Internet DRAFT - draft-lhan-problems-requirements-satellite-net
draft-lhan-problems-requirements-satellite-net
Network Working Group L. Han, Ed.
Internet-Draft R. Li
Intended status: Informational A. Retana
Expires: 7 July 2024 Futurewei Technologies, Inc.
M. Chen
L. Su
Tianji. Jiang
China Mobile
4 January 2024
Problems and Requirements of Satellite Constellation for Internet
draft-lhan-problems-requirements-satellite-net-06
Abstract
This document presents the detailed analysis about the problems and
requirements of satellite constellation used for Internet. It starts
from the satellite orbit basics, coverage calculation, then it
estimates the time constraints for the communications between
satellite and ground-station, also between satellites. How to use
satellite constellation for Internet is discussed in detail including
the satellite relay and satellite networking. The problems and
requirements of using traditional network technology for satellite
network integrating with Internet are finally outlined.
Status of This Memo
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Satellite Network Integrated with Internet . . . . . . . . . 7
4.1. Protocol Stack for Satellite Networking with Different
Technologies . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Use Satellite Network for Broadband Access . . . . . . . 9
4.3. Use Satellite Network with 3GPP Wireless Access
Network . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Recent Development and Study in 3GPP for Satellite
Network . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Problems and Requirements for Satellite Constellation for
Internet . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Common Problems and Requirements . . . . . . . . . . . . 15
5.2. Satellite Relay . . . . . . . . . . . . . . . . . . . . . 17
5.2.1. One Satellite Relay . . . . . . . . . . . . . . . . . 17
5.2.2. Multiple Satellite Relay . . . . . . . . . . . . . . 18
5.3. Satellite Networking by ISL . . . . . . . . . . . . . . . 20
5.3.1. L2 or L3 network . . . . . . . . . . . . . . . . . . 20
5.3.2. Inter-satellite-Link Lifetime . . . . . . . . . . . . 20
5.3.3. Problems for Traditional Routing Technologies . . . . 21
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Basics of Satellite Constellation . . . . . . . . . 29
A.1. Satellite Orbit . . . . . . . . . . . . . . . . . . . . . 29
A.2. Coverage of LEO and VLEO Satellites and Minimum Number
Required . . . . . . . . . . . . . . . . . . . . . . . . 29
A.3. Real Deployment of LEO and VLEO for Satellite Network . . 32
Appendix B. Communications for Satellite Constellation . . . . . 33
B.1. Dynamic Ground-station-Satellite Communication . . . . . 34
B.2. Dynamic Inter-satellite Communication . . . . . . . . . . 35
B.2.1. Inter-satellite Communication Overview . . . . . . . 35
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B.2.2. Satellites on Adjacent Orbit Planes with Same
Altitude . . . . . . . . . . . . . . . . . . . . . . 38
B.2.3. Satellites on Adjacent Orbit Planes with Different
Altitude . . . . . . . . . . . . . . . . . . . . . . 40
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
Satellite constellation for Internet is emerging. Even there is no
constellation network established completely yet at the time of the
publishing of the draft (June 2021), some basic internet service has
been provided and has demonstrated competitive quality to traditional
broadband service.
This memo will analyze the challenges for satellite network used in
Internet by traditional routing and switching technologies. It is
based on the analysis of the dynamic characters of both ground-
station-to-satellite and inter-satellite communications and its
impact to satellite constellation networking.
The memo also provides visions for the future solution, such as in
routing and forwarding.
The memo focuses on the topics about how the satellite network can
work with Internet. It does not focus on physical layer technologies
(wireless, spectrum, laser, mobility, etc.) for satellite
communication.
2. Terminology
LEO Low Earth Orbit with the altitude from 180 km to
2000 km.
VLEO Very Low Earth Orbit with the altitude below 450 km
MEO Medium Earth Orbit with the altitude from 2000 km
to 35786 km
GEO Geosynchronous orbit with the altitude 35786 km
GSO Geosynchronous satellite on GEO
ISL Inter Satellite Link
ISLL Inter Satellite Laser Link
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3GPP 3rd Generation Partner-ship Project
NTN Non-Terrestrial Network, it includes satellite
networks (satellite could be on GEO, MEO, LEO or
VLEO), high altitude platform systems (HAPS) and
other types of air-to-ground networks
EIRP Effective isotropic radiated power
P2MP Point to Multiple Points
GS Ground Station, a device on ground connecting the
satellite. In the document, GS will hypothetically
provide L2 and/or L3 functionality in addition to
process/send/receive radio wave. It might be
different as the reality that the device to
process/send/receive radio wave and the device to
provide L2 and/or L3 functionality could be
separated.
SGS Source ground station. For a specified flow, a
ground station that will send data to a satellite
through its uplink.
DGS Destination ground station. For a specified flow,
a ground station that is connected to a local
network or Internet, it will receive data from a
satellite through its downlink and then forward to
a local network or Internet.
PGW Packet Gateway
UPF User Packet Function
NodeB The base station in 3G
eNodeB The base station in 4G
gNB gNodeB, the base station in 5G
PE router Provider Edge router
CE router Customer Edge router
P router Provider router
LSA Link-state advertisement
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LSP Link-State PDUs
L1 Layer 1, or Physical Layer in OSI model [OSI-Model]
L2 Layer 2, or Data Link Layer in OSI model
[OSI-Model]
L3 Layer 3, or Network Layer in OSI model [OSI-Model],
it is also called IP layer in TCP/IP model
BGP Border Gateway Protocol [RFC4271]
eBGP External Border Gateway Protocol, two BGP peers
have different Autonomous Number
iBGP Internal Border Gateway Protocol, two BGP peers
have same Autonomous Number
IGP Interior gateway protocol, examples of IGPs include
Open Shortest Path First (OSPF [RFC2328]), Routing
Information Protocol (RIP [RFC2453]), Intermediate
System to Intermediate System (IS-IS [RFC7142]) and
Enhanced Interior Gateway Routing Protocol (EIGRP
[RFC7868]).
3. Overview
The traditional satellite communication system is composed of few GSO
and ground stations. For this system, each GSO can cover 42% Earth's
surface [GEO-Coverage], so as few as three GSO can provide the global
coverage theoretically. With so huge coverage, GSO only needs to
amplify signals received from uplink of one ground station and relay
to the downlink of another ground station. There is no inter-
satellite communications needed. Also, since the GSO is stationary
to the ground station, there is no mobility issue involved.
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Recently, more and more LEO and VLEO satellites have been launched,
they attract attentions due to their advantages over GSO and MEO in
terms of higher bandwidth, lower cost in satellite, launching, ground
station, etc. Some organizations [ITU-6G][Surrey-6G][Nttdocomo-6G]
have proposed the non-terrestrial network using LEO, VLEO as
important parts for 6G to extend the coverage of Internet. 3GPP has
been working on the NTN integration with 5G and beyond. SpaceX has
started to build the satellite constellation called StarLink that
will deploy over 10 thousand LEO and VLEO satellites finally
[StarLink]. China also started to request the spectrum from ITU to
establish a constellation that has 12992 satellites
[China-constellation]. European Space Agency (ESA) has proposed
"Fiber in the sky" initiative to connect satellites with fiber
network on Earth [ESA-HydRON].
When satellites on MEO, LEO and VLEO are deployed, the communication
problem becomes more complicated than for GSO satellites. This is
because the altitude of MEO/LEO/VLEO satellites are much lower. As a
result, the coverage of each satellite is much smaller than for GSO,
and the satellite is moving very fast on the ground reference and not
relatively stationary to the ground. This will lead to:
1. More satellites than GSO are needed to provide the global
coverage. Appendix A will brief the satellite orbit parameters;
analyze the coverage area, and the minimum number of satellites
required to cover the earth surface; discuss the real deployment
for LEO satellite network.
2. The point-to-point communication between satellite and ground
station can only last a few minutes. Mobility issue has to be
considered. Detailed analysis about the lifetime of
communication is done in Appendix B.1.
3. The inter-satellite communication is needed, and all satellites
need to form a network. details are described in Appendix B.2
that includes the communication between satellites on different
orbit and different geographic areas.
In Section 4, we will discuss couple of topics of satellite network
integration with Internet, such as using satellite network for
broadband access and wireless access, the current 3GPP works for
satellite network in 5G and beyond.
