GE MDS iNET Series, Справочное руководство

Страница: 130 / 173

118 MDS iNET Series Reference Manual 05-2806A01, Rev. L
The radio goes through a listening and learning period at start-up before it will send any packets over either
of its ports. This is about 10 seconds after the CPU’s operating system has finished its boot cycle.
The bridge code in the transceiver operates and makes decisions about packet forwarding just like any other
bridge. The bridge code 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 remotes.
• If the destination address is not known, forward the packet to all remotes.
• If the destination address is known, forward the packet to the port that the destination is known to be
on (usually the RF port).
• The bridge code uses Spanning Tree Protocol (STP) to prevent loops from being created when con-
necting bridges in parallel. For example, connecting two remotes to the same wired LAN could cre-
ate 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.
4.4.2 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.
4.4.3 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—low traffic and good RF signal path—the latency for units operating in the TCP
mode, will typically be around 5 ms in each direction. However, when UDP multicast traffic is transported,
the outbound packet latency (from AP to remote) is dependent on the beacon period.
UDP multicast packet latency can be minimized by setting the
Beacon Period
to
Fast
(52 ms). Changing
beacon rate to
Fast
will result in an average latency of 31 ms, assuming outbound packets wait for a beacon
transmission 50% of the time (26ms) plus the normal packet latency (5 ms).
4.4.4 Data Compression
Enabling this option uses an LZO compression algorithm for over-the-air data. Varying levels of data reduc-
tion are achieved depending on the nature of the data. Text files are typically the most compressible,
whereas binary files are the least compressible. On average, a 30% increase in throughput can be achieved
with compression enabled.
Compression is used on data packets of 100 bytes or more, including Ethernet, IP, and TCP/UDP headers.
4.4.5 Packets-per-Second (PPS)
The iNET-II radio has a limit of approximately 140 PPS (70 PPS in iNET). 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).