ND SatCom SKYWAN IDU 7000, Manual

Page: 55 / 154

2010-10-26 Network Design and Engineering Guide 51
General Carrier Design
From User Traffic to Satellite Link Carriers
2.5 From User Traffic to Satellite Link Carriers
After the estimation of the required user data rate per carrier, the calculation of the respective
bandwidth on the satellite link must consider the following (refer to the steps in procedure de-
scription below):
-
Step 1 and 2: Encapsulation of IP and Frame Relay packets on the satellite link layer .
- Step 3: Added redundancy bits for Forward Error Correction (FEC) functionality.
- Step 4: Added synchronization symbols to ensure demodulator synchronization even at
low Signal-to-Noise levels.
- Step 5: Transmission gaps between time slots to avoid signal collisions due to transmission
time inaccuracies of the stations.
- Step 6: Added time slots for signaling data like reference or request slots.
- Step 7: Minimal frequency spacing between adjacent carriers.
Taking these steps into account, it is possible to derive the required carrier bandwidth from the
user data rate on the IP or Frame Relay network level. This calculation is no trivial task, but is
supported by the “ND Satcom TDMA Calculator Tool” which will be discussed in the next sec-
tion.
To illustrate the individual procedures we discuss the example of forwarding a LAN packet over
satellite.
1. On reception of an Ethernet packet on the LAN port, the Sky WAN
®
IDU will first strip the
Ethernet header. The Sky WAN
®
IDU operates as an IP router, therefore Ethernet infor-
mation will not be transported over the satellite link. After inspection of the IP destination
address the IDU will perform the routing procedure to decide if the packet has to be for-
warded over one of the available satellite link carriers. If that is the case it will enqueue the
packet in the corresponding transmit queue taking into account also potential quality of
service (QoS) definitions . During this process, the IP packet is encapsulated in a Satellite
Link Layer (SLL) frame which adds a 2 Byte header and a 4 Byte CRC checksum to the
packet, refer to figure 2-32.
Figure 2-32 SLL Encapsulation
2. If a timeslot on the satellite carrier is available the payload of the timeslot’s gross con-
tainer is filled up with enqueued SLL frames. If such a frame does not fit into the remain-
ing part of the container, it may be fragmented and the remaining fragment will be put in
the next container. This procedure adds a 2 Byte descriptor to each fragment, additionally
each gross container includes a 9 Byte TDMA header; refer to upper part of figure 2-33. If
there are not enough user data to fill up a container the remaining part is filled with dummy
bits.
i
The size of the gross container can be configured in a Sky WAN
®
network.
Possible gross container size range: 100-3000 Byte.