Comparison of Type 2 and Type 4 Services for MIL-STD-188-220B
Dr. David J. Thuente 1
Timothy E. Borchelt
Department of Computer Science
Indiana University Purdue University Fort Wayne
Fort Wayne, Indiana 46815
email: thuente@ipfw.edu
Raytheon Systems Company
Fort Wayne, Indiana, 46808
email: teborc@ftw.rsc.raytheon.com
Abstract - Numerous papers at recent MILCOM
conferences have proposed and examined different media
access control (MAC) algorithms. Many of these have
been for combat net radios or for MIL-STD-188-220B.
The two most visible and perhaps viable MAC algorithms
from MIL-STD-188-220B are deterministic adaptable
priority network access delay (DAP-NAD) and radio
embedded network access delay (RE-NAD). Analyses of
these algorithms and their variations have been
published. These comparative analyses of parts of MILSTD-188-220B have not addressed an equally
fundamental aspect of the datalink layer protocol: the
type of service supported.
MIL-STD-188-220B has
defined four types of service: Types 1, 2, 3, and 4. Type 2
and 4 are generally used to reliably transfer large
amounts of data because they allow concatenation and
acknowledgments.
Type 2 services are connection
oriented with decoupled acknowledgments based on
HDLC protocols. Type 4 services are connectionless with
individual decoupled acknowledgments for every packet
received. Type 1 and 3 are connectionless operations with
Type 1 being unacknowledged and Type 3 requiring an
immediate acknowledgment (also called coupled
acknowledged Type 1 in MIL-STD-188-220B). There has
been no published analysis of Type 2 and 4 services and
yet this is a fundamental choice for most reliable
transmissions on networks that will use MIL-STD-188220B.
(FS) system have been used to drive the model. Most of
the comparative studies for Type 2 and Type 4 were done
using DAP-NAD. We show, for our example networks,
that if they are lightly or moderately loaded in an errorfree environment, then the performance of the Type 2 and
Type 4 services for average message latency and for the
completion of fire missions are relatively close to each
other. In general, Type 2 slightly outperforms Type 4
under these conditions. When the load on the network
becomes heavier and when errors are introduced, Type 2
significantly outperforms Type 4 in all measures of
network performance. The results also show that the
maximum window size of 20 for Type 4 services can be a
significant constraint on the performance of the Type 4
protocol. Comparative results for Type 2 and Type 4
using RE-NAD showed that Type 2 outperformed Type 4
much more completely for RE-NAD than for DAP-NAD.
The results in this paper show that DAP-NAD is
significantly better than RE-NAD with both Type 2 and
Type 4 services. All of these results have direct and
immediate applications to networks that are to use MILSTD-188-220B.
As an outgrowth of the work begun on the Advanced
Field Artillery Tactical Data System (AFATDS) program,
a detailed model for the analysis and evaluation of many
aspects of MIL-STD-188-220B has been built. This model
has been used to develop many important results about
the performance of MIL-STD-188-220B for various
network configurations and message types and loads. The
primary emphasis in this paper is on the comparative
results for Type 2 and Type 4 services.
The message behavior (threads, sizes, frequency,
dependencies, sequences, etc.) of an actual fire support
1
This work was performed, in part, while the author was a
consultant at Raytheon Systems Company C3I Segment, Tactical
Systems division and where he continues as a consultant on
communications protocols.
I. INTRODUCTION
Efficient utilization of the available bandwidth is critical to
the use of combat net radios. Two of the key issues for this
efficient utilization are the MAC algorithms and the types of
service provided.
Some results presented at previous
MILCOM conferences have been on the performance,
improvements, or variations of RE-NAD and on extensions
of the RE-NAD protocol to better transmit compressed voice.
See for example, [1], [6], [10], [11], [13], [14] and references
within those papers. All of the extensions and improvements
still rely on the type of service for the reliable delivery of
packets. The results presented here are the first time, to our
knowledge, any study has shown the conditions for and the
magnitude of the benefits of Type 2 services. There have not
been any published studies comparing the efficiency of the
types of service for MIL-STD-188-220B.
