Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
33
3-3. Flow Control
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. 12 Gigabit Web Smart Switch with PoE) 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 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.
where
k = min (n, 10)
