CHAPTER 1: INTRODUCTION
Page 1-12
EMME USER’S GUIDE
The bridge is considered a node on the network and performs store and
forward functions for packets on each network. This contrasts with a
repeater which repeats the signal bit by bit from one side of the network
to the other. The bridge actually reads each packet, checks the packet for
accuracy, then decides whether the packet should be sent to the other
network based on the destination address. If the other network is busy, it
is the responsibility of the bridge to store the packet, for a reasonable
time, until the transmission can be made.
The bridge is also responsible for handling collisions. If a collision
happens as the bridge is transmitting onto the second network, the bridge
is responsible for the back off and retransmission process. The original
sending node is not made aware of the collision. It assumes the packet has
been sent correctly. If the bridge is unable to send the packet to its final
destination, the original sending station, expecting some response from
the device it was attempting to contact, will “time out” and depending on
the protocol attempt retransmission.
1.5.1 Filtering and Forwarding
The bridge decides whether to forward or filter a packet based on the
physical location of the destination device with respect to the source
device. A bridge dynamically learns the physical location of devices by
logging the source addresses of each packet and the bridge port the packet
was received on in a table called the Source Address Table (SAT).
The EMME supports the Special Filtering Database. This feature allows
the networks manager to define bridge filters above and beyond the
normal source and destination filters. The Special Filtering Database
allows filters to be configured for source, destination, type field, and a
flexible 64-byte offset to filter on information within the data portion of
the packet. A 64-byte window of data can search the data portion of the
packet to make filtering and forwarding decisions.