Finally, the problems and requirements for satellite network
integration with Internet will be discussed and analyzed in
Section 5.
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As the 1st satellite constellation company in history, the SpaceX/
StarLink will be inevitably mentioned in the draft. But it must be
noted that all information about SpaceX/StarLink in the draft are
from the public. Authors of the draft have no relationship or
relevant inside knowledge of SpaceX/Starlink.
4. Satellite Network Integrated with Internet
Since there is no complete satellite network established yet, all
following analysis is based on the predictions from the traditional
GEO communication. The analysis also learnt how other type of
network has been used in Internet, such as Broadband access network,
Mobile access network, Enterprise network and Service Provider
network.
To integrate the satellite network with Internet, many other
technologies are needed to provide the functions on different
layeres. Currently, there are four major international SDO (standard
organizations) involved in the development of different technologies:
IETF, 3GPP, CCSDS (Consultative Committee for Space Data Systems
[CCSDS]), DVB (Digital Video Broadcast [DVB]). Section 4.1 will
discuss the different protocol stacks based on different combinations
of technologies from different SDOs.
As a criteria to be part of Internet, any device connected to any
satellite should be able to communicate with any public IP4 or IPv6
address in Internet. There could be three types of methods to
deliver IP packet from source to destination by satellite:
1. Data packet is relayed between ground station and satellite.
For this method, there is no inter-satellite communication and
networking. Data packet is bounced once or couple times between
ground stations and satellites until the packet arrives at the
destination in Internet.
2. Data packet is delivered by inter-satellite networking.
For this method, the data packet traverses with multiple
satellites connected by ISL and inter-satellite networking is
used to deliver the packet to the destination in Internet.
3. Both satellite relay and inter-satellite networking are used.
For this method, the data packet is relayed in some segments and
traverse with multiple satellites in other segments. It is a
combination of the method 1 and method 2.
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Using the above methods for IP packet delivery via satellite network,
we will have two typical use cases for satellite network. One is for
the general broadband access (see Section 4.2), another is for the
integration with 3GPP wireless network including 4G and 5G (see
Section 4.3 and Section 4.4).
It must be noted that we use 3GPP as an use case example does not
mean other technologies cannot be used, e.g., using DVB instead of
3GPP for the satellite network integration. We use 3GPP is because
3GPP has done the most thorough research and produced lots of
solutions for satellite networking, such as using 5G NR for satellite
up link and down link, use transparent pay load and regenerative pay
load for different scenarios, etc. (see Section 4.3).
4.1. Protocol Stack for Satellite Networking with Different
Technologies
Figure 1 illustrates three typical protocol statcks that use
different technology combinations. This does not include the use of
private technologies for wireless and Link Layer such as Starlink.
The stacks show obviously that TCP/IP is the common technologies,
IETF has to (at least) provide the L3 and L4 technologies for
satellite networking integrated with Internet.
+-------------------+ +--------------------+ +-------------------+
|5G upper layers(SA)| | CCSDS upper layers | | DVB upper layers |
+-------------------+ +--------------------+ +-------------------+
| TCP/UDP | | TCP/UDP | | TCP/UDP |
| IP | | IP | | IP |
+-------------------+ +--------------------+ +-------------------+
| SDAP | | IPoC | | IPoGSE |
| PDCP | | | | |
| RLC | | USLP | | DVB MPE/GSE |
| MAC | | | | |
+-------------------+ +--------------------+ +-------------------+
| 3GPP RF; | | CCSDS RF; | | DVB S2/S2X/T2; |
| 3GPP Spectrum;FSO | | CCSDS Spectrum;FSO | | DVB Spectrum; FSO |
+-------------------+ +--------------------+ +-------------------+
3GPP Stacks CCSDS Statks DVB Stacks
Figure 1: Protocol Statcks for Different Standard Technologies
Some meaning of symbols in Figure 1 are as follows:
SA Service Architecture
SDAP SService Data Adaption Protocol
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PDCP Packet Data Convergence Protocol
RLC Radio Link Control
MAC Medium Access Control
RF Radio Frequency
FSO Free Space Optics, provided by ITU
IPoC IP over CCSDS
USLP Unified Space Link Protocol
IPoGSE IP over Generic Stream Encapsulation
MPE Multiprotocol Encapsulation
GSE Generic Stream Encapsulation
S2 Digital Video Broadcasting Satellite Second
Generation
S2X Digital Video Broadcasting Satellite Second
Generation Extension
T2 Digital Video Broadcasting Satellite Second
Generation Terrestrial
4.2. Use Satellite Network for Broadband Access
For this use case, the end user terminal or local network is
connected to a ground station, and another ground station is
connected to Internet. Two ground stations will have IP connectivity
via a satellite network. The satellite network could be by satellite
relays or by inter-satellite network.
Follows are typical deployment scenarios that a Satellite network is
used for broadband access of Internet.
1. The end user terminal access Internet through satellite relay
(Figure 2 for one satellite relay, Figure 3 for multiple
satellite relay).
2. The end user terminal access Internet through inter-satellite-
networking
(Figure 4).
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3. The local network access Internet through satellite relay
(Figure 5 for one satellite relay, Figure 6 for multiple
satellite relay).
4. The local network access Internet through inter-satellite-
networking
(Figure 7).
S1----\ /-----------\
/ \ / \
T---GW--GS1--S2--GS2-------PE Internet +
\ / \ /
\---S3/ \-----------/
Figure 2: End user terminal access Internet through one satellite
relay
S1----\ S4----\ /-----------\
/ \ / \ / \
T---GW--GS1--S2--GS2---S5--GS3---PE Internet +
\ / \ / \ /
\---S3/ \---S6/ \-----------/
Figure 3: End user terminal access Internet through multiple
satellite relay
S1-----S2-----S3--\ /----------\
/ \ / \
T---GW--GS1--S4----S5---S6---GS2-------PE Internet +
\ / \ /
\---S7----S8----S9/ \----------/
Figure 4: End user terminal access Internet through inter-
satellite-networking
/-----------\ S1----\ /-------\
/ \ / \ / \
+ Local network CE------GS1--S4--GS2-------PE Internet +
\ / \ / \ /
\-----------/ \---S7/ \-------/
Figure 5: Local network access Internet through one satellite relay
/-----------\ S1----\ S4----\ /-------\
/ \ / \ / \ / \
+ Local network CE----GS1--S2--GS2--S5--GS3---PE Internet +
\ / \ / \ / \ /
\-----------/ \---S3/ \---S6/ \-------/
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Figure 6: Local network access Internet through multiple
satellite relay
/-----------\ S1-----S2-----S3---\ /------\
/ \ / \ / \
+ Local network CE------GS1--S4----S5---S6---GS2-------PE Internet+
\ / \ / \ /
\-----------/ \---S7----S8----S9/ \------/
Figure 7: Local network access Internet through inter-satellite-
networking
In above Figure 2 to Figure 7, the meaning of symbols are as follows:
T The end user terminal
GW Gateway router
GS1, GS2, GS3 Ground station with L2/L3 routing/switch
functionality.
S1 to S9 Satellites
PE Provider Edge Router
CE Customer Edge Router
The above configuration may have different variations, e.g., the GW
and GS functions can be merged into one ohysical devices.
4.3. Use Satellite Network with 3GPP Wireless Access Network
For this use case, the wireless access network (4G, 5G) defined in
3GPP is used with satellite network. By such integration, a user
terminal or local network can access Internet via 3GPP wireless
network and satellite network. The End user terminal or local
network access Internet through satellite network and Mobile Access
Network. There are two cases: 1) From mobile access network to
satellite network or 2) From satellite network to mobile access
network, Satellite network includes inter satellite network and relay
network. See Figure 8 for mobile access network to satellite
network, and Figure 9 for satellite network to mobile access network.
+--------------+ +-------------+ +---------+ +--------+
| T or | |Mobile Access| |Satellite| |Internet|
| Local network+----+ Network +----+ Network +----+ |
+--------------+ +-------------+ +---------+ +--------+
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Figure 8: End user terminal or local network access Internet
through Mobile Access Network and Satellite Network
+--------------+ +---------+ +-------------+ +--------+
| T or | |Satellite| |Mobile Access| |Internet|
| Local network+---+ Network +---+ Network +---+ |
+--------------+ +---------+ +-------------+ +--------+
Figure 9: End user terminal or local network access Internet
through Satellite Network and Mobile Access Network
4.4. Recent Development and Study in 3GPP for Satellite Network
3GPP SA Working Groups (WG) feature a couple of satellite-related
projects (or SIDs).
