1. REGENERATIVE SATELLITE MESH SYSTEM FOR REAL-
TIME MULTI-PARTY MULTIMEDIA TRAFFIC
R. Gopal*, S. Arnold †
Hughes Network Systems, LLC (HUGHES) 11717 Exploration Lane, Germantown, MD 20876, USA.
*rajeev.gopal@hughes.com, †steven.arnold@hughes.com
Keywords: Geo Satcom, H.323, RSM-A, Diffserv, Multicast Secondly, ground-based replication is required to avoid
excessive bandwidth needed to uplink multiple copies of the
Abstract same data stream from a given source terminal. Either of
these deficiencies essentially rules out the use of transponded
satellites for transporting multi-party, real-time traffic in a
Regenerative satellites with on-board packet processing can responsive and scalable fashion.
provide full-mesh, single-hop connectivity between two or
among many terminals without requiring a terrestrial Hub for With regenerative satellites, a given source terminal sends
traffic routing and control. By implementing resource only a single, real-time packet stream that is replicated by the
allocation and packet replication in the payload, these onboard satellite packet switch. The resultant multiple copies
satellites can extend the single-hop benefits to a multi-site, are then transmitted to all the destination terminals
real-time collaborative environment. This paper summarizes participating in the multi-party conferencing. Furthermore,
the key features of the ETSI and TIA Regenerative Satellite Ka-band systems employ smaller ground antenna sizes than
Mesh A (RSM-A) standards and enumerates the capabilities those operating in the Ku-band at similar power levels,
of a pioneering implementation, the Hughes SPACEWAY® facilitating easy installation at customer sites.
3 satellite system, which was launched into commercial
service over North America in April 2008. Capabilities such The SPACEWAY 3 [13] packet processing architecture
as spot beam forming, dynamic satellite communication leverages the differentiating features of the ETSI [1] and TIA
hardware to beam mapping, and adaptive resource control RSM-A [2] standards and is summarized as follows:
enable a responsive transport system for inelastic packet
streams associated with high data-rate video and audio traffic 1. Data plane packet queuing, scheduling, and satellite
for real-time interactions. It also describes an IP multicasting transport that define the delay and jitter
architecture over RSM-A, aligned with H.323 and SIP characteristics of the RSM-A transport;
standards, for managing real-time, point-to-point and multi- 2. Time Division Multiple Access (TDMA) resource
party video conferencing applications. Proceedings of IET ICSSC 2009 allocation that is critical in ensuring the satellite link
is multiplexed for enhanced capacity utilization, but
not at the cost of real-time needs of inelastic traffic;
1 Introduction 3. Short term uplink and downlink capacity assurance
(for example, at packet flow level), by using formal
Geosynchronous satellite systems have dominated both admission control of a specific packet flow for
communications and broadcast applications because of large, unicast (point-to-point) or multicast (for multi-party)
continent-wide coverage areas and compact, low cost ground traffic; and
terminals and antennas that are stationary with respect to the 4. Longer term capacity management that deals with
satellite. There is, however, the cost of propagation delay of uplink and downlink reservations for aggregate
about 250 ms per hop, which borders on the acceptability of flows.
audio and video exchanges typical of interactive
communication between two parties. While not perceptible These features guarantee the required packet Quality-of-
when broadcasting television channels, doubling the Service (QoS) for enabling High Definition (HD)
propagation delay severely compromises video conferencing transmission (with at least 720p resolution), multi-party video
quality, even if bandwidth capacity for high definition conferencing with compact terminals, and system scalability
transmissions is available on a given satellite. Indeed, full- from one to several thousands of such video conferencing
mesh satellite networking is almost a prerequisite for high groups per satellite system.
definition video conferencing that is now increasingly
emerging as a replacement for face-to-face meetings. The SPACEWAY 3 IP-based architecture supports multiple
standard techniques for encoding and compressing video and
Conventional bent-pipe transponded satellites have two audio streams. This allows both Standard and High
primary limitations in supporting interactive, multi-party Definition coding to be uplinked from a satellite terminal with
communications requirements. Firstly, the transponded a small sub 1 meter Ka-band antenna, easily deployable in
terminals can receive only a single source at a time. residential areas. Several such terminals can be used in the

2. multi-party scenario to allow up to 6 simultaneous parties 2. RSM-A Standard
communicating interactively. The SPACEWAY 3 system
employs recurring (rate for inelastic, real-time traffic) and Satellite networking is reaching an inflection point, driven by
just-in-time (volume for elastic, best effort traffic) TDMA the need for larger coverage, capacity, and ease of
allocations directly from the satellite. Commensurate with deployment. Satellite networks are now widely used for a
the wide range of user data rate service requirements, RSM-A variety of broadband applications, and can significantly
supports a low delay and low jitter rate transport for real-time extend the scope of distributed information systems.
