User's Manual

12 MDS Mercury 16E Technical Manual MDS 05-6302A01, Rev. A
3.3 ARQ and Hybrid ARQ
Automatic Retransmission Request (ARQ) enables retransmission of
erroneous or lost data packets. Hybrid ARQ (HARQ) combines forward
error correction with ARQ retransmissions to improve performance at
lower RF signal levels.
With ARQ, the receiver discards erroneous packets and requests
retransmission. With HARQ, erroneous packets are saved by the
receiver and combined with the retransmitted data. Generally, HARQ
provides better throughput than ARQ. While ARQ and HARQ can be
enabled at the same time, it is not recommended to do so because
throughput will be less than if either ARQ or HARQ was enabled on its
own.
ARQ and HARQ can be enabled or disabled in the ARQ/HARQ Settings
table of the Configuration-Radio page on the Base Station.
ARQ Setup
ARQ utilizes a sliding window approach where a “window” of blocks
can be transmitted without receiving acknowledgement from the
receiver. ARQ blocks that are unacknowledged will be resent. You can
specify the block and window size at the Base Station, as well as Block
Lifetime, Transmitter Delay, and Receiver Delay.
• ARQ Block Size - The size, in bytes, of the block of data to be
considered for retransmission.
• ARQ Window Size - The number of blocks of ARQ data that
can be transmitted without receiving an acknowledgment.
• ARQ Block Lifetime - The maximum period, in milliseconds,
that the ARQ block is considered still valid and can be retrans-
mitted.
• ARQ Transmitter Delay - The amount of delay time, in millisec-
onds, at the transmitter.
• ARQ Receiver Delay - The amount of delay time, in millisec-
onds, at the receiver. The Receiver Delay taken together with
the Transmitter Delay determines the total ARQ retry timeout.
Use the Configuration - Radio page to set ARQ parameters on the Base
Station. ARQ/HARQ settings are located at the bottom of the page.