HIGHER LAYER PROTOCOLS (TCP/IP) OVER SATELLITES1

TCP/IP

Transport control protocol/Internet protocol (TCP/IP) is the protocol suite
on which the Internet is based. TCP/IP is now very widely deployed. However, it
was developed without taking into consideration its performance over very high
speed (fiber optic) links or long-delay (satellite) links, with the result that
efforts are now underway to remedy some of the shortcomings that are evident on
links that have large bandwidth-delay product. At the present time, considerable
low bit rate TCP/IP commercial traffic is being carried over GEO satellites.
With suitable TCP/IP performance enhancements, data rates in excess of 500 Mbps
have been demonstrated over GEO located satellites."

Internet Protocol

IP is a network layer protocol whose function is to permit data traffic to
flow seamlessly between different types of transport mechanisms (Ethernet, ATM,
Frame Relay, etc). IP resides in the terminal devices and in routers which
function as switches in the network, routing datagrams (packets) towards their
destination based on an address field contained in the datagram. Figure 4.21
shows the protocol stack for a network operating on TCP/IP.

The routers in the network are required to translate between different
addressing schemes. For example, local area networks operating on the IEEE 802
LAN standard address attached devices with 16 or 48-bit binary addresses. An
X.25 public packet-switching network, on the other hand, uses 12-digit decimal
addresses. IP provides a global addressing scheme and a directory service. The
current version (Ipv4) of IP has address space limitations that threaten to
inhibit the growth of the Internet, with the result that a new version (Ipv6) is
under development (Stallings 1997).

Fig. 4.21. The protocol stack for a
network running on TCP/IP (Stallings 1997).

Routers are also required to handle differences in the size of packets that
can be carried on different networks. X.25 networks commonly operate with
packets having a maximum size of 1,000 bytes-in contrast to Ethernet, which
permits packets of 1,500 bytes. To overcome these differences datagrams may have
to be broken into smaller packets (this is known as fragmentation) and
reassembled when they reach their destination.

The IP protocol does not guarantee delivery, or that packets will arrive in
the proper sequence. (Packets can get out of order since they may follow
different paths through the network, thereby encountering different amounts of
delay.) Packets can fail to be delivered for several reasons. If the network
becomes congested one or more routers may become overloaded and their buffers
may begin to overflow. Rather than simply discarding all newly arriving packets,
the routers are programmed discard packets in a random fashion to prevent buffer
overflow. This is best implemented in a "fair" way so that the data stream
having the largest volume suffers the largest number of dropped packets. The
links in the network are not error free so that it is possible for a packet's
address to become corrupted making the packet undeliverable. Again it must be
discarded lest the network become clogged with undeliverable traffic. In sum, IP
is engineered to make a best effort to deliver a message but does not guarantee
to do so.

Transport Control Protocol (TCP)

It is the function of the TCP protocol residing in the end devices
(computers)-see Figure 4.21-to ensure the proper delivery of a complete message.
TCP achieves this by assigning each byte of information a unique sequence
number. The receiver keeps track of these sequence numbers and sends
acknowledgements (ACKS) to indicate that it has received each datagram up to a
particular byte number.

Window Size

A problem for links via geostationary satellites that involve a response time
of almost 0.5 seconds is that TCP will not allow for more data to be sent beyond
a certain "window" size before receiving an acknowledgement. This is currently
set at 64 kB and is limited by the fact that only 16 bits are available in the
header to describe the packet size. This limits the throughput to 216
bytes divided by the response time (round trip delay) of the circuit. For a GEO
path with a round trip delay of 600 msec this corresponds to approximately 840
kbps (Partridge and Shepard 1997).

Figure 4.22 shows the effect of the round trip time (RTT) on throughput as a
function of window size. It can be seen that a long terrestrial fiber link with
a 200 msec round trip time would be limited to 2.6 Mbps by the maximum window
size of 64 kbytes. Because TCP resides in the users' computers the only way to
"spoof" it is to place at the forwarding earth station a terminal device that
acknowledges receipt of data segments as if it were the distant receiver.
However this has its own drawbacks as discussed below.