traffic, while sharing the satellite domain with best effort, Conventional satellite networks have been supported mainly
elastic user data. The QoS architecture ensures that packet by transponded architectures which do not have any onboard
delay and jitter is bounded within the satellite domain despite digital processing. In this structure, a ground terminal sends
the use of a multiple access scheme, while optimizing system traffic up to the satellite, which reflects it back to a terrestrial
utilization for best effort traffic. The system also provides IP- Hub where routing information is applied, and then is sent
based addressing, routing, multicast, and IP Diffserv QoS back up in a second hop to be reflected into the coverage area
support for end user networks. This allows the use of containing both the transmission and destination terminal(s).
industry standards such as H.323 [3] and Session Initiation This conventional transponded architecture has for many
Protocol (SIP) [10] for managing audio and video years provided cost-effective networking services over large
conferencing in point-to-point and multi-party modes. coverage areas using geosynchronous satellites. However, it
presents significant capacity and connectivity limitations
An initial Windows/Linux-based prototype with open source when implementing interactive full-mesh and multi-party
software video and audio codecs (coders and decoders) was video applications.
adequate to validate the architectural approach and to conduct
preliminary tests for characterizing network configuration and Regenerative satellite system architectures such as
transport for multi-party environment. Next, the testing SPACEWAY 3 employ onboard digital packet processing,
environment was enhanced by introducing hardware-based thus enabling capabilities such as onboard resource control
encoders, which eliminated the software coding and decoding and packet switching. Besides increasing capacity, such
delays inherent in the initial prototype. An early version of architectures facilitate innovative networking topologies, and
the hardware-based prototype was configured with off-the- in particular, true full-mesh, single hop and multicast
shelf codec components and was successfully used with connectivity across ground terminals. Standardized by ETSI
partners for demonstration purposes over a period of five and TIA for Ka-band usage, the pioneering RSM-A
months. A more advanced configuration with the use of High architecture is now an integral part of satellite network
Definition off-the-shelf video tele-conferencing (VTC) services now available to enterprises, government agencies,
encoding has also been successfully used and tested in a and consumers throughout North America. Details of the
point-to-point mode. RSM-A architecture have been described in other
publications, and a brief overview is presented here.
A combination of high capacity, full-mesh satellite transport
and the VTC architecture enables the use of fully functional The RSM-A architecture, with packet processing and
multimedia collaboration in a cost-effective fashion that can bandwidth-on-demand functions hosted by the satellite,
easily be deployed over a large geographical area. In addition provides a capable platform for IP packet services with
to commercial use, such applications are indispensable for differentiated QoS. The RSM-A architecture can, for
dealing with disaster and security situations that are difficult example, send ad hoc information directly to the tactical users
for other networking transports due to their wide coverage, when and where needed, and concurrently gather strategic or
high capacity, and dynamic connectivity requirements. raw data from the field. Tactical users on the ground can
simultaneously monitor and direct sensors while receiving
This paper is organized as follows. The next section processed data and also engage in interactive real-time
provides an overview of the RSM-A standard, which is multimedia conferencing for decision making and situational
followed by a summary of the relevant capabilities of the awareness.
SPACEWAY 3 system that was developed with the standard.
This is followed by an overview of the IP-based VTC Packet processing in the satellite is symbiotic with the use of
architecture and associated packet networking transport multiple spot beams in both uplink and downlink, leading to
requirements. IP multicast is described next, which is an order of magnitude capacity gain because of the spatial
required for efficient use of networking resources and for frequency reuse. Employing a number of satellite
minimizing transmission delay. This is followed by the demodulators and a programmable mapping between the
description of an architecture for a multi-party video demodulators and the beams further enhances dynamic
conferencing system that uses the packet replication bandwidth capacity allocation. These advanced capabilities
capability of the SPACEWAY 3 system. This architecture can now easily be used by a wide variety of end user
has been prototyped with off-the-shelf software and hardware applications, leveraging the IP protocol of the system to
components, which is the topic of the next section. Finally, ensure interoperability.
some experimental results are provided, followed by our
concluding remarks and guidelines for future work.

