intersatellite link (isl) networks: topological design ...€¦ · markus werner atm-sat workshop,...

ATM-Sat Workshop, 12 December 2000, Oberpfaffenhofen KN-DNMar

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Intersatellite Link (ISL) Networks: Topological Design, Routing and

Network Dimensioning

Markus Werner

DLR Oberpfaffenhofen


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4 Why (still) ISLs ? ... and what about ISLs in ATM-Sat ?

4 Which ISL network topology ?

h from the Iridium heritage to a modern (sustainable) design !

4 “ATM-based“ ISL routing concept:

h a discrete-time dynamic routing framework derived from core ATM principles rather than an ATM implementation !

4 Integrated ISL routing and network dimensioning

4 Routing towards an ISL future:

h ATM+IP ? ↔ all IP ? ↔ MPLS ↔ optical networks . . .

4 A conclusion: Good concepts are sustainable - sustainable concepts are good!

Outline


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4 Iridium&ISLs: final desaster or lessons learnt (to learn) ?

4 Teledesic: ongoing dream or diminishing nightmare ?

4 Backbone-in-the-sky (not only “Internet-in-the-sky“) as a sustainable concept

4 A sustainable ISL networking concept will pay off one (resurrection) day !

Why (still) ISLs ?


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... and what about ISLs in ATM-Sat ?

4 We don‘t have (short-term) stock options in ISL networks

4 No “early-to-market“ strategy, but strategic project ...

4 ... consequently re-defining or just recalling the orientation/direction:

h “friendly“ scenarios: – high-capacity multicast– fixed terminals – aggregated traffic ...

h prospective markets: – high quality Internet– particular global VPNs– high-speed and reliable global information distribution– trunking market niches ...

h driving technologies: – MPLS– optical networking– λ switching ...


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Scenario

Space segment:ISL trunk network

Earth segment: Wireline backbones

Air interface:OD traffic

ISLSubnetwork

ATM, IPUp/Downlink:- Handover- Sat. Diversity


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Problems with (Near-)Polar Orbits: The Iridium HeritageSatellite constellation:

"Seam" betweencounter-rotating orbits:

Snapshot of footprints and ISL topology:

intra-plane ---- inter-plane ISLs

Drawback: two kinds of irregularityno ISLs crossing the "seam"deactivation of inter-plane ISLs in polar regions-> considerable path rerouting requirements !


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The Inclined Walker “Delta“ Constellation M-StarConstellation:

regular phasing, phasing factor 5absolute symmetry of the orbital constellationno "seam" like in polar constellations

orbit altitude 1350 kmorbit period 113 min# of satellites 72# of orbits 12inclination 47°

Planar projection (schematic):

M-Star utilizes the promising combination of “delta“ constellation pattern and optical ISLs

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Orbits


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Reference ISL Topology for M-Star

Snapshot at t=0:

All links are permanently maintained over the whole orbit period !


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Network Design: Concepts and Methods

4 Network design = topology + routing + dimensioning

h usually an iterative process

4 ISL network design:

h extreme challenges: dynamic topology, traffic variation

h design process must be simple !

4 Ingredients for “simple“ ISL network design:

h hierarchical “open-loop“ design:begin {topology; routing; dimensioning} end

h top-down decomposition

h standard modules (shortest path search, LP optimization, ...)

h abstraction of dynamics: discretization and virtualization


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Discrete-Time Dynamic Routing FrameworkOff-line Routing Framework

t=T-∆t s=S-1

t=∆t s=1

t=0 s=0Instantaneous

Virtual Topology Setup I-VTS(s)

t=2∆t s=2

Discrete-Time VirtualTopology Design DT-VTD

(1,2)

(1,3)

(N-1,N)

K Shortest Path Search

KSPA

OD pairs∀

ProcessingOD)(kK →

flexible k , path grouping / path reordering

Topological DesignPermanent Physical Topology Design P-PTD

Topological design of ISL networkwith permanent (non-switched) physical links

Topological design of ISL networkwith permanent (non-switched) physical links

for Walker delta constellations

Capacity DimensioningDiscrete-Time Multi-Step Dimensioning DT-MSD

Single-stepdimensioning

+ post-processing

Linkedsingle-step

dimensioning

Multi-stepdimensioning:

Dynamic optimiz.