Fig. 4.22. Maximum throughput for a
single TCP connection as
a function of window size and round
trip time (RTT).

The Internet Engineering Task Force (IETF) has been at work recommending
changes to TCP/IP to overcome this and other limitations inherent in the current
design. An increase in the size of the window to 230 bytes is
proposed (RFC 1323) which would raise the throughput over a GEO satellite link
to about 15 Gbps. Since even at Q/V-band, satellite frequency assignments are
likely to have no more than 3 GHz bandwidth, this will probably not impose any
limitation for the foreseeable future.

Selective Acknowledgement

TCP ensures the complete delivery of data over a link by retransmitting
anything for which it does not receive an acknowledgement. That is, it
retransmits everything that was sent since the last acknowledged datagram. This
"ARG" scheme is clearly inefficient in a situation where many bytes in a packet
were received correctly and only one or two arrived corrupted. In these
situations it is preferable to retransmit only the corrupted information i.e.,
perform a selective acknowledgement. TCP has yet to be modified and widely
deployed with this capability. It is, however, possible to implement this
capability on satellite links by introducing suitable interface units at the
earth stations at each end of the link.

A relatively straightforward modification to TCP that goes some way towards
remedying its current shortcomings has been approved by the IETF. This permits
the acknowledgement of datagrams received correctly, but out of order. This new
feature has been termed selective acknowledgement (SACK).

Congestion Control

TCP employs two strategies for avoiding or mitigating congestion in the
network. The first scheme is a "slow start" mechanism. Under this protocol a
transmission commences with the sending of a single segment (datagram) of
information. The size of a segment is negotiated between sender and receiver at
the start of transmission, and may be limited by features of the network, but
typically might be 1,000 bytes. Once this is acknowledged two segments are sent,
then four, eight, etc. This exponential growth continues until limited by the
maximum window size. This slow start algorithm will cause the throughput on long
delay links to rarely reach its maximum. It is particularly troublesome when
transmitting Web pages formatted by HTTP, since TCP treats each item in the
image as requiring a separate transmission sequence. An IETF proposal to reduce
the impact of the slow start protocol is to commence by sending four
segments.

In the event the network becomes congested and a packet is dropped (or lost
due to error) the sender will fail to receive an acknowledgement. Regardless of
the cause, the sender is obliged to assume that the problem is congestion and
institute a congestion control algorithm. This requires that the sending rate be
immediately reduced to about half, and then increased only gradually (by one
segment at a time). This introduces a linear increase as opposed to the initial
exponential increase. On long delay circuits the consequences of this congestion
algorithm are particularly severe since it now takes an inordinately long time
to reach maximum throughput. This is also potentially very severe for satellite
circuits with their higher error rates, since any loss is interpreted as being
caused by congestion. The best means of avoiding this error loss problem appears
to be to operate the link with sufficient (concatenated) coding to ensure very
low BER. Commercial devices such as COMSAT Link Accelerator for IP, CLA-2000/IP,
address this problem in a similar manner as described before for ASIU.
CLA-2000/IP invokes link error dependent amount of Reed Solomon outer coding,
resulting in a very low TCP packet error ratio. This dynamic adaptive coding
method, coupled with the data compression, improves considerably the throughput
of applications such as file transfer protocol (FTP) running over TCP, as
illustrated in Figure 4.23.

Fig. 4.23. Improvement in the throughput
of data over a satellite link as a Function of bit error rate with and without a
link conditioning unit (ASIU). The example shown here is for a commercial unit
(COMSAT Link Accelerator).

Current Research

Current research involving TCP over satellite channels is generally focused
in two areas. The first area is the slow start algorithm. By beginning slow
start by sending more than 1 segment, transfer time can be reduced by several
round trip times. This change has been shown to be effective in the satellite
environment. This change is being proposed in the IETF by researchers from NASA
Lewis Research Center, Lawrence Berkeley Laboratory and BBN. In addition,
researchers at NASA Lewis Research Center and Ohio University are further
investigating the impact of this proposed change.