For Release 18, 3GPP has finished the project 'Study on 5G System
with Satellite Backhaul' [TR-23.700-27] and 'Study on 5GC enhancement
for satellite access Phase 2' [TR-23.700-28].
For Release 19, 3GPP will study more topics for satellite network
used for 5G system, such as Regenerative payload generic architecture
study, Store and Forward Satellite operation, etc.
One key aspect is to investigate the potential architecture
requirements and enhancements to deploy UPFs on satellites (LEO/MEO/
GEO) with gNBs on the ground. Specifically, it targets at enhancing
the local-switching capability for UE-to-UE data communication when
UEs are served by UPFs on-board satellite(s). Similarly, the SA1 WG
proposed a new satellite-based SID in which the service end points
(could also be called UEs in a broader sense) may continuously move
in a fast way. The UEs can be ships, boats, and cars, etc., which
are located in remote regions that need the connection to LEO's for
achieving communication.
In all the SIDs, satellite based backhaul is important for mission
critical scenarios in remote areas. Here, we want to clarify that
while 3GPP documents TS 23.501 [TS-23.501] and 23.502 [TS-23.502]
specify that a ground base station, i.e., gNB, may have multiple
types of satellite backhauls (BH), e.g., GEO BH, LEO BH and LEO-BH
with ISL, this use case focuses specifically on the LEO-BH with ISL.
ISL stands for inter-satellite link.
Clearly, when a satellite backhaul involves multi-hop ISL path
connected via different satellites, the capabilities provided by the
satellite path would be changed and adjusted dynamically. For
example, in the LEO case, the peering relationship between two
neighboring satellites changes roughly every 5 minutes thanks to the
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orbital movement (see Table 2). This will definitely impair the
networking performance and stability, and, in worst case, may cause
the loss of connectivity. Even if some overlay tunneling mechanisms
could be used to address the multi-hop ISL issue, the extra delay and
potentially less bandwidth as introduced naturally by the ever-
changing backhaul path would still impact the traffic engineering
over the links.
The following diagram Figure 10 demonstrate the dynamic
characteristics of satellite backhaul between two UEs. In the
figure, UEs are connected, via gNBs, to UPFs on-board satellites.
Both UPFs are connected via multi-hop ISLs to the 5G core (5GC) on
the ground. There are two different multi-hop ISL paths: o A UE has
to rely on a multi-hop ISL path to connect to 5GC on the ground. o
When two UEs intend to communicate via the local data switching on
satellite(s), some new ISL-based peering has to be established which
would bring in the multi-hop ISL scenario. For example, the ISL
between the Sat#1 and Sat#2 helps form a multi-hop path (marked N19
in the diagram) between the two UEs. Note that if the UPF-based
local data switching involves only one UPF, then it is designated as
intra-UPF local switching and relatively simpler. This is compared
to the case of inter-UPF local switching as shown in the diagram.
Sat#: Satellite GS: Ground Station
UPF: User Plane Function (5G) gNB: Next Generation NodeB
Sat#1
+----------+ +--------+
| UPF#1 | (ISL) | Sat#.. |
UE#1--gNB#1--|(on-board)|- - - -| |-----+
+----------+ +--------+ |
: | --------
:(N19) v / \
:(ISL) GS ---+ 5G Core +
: ^ \ /
+----------+ +--------+ | --------
| UPF#2 | (ISL) | Sat#.. | |
UE#2--gNB#2--|(on-board)|- - - -| |-----+
+----------+ +--------+
Sat#2
Figure 10: Use Satellite network as back haul for 5G
In this diagram, both UEs are served by different satellite
backhauls. If the local data switching via LEO UPFs on-board could
be established (via the N19 ISL forwarding), then the system
efficiency and QoE improvement would be achieved. Here, since UEs
are served by different satellites, a multi-hop ISL scenario must be
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supported. But, this scenario posts challenges due to the dynamic
satellite network topology and distinguished transmission
capabilities from different satellites.
For example, if the UE-to-UE session has to maintain a service over
longer time (> 5 minutes) such that the Sat#1 and Sat#2 move apart,
then a new ISL path with potentially a new N19-ISL might be
established. In worst case, if newly-involved satellites in the path
happen to be polar-orbit ones and they do not support cross-seam
ISLs, the communication latency may change dramatically when cross-
seam transits or leaves. In another example, if both UEs belong to
the same entity and need to form a 5G-VN group, then the 5G LAN-type
service with PSA UPF-based local-switching must be applied among
them.
Regardless, more efficient satellite communication mechanisms must be
adopted, e.g., running efficient satellite-based routing protocols,
establishing tunnels between LEO UPFs on-board, etc., for better
local-data switching.
Further, 5GS may collaborate with satellite networks to improve QoS.
One 5GC NF (i.e., SMF) can initiate UP path monitoring, and
accordingly receive UP path monitoring results indicating observed
delay. After that, the SMF takes corresponding actions like further
verifying network statistics, updating sessions, etc. The
coordination with the satellite networks would improve the process,
which suggests satellites networks respond better to the (monitor-
based) polling from 5GS.
One more thing we want to point out is that, while the propagation
delay of satellite backhaul paths may change dramatically with the
movement of satellite, this kind of change normally be periodic and
can be well predicated based on the operation information of
satellite constellation. Thus, making use of these information would
also help for better services.
5. Problems and Requirements for Satellite Constellation for Internet
As described in Section 4, satellites in a satellite constellation
can either relay internet traffic or multiple satellites can form a
network to deliver internet traffic. More detailed analysis are in
following sub sections. There might have multiple solutions for each
method described in Section 4, following contexts only discuss the
most plausible solution from networking perspectives.
Section 5.1 will list the common problems and requirements for both
satellite relay and satellite networking.
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Section 5.2 and Section 5.3 will describe key problems, requirement
and potential solution from the networking perspective for these two
cases respectively.
5.1. Common Problems and Requirements
For both satellite relay and satellite networking, satellite-ground-
station communication must be used, so, the problems and requirements
for the satellite-ground-station communication is common and will
apply for both methods.
When one satellite is communicating with ground station, the
satellite only needs to receive data from uplink of one ground
station, process it and then send to the downlink of another ground
station. Figure 2 illustrates this case. Normally microwave is used
for both links.
Additionally, from the coverage analysis in Appendix A.2 and real
deployment in Appendix A.3, we can see one ground station may
communicate with multiple satellites. Similarly, one satellite may
communicate with multiple ground stations. The characters for
satellite-ground-station communication are:
1. Satellite-ground-station communication is P2MP.
Since microwave physically is the carrier of broadcast
communication, one satellite can send data while multiple ground
stations can receive it. Similarly, one ground station can send
data and multiple satellites can receive it.
2. Satellite-ground-station communication is in open space and
not secure.
Since electromagnetic fields for microwave physically are
propagating in open space. The satellite-ground-station
communication is also in open space. It is not secure naturally.
3. Satellite-ground-station communication is not steady.
Since the satellite is moving with high speed, from Appendix B.1,
the satellite-ground-station communication can only last a
certain period of time. The communication peers will keep
changing.
4. Satellite-to-Satellite communication is not steady.
For some satellites, even they are in the same altitude and move
in the same speed, but they move in the opposite direction, from
Appendix B.2.2, the satellite-to-satellite communication can only
last a certain period of time. The communication peers will keep
changing.
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5. Satellite-to-Satellite distance is not steady.
For satellites with the same altitude and same moving direction,
even their relative position is steady, but the distance between
satellites are not steady. This will lead to the inter-
satellite-communication's bandwidth and latency keep changing.
6. Satellite physical resource is limited.
Due to the weight, complexity and cost constraint, the physical
resource on a satellite, such as power supply, memory, link
speed, are limited. It cannot be compared with the similar
device on ground. The design and technology used should consider
these factors and take the appropriate approach if possible.
The requirements of satellite-ground-station communication are:
R1. The bi-directional communication capability
Both satellites and ground stations have the bi-directional
communication capability
R2. The identifier for satellites and ground stations
Satellites and ground stations should have Ethernet and/or IP
address configured for the device and each link. More detailed
address configuration can be seen in each solution.
