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The VoIP Phone System: Packet Switching
Data networks do not use circuit switching. Your Internet
connection would be a lot slower if it maintained a constant connection to
the Web page you were viewing at any given time. Instead, data networks
simply send and retrieve data as you need it. And, instead of routing the
data over a dedicated line, the data packets flow through a chaotic
network along thousands of possible paths. This is called packet switching.
While circuit switching keeps the
connection open and constant, packet switching opens a brief connection --
just long enough to send a small chunk of data, called a packet, from one
system to another. It works like this:
- The
sending computer chops data into small packets, with an address on each
one telling the network devices where to send them.
- Inside of each packet is a payload. The payload is a piece of the e-mail,
a music file or whatever type of file is being transmitted inside the
packet.
- The sending computer sends the packet to a nearby router and forgets
about it. The nearby router send the packet to another router that is
closer to the recipient computer. That router sends the packet along to
another, even closer router, and so on.
- When the receiving computer finally gets the packets (which may have all
taken completely different paths to get there), it uses instructions
contained within the packets to reassemble the data into its original
state.
Packet switching is very efficient. It
lets the network route the packets along the least congested and cheapest
lines. It also frees up the two computers communicating with each other so
that they can accept information from other computers, as well.
The Advantage
VoIP technology uses the Internet's
packet-switching capabilities to provide phone service. VoIP has several
advantages over circuit switching. For example, packet switching allows
several telephone calls to occupy the amount of space occupied by only one
in a circuit-switched network. Using PSTN, that 10-minute phone call we
talked about earlier consumed 10 full minutes of transmission time at a
cost of 128 Kbps. With VoIP, that same call may have occupied only 3.5
minutes of transmission time at a cost of 64 Kbps, leaving another 64 Kbps
free for that 3.5 minutes, plus an additional 128 Kbps for the remaining
6.5 minutes. Based on this simple estimate, another three or four calls
could easily fit into the space used by a single call under the
conventional system. And this example doesn't even factor in the use of
data compression, which further reduces the size of each call.
Let's say that you and your friend both have service through a
VoIP
provider. You both have your analog phones hooked up to the
service-provided ATAs. Let's take another look at that typical telephone
call, but this time using VoIP over a packet-switched network:
1- You pick up the receiver, which sends a signal to
the ATA.
2- The ATA receives the signal and sends a dial tone. This lets you know
that you have a connection to the Internet.
3- You dial the phone number of the party you wish to talk to. The tones
are converted by the ATA into digital data and temporarily stored.
4- The phone number data is sent in the form of a request to your VoIP
company's call processor. The call processor checks it to ensure that it
is in a valid format.
5-The call processor determines to whom to map the phone number. In
mapping, the phone number is translated to an IP address (more on this
later). The soft switch connects the two devices on either end of the
call. On the other end, a signal is sent to your friend's ATA, telling it
to ask the connected phone to ring.
6- Once your friend picks up the phone, a session is established between
your computer and your friend's computer. This means that each system
knows to expect packets of data from the other system. In the middle, the
normal Internet infrastructure handles the call as if it were e-mail or a
Web page. Each system must use the same protocol to communicate. The
systems implement two channels, one for each direction, as part of the
session.
7- You talk for a period of time. During the conversation, your system and
your friend's system transmit packets back and forth when there is data to
be sent. The ATAs at each end translate these packets as they are received
and convert them to the analog audio signal that you hear. Your ATA also
keeps the circuit open between itself and your analog phone while it
forwards packets to and from the IP host at the other end.
8- You finish talking and hang up the receiver.
9- When you hang up, the circuit is closed between your phone and the ATA.
10- The ATA sends a signal to the soft switch connecting the call,
terminating the session. Probably one of the most compelling advantages of
packet switching is that data networks already understand the technology.
By migrating to this technology, telephone networks immediately gain the
ability to communicate the way computers do.
It will still be at least a decade before
communications companies can make the full switch over to VoIP. As with
all emerging technologies, there are certain hurdles that have to be
overcome.
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