In addition to starting with a larger number of segments, NASA Lewis Research
Center is investigating alternate methods for generating and utilizing
acknowledgments that will provide more rapid speedup during slow start. This
will be especially useful in the long-delay satellite environment, but should
benefit all networks including terrestrial networks.

The second broad area for study is loss recovery. Traditionally TCP has used
the lack of an acknowledgment from the receiver to indicate a segment was
dropped. However, with the recent introduction of a selective acknowledgment
option for TCP, the TCP sender is able to better manage which segments are
retransmitted, as it has more information. With SACK, instead of the receiver
returning the highest in-order segment received, it informs the sender about all
the segments it has received (and therefore, all the segments that have not been
received). This allows the sender to implement network-friendly retransmission.
In addition, since the sender knows much more about the state of the network it
can safely determine when it is appropriate to inject new segments into the
network during recovery. This allows better utilization of the network and
therefore better performance.

IETF TCP Over Satellite Working Group

The TCPSAT Working Group of IETF is chartered to produce two documents
(Glover, Allman) for those working with and studying satellite networks. The
first document outlines the current standard mechanisms that can improve the
bandwidth utilization of TCP over satellite channels. The second document
outlines areas currently being researched and areas for future study. The
mechanisms in the second document may be useful for private satellite networks,
but have not yet been judged to be safe for use in a shared network such as the
Internet.

Interoperability with Terrestrial Networks

It has become increasingly evident that the full potential of the emerging
national and global information infrastructure (NII/GII) depends on the
inclusion of satellite networks in telecommunication networks. Full inclusion
requires achievement of seamless interoperability of satellite networks with
terrestrial networks as both evolve to provide end-to-end services.

This panel observed that the interoperability issues are being addressed
worldwide on several fronts. Standards, of course, are key to interoperability.
The development of applications that are based on integrated networks also plays
a role. This section highlights the research activity that is being conducted to
address interoperability issues worldwide.

In Europe, the activity tends to fall into the service-driven category. Major
North American activity tends to be technology-driven, while Asian activities
are more evenly divided, although there are exceptions and changes.

In order to integrate satellites into the emerging global information
infrastructure, both technology-driven and service-driven programs are
necessary. Neither one is necessarily superior to the other; in fact, in either
case, success is dependent upon implementation.

Europe's Experimental Programs

The European ACTS Program

Advanced Communications Technology and Services, known simply as ACTS, is one
of the specific programs of the Fourth Framework Program of European Union (EU)
activities in the field of research and technological development and
demonstration (1994 to 1998). In fact, it is the focus of the EU's research
effort to accelerate deployment of advanced communications infrastructures and
services, and is complemented by extensive European research in the related
fields of information technology and telematics. ACTS research strongly
complements a broad range of EU policy initiatives, examples of which
include:

improving the competitiveness of European enterprises in global markets

achieving sustainable economic growth

creating more employment opportunities and new way of working

strengthening the single market through the development of trans-European
networks

ACTS builds on the work of the earlier RACE programs (Research and
Development in Advanced Communications Technologies for Europe, 1985-1995),
which were established to help introduce Integrated Broadband Communications
(IBC), taking into account the evolving ISDN and national introduction
strategies, and to bring about EU-wide services by 1995. Independent assessments
have confirmed that RACE broadly achieved this objective, and that such
technologies are beginning to be deployed in European countries either in
specialized scientific networks or, in a few cases, as limited public
services.

European Space Agency (ESA)

Like NASA, ESA is heavily involved in satellite-terrestrial interoperability
R&D. ESA provided the following material from a presentation given at an ATM
workshop in Paris in July of 1996.

