Manual Draytek VigorSwitch G1240 (page 29 of 83) (English)

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VigorSwitch G1240 User’s Guide

the data field and FCS field in turn. The followings summarize what a MAC does before

transmitting a frame.

1. MAC will assemble the frame. First, the preamble and Start-of-Frame delimiter will be

put in the fields of PRE and SFD, followed DA, SA, tag ID if tagged VLAN is applied,

Ethertype or the value of the data length, and payload data field, and finally put the

FCS data in order into the responded fields.

2. Listen if there is any traffic running over the medium. If yes, wait.

3. If the medium is quiet, and no longer senses any carrier, the MAC waits for a period of

time, i.e. inter-frame gap time to have the MAC ready with enough time and then start

transmitting the frame.

4. During the transmission, MAC keeps monitoring the status of the medium. If no

collision happens until the end of the frame, it transmits successfully. If there is a

collision happened, the MAC will send the patterned jamming bit to guarantee the

collision event propagated to all involved network devices, then wait for a random

period of time, i.e. backoff time. When backoff time expires, the MAC goes back to the

beginning state and attempts to transmit again. After a collision happens, MAC

increases the transmission attempts. If the count of the transmission attempt reaches 16

times, the frame in MAC’s queue will be discarded.

Ethernet MAC transmits frames in half-duplex and full-duplex ways. In halfduplex

operation mode, the MAC can either transmit or receive frame at a 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 worstcase, 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 10Mbps LAN, it’s 2500 meters, in 100Mbps LAN, it’s approximately

200 meters and in 1000Mbps, 200 meters. According to the theory, it should be 20 meters.

But it’s not practical, so the LAN diameter is kept by using to increase 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 format suitable for 10M, 100M and 1000M

Ethernet, and some parameter values that shall be applied to all of these three types of

Ethernet.

Actually, the practice Gigabit Ethernet chips do not feature this so far. They all have their

chips supported full-duplex mode only, as well as all network vendors’ devices. So this

criterion should not exist at the present time and in the future. The switch’s Gigabit module

supports only full-duplex mode.

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Draytek VigorSwitch G1240 Manual

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