Throughput for the PMP 54400 Access Point (AP) and Subscriber Module (SM)
Motorola Point-to-Multipoint (PMP) Solutions OFDM Technical Notes and Clarifications:
To achieve 20 Mbps throughput for the PMP 54400 radio, benchmarking should be done with a configuration of 1 AP supporting 3 SMs. Specifications for the publicized aggregate throughput for PMP 100 series FSK and PMP 400 series OFDM radios are based on this system setup and are used consistently in the Motorola testing labs for performance testing.
The throughput for very high uplink ratios (i.e. 20/80 or 10/90) cannot be maximized in a single-threaded system of 1 AP and 1 SM. In a single SM scenario, there is a considerable amount of inactive time between transmissions as the AP empties the data buffer and waits for ACKs from the network. Thus, the SM may be waiting for a bandwidth grant while the AP is still emptying its buffer. It is recommended to limit the maximum uplink to 80 to allow the AP to send the necessary frames for transmissions.
The Motorola PMP system, both 100 series FSK and 400 series OFDM, is designed to optimally support the multi-point situation. Motorola PMP protocols include requests for bandwidth from the SM and bandwidth grants from the AP. Motorola PMP 400 APs are designed to support multiple SMs. Each SM requests bandwidth from the AP and waits for a bandwidth granted message to begin the transfer of data. Each SM is given its own buffer to support data transfer. The buffer must be emptied before the requesting SM will be sent a bandwidth grant. The round robin algorithm moving between SMs more effectively uses the AP’s CPU.
Packet size is also a consideration. Very small packets can impact throughput also. There is additional overhead for bandwidth requests, bandwidth grants, and ACKs when using small packets. The standard for testing larger packets is 1,500 byte packets.
The customer should consider if their specific application is more suited to PTP. Backhauls do not use the same protocol as the AP. Thus, the overhead of the message frames required to pass data is less. The use of PTP could increase the throughput for the customer.