ANSI SCTE 95-2018 pdf download.HMS Inside Plant HMTS Theory of Operation
4.1 Separation of RX and TX RF ports
The design of the HMTS MIB is one where the operations of the transmitter and ports are considered independent of each other. This does not mean that the HMTS must physically have separate transmitter and receiver cards, only that they be treated as separate MIB entities with the ability to associate one or more receivers with a single transmitter. RF and RS-485 ports both share a common port definition through use of a media type flag. There is no restriction on the use of return path switches on RF receivers except that each return path port must then appear as a separate RF receiver in the receiver port table. The naming convention used for each port must be able to handle the use of this switch. The HMTS MIB does not limit the number of receivers or transmitters in any vendor implementation. An important aspect of the separation of receiver and transmitter is that the receiver should be able to properly respond to a HMS TALKRQST regardless of when it is received. This modeless operation allows for a looser implementation of the HMS contention mode, allowing a HMTS to catch requests that may be slightly out of sync with the clock of the HMTS.
4.2 HMTS support for RF channel definition
The HMTS allows the user to define RF forward and return path channels. This is done using one return path frequency value per RF receiver and two forward path frequency values per RF transmitter. Two forward path frequencies are necessary, as the HMTS needs to be able to command transponders to switch frequencies while still being tuned to the old forward path frequency. The HMTS provides the user with two methods to control how forward channel switching occurs. The first is the manual method, in which the external management system will take control of setting both the transponder forward channel and the HMTS forward channel values separately 1 . The second automatic method is optional, but places control of switching the HMTS forward channel under control of the HMTS whenever the transponder forward channel definition is changed. Exactly how this automatic method is implemented will be left up to each vendor. The end result will be that when the user sets the transponder forward channel value, the HMTS will take actions to cause the transponders to switch to the new channel definition and then finally cause the HMTS forward channel value to be changed to the new value. Recommended actions: Whenever the forward port transponder frequency is changed, the HMTS should immediately send the HMS channel definition PDU multiple times while obeying the HMS MAC broadcast delay limitation. The HMTS must be aware that since each transmitter can have more than one receiver. Each receiver may also have it’s own return path frequency. Thus multiple HMS channel definition PDU messages may need to be sent (one for each unique return path frequency) whenever the forward path frequency is changed.
4.3 Optional: Return Path Multicast Addressing
The broadcast/multicast table can be used to define a multicast address that is to be associated with all HMS transponders on a single return path. This return path feature provides a method of addressing a specific sub- group of transponders based upon the return path they are attached to. It is the responsibility of the HMTS to ensure that each transponder’s common MIB be updated (during the registration process) to reflect the multicast address assigned to a return path. Note: this is an optional HMTS feature – an HMTS that implements the HMTS MIB is not required to implement also return path multicast addressing.ANSI SCTE 95-2018 pdf download
