Onboard baseband processing architectures for future satellite broadband integrated services digital networks (B-ISDN's) are addressed. To assess the feasibility of implementing satellite B-ISDN services, critical design issues, such as B-ISDN traffic characteristics, transmission link design, and a trade-off between onboard circuit and fast packet switching, are analyzed. Examples of the two types of switching mechanisms and potential onboard network control functions are presented. A sample network architecture is also included to illustrate a potential onboard processing system

Faris, Faris

Inukai, Thomas

Shyy, Dong-Jye

English

NASA Technical Reports Server

N92"14203
ON-BOARD PROCESSING ARCHITECTURES
FOR SATELLITE B.ISDN SERVICES 1
Thomas Inukai, Dong-Jye Shyy, and Faris Faris
COMSAT Laboratories
Clarksburg, Maryland 20871
SUMMARY
This paper addresses on-board baseband processing architectures for future satellite broadband
integrated services digital networks (B-ISDNs). To assess the feasibility of implementing satellite
B-ISDN services, critical design issues, such as B-ISDN traffic characteristics, transmission link
design, and a trade-offbetween on-board circuit and fast packet switching, are analyzed.
Examples of the two types of switching mechanisms and potential on-board network control
functions are presented. A sample network architecture is also included to illustrate a potential
on-board processing system.
1. INTRODUCTION
The B-ISDN will likely play a major role in the future telecommunications networking for
providing high speed integrated services to network users such as broadband video-telephony,
broadband videoconference, high volume file transfer, high speed telefax, high definition TV
(HDTV), and broadband videotext. It is envisaged that this technology will have an impact on
satellite communications. This paper addresses some of the key design issues and alternate on-
board switching architectures for a satellite-based B-ISDN.
The paper presents examples of B-ISDN traffic characteristics and identification of potential
satellite applications of B-ISDN services. One of the primary concerns in designing a satellite B-
ISDN system is the design of a transmission link that can support very high data rates ranging
from 155 Mbit/s to over 1 GbitYs with an availability that is comparable to terrestrial-based
services. A summary of link analysis for Ku- and Ka-bands is included in the paper.
One of the most critical design issues for on-board processing satellites is the selection of an on-
board baseband switching architecture. Circuit switching and fast packet switching are probably
the two most common architectures. A trade-off analysis is performed for these architectures on
the capability of handling circuit- and packet-switched traffic and the impact of traffic
reconfiguration. Several switch structures for the two types of switching are also illustrated.
Potential satellite network architectures for B-ISDN include baseband-switched TDMA (SS-
TDMA), TDMA up-link/TDM down-link, TDM up-link/TDM down-link, and hopping beam TDMA.
SS-TDMA requires simpler on-board hardware and provides flexible connectivity among the same
type of earth stations. TDMA up-link/TDM down-link allows the use of shared up-link capacity by
multiple earth stations, while optimizing down-link transmission on a single carrier. It will
require an on-board baseband processor for rate conversion and interconnection of user traffic.
TDM up-link/TDM down-link is particularly suited for trunking applications and circuit-switched
B-ISDN traffic. As in the previous case, an on-board processor provides rate conversion and
connectivity. Thin-route traffic can be efficiently carried by hopping beam TDMA to dynamically
1This paper is based on work performed at COMSAT Laboratories under the sponsorhip of the National Aeronautics and
Space Administration (NASA) under Contract No. NASW-4528.
https://ntrs.nasa.gov/search.jsp?R=19920004985 2020-03-17T14:26:51+00:00Z
allocate the necessary capacity to different dwell areas. The paper presents a sample network
architecture with TDMA up-link and TDM downlink and on-board fast packet switching.
2. SATELLITE APPLICATIONS
The B-ISDN supports a wide variety of communications services. Although the B-ISDN is being
developed primarily for terrestrial networking using fiber optic cables, satellites are well
positioned in complementing terrestrial-based B-ISDN services. The satellite-based system has
inherent capabilities of providing multipointfaroadcast transmission, connectivity between any two
points within a beam coverage, quick reallocation of space segment capacity, and distance-
