One thing to understand is that the performance numbers provided in the data sheet specifically state "Up to" the specific Mbps number.
You can read this as stating that in optimal conditions the 5540/AIP-SSM 20 combination can performance "Up to" 500 Mbps.
Which means in most real world situations the performance will actually be lower.
Let me compare this to performance numbers we state for the IPS-4260 and IPS-4270 IPS Appliances.
We have gone to a method of stating 2 different performance numbers for each platform. We have two primary performance tests that are run against the platforms. The first is what we call a Transactional test, and the second is what we call a Media Rich test. The Transaction test is configured to use short lived connections downloading smaller files. The Media Rich test is configured to use longer lived connections downloading larger files.
The IPS-4260 is rated at 1 Gbps for Transactional and 2 Gbps for Media Rich.
The IPS-4270 is rated at 2 Gbps for Transactional and 4 Gbps for Media Rich.
The theory is that most customer networks will fall between these 2 types of traffic and so the REAL performance of the sensors on your network would be somewhere between the Transactional and Media Rich ratings.
(NOTE: There are a few exceptions where the REAL performance may actually fall below the Transactional rating. This usually happens in situations where the customer is running a web server used for higher rates of small transactions than what is simulated in our Transactional performance test. By the same token there are also a few sites where REAL performance is even higher than our Media Rich test because the web sites serves out extremely large files.)
So if you are a typical customer (and not one of the extremes) you would expect performance somewhere between the Transactional rating and the Media Rich rating.
So we've told you what the Transactional and Media Rich ratings are for the Appliances, but you won't find those terms when looking at the Data Sheets for the AIP-SSM combinations.
What you would need to know is that the "Up to" performance numbers documented in the AIP-SSM data sheets is the equivalent of the Media Rich performance test rating.
So the 500Mbps for the AIP-SSM 20 and ASA 5540 is how that combination did in the Media Rich test.
As for the Transactional test ratings, these have not been published for the AIP-SSMs.
However, I can give you some rough estimations.
AIP-SSM 10 => 100 Mbps
AIP-SSM 20 => 200 Mbps
AIP-SSM 40 => 400 Mbps
(Note: these are just rough numbers. The actual performance with the Transactional test will vary depending on the ASA chassis that the AIP-SSM is placed within)
So for your AIP-SSM-20 and ASA 5540 combination you could expect a REAL performance somewhere between the rough 200 Mbps Transactional rating, and the data sheet documented 500 Mbps Media Rich rating. The REAL performance in your network will depend on whether your traffic is more like our Transactional test or more like our Media Rich test.
Here is a link to the White Paper that talks about the 2 tests I mentioned above:
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps5729/ps5713/ps4077/prod_white_paper0900aecd806e7283.html
