LAN9117 Datasheet, PDF (17/114 Pages) SMSC Corporation – HIGH PERFORMANCE SINGLE-CHIP 10/100 NON-PCI ETHERNET CONTROLLER

LAN9117
3.2 Flow Control
The LAN9117 Ethernet MAC supports full-duplex flow control using the pause operation and control frame. It also sup-
ports half-duplex flow control using back pressure.
3.2.1 FULL-DUPLEX FLOW CONTROL
The pause operation inhibits data transmission of data frames for a specified period of time. A Pause operation consists
of a frame containing the globally assigned multicast address (01-80-C2-00-00-01), the PAUSE opcode, and a param-
eter indicating the quantum of slot time (512 bit times) to inhibit data transmissions. The PAUSE parameter may range
from 0 to 65,535 slot times. The Ethernet MAC logic, on receiving a frame with the reserved multicast address and
PAUSE opcode, inhibits data frame transmissions for the length of time indicated. If a Pause request is received while
a transmission is in progress, then the pause will take effect after the transmission is complete. Control frames are
received and processed by the MAC and are passed on.
The MAC also transmits control frames (pause command) via both hardware and software control. The software driver
requests the MAC to transmit a control frame and gives the value of the PAUSE time to be used in the control frame.
The MAC Function constructs a control frame with the appropriate values set in all the different fields (as defined in the
802.3x specification) and transmits the frame to the MII interface. The transmission of the control frame is not affected
by the current state of the Pause timer value that is set because of a recently received control frame.
3.2.2 HALF-DUPLEX FLOW CONTROL (BACKPRESSURE)
In half-duplex mode, back pressure is used for flow control. Whenever the receive buffer/FIFO becomes full or crosses
a certain threshold level, the MAC starts sending a Jam signal. The MAC transmit logic enters a state at the end of cur-
rent transmission (if any), where it waits for the beginning of a received frame. Once a new frame starts, the MAC starts
sending the Jam signal, which will result in a collision. After sensing the collision, the remote station will back off its trans-
mission. The MAC continues sending the jam to make other stations defer transmission. The MAC only generates this
collision-based back pressure when it receives a new frame, in order to avoid any late collisions.
3.2.3 VIRTUAL LOCAL AREA NETWORK (VLAN)
VLAN is a means to form a “broadcast domain” without restriction on the physical or geographical location on the mem-
bers of that domain. VLAN can be implemented in any number of different factors, such as:
• Physical port
• MAC address
• Layer-3 unicast address
• Multicast address
• Date/time in combination with MAC address, etc.
An example of a VLAN is depicted in Figure 3-1, "VLAN Topology". It demonstrates the freedom from physical constraint
on the network, and the ability to divide a single switched network into a smaller broadcast domain.
Moreover, VLAN offers a number of other advantages, such as:
Configurability: Changes to an existing VLAN can be made on the network administrative level, rather than on the
hardware level. A member of a VLAN can thus change its MAC address or its port and still be a member of the same
VLAN. Extra routing is not necessary.
Security: VLAN can improve security by demanding a predefined authentication before admitting a new member to the
domain.
Network efficiency: Allows shielding one system resource from traffic not meant for that resource. A workstation in one
VLAN is shielded from traffic on another VLAN, increasing that workstation’s efficiency.
Broadcast containment: Leakage of broadcast frames from one VLAN to another is prevented.
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