The Internet of Things (IoT) is the network of things (physical devices, vehicles, home appliances, and other items embedded with electronics, software, sensors, actuators)  connected through internet which enables these things to exchange data. This creates opportunities for more direct integration of the physical world into computer-based systems, resulting in efficiency improvements, economic benefits, and reduced human exertions.

In simple terms we can say IoT is a system of things integrated with sensors, softwares, electronics which are connected to each other & can exchange data or Information with other connected devices.

What is a Gateway?

A gateway is a piece of networking hardware used in telecommunications for telecommunications networks that allows data to flow from one discrete network to another. Gateways are distinct from routers or switches in that they communicate using more than one protocol and can operate at any of the seven layers of the open systems interconnection model (OSI).The term gateway can also loosely refer to a computer or computer program configured to perform the tasks of a gateway, such as a default gateway or router.

What is a IoT Gateway?

As IoT grows and billions of connected devices enter the world, one of the most critical components of future internet of things systems may be a device known as an IoT gateway. The importance of IoT gateways is understandable .

An Internet of Things (IoT) gateway is an integral part of an IoT ecosystem, handling communication with local sensors and remote users among a suite of other functions.

What is IoT edge gateway ? An Internet of things gateway provides the bridge between IoT devices in the field, the cloud, and user equipment such as smartphones. The IoT gateway provides a communication link between the field and the cloud and can provide offline services and real-time control over the devices in the field. Simple IoT gateway definition we can say IoT gateway aggregates sensor data, translates between sensor protocols, processes sensor data before sending it.

The interconnected devices communicate by using lightweight IoT gateway protocols like bus-based (DDS, REST, XMPP) and broker based (AMPQ, CoAP, MQTT, JMI) that don't require extensive CPU resources.To handle any needed protocol conversion, database storage or decision making, IoT nodes use separate IoT gateways in order to supplement the low-intelligence of devices.

The sensors generate huge amount of data per second and IoT gateway hardware provides a place to preprocess that data locally before sending it on to the cloud. When data is collected and analyzed at the edge, it minimizes the amount of data that needs to be forwarded on to the cloud and this is the use of iot gateway or we can say IoT gateway functionality.

IoT Gateway Architecture:

If you are thinking why you need an IoT Gateway, With IoT solutions, we have to deal with tons of devices working in the field and because the nature of these devices is very different like web, desktop, smart phones etc we need an intermediate architectural element that will act as a proxy between the world of field devices and the enterprise data center. That's why What we need is an IoT gateway.

The IoT gateway software or IoT gateway hardware is not just a dump proxy that forwards data from sensors to backend services. Sending all the information collected by sensors to the cloud or storage data center would be highly ineffective in terms of performance and network utilization. The IoT gateway will perform the pre-processing of information in the field, before they’re sent to the data center and it is the main function of the iot gateway.

An IoT ecosystem is made up of many components, or layers. The bottom layer contains the sensors and devices. The gateway acts as a secure intermediary between these sensors, devices and the cloud. The top layer handles overall monitoring and management of the IoT ecosystem, with analytics of the collected and persisted data, including applications.

IoT gateways consist of a hardware platform and an operating system and expose a rich set of services for your IoT application. The gateway software installed on the device is responsible for collecting data from the sensor, pre-processing that data, and sending the results to the data center.

A large number of manufacturers are involved in the IoT gateways design and production and some of iot gateway providers or iot gateway hardware manufacturers include CISCO, Harman International Industries, Advantech, ADLINK, Supermicro, Intel (intel iot gateway) , and NXP.

Are there any opensource IoT gateway or free iot gateway available ? Thingsboard is a iot gateway open source. The ThingsBoard IoT Gateway is an open-source solution that allows you to integrate IoT devices connected to legacy and third-party systems with ThingsBoard. ThingsBoard is an open-source IoT platform that enables rapid development, management and scaling of IoT applications. Refer the link for more details.

Refer the link for iot gateway wiki.