Siemens ESM - Manual (Page 30)

Functions
Industrial Ethernet OSM/ESM
26 C79000-Z8976-C068-10
Deleting Addresses
An OSM/ESM monitors the age of the addresses it has learnt - address entries that
exceed a certain age (aging time, default setting on the OSM/ESM 40 seconds) are
deleted again by the OSM/ESM. If a packet with a source address matching the
address entry is received before the aging time elapses, the address entry is
retained and the age of the address is set to 0 again. When the OSM/ESM is
restarted, the address entries are also deleted. If a packet is received by an
OSM/ESM for which there is no address entry, the OSM/ESM distributes it to all
ports.
Setting the Transmission Rate, Autonegotiation
The electrical ports of the OSM/ESM are set to autonegotiation mode as default
(autosensing).
They automatically detect the transmission rate (10 or 100 Mbps) at which the
attached device or attached network segment operates and set themselves to this
rate. If the partner device also supports the autonegotiation mode, the devices
further negotiate whether they will exchange data with each other in the half duplex
or full duplex mode.
As a result of the automatic adaptation to the transmission rate of the attached
DTEs, existing network segments operating at 10 or 100 Mbps can be
interconnected simply using OSMs/ESMs.
Note
If the partner device connected to a port of an OSM/ESM does not support the
autonegotiation mode (for example OSM Version 1), the port of the partner device
must be set to half duplex mode or the port properties of the OSM/ESM must be
configured.
Packets with the VLAN Priority Tag
OSMs/ESMs support frames with VLAN tags according to IEEE 802.1Q.
VLAN tags are additional information entered in the frame header. This additional
information includes a priority field according to IEEE 802.1D. This field can have
values from 0 through 7 and allows frames to be assigned a priority from 0 (lowest
priority) through 7 (highest priority).
Frames with priority 0 through 3 are processed with low priority in the OSMs/ESMs
and frames with priority 4 through 7 are processed with high priority.
Frames with a non-learned unicast destination address are an exception. These
are always handled with low priority.