It has been noted that TCP tput observed on an ePMP link is lower than the wireless link test tool or UDP tput when the test is performed using Mikrotik router boards and the built in bandwidth tester.
Cambium believes that the lower TCP tput observed is in direct co-relation to both TCP layer configuration parameters as well as test conditions, all of which can contribute to the lower tput
Default TCP Window Size
TCP Window Scaling (RFC 1323 – TCP Extensions for High Performance)
Latency
In a real world backhaul or access point scenario, there will be multiple TCP sessions established over a link. The performance of each single TCP session will vary depending on network conditions and TCP layer configuration parameters at each endpoint. Therefore, the lower tput observed over a single TCP link with a very small window size does not represent the overall capacity of the link and will not be an issue in a typical deployment.
In the case of a single stream TCP session and a smaller TCP window size such as 8Kb, the latency of the link will impact the achievable tput, as predicted by the TCP Bandwidth-Delay product. In this case, the lower the latency the higher the tput for a single TCP session test.
In tests performed at Cambium, maximum TCP tput was achieved using Mikrotik Bandwidth tester tool and two router boards on each end of an ePMP PTP link. The topology of the link is as follows:
It was observed that with Router OS version 6.2X, the default TCP parameters utilized by the router boards allowed for a large TCP RX Window Size and Selective ACKs option, by enabling the TCP extensions in RFC1323 by default.
With these default parameters, the ePMP link demonstrated the maximum tput at 20MHz, TDD 75/25 scheduler mode as seen by the screen capture below.
TCP DL (single stream unidirectional):
TCP UL (single stream unidirectional):
The wireshark capture of actual TCP traces during a single stream TCP test also highlighted the default parameters used.
In conclusion, Cambium is recommending that the tests are run with the latest MT Router OS version to demonstrate the maximum TCP tput or increasing the TCP session count during the tests.