Performance and Setup Guide
for the NOS TCP/IP Protocol Used on HF Near-Vertical-Incidence-Skywave
(NVIS) Radio Paths
This note describes two years of testing of the Transmission Control
Protocol and Internet Protocol (TCP/IP) protocol stack over near-vertical-incidence-skywave
(NVIS) propagation paths in Massachusetts and New Hampshire. Results
of the testing bear on the evaluation of a number of COTS and developmental
systems that transfer data over high frequency (HF) radio.
Among these are systems being considered in "dual-use" HF
radio configurations on board military aircraft being modified under
the US Air Force Global Air Traffic Management (GATM) upgrade program.
One use of such radio systems is communication of civilian air traffic
control and "airline operational control" data over commercial
networks. A second use is communication of encrypted military command
and control data over purely military networks. Our results provide
throughput and probability of correct message delivery baselines for
both modes of use.
Our study also allows baseline performance comparison for systems that
exchange data between tactical ("short-haul") military users
over beyond-line-of-sight radio links, and for systems that can communicate
IP-addressable data among platforms being integrated into the worldwide
US military Global Grid. In the last case, the TCP/IP protocol stack
we have evaluated can itself be used in preliminary studies of direct
transmission of TCP/IP datagrams over half-duplex radio.
To aid the use by others of the shareware stack we evaluated, we have
expanded the scope of our report to include not only performance results
but also selected information on how to set up and effectively operate
the stack over error-prone radio links. The report is not, however,
a comprehensive manual on configuration and operation of TCP/IP over
radio.
NVIS paths are relatively short radio paths (ground distances from
about 20 to 400 miles) used for tactical military communications and
various civilian purposes. Difficult conditions are often encountered
during NVIS communications. Among these conditions are multipath interference
at sunrise and sundown, high D-layer signal absorption at midday and
strong interference from long-distance broadcast stations at night.
Protocol performance evaluations over NVIS links therefore provide conservative
(i.e., lower-bound) assessments of the long-distance HF skywave communications
normally associated with the concept of "shortwave" radio.
The tests are part of an on-going, multi-year program to assess and
compare reliable, modern systems for communicating data over HF radio.
Such systems comprise, at each station, an HF radio and antenna, a controller
(usually a computer) that runs a user interface, and an HF modem that
does waveform generation, signal processing and forward error correction.
The software that implements a particular data transmission protocol
runs either on the controller or in the modem or both.
"Reliable" in the sense of this report means that the protocol
includes an automatic repeat request (ARQ) sub-protocol that causes
a station receiving erroneous and uncorrectable data to ask the transmitting
station to repeat it.
Although the standard TCP/IP stack was not developed for use on radio
channels, its performance (in half-duplex modes) is still good enough
for fielded military use and to serve as a baseline for the performance
of (non-TCP) ARQ protocols that are tailored to HF channels. The TCP/IP
stack used for these tests is the Net Operating System (NOS), a 16-bit,
DOS-based stack developed by amateur radio operators and subsequently
used commercially and by the military. By installing special software
"shims" we have interfaced IP-based 32-bit Windows 95 applications
such as E-mail clients and Worldwide Web (WWW) browsers with NOS and
have sent E-mail and other TCP data over NVIS links using standard Windows
95 applications. This process is described in the note.
Because of their inability to distinguish between contention and channel
errors (to be discussed below), standard (RFC-1122-compliant) TCP/IP
stacks like NOS are ill suited to highly efficient use over radio channels.
In spite of this, performance over HF links that is acceptable in many
data-communication applications can be achieved by adapting the stack's
window- and segment-size parameters, and the HF modem's data rate
and interleaver settings, to noise and propagation conditions on the
channel. Our experience with this approach, which is meant to defeat
standard TCP's unsuitable contention-control protocols, is also
discussed in the note.
TCP/IP over HF Radio, IP-based Applications over HF Radio, Tactical
Data Communications, Near-Vertical-Incidence- Skywave, NVIS, MIL-STD-188-110A
Modems, Serial-tone Modems, NOS, Pinging, Radio Data Networks, SMTP,
ICMP, FTP