3. From a development perspective, it is important to strike the specific carrier frequencies and timeslots for each
right balance between the satellite payload complexity and the transmitting terminal. By controlling the number of timeslots
necessary set of onboard networking functionality. As and using a specific digital coding scheme, the BoD function
illustrated in Figure 1, RSM-A incorporates layer 1 (physical) can adapt to changing user traffic and environmental
and layer 2 (data link) packet networking functions. The conditions. In addition, RSM-A prescribes differentiated
figure shows the distribution of IP network, packet control packet delivery services for providing QoS to upper layers.
protocol, and packet lower layer functions across satellite, Both packet processing and resource control functions are
Network Operations and Control Center (NOCC), and under policy-based ground control for further user priority
terminal segments with respect to the core RSM-A functions. considerations.
The end user networks, represented as Subnet 1 and Subnet 2,
can use standard, dynamic IP routing protocols with their As we describe later, the core RSM-A functionality can be
corresponding terminals. The NOCC, deployed in the ground extended and enhanced for a specific system instantiation
segment, provides addressing, QoS signaling, and multicast with value-added capabilities. This includes multiple spot
support for making network level decisions communicated to beams for providing more capacity with spatial reuse; a
the end user networks with the use of IP protocols on the configurable satellite demodulator with beam mapping for
terminals. dynamic capacity allocation [12]; policy-based network
management [11]; and differentiated end user IP packet
An RSM-A compliant satellite typically implements packet services [13].
switching in hardware. Some RSM-A control functions, such
as Bandwidth-on-Demand (BoD) and MAC processing, The RSM-A architecture supports the always-on concept for
typically use software implementation. Because of recent IP satellite terminals. A new terminal registers with the
advances, it is now possible to implement more advanced satellite network to get the MAC address, security keys, and
features within the satellite, such as full layer 3 packet associated management and terminal user port IP address. A
routing. These higher layer functions enable extended policy set of basic configuration profiles for default service are
control and routing possibilities across multiple satellites in a automatically downloaded from the NOCC, while specific
constellation, using inter-satellite links. However, dynamic profiles for additional services, such as multicast and full-
IP routing, full-mesh, packet replication, QoS and packet mesh connections, are provided selectively based on
statistical multiplexing gain are possible with RSM-A, even if configuration and user priorities.
using only a layer 2 packet switch in the satellite.
3. SPACEWAY 3 Implementation of RSM-A
Sub- Sub-
Net Net SPACEWAY 3 is the first commercial instance of an RSM-A
1 T1 T2 2
IP Network Layer
based satellite system [13]. The end user equipment (hosts or
routers) connect to the satellite terminal with Ethernet cable
NOCC and interoperate with distant networks using the Internet
Control Protocol Layer Engineering Task Force (IETF) standard IP protocols. The
system provides the following capabilities on a terminal, all
Satellite concurrently with multiple full mesh interactions:
(Packet) Data Link Layer • High performance guaranteed QoS needed for voice
(Packet) Physical Layer and video applications.
• On-board BoD needed for bursty best-effort IP
Core RSM-A Functions
traffic.
Figure 1. RSM-A Core Functions and a System Instantiation • Rate-based virtual connections (both scheduled and
on- demand) with committed resources for real-time
The RSM-A specifications include the physical layer (e.g., packet services (both unicast and multicast)
channel coding, modulation, radio transmission and reception, • Multicast with packet replication in the satellite to
frame structure, radio link control, and synchronization); eliminate unnecessarily replicated uplink
satellite medium access control; satellite link control; transmissions.
broadband satellite multimedia services and architectures; and • Dynamic scalable routing for a group of terminals.
the security specification. Per RSM-A, radio transmission
from a transmitting terminal is demodulated and decoded in Figure 2 shows the uplink beam pattern of SPACEWAY 3.
the satellite to extract the fixed size packets along with their The uplink beams are narrow in order to provide dramatic
destination MAC address. The satellite packet switch uses increases in system capacity due to frequency reuse among
the MAC address to place a packet on the destination the spot beams. Each uplink beam is itself composed of 7
downlink beam, and subsequently the packet is received by downlink beams. The on-board digital satellite processor
the destination terminal. provides separation of the radio uplink from the downlink by
re-generating the digital signal and thus boosting signal
TDMA coordinates packet transmissions in the shared space strength at the receiving terminal.
media, where a satellite-based, BoD function allocates

4. The uplink physical layer uses a 96 ms TDMA frame with 32 point-to-point communication, many Internet services now
slots of 3 ms duration each. There are 8 frames in a super provide low cost or even free multi-party video conferencing,
frame of approximately 1 second duration. The 500 MHz Ka- employing a centralized bridge that aggregates all incoming
band uplink is divided into 8 sub-bands of 62.5 MHz each and traffic and then redistributes it to all participants.