insensitive cost. Use of multiple spot beams and on-board rate conversion/switching will provide a
larger capacity, additional flexibility, and lower user terminal cost. The satellite B-ISDN system
can be used for direct (mesh) interconnection among users (UNI-UNI), interconnection between a
user and a switching node (UNI-NNI), and interconnection between switching nodes (NNI-NNI).
Figure 1 illustrates these options.
B-tSDN USER
Figure 1. A satellite B.ISDN can provide direct interconnection between users,
switching centers, and a user and a switching center.
A variety of B-ISDN services can be supported by a satellite network. Table 1 depicts examples of
satellite B-ISDN services and their typical traffic characteristics. The B-ISDN interface rates are
155.52 Mbit/s (51.84 Mbit/s for SONET STS-1) or higher, but their information rates can be as
small as 64 kbit/s. Traffic can be extremely bursty such as in LAN/MAN interconnection, or can be
a steady flow of high speed data as in video program distribution. A satellite network must be
flexible to accommodate a wide range of transmission rates and different degrees ofburstiness. In
addition, Asynchronous Transfer Mode (ATM) is a packet-based transmission mechanism that
supports connection-oriented as well as connectionless services at various bit rates. Thus, a
satellite system design must take into account these new environments that are significantly
different from the traditional circuit-oriented system.
Table 1. Traffic Characteristics of Satellite B.ISDN
BIT RATE RANGE SOURCE
SAMPLE APPLICATION (MbiVs) BURSTINESS TRAFFIC TYPE
Video Distribution
Live Newscast/Broadcast
Science Data Distribution
Supercomputer Networking
Private Networking
Trunking
Emergency Communications
Thin-Route Networking
45 - 140
20 - 45
30 - 300
30 - 1000
0.1 - 100
52 - 2500
2 - 45
0.1 - 10
Low
Low
Medium
H_gh
I_gh
Low
Low to Medium
Medium to High
Circuit
Circuit
Circuit/Packet
Packet (Circuit)
Packet (Circuit)
Circuit
Circuit (Packet)
Packet (Circuit)
3. TRANSMISSION LINK DESIGN
As seen in the previous section, the satellite B-ISDN network must support a wide range of bit
rates from as low as 64 kbit/s to over 1 Gbit/s. A link analysis was performed for typical Ku- and
Ka-band satellites to determine typical bit rates that can be supported by various earth station
sizes (HPA and antenna). A Ku-band spot beam satellite assumes INTELSAT-VII or typical
domestic satellite parameters with a Gfr of 5 dB/K and an EIRP of 50 dBw, and a Ka-band
satellite has a Gfr of 20 dB/K and an EIRP of 60 dBw (ACTS parameters). It is also assumed that
these satellites perform on-board regeneration as well as FEC decoding and recoding. The results
of link analysis are shown in Table 2.
Table 2. Link Analysis Results (QPSK Modulation, BER=IO-8)
ANTENNA HPA BIT FEC UPLINK DOWNLINK
FREQ. DIAMETER SIZE RATE CODING MARGIN MARGIN
BAND. (m) (watts) (Mbit/s) RATE (dB) (dB)
Ku
Ka
12.0 100 620 0+75 10.8 12.5
9.0 100 620 0.75 8.3 10.0
7.5 100 310 0.5 10.8 12.5
5.0 50 155 0.5 7.3 12.0
2.4 50 50 0.5 5.8 10.5
2.4 20 10 0.5 8.9 10.5
1.2 20 10 0.5 2.8 4.5
12.0 100 1240 0.75 22.8 15.8
9.0 100 1240 0.75 20.3 13.3
5.0 50 620 0.5 16.3 12.3
5.0 50 310 0.875 17.3 13.3
2.4 50 155 0.5 15.9 11.9
2.4 20 50 0.5 16.9 16.9
1.2 20 10 0.5 17.8 10.8
The results indicate that an information rate of over 1 GbitYs can be supported by a large Ka-band
earth station, and small VSAT class terminals can be used for bit rates of up to 155 MbitYs. A
relaxed BER requirement, e.g. 10 "6, will result in a greater margin, a smaller antenna/HPA size,
or a higher bit rate. If an additional margin or a lower threshold BER, e.g. 10 "11, is needed, a
high-rate outer code, such as a Read-Solomon code, may be used with a slight increase in the
required bandwidth.
3
4. CIRCUIT OR FAST PACKET SWITCHING?
On-board baseband switching provides interconnection of user earth stations operating at different
bit rates and access schemes. There are two types of switching architectures that can be used for
baseband switch implementation: (a) circuit switching and (b) fast packet switching. Circuit
switching uniquely maps uplink time slots to downlink time slots and guarantees their connections
until a controller deallocates the assigned slots. In fast packet switching, data are packetized into
a fixed format with a routing header, and packets from earth stations are routed to the destination
beams (or carriers) according to header information 2. No fixed mapping exists between uplink
slots and downlink slots. However, there is a potential on-board buffer overflow problem. A
comparison of the two switching architectures is shown in Table 3.