The authors have presented results at MILCOM
conferences comparing RE-NAD and DAP-NAD for Types 1
and 2 and have discussed an OPNET model for other MILSTD-188-220B features. Earlier results presented at the
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Tactical Communications Conference [9] developed some
improvements and optimizations for DAP-NAD as part of the
AFATDS modeling work.
Extensions to the original
AFATDS model were accomplished in conjunction with its
application to two other projects as well as some Raytheon
sponsored IR&D. The work presented in this paper is an
extension of that original AFATDS OPNET work but is not
part of the AFATDS development. The original model of the
AFATDS communication system was verified and validated
and the description of that effort was described in [8], [9].
The primary emphasis in this paper is on the comparative
results for Type 2 and Type 4 services for various network
configurations and MAC algorithms.
II. NETWORK AND SCENARIO CONFIGURATIONS
Two different sized networks are used for the simulation
scenarios. Similar network configurations are expected to be
used in actual FS situations. The smaller network, called
Net_A has seven nodes: six fire units, FU_As, and an
operations center, OC_A. The OC_A generates all of the fire
missions. There are two types of fire missions generated:
thread 1 has a Subsequent Adjust and consists of a sequence
of six messages with corresponding ACKs plus two
additional messages. Thread 2 does not have a Subsequent
Adjust message and consists of a sequence of four messages
with corresponding ACKs. The average message size is 75
bytes with a range of 20 to 400 bytes. The Type 2
acknowledgments (when needed) and the Type 4
acknowledgments are eight bytes each. The second network,
called Net_AB, has 14 nodes: two operations centers, OC_A
and OC_B, with each operations centers controlling its
corresponding set of six fire units called FU_As and FU_Bs.
All fire missions are generated exponentially at the OC_A
or OC_B and randomly distributed to their respective fire
units. A load of 600 fire missions on Net_A means that 300
fire missions of Thread 1 and 300 fire missions of Thread 2
are generated at OC_A. A load of 600 fire missions on
Net_AB means that 150 fire missions of Thread 1 and 150
fire missions of Thread 2 are generated at OC_A and 150 fire
missions of Thread 1 and 150 fire missions of Thread 2 are
generated at OC_B. The fire mission load is distributed
randomly across the two operation centers and subsequently
across their fire units. In addition, situational awareness (SA)
type messages of 30 bytes each are generated exponentially
with mean of 60 seconds at every node. This data is
transmitted to all other nodes using unacknowledged Type 1
services.
The OPNET model developed includes modified pipeline
stages that provide an accurate simulation of the ReedSolomon (32,12) error correction algorithm. The ReedSolomon (RS) model incorporates 32-hop barberpole
interleaving for an input RS data block size of 1440 bits and
RS transmission block size of 4608 bits. Each transmission
unit includes a frequency hoping synchronization block of
2304 bits and the first RS block of each transmission unit
includes a message indicator and header information. See
[2], [3], [4], [7] for a discussion of RS and structure of the RS
error blocks. RS error correction is not part of MIL-STD188-220B but 24/12 Golay error correction is part of the
standard. Earlier papers [10], [11] that compared DAP-NAD
and RE-NAD were based on MIL-STD-188-220B using
Golay error correction. However, RS error correction is
generally more efficient and is an error correction technology
used in some of the radios currently used on tactical
networks.
The error correction block size for 24/12 Golay with time
dispersal coding (TDC) is 384 bits versus 4608 bits for RS.
A careful analysis of message transmission blocks shows the
effect of the Type 4 acknowledgments would be more
pronounced for Golay than it is for RS. The reason is that for
most cases several 64 bit Type 4 acknowledgment packets
can be included in the 4608 bits of the RS error block with no
additional bits transmitted since the RS error block must be
padded with fill bits to the 4608 bits. Consequently, there is
usually no additional cost for the Type 4 acknowledgment
packet. However, the additional bits will frequently require
an additional 384 bit TDC Golay error block.
Each of the FS threads has delay time for human/machine
interaction or support built into the threads. Threads 1 and 2
have about 105 and 65 seconds, respectively, of delay
incorporated into their execution. The results reported give
the total execution time for the various threads. Many of the
graphs for the completion time of fire threads are scaled to
account for this delay time. The reporting of the packet hop
times or message latency is not scaled in this fashion since
these values represent the actual times.