used with a 4 color reuse (with right and left hand circular
polarization). There are hundreds of demodulators to support For more formal communication with a better QoS, custom
the raw aggregate capacity of 10 Gbps on the satellite and VTC equipment is now available from multiple sources. The
each can be assigned to a particular uplink spot beam to listen use of IP-based standards has increased interoperability
to the assigned uplink frequency ranges. A demodulator can across VTC equipment from different vendors. Many
be configured to support a combination of approximately 16, vendors of the latest VTC equipment and software
2, and 0.5 MHz carriers, aggregating to 62.5 MHz per sub- implementations are now employing MPEG-4 compression
band. A SPACEWAY 3 terminal is assigned a specific standards and are able to provide high quality imagery at
frequency carrier, frame, and slot in a dynamic fashion by the relatively low bit rates. There are also many different types
satellite BoD function, depending on its uplink data rate of audio compression standards with varying degrees of voice
needs. The end user terminals generally use 2 and 0.5 MHz quality and bit rate requirements in the marketplace.
carriers, while the larger gateways typically use 16 MHz
carriers. There are two broad families of control protocols that are
currently used for multimedia conferencing over packet
The SPACEWAY 3 downlink uses a Time Division networks. H.323 is the ITU Telecommunication
Multiplexing (TDM) scheme where each of the 24 concurrent Standardization Sector (ITU-T) recommendation that defines
beams (12 left and 12 right hand circularly polarized) can protocols for audio and video conferencing over IP. Session
transmit the full 500 MHz downlink. These hopping beams Initiation Protocol (SIP) is specified by IETF and can be used
illuminate the cells where the user traffic is directed based on to manage call setup for audio and video conferencing. H.323
individual packet header information. A terminal in any one provides more specific guidance on individual aspects of
of the 784 downlink cells sees all packets destined for its multimedia conferencing, resulting in better interoperability
respective downlink cell, filtering those packets addressed to across equipment from different vendors. SIP, on the other
it and providing them to the upper layers. hand, is more flexible in allowing greater improvisation
within a vendor-specific implementation. In this paper, we
have used the H.323 terminology to describe the point-to-
point and multipoint aspects of video conferencing. Our
multipoint architecture is agnostic to the use of H.323 or SIP
since they are all IP-based.
4.1 H.323 Protocol Family
H.323 is widely supported by multiple manufacturers and is
now available in dedicated VTC equipment and software
implementations. H.323 uses End Points for users;
Gatekeeper for call routing, addressing and authentication,
bandwidth management and billing; and Gateways to connect
to other networks. These are all logical entities which can be
ST implemented in one or more physical device. An H.323 zone
ST has one Gatekeeper and can have multiple Gateways, all
connected through an IP network within the zone.
A point-to-point H.323 call can be arbitrated by a Gatekeeper,
or directly dialed between two End Points. Simultaneous
Figure 2. SPACEWAY 3 System Uplink and Downlink videoconferencing among three or more remote points is
possible by means of an H.323 Multipoint Control Unit
4. IP Video Conferencing over Satellite (MCU). An MCU is essentially a conferencing bridge that
interconnects calls from several sources. All parties call the
IP video conferencing has become more commonplace with MCU unit, or the MCU unit itself can call the parties that are
the increasing availability of broadband connections, going to participate, in sequence. There are MCUs which are
advanced video codecs implemented in software, and implemented in software and others which are a combination
affordable video cameras and computers. Many consumer of hardware and software. An MCU is characterized
laptop computers now include built-in web cameras, speakers, according to the number of simultaneous calls it can handle,
and access to software such as Google chat and Skype that are its ability to conduct transposing of data rates, coding
revolutionizing casual collaboration and social interaction schemes, other protocols, and features such as Continuous
across continents and indeed across the globe. In addition to Presence, in which multiple parties can be seen onscreen at

5. once. H.323 is a comprehensive protocol family and specifies multipoint call exchanges voice and video directly with the
both basic services and supplementary services, such as call other stations. This decentralized architecture does not
transfer, park, pick-up, and hold. There are many aspects of require any centralized MCU. Such a decentralized
tele-conferencing that are relevant from a networking configuration avoids both the extra relays to and from the
perspective and we have selected H.323 to illustrate them. MCU and the End Units, and trans-coding at the MCU, which
Table 1 lists important protocols in the H.323 family in ultimately leads to higher quality. Additionally, the End
support of packet-based teleconferencing. Units do not need to coordinate with a centralized MCU,
which may result in operational simplicity. However, this
Function H.323 Description decentralization comes at the expense of increased network
Example bandwidth, especially when the network is not providing IP
Re- H.225.0 Registration, Admission multicast. This is the case since every station must transmit
gistration and Status used between to every other station directly. So what is gained in
Endpoint and a Gatekeeper minimizing delay with no MCU relays is lost in extra
Call H.225.0 Call Signaling used bandwidth when using multiple point-to-point (unicast) data
Signaling between any two H.323 streams among the End Points.