Table 3. Comparison of Circuit and Fast Packet Switching
SWITCHING CIRCUIT-SWITCHED PACKET-SWITCHED
ARCHITECTURE TRAFFIC TRAFFIC TRAFFIC RECONFIGURATION
CIRCUIT
SWITCHING
FAST PACKET
SWITCHING
Efficient Bandwidth
Utilization
• Can Accommodate
Circuit-Switched Traffic
+ Somewhat Higher
Overhead Due to Packet
Headers
• Very Inefficient
Bandwidth Utilization
• Inflexible Connectivity
On-Board Congestion
May Occur
• Reprogramming of On-Board Switch
Control Memories
• Reconfiguration of Earth Station
Time/Frequency Plans for Each
Circuit Setup
• Difficult to Implement Autonomous
Private Networks
• Self-Routing
• Does not Require Control Memory for
Routing
• Reeonfiguration of Earth Station
Time/Frequency Plans for Major
Traffic Changes
• Easy to Implement Autonomous
Private Networks
From this table, the following conclusions can be made:
a. Select circuit switching for circuit-switched traffic without frequent traffic
reconfiguration.
b° Select fast packet switching for packet-based traffic or circuit-switched traffic
with frequent channel reconfiguration. Special consideration must be given
to the on-board congestion problem.
The congestion problem for fast packet switching can be completely eliminated for circuit-switched
traffic by allocating a desired capacity (on a call-by-call basis) for both uplink and downlink
carriers. For packet-switched traffic, some form of flow/congestion control is needed to minimize
on-board buffer overflow. The techniques include the following: (a) a dynamic allocation of fixed
capacity from a transmit earth station to each downlink carrier, (b) call admission control at the
earth station, (c) on-board capacity allocation based on current queue status, (d) feedback control,
and (e) a combination of these. The goal of flow/congestion control is to achieve a certain packet
2On-Board Processing Satellite Network Architecture and Control Study, Final Report, NASA Contract NAS3-24886,
Prepared by COMSAT Laboratories, June 1987. Also, see a paper by S. J. Campanella and B. A. Pontano, "Future
Switching Satellite," AIAA 12th International Communication Satellite Systems Conference, Virginia, pp. 264-273,
March 13-17, 1988.
4
lossratio within a satellite systemsuch that nosignificant degradation results in the end-to-end
performance. Someof these techniques have been successfully tested at COMSAT Laboratories.
The on-board switch selection described above is a general guideline based on a qualitative trade-
off and should be carefully evaluated for specific applications, considering a total system capacity,
traffic types, user traffic volume, and network connectivity.
5. ON-BOARD BASEBAND SWITCHING ARCHITECTURES
The simplest form of a baseband circuit switch Consists of a space switch, such as the one used in
the ITALSAT system, to provide SS-TDMA operation. The space switch allows dynamic
interconnection among uplink and downlink spot beams according to the switch state
configurations stored in the on-board control memory. It provides high speed interconnection most
efficiently with a minimum amount of hardware and is also capable of providing multicast
connectivity. A typical B-ISDN application of SS-TDMA includes HDTV program distribution,
trunking, and cable restoration.
More flexible circuit switching to provide rate conversion and channel demultiplexing/multiplexing
is achieved by the use of on-board data memory. Examples of this type of switch are a common
memory (T-stage) switch, a distributed output memory with a parallel data bus (S-T), and a
distributed input/output memory with a space switch (T-S-T). Figure 2 illustrates these switch
structures. Also shown in the figure is a switch structure using a high speed fiber optic ring. The
common memory structure possibly requires the least amount of hardware, but its capacity is
limited by the memory access speed. The capacity of a distributed output memory structure
depends on the speed of the parallel bus, and a careful design is required to avoid signal
interference among the bus lines. The T-S-T structure is modular in design and can accommodate
a larger capacity, but it may require a larger buffer size. The fiber optic bus is also modular in
design, embodies a fault-tolerant structure, and can achieve a high throughput. Selection of a
particular switch structure must consider the throughput requirement, a redundancy structure,
and mass/power requirements.
Common Memory Structure Distributed Output Memory
Time-Space-Time (TST) Structure High-Speed Fiber Optic Ring
Figure 2. Examples of Circuit Switch
Fast packet switching allows dynamic routing of small packets to desired destination beams solely