ESA Research and Development Activities on ATM

Interconnection of ATM LANS:

partly in the frame of COST 226, several companies involved (Johanneum
Research, Uni-Salzburg, Spar Aerospace, Matra, etc.)

mf-TDMA satellite access scheme

symbol synchronous tx scheme

News on demand (Logica)

focus on applications

selection of test cases

performance evaluation

System studies (CSEM, Spar) :

ATM-based satellite system evolution scenario

architectural solutions to support ATM traffic via satellite

traffic modeling and performance simulations

Technology studies (Spar) :

ATM on-board switch technology

low-cost earth stations components

efficient access schemes

reconfigurable multi-beam front-end

Results of ESA developments:

Switched VSAT (XVSAT)

Interconnection of ATM LANs (SAIT)

ATM for service multiplexing and switching building on technology
available

hubless and fully meshed Ku-band VSAT system

BoD SCPC-based DAMA system provides IP-based LAN interconnection, will
provide direct ATM UNI

Status of the Project:

ATM-based IWU developed and tested

Q.930 based call-handler developed, under testing

System integration under way

Installation of three sites planned end summer '96

Switched VSAT- XVSAT (follows)

System verification until end '96

System demonstration (6 months) planned first half '97

Contract for extension ATM UNI under negotiation

Industrial Team

SAIT Systems (call handling, management system)

MPR Teltech (ATM Service Adapters, tx adapters)

EB/NERA (BoD, SCPC DAMA system)

Telenor (operations of the DAMA system)

Satellite Rf equipment for Broadband Communications.

Modem at 155 Mbits (Newtec) provides fiber-like performance over
satellite links at SDH rates

VANTAGE - VSAT ATM Network Trials for Applications Groups Across
Europe

VANTAGE will unite the service flexibility of ATM and the access flexibility
of satellites to provide a pan-European interconnection service. This project
will include trials with a variety of earth station technologies and access
techniques to interconnect isolated users, to integrate National Host networks
out to remote users, and to show that ATM's flexibility in the management of
signaling allows satellites to be seamlessly embedded in the terrestrial
network.

VANTAGE will implement a novel architecture using a conventional transparent
satellite, with its earth stations, as a distributed ATM switch. This project
will be the first time such an innovative architecture has been used.

A series of three major trials, each of several weeks duration, will be
mounted, with at least four simultaneous sites provided from project resources.
User access will be by standard interfaces (from a few bits to many megabits per
second), and total capacity will be increased through the project to over
20Mbps.

Key issues. By using this approach VANTAGE will allow
"switch-in-the-sky" capabilities to be provided and operationally tested while
avoiding the need for high cost, high risk, specialist satellite development or
major terrestrial infrastructure.

Relationship to previous work. The VANTAGE project will elaborate on
concepts developed and techniques explored and evaluated in the former RACE
program.

Expected Achievements. VANTAGE will provide a service platform
allowing a variety of other projects to develop and evaluate their applications
and technologies. For some projects this platform may provide the only
interconnection possibility. VANTAGE will provide a unique opportunity for
empirical exploration of the capabilities, strengths, weakness, and impact of
the IBC network in advance of its wide terrestrial deployment. VANTAGE will
demonstrate the capability of European industry to develop, and of European
government to coordinate, world-leading equipment, systems and services.

To the extent that the system is ultimately exploited, VANTAGE provides the
opportunity for Europe to establish world leadership with a "first" operational
system setting de facto international standards.

Expected Impact. VANTAGE will provide the enabling technology to
extend a variety of (many novel) applications and services beyond their current
urban limits.

Relationship to other Projects. In addition to testing applications
from within the project, VANTAGE will also offer a connection service to
applications from other ACTS projects and to national hosts.

Japan's Experimental Programs

Communications Research Laboratory (CRL)

Although not specifically targeted at SDH, WTEC panelists were informed by
the CRL project managers that one of the key issues to be addressed in CRL's
program is the ability of current and next-generation satellites to interoperate
with international, SDH-based, undersea fiber optic cable systems. Note that
this project includes both technology-driven and service-driven research
thrusts. Because of its importance to SDH-satellite interoperability, however,
we include it only in this section.

The laboratory has two principal goals: first, the international
interconnection of ultra high-speed telecommunication networks. This is expected
to be achieved by the installation of an experimental facility on the premises
of CRL, which would have state-of-the art equipment. The second goal is research
and development to enhance international multimedia applications. Again, cutting
edge equipment is being installed so that the research can lead to the
development of international standards for applications, such as international
transactions.

