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PRV-1059 User Manual
Half- and Full-Duplex Flow Control
Half- and Full-duplex flow control is used to throttle the end station to avoid dropping packets during
network congestion. When the free buffer space is almost empty or the output queue is almost full, the
MACs force a collision in the input port when sensing an incoming packet. Full-Duplex uses frame
pauses to handle the congestion.
Address Management
The switching function of the module involves the learning how to switch packets to the correct MACs, and only
to the correct ones. The switch learns what port to which an end station is connected by remembering each
packet’s Source Address, and the port number the packet came in on.
When a packet is directed to an unlearned MAC address, the packet is transmitted to all of the ports. When an
end-station responds back, its address is learned and stored until it is aged (see section 3.4.4). Once a MAC
address/port is learned, all future packets directed to that address will be forwarded solely to that port.
When an end station is moved from one port to another, a new MAC address/port number association will have
to be learned, and the new one replaced. For this to occur, the old association needs to expire or “age”. These
issues are handled by the “Aging” function (see section 3.4.4).
Address Translation Unit
The Lookup Engine or Address Translation Unit (ATU) reads the Destination Address (DA) and Source
Address (SA) from each received packet, performing all address searching, learning and aging functions
for all five ports at “wire speed” rates. It uses a hashing technique for quick storage and retrieval. If
there is a hash collision, there is a four entry bin to hold MAC address with the same hash. The address
table has storage for up to 1024 entries in the embedded SRAM, and has a default 300 second (5 min)
aging time.
Address Searching
The address search engine searches the address database to obtain the output port number, called the
Destination Port Vector (DPV), for each frame’s Destination Address (DA). It arbitrates destination
address requests from the ports and grants one lookup at a time. The MAC address is hashed, and then
data is read from the SRAM table and compared to the MAC address for a match. If the compare
matches, the DPV is returned to the output port queue manager, where is may be modified by the
VLANTable data. When no match is found, a unique default DPV is returned to the output port queue
manger, which typically sends the frame to all ports (to learn the MAC address).
Address Learning
An address-learning engine is used to learn the source address. Up to 1024 MAC address/port number
mappings can be stored in the address table.
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