rohc

About the feedback channel

Asked by regulararmy

Hello Didier,
          when can we think the radio link as "no feedback channel", only it depends on the decompressor is not associated with the compressor ? if not, what are the other elements for definding or deciding the "no feedback channel"? Thanks.

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Didier Barvaux
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regulararmy (regulararmy) said : 		#1

            I think the "no feedback channel" is depends on the data traffic types, if the data business type is Unidirectional, such as '' Broadcast Multicast Service‘,then it is "no feedback", it does not depend on the The actual physical channel, such as the 'simplex radio communication' . is it right? Thanks.

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Best Didier Barvaux (didier-barvaux) said : 		#2

Hello,

All links may have feedback channels. Some link types make this easier than others. Bidirectional links such as Ethernet, ADSL, ISDN can easily have a feedback channel. Unidirectional links such as unidirectional satellite cannot have a feedback channel, except if they are associated with an additional return link (ADSL, ISDN...).

The ROHC protocol was designed to work for both unidirectional and bidirectional links. ROHC handles unidirectional links with an additional return link the same way it handles bidirectional links. Please note that ROHC can work in unidirectional mode on a bidirectional link if one wishes to do so. In your multicast example, you may use no return link or use a unicast return link as you wish.

A developer tells the ROHC library to work in bidirectional mode by associating a ROHC compressor object to a ROHC decompressor object upon decompressor creation. If NULL is given, the unidirectional mode is used. If a valid object is given, the bidirectional mode will be used.

Regards,
Didier

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regulararmy (regulararmy) said : 		#3

      Thank you for your answers. However, According to what you said, Having feedback channel or not depends on The actual physical channel, all links may have feedback channels. Some link types make this easier than others. A developer should tell the ROHC library to work in bidirectional mode if the link “make this easier”, otherwise,as for the links which can not get the feedback channel easily ,such as unidirectional satellite, the ROHC library should work in Unidirectional mode. Is it right?

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Didier Barvaux (didier-barvaux) said : 		#4

No, it isn't. On bidirectional links, you may configure the ROHC library to work in unidirectional or bidirectional modes as you wish. On unidirectional links, you must configure the ROHC library to work in unidirectional mode only. On undirectional links that are associated with a return link (undirectional or bidirectional), you may configure the ROHC library to work in unidirectional or bidirectional modes as you wish.

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regulararmy (regulararmy) said : 		#5

      sorry, maybe my question is not clearly elaborated.However, Now my real question is : the "bidirectional links" or "unidirectional links" you said above is the actual physical channels, or the virtual channel??
     "On undirectional links that are associated with a return link (undirectional or bidirectional)",I can not understand that sentence clearly, How can a undirectional link can be associated with a return link , if so,then it should be called bidirectional link? and if so, How can we distinguish the undirectional link and bidirectional link?
        Thank you very much.

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Didier Barvaux (didier-barvaux) said : 		#6

Hello,

> sorry, maybe my question is not clearly elaborated.However, Now
> my real question is : the "bidirectional links" or "unidirectional links"
> you said above is the actual physical channels, or the virtual channel??

What do you exactly mean by physical or virtual channels?

> "On undirectional links that are associated with a return link
> (undirectional or bidirectional)",I can not understand that sentence
> clearly, How can a undirectional link can be associated with a return
> link , if so,then it should be called bidirectional link? and if so, How
> can we distinguish the undirectional link and bidirectional link?

I gave you some example in my previous answers: if you have 2 modems, one to access a unidirectional satellite network and one to access a ADSL network, you could use the unidirectional satellite network as forward link and the ADSL network as return link. The association of the 2 links makes a bidirectional link.

Regards,
Didier

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regulararmy (regulararmy) said : 		#7

physical channel means ：such as coaxial-cable, fiber optic，or radio frequency channel，as for the the virtual channel, I think ,such as undirectional “ ping ” data traffic business, or bidirectional "UDP data Bagging" business,and so on.

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regulararmy (regulararmy) said : 		#8

Thank you.

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regulararmy (regulararmy) said : 		#9

Thanks Didier Barvaux, that solved my question.