Afar NX-2E1, Руководство оператора

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NetCrossing Gateway NX-2E1/T1 Operator’s Manual
2-1
2 THEORY OF OPERATION
The NetCrosing Gateway 2E1/T1 models includes three User ports: one to connect to an Ethernet
Local Area Network (LAN), and the other two (E1/T1) to connect to up to two Serial Time Division
Multiplexed (TDM) devices. The gateway processes the LAN and TDM data differently, but in
general, data from all three ports is sent to the Ethernet Wide Area Network (WAN) port.
Conversely, data received in the WAN port is processed by the gateway and sent to the LAN or the
TDM ports as appropriate.
2.1 Wide Area Network (WAN)
The Gateway WAN port is connected to a “Wide Area Network”, through which the gateway can
reach other gateways. The
type of WAN network connecting the various gateways is an important
parameter and should be configured accordingly.
If the WAN network consists of a single Ethernet, possibly including Ethernet Bridges, the network
type is classified as a
bridge network. In this case the gateways can reach each other by sending
packets addressed to the other gateway physical address . This physical address corresponds to the
unit serial number, which is pre-configured at the factory.
If the WAN network includes Internet Protocol (IP) routers that switch IP packets to their final
destination, the network type is a
route network. In this case each gateway must be configured with
its own IP address and the gateways send the packets to each other’s IP addresses. In a routed
network the gateways execute the Address Resolution Protocol (ARP) to translate the destination IP
address into the physical address of the first router in the path.
The route network is the most generic and you can always operate a bridge network in route mode.
However, in bridge mode the packet overhead is considerably smaller. This translates into higher
effective throughput which may be important if the WAN network connecting the gateways is
limited.
2.2 LAN Port
The Local Area Network (LAN) port can be connected to a user LAN or directly to the Ethernet port
of a PC. The gateway implements a self learning “bridging” algorithm that transfers the Ethernet
packets, as appropriate, between the LAN and the WAN ports.
Both ethernet ports are configured in “promiscuous” mode, i.e., the gateway examines all the Ethernet
packets present in either port. Since these Ethernet packets contain a “source” and “destination”
address, the gateway quickly learns the addresses of all the stations that are directly reachable in each
network (all the “source” addresses of packets flowing in that port are reachable).
With this information on hand, the gateway examines the destination address of every Ethernet packet
received and makes one of the following decisions: