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3-3. Flow Control
Flow control is a mechanism to tell the source device to stop sending a 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. 16-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.
Inter-frame Gap time
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.
Collision
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 un-discriminated. 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 of the 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 exceed 16. If this happens,
the frame will be discarded and backoff time will also be reset.
where
k = min (n, 10)