R3. The capability to decide where the IP packet is forwarded to.
In order to send Internet traffic or IP date to destination
correctly, satellites and ground station must have Ethernet hub
or switching or IP routing capability. More detailed capability
can be seen in each solution.
R4. The protocol to establish the satellite-ground-station
communication.
For security and management purpose, the satellite-ground-station
communication is only allowed after both sides agree through a
protocol. The protocol should be able to establish a secured
channel for the communication when a new communication peer comes
up. Each ground station should be able to establish multiple
channels to communicate with multiple satellites. Similarly,
each satellite should be able to establish multiple channels to
communicate to multiple ground stations.
R5. The protocol to discover the state of communication peer.
The discover protocol is needed to detect the state of
communication peer such as peer's identity, the state of the peer
and other info of the peer. The protocol must be running
securely without leaking the discovered info.
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R6. The internet data packet is forwarded securely.
When satellite or ground station is sending the IP packet to its
peer, the packet must be relayed securely without leaking the
user data.
R7. The internet data packet is processed efficiently on
satellite
Due to the resource constraint on a satellite, the packet may
need more efficient mechanism to be processed on satellite. The
process on satellite should be very minimal and offloaded to
ground as much as possible.
5.2. Satellite Relay
One of the reasons to use satellite constellation for internet access
is it can provide shorter latency than using the fiber underground.
But using ISL for inter-satellite communication is the premise for
such benefit in latency. Since the ISL is still not mature and
adopted commercially, satellite relay is a only choice currently for
satellite constellation used for internet access. In
[UCL-Mark-Handley], detailed simulations have demonstrated better
latency than fiber network by satellite relay even the ISL is not
present.
5.2.1. One Satellite Relay
One satellite relay is the simplest method for satellite
constellation to provide Internet service. By this method, IP
traffic will be relayed by one satellite to reach the DGS and go to
Internet.
The solution option and associated requirements are:
S1. The satellite only does L1 relay or the physical signal process.
For this solution, a satellite only receives physical signal, amplify
it and broadcast to ground stations. It has no further process for
packet, such as L2 packet compositing and processing, etc. All
packet level work is done only at ground station. The requirements
for the solution are:
R1-1. SGS and BGS are configured as IP routing node. Routing
protocol is running in SGS and BGS
SGS and BGS is a IP peer for a routing protocol (IGP or BGP). SGS
will send internet traffic to DGS as next hop through satellite
uplink and downlink.
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R1-2. DGS must be connected with Internet.
DGS can process received packet from satellite and forward the
packet to the destination in Internet.
In addition to the above requirements, following problem should be
solved:
P1-1. IP continuity between two ground stations
This problem is that two ground stations are connected by one
satellite relay. Since the satellite is moving, the IP continuity
between ground stations is interrupted by satellite changing
periodically. Even though this is not killing problem from the
view point that IP service traditionally is only a best effort
service, it will benefit the service if the problem can be solved.
Different approaches may exist, such as using hands off protocols,
multipath solutions, etc.
S2. The satellite does the L2 relay or L2 packet process.
For this solution, IP packet is passing through individual satellite
as an L2 capable device. Unlike in the solution S1, satellite knows
which ground station it should send based on packet's destination MAC
address after L2 processing. The advantage of this solution over S1
is it can use narrower beam to communicate with DGS and get higher
bandwidth and better security. The requirements for the solution
are:
R2-1. Satellite must have L2 bridge or switch capability
In order to forward packet to properly, satellite should run some
L2 process such as MAC learning, MAC switching. The protocol
running on satellite must consider the fast movement of satellite
and its impact to protocol convergence, timer configuration, table
refreshment, etc.
R2-2. same as R1-1 in S1
R2-3. same as R1-2 in S1
In addition to the above requirements, the problem P1-1 for S1 should
also apply.
5.2.2. Multiple Satellite Relay
For this method, packet from SGS will be relayed through multiple
intermediate satellites and ground station until reaching a DGS.
This is more complicated than one satellite relay described in
Section 5.2.1.
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One general solution is to configure both satellites and ground-
stations as IP routing nodes, proper routing protocols are running in
this network. The routing protocol will dynamically determine
forwarding path. The obvious challenge for this solution is that all
links between satellite and ground station are not static, according
to the analysis in Appendix B.1, the lifetime of each link may last
only couple of minutes. This will result in very quick and constant
topology changes in both link state and IP adjacency, it will cause
the distributed routing algorithms may never converge. So this
solution is not feasible.
Another plausible solution is to specify path statically. The path
is composed of a serials of intermediate ground stations plus SGS and
DGS. This idea will make ground stations static and leave the
satellites dynamic. It will reduce the fluctuation of network path,
thus provide more steady service. One variant for the solution is
whether the intermediate ground stations are connected to Internet.
Separated discussion is as below:
S1. Manual configuring routing path and table
For this solution, the intermediate ground stations and DGS are
specified and configured manually during the stage of network
planning and provisioning. Following requirements apply:
R1-1. Specify a path from SGS to DGS via a list of intermediate
ground stations.
The specified DGS must be connected with internet. Other
specified intermediate ground stations does not have to
R1-2. All Ground stations are configured as IP routing node.
Static routing table on all ground stations must be pre-
configured, the next hop of routes to Internet destination in any
ground station is configured to going through uplink of satellite
to the next ground station until reaching the DGS.
R1-3. All Satellites are configured as either L1 relay or L2
relay.
The Satellite can be configured as L1 relay or L2 relay described
in S1 and S2 respectively in Section 5.2.1
In addition to the above requirements, the problem P1-1 in
Section 5.2.1 should also apply.
S2. Automatic decision by routing protocol.
This solution is only feasible after the IP continuity problem (P1-1
in Section 5.2.1) is solved. Following requirements apply:
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R2-1. All Ground stations are configured as IP routing node.
Proper routing protocols are configured as well.
The satellite link cost is configured to be lower than the ground
link. In such a way, the next hop of routes for the IP forwarding
to Internet destination in any ground station will be always going
through the uplink of satellite to the next ground station until
reaching the DGS.
R2-2. All Satellites are configured as either L1 relay or L2
relay.
The Satellite can be configured as L1 relay or L2 relay described
in S1 and S2 respectively in Section 5.2.1
In addition to the above requirements, the problem P1-1 in
Section 5.2.1 should also apply.
5.3. Satellite Networking by ISL
In the draft, satellite Network is defined as a network that
satellites are inter-connected by inter-satellite links (ISL). One
of the major difference of satellite network with the other type of
network on ground (telephone, fiber, etc.) is its topology and links
are not stationary, some new issues have to be considered and solved.
Follows are the factors that impact the satellite networking.
5.3.1. L2 or L3 network
The 1st question to answer is should the satellite network be
configured as L2 or L3 network? As analyzed in Appendix A.2 and
Appendix A.3, since there are couple of hundred or over ten thousand
satellites in a network, L2 network is not a good choice, instead, L3
or IP network is more appropriate for such scale of network.
5.3.2. Inter-satellite-Link Lifetime
If we assume the orbit is circular and ignore other trivial factors,
the satellite speed is approximately determined by the orbit altitude
as described in the Appendix B.1. The satellite orbit can determine
if the dynamic position of two satellites is within the range of the
inter-satellite communication. That is 2000km for laser
communication [Laser-communication-range] by Inter Satellite Laser
Link (ISLL).
When two satellites' orbit planes belong to the same group, or two
orbit planes share the same altitude and inclination, and when the
satellites move in the same direction, the relative positions of two
satellites are relatively stationary, and the inter-satellite
communication is steady. But when the satellites move in the
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opposite direction, the relative positions of two satellites are not
stationary, the communication lifetime is couple of minutes. The
Appendix B.2.2 has analyzed the scenario.
When two satellites' orbit planes belong to the different group, or
two orbit planes have different altitude, the relative position of
two satellite are unstable, and the inter-satellite communication is
not steady. As described in Appendix B.2, The life of communication
for two satellites depends on the following parameters of two
satellites:
1. The speed vectors.
2. The altitude difference
3. The intersection angle
From the examples shown in Table 4 to Table 7, we can see that the
lifetime of inter-satellite communication for the different group of
orbit planes are from couple of hundred seconds to about 18 hours.
