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3-3. Flow Control
Flow control is a mechanism to tell the source device stop sending frames 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 also asymmetric flow controls for special applications. In
Gigabit Ethernet, both symmetric flow control and asymmetric flow control are
supported. Asymmetric flow control only allows transmitting PAUSE frame in
direction way from one side, the other side does not but only receive and discard
the flow control information. Symmetric flow control allows both two ports to transmit
PASUE frames to 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, at 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 will always occur
and interfere with each other. This results the carrier signal distorted and un-
discriminated. When a collision is detected during a frame transmission, the
transmission will not stop immediately but, instead, continue transmitting until the
rest bits specified by jamSize are completely transmitted. This guarantees the
duration of collision to be 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 frames. The backoff time is
determined by the formula below. When the times of collision is increased, the
backoff time is getting longer until the collision times excess 16. If this happens, the
frame will be discarded and backoff time will also be reset.
where
k = min (n, 10)