CTC Union ET100/G64, Руководство пользователя

Страница: 12 / 20

ET100/G64 LAN-WAN Bridge
1.5 Theory of Operation
A bridge is used to connect networks locally or remotely such that they appear to the user to be the same
network. An Ethernet LAN bridge will connect two LAN segments at the Data Link Layer (ISO Layer 2).
At this layer, the MAC (Media Access Control) addresses are used for low level addressing to send
information to devices. The bridge builds tables of MAC addresses for each network segment based on the
source and destination addresses of the packets it receives and forwards, then filters the traffic not destined
for the remote network.
The Ethernet-WAN bridge will connect two remote Ethernet networks over bit stream interfaces such as
that of modems or DSU/CSUs. One method to do this is to use HDLC, an international standard set by the
ISO, a set of protocols for carrying data over a link with error detection/correction and flow control.
The ET100/G64 utilizes both Ethernet Bridging and HDLC encapsulation to provide a connection
between LANs over bit stream architectures. The LAN side of the
ET100/G64
receives an Ethernet
packet and examines its destination MAC address. If it knows the MAC is on the local network then it
simply drops the packet. Otherwise, if it knows the packet destination is on the remote side, or if it cannot
be determined because its MAC cannot be found in the table, then it forwards it. During forwarding, the
packet is processed for transmission across the WAN link. Here is where the Ethernet packet is
encapsulated in HDLC.
When the HDLC packet is received on the remote side unit's data port, the packet is checked for
transmission errors, and the original Ethernet packet(s) is recovered and sent out the remote's LAN port
completing the transmission. Here is the typical application of the ET100/G64 .
Figure 5. Typical application of ET100/G64 LAN-WAN Bridge.
In the above application, the ET100/G64 provides LAN bridging between two LAN segments using a
G.703 64K co-directional transmission network.
12