JPS Communications NXU-2, Инструкция по установке и эксплуатации

Страница: 51 / 53

NXU-2 Operations Manual
JPS Communications, Inc.
7-1
7 Theory of Operation
The NXU-2 Network Extension Unit consists of two main assemblies – the network processor
and the digital signal processor (DSP). The network processor, a Motorola Coldfire
MCF5206e, handles all the Internet Protocol (IP) related tasks, and provides an Ethernet
interface to the network. The DSP, a Texas Instruments TMS320VC5409, handles all the
audio-related tasks, including the voice compression and decompression.
The NXU-2 can be configured as a client or a server. Servers can only accept IP connections
from clients, and clients can only make and break connections from servers. Once a connection
is established, however, the operation of an NXU-2 is the same regardless of whether it’s a
client or a server. This operation is outlined below.
When power is applied to an NXU-2 it either waits for a connection (if it’s a server) or attempts
a connection to a server (if it’s a client). The server it attempts to connect to is the one that has
an IP address identical to the SRVRIP address programmed in the client. This connection is a
standard TCP/IP connection on port 1221. Once a connection is established each NXU-2 DSP
begins converting analog data into digital data and compressing it to reduce the amount of
bandwidth it will take to send it across the network to the associated unit. This
conversion/compression process runs continuously, even if data is not currently being sent
across the network. The network processor on each NXU-2 shares a common area of memory
with the unit’s DSP processor, allowing data to be exchanged between the two processors
quickly and easily. When the network processor sees the unit’s COR input line go active it
collects the frames of compressed digital audio from the DSP and packages them into packets
for transmission across the network. These audio packets are sent to the NXU-2 at the other end
of the link using UDP on port 1221. In addition to the audio information the packets also
contain information about the status of the COR and AUX IN lines.
When these packets are received at the other end of the link, the receiving network processor
separates the audio from the status information and updates the unit’s PTT output and AUX
OUT lines based on this status information. The audio frames are then sent to the DSP for
decompression. When the DSP has completed the decompression of a frame, it sends the
resulting samples to the digital-to-analog (D/A) converter; the resulting analog audio signal is
available at the units audio output port. This process can run in both directions simultaneously
since the NXU-2 is capable of full duplex operation.
Transmission of RS-232 data is handled solely by the network processor, and is sent using TCP
on port 1221. If COR is not active the NXU-2 will send an empty packet every four seconds in
order to keep the connection from timing out.
The DSP master clock is the source of timing for A/D and D/A conversions as well as for
transmission of packets across the network. The buffer management software in the NXU-2 can
account for slight differences in master clock frequencies on each end, and can account for
network jitter or packets, which arrive late.