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.
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 con- nected 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 tele- phony 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 destina- tion 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 pack- ets 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.
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