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4 Voice Over IP
About Voice over IP (VoIP)
SoTCP Operation
Introduction This section describes the operation of SoTCP protocol. SoTCP provides the
encapsulation method for carrying voice traffic over an IP network.
SoTCP Packet
Format
The SoTCP protocol terminates voice traffic received from a voice device and
prefixes the traffic with a 3-byte SoTCP header or 4-byte SoTCP header, depending
on the version of SoTCP protocol. Figure 3 illustrates the SoTCP packet.
Figure 3. SoTCP Packet Format
The SoTCP header contains information that will allow a destination IP node to
reconstruct the voice packet at the remote end.
Transport
Mechanism
To carry voice traffic over IP, SoTCP uses two Transport layer protocols, the
Transport Control Protocol (TCP) and User Datagram Protocol (UDP).
Each TCP or UDP session may carry multiple switched virtual circuits (SVCs).
SoTCP uses SVC call setup and routing procedures to establish voice SVCs.
3 or 4 Byte SoTCP Header
Data
Transport
Layer
Protocol
Traffic Carried Description
TCP Voice Signalling Traffic SoTCP uses TCP to carry voice signalling
traffic because TCP provides reliable,
sequenced packet delivery. Voice
signalling traffic encapsulated by SoTCP
must arrive at the destination node without
error or delay. TCP implements error
recovery for duplicate or lost packets.
UDP Audio and Fax SoTCP transports audio and fax traffic
over UDP session. Unlike TCP, UDP
provides fast and simple transaction
services with minimal protocol overhead.
UDP provides a best-effort, transport
mechanism only and does not ensure
reliable delivery.