PSTN (Public Switched Telephone Network) is a switched network used for global voice communications. This network has approximately 800 million users and is the largest telecommunications network in the world today.

The voice VLAN function allows the access port to transmit voice data streams from IP phones. When a Cisco IP phone is connected to a switch, it will specify the Layer 3 IP priority and Layer 2 Class of Service (CoS) values in the voice data stream sent;

SIP sessions use up to four main components: SIP user agent, SIP registration server, SIP proxy server, and SIP redirect server.

It is not a single protocol, but a collection of protocols and services that provide security for IP networks. It provides high-quality, interoperable, and cryptographic-based security guarantees for data transmitted on the Internet.

SSL is called Secure Sockets Layer. It is a security protocol that guarantees privacy. SSL can prevent the communication between the client and the server from being intercepted and eavesdropped.

The router is generally selected in an underground weak current room (far away from a strong current room to avoid strong electromagnetic interference). Pay attention to ventilation and keep it dry. It is best to have a cabinet and put it together with the core switch.

Whether it is OSPFv2 or OSPFv3, the basic operating principles of the OSPF protocol are the same. However, due to the different meanings of the IPv4 and IPv6 protocols and the size of the address space, the differences between them are bound to exist.

Spam refers to emails sent forcibly without the user's permission. The emails contain advertisements, viruses, and other content. For users, in addition to affecting normal mail reading, spam may also contain harmful information such as viruses;

IPSec (Internet Protocol Security) is a set of open network security protocols formulated by IETF (Internet Engineering Task Force). It is not a single protocol, but a collection of protocols and services that provide security for IP networks.