or or


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Path Grouping for Origin-Destination (OD) Pairs

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Hop-based path groupingfor example OD pair:

Clear path group separation by cost ranges(costs = accumulated propag. + processing delays)

PG separation extends over all steps; no overlaps of cost ranges!Ordering of paths (KSPA) can only vary within a group

Select k* = k(OD) such that k*-path set always forms a complete path group


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Network Dimensioning: Target Functions

ISL capacity requirements

Bandwidth & RF power

Satellite capacity requirements

Processing power & buffer sizes

DC power

Size & weight of satellite System costs

positive “side effect“: better utilization of installed network capacity

Candidate target functions: TF1: Minimize worst case link (WCL) load

TF2: Minimize worst case node (WCN) load

LEO constellation dynamics --> every sat/link encounters worst case sometime--> all sats/links to be dimensioned accordingly


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Network Dimensioning: Approaches

ES: Equal sharing of total OD traffic between all k OD pathsFO: Linear optimization of OD traffic distribution on k alternative paths

without specific constraintsBO: Linear optimization of OD traffic distribution on k alternative paths

with additional constraints

Discrete-Time Multi-Step Dimensioning DT-MSD

Single-stepdimensioning

+ post-processing

Linkedsingle-step

dimensioning

Multi-stepdimensioning:

Dynamic optimiz.

or or

isolated step history-based

Equal Sharing (ES) iES hES

Bounded Optimization (BO) iBO hBO

Full Optimization (FO) iFO hFO


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LP Optimization Approach: Isolated StepFor each time step s = 1 ... S :

Post-processing : Take maximum of all S minimax values

for k=3:

7668optimization

variables

144constraints

2556constraints

7668constraints

iFO

iBO

)}({max pnlln

∀

subject to

wnnwPpp

pw ∀= ∑∈

,,

minimizeminimax problem

lnnnwPpp

ppl

Wwwl ∀=≥ ∑∑

∈∈

,,,

max δ

)(max pnn

subject to

minimizelinear optimization problem

wnnwPpp

pw ∀= ∑∈

,, OD/path capacity constr.

minimize WCL capacity

link capacity constraint

pnn wp ∀∈≤≤ ,]10[,0 Kααpath cap. upper bounds


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Performance of Isolated Step Optimization from here, all numerical examples assume network-uniform traffic,i.e., a normalized symmetrical traffic load of 1 between all OD pairs

Worst Case Link (WCL) Load: iES - iBO - iFO


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History-Based Optimization: Rationale

4 Modeling deficiencies of isolated step approach:

h assumes uncorrelated demand pair capacities

h neglects “history“ of single calls

h implicitly assumes that all calls are freely (re)routable in each step

⇒ isolated step optimization results are “too good“ (considering QoS)

4 History-based approach:

h guarantees that remaining (old) calls stick to the once chosen path:

→ avoid uncontrolled delay offset→ reduce dropping probability→ avoid unnecessary signaling

h takes care of incoming rerouted calls (from other OD pairs; sat. handover!)


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History-Based Optimization: Modeling

4 Call model:

h determines the ratio between old and new traffic

h classical Erlang model: Poisson arrival, negative exponential holding time

4 Rerouting model:

h sub-classifies the old traffic into remaining and rerouted

h based on a handover model for serving satellites (source/destination)

With ODT(s) = OD demand pair Traffic at step s:

total ODT(s) = new ODT(s) + rerouted ODT(s) + remaining ODT(s)

old ODT(s)freely (re)routable


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From Isolated Step to History-Based Optimization

lnnnwPpp

ppl

Wwwl ∀=≥ ∑∑

∈∈

,,,

max δ

)(max pnn

subject to

minimizeisolated step approach

wnnwPpp

pw ∀= ∑∈

,, OD/path capacity constr.

minimize WCL capacity

link capacity constraintFO

BO pknn wp ∀∈≤≤ ,]1/1[,0 Kααupper path cap. bounds

history-based approach

lower path capacity bounds

K,remwpp nnn α≤≤

lower and upper path cap. bounds

pnn pp ∀≤ ,rem

unchanged

plus:

in other words:rernewremopt 1 pppp nnnn +=−=


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WCL Load: Isolated Step vs History-Based

number of alternative OD paths k

0 1 2 3 4 5 6 7 8

WC

L tra

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load

(mea

n)