Message precedence can be set to urgent, priority or
routine.
According to MIL-STD-188-220B, Type 4
acknowledgments have the same priority as the messages that
they are acknowledging. Type 2 messages piggyback their
acknowledgments on any precedence message but can set
their acknowledgments to any precedence level if a separate
ACK packet is generated. The FS messages were determined
to have priority precedence and hence, for Type 4, so did
their acknowledgments. The effect on the network of priority
packets for Type 4 acknowledgments was that the network
stayed in the priority precedence for the entire simulation
unless the network was extremely lightly loaded. For all of
our Type 4 simulation runs, the precedence of the network
remained at priority. In order to fairly compare Type 2 and
4, the precedence of all messages for Type 2 was therefore
set at priority. Simulation has shown that the precedence
effects for DAP-NAD and RE-NAD can be significant but
much work needs to be done before any definitive results can
be presented. (A detailed study of message and network
precedence and their effect on the performance of the
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packets being retransmitted while Type 4 retransmits packets
when
communication system used on tactical networks is
warranted.)
All simulation runs were executed for 13,000 seconds with
the SA type messages being generated by all fire units for the
entire simulation run. The FS threads were generated from
the beginning until 10,000 seconds. This allowed all FS
threads to run to completion unless an intermediate message
in the thread was not able to be received by the necessary
node. The thread then becomes suspended and does not
complete.
The simulation runs include several sets where significant
random noise is introduced on the transmission medium to
test how the networks respond to errors. The random noise is
of sufficient magnitude that it partially disrupts the
communication channel and the RS error correction is not
adequate to correct the subsequent errors. The noise level
was set high enough so that RS could not correct many of the
resulting errors but sufficiently low so that most FS threads
could run to completion. It also needed to be high enough to
test the retransmission differences between Type 2 and Type
4. The effect of the channel noise is that approximately 30%
of the packets are not received correctly. Noise was used to
introduce errors only for the DAP-NAD transmissions since
DAP-NAD was the primary MAC algorithm used. All results
in this paper use DAP-NAD unless explicitly stated
otherwise. Noise was not used with RE-NAD because
transmission failures resulting from collisions (primarily soft
collisions) cause more than sufficient retransmissions.
III. PROTOCOL CONFIGURATIONS
The Type 2, Type 4, DAP-NAD, and RE-NAD protocols
will be discussed only briefly and the reader is referred to
MIL-STD-188-220B for a complete description. There are
obviously too many details to be included in a paper of this
length. Type 2 services are connection oriented with
decoupled acknowledgments and are based on the HDLC
(high-level datalink control) protocol. Type 2 services
provide high reliability because of the connection state but do
not require individual acknowledgments. In most heavily
used networks, Type 2 acknowledgments are included in
other packet headers via piggy backing and incur little or no
additional overhead.
Type 4 uses decoupled
acknowledgments (separate ACK packet) for every
individual Type 4 packet. For reliable transfer of large
amounts of data, Types 2 and 4 are generally used because of
the decoupled nature of the acknowledgments facilitates
concatenation. Type 2 is considered more efficient since it
does not require separate acknowledgments for each packet.
For both Type 2 and Type 4, the acknowledgment packets are
only 64 bits and so the additional overhead for Type 4 may
not be that significant. However, Type 4 differs substantially
from Type 2 when there are network transmission errors.
Type 2 implements selective retransmission with only lost
TABLE I. Type 2 and Type 4 Parameters
Type 4
Parameters
Type 2
ACK_timer
38.0
38.0
P_Bit_timer
19.0
Reject_timer
38.0
Response_Delay_timer
7.0
Max_Retransmissions
3
3
K_Window
64
20/100
K2_Threshhold
32
K3_Threshhold
64
the packet or its acknowledgment is lost. Type 4 will
retransmit the packets when its ACK timer expires whereas
Type 2 will issue a request to the receiver for an
acknowledgment. The Type 2 and Type 4 parameters are
given in Table I.
For Type 2, the K_Window parameter represents the
maximum number of outstanding (unacknowledged)
information frames allowed on a connection and it did not
constrain the network performance in any way. The need for
a large Type 2 K_Window is further limited by the response
delay timer and the K2 threshold. For Type 4, the
K_Window parameter represents the maximum number of
unacknowledged frames allowed for a transmitting station or
node. The range of K_Window parameter for Type 4 is 5 to
20 according to MIL-STD-188-220B [5, Appendix E].
Several sets of simulations were executed with the
K_Window parameter set to 20 to show the limitation it has
on the performance of the networks. Most of the simulations
were run with this parameter set to 100 so as to test other
performance aspects of Type 2 versus Type 4. For almost all
simulation runs, K_Window = 100 for Type 4 was adequate
to not impact network performance.
DAP-NAD is a method of generating network access
delays, which provides every subscriber an equal opportunity
to use the network. It is a dynamic protocol and is
deterministic only in that each subscriber can determine the
maximum amount of time before its next network access
opportunity. From any node’s point of view, if no other
subscriber starts transmitting before that node’s networkaccess-time occurs then that node will transmit. The formulas
for computing the access slots are given in [5], [8].
The RE-NAD algorithm dynamically controls network
access based on network load, network topology, and load
factors. RE-NAD uses two levels of algorithms for media
access control:
(1) modem to radio; the continuous scheduler,
defined in the standard, and
(2) radio to radio; the radio embedded portion, not
defined in the standard.
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RE-NAD uses a continuous scheduler interval computed as
sum of fixed part and a random part; range is 1.0 to 30.0
seconds. The second level of the algorithm uses contention
slots to access the medium. Both levels of RE-NAD’s MAC
are, in part, random.
The contention slot period of RE-NAD is not part of MILSTD-188-220B. The contention period using the CSMA
contention slots follows the protocol given by Whitehall [12].
The radio embedded part of the medium access control
scheme is based on voice, urgent, priority, and routine
messages each being assigned a group of slots in which they
may be transmitted. Various slot configurations were used
during tests and tuning of RE-NAD. The slot configurations
used for the simulation were the most favorable for this
simulation application of any slot configuration tested. The
slot groups may overlap and the slots are numbered starting
at zero. The slot duration was 0.018 seconds and there are
nine priority slots beginning at slot 12.
The net_busy_detect_time = 0.1 seconds was used for all
RE-NAD and DAP-NAD simulation runs. This time is
appropriate for radio configurations that have a squelch pin,
which includes most current production radios used on
tactical networks. It is important for network performance
that the squelch pin be used and configured into the network
operation. For the simulation, the timers used for DAPNAD were TP_time = 0.11 and the DTETURN_time = 0.01
seconds.
Larger values of the net_busy_detect_time
significantly decrease the performance for Type 2 and 4 and
for RE-NAD and DAP-NAD. The raw bit capacity of the
networks was assumed to be 16,000 bps. After RS
encoding, that capacity is reduced to approximately 5000
bps.
This does not include any overhead for the
synchronization or COMSEC headers. The simulation
accounted for all such headers.
present the following results. All simulation results presented
are the average of a number of simulation runs with different
random number seeds. The metrics of particular interest for
this paper are the time to completion of the FS threads and
the packet-hop-time (latency) for Type 2 and Type 4 packets.
The latency time is the time from when the packet is queued
for transmission at the transmitting node until it is received
at the datalink layer by the receiving node. This represents
the individual packet latency on a single link.
The
completion times of the FS threads are important for FS
applications. Generally, these two measures of performance
are consistent and may vary at different rates but this is not
always the case. SA type messages are generated by every
fire unit at an average rate of one every 60 seconds and their
latency is reported. This is not a key performance parameter
since the average latency from the fire units is generally
reasonable. The performance benchmarks given in Figs. 1
and 2 are for error free networks with the K_Window = 100
for Type 4 service. The scenario for Figs. 1, 2, and 3 is the
most favorable to the optimum performance of the networks
and the protocols.
Ne t_ A A v e r a g e Fir e M is s io n C o m p le t io n
( DA P- NA D)
300
T im e ( se c )
250
Th r e a d 1 Ty p e 4
200
Th r e a d 1 Ty p e 2
Th r e a d 2 Ty p e 4
150
Th r e a d 2 Ty p e 2
100
50
200 300 400 500 600 700 800 900
The message precedence assumptions of priority were used
for the simulation in order to make a fair comparison
between Type 2 and Type 4 services. As indicated earlier,
Type 4 acknowledgments are at the same priority as the
original message. We assigned all messages the priority
precedence which forced the network to stay in priority
precedence. A great deal more should be done with the
effect of precedence on network performance for the various
protocols. The effect could be quite significant and a
separate study should be initiated to do a careful analysis. It
is not addressed in this paper.
L o a d ( m is s io n s p e r h o u r )
Fig. 1. Comparison of Types 2 and 4 Fire Mission
Completion Times on Net_A
IV. SIMULATION RESULTS
Benchmarks of Type 2 and Type 4 performance for Net_A
and Net_AB are presented in Figs. 1, 2 and 3. These results
are also used to make comparisons with subsequent
simulation runs. Fig. 1 graphs the completion time of the two
FS thread types on the seven node network, Net_A. Type 4
services clearly reach a knee in the time-to-completion curve
around a load of 800 fire missions per hour. Recall that these
fire mission threads have short message sequences. Type 2
services provide better performance throughout the whole
loading range and superior performance at the heaviest loads.
There are many performance metrics of interest and more
than 48 metrics are collected per node as well as numerous
global statistics for each simulation run. More than a
thousand simulation runs have been made and analyzed to
The completion times of the FS threads for the 14 node
network, Net_AB, are given in Fig. 2. The performance knee
for the seven node network is much sharper than the knee in
the 14 node network. The reason is that the traffic generated
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by the 800 or more fire missions and their extra ACK packets
for Type 4 on the single operations center is too much for the
single node to handle and messages must wait there longer
before transmission. Recall that for the 14 node network, the
fire thread load is split between two operation centers. The
time to completion increases more steeply for the larger
network but not as dramatically at the knee of the curve.
This reflects the slight decrease in performance as the
network size increases. For the lighter loads, Type 2 and
Type 4 performance is very close but Type 2 consistently
performs better than Type 4.
Ne t _ A B A v e r a g e Fir e M is s io n C o m p le t io n
( D A P- N A D )
300
of unacknowledged packets to all destinations from any
individual transmitting node to 20 packets. Most of the
simulations were run with a maximum window size of 100 at
each node. Several simulations for Type 4 with K_Window
size equal to 20 were run to determine the effect this has on
the performance of the system under various configurations.
When Net_AB was simulated using RE-NAD, Type 4, load
500, and K_Window = 100, the average and maximum
number of unacknowledged messages averaged over the two
operations centers was approximately 28 and 70 respectively.
Similarly, on Net_A with errors using DAP-NAD, the
average and maximums were 41 and 70 respectively. For
Net_A with 30% errors, the effect on thread completion time
is given in Table II. Obviously, the 500 fire mission load is
beyond the capacity of the network with K_Window = 20.
T im e ( s e
250
TABLE II. Comparison of Thread Completion Times for
Type 2 and Type 4 with a Load of 500 on Net_A with Errors
(30%) with 100 and 20 Window Size
Th r e a d 1 Ty p e 4
200
Th r e a d 1 Ty p e 2
Th r e a d 2 Ty p e 4
150
Th r e a d 2 Ty p e 2
Type 2
64 Window
100
50
200 300 400 500
600 700 800 900
Thread 1
Thread 2
L o a d ( m is s io n s p e r h o u r )
Fig. 2. Comparison of Type 2 and Type 4 Fire Mission
Completion Times on Net_AB.
Ne t _ A a n d Ne t _ A B L a t e n c y
( DA P- NA D)
60
T im e ( s e c
50
40
Ne t_ A B Ty p e 4
Ne t_ A B Ty p e 2
30
Ne t_ A Ty p e 4
Ne t_ A Ty p e 2
20
10
0
200 300 400 500 600 700 800 900
L o a d ( m is s io n s p e r h o u r )
Fig. 3. Comparison of Type 2 and Type 4 Latency on
Net_A and Net_AB
The packet latency times for Type 2 and Type 4 packets for
both Net_A and Net_AB are given in Fig. 3. The sharp
increase in the packet latency for Net_A at 800 fire mission
again reflects the difficulty of having a single node with a
very heavy traffic load to transmit. For the Net_AB, this load
is split between the two operations centers and it makes a
more balanced network and a more efficient utilization of
network resources.
MIL-STD-188-220B stipulates that Type 4 (but not Type
2) services have a maximum of 20 packets (range 5 – 20) in
the message window (K_Window). This limits the number
269.2 sec.
174.0 sec.
Type 4
100
Window
554.1 sec.
365.3 sec.
Type 4
20 Window
3165.7 sec.
2350.2 sec.
Fig. 4 gives the graph for the average latency times for the
same Net_A with errors. The window restriction of 20 for
Type 4 makes these packet latencies become unacceptable
and go off the chart. Fig. 4 also gives the packet latency for
the SA (Type 1) packets. SA type data continues to be
delivered in a timely manner even under the most adverse
conditions. The average delivery times for the Type 1
packets varies from about 2 seconds for the 200 fire mission
load to about 4 seconds for the 500 fire mission load. As can
be seen from the bottom two lines in the graph of Fig. 4, the
effect on Type 1 message latency of using Type 2 or Type 4
for FS messages was not as significant as many other
message latencies in this study. The reason for this is that
Type 1 messages are generated by all nodes at approximately
the same rate instead of having just two nodes generate the
initial messages for the FS threads. Again, the performance
of Type 1 messages consistently did better using Type 2
rather than Type 4 for the FS messages.
The effect of Type 4 (K_Window of 20 and 100) versus
Type 2 on the completion time of fire threads is shown in
Fig. 5. A comparison of Fig. 5 and Fig. 1 shows that the
performance of the network has dramatically decreased due
to errors for the Type 4 services. Type 2 continued to
perform quite well. It should be noted that the fire mission
load for Figs. 4 and 5 varies from 200 to 500 fire missions
per hour whereas the load for Figs. 1, 2, and 3 goes up to 900
fire missions per hour.
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was used since that would be less advantageous to DAPNAD (generally considered better on smaller nets) for any
possible comparisons between MAC algorithms.
The
network for RE-NAD had no external noise. Fig. 6 gives the
packet latency. The latency for Type 1 packets was
acceptable across the entire loading of FS threads. Again,
Type 2 for the FS messages allowed better performance for
the Type 1 messages than did Type 4. However, the message
latency for Type 4 messages was clearly unacceptable for
even modestly loaded networks. Type 2 performed well for
the all fire mission loads using RE-NAD. However,
comparison of Fig. 6 (RE-NAD) and Fig. 3 (DAP-NAD)
shows the message latency was much lower for DAP-NAD
with an average of approximately 5 seconds for DAP-NAD
versus 36 seconds for RE-NAD at the 500 fire mission per
hour load. Similar observations can be made for the thread
completion times using Fig. 7 for RE-NAD and Fig. 2 for
DAP-NAD.
200
180
160
140
120
100
80
60
40
20
0
Type 4 (w indow
100)
Type 2
Type 1 (w ith Type 4
w indow 100)
Type 1 (w ith Type 2)
Type 4 (w indow 20)
200
300
400
500
Load (m is s ions pe r hour )
Fig. 4. Effect of Errors on Type 2 and Type 4
(100 and 20 Window) on Latency on Net_A
Ne t_ A T h r e a d Co m p le tio n w ith 3 0 % Er r o r s
(DA P-NA D)
800
600
500
Th r e a d 2 Ty p e 4
( w in d o w 2 0 )
80
Th r e a d 1 Ty p e 4
70
Ty p e 4
60
400
T im e ( s e c )
Th r e a d 1 Ty p e 2
Th r e a d 2 Ty p e 4
20
Th r e a d 2 Ty p e 2
T y p e 1 ( w ith T y p e
2)
10
L o a d ( m is s io n s p e r h o u r )
Fig. 5. Effect of Errors on Type 2 and Type 4
Mission Completion Times on Net_A
0
0
0
0
50
45
40
10
0
0
0
500
0
400
30
300
0
200
T y p e 1 ( w ith T y p e
4)
30
25
100
40
0
200
Ty p e 2
50
20
300
15
T im e ( s e c )
N e t_ A B L a te n c y
(R E -N AD )
Th r e a d 1 Ty p e 4
( w in d o w 2 0 )
700
35
Time (sec)
Ne t_AB Late ncy w ith 30% Errors
(DAP-NAD)
L o a d ( m is s io n s p e r h o u r )
DAP-NAD was used as the MAC algorithm for all of the
previous results.
