TCP/IP Tutorial and Technical Overview - IBM Redbooks

Call Agent
The Call Agent serves several purposes. It can use a TCP/IP link to manage
multiple Media Gateways. Primarily, however, it manages the following functions:
► Billing
► Call routing
► Network signalling
Phone handset
There are two different types of handsets:
Softphone The relationship between a traditional phone handset and a
softphone is similar to the relationship between a switchboard
and a softswitch. The functionality of the softphone is provided
through software, often running on a personal computer. The
hardware involved typically consists of any combination of
computer sound cards, headphones, or speakers, and a
microphone. This implementation is currently included in
common instant message programs and is often used in
PC-to-PC calls.
Hardphones The hardphone and the traditional handset correspond almost
one-to-one. The hardphone is simply a traditional telephone
handset that is VoIP capable.
Both types of VoIP phones are telephones capable of carrying voice traffic using
the Session Initiation Protocol (see 20.2, “Session Initiation Protocol (SIP)
technologies” on page 730) over a broadband connection.
Analog Terminal Adapters (ATA)
Analog Terminal Adapters (ATA) perform analog and digital conversions
between a traditional analog phone and the broadband modem, allowing users to
use the VoIP service with an existing phone. The ATA functions much as a
Media Gateway does, translating data between analog and digital
communication. ATAs typically have two ports: a telephone jack (FXS port) and a
LAN port. This enables a user to plug into the FXS port an analog telephone, and
then attach the ATA to the network. The ATA can then communicate with a VoIP
server using protocols such as H.323, SIP, MGCP, or IAX.
Coder-decoder (CODEC)
The processing of digitizing a voice sample, or converting a digitized signal into
an analog signal, requires the use of a coder-decoder. Each softphone,
hardphone, or ATA must implement a CODEC in order to implement VoIP. There
Chapter 20. Voice over Internet Protocol 727