Wednesday, February 9, 2011

Making internal calls | Making external calls (chapter 1)

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 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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