Amateur Radio - Free and Open Source Software

Number I' on your FeedbKil card
TexNet
Packet-Switching Network
An overview ofa highly successful
and efficient packet radio network.
by Greg Jones WD51 VD
C
o
nce ived by Tom McDermott
N5EG an d T o m
Asche nb re nne r WB5PUC in the
summer o f 1985, the Texas network
began as a small summer projeer.
One year later, the result wa s
TexNcl- an inexpe nsive , mu lli-resou
rce, Iour-pon . hi gh-speed
" backbo ne ," datagram-based amateur
packet switchi ng system.
Texjcet allows Texas packet rad io
operators 10 commun icate effecti
vely over di stances of several
hundred miles in real time , and is
currently beli eved 10 be the longest
and fastest 9600 baud amateur network
in the United Stales (see Figure
I).
S)'stem Definition
TexNel , a datagram-based octwork,
ackno wledges packets at
each step o f the path, ope rate s with
minimal time delay , and provides
user serv ices as we ll as information
about network operations . Operation
is at 9600 baud on 450 MHz,
with typical local user access at
1200 baud A FS K on IWO meters or
220 MHz. If necessary, inter- node
trunks can run at any of the lower
speeds. and the primary or secondary
user port will support other
baud rates and modulation techniques .
The system is completely compat ible with
both versions of the AX .25 protocol specifications
for user connections. The network
itse lf communicates between its own nodes
using AX .25 as the data-link layer two protocol
and T EXNET-IP as the layer three network
protocol. The TEXNET-IP prot ocol
adds only fi ve bytes of overhead to the front
of every packet inside the network.
TEXNET-IP is tran sparent 10 all users
because the entry and exit nodes translate
the users' packets to T EXNET-IP and back
again (see Figure 2). The terminating nodes
du ring a use r connect ion mainta in tables that
specify how each user is connected.
The purpose of an intermediate node is to
perform transit-routing only. A TexNet node
ope rates with no fixed routing assignments; a
node's routing table is generated upon startup
54 73 Amaleur Radio • OCtober. 1989
Photo A. Top l ·it'K'• • •
.JJII-.l1II
Photo B . . " andf rom panel ofa TexNet Networs: Comrol Processor.
Michixan Textvet Node constructed by Jay Nugem WB8TKL.
and updated as new nod es begin operations or
as current nodes are reset. This allows a node
to mai ntain primary and secondary rout ing to
all other nodes in the network (the use r command
ROUTE describes this in more detail).
TexNet supports 255 nodes pe r network using
the same network ID , and there are 255
netwo rk IDs av ailable.
Hardware
The heart ofa TexNet node is a partit ioned
PC board composed ofthe NCP (node control
processor). a 9600 baud TPRS (Texas Packet
Radio Society) FSK modem. and a I200 baud
A FSK modem. (Figure 3 shows a block d iagram
of a Te xNel node configuratio n.I The
third port is left free for the attachment ofany
kind of modem (land-line, 2400 baud , PSK).
The 1200 baud AFSK modem is similar to the
T APR TNC-I modern. The 9600 baud FS K
•
modem is a redesigned
K9 NG
modem, with improved receive filters.
The NCP u nit contains a
Z80A CPU operating at 4 MHz,
32K EPROM , 40K RAM memory,
two sioro serial comm unications
ICs for the serial HOLC po rts, and
aCTC .
Careful design in both softwa re
and hardware was necessary 10 allow
all th ree po rts to ru n at 9600
bps. TPRS decided to develop its
own board 10 keep the cost down
and to include two special circuits,
a reliable crys tal oscillator and a
fail-safe state mac hine called " fire
code." Fire code is an EPRO M­
based logic circuit that monitors
the IP data and dock lines (completely
independent of the processor
and communications ICs) for
the presence of a 72-bit un ique sequence
comm andi ng the node 10
reset. This 72 ,bit sequence is progra
mmed into the EPROM along
with its slate mac hine. The mean
lime between false activation is calculated
to be considerably more
than one million years.
The local console port on the
NCP supports the local console,
control points, and weather interface
. The control point s let the
node control and monitor status items connected
to the node. Some of the uses for
control points are to check the status of emergency
power at site, 10 check if the power
level ofbatteries is too low o r too high, and to
turn devices on and off.
A TexNet daughterboard lets the NC P interface
with an optional Winchester hard disk
controller. The daught crhoard supports the
ha rd disk feature, that in turn suppo rts the
packet message server (PMS) and weather
feed . The deugbterboard also supports additional
control points. T he iocal console
weather input allows data from the National
w eather Service 10 be fed in at either 1200
bps RS-232 or 75 baud Baudot : th is data is
then stored on the PM S.
The other key aspect of how well the hardware
operates is not the TexNet hardware,
bUI the backbone rad io . Th e performance o f