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.