Gateways allow communication between networks that use different communications architectures by converting between the different protocols. There are as many types of gateways as combinations of architectures and protocols. One type, called a protocol converter, changes a protocol from one architecture to another. For example, a gateway can connect a TCP/IP network to an SNA network by converting between their protocols. Protocol converters must operate at all protocol levels above the data link layer and handle any differences or missing layers between the protocols being converted.

1. GATEWAYS

2. As you have seen, bridges and routers can be used in
networks that run more than one communications
architecture. But they cannot interconnect nodes that use
different architectures. TCP/IP nodes can communicate with
other TCP/IP nodes, but not with DNA nodes. There are as
many types of gateways possible as there are combinations of
communications architectures and application level protocols.

3. One type of gateway, called a protocol converter, changes a
protocol from that of one communications architecture to that of
another. An example of a protocol converter is a node that can connect a
TCP/IP network to an SNA network, as shown in the Gateway (Protocol
Converter) diagram below. Another example is a node that converts OSI
MOTIS mail to SMTP for TCP/IP delivery. A gateway handles differences
in upper level protocols. To illustrate this point we will use two
combinations of protocols, SNA and TCP/IP to illustrate gateways.

4. Figure 8-11. Gateway (Protocol Converter)

5. A protocol converter must operate at all of the
protocol levels above the Data Link Layer and must be
transparent to the processes in those layers at each end of
the connection. The Protocol Conversion diagram below
depicts the correspondence between TCP/IP and SNA
through their mutual correspondence to OSI. It is not an
exact match.

6. Figure 8-12. Protocol Conversion

7. Two problems are apparent:
1. The protocols do not correspond exactly and cannot be independently
converted. Instead their conversion must be handled as an integrated task. For
example, when converting from IP to SNA Path Control protocol, elements of the
IP protocol must also be reflected in the SNA Transmission Control Protocol.
2. Layers are missing. For example the conversion program must handle the
Function Management protocol even though that layer does not exist on the
TCP/IP side.

8. The task of protocol conversion can also be viewed in relation to the
data stream. See the SNA and TCP/IP Conversion diagram below. The
converter sees datagrams from the TCP/IP side and BIUs from the SNA side. A
conversion of header data must be performed by the conversion program, i.e.,
the datagram header must be removed and headers for the BLU, BIU and RU
with the corresponding information must be built. Note that protocols
converters must also perform the function of a router, maintaining consistent
addresses throughout a network. If the address format of the different types
of networks vary, the gateway must handle this as well.

9. Figure 8-13. SNA and TCP/IP Conversion

10. Gateways and Remote Access. The term "gateway" is
also used to describe a router. These gateways are also
referred to as remote gateways. For example, Internet routers
are called gateways. The term is also used for protocol
converters. A modem gateway is another example of a
access gateway. Modem gateways allow multiple remote
asynchronous terminals to connect to a network through
modems.