This fact will impact the routing technologies used for satellite
network and will be discussed in Section 5.3.3.
5.3.3. Problems for Traditional Routing Technologies
When the satellite network is integrated with Internet by traditional
routing technologies, following provisioning and configuration (see
Figure 11) will apply:
1. The ground stations connected to local network and internet are
treated as PE router for satellite network (called PE_GS1 and
PE_GS2 in the following context), and all satellites are treated
as P router.
2. All satellites in the network and ground stations are configured
to run IGP.
3. The eBGP is configured between PE_GS and its peered network's PE
or CE.
The work on PE_GS1 are:
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* The local network routes are received at PE_GS1 from CE by eBGP.
The routes are redistributed to IGP and then IGP flood them to all
satellites. (Other more efficient methods, such as iBGP or BGP
reflectors are hard to be used, since the satellite is moving and
there is no easy way to configure a full meshed iBGP session for
all satellites, or configure one satellite as BGP reflector in
satellite network.)
* The internet routes are redistributed from IGP to eBGP running on
PE_GS1, and eBGP will advertise them to CE.
The work on PE_GS2 are:
* The Internet routes are received at PE_GS2 from PE by eBGP. The
routes are redistributed to IGP and then IGP flood them to all
satellites. (Similar as in PE_GS1, Other more efficient methods,
such as iBGP or BGP reflector cannot be used.)
* The local network routes are redistributed from IGP to eBGP
running on PE_GS2, and eBGP will advertise them to Internet.
/--------\ S1---S2----S3----\ /------\
/ \ / IGP domain \ / \
+ Local net CE--eBGP--PE_GS1---S4---S5---PE_GS2--eBGP--PE Internet +
\ / \ / \ /
\--------/ \---S6---S7---S8/ \------/
Figure 11: Local access Internet through inter-satellite-networking
Local access Internet through inter-satellite-networking
On PE-GS1, due to the fact that IGP link between PE_GS1 and satellite
is not steady; this will lead to following routing activity:
1. When one satellite is connecting with PE_GS1, the satellite and
PE_GS1 form a IGP adjacency. IGP starts to exchange the link
state update.
2. The local network routes received by eBGP in PE_GS1 from CE are
redistributed to IGP, and IGP starts to flood link state update
to all satellites.
3. Meanwhile, the Internet routes learnt from IGP in PE_GS1 will be
redistributed to eBGP. eBGP starts to advertise to CE.
4. Every satellite will update its routing table (RIB) and
forwarding table (FIB) after IGP finishes the SPF algorithm.
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5. When the satellite is disconnecting with PE-GS1, the IGP
adjacency between satellite and PE_GS1 is gone. IGP starts to
exchange the link state update.
6. The routes of local network and satellite network that were
redistributed to IGP in step 2 will be withdrawn, and IGP starts
to flood link state update to all satellites.
7. Meanwhile, the Internet routes previously redistributed to eBGP
in step 3 will also be withdrawn. eBGP starts to advertise route
withdraw to CE.
8. Every satellite will update its routing table (RIB) and
forwarding table (FIB) after the SPF algorithm.
Similarly on PE_GS2, due to the fact that IGP link between PE_GS2 and
satellite is not steady; this will lead to following routing
activity:
1. When one satellite is connecting with PE_GS2, the satellite and
PE_GS2 form a IGP adjacency. IGP starts to exchange the link
state update.
2. The Internet routes previously received by eBGP in PE_GS2 from PE
are redistributed to IGP, IGP starts to flood the new link state
update to all satellites.
3. Meanwhile, the routes of local network and satellite network
learnt from IGP in PE_GS2 will be redistributed to eBGP. eBGP
starts to advertise to Internet peer PE.
4. Every satellite will update its routing table (RIB) and
forwarding table (FIB) after IGP finishes the SPF algorithm.
5. When the satellite is disconnecting with PE-GS2, the IGP
adjacency between satellite and PE_GS2 is gone. IGP starts to
exchange the link state update.
6. The internet routes previously redistributed to IGP in step 2
will be withdrawn, and IGP starts to flood link state update to
all satellites
7. Meanwhile, the routes of local network and satellite network
previously redistributed to eBGP in step 3 will also be
withdrawn. eBGP starts to advertise route withdraw to PE.
8. Every satellite will update its routing table (RIB) and
forwarding table (FIB) after the SPF algorithm.
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For the analysis of detailed events above, the estimated time
interval between event 1 and 5 for PE_GS1 and PE_GS2 can use the
analysis in Appendix B.1. For example, it is about 398s for LEO and
103s for VLEO. Within this time interval, the satellite network
including all satellites and two ground stations must finish the
works from 1 to 4 for PE_GS1 and PE_GS2. The normal internet IPv6
and IPv4 BGP routes size are about 850k v4 routes + 100K v6 routes
[BGP-Table-Size]. There are couple critical problems associated with
the events:
P1. Frequent IGP update for its link cost
Even for satellites in different orbit with the steady relative
positions, the distance between satellites is keep changing. If
the distance is used as the link cost, it means the IGP has to
update the link cost frequently. This will make IGP keep running
and update its routing table.
P2. Frequent IGP flooding for the internet routes
Whenever the IGP adjacency changes (step 1 and 5 for PE_GS2), it
will trigger the massive IGP flooding for the link state update
for massive internet routes learnt from eBGP. This will result in
the IGP re-convergency, RIB and FIP update.
P3. Frequent BGP advertisement for the internet routes
Whenever the IGP adjacency changes (step 3 and 7 for PE_GS1), it
will trigger the massive BGP advertisement for the internet routes
learnt from IGP. This will result in the BGP re-convergency, RIB
and FIB update. BGP convergency time is longer than IGP. The
document [BGP-Converge-Time1] has shown that the BGP convergence
time varies from 50sec to couple of hundred seconds. The analysis
[BGP-Converge-Time2] indicated that per entry update takes about
150us, and it takes o(75s) for 500k routes, or o(150s) for 1M
routes.
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P4. More frequent IGP flooding and BGP update in whole satellite
network
To provide the global coverage, a satellite constellation will
have many ground stations deployed. For example, StarLink has
applied for the license for up to one million ground stations
[StarLink-Ground-Station-Fcc], in which, more than 50 gateway
ground stations (equivalent to the PE_GS2) have been registered
[SpaceX-Ground-Station-Fcc] and deployed in U.S.
[StarLink-GW-GS-map]. It is expected that the gateway ground
station will grow quickly to couple of thousands
[Tech-Comparison-LEOs]. This means almost each satellite in the
satellite network would have a ground station connected. , Due to
the fact that all satellites are moving, many IGP adjacency
changes may occur in a shorter period of time described in
Appendix B.1 and result in the problem P1 and P2 constantly occur.
P5. Service is not steady
Due to the problems P1 to P3, the service provider of satellite
constellation is hard to provide a steady service for broadband
service by using inter-satellite network and traditional routing
technologies.
As a summary, the traditional routing technology is problematic for
large scale inter-satellite networking for Internet. Enhancements on
traditional technologies, or new technologies are expected to solve
the specific issues associated with satellite networking.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
Security considerations for communication between satellite and
ground station, or between satellites are described in corresponding
sections. There is no extra security issue introduced by this memo.
8. Contributors
9. Acknowledgements
10. References
10.1. Normative References
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[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC7142] Shand, M. and L. Ginsberg, "Reclassification of RFC 1142
to Historic", RFC 7142, DOI 10.17487/RFC7142, February
2014, <https://www.rfc-editor.org/info/rfc7142>.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
DOI 10.17487/RFC2453, November 1998,
<https://www.rfc-editor.org/info/rfc2453>.
[RFC7868] Savage, D., Ng, J., Moore, S., Slice, D., Paluch, P., and
R. White, "Cisco's Enhanced Interior Gateway Routing
Protocol (EIGRP)", RFC 7868, DOI 10.17487/RFC7868, May
2016, <https://www.rfc-editor.org/info/rfc7868>.
10.2. Informative References
[KeplerianElement]
"Keplerian elements",
<https://en.wikipedia.org/wiki/Orbital_elements>.
[GEO-Coverage]
"Coverage of a geostationary satellite at Earth",
<https://www.planetary.org/space-images/coverage-of-
a-geostationary>.
