GE MDS ORBIT MCR Technical Manual page 204

The bridge learns the clients' locations by analyzing the source address of incoming frames from all
attached networks (LAN and WiFi network). For example, if a bridge sees a frame arrive on LAN port
from Host A, the bridge concludes that Host A can be reached through the segment connected to LAN
port. Through this process, the bridge builds a forwarding table (the learning process). When a frame is
received on one of the bridge's interfaces, the bridge looks up the frame's destination address in its
forwarding table. If the table contains an association between the destination address and any of the
bridge's ports aside from the one on which the frame was received, the frame is forwarded out the
indicated port. If no association is found, the frame is flooded to all ports except the inbound port.
Broadcasts and multicast also are flooded in this way.
Typically, for LAN/WiFi-to-Cellular Router use case (a.k.a. LAN/WiFi HotSpot), the LAN and WiFi
interface (acting as an Access Point) are bridged. However, for security and bandwidth considerations, a
user might want to remove LAN and WiFi networks from the bridge (i.e., configuring LAN and WiFi
networks as separate IP networks). In this network setup, broadcast/multicasts data packets coming into
WiFi are not directed out the LAN connection and vice versa.
The bridged network is addressable via bridge interface (a virtual interface). The interfaces that are in the
bridge are called bridged interfaces. The interfaces that are not in the bridge are called routed interfaces.
Bridging is performed between bridged interfaces. Routing is performed between routed interfaces. The
bridge interface itself is a routed interface.
NOTE
The Cellular interface cannot be added to the bridge and is, therefore, a routed interface.
However, a GRE interface in 'ethernet-over-gre' mode can be configured to operate over Cell
interface and added to a bridge to enable tunneling of layer-2 traffic over the cellular network.
Refer to section on GRE for more details. Advanced details of networking concepts such as
routing and bridging are outside the scope of this manual but are available through various
training materials freely available on the Internet.
Theory of Operation
Refer to Figure 3-110 below for this discussion. In a typical application, the MCR-4G provides cellular
connectivity to locally connected devices that are located on the user's local/internal/private LAN or WiFi
network. The MCR-4G acts as an Access Point on the Wi-Fi interface, providing connectivity to Wi-Fi
clients. The Wi-Fi traffic is combined with the local Ethernet port traffic through a Layer 2 bridge. The
serial interface is matched to a terminal server that encapsulates serial data over a TCP or UDP
connection.
The MCR-4G provides Network Address Translation (NAT) (both Masquerading and Port Forwarding)
as well as Firewalling between the cellular data interface (WAN side) and the local network
(LAN/WiFi). The MCR-4G can also act as a VPN client to provide a secure tunnel for LAN data to the
user's local network (LAN/WiFi). This configuration obviates the need for NAT, as the back-office
network behind the VPN Concentrator (VPNC) can address the local LAN or WiFi network directly via
the secure tunnel.
204 MDS Orbit MCR/ECR Technical Manual MDS 05-6632A01, Rev. F