Comtech EF Data CDM-625A - Manual (Page 186)

CDM-625A Advanced Satellite Modem MN-CDM625A
Front Panel Operation Revision 3
6–60
When the rate at which packets received from the LAN side approaches the maximum
rate the modem can handle, the FIFO acts as an elastic buffer that will store a packet,
or packets, on a temporary basis. If the WAN Buffer capacity is exceeded, it
will drop IP packets at exact packet boundaries. This causes the minimum impact to
the IP link.
What does the WAN Buffer size mean?
The WAN Buffer is sized in milliseconds. It can range from a minimum of 20 ms to a
maximum of 400 ms. The maximum number of bits the WAN Buffer can hold is given
by:
Size in bits = modem Tx bit rate * buffer size
(where Tx bit rate is expressed in kbps, and buffer size is expressed in milliseconds)
For example, if Tx bit rate = 1000 kbps, and WAN Buffer size = 100 ms, size in bits =
100,000.
The corresponding size in bytes = 12.5 kbytes.
Therefore, in the example shown, the WAN Buffer can store just over 8 packets with a
size of 1500 bytes.
Does the WAN Buffer size affect latency?
Yes and No:
• When Yes – When the LAN is over-driving the modulator (the rate at which
packets are being received exceeds the capacity of the data rate set for the
modulator) the WAN Buffer will constantly fill, and this will add latency, up to the
maximum programmed size. If the LAN constantly over-drives the Modulator,
packets will be dropped, and re-transmission will be required.
• When No – When the router or other device on the LAN side is effectively rate-
limiting the flow of data to the modulator (to a value less than the Tx data rate of
the Modulator), the WAN Buffer should not be filling, and will not add latency to
the link. Momentary packet congestion may cause the WAN Buffer to fill (and
hence increase latency on temporary basis) but as soon as the congestion clears
the latency will return to near zero.
What about Pause Frames?
In cases where the router (or other attached device) does not have the capability to
rate limit, Pause Frame Flow Control may be used to stop the WAN Buffer from
dropping packets. In this case, if the WAN Buffer depth exceeds 80%, a Pause Frame
will be sent back to the LAN to instruct the router (or other attached device) to stop
sending data until the WAN Buffer depth has reduced.