karn (01/11/83)
LOCAL AREA NETWORKS VIA PACKET RADIO MEDIUM
Den Connors
University Computer Center
University of Arizona, Tucson, Arizona
Abstract
Investigations are underway by the Amateur
Radio community to incorporate digital communica-
tions techniques into the Amateur Service. The
network system utilizes simple techniques, using
normal terminals and personal computers, and a wide
variety of amateur radio transceivers. Complete
local area network protocols are realized by
incorporating a micro-processor based interface
unit to connect the terminal device and radio.
Physical-level protocol involves half-duplex trans-
missions with frequency-modulated transceivers.
Channel activity is sensed by the packet radio, and
negative acknowledgement used to determine if
retransmission of lost packets is required. A
data-link control is also implemented, and two
connecting protocols are being tested on top of
these lower levels. The TAPR/AMSAT protocol
provides multiple point-to-point connections on the
channel. The TAPR/DA (dynamic-addressing) protocol
implements a full datagram network, and uses a
robust network control scheme to provide near-
optimum channel utilization and network tolerance
to a number of error conditions related to open
radio channels.
Introduction
The Tucson Amateur Packet Radio group is
currently coordinating a series of tests with
Amateur Radio operators in eighteen major U.S.
cities, to investigate the use of digital computer
and terminal communications using packet radio
techniques. Initial investigations are being
performed on the Amateur Radio Service VHF bands,
using half-duplex transmissions. Two sets of
experiments are being undertaken; the first is
testing multiple simultaneous connections on a
single shared frequency, while the second tests a
new form of efficient radio frequency local area
network.
A general description of the system is given
in terms of the signals and medium access. This is
followed by information on the hardware imple-
mentation. A more detailed description of the
signalling technique, data link protocol and the
two types of link and network protocols are
described.
Physical and Medium Access Protocols
Both protocol tests are using a common medium.
Access to the radio channel is accomplished using
audio frequency shift keying within the normal
audio communications passband. The packet radios
are able to detect digital communications on the
channel, and will defer transmissions while there
is activity on the channel. Collisions,
interference and unusual conditions (fading,
multipath, etc.) which cause unsuccessful trans-
missions will result in no acknowledgement, and an
automatic retransmission will result after a set
delay. Multiple unsuccessful transmissions will
produce an error indication at the data source.
Successful transmissions will result in an acknow-
ledgement being transmitted to the originating
station. The acknowledgements may occur after each
packet, or may be deferred for up to seven packets,
using standard high-level bit-oriented data link
protocol (HDLC), implemented with large-scale
integrated circuitry.
This multiple-access protocol is therefore
based on on-channel data sensing, with negative
acknowledgement causing retransmission (data-sensed
multiple-access with collision-detection by
negative acknowledgement, or DSMA/NAK). It should
be noted that the normal LAN technique of
carrier-sensing is not appropriate to the Amateur
Radio Service, where all manner of signals are apt
to impinge on the data channel. Use of much less
occupied bands is possible, and conversion to
carrier-sensing trivial. However, the choice of
using the more heavily-used bands was made to
provide exposure to the general population, and
promote acceptance of this new mode.
Hardware
The Tucson Amateur Packet Radio Corporation
(TAPR) has created a single-board packet radio
terminal node controller (TNC) which interfaces
standard microcomputers or data terminals to
commonly available Amateur Radio transceivers.
Initial investigations in Tucson are using the
two-meter band (144-148MHz), and both this band and
others will be used in the other seventeen test
sites. This band was chosen for maximum visibility
within the Amateur community.
The transceiver interface is performed without
modification to the unit by using the three nor-
mally-available signal lines; audio input
(microphone), audio out (speaker) and transmit
(push-to-talk). This ease of equipment config-
uration was the primary reason to use audio-
frequency shift keying, and to keep the modulating
signals within the audio passband.
The digital data from the terminal or computer
is fed to the packet radio interface board either
through a standard RS-232 serial interface port, or
by using a custom parallel interface. TTL-level
lines are provided on the board for these
connections, and a wire-wrap area is also included
for non-standard interconnects.
The TNC accepts the serial or parallel data
and formats the data into packets, using the
standard HDLC format as described below. These
serial packets are encoded using a non-return-
to-zero inverted (NRZI) code to assist in providing
an audio signal for the demodulator to lock on,
using phase-locked loops. This NRZI data is
presented to the audio FSK modulator, and the
modulated signal is fed to the audio input of the
transceiver.
The received signal appears as an AFSK signal
on the audio output of the radio. A side benefit
of using standard Amateur radios is the avail-
ability of a speaker for monitoring the channel
while the experiments are in progress, although
experience suggests that continued monitoring can
be most distracting.
The signalling tones used to modulate the
radio were chosen to be compatible with those of
the Bell 202 half-duplex modem set of standards.
This was done both to use a well-tested pair of
frequencies with much operating experience, and to
allow earlier experimenters with surplus 202-type
modems to use their equipment, expanding the size
of the test community. The main drawback to the
202-type modem is the limitation of 1200
bits/second of data transmission. Further experi-
mentation is planned for higher-speed phase-shift
keying modems within radios built expressly for
packet radio service.