[Nttdocomo-6G]
"NTTDPCOM 6G White Paper",
<https://www.nttdocomo.co.jp/english/binary/pdf/corporate/
technology/whitepaper_6g/
DOCOMO_6G_White_PaperEN_20200124.pdf>.
[ITU-6G] "ITU 6G vision", <https://www.itu.int/dms_pub/itu-
s/opb/itujnl/S-ITUJNL-JFETF.V1I1-2020-P09-PDF-E.pdf>.
[Surrey-6G]
"Surrey 6G vision",
<https://www.surrey.ac.uk/sites/default/files/2020-11/6g-
wireless-a-new-strategic-vision-paper.pdf>.
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[OSI-Model]
"OSI Model", <https://en.wikipedia.org/wiki/OSI_model>.
[StarLink] "Star Link", <https://en.wikipedia.org/wiki/Starlink>.
[China-constellation]
"China Constellation", <https://www.itu.int/ITU-
R/space/asreceived/Publication/DisplayPublication/23706>.
[ESA-HydRON]
"HydRON: Fiber in the sky",
<https://www.esa.int/ESA_Multimedia/Videos/2021/04/
HydRON_Fibre_in_the_sky>.
[SpaceX-Non-GEO]
"FCC report: SPACEX V-BAND NON-GEOSTATIONARY SATELLITE
SYSTEM", <https://fcc.report/IBFS/SAT-LOA-
20170301-00027/1190019.pdf>.
[Satellite-coverage]
Alan R.Washburn, Department of Operations Research, Naval
Postgraduate School, "Earth Coverage by Satellites in
Circular Orbit",
<https://faculty.nps.edu/awashburn/Files/Notes/
EARTHCOV.pdf>.
[Microwave-vs-Laser-communication]
International Journal for Research in Applied Science and
Engineering Technology (IJRASET), "Comparison of Microwave
and Optical Wireless Inter-Satellite Links",
<https://www.ijraset.com/fileserve.php?FID=7815>.
[Laser-communication-range]
"Interferometric optical communications can potentially
lead to robust, secure, and naturally encrypted long-
distance laser communications in space by taking advantage
of the underlying physics of quantum entanglement.",
<https://www.laserfocusworld.com/optics/article/16551652/
interferometry-quantum-entanglement-physics-secures-
spacetospace-interferometric-communications>.
[BGP-Table-Size]
"BGP in 2020 - BGP table",
<https://blog.apnic.net/2021/01/05/bgp-in-2020-the-bgp-
table/>.
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[BGP-Converge-Time1]
"BGP in 2020 - BGP Update Churn",
<https://labs.apnic.net/?p=1397>.
[BGP-Converge-Time2]
"Bringing SDN to the Internet, one exchange point at the
time",
<https://www.cs.princeton.edu/courses/archive/fall14/
cos561/docs/SDX.pdf>.
[StarLink-Ground-Station-Fcc]
"APPLICATION FOR BLANKET LICENSED EARTH STATIONS",
<https://fcc.report/IBFS/SES-LIC-INTR2019-00217/1616678>.
[SpaceX-Ground-Station-Fcc]
"List of SpaceX applications for ground stations",
<https://fcc.report/IBFS/Company/Space-Exploration-
Technologies-Corp-SpaceX>.
[Tech-Comparison-LEOs]
"A Technical Comparison of Three Low Earth Orbit Satellite
Constellation Systems to Provide Global Broadband",
<http://www.mit.edu/~portillo/files/Comparison-LEO-IAC-
2018-slides.pdf>.
[StarLink-GW-GS-map]
"StarLink gateway ground station map",
<https://www.google.com/maps/d/u/0/
viewer?mid=1H1x8jZs8vfjy60TvKgpbYs_grargieVw>.
[UCL-Mark-Handley]
"Using ground relays for low-latency wide-area routing in
megaconstellations",
<https://discovery.ucl.ac.uk/id/eprint/10090242/1/hotnets-
ucl.pdf>.
[TR-23.700-27]
3GPP, "Study on support of satellite backhauling in 5GS",
December 2022.
[TR-23.700-28]
3GPP, "Study on 5GC enhancement for satellite access Phase
2", December 2022.
[TS-23.501]
3GPP, "System architecture for the 5G System (5GS)",
December 2021.
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[TS-23.502]
3GPP, "Procedures for the 5G System (5GS)", December 2021.
[CCSDS] CCSDS, "Consultative Committee for Space Data Systems",
<https://public.ccsds.org/default.aspx>.
[DVB] DVB, "Digital Video Broadcasting",
<https://en.wikipedia.org/wiki/DVB>.
Appendix A. Basics of Satellite Constellation
This section will introduce some basics for satellite such as orbit
parameters, coverage estimation, minimum number of satellite and
orbit plane required, real deployments.
A.1. Satellite Orbit
The orbit of a satellite can be either circular or ecliptic, it can
be described by following Keplerian elements [KeplerianElement]:
1. Inclination (i)
2. Longitude of the ascending node (Omega)
3. Eccentricity (e)
4. Semimajor axis (a)
5. Argument of periapsis (omega)
6. True anomaly (nu)
For a circular orbit, two parameters, Inclination and Longitude of
the ascending node, will be enough to describe the orbit.
A.2. Coverage of LEO and VLEO Satellites and Minimum Number Required
The coverage of a satellite is determined by many physical factors,
such as spectrum, transmitter power, the antenna size, the altitude
of satellite, the air condition, the sensitivity of receiver, etc.
EIRP could be used to measure the real power distribution for
coverage. It is not deterministic due to too many variants in a real
environment. The alternative method is to use the minimum elevation
angle from user terminals or gateways to a satellite. This is easier
and more deterministic. [SpaceX-Non-GEO] has suggested originally
the minimum elevation angle of 35 degrees and deduced the radius of
the coverage area is about 435km and 1230km for VLEO (altitude
335.9km) and LEO (altitude 1150km) respectively. The details about
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how the coverage is calculated from the satellite elevation angle can
be found in [Satellite-coverage].
Using this method to estimate the coverage, we can also estimate the
minimum number of satellites required to cover the earth surface.
It must be noted, SpaceX has recently reduced the required minimum
elevation angle from 35 degrees to 25 degrees. The following
analysis still use 35 degrees.
Assume there is multiple orbit planes with the equal angular interval
across the earth surface (The Longitude of the ascending node for
sequential orbit plane is increasing with a same angular interval).
Each orbit plane will have:
1. The same altitude.
2. The same inclination of 90 degree.
3. The same number of satellites.
With such deployment, all orbit planes will meet at north and south
pole. The density of satellite is not equal. Satellite is more
dense in the space above the polar area than in the space above the
equator area. Below estimations are made in the worst covered area,
or the area of equator where the satellite density is the minimum.
Figure 12 illustrates the coverage area on equator area, and each
satellite will cover one hexagon area. The figure is based on plane
geometry instead of spherical geometry for simplification, so, the
orbit is parallel approximately.
Figure 13 shows how to calculate the radius (Rc) of coverage area
from the satellite altitude (As) and the elevation angle (b).
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x
| |
| |
x *********
| * | *
| * | *
********* x *
* | * | *
* | * | *
* x *********
* | * |
* | * |
********* x
| |
| orbit 2 ^ north
x |
| |
| |
orbit 1 +-------> east
Figure 12: Satellite coverage on ground
|<--- 2*Rc --->|
+ Satellite
/|
/ |
/ |
/ b |
/-\ +
/ * | __Earth surface
/ * | /
/ *_----+----__
+ +
* *
* *
* 2*a *
* ___ *
*- -*
* *
* *
* Earth center
Figure 13: Satellite coverage estimation
x The vertical projection of satellite to Earth
Re The radius of the Earth, Re=6378(km)
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As The altitude of a satellite
Rc The radius (arc length) of the coverage, or, the arc length of
hexagon center to its 6 vertices. Rc=Re*(a*pi)/180
a The cap angle for the coverage area (the RC arc). a =
arccos((Re/(Re+As))*cos(b))-b.
b The least elevation angle that a ground station or a terminal can
communicate with a satellite, b = 35 degree.