Project Objectives. One objective is to improve the telecommunications
network infrastructure for Japanese corporations for communication between these
corporations and also for communicating with overseas corporations. This is
expected to promote the transformation of the Japanese economic structure by
developing new markets, growth in sophistication of existing operations, and an
increase in import volumes. This will relieve the negative impact of the
appreciation of the yen through expansion of domestic demand for goods and
services.

Another objective is to provide leadership for G7 international joint
projects in order to contribute to the international standardization in terms of
interconnectivity and interoperability of broadband network applications.
Additional objectives are as follows:

Promotion of standardization

Promotion of efforts to construct global broadband network and relevant
applications development

Promotion of various new businesses utilizing broadband network
technology.

Asia-Pacific Information Infrastructure Testbed

Kansai Advanced Research Center was established in May 1989 as a major
facility to perform basic research under CRL. The center performs basic research
in the areas of information science and technology, material science and laser
technology and biological information science with the objective of developing
advanced technology of the future in the areas of information processing and
communications. Also near its facility, the Asia Pacific Information
Infrastructure (APII) Technology Center has been established. Details are
provided below.

Asia Pacific Information Infrastructure (APII) Technology Center. With
the aim of achieving a leading role in the establishment of the information
infrastructure in the Asia-Pacific region, the APII Technology Center has been
constructed in Kobe City.

To respond appropriately to the globalization of information communications,
there is a need to establish an information infrastructure that crosses national
borders while giving full consideration to the social and cultural diversity as
well as different levels of economic and technological development in the
Asia-Pacific region.

International joint research and experiments on a multimedia information
network are being planned. The final goal is to conduct remote joint research
and distance learning activities and to establish capabilities in telemedicine,
teleshopping, and an electronic museum. A related goal is to promote the use of
this network, and to create an environment in which as many countries in the
Asia-Pacific region as possible can participate.

The APII Technology Center offers group technical training courses on
multimedia information and communications technology to the engineers from the
Asia-Pacific region. The technical training program consists of trends in
multimedia information technology, internet utilization technology, video on
demand (VOD), application techniques and cyber space application techniques.

The APII Technology Center has been built by the Ministry of Posts and
Telecommunications. This facility uses various systems, including Internet,
"cyberspace" and VOD, which are offered through an ATM-LAN. The facility carries
out joint development of applications and network interconnection technologies,
and trains technicians.

At the APEC informal meeting of heads of government held in Manila in 1996,
Prime Minister Hashimoto called for, among other things, various kinds of
experiments and training to be undertaken that would use this facility as a
test-bed, with the goal of becoming a nucleus in the Asian-Pacific information
and communications infrastructure (APII).

In addition to the ATM-LAN, which has been installed as a local area network
facility, the center is connected to external experimental facilities via N-ISDN
and high speed private lines. At the time of the WTEC visit, this facility was
connected to external networks with six N-ISDN lines, and was connected to Tokyo
via a 45 Mbps high speed private line.

Canada's Experimental Programs

Communications Research Centre (CRC) is the Canadian government's leading
communications research facility. It is responsible for conducting leading edge
R&D to develop the Canadian communications infrastructure. Its key objective
is to support Canadian telecommunications firms in their efforts to remain
globally competitive.

Broadband Applications and Demonstration Laboratory

The WTEC team was given a tour of the BADLAB, which is designed to
demonstrate and test "Information Highway" applications using ATM fiber optic
networks, with network extension via satellite and wireless. The staff has
performed several experiments to investigate satellite/terrestrial
interoperability based on ATM architecture.

BADLAB is Canada's ATM gateway to high speed communications networks around
the world. It is a major node on the CANARIE National Test Network and an active
partner in the Ottawa Carleton Research Institute Network Inc. (OCRInet). BADLAB
is also connected to Europe through Teleglobe Canada's CANTAT-3 transatlantic
fiber optic cable, and will use satellites to connect to Japan. The lab is
actively working with its European partners on broadband interoperability and
applications trials.

