Comtech EF Data SLM-5650A Installation And Operation Manual page 506

In a point-to-multipoint system, at the hub a split-path topology is used and traffic in the Forward
Link and Return Link directions may be processed by different SLM-5650A modems.
Figure H-7 shows a typical Hub Data Traffic Network for one Layer 2 Network ('P1/P2/A' shown in
red). The other two networks ('P3/P4/B' shown in blue and 'P5/C' shown in green) can be used in
a similar fashion.
Note that the Hub Demodulators used in this configuration are SLM-5650A modems configured
for operation with the optional 'Demodulator only' FAST Feature enabled.
In the Forward Link direction, all traffic is transmitted by the Hub TDM Modem. Packets received
from the Hub Data Network are received by the Hub TDM Modem on a Data LAN Port (P1) and
forward to the Data WAN.
The packets received by Return Link channels can come into the Hub in one of two ways:
1. They are processed by the Hub TDM Modem for Self-Organizing Time Division Multiple
Access (STDMA) channels.
2. Hub Demodulators are used for Single Channel per Carrier (SCPC) channels.
In Figure H-7, the external hub switch supporting data traffic is shown as two separate switches:
The Hub LAN Data Switch, and the Hub Demodulator Data Switch. It is important to note that this
is a functional concept only – the physical implementation of this switch can be accomplished in
one of three ways:
1. Two separate switches, as illustrated in Figure H-7.
2. One physical switch, partitioned into two logical switches using port isolation. Many
commercially available managed switches have this capability.
3. A data-path router directly connected to the Hub TDM Modem's P1 port, with the
functionality of the Hub Demodulator Data Switch implemented by means of daisy-
chaining the Hub Demodulator units Data LAN ports (i.e., connecting the P1 port of one
Hub Demodulator to the P2 port of the next Hub Demodulator). This third method has the
advantage of reducing the required Hub equipment. However, it has the disadvantage of
adding between 5 µsec and 0.01 ms of delay per Hub Demodulator unit traversed,
depending on packet size (64 – 1500 bytes @ 1000 Mbps GigE negotiation between Hub
Demodulators and the Hub TDM Modem).
Note also that for return link packets arriving via SCPC channels, you must make sure to avoid
potential issues associated with the split path topology inherent in the point-to-multipoint system
architecture. In order to forward the packets received from the Hub Demodulator to the Hub
expansion port P2, MAC learning on the HUB demodulators switch should be disabled, as
indicated by Data Type 'D' in Figure H-7.
Appendix I H–14
SLM-5650A Satellite Modem
Revision 10
MN-SLM-5650A