Gateways

There's a few definitions of what a Gateway is. A common term is the Default Gateway that is used when configuring TCP/IP on a NIC. This refers to the layer 3 device that forwards traffic off the Local Area Network (LAN) to the rest of the world. Often the router that is used to connect a LAN to the Internet will provide extra services. It will have the added capability to direct and filter Application Layer ports to specific devices such as web servers, ftp, servers and e-mail servers. In this mode, it is called a firewall

Another less common definition of a Gateway is the Hardware/Software device that is used to interconnect LANs and WANs with mainframe computers such as DECnet and IBM's SNA.

Gateway's OSI Operating Layer - A Gateway operates at the Transport Layer and above. Typically translating each source layer protocol into the appropriate destination layer protocol. A mainframe gateway may translate all OSI Model layers. For example, IBM's SNA (System Network Architecture) does not readily conform to the OSI Model and requires a gateway to tranlate between the two architectures.

Gateway Segment to Segment Characteristics - There can be major differences between "local" and "distance" segments. As can be seen from the above diagram, the 2 Networks appear as if they are from other planets. Mainframes are based on a central number crunching CPU with terminals connected. All information displayed on the terminals is controlled by the central CPU.

LANs consist of distributed CPUs that share data and files. This leads to a unique problem in connecting the two architectures that requires a gateway.

Gateway Addressing - The gateway addressing depends on which OSI layers are translated. It could be all layers!

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Copyright July 2013 Eugene Blanchard