BADLAB is collaborating with the Government Telecommunications and
Informatics Services (GTIS) to explore broadband service options for a range of
government clients across Canada. BADLAB is connected to GTIS through a 155 Mbps
line. The lab is currently connected to OCRInet through two 45 Mbps links, with
the capacity to upgrade to 155 Mbps. The objective of this lab is to test and
demonstrate various ATM networks such as OCRInet, Rnet, Wnet, LARG*net and
others as they come on line, making use of BADLAB's satellite link capability
for network extension and to make the lab available to industry, especially
small and medium-sized high technology R&D companies across Canada, to
develop applications that may be of commercial value.

The broadband applications experiments are being conducted in the following
areas:

Education: distance education and training through enhanced video
conferencing with shared applications capability

Medical: patient diagnosis and physician consultations between hospitals
in different parts of the country and European medical institutes

Scientific Applications: computer reconstruction of human profiles
transmitted via ATM for simultaneous analysis by police and forensic experts
at different locations

Engineering: collaborative designs that can be worked on simultaneously at
multiple sites; remote control of robotic devices from thousands of kilometers
away, using telemetry carried via ATM over satellite or ground-based fiber
optic cable

Government service: CRC and GTIS are exploring applications

U.S. Experimental Programs

In mid-1995 the U.S. satellite industry responded to the call for a NII/GII
initiative. In a White House briefing to Vice President Gore, the Satellite
Industry Task Force (SITF) brought attention to five major areas of concern that
would affect the ability of satellites to play an effective role in the
NII/GII:

lower cost access to space

regulatory matters (including spectrum and orbital assignments)

trade and security issues

standards, protocols, and interoperability

enabling technologies and applications

Subsequently, the U.S. satellite industry organized a group under the
auspices of the Telecommunications Industry Association (TIA) to deal with these
issues. Using TIA as a forum, the satellite industry has now successfully begun
to address the standards, protocols and interoperability issues. At its
inception, the Satellite Communications Division (SCD) of the TIA created two
subgroups, TR34.1 and TR34.2, to address technical issues. It delegated to the
TR34.1 subgroup those technical matters associated with protocol and
interoperability issues. Spectrum and orbital utilization issues were delegated
to the TR34.2 subgroup.

To date, both TR34.1 and TR34.2 have made progress in assuring that satellite
systems can be seamlessly and transparently integrated into the NII/GII. They
continue to be effective, working both internally and through the various
standards-making bodies, in influencing the development of standards and
regulatory matters.

Also, satellite interoperability experiments are being conducted. These
experiments are driven by either application and service programs or by
standards and technology programs. The experiments are listed in Table 4.6.

ATM Protocol

The TR34.1 subgroup thus far has focused on several ATM protocol related
issues affecting satellite networks. These include ATM speech, ATM quality of
service considerations, wireless ATM considerations, and ATM traffic management
considerations.

In the area of ATM speech, TR34.1 has been successful in promoting
satellite-friendly specifications for the ATM adaptation layer protocols through
the ITU-T SG13 and through the ATM Forum. Similarly, TR34.1 has been successful
in working with the ATM Forum to develop a Network Architectures and
Requirements Document, which establishes the key interoperability specifications
for both fixed and mobile satellite networks, including systems employing
onboard ATM switches. Also included in that document are the radio access layer
specifications for satellite ATM networks for a variety of network scenarios.
The details on these proposed standards are available through the COMSAT website
(http://ww.comsat.com). Additional work is needed to establish
satellite-friendly specifications for packet error rates; packet loss rates;
packet delay and delay variation specifications for AAL2; and for speech
compression protocols, congestion control procedures; the multiplexing of voice,
video, and data; and echo cancellation.

Table 4.6
Satellite ATM
Projects

Project Name

Development Team

Transponder

Access Method

Speed

Vantage

BT Telecom

Ku-band

Point-to-Point

2 Mbps

QoS over an IDR Link

AT&amp;T, KDD (Japan), Tetstra (Australia)

Ku-band

Point-to-Point

44.736 Mbps

ATM Satellite Experiments

AFRL (U.S.)

