Draytek VigorSwitch G1260, Руководство пользователя

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VigorSwitch G1260 User’s Guide 26
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Flow control is a mechanism to tell the source device stopping sending frame for a
specified period of time designated by target device until the PAUSE time expires. This is
accomplished by sending a PAUSE frame from target device to source device. When the
target is not busy and the PAUSE time is expired, it will send another PAUSE frame with
zero time-to-wait to source device. After the source device receives the PAUSE frame, it
will again transmit frames immediately. PAUSE frame is identical in the form of the MAC
frame with a pause-time value and with a special destination MAC address
01-80-C2-00-00-01. As per the specification, PAUSE operation can not be used to inhibit
the transmission of MAC control frame.
Normally, in 10Mbps and 100Mbps Ethernet, only symmetric flow control is supported.
However, some switches (e.g. 24-Port GbE Web Smart Switch) support not only
symmetric but asymmetric flow controls for the special application. In Gigabit Ethernet,
both symmetric flow control and asymmetric flow control are supported. Asymmetric flow
control only allows transmitting PAUSE frame in one way from one side, the other side is
not but receipt-and-discard the flow control information. Symmetric flow control allows
both two ports to transmit PASUE frames each other simultaneously.
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After the end of a transmission, if a network node is ready to transmit data out and if there
is no carrier signal on the medium at that time, the device will wait for a period of time
known as an inter-frame gap time to have the medium clear and stabilized as well as to
have the jobs ready, such as adjusting buffer counter, updating counter and so on, in the
receiver site. Once the inter-frame gap time expires after the de-assertion of carrier sense,
the MAC transmits data. In IEEE802.3 specification, this is 96-bit time or more.
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Collision happens only in half-duplex operation. When two or more network nodes
transmit frames at approximately the same time, a collision always occurs and interferes
with each other. This results the carrier signal distorted and undiscriminated. MAC can
afford detecting, through the physical layer, the distortion of the carrier signal. When a
collision is detected during a frame transmission, the transmission will not stop
immediately but, instead, continues transmitting until the rest bits specified by jamSize are
completely transmitted. This guarantees the duration of collision is enough to have all
involved devices able to detect the collision. This is referred to as Jamming. After jamming
pattern is sent, MAC stops transmitting the rest data queued in the buffer and waits for a
random period of time, known as backoff time with the following formula. When backoff
time expires, the device goes back to the state of attempting to transmit frame. The backoff
time is determined by the formula below. When the times of collision is increased, the
backoff time is getting long until the collision times excess 16. If this happens, the frame
will be discarded and backoff time will also be reset.