Ns The minimum number of satellites on one orbit plane, it is equal
to the number of the satellite's vertical projection on Earth,
so, Ns = 180/(a*cos(30))
No The minimum number of orbit (with same inclination), it is equal
to the number of the satellite orbit's vertical projection, so,
No = 360/(a*(1+sin(30)))
For a example of two type of satellite LEO and VEO, the coverages are
calculated as in Table 1:
+============+=======+=======+========+========+
| Parameters | VLEO1 | VLEO2 | LEO1 | LEO2 |
+============+=======+=======+========+========+
| As(km) | 335.9 | 450 | 1100 | 1150 |
+------------+-------+-------+--------+--------+
| a(degree) | 3.907 | 5.078 | 10.681 | 11.051 |
+------------+-------+-------+--------+--------+
| Rc(km) | 435 | 565 | 1189 | 1230 |
+------------+-------+-------+--------+--------+
| Ns | 54 | 41 | 20 | 19 |
+------------+-------+-------+--------+--------+
| No | 62 | 48 | 23 | 22 |
+------------+-------+-------+--------+--------+
Table 1: Satellite coverage estimation for
LEO and VLEO examples
A.3. Real Deployment of LEO and VLEO for Satellite Network
Obviously, the above orbit parameter setup is not optimal since the
sky in the polar areas will have the highest density of satellite.
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In the real deployment, to provide better coverage for the areas with
denser population, to get redundance and better signal quality, and
to make the satellite distance within the range of inter-satellite
communication (2000km [Laser-communication-range]), more than the
minimum number of satellites are launched. For example, different
orbit planes with different inclination/altitude are used.
Normally, all satellites are grouped by orbit planes, each group has
a number of orbit planes and each orbit plane has the same orbit
parameters, so, each orbit in the same group will have:
1. The same altitude
2. The same inclination, but the inclination is less than 90
degrees. This will result in the empty coverage for polar areas
and better coverage in other areas. See the orbit picture for
phrase 1 for [StarLink].
3. The same number of satellites
4. The same moving direction for all satellites
The proposed deployment of SpaceX can be seen in [SpaceX-Non-GEO] for
StarLink.
The China constellation deployment and orbit parameters can be seen
in [China-constellation].
Appendix B. Communications for Satellite Constellation
Unlike the communication on ground, the communication for satellite
constellation is much more complicated. There are two mobility
aspects, one is between ground-station and satellite, another is
between satellites.
In the traditional mobility communication system, only terminal is
moving, the mobile core network including base station, front haul
and back haul are static, thus an anchor point, i.e., PGW in 4G or
UPF in 5G, can be selected for the control of mobility session.
Unfortunately, when satellite constellation joins the static network
system of Internet on ground, there is no such anchor point can be
selected since the whole satellite constellation network is moving.
Another special aspect that can impact the communication is that the
fast moving speed of satellite will cause frequent changes of
communication peers and link states, this will make big challenges to
the network side for the packet routing and delivery, session control
and management, etc.
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B.1. Dynamic Ground-station-Satellite Communication
All satellites are moving and will lead to the communication between
ground station and satellite can only last a certain period of time.
This will greatly impact the technologies for the satellite
networking. Below illustrates the approximate speed and the time for
a satellite to pass through its covered area.
In Table 2, VLEO1 and LEO3 have the lowest and highest altitude
respectively, VLEO2 is for the highest altitude for VLEO. We can see
that longest communication time of ground-station-satellite is less
than 400 seconds, the longest communication time for VLEO ground-
station-satellite is less than 140 seconds.
The "longest communication time" is for the scenario that the
satellite will fly over the receiver ground station exactly above the
head, or the ground station will be on the diameter line of satellite
coverage circular area, see Figure 12.
Re The radius of the Earth, Re=6378(km)
As The altitude of a satellite
AL The arc length(in km) of two neighbor satellite on the same orbit
plane, AL=2*cos(30)*(Re+As)*(a*pi)/180
SD The space distance(in km) of two neighbor satellite on the same
orbir plane, SD=2*(Re+As)*sin(AL/(2*(Re+As))).
V the velocity (in m/s) of satellite, V=sqrt(G*M/(Re+As))
G Gravitational constant, G=6.674*10^(-11)(m^3/(kg*s^2))
M Mass of Earth, M=5.965*10^24 (kg)
T The time (in second) for a satellite to pass through its cover
area, or, the time for the station-satellite communication. T=
ALs/V
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+============+=======+========+========+========+========+
| Parameters | VLEO1 | VLEO2 | LEO1 | LEO2 | LEO3 |
+============+=======+========+========+========+========+
| As(km) | 335.9 | 450 | 1100 | 1150 | 1325 |
+------------+-------+--------+--------+--------+--------+
| a(degree) | 3.907 | 5.078 | 10.681 | 11.051 | 12.293 |
+------------+-------+--------+--------+--------+--------+
| AL(km) | 793 | 1048 | 2415 | 2515 | 2863 |
+------------+-------+--------+--------+--------+--------+
| SD(km) | 792.5 | 1047.2 | 2404 | 2503.2 | 2846.1 |
+------------+-------+--------+--------+--------+--------+
| V(km/s) | 7.7 | 7.636 | 7.296 | 7.272 | 7.189 |
+------------+-------+--------+--------+--------+--------+
| T(s) | 103 | 137 | 331 | 346 | 398 |
+------------+-------+--------+--------+--------+--------+
Table 2: The time for the ground-station-satellite
communication
B.2. Dynamic Inter-satellite Communication
B.2.1. Inter-satellite Communication Overview
In order to form a network by satellites, there must be an inter-
satellite communication. Traditionally, inter-satellite
communication uses the microwave technology, but it has following
disadvantages:
1. Bandwidth is limited and only up to 600M bps
[Microwave-vs-Laser-communication].
2. Security is a concern since the microwave beam is relatively wide
and it is easy for 3rd party to sniff or attack.
3. Big antenna size.
4. Power consumption is high.
5. High cost per bps.
Recently, laser is used for the inter-satellite communication, it has
following advantages, and will be the future for inter-satellite
communication.
1. Higher bandwidth and can be up to 10G bps
[Microwave-vs-Laser-communication].
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2. Better security since the laser beam size is much narrower than
microwave, it is harder for sniffing.
3. The size of optical lens for laser is much smaller than
microwave's antenna size.
4. Power saving compared with microwave.
5. Lower cost per bps.
The range for satellite-to-satellite communications has been
estimated to be approximately 2,000 km currently
[Laser-communication-range].
From Table 2, we can see the Space Distance (SD) for some LEO
(altitude over 1100km) are exceeding the celling of the range of
laser communication, so, the satellite and orbit density for LEO need
to be higher than the estimation values in the Table 1.
Assume the laser communication is used for inter-satellite
communication, then we can analyze the lifetime of inter-satellite
communication when satellites are moving. The Figure 14 illustrates
the movement and relative position of satellites on three orbits.
The inclination of orbit planes is 90 degrees.
+ North pole
/|\
| s |
s | s
/ s \
s | s
| s1 |
s4 | s6
| s2 | -------- Equator
s5 | s7
| s3 |
s | s
\ s /
s | s
| s |
\|/
+ South pole
Figure 14: Satellite movement
There are four scenarios:
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1. For satellites within the same orbit
The satellites in the same orbit will move to the same direction
with the same speed, thus the interval between satellites is
relatively steady. Each satellite can communicate with its front
and back neighbor satellite as long as satellite's orbit is
maintained in its life cycle. For example, in Figure 14, s2 can
communication with s1 and s3.
2. For satellites between neighbor orbits in the same group at
non-polar areas
The orbits for the same group will share the same orbit altitude
and inclination. So, the satellite speed in different orbit are
also same, but the moving direction may be same or different.
Figure 15 illustrates this scenario. When the moving direction
is the same, it is similar to the scenario 1, the relative
position of satellites in different orbit are relatively steady
as long as satellite's orbit is maintained in its life cycle.
When the moving direction is different, the relative position of
satellites in different orbit are un-steady, this scenario will
be analyzed in more details in Appendix B.2.2.
3. For satellites between neighbor orbits in the same group at
polar areas
For satellites between neighbor orbits with the same speed and
moving direction, the relative position is steady as described in
#2 above, but the steady position is only valid at areas other
than polar area. When satellites meet in the polar area, the
relative position will change dramatically. Figure 16 shows two
satellites meet in polar area and their ISL facing will be
swapped. So, if the range of laser pointing angle is 360 degrees
and tracking technology supports, the ISL will not be flipping
after passing polar area; Otherwise, the link will be flipping
and inter-satellite communication will be interrupted.