CRC (Canada)

NCCOSC (U.S.)

Ku-band

Point-to-Point

Multicast

44.736 Mbps

RACE

Eutelsat

Alcatal

Ku-band

Point-to-Point

34 Mbps

ACTS

NASA Lewis Research Center

Ka-band

microwave

switch matrix

TDMA

622 Mbps

Bandwidth on Demand

COMSAT

Interstate

Ku-band

MF-TDMA

2-8 Mbps

Teledesic* Broadband/Global/Net

Teledesic

Ka-band

LEO Onboard Processing

MF-TDMA

16 kbps - 2 Mbps

* 1st of many proposed systems

In the area of ATM traffic management, Raj Jain (Ohio State University),
working under the sponsorship of NASA Lewis Research Center, is exploring the
performance characteristics of new ATM networking concepts. These include
simulations of available-bit-rate (ABR) and unspecified-bit-rate (UBR) traffic
flows versus constant-bit-rate (CBR) traffic flows. Additionally, Jain is
investigating the performance characteristics of guaranteed-frame-rate (GFR)
networking concepts, point-to-multipoint service concepts, and
multipoint-to-point service concepts for high latency satellite links. He is
working closely with the ATM Forum and the Internet Engineering Task Force
(IETF) in the development of these concepts. His work is available for review on
the OSU website: http://www.cis.ohio-state.edu/~jain.

In the area of ATM quality of service, NASA Lewis Research Center (LeRC) has
conducted a series of experiments to characterize the effects of link bit error
rates and errored cell losses on video picture quality. The objective of this
work is to ensure that the requirements for ATM video standards are not overly
stringent, thereby needlessly burdening satellite network interoperability
requirements. The results of this work have been presented to the ATM Forum.

TCP/IP Protocol for Satellites

The performance issues of TCP/IP over high-latency links are being
investigated by the Internet Engineering Task Force (IETF), by TR34.1 through
its Internet Protocol over Satellite (IPoS) group, and by Craig Partridge (BBN
and the IETF) under a grant from NASA LeRC. The majority opinion has been that
there must be a fundamental change to the protocol to make it work efficiently
over high-latency satellite links. The minority opinion, promoted primarily by
van Jacobson and others in the IETF, is that globally modifying the installed
base of TCP applications it is not only impractical, but that it is totally
unnecessary. Instead, they feel it should be possible to correct the problem by
changing the slow-start and ack/nak processes that reside only in the earth
stations themselves. Simulation studies need to be done to confirm or disprove
that conjecture.

The Advanced Communication Technology Satellite (ACTS) program at NASA Lewis
Research Center has demonstrated that it is possible to create an all-digital
Ka-band system that can overcome rain fade. The ACTS is a TDMA-based system that
uses spot beam (or multibeam) technology, on-board storage and processing, and
all-digital transmission. ACTS can transfer TCP/IP data at OC-12 speeds (622
megabits per second). Additional information can be obtained at
http://acts.lerc.nasa.gov.

Common Air Interface for Communication Satellites

One of the most significant outcomes of the organization of the satellite
industry under the TIA has been the cooperative efforts that have been mounted
within the industry to standardize the air interface for mobile communications.
This activity, which is being managed by TR34.1, was initiated jointly by Hughes
and Ericsson. The objective is to reduce the cost of satellite transceivers
through economies of scale. To date, the group has agreed on a top-down approach
in developing the standard, and is in the process of drafting a specification.
The draft is available for review on the COMSAT website
(http://www.comsat.com).

Hybrid Access Services

Currently, a number of standards bodies are in the early stages of developing
hybrid service concepts that would provide IP based access to traditional
telephony services, and conversely, circuit-switched access to IP based content.
Work is on-going in the ITU-T under SG13 and SG16, in ETSI under project
TIPHON, in the IETF under working groups MUSIC, AVT, and
PINT, and in the Voice-Over-IP Forum of the Multimedia Teleconferencing
Consortium (IMTC).

1Prakash Chitre of COMSAT Laboratories contributed to this
section.

Published: December 1998; WTEC
Hyper-Librarian