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The H.323-series protocols are
implemented in layer 5 of the TCP/IP model and run over either TCP or UDP. The
H.323 series supports simultaneous voice and
data transmission and can
transmit binary messages that are encoded using basic
encoding rules. H.323 provides a
unique framework for security ,user authentication, and authorization and
supports conference calls and multipoint
connections , as well as
accounting and call-forwarding services. H.323 components
1. Multimedia terminal: a
multimedia terminal is designed to support video and
data traffic and to provide
support for IP telephony.
2. DNS Server: As in SIP, a DNS
server maps a domain name to an IP address.
3. Gateway: the gateway is a
router that serves as an interface between the IP telephone system and the
traditional telephone network.
4. Gate keeper: the gatekeeper is
the control center that performs all the location and signaling function. The
gatekeeper monitors and coordinates the activities of the gateway. The gateway
also performs some signaling functions.
5. Multicast or multipoint
control uint: this unit provides some multipoint services such as conference
calls
When user1 communicates with its
DNS server, the signaling use both TCP and UDP, and partitioned into the
following five steps
1. Call setup
2. Initial communication
capability
3. Audio/video communication
establishment
4. Communications
5. Call termination
When user1 dials user2’s
telephone number , the first set of signals are
exchanged between these two users
in conjunction with opening a TCP
connection.
TCP-SYN,TCP-SYN-ACK and then
TCP-ACK signals are generated between the two users. At this point, theH225.0
SETUP ON TCP signal informs the called party that the connection can be set up
on TCP. The users can now request a certain bandwidth from the associated
gatekeeper server to register with. At step2, all the end points communication
capabilities available over TCP are exchanged. Step3 implements the
establishment of a logical channel , which in H.323 is unidirectional. At the
end of this phase, two end points are set for communications.
Step 4 comprises the
communication between two users. This phase is handled using RTP over UDP. At
this step any kind of media flow can be considered, depending on the size and
type of the channel established in step4. Finally, at step 5, the call is
terminated by either user. , user2 initaites the termination of the call by
closing the logical channel in H.245 and disconnecting the call from
the gatekeeper.
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