Hardware also exists on board to handle packet
encoding and decoding, buffering, and system initi-
alization and calibration. All components neces-
sary to provide the channel access and data link
control, as well as network protocol, are also
contained on the TNC. A built-in modem and power
supply minimize the complexity of installation.
Data Link Control
Both protocols being investigated use similar
data-link protocols, although there are differences
in addressing modes. An HDLC controller integrated
circuit provides most of the circuitry for building
and disassembling the packets. HDLC framing
signals, address bytes, control byte and trailing
cyclic redundancy check bytes are employed without
modification. Successful packets are acknowledged
on receipt, using standard HDLC control modes.
Each packet is checked for correct CRC calculation,
and the acknowledgement packet is withheld if the
CRC is incorrect. This forces a retransmission by
the source station. Retransmitted packets are
thereby forced either by incorrect CRC calculation
or by having been interfered with (collisions or
unwanted signals). Collisions of packets are
minimized by the data-sensing capabilities of the
packet radios.
The decision of the next time to retransmit a
packet will be weighted by a binary-exponent random
back-off algorithm, similar to that used by
Ethernet-like systems. This minimizes the possi-
bility of continuing collisions by radios with
nearly-synchronous retransmission attempts.
Unlike most LAN media, the open radio channel
susceptibility to intentional and unintentional
interference adds complexity to the link-level
algorithms used to reschedule transmissions.
Another problem is the phenomenon of "capture" of
the receiver demodulator by FM signals stronger
than that of the desired signal.
Connection-Oriented Protocol
The first protocol used to exercise the packet
radio system was one developed simultaneously by
groups in the Los Angeles and Washington, D.C.
areas, and approved and "standardized" at a meeting
sponsored by the Radio Amateur Satellite
Corporation (AMSAT) in October of 1982. The
implementation of this TAPR/AMSAT protocol provides
a means of connecting pairs of packet radios with a
communication channel transparent to other users of
the shared channel. While lacking some of the
capabilities of a full local area network, this
protocol nonetheless provides a means of exper-
imenting with channel capacity, access algorithms
and system parameters.
The TAPR/AMSAT implementation provides the
same types of functions as the link-access protocol
of X.25. Connection and disconnection, error
indications and control, and flow control are all
provided. Virtual circuits are supported on the
channel and channel-wide monitoring is available.
The addressing scheme is unique, in that the
Amateur Radio Service has associated with each
station a single call sign, that of the station
owner/operator. The packet radios have made use of
this uniqueness to provide unambiguous addressing,
both within LAN's and across future internetwork
gateways. A serious drawback is the requirement of
seven bytes for both sending and receiving
addresses, but on the half-duplex channels
currently in use, this added overhead is not
noticeable.
A further enhancement allows extension of the
network to larger geographical areas with any of
the stations acting as a store-and-forward packet
repeater. Stations requiring repeat services use
bits in the HDLC control word to signal the need to
have a message repeated, and repeated messages turn
off this bit to prevent possible multiple
acknowledgements.
Although this protocol is designed primarily
for point-to-point links within a larger multi-
point network configuration, it does lend itself
well to some of the needs of the shared-channel
packet system, including terminal-to-host and
host-to-host connections.
Datagram Protocol with Dynamic Addressing
The second protocol being used in the packet
radio experiment test beds has a more efficient
addressing scheme, providing each station with a
mechanism to obtain a one-byte address when first
accessing the network. This dynamic-addressing
protocol, called TAPR/ DA, provides a full network
configuration with connectionless protocol and a
centralized, failure-tolerant network control
mechanism.
The unique aspect of this LAN protocol is this
utilization of dynamic address allocation. A
network control station (NCS) monitors for stations
attempting to sign on, and replaces the amateur
radio call sign sent to the NCS with a single byte
address for efficiency. Each station then has
available from the NCS a list of users (amateur
call signs) and corresponding assigned addresses.
This list is the system status table. For
robustness and ease of switching to other radio
channels, the first station on any given frequency
will automatically be chosen as the NCS, and in the
event of loss of this station, the next station
attempting transmissions which require the NCS will
become the NCS.
This protocol also supports a store-and-
forward repeat capability. The possibility of
hearing stations in a different network geo-
graphically remote, but using the same frequency
requires special algorithms. The conditions of
multiple store-and-forward digipeaters, network
control stations or identically-addressed stations
must be detected and corrected.
Future Research
Currently experiments are underway to further
extend network topology through the use of
specified stations as network-to-network links,
using separate radio channels. Additionally, the
use of amateur radio satellites is being planned.
Acknowledgements
The author wishes to acknowledge the efforts
of Lyle Johnson, who is the principal architect of
the terminal node controller hardware. Margaret
Morrison, Harold Price and David Henserson wrote
the TAPR/AMSAT software for the system, and Marcus
Chamberlin and David McClain produced the TAPR/DA
software. Design credit also goes to all of the
above, as well as Charles Green, Mark Baker and Dan
Morrison for several design concepts. Further
gratitude is given to the 180 hams who have
volunteered a wide variety of resources to help
test the system.