entities in order to establish
communication Individual audio/video stream from each
Control H.245 Messages and procedures End Point aggregated, transcoded, and
Protocol used for capability distributed individually to all end units
by MCU
exchange, opening and
closing logical channels for Multipoint
audio, video and data, IP Network Controller
control and indications Supporting only point-to- Unit (MCU)
Real-time RTP Realtime protocol for point unicast connectivity
Data (IETF) sending or receiving
Transport multimedia information
(voice, video, or text)
between any two entities
RTCP (IETF) RT control protocol End Unit End Unit End Unit
Audio G.711 64 kbps data rate with high One Two Three
Codec quality PCM
Figure 3. Centralized Multipoint Conferencing without
G.723, G.726 Varying rates and voice
Multicast Requires Double Hop
quality
G.729 8 kbps data rate
For a satellite-based IP transport, both delay and bandwidth
Video H.261 CIF (352x288 luma with
are fundamental drivers and need to be optimized
Codec 176x144 chroma) and QCIF concurrently. With satellites in geosynchronous orbit, a
(176x144 luma with 88x72
single hop propagation delay is minimally on the order of
chroma)
250ms. The use of a centralized MCU requires two hops,
H.263 MPEG-1 and MPEG-2 doubling this propagation delay to at least 500ms. When
compression combined with the network processing and device processing
H.264 MPEG-4 AVC (part 10) overheads at the MCU, the overall delay would far exceed
compression 500 ms! The use of network-based IP multicast can avoid
this penalty. However, for a satellite network it can only be
Table 1. H.323 Protocol Family for Multimedia Conferencing avoided if packet replication is performed onboard the
satellite itself, which is infeasible with bent-pipe transponded
There are additional protocols in the H.323 family. The satellites.
H.235 series describes security; the H.239 series describes
dual stream use in videoconferencing with usually one for RSM-A facilitates packet processing and packet replication
live video and, the other for still images; the H.450 series onboard the satellite, thus enabling responsive multipoint
describes various supplementary services; and the H.460 VTC with regenerative satellites (as shown in Figure 4) using
series defines optional extensions such as address translation the standard IP multicast transport. The H.323 standard
and Firewall traversal. supports IP multicast for such multipoint scenarios. Thus, the
participating End Units in a decentralized H.323 multipoint
4.2 H.323 Multipoint Support mode can use IP multicast for transporting audio and video
H.323 also supports multipoint conferencing with the use of data to the other End Points. The End Points can continue to
an MCU, either as a centralized standalone device, shown in use point-to-point IP transport for control plane signaling via
Figure 3, or embedded in an End Unit. Alternatively, a the MCU.
decentralized architecture is possible where each station in a

6. 5. IP Multicast with RSM-A represented by the multicast address. Multicast routing
ensures that at each network node in the path, the right egress
IP multicast is a key enabler for satellite-based multipoint link will be determined for scheduling the packet. In case the
VTC. The IP multicast scheme involves a variety of routing node serves as a branching point for the multicast tree, a
and QoS standards specified by IETF. This includes multicast packet is replicated so that a copy of the packet can
multicast extensions for standard routing protocols, such as a be propagated on each egress link corresponding to a
multicast version of unicast routing protocols, and a protocol multicast tree branch. Actual scheduling of such packets
independent multicast (PIM) [4] that can use any underlying within that network node is driven by the QoS scheme used
IP unicast routing protocol for making optimal multicast for multicast traffic.
routing decisions. A key objective of these routing protocols
is to ensure loop free identification of multicast paths guided 5.2 Satellite Transport for Multicast
by a multicast tree. The multicast tree can either be a custom
source-specific multicast tree (e.g., for PIM source specific A satellite-based IP network is quite different from traditional
mode), or a shared multicast tree anchored at Rendezvous terrestrial networks. A transponded satellite serves as a high
Points used in the PIM sparse mode. capacity link shared among a large number of terrestrial
Single data uplink networks. Multiple access schemes such as TDMA are
from an End Unit is utilized to share this space link among various terminals
Packet
Switch
replicated by packet belonging to one or more networks. The RSM-A architecture
switching satellite to enhances both capacity and sharing of the space transport by
all destinations
(shown here only for using multiple spot beams and a satellite packet switch. A
End Unit One) packet processing satellite serves as a space-based switch
with a large fan-in and fan-out, as it virtually connects all
MCU involved terminals and networks in its footprint. This large number of
only in control adjacent connections for a single space-based network node
plane signalling creates scalability challenges for every packet control plane
in point-to-point IP
mode Multicast protocol, including addressing, routing, and QoS.