based on their routing headers. The routing function is performed with a hardware-oriented self-
routing architecture. In fast packet switching, the bandwidth assigned to each connection can be
instantaneously varied, which circuit switching cannot achieve. The interconnection network of
the packet switch decides the performance of the switch such as delay and throughput. The circuit
5
switch structures describedabove,exceptT-S-T,canalsobeusedfor fast packet switching, but the
switch throughput is limited by the speedof the sharedmemoryor the sharedmedium.
Thespace-divisionnetwork can setup multiple connectionsfrom different input ports to different
output ports. Sincepacket transfer is donein parallel, the switching network speedis reduced
comparedwith the shared-memoryand shared-mediumschemes.The other advantageis that
routing control canbe distributed over the switching fabric. Examplesof spacedivision switching
networks areshownin Figure 3.
Sorted-Banyan Based Network
ER BANYAN
,K _X_ NETWORK I
Multicast Switch
MU LTICAST PORT
Figure 3. Examples of Fast Packet Switch Using a Space Division
Switching Network
The space-division switching network can be implemented either in electronics, optoelectronics, or
optics. The maximum data rate of a switch is a function of the device technology and the delays of
the links. The VLSI technologies such as GaAs can support a data rate of up to several thousand
Mbit/s. Other fast VLSI technologies are ECL and high speed CMOS.
Fast packet switching is particularly suited for providing dynamic interconnection of users with
bursty traffic. Its application is not limited to only ATM users but includes lower speed traffic
users with N-ISDN circuit-switched or packet-switched (X.25/frame relay) traffic.
In the conventional transparent satellites, network control functions, such as allocation of space
and ground segments, demand assignment processing, and earth station monitor and control are
performed by a ground-based network control station (NCS) through orderwire communication
channels. The NCS is often equipped with sophisticated computer systems to perform real-time
and batch processing to support dynamic network operation. Implementation of network control
functions on board the satellite not only simplifies ground-based network control, but also provides
a more efficient, reliable, and simpler interface (i.e., direct error-free connection between the on-
board processor and the controller) and a lower ground segment cost. Some of the key on-board
network controller (OBNC) functions are shown in Figure 4.
i
......... _ OBP MONITOR & CONTROL
ON-BOARD NETWORK CONTROLLER
Figure 4. On-Board Network Controller Functions
6
To minimize the complexity of implementing thesefunctions,expert systemsand/or neural
network technologymay be fully utilized.
7. SAMPLE NETWORK ARCHITECTURE
A sample network architecture for an advanced on-board processing system is illustrated in Figure
5. The system consists of 15 Ka-band steerable beams and 13 Ku-band fixed spot beams, both
covering the CONUS. The Ka-band beams can be parked on any of the 87 spot beam areas
according to the beam traffic requirements. Each beam supports 155 Mbit/s TDMA transmission.
The Ku-band system operates on an uplink TDMA bit rate of 5 Mbit/s and 10 carriers per beam
and a single carrier TDM downlink at 50 Mbit/s. The Ku-band system is primarily used by low
traffic users, for example, with N-ISDN interfaces. The two types of systems are interconnected by
a 20 x 20 155-Mbit/s baseband switch matrix (BSM) and an 18 x 18 fast packet switch. The system
capacities are respectively 2.3 Gbit/s and 650 Mbit/s for Ka- and Ku-bands, and a simple non-
blocking switch structure shown in Figure 2 may be used for fast packet switch implementation.
Ku-BAND - 1 _o _oOT BEAMS
Ks-BAND - 0.5 ° SPOT BEAMS
Figure 5. Sample Network Architecture
8. CONCLUSION
Satellite systems can provide B-ISDN services at bit rates of up to 1.24 Gbit/s with an antenna size
of 9 m (Ka-band). However, user bit rates can be significantly smaller than this upper bound in
most applications. A typical information rate may vary in the range of 100's kbit/s to 10's Mbit/s
with occasional requirements at 52 Mbit/s and at up to 155 Mbit/s. In this scenario, cost-effective
satellite integrated digital services can be provided to users with an antenna size of 1.8 - 2.4 m,
and the use of fast packet switching on board the satellite will simplify overall network control and
provide additional flexibility.
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On-board processing architectures for satellite B-ISDN services

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