VoIP also makes possible other services that older telephony systems can’t provide. VoIP telephony services are interoperable, meaning that they work well over all kinds of networks. They are also highly portable, which means they will work with any IP-enabled device such as an IP telephone, a computer, or even a personal digital assistant (PDA).
IP telephony works by taking traditional voice signals and converting them to a form that can be easily transmitted over a local area network. Thus, the heart of IP telephony is the same as traditional data networking with computers. IP-enabled phones handle the voice-to-data conversion well, but don’t be misled — implementing VoIP doesn’t mean that everyone has to use IP-enabled phones. The best VoIP providers implement IP telephony in a manner that protects your investment in existing telephone equipment, even if you have analog telephone stations. (You’ll find more on this topic in topic 10.)
All IP phones have one important thing in common: a built-in network interface card (NIC), just like a computer uses. The NIC is critical for any network device because it provides the device with a physical address and a way to communicate over the network.
The physical address supplied by a NIC is called a MAC address. MAC stands for media access control. The MAC address uses a standardized address and is usually represented by six hexadecimal numbers separated by dashes. For example, the following is a valid MAC address: 00-0A-E4-02-7B-99.
To support IP telephony, a server is typically dedicated to run the software used to manage calls. Servers are just like personal computers, except they have more memory, speed, and capacity. The server stores the database that contains all the MAC addresses corresponding to all the IP telephone extensions assigned to users. Depending on the size of the LAN and the number of users, you may use more than one server. For example, some LANs running IP telephony dedicate a server just for handling voice mail.
Depending on the size of the LAN, one or more devices known as switches are installed. These switches are boxes that have a series of ports into which all LAN-addressable devices ultimately connect. (Examples of LAN-addressable devices include computers, printers, wireless access devices, gateways, and storage devices.) Usually the switches are set up in the communications closets around the LAN, and they operate 24/7. All the switches are interconnected, often with fiber-optic cable.
In a nutshell, all network devices, including your IP telephone, must physically connect to the LAN through a port on a switch.
Calling over a computer network
Voice over Internet protocol is often taken to mean basically what it states: Voice traveling over the Internet. When VoIP was developed,
it worked only with the Internet. Today, VoIP works on all other major network types, including those used throughout the corporate sector.
Making internal calls
When you want to call a coworker at your same location, you dial the phone number corresponding to the person’s name. The signals are packetized and sent to the managing server, where the packet picks up the MAC address of the person you’re calling. Next, the packet is forwarded to the switch, then to a particular port on that switch, and finally to the IP telephone connected to the port. The coworker’s telephone rings. When the coworker picks up the receiver or answers the call, a virtual connection is established between the coworker and yourself for the life of the call. IP telephony does all this at lightning speed.
Making external calls
The process of calling a coworker at an offsite location varies only a little. The call is still initiated in the same way. But because the coworker is connected to a different LAN, the local server sends the call not to a switch located on your LAN but through the company’s WAN (wide area network). This is where IP telephony technically becomes VoIP.
Each LAN in a multilocation network is connected to the larger WAN. If you’re located at the company’s headquarters in Pittsburgh, and you call a coworker located at the office in Los Angeles, your call begins as an IP telephony call on your LAN. It then travels from your LAN through a gateway, switch, or router that is programmed to re-packetize your call and encode the VoIP packet with additional information, such as the address for the destination LAN.
Network gurus refer to the process of packetizing your voice telephone call as encapsulation. A good analogy for this fancy techno-term is putting a letter into an envelope for mailing. The difference is that these encapsulated packets contain the content of the telephone conversation in digitized form.
To participate in the company’s VoIP WAN, each LAN needs at least one edge device, such as a router, a switch, or a gateway. An edge device is just that — a device that sits on the boundary, or edge, of your local network and
provides a connection to external networks. Depending on the company’s network design, these edge devices can even have multiple interfaces that connect them to more than one outside network. The edge devices take care of all the IP telephony traffic going off-LAN by encapsulating the signals into packets, encoding the packets with the correct addressing information, and forwarding the packets out onto the WAN, where they make their way in a packet-switched manner to their respective destinations.
When the packets arrive at the destination LAN, the edge device on that LAN breaks down the VoIP packets and forwards them internally to the server that manages IP services. From this point, the rest of the process is similar to IP telephony services described in the preceding section: The phone rings, the person being called answers, and a virtual circuit is established between the caller and the receiver.