End Unit In SPACEWAY 3, dynamic IP routing is implemented with a
One scalable routing architecture [13]. It uses IETF protocols,
Figure 4 Decentralized Multipoint VTC with Packet such as RIPv2, on the terrestrial user interface to learn routes
Replication in the Satellite that are efficiently distributed to all other terminals in the
routing domain, using a terminal acting as a route server for
5.1 Multicast QoS that domain. Satellite uplink and downlink resources are
managed at individual flow and aggregate levels using a
Both IETF Intserv [5] and Diffserv [9] architectures can connection admission control function that tracks actual
provide QoS for multicast packet transport. Intserv uses capacity used in all uplink (source) and downlink cells where
control plane signaling and specifies Resource Reservation destination terminals are located. This capacity management
protocol (RSVP) [6], which reserve resources at each node has an additional time dimension that allows pre-reservation
between the source(s) and destination(s). These flow-specific of edge-to-edge capacity for a particular enterprise by the
reservations are managed at each network node in a service provider serving multiple enterprises [12]. These
distributed fashion, using soft states in the nodes that need to capacity pools can then be used for unicast or multicast
be periodically refreshed with RSVP signalling between the transport. Flow-level connection admission can be triggered
end hosts. Diffserv, on the other hand, is more scalable as it by configured policy at the terminal (e.g., by time or by using
uses the concept of traffic classes specified by the DSCP packet header information from the data plane).
(Diffserv Code Point) field in every data packet. Using 6 bits
in the DSCP field, multiple types of traffic can be specified For multicast packet transport, there could be multiple
(e.g., expedited forwarding, assured forwarding, and best destination cells for the user traffic. These destination cells
effort), and used at each network node for QoS decisions. A are either statically configured or determined by a host
network node in the traffic path provides appropriate per hop wanting to joint a multicast group, resulting in IGMP
behavior for a traffic class that can either be pre-provisioned, signaling supported by the terminals. In either case, the
or use a dynamic bandwidth broker with formal admission system knows whether it has to send replicated packets in a
control. Diffserv is more scalable, as individual flow states downlink cell that contains a terminal supporting destination
do not need to be maintained at each network node. hosts. The packet switch in the satellite replicates multicast
packets so that a copy can be provided to each such cell with
IP multicast packets use the Internet class D address (ranges interested recipients. Internal SPACEWAY 3 signaling
from 224.0.0.0 to 239.255.255.255), reserved especially for between terminals and the NOCC for connection admission
multicast. A multicast address is used by a source in every control ensures (corresponding to IP multicast) that a packet
data packet that needs to be distributed to multiple destination replicating satellite transport is available to the ground IP
points which are the members of the same multicast group networks using SPACEWAY 3. The end user multicast

7. traffic from a source enters an RSM-A terminal using ingress terminal is optionally available to provide some
standard IP protocols. Actual capacity for a multicast uplink elasticity to the underlying satellite transport. Thus any
and downlink is used only when either a time-based (with traffic over the committed rate can be queued and transmitted
scheduled pre-provisioning) or a policy based (source using volume packet delivery service (PDS). Thus, burstiness
address, destination address, DSCP, port number, protocol can be handled gracefully without committing to a larger peak
number in data packets) trigger initiates a multicast rate.
SPACEWAY 3 virtual connection.
6. Prototype Implementation
A connection request is made by the terminal and based on
terminal priority, ownership, and available capacity pools the Several multipoint prototypes were developed using
request can be granted by the NOCC. The TDMA grants SPACEWAY 3-based services that have been available since
from the satellite use the rate transport to minimize jitter and April 2008. Most of the networking capabilities used in these
delay (with recurring timely slot allocations) for multicast. prototypes are commercially available. However, certain
The IP multicast packets are uplinked with high priority rate aspects of these multicast prototypes used advanced features
allocations and, within the satellite switch, replicated so that in a controlled engineering environment.
