User's Manual

58 MDS Mercury 16E Technical Manual MDS 05-6302A01, Rev. B
bridge running STP sends out Bridge Protocol Data Units
(BPDUs) at regular intervals so that the spanning tree can be
built and maintained. BPDUs are 60-byte multicast Ethernet
frames.
NOTE: STP expected availability is mid-2012.
7.2 Distance-Throughput Relationship
Distance affects throughput. Because of timers and other components of
the protocol, there is a practical distance limit of 30 miles (48 km) for
reliable operation. After this, although data still flows, the throughput
will begin to drop and latency will increase, due to additional retries
between the radios. Packets may start to be dropped. Some applications
may tolerate this; others may not. Repeater stations may be used to
extend the range.
7.3 Data Latency—TCP versus UDP Mode
The latency of data passing through a network will depend on user data
message length, the overall level of traffic on the network, and the
quality of the radio path.
Under ideal conditions—and without the use of QoS—with low traffic
and good RF signal path, the latency for units operating in the TCP
mode will typically be around 50 ms in each direction.
7.4 Packets-per-Second (PPS)
The radio has a data speed limit of approximately 1000 PPS. Consider
this restriction when planning your network, especially when smaller
packets are expected to make up the majority of the traffic as is the case
with VoIP (Voice over IP). Actual speed can vary depending on network
conditions. Consult your system designer for more detail.
7.5 Subscriber-to-Subscriber Traffic
When sending frames from an endpoint connected to one Subscriber to
another endpoint with a different Subscriber, the throughput will be
halved at best. This is because all frames must go through the Base Sta-
tion and thus are transmitted twice over the same radio system. There-
fore, in the previous 100-byte UDP example, the number of over-the-air
bytes will be 380 bytes (190 bytes x 2) if the frame has to go sub-
scriber-to-subscriber.