Gateway:
A gateway is a communications processor that connects dissimilar networks by providing the translation from one set of protocols to another. It acts as an interface between two networks with different protocols and allows them to communicate with each other seamlessly.

Functions of a Gateway:
- Protocol Translation: One of the main functions of a gateway is to translate the protocols used by different networks. It receives data packets from one network, strips off the original protocol, and then encapsulates the data in the appropriate protocol for the destination network. This allows the data to be understood and transmitted by the receiving network.

- Data Conversion: In addition to protocol translation, a gateway may also perform data conversion. This involves converting data formats, such as converting between ASCII and EBCDIC formats, to ensure compatibility between the two networks.

- Address Mapping: Gateways often perform address mapping, which involves mapping the addresses used by one network to the addresses used by another network. This allows data to be properly addressed and delivered to the correct destination.

- Security: Gateways also play a crucial role in network security. They can act as a firewall, filtering and blocking unauthorized access attempts from one network to another. They can also encrypt and decrypt data to ensure secure transmission between networks.

Comparison with other Network Devices:
- Bridge: A bridge connects two similar networks and operates at the data link layer of the OSI model. It forwards data packets based on MAC addresses and does not perform protocol translation. Unlike a gateway, a bridge does not connect dissimilar networks.

- Router: A router is a network device that connects multiple networks and forwards data packets based on IP addresses. It operates at the network layer of the OSI model. While a router can connect dissimilar networks, it primarily focuses on routing packets based on IP addresses rather than performing protocol translation.

- Modem: A modem is a device that modulates and demodulates data signals to enable communication over a specific medium, such as telephone lines or cable. It is used to connect a computer or network to an internet service provider. Unlike a gateway, a modem does not perform protocol translation or connect dissimilar networks.

In conclusion, a gateway is a communications processor that connects dissimilar networks by providing protocol translation, data conversion, address mapping, and security features. It acts as an interface between two networks with different protocols, allowing them to communicate effectively.