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240hFOiFOhESiES

(mean call holding time: 3 min)


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Link Load over Time

4 periodic behaviour: peaks at higher latitudes (shorter inter-orbit links)

4 inter-orbit ISLs are the critical ones

north south

equator equator

for shortest path routing (no optimization):


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Link Load Distribution in the Network: hFO vs iFO

PL identification number0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

PL tr

affic

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iFOhFO

Snapshot at t=0:

inter-orbit

intra-orbit


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Link Load Distribution in the Network: hFO vs iFO

Snapshot at t=0:

PL number (descending load value sorted)0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

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hFO inter-orbithFO intra-orbit

iFO inter-orbitiFO intra-orbit

inter-orbit

intra-orbit


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... and all that on the map:without optimization (shortest path routing):

with optimization (multipath/alternate routing):


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Conclusions (on Work Done)

Network design process for LEO ISL networks:

Topological design and dynamic routing framework

Network dimensioning: (a) heuristic design rules, (b) LP optimization approach

Advanced history-based approach shows limited reduction of optimization gain

Simple isolated step approach remains a low-complexity method for “quick-and-dirty“ dimensioning


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A Motivation for Ongoing Work:Connection-Oriented (CO) vs Connectionless (CL) ?Looking from a CO engineer‘s point of view:

• ISL topologies are (deterministically) dynamic• end user connections must be handed over between serving satellites

==> “connections“ must be broken up and rerouted in predictable steps

Looking from a CL engineer‘s point of view:

• ISL topologies are dynamic (-> routing updates ?!); YES, BUT:• ISL dynamics is predictable, periodic• ISL topologies are regular (in terms of nodes and links)

==> use nice properties, become “a bit more CO“?

Looking from an unbiased engineer‘s point of view:

==> Combine benefits, be SCO (sub-connection oriented)!


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Packet-Oriented ISL Networking4 Ongoing joint research area of Institut Jožef Stefan (IJS) Ljubljana and DLR

h partly within COST 252 Action

h ISL network simulator on packet level implemented at IJS:– Dijskstra SPA for route calculation– traffic adaptive components using queue status information– generic on-board packet generation approach– interface to up/donwlink modules and corresponding traffic input

4 Some results:

h constraint-based routing combines topology and load information

h adaptive routing concept and protocol implementation verified

h some performance studies (routing update invterval, WCL load, ...)

h alternate link routing proposed for “optimized“ flows ...

4 ... but still pure IP ... no systematic traffic engineering ... -> IP over ATM??


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Multi-Protocol Label Switching (MPLS)4 THE solution for future packet-switched backbones (?)

4 THE way to integrate ATM and IP (?), combining the benefits

h calculate CO paths (ATM) using routing protocols like OSPF (IP)

h establish these Label Switched Paths (LSPs) using MPLS unique protocols

h allow frame and cell forwarding on the same link & in the same router

4 Appeal of and issues for MPLS-based ISL networking

hMPLS is a technique dedicated to backbones

h a “blue-sky“ ISL network may be an ideal basis to exploit the fully potential of MPLS in a homogeneous global MPLS domain

h each satellite acts as edge router (serving ground) and core router (transit)

h importance of traffic engineering; potential use of proposed dimensioning methods, alternate routing approaches ...

h use OSPF and LDP protocols within the proposed routing framework ?!


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Optical ISL Networking4 Key role of optical communications for current/future backbone networks

h (D)WDM - (dense) wavelength division multiplex

h wavelength-routing or λ-switching

h form optical transport network (OTN) by circuit-switched lightpaths

4 For future ISL backbones:

h low mass, size and power consumption of optical ISL terminals

h PAT requirements for inter-plane ISLs can be met by laser technology

h space OTN concepts are really close to the developed connection-oriented routing and dimensioning framework

4 Research on space OTN has been initiated

h lightpath assignment, (even more) simplified routing, new dimensioning issues, interoperation between optical and electronic domain (esp. for efficient resource utilization, flexible traffic flow multiplexing, ...)


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Another Conclusion

Good concepts are sustainable –

sustainable concepts are good

(and both are required)

Just do it !

intersatellite link (isl) networks: topological design ...€¦ · markus werner atm-sat workshop,...

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