zillah Posted May 13, 2005 Share Posted May 13, 2005 NRM= Normal Response ModeIn this scenario we have one primary station and multiple secondary startions. A primary station can send commands; a secondary station can only respond.The NRM is used for both point-to-point and multiple-point links.ABM: Asynchronous Balanced ModeIn this scenarion, the link is point-to-point, and each station can function as a primary and a secondary.What is it meant by sending command and responding ?I think it has been meant by point-to-point, for the link, not for point-to-point protocol,,,,,Has not it? Quote Link to comment Share on other sites More sharing options...
Ragnar Paulson Posted May 13, 2005 Share Posted May 13, 2005 NRM= Normal Response ModeIn this scenario we have one primary station and multiple secondary startions. A primary station can send commands; a secondary station can only respond.The NRM is used for both point-to-point and multiple-point links.ABM: Asynchronous Balanced ModeIn this scenarion, the link is point-to-point, and each station can function as a primary and a secondary.What is it meant by sending command and responding ?I think it has been meant by point-to-point, for the link, not for point-to-point protocol,,,,,Has not it?<{POST_SNAPBACK}> What is the context of the question? In NRM, the secondary's can only respond to queries from the master, the master sends data to the secondary as commands, when it wants to receive data it sends a poll and the secondary responds. In ABM, either station may transmit commands at any time, and the peer station responds, data is sent as a command.Acknowledgements are responses. It's slightly more complex than that but that is the gist of it.point-to-point in the above context referse to the network architecture as you surmise and not the protocol.PPP (point to point) protocol was derived from HDLC. It's a variant for use in asynchronous environments. Surprisingly, it is used best in point to point network architectures.Ragnar Quote Link to comment Share on other sites More sharing options...
zillah Posted May 13, 2005 Author Share Posted May 13, 2005 (edited) What is the context of the question?What is it meant by sending command and responding ?the master sends data to the secondary as commandsCould you please explain more?It's a variant for use in asynchronous environmentsAre HDLC or PPP used for asynchronous envirnmonets only? My guess they should not be used for asynchronous envirnmonets onlyBecause they are WAN layer 2 protocols wich are long distance. And Cisco says : Asynchronous serial connections are commonluy used with short distance connections. Edited May 13, 2005 by zillah Quote Link to comment Share on other sites More sharing options...
Ragnar Paulson Posted May 13, 2005 Share Posted May 13, 2005 What is it meant by sending command and responding ?Could you please explain more?Are HDLC or PPP used for asynchronous envirnmonets only? My guess they should not be used for asynchronous envirnmonets onlyBecause they are WAN layer 2 protocols wich are long distance. And Cisco says : Asynchronous serial connections are commonluy used with short distance connections.<{POST_SNAPBACK}> The answers can be long and complex, it might be easier if I knew what the problem is you are trying to solve. IE why do you need to know. Command/Response is just a nomenclature within the protocol. You could say poll/response, you could say A/B, they're just names. From a protocol point of view each message is either a command (which requires a response) or a response to a command. Unless you are trying to implement the protocol, you don't really have to worry about which messages are commands and which are responses. HDLC is used in synchronous environments. PPP is usually used in asynchronous environment. When HDLC is used in WAN layer two, you are usually talking about X.25, where layer two is HDLC, aka LAPB. Again with the nomenclature, as HDLC is all encompassing, including SDLC (NRM), LAPB (ABM), multilink protocols, and ISDN layer 2 (LAPD), etc. etc. Are you trying to implement some form of HDLC (don't ... just buy it, its a lot cheaper and bound to be more robust), or are you trying to troubleshoot an HDLC connection between two computers?Ragnar Quote Link to comment Share on other sites More sharing options...
zillah Posted May 14, 2005 Author Share Posted May 14, 2005 Thanks for your explaination IE why do you need to know.Because when I read the Book "Data Communications and Networking" By Forouzan, I want to understand concept deeply.I will go through what you mentioned and I will discuss with you any ambiguity Quote Link to comment Share on other sites More sharing options...
Ragnar Paulson Posted May 15, 2005 Share Posted May 15, 2005 Fire away and I'll try to answer what I can. It took me a long time to grasp the concepts though and it didn't come from any text book I'm not good at learning that way I guