IEEE - Aerospace and Electronic Systems - September 2023 - 25

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case of link disruption. Extensive work is needed to have a
solid performance evaluation of LTP for reliable file
delivery over a relay-based communication architecture
between the space station and the ground stations.
In this article, a study of the effect of link disruption
events on LTP using a PC-based experimental infrastructure
is presented. The main contribution of this article is
an experimental performance evaluation of LTP data
transfer experiencing link disruptions between the space
station and the ground stations. Realistic data transmission
experiment results are presented with a focus on the effect
of link disruption which happened during data transmission.
The analysis is done over both a clean channel (i.e.,
without data loss caused by channel error) and a lossy
channel having a high channel error rate.
To the best of our knowledge, this article presents the
first experimental study for effect oflink disruptions on the
use ofLTP for reliable file delivery over a relay-based communication
architecture between the ISS and the ground
stations. The quantitative results collected using an experimental
method are useful in characterizing the operation
and transmission performance ofLTP for reliable file delivery
from the space station to the ground stations.
DTN PROTOCOLS AND RELIABLE TRANSMISSION
MECHANISMS OF LTP FOR SPACE
As mentioned, DTN was intended to serve as the baseline
technology to implement space networks. As the core
protocol of DTN, BP was designed to build a " store-andforward "
overlay network for custody-based data transmission
service to DTN. Figure 1 illustrates the BP-based
DTN protocol stacks that can be deployed in space networks
[18, with changes]. As the overlaying protocol of
space DTN, BP utilizes the service of the underlying data
transport protocols through an interfacing " convergence
layer adapter " (CLA) for reliable or unreliable data
SEPTEMBER 2023
delivery. As illustrated, a variety ofdata transport protocols
are employed to be operable underneath BP to meet various
user requirements from heterogeneous networks. These
data protocols may be the most widely deployed TCP,
UDP, or the recently developed LTP.
To improve data delivery efficiency and maximize link
utilization, LTP is designed to operate on multiple concurrent
transmissions of data blocks in a continuous manner.
This is implemented using the transmission " sessions. " A
" session " is defined as the sequence of segment exchanges
ofLTP undertaken for the successful transmission ofa single
data block. LTP enables multiple block sessions running
in different stages of data transmission concurrently.
One session corresponds to transmission of a single block
per second. The data size of a block is equal to the maximum
data rate ofthe channel per second. The transmission
mechanism of block sessions is expected to maximize the
space link utilization by fully using the data link capacity.
On the other hand, it serves as a flow control to LTP.
Selective data transmission mechanisms are available
for LTP: LTP service with respect to any single block can
be reliable, unreliable, or both [5]. That is, LTP may transmit
an entire block either reliably (in the manner of TCP)
or unreliably (in the manner of UDP), or alternatively it
may transmit part ofa block reliably and the rest unreliably.
Reliable transmission of a single block or a portion of it is
effected by designating the data bytes as " red " data while
the rest data is designated as " green " data.
As this study is mainly concerned with the reliable
transmission service of LTP used in space station communications,
we only present an overview of the transmission
ofa block when it is configured to be entirely " red. " For the
transmission of a " green " block or a mix of " green " and
" red " parts of a block, refer to [5]. The " red " data block is
generally segmented as multiple red LTP data segments
(RDS) depending on the block size and the data link frame
size with the last segment flagged as an asynchronous
checkpoint (CP). The CP segment also serves as the end of
IEEE A&E SYSTEMS MAGAZINE
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IEEE - Aerospace and Electronic Systems - September 2023

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