2000, 2001 Cirronet
Incorporated
6
M-2410-0007 Rev -
SEM Operation
Overview
SEM devices are wireless Ethernet modems that perform bridging functions in point-to-
point or point-to-multipoint configurations. SEM products are designed to connect
remote network segments together while keeping the data traffic between the network
segments to a minimum. SEM products use MAC-layer addresses to learn on which
network segment a device is located.
A master SEM can connect up to 5 remote slave SEMs. The SEM2410 is a single channel
device with an over-the-air data rate of 460Kbps providing 230Kbps data throughput.
The radio modems in the SEMs are factory configured with optimum settings for typical
point-to-point applications. The radio parameters can be configured to optimize data
throughput, latency and range for whatever the application. In addition, the radios in the
SEMs can be configured to allow multiple SEM networks to be co-located.
Point-to-Point Mode
In point-to-point operation, one SEM is configured as the Master and the other is
configured as a Slave. While this is necessary for operation, it does not matter which
SEM is the master and which is the slave. The radio in the master operates as the base
radio. Configuring the SEM as the master automatically configures the radio in the
master as a base radio. Similarly, configuring a SEM as a slave automatically configures
the slave radio as a remote radio.
When a master SEM is powered on, it becomes active immediately, even if no slave
SEMs are detected. It will attempt to send packets addressed to devices that it thinks are
not on its local network. When a slave SEM is powered on, it listens for a master SEM
and attempts to register with the radio in the master. This detection and registration
process can take up to 2 seconds. During this time, no packets will be sent or received
over the RF link by the slave.
If the application is such that more than one point-to-point link needs to located in the
same area, each master/slave pair must be assigned different network numbers. This will
allow the SEMs to identify the appropriate other SEM to which they should
communicate. Because different network numbers have different hopping sequences, this
also allows various pairs to operate in the same area without interfering with each other.
Refer to the section on radio commands for details on setting network numbers.
Point-to-Mulitpoint Mode
In point-to-multipoint mode multiple slave SEMs link with a single master SEM. Similar
to the point-to-point mode, each slave must register with the master, a process that can
take up to 2 seconds. All of the radio addressing and registration occurs automatically
and is transparent to the application. A maximum of 5 slave SEMs can be connected to a
master SEM. The amount of data a slave can transmit depends on the number of slaves
connected and the hop duration of the radio network. Because packets from one slave that
are intended for another slave must pass through the master SEM, the master SEM should