copies can be scheduled for downlink cells where the
destination terminals are present. On receiving multicast Multipoint prototyping was done in three increments. The
packets, a destination terminal uses its IP routing tables and first increment focused on uni-directional streaming of audio
packets are transmitted to their recipients in the attached user and video data to multiple recipients. The implementation
networks. was done using open software (VideoLAN VLC media
player) on Linux and PC platforms. The second increment
From the end user perspective, the SPACEWAY 3 terminals involved interactive bi-directional transmission of audio and
provide IP interoperability for multicast. In addition, the use video data, as would be encountered in a video conference.
of formal admission control ensures high priority transport The third increment involved the use of off-the-shelf VTC
(per hob behavior) for minimal delay and jitter. The use of equipment with High Definition capability that requires less
multiple DSCP marking allows the use of dynamic and than 1.5 Mbps transport.
policy-based capacity. For example different DSCP values
can be used to select specific high priority bandwidth such as These prototypes used Hughes HN9500 and HN9000
384 kbps, 786 kbps, and 1312 kbps (with a granularity of 16 terminals to access the RSM-A system implementation. An
kbps). HN9500 terminal, for example, includes both IP router
functionality for the end user interface, and modem
It is possible to interface with RSVP to support the Intserv functionality for the satellite interface. The end user network
scheme for specific data rate requests with RSM-A. can be connected to an HN9500 using a standard Ethernet
However, the use of DSCP is pivotal in the triggering of cable. The HN9500 uses the RSM-A standard for uplink and
packet transport service in the satellite domain without any IP can transmit up to 2 Mbps and receive up to 30 Mbps of
control plane signaling. The use of DSCP (and other fields RSM-A packets. The In Door Unit (IDU) uses two CAT 5
such as source and destination addresses) marked user data Coaxial cables for connecting it to the Out Door Unit (ODU)
packets ensures that high priority rate resources are actually that includes a 98 cm Ka-band antenna and a TRU (transmit
allocated in the satellite domain only in the presence of a and receive unit) with a 2 Watt radio amplifier. The distance
specific type of user traffic. Otherwise, the rate resources can from the IDU to ODU can be up to 300 feet in the standard
be statically multiplexed for other multicast and unicast users configuration. The terminals were pointed to operate over the
requiring better QoS. SPACEWAY 3 satellite located at 94.54° West location,
providing coverage across North America and major South
For guaranteed multipoint transport, it is possible to pre- American cities.
provision satellite uplink and downlink rate resources in
advance with the use of scheduled virtual connections. These A typical service over SPACEWAY 3 provides Internet
reservations have specific start and end times and recurring access to consumers via the Hughes Gateways. A Gateway
patterns (e.g., scheduled VTC every Monday morning from 9- is essentially a large terminal with tens of Mbps uplink and
10 AM). These reserved connections can pre-empt other downlink capacity with a dedicated connection to the Internet.
traffic, including on-demand connections triggered with the The multipoint prototypes described here used the terminals
use of DSCP-marked traffic. Other than advance reservations with access to the Internet via Hughes Gateways. However,
and pre-emption possibility, there is no other QoS difference the tests used full-mesh connectivity where the Gateways did
between on-demand and scheduled connections. not have any role in data or control plane interactions. The
multicast and connection configurations for the terminals
For fixed rate audio conferencing (e.g., with the use of G.711 were done using the NOCC in a dedicated End User Group
codec requiring 64kbps transport), rate allocations is the which separated the prototype terminals from the other
appropriate user data transport service. Any packets over the terminals using commercial services over SPACEWAY 3.
committed rate can be policed and dropped at an ingress
terminal. For variable rate codecs, a longer queue at an

8. 6.1 Software Codec as End Unit codec converted the NTSC video stream into MPEG-2
compressed IP packets which used the respective multicast
The multi-party exchange of multimedia traffic is class D address.
encountered in many scenarios. In one-way streaming, the
source can distribute audio and video information to multiple An echo-canceller was also used to cancel voice spurts
destinations in real-time. A use of multicast allows a single received at the originating site via the loud speaker at a
uplink carrying traffic that gets replicated in the satellite. remote site feeding back into the remote microphone and
Since immediate response to audio and video data is often not transmitted back over the satellite. Configuration for off-the-
required, streaming allows certain elasticity in the underlying shelf equipment was done using a web interface, and standard
network transport. This allows for slightly relaxed delay and definition picture format (resolution 352x288) was used for
delay variation characteristics. However, best effort network images. Each terminal used 1.3 Mbps uplink and received 3.9
transport may lead to excessive delay and perhaps packet loss Mbps downlink (1.3 Mbps times 3). Each site had an NTSC
which may compromise the quality of streaming sessions. TV screen that showed all four video streams, one local and
three remote. The multicast-based multipoint video
The software prototypes used a collection of four terminals, conferencing demos over SPACEWAY 3 were conducted in
geographically separated from each other so that packet private and public forums. This included a public conference
replication in the satellite could be used. The source terminal in North America during the second half of 2008.
