The TCP/IP layers

TCP/IP is first and foremost a group of networking protocols. Protocols are
the rules that govern how network traffic gets packaged electronically for
transmission over a network. Some TCP/IP protocols are used strictly for
data networking, some are used strictly for VoIP telephony, and some are
used by both data and VoIP. Each protocol corresponds to one of five possible
layers that make up the TCP/IP model:
Application: Special protocols at this layer ensure the quality and deliverability
of VoIP packets.
Transport: The user datagram protocol (UDP) at this layer transports
the VoIP packets from start to finish, which in this case means from
caller to receiver and vice versa.
Internetwork: At this layer, IP addressing is added to the packet. Every
VoIP phone or computer acting as a VoIP phone gets a unique IP address
that routes delivery of VoIP packets to and from the caller and receiver
during the life of the call.
Network interface: At this layer, MAC addressing is added to the packet.
(The MAC address is supplied by the NIC required for all network
devices.)
Physical: This layer converts all packets to electro or electro-optical signals
to be carried over the local or external network.
Each layer is associated with one or more protocols. A packet must traverse
all five layers: once when the packet is sent and again when it is received.
Basically, the VoIP packet originates with the caller. The packet travels down
all five layers on the caller’s side of the network and gets packaged with the
correct protocols at each layer. After the packet reaches the lowest layer, the
physical layer, it is sent over the network to its destination. When the packet
reaches its destination, it makes its way up through the layers and gets
unpackaged. When it reaches the application layer of the receiver, the packet
is translated into a voice signal that the receiver hears.

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Looking the TCP or IP Model

Many people marvel at the very thought that the POTS method of placing
telephone calls can be replaced by a technology that essentially runs on the
computer network. They are also startled by the many new and exciting features
that come with VoIP. However, people also question how VoIP can possibly
work and are a bit suspicious about whether VoIP can really live up to all
the claims.
The answer can be found in the very same model that has been supporting
data-only networking since the inception of the Internet more than twentyfive
years ago: the TCP/IP model.
Pronounced “t, c, p, i, p,” the model uses a five-layer approach to networking.
TCP/IP is adapted to enable it to also support VoIP. TCP/IP has proven to be
just as effective with packetizing telephony as it has been for many years
with packetizing computer data.
To fully understand VoIP, it pays to know a little about the technical underpinnings
that make it work over the network of your choice. In this section,
I describe the layers of the TCP/IP model in relation to computer networks.
Then I insert into this content the parts that change when TCP/IP supports
VoIP.

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The Gaining Flexibility with VoIP

VoIP is not just about making and receiving telephone calls; it’s about a whole
new way of communicating. Sure, it includes telephone calls, but there is so
much more to the VoIP telephony picture. VoIP integrates most if not all
other forms of communication. You can even run videoconferencing to your
desktop.
With VoIP, your company enjoys increased productivity and customer satisfaction.
These improvements are typically realized through the flexibility
offered by enhanced calling features. A few calling features, such as voice
mail and call transfer, have been around in the POTS world for quite some
time. On the other hand, integrating data, voice, and video applications to
run over a single network and work with wireless phones are more recent
innovations made possible by IP telephony.
Following are some enhanced calling features made possible by IP telephony:
Vemail: Before IP telephony and VoIP, you accessed voice mail through a
telephone and accessed e-mail through a computer. With VoIP, you can
read your voice mail on your computer screen and listen to your e-mail
through an IP-enabled telephone. The new term for this converged feature
is vemail (pronounced “v-e-mail”).
Web surfing: Because VoIP operates with the same set of IP rules and
protocols that support Web-based applications, it is possible to access
the Web with an IP-enabled telephone. If you have an IP telephone with a
large enough screen, it can display Web pages or a list of your favorite
Web links. For instance, you could use your phone to view your stock
exchange trading status or the current weather forecast.
In an IP telephony world, these calling features (and many more) are available
with no monthly recurring charges. VoIP, with all of its many benefits, is
quickly replacing traditional POTS-based technologies. VoIP is even becoming
a superior replacement for many former computer-only applications.
One of the big stories with VoIP is the many new and exciting features that
increase your ability to be agile and mobile. You no longer have to say “I’ve
got to get to a phone!” VoIP can be on your desk, computer, mobile phone, or
PDA. It can be hardwired or have no wires at all. This flexibility is astounding
to those familiar with traditional telephony.
If you have a mobile user base, be sure to check out IP soft phones. A soft
phone is software that works on a laptop computer or pocket PC and provides
most of the functionality of a traditional desk phone. If a user can connect
to a network, the soft phone provides a way to reap the benefits of IP
telephony regardless of location.

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To Eye for IP Telephony

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 Chapter 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.

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POTS to Packets VoIP

VoIP technology enables traditional telephony services to operate over computer networks using packet-switched protocols.

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the Beginning, There Was POTS

Before digital networking took off, everyone had to use the one and only POTS, which stands for plain old telephone service (honestly, it does).

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The Getting Down to Business with VoIP

Technological innovation is hurling itself upon us once again. This time, it’s coming in the form of improving the way we communicate, bringing with it new capabilities that change the meaning of the phrase telephone call.

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