4. For satellites between different orbits in the different group
The orbits for the different group will have different orbit
altitude, inclination and speed. So, the relative position of
satellite is not static. The inter-satellite communication can
only last for a while when the distance between two satellite is
within the limit of inter-satellite communication, that is 2000km
for laser [Laser-communication-range], this scenario will be
analyzed in more details in Appendix B.2.3
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i+N/2 i+1+N/2 i+2+N/2
/ \ / \ / \
/ \ / \ / \
S1 \ S2 \ S3 \
/ S4 / S5 / S6
/ \ / \ / \
/ \ / \ / \
i-1 i i+1
* The total number of orbit planes are N
* The number (i-1, i, i+1,...) represents the Orbit index
* The bottom numbers (i-1, i, i+1) are for orbit planes on
which satellites (S1, S2, S3) are moving from bottom to up.
* The top numbers (i+N/2, i+1+N/2, i+2+N/2) are for orbit
planes on which satellites (S4, S5, S6) are moving from up
to bottom.
Figure 15: Two satellites with same altitude and inclination (i)
move in the same or opposite direction
\ /
P3 P4
\ /
\/
/\
/ \
P1 P2
/ \
* Two satellites S1 and S2 are at position P1 and P2 at time T1
* S1's right facing ISL connected to S2's left facing ISL
* S1 and S2 move to the position P4 and P3 at time T2
* S1's left facing ISL connected to S2's right facing ISL
Figure 16: Two satellites meeting in the polar area will change
its facing of ISL
B.2.2. Satellites on Adjacent Orbit Planes with Same Altitude
For satellites on different orbit planes with same altitude, the
estimation of the lifetime when two satellite can communicate are as
follows.
Figure 17 illustrates a general case that two satellites move and
intersect with an angle A.
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^ V2
/
/
+-
/ \ A
-------------+----+----> V1
/
/
Figure 17: Two satellites (speed vector V1 and V2) intersect with
angle A
More specifically, for orbit planes with the inclination angle i,
Figure 18 illustrates two satellites move in the opposite direction
and intersect with an angle 2*i.
^ move from south to north
\ /
\ /
\ /\
\/ | A = 2*i
/\ |
/ \/
/ \
/ V move from north to south
Figure 18: Two satellites with same altitude and inclination (i)
intersect with angle A=2*i
Follows are the math to calculate the lifetime of communication.
Table 3 are the results using the math for two satellites with
different altitudes and different inclination angles.
Dl The laser communication limit, Dl=2000km
[Laser-communication-range]
A The angle between two orbit's vertical projection on Earth.
A=2*i
V1 The speed vector of satellite on orbit1
V2 The speed vector of satellite on orbit2
|V| the magnitude of the difference of two speed vector V1 and
V2, |V|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2). For
satellites with the same altitude and inclination angle i, V1=V2,
so, |V|=V1*sqrt(2-2*cos(2*i))=2V1*sin(i)
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T The lifetime two satellites can communicate, or the time of two
satellites' distance is within the range of communication, T =
2*Dl/|V|.
+============+=======+=======+=======+======+=======+=======+
| i (degree) | 80 | 80 | 65 | 65 | 50 | 50 |
+============+=======+=======+=======+======+=======+=======+
| Alt (km) | 500 | 800 | 500 | 800 | 500 | 800 |
+============+=======+=======+=======+======+=======+=======+
| |V| (km/s) | 14.98 | 14.67 | 13.79 | 13.5 | 11.66 | 11.41 |
+------------+-------+-------+-------+------+-------+-------+
| T(s) | 267 | 273 | 290 | 296 | 343 | 350 |
+------------+-------+-------+-------+------+-------+-------+
Table 3: The lifetime of communication for two LEOs (with
two altitudes and three inclination angles)
B.2.3. Satellites on Adjacent Orbit Planes with Different Altitude
For satellites on different orbit planes with different altitude, the
estimation of the lifetime when two satellite can communicate are as
follows.
Figure 19 illustrates two satellites (with the altitude difference
Da) move and intersect with an angle A.
^ V2
/
/
-------+ /
Da /| +-
/ |/ \ A
----------/--+----+----> V1
/ /
/
/
/
Figure 19: Satellite (speed vector V1 and V2, Altitude difference
Da) intersects with Angle A
Follows are the math to calculate the lifetime of communication
Dl The laser communication limit, Dl=2000km
[Laser-communication-range]
Da Altitude difference (in km) for two orbit planes
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A The angle between two orbit's vertical projection on Earth
Vl The speed vector of satellite on orbit 1
V2 The speed vector of satellite on orbit 2
|V| the magnitude of the difference of two speed vector V1 and
V2, |v|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2)
T The lifetime two satellites can communicate, or the time of two
satellites' distance is within the range of communication, T =
2*sqrt(Dl^2-Da^2)/|V|
Using formulas above, below is the estimation for the life of
communication of two satellites when they intersect. Table 4 and
Table 5 are for two VLEOs with the difference of 114.1km for
altitude. (VLEO1 and VLEO2 on Table 2). Table 6 and Table 7 are for
two LEOs with the difference of 175km for altitude (LEO2 and LEO3 on
Table 2).
+============+=======+=======+
| Parameters | VLEO1 | VLEO2 |
+============+=======+=======+
| As(km) | 335.9 | 450 |
+------------+-------+-------+
| V (km/s) | 7.7 | 7.636 |
+------------+-------+-------+
Table 4: Two VLEO with
different altitude and
speed
+============+=======+=======+=======+========+========+========+
| A (degree) | 0 | 10 | 45 | 90 | 135 | 180 |
+============+=======+=======+=======+========+========+========+
| |V| (km/s) | 0.065 | 1.338 | 5.869 | 10.844 | 14.169 | 15.336 |
+------------+-------+-------+-------+--------+--------+--------+
| T(s) | 61810 | 2984 | 680 | 368 | 282 | 260 |
+------------+-------+-------+-------+--------+--------+--------+
Table 5: Two VLEO intersects with different angle and the life of
communication
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+============+=======+=======+
| Parameters | LEO1 | LEO2 |
+============+=======+=======+
| As(km) | 1150 | 1325 |
+------------+-------+-------+
| V (km/s) | 7.272 | 7.189 |
+------------+-------+-------+
Table 6: Two LEO with
different altitude and
speed
+============+=======+=======+=======+========+========+========+
| A (degree) | 0 | 10 | 45 | 90 | 135 | 180 |
+============+=======+=======+=======+========+========+========+
| |V| (km/s) | 0.083 | 1.263 | 5.535 | 10.226 | 13.360 | 14.461 |
+------------+-------+-------+-------+--------+--------+--------+
| T(s) | 47961 | 3155 | 720 | 390 | 298 | 276 |
+------------+-------+-------+-------+--------+--------+--------+
Table 7: Two LEO intersects with different angle and the life
of communication
Appendix C. Change Log
* Initial version, 07/03/2021
* 01 version, 10/20/2021
* 02 version, 2/13/2022
* 03 version, 7/5/2022
* 04 version, 1/4/2023
* 05 version, 7/5/2023
* 06 version, 1/3/2024
Authors' Addresses
Lin Han (editor)
Futurewei Technologies, Inc.
2330 Central Expy
Santa Clara, CA 95050,
United States of America
Email: lhan@futurewei.com
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Richard Li
Futurewei Technologies, Inc.
2330 Central Expy
Santa Clara, CA 95050,
United States of America
Email: rli@futurewei.com
Alvaro Retana
Futurewei Technologies, Inc.
2330 Central Expy
Santa Clara, CA 95050,
United States of America
Email: alvaro.retana@futurewei.com
Meiling Chen
China Mobile
32, Xuanwumen West
BeiJing 100053
China
Email: chenmeiling@chinamobile.com
Li Su
China Mobile
32, Xuanwumen West
BeiJing 100053
China
Email: suli@chinamobile.com
Tianji Jiang
China Mobile
1525 McCathy Blvd.
Milpitas, CA 95035,
United States of America
Email: tianjijiang@chinamobile.com
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