Digital Voice Systems Net-2000-P25, Руководство пользователя

Страница: 24 / 48

Net-2000™ Voice Codec Unit
User’s Manual
Page 16
DVSI Confidential Proprietary
Section 4 – Net-2000™ VCU Interface Protocol
Interface Protocol
Protocol Packet Description
The UDP 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 (coded speech)
that is sent to the decoder to synthesize speech. Voice packets contain voice data that is sent to the
encoder to compress into channel data. 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 UDP 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.
PC source code is provided on the CD included with the VCU that implements the protocol and can
be used with the VCU in File Mode to encode and decode speech.
Packet Header
Each packet in the protocol starts with a six-byte header containing a 16-bit integer indicating the
overall length of the packet in bytes, a 16-bit integer indicating the type of the packet, and a 16-bit
sequence number. The six-byte packet header for all data packets is shown in Table 2. An “x” is
used in all the tables in this description to indicate that a bit can take a value of 0 or 1.
Packet Header
Bytes
(msb-lsb)
Bits
7 6 5 4 3 2 1 0
Bits
7 6 5 4 3 2 1 0
Parameter Name
0 – 1 0 0 0 0 0 0 0 0 x x x x x x x x len
2 – 3 0 0 0 0 0 0 0 0 x x x x x x x x type
H
e
a
d
e
r
4 – 5 x x x x x x x x x x x x x x x x seq
Table 6 Channel Packet Header