Explaining "Link Test with Multiple VCs" on the 450m

Credit to Peter for posting this as an answer to a customer question originally, but I think it's a very useful explanation.

To explain the columns in the Link Test results I will use the following results obtained from a Cambium test sector.    

The columns in the first table, are :

VC                               : virtual circuit queue

Rate                            : delivered data rate averaged during the test 

Efficiency                   : fragment success rate during the link test

Transmit fragments : number of fragments transmitted during the test

Receive fragments   : number of fragments received during the test

Downlink rate           : downlink rate adapt modulation mode at the end of the test

The first row details the totals for the test showing an aggregate of 211.25 Mbps Subsequent rows detail results for individual VCs 

The second table details the grouping statistics achieved during the link test. The columns are :

Group size               : each of the possible spatially multiplexed group sizes from 1 to 7

% distribution         : the proportion of transmissions using this group size

average slot count : average number of slots per TDD used for transmissions with this group size

For these results all transmissions achieved a grouping of 3. VCs 18 and 21 are always grouped with one of the remaining four VC. None of the remaining 4 VCs could be grouped because the spatial seperation were too small.  Consequently transmissions occur to VCs 18 and 21 every TDD cycle. and transmissions to the remaining VCs every 4 TDD cycles due to the round robin scheduler.  

The reported TX fragment count for VC 18 is also consistent with the estimate based on

     6 fragments/slot due to 64QAM modulation

x 55 slots/TDD (reported in the second table)

x 400 TDD/s (configured TDD rate) 

x 10 s test time


Further commentary:

When running the Link Test with Multiple VCs, data rates per VC may be lower than that initially expected. In this test, the AP is offering data to all VCs participating in the test. Consequently the AP scheduler has to share the capacity of the wireless medium amoung all the active VC. It is this sharing which reflects the lower rate per VC compared to a test performed on a VC with no other traffic on the sector.

cnMedusa has been optimised for maximising the minimum rate that can be delivered to SMs under busy conditions. This enables the service provider to increase the number of end customers that can receive video on demand during peak time. 

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