DAP-NAD is a deterministic high
performance MAC algorithm and was ideal for testing Type
2 versus Type 4. The other most important MAC algorithm
in MIL-STD-188-220B is RE-NAD. Type 2 and 4 services
needed to be tested with it as well. The larger net, Net_AB,
Fig. 6. Type 2 and Type 4 Latency for RE-NAD on Net_AB
Ne t_A B Th rea d Co m plet ion
(R E -N AD )
1 00 0
9 00
8 00
T im e ( sec )
Network utilization, viewed as raw bits seen on the
network, for the Net_A with errors (the networks for Figs. 4
and 5) is 6% higher for Type 4 than for Type 2 for the 500
load with the 100 window. The reason is that Type 4
retransmits packets when the sender has its ACK_timer
expire even if the packets were correctly delivered and only
the ACK was destroyed.
Type 2 issues only an
acknowledgment request. The network utilization with DAPNAD gets as high as 85% with many transmission units at the
user configurable maximum size of 57600 bits and averaged
over 25,000 bits. Each Type 4 and Type 2 transmission unit
had on average 8.9 and 4.2, respectively, packets. Half of the
Type 4 packets must be datalink layer acknowledgments. The
Type 4 retransmissions do allow slightly shorter fire thread
completion times for the very lightly loaded networks. This
is the only known case where Type 4 outperformed Type 2.
7 00
Threa d 1 Ty pe 4
6 00
Threa d 1 Ty pe 2
5 00
Threa d 2 Ty pe 4
4 00
Threa d 2 Ty pe 2
3 00
2 00
1 00
10 0 1 50 2 00 2 50 3 00 3 50 4 00 45 0 5 00
L oa d (m i ss io ns p e r h ou r )
Fig. 7. Type 2 and Type 4 Thread Completion Times
RE-NAD on Net_AB
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V. CONCLUSIONS
The results presented here and the simulations showed that
Type 2 services are, in general, more efficient than Type 4
services for FS type networks in all measures of network
performance. Type 2 is dramatically more effective than
Type 4 when any of the following hold:
(1) the network load approaches maximum capacity, or
(2) when the network has a reasonable number of packets
received in error and the load was at least moderate, or
(3) when using RE-NAD instead of DAP-NAD.
These results were obtained using FS threads and SA type
messages to load the network and were consistent for both
the seven and 14 node network. The large size of the RS error
block mitigates some of the effects of the Type 4
acknowledgments on error free lightly loaded networks.
The MIL-STD-188-220B Type 4 parameter (K_Window)
for the maximum number of frames outstanding from a node
has the range of 5 to 20 packets. This is clearly a severe limit
on the performance of the network under error or under
heavy load conditions. For even moderately loaded error free
networks using RE-NAD, the maximum window of 20 for
Type 4 was an additional limit on the performance of RENAD. It is strongly recommended that this parameter value
be changed if Type 4 services are going to continue to be
used. The K_Window size of 64 for Type 2, which is half of
its maximum size allowed by MIL-STD-188-220B, was
never a constraint for Type 2 network performance and was
never reached. As was mentioned, message and network
precedence are very important for the performance of these
protocols. A thorough study needs to be made if the MILSTD-188-220B protocols are going to be used on tactical
networks.
RE-NAD did not perform as well as DAP-NAD in general.
This was shown by using Figs. 6 and 3 to compare message
latency and Figs. 7 and 2 to compare thread completion times
for both Type 2 and Type 4. DAP-NAD with either Type 2 or
Type 4 performed very well and much better than RE-NAD.
Figs. 2, 3, 6 and 7 show that Type 4 with RE-NAD
performed significantly less well than any other protocol
combination.
These results have significant implications for tactical
networks used for FS. The results presented here for the
seven and 14 node networks indicate that the results will
scale to larger networks with only modest decreases in the
performance. For modest sized tactical internets, Type 2 and
DAP-NAD appears to be the choice for performance.
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