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MDS Orbit MCR-4G Technical Manual
MDS 05-6628A01, Rev. B
Bridging
Understanding
The unit supports transparent bridging of LAN and WiFi networks. The bridge forwards traffic between
LAN and WiFi networks at the layer-2 of OSI model. This allows LAN and WiFi clients to be in the same
IP sub-network.
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 multicasts 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 inter-
face (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 is a routed interface.
NOTE:
The Cellular modem by its nature can never be added to the bridge and is, therefore, a routed inter-
face. 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 15
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.
