Net Optics GigaBit Fiber SFP Port Aggregator Tap - Manual (Page 8)

GigaBit Fiber Port Aggregator Tap
4
Application Diagrams: Memory Operation
All traffic that passes through the Tap is sent to the monitoring device NIC on
a first-in, first-out basis, including traffic that is temporarily stored in memory.
(If two packets enter at the same time then one packet is processed while the
other is stored briefly in memory, preventing collisions.)
When there is a burst of data, traffic in excess of the NIC's capacity is sent to
the Tap's memory . Up to 512 megabytes of data per side of the full-duplex
stream can be stored in memory. Memory continues to fill until its capacity is
reached, or the burst ends – whichever comes first.
In both cases, the Tap applies a first-in, first out procedure, processing stored
data before new data from the link. If memory fills before the burst ends, the
memory stays filled as the stored data is processed – data that leaves the buffer
is immediately replaced. If the burst ends before the memory fills, memory
clears until the full megabyte of capacity is available, or another until another
burst in excess of the NIC's capacity requires additional memory .
The following three diagrams illustrate a simple example of a 1000 Mbps NIC
moving from 80% utilization, to 140% utilization, then back to 80%
utilization .
Figure 1: Aggregated traffic is less than NIC's capacity
Using a single NIC, the monitoring
device receives all combined traffic
from Side A and Side B, including
physical layer errors.
GigaBit SX SFP
Port Aggregator Tap
Span Port 2
Span Port 1
Monitoring Device
1
Side A Side B
Side A +
Side B
State 1: Side A + Side B is less than or equal to 100%
of the NIC's receive capacity.
Example: On a 1000 Mbps Span port, Side A is at 300 Mbps and Side B is at 500 Mbps.
The NIC receives 800 Mbps of traffic (80% utilization), so no memory is required
for the monitoring device NIC to process all full-duplex traffic.
1 2 Network
OUT IN OUT IN
A B Port Aggregator
1 Network Monitor 2 A B