Comtech EF Data SLM-5650A - Manual (Page 416)

SLM-5650A Satellite Modem Revision 8
Appendix E MN-SLM5650A
E–12
In the Forward Link direction, all traffic is transmitted by the Hub Time-Division Multiplexing
(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 E-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 E-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, care must be taken 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 E-7.