Specifications

Net-2000™ VCU Users Manual
Version 4.0, December, 2013
Interface Protocol
Wh
en the VCU is in VOIP Mode it exchanges Decode Command Packets with another VCU in real time to provide
a full duplex communication channel. The encoder in a VCU compresses speech from its analog handset and 4-wire
sources and transmits it to a receiving VCU using Decode Command Packets. The decoder in the VCU synthesizes
the channel data from a received Decode Command Packet into speech and sends it to its analog interfaces.
When the VCU is in File Mode, it 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 Encode Command Packet is
received the VCU encodes the voice data in the packet and responds to the sender with a Channel Response Packet
containing the encoded data. If a Decode Command Packet is received, the VCU decodes the channel data in the
packet and responds to the sender with a Voice Response Packet containing 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.
6.3.1 Packet Header
Every 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 8. An “x” is used in all the tables in this description to indicate
t
hat a bit can take a value of 0 or 1.
Packet Header
16 bit Word
15 14 13 12 11 10 9 8 6 5 4 3 2 1 0
Bytes
(msb-lsb)
Byte (n) Byte (n+1)
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
Header
4 – 5 x x x x x x x x x x x x x x x x seq
Table 8 Packet Header
Bytes 0 and 1 hold the 16-bit length of the packet (see Table 9 len Header ). Maximum packet length is 514 bytes.
B
ytes 2 and 3 hold the 16-bit packet type (see Table 10 type ). Bytes 4 and 5 hold the 16-bit sequence number
param
eter (see Table 11 seq Header .) The sequence number of a response t
o a command packet is the command
packet sequence number plus one. The sequence number is used as a method to detect that no data packets are
dropped, so it is important to ensure that consecutive sequence numbers are transmitted for voice and channel data
packets during operation.
len = Length (in bytes) of total packet (including header)
16 bit Word
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Byte 0 Byte 1 Decimal Value
Minimum Value 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 6 bytes
Maximum Value 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 514 bytes
Table 9 len Header Parameter
(Subject to Change) Page 37
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