User's Manual

MDS 05-6302A01, Rev. A MDS Mercury 16E Technical Manual 49
The bridge in the transceiver operates and makes decisions about packet
forwarding just like any other bridge. The bridge builds a list of source
MAC addresses that it has seen on each of its ports.
There are a few general rules that are followed when a packet is received
on any port:
• If the destination address is a multicast or broadcast address,
forward the packet to all ports.
• If the destination address is not known, forward the packet to
all ports.
• If the destination address is known, forward the packet to the
port that the destination is known to be on.
• Spanning Tree Protocol (STP)* is used by the bridge to pre-
vent loops from being created when connecting bridges in
parallel. For example, connecting two remotes to the same
wired LAN could create a loop if STP was not used. Every
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 will be available in 2012.
8.2 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.
8.3 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.
8.4 Packets-per-Second (PPS)
The radio has a limit of approximately 800 PPS. Consider this restriction
when planning your network, especially when smaller packets are