98 lines
4.1 KiB
Markdown
98 lines
4.1 KiB
Markdown
---
|
|
title: Topology, handover and routing
|
|
tags: isl, handover, intersatellite, routing, disruption
|
|
summary: When there is more than one way across the network, the question of which way, and what it costs when the choice changes.
|
|
---
|
|
|
|
# Topology, handover and routing
|
|
|
|
A constellation network is not a set of point-to-point links to ground. It
|
|
is a graph whose nodes move, and every problem of graph routing has to be
|
|
re-asked every time the graph changes shape.
|
|
|
|
## Definition
|
|
|
|
An **inter-satellite link** is a radio link between two satellites, giving
|
|
laser or radio crosslink connectivity between them. **Handover** is the
|
|
transfer of an active session from one satellite to another. **Routing** is
|
|
the choice of path across the graph. **Disruption tolerance** is the
|
|
mechanism that keeps a session alive through a handover.
|
|
|
|
The distinction that matters operationally is between a link that is planned
|
|
and a link that exists opportunistically, because the first can be scheduled
|
|
and the second cannot be.
|
|
|
|
## Mathematics
|
|
|
|
### Visibility of a crosslink
|
|
|
|
Two satellites can see each other when the elevation of one above the other's
|
|
horizon is above a mask, which is the same geometry as the ground case with
|
|
the observer at orbit altitude. The consequence is that crosslinks are
|
|
intermittent: a pair of satellites in adjacent planes are in contact for a
|
|
fraction of their relative motion, not continuously, and the contact interval
|
|
is shorter for lower orbits.
|
|
|
|
### Why handover costs
|
|
|
|
The cost of a handover is not the signalling; it is the buffering. A session
|
|
in flight holds state, and that state must be transferred or reconstructed on
|
|
the far side. The cost rises with the data rate on the session and with how
|
|
much of the protocol's state is not reconstructible from the data itself.
|
|
|
|
This is why a system that can tolerate disruption pays for it in the
|
|
protocol rather than in the network, and why the disruption-tolerant
|
|
protocols are built around the assumption that a handover is normal.
|
|
|
|
### Graph shape over time
|
|
|
|
Because nodes move, the adjacency matrix is a function of time, and a route
|
|
optimal at one instant is not optimal at the next. Two approaches follow:
|
|
|
|
- **Scheduled routing**, which uses a precomputed time-varying topology and
|
|
plans against it. Predictable, and limited to links that exist on schedule.
|
|
- **Opportunistic routing**, which uses whatever is present. Reaches further,
|
|
and can fail at a moment when nothing can be done.
|
|
|
|
### Capacity per node
|
|
|
|
A satellite is a network node with a fixed set of radio terminals. Its
|
|
throughput is therefore shared between every session it carries, and the
|
|
capacity available to one session falls as the constellation gets busier at
|
|
the same location.
|
|
|
|
## Unit conventions
|
|
|
|
- Crosslink data rate in bits per second per link; aggregate in the same.
|
|
- Handover interruption time in milliseconds.
|
|
- Link elevation in degrees; the horizon geometry uses the same quantities as
|
|
the ground case.
|
|
|
|
## Applicability
|
|
|
|
Routing decisions are constrained by the physics long before they are
|
|
constrained by the protocol. A route that needs a link which is only present
|
|
for 30 seconds per orbit is not a route; it is a hope.
|
|
|
|
The hardest case is not the dense shell, where a route exists, but the sparse
|
|
case, where sometimes no path crosses a given region for minutes. Designs
|
|
handle it by accepting the disruption and reconstructing state rather than by
|
|
searching for a route that does not exist.
|
|
|
|
Capacity per node explains a behaviour that looks anomalous from outside: a
|
|
service gets slower in a city during busy hours even though coverage is
|
|
unchanged, because the same satellites are serving more sessions.
|
|
|
|
## Standards
|
|
|
|
- Inter-satellite link characterisation and the laser physical layer: the
|
|
CCSDS Inter-Satellite Laser Communications specifications
|
|
- Session continuity and disruption tolerance: the recommendations for
|
|
handover and for the service data, in the CCSDS Space Link Protocol
|
|
family
|
|
|
|
## See also
|
|
|
|
- [[access-and-availability]] for the ground access a handover moves between
|
|
- [[constellations]] for the geometry that decides adjacency
|
|
- [[interference]] for the physical layer under the topology |