High Performance Single-Chip 10/100 Non-PCI Ethernet Controller
Datasheet
3.2
Flow Control
The LAN9117 Ethernet MAC supports full-duplex flow control using the pause operation and control
frame. It also supports 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 parameter 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
3.2.3
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 current 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 transmission. 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.
Virtual Local Area Network (VLAN)
VLAN is a means to form a “broadcast domain” without restriction on the physical or geographical
location on the members of that domain. VLAN can be implemented in any number of different factors,
such as:
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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.
SMSC LAN9117
Revision 1.1 (05-17-05)
DATA2S3HEET
SMSC [ SMSC CORPORATION ]
