two most-distant devices in the same collision

domain, when one sends the frame first, and the second sends the frame, in the

worst case, just before the frame from the first device arrives. The collision happens

and will be detected by the second device immediately. Because of the medium

delay, this corrupted signal needs to spend some time to propagate back to the first

device. The maximum time to detect a collision is approximately twice the signal

propagation time between the two most-distant devices. This maximum time is

traded off by the collision recovery time and the diameter of the LAN.

In the original 802.3 specification, Ethernet operates in half duplex only.

Under this condition, when in a 10 Mbps LAN, it’s 2500 meters, in a 100 Mbps LAN,

it’s approximately 200 meters, and in 1000 Mbps, 200 meters. According to the

theory, it should be 20 meters. But it’s not practical, so the LAN diameter is

maintained by increasing the minimum frame size with a variable-length non-data

extension bit field which is removed at the receiving MAC. The following tables are

the frame formats suitable for 10 M, 100 M and 1000 M Ethernet, and some

parameter values that shall be applied to all of these three types of Ethernet.

Actually, the currently-available Gigabit Ethernet chips do not feature this at

this time. They all have their chips supported in the full-duplex mode only, and this

applies to all network vendors’ devices as well. So, this criterion should not exist at

the present time or in the future. The switch’s Gigabit modules support only the full-

duplex mode.

Fig. 3-4 Gigabit Ethernet Frame

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