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6 Voice Over IP
About Voice over IP (VoIP)
Establishing a
Session
Before voice traffic can pass between the voice ports over the network, the source IP
must establish a TCP and UDP session with the destination IP node. Each session is
uniquely identified by a source IP address, destination IP address, source port
number, and destination port number. The source and destination node exchange the
following call packets to establish a session:
Establishing a
Connection
After opening a TCP and UDP session, the source node must establish an SoTCP
connection with the destination IP node. Each TCP session support multiple
connections or SVCs. A unique connection identifier identifies each connection
multiplexed on a TCP session. The source and destination IP nodes exchange the
following call packets to establish a connection:
Stage Packet Transferred Description
1 ACP Call Request The voice port sends an ACP (Adjacent
Channel Protocol) Call Request to inform
the source IP node that it wishes to send
voice over the network to the destination
voice port.
Open UDP When a voice port attempts to make a call,
the Source IP node opens a UDP channel.
2 TCP Call Request To open a session, the source IP node sends a
TCP call request to the destination IP node.
TCP Call Accept The destination IP node responds to the Call
Request by sending a Call Accept, and
establishes a TCP session.
Stage Packet Transferred Description
1 SoTCP Internal Call
Request
The source IP node sends an internal
SoTCP call request packet with the UDP
port number included, to the destination IP
node. The destination IP node examines the
facilities field in the header to determine
the presence of voice traffic. If voice traffic
is present, the destination IP node opens a
UDP channel.
The destination IP node uses the UDP port
number in the internal SoTCP call request
packet to identify the destination port for
UDP packets.
2 ACP Call Request The destination IP node sends an ACP call
request to the remote voice port.
ACP Call Accept To accept the call, the voice port responds
with an ACP Call accept packet.