Chapter 39. Gateways
A Gateway is the Hardware/Software device that is used to interconnect LANs & WANs using mainframe computers (such as DECnet and IBM's SNA).
Often, the router that is used to connect a LAN to the Internet will be called a gateway. It will have added capability to direct and filter higher layer protocols (layer 4 and up) to specific devices (such as web servers, ftp servers and e-mail servers).
Gateway's OSI Operating Layer
A Gateway operates at the Transport Layer and above and it typically translates 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 translate between the two architectures.
Gateway Segment to Segment Characteristics
There can be major differences between "local" and "distance" segments. As shown in 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 (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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