User's Manual

System Architecture and Protocol Guide – 9-05 5 © 2005 SkyPilot Networks, Inc.
this message records the link on which the message was received, the SkyGateway identifier and the associated cost. These
SkyExtender and SkyConnector nodes utilize this cost data to select the optimal, lowest cost link on which to forward data.
Every SkyExtender forwards a message specifying its lowest cost path to a given SkyGateway. In the case of a network with
multiple SkyGateways, each SkyExtender forwards a cost message for each SkyGateway. The SkyExtenders forward the cost
message(s) to any and all neighboring nodes.
Routing Cost
The routing cost is calculated as follows:
1) A SkyGateway generates a routing cost message based on the cost table shown below. The values in this table represent a
weighted inverse relationship to modulation level with double the weight given to downstream capacity. This weighting towards
higher modulation has the effect of focusing system traffic on the SkyGateway links with the greatest capacity, thus
maximizing the overall capacity of the entire SkyPilot network.
SkyGateway Link Cost Table
Raw Mod Rate
6Mbps 9Mbps 12Mbps 18Mbps 24Mbps 36Mbps 48Mbps 54Mbps
Downstream
85 56 41 27 20 13 9 8
Upstream
43 28 21 14 10 7 5 4
For example, consider the cost calculated for a single SkyGateway link with the same overall aggregate capacity. The total cost of
a link with 36 Mbps downstream modulation (cost of 13) and 18 Mbps upstream modulation (cost of 14) link is 27; the total cost of a
link with 24 Mbps downstream modulation (cost of 20) and 24 Mbps upstream modulation (cost of 10) is 30; and the total cost of a
link with 12 Mbps downstream modulation (cost of 41) and 36 Mbps upstream modulation (cost of 7) is 48. In this example, the first
link with a cost of 27 will be selected first.
2) A SkyExtender selects the lowest cost to a given SkyGateway. For each SkyExtender link, the cost is calculated using a
formula and values from the table below. The total cost of the link is the sum of the lowest incoming link cost increased by 10%
plus the outgoing link cost determined by its downstream and upstream rates. The algorithm is as follows:
Total_link_cost = (lowest_incoming_cost * 1.1) + outgoing_link_cost (from the table below)
SkyExtender Link Cost Equation and Table
Raw Mod Rate
6Mbps 9Mbps 12Mbps 18Mbps 24Mbps 36Mbps 48Mbps 54Mbps
Downstream
72 48 36 24 18 12 9 8
Upstream
36 24 18 12 9 6 5 4
The figure below illustrates an example of routing costs in a relatively small network with seven SkyExtenders and a single
SkyGateway. As each SkyExtender is added to the network, it determines its route selection based on either the direct link cost
(first table) or the cost calculation above.