was connected to a PC running VLC software with
compressed files, both audio and video, coded for multiple 6.3 Off-the-Shelf HD VTC Equipment
picture resolutions. During replay, each coding (using
MPEG-4 and 25 frames per second) resulted in a specific The third prototyping increment used off-the-shelf VTC
average bit rate requirement for the multicast transport. equipment. The selected state-of-the-art equipment employs
These measures are provided in Table 2 . advanced MPEG-4 compression techniques to deliver High
Definition video at a sub T1 (1.5 Mbps) data rate. As of the
Picture Resolution Aspect Average time of writing of this paper, the VTC prototyping increment
Ratio Bit Rate demonstrated only the point-to-point (unicast) mode to
192x144 4/3 109 kbps validate the use of out-of-the-box VTC capability with RSM-
640x480 4/3 375 kbps A, employing H.323 signaling.
1280x720p Normal 16/9 828 kbps
1280x720p High 16/9 1703 kbps Micro
phone
Packet Switch
Table 2. Average Bit Rate for Software Codec Prototypes
Echo
The raw movie footage used for these tests dealt mostly with Canceller
End Unit
human interviews, similar to a VTC environment. Audio was
One
coded identically for all tests, using 48 kbps bit rate. Each Loud
multimedia file had both video and audio streams. Only one Speaker
source terminal was used to send the audio/video data with a
Similar equipment
specific class D multicast address on the source uplink, and H/W configuration at all
SD video H/W NTSC
the other three terminals listened to the specific multicast MPEG-2
MPEG-2 other terminals
Camera Codec TV
address. This scenario used a statically configured multicast Codec
group that included the terminals in the same terminal group. Figure 5. Network Configuration for a Terminal Site using
Hardware Codecs and Echo Canceller
6.2 Off-the-Shelf Hardware Codecs and Echo Canceller
The software-based prototype increment validated the end-to- Table 3 shows some of the relevant end-to-end performance
end multimedia transmission over RSM-A, leveraging IP measures of the HD video conferencing over SPACEWAY 3.
multicast. The experimental work helped identify the Note that the packet QoS for multicast transport is very close
underlying SPACEWAY 3 configuration for the future to point-to-point QoS when rate based PDS is used. The
increments. It also provided a test and measurement additional overhead for replicating packets within the satellite
environment to characterize the packet QoS provided by the is minimal as it is performed in hardware by a 10 Gbps
satellite domain. The next step was to replicate the canonical switch.
multicast architecture with four sources, each transmitting
multimedia traffic for their respective class D multicast Primarily designed with terrestrial connectivity in mind, a
address. The other three sites listened to the class D multicast typical VTC equipment configuration should support both
address of the fourth site sourcing that traffic. Each site used point-to-point and multi-party video conferencing. The
a collection of off-the-shelf hardware based codecs, as shown multipoint configuration, however, could involve a
in Figure 5. Audio was generated using a microphone and centralized use of an H.323 MCU that can essentially
video using a Standard Definition NTSC video camera. The aggregate unicast traffic from each participant and then
distribute it to all other participants. This easier

9. configuration does not require the use of IP multicast, which High Definition video conferencing for multiple parties with
is not universally supported by all IP service providers. A easily deployable terminals by providing a single hop
typical VTC equipment kit supports the H.323 standard which transport. This unique capability bodes well in the future for
does specify, though optionally, the use of multicast data increasing deployment of RSM-A architectures for important
transport for decentralized multipoint videoconferencing. A yet difficult to support domains dealing with health, defense,
more detailed configuration involving the VTC equipment is and emergency response.
needed to explore and test IP multicast compatibility with
RSM-A compliant transport. This work is currently in References
progress.
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Parameter Value (ETSI), “Technical Specification 102 188 and 102 189,
Satellite Earth Stations and Systems; Regenerative
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Video Jitter 9 ms published TIA-1040.1.07 - Regenerative Satellite
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Sparse Mode (PIM-SM): Protocol Specification,” RFC
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[6] R. Braden et al., “Resource ReSerVation Protocol
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(using unicast) and data (using multicast) planes.
Video conferencing has emerged as a universal tool for
critical applications such as distributed decision-making,
telemedicine, security, and disaster management. Satellites
provide continent-wide and remote coverage for supporting
such applications, though conventional transponded satellites
are limited to a point-to-point mode in order not to
compromise on transport delay. RSM-A facilitates the use of