# Design and Implementation of a Smart Intercom System through Web Services on Web of Things
## 2 Related Research  
In order to improve the problem of the traditional intercom, an article created a door phone prototype using an IP network system with a SIP (Session Initiation Protocol) server and many SIP clients. The IP door phone is one of the SIP clients in the system and provides basic door phone functionality. Visitors can use voice and video calls to talk to residents and control the device during the ring [11]. There is also a purpose in designing and implementing a free intercom system that uses cell phones and IP phones for free through a wireless network, which uses a SIP to allow users to make voice, video, and SMS conversations [12]. The IP phone is used as a door phone system under the SIP server, called VoIP Intercom (Voice over Internet Protocol). Once IP telephony (Internet Protocol telephony) software is installed on a resident's or a visitor's smartphone, they can talk to each other through the system at any time. However, since there is still a fixed device in the house, it is impossible to know that a visitor is calling if the resident is not at the handset. Another important issue is that if the visitors do not register and install the software beforehand, there is no way to contact the residents. Another study used a wireless transceiver module to implement a cost-effective home door intercom [13]. Furthermore, a smart intercom has been constructed to provide live video feeds from a visitor to the resident by cell phone so that the resident can remotely control the visitor's movement [14]. One paper designed an Android smartphone-based instant messaging application that enables users to enter their user data on the login screen. The system verifies the information and displays the intercom interface. Users can select a channel to enter the specified channel and talk within the channel without interfering with each other, and users can choose any channel and exit after talking. In addition, users can invite other online users to enter the temporary channel to achieve the function of intercom communication [15]. The use of apps on smartphones as intercoms is referred to in this paper as a mobile intercom. Although this system eliminates the need for a handset in the home, it allows residents to receive visitor notifications from anywhere. However, it does lose the functionality of the original intercom to view visitors and open the door. The visitors cannot contact the resident without prior installation of the intercom software. There are three types of intercoms: the traditional intercom, the VoIP intercom, and the mobile intercom, as listed in Table 1. The traditional intercom has the highest cost, and the mobile intercom is the lowest solution. Traditional intercom systems usually have a dedicated system. In contrast, the VoIP intercom and the mobile intercom can be installed with a specific app on the mobile phone, allowing visitors and residents to contact each other easily. However, visitors or residents who do not have the app cannot contact each other. In addition, a laboratory cloud-based video intercom access control system based on NFC (Near-field communication) technology was designed, and the experimental results proved that the combination of NFC technology and the cloud platform enhanced the load capacity and security of the system [16]. As a simple password is no longer foolproof, it is important for homeowners to provide tenants with a secure door-locking system. Some smart door lock systems are designed to set a code for a new tenant each time, and the tenant can use a one-time password from an Android app to unlock the door. Another is an automated door lock system designed using a chatbot that uses a PIN (personal identification number) instead of a lock key. Keyless approaches are convenient for both the owner and the tenant because they utilize IoT capabilities to monitor and grant access to visitors [17]. The user can also open or close the lock by installing a developed application on the cell phone.