Net-2000™ Voice Codec Unit
User’s Manual
Page 27
Section 3 – Net-2000™ VCU Control
DVSI Confidential Proprietary
Interface Protocol
Introduction
The VCU implements a channel data interface using a special protocol designed by DVSI. The Net-
2000™ VCU packet is described from the point-of view of the Net-2000™ VCU. Although the data
packet structure is described in some detail here, the reader should also refer to the listing of files in
the C-language header file section for data structure layout, data values, etc.
Serial Port Protocol
The RS-422 interface protocol is exactly the same as the Ethernet protocol interface but, with an
extra 16-bit sync word so that sync up can done in the middle of a transmission.
Ethernet Protocol
The Net-2000™ VCU uses port 4000 for communication of channel and voice data across the
Ethernet interface.:
Port 4000
-- the Net-2000™ VCU sends channel/voice data over this port when the channel/voice
interface is Ethernet. It also uses this port for vocoder commands. The default port number is 4000,
but a different port number can be assigned by using the web interface control and selecting the Net-
2000™ VCU settings button.
Note:
Port numbers 4001, 4002 and 4003 are reserved by the Net-2000™ VCU and
must not be used. Valid port numbers are greater than 1024 and less than 65535.
Protocol Packet Description
The Ethernet protocol of the Net-2000™ VCU’s uses port 4000. Byte order for all 16-bit data types
(“short” in the C-language data structure) is network order (big endian), so bytes with lower indexes
are the most significant. Since PC’s are little endian machines it is necessary to byte-swap when
using this protocol with a PC.
There are four packets defined for the VCU protocol: a Channel packet, a Voice packet, a Reset
packet, an Ack packet and a Nack packet. Channel packets contain channel data that are output by
the encoder (coded speech) and are inputs to the decoder to synthesize speech. Voice packets
contain voice data that are inputs to the encoder to compress into channel data and outputs of the
decoder. The Reset packet is used to re-initialize the encoder and decoder state data structures so
that repeatable results can be generated for a given input. Ack packets are sent in response to
Reset packets. Nack packets are sent in response to anything else.
When the VCU is in VOIP Mode it exchanges Channel packets with another VCU in real time to
provide a full duplex communication channel. The decoder in the VCU synthesizes the channel data
into speech and sends it out the handset and 4-wire voice interfaces. Only Channel packets are
transmitted in VOIP mode. Voice, Reset, and Acknowledge packets are not used.
When the VCU is in File Mode is monitors port 4000 (or a different port if one has been selected
using the web interface) for incoming packets. If a Reset packet is received the VCU resets the
encoder and/or decoder as specified in the packet and responds to the packet sender with an ACK
packet. If an Voice packet is received the VCU encodes the data in the packet and responds to the
sender with a Channel packet containing the encoded data. If a Channel packet is received the VCU
decodes the packet and responds to the sender with an Voice packet containing the synthesized
speech. In File Mode it is possible to produce bit-exact, repeatable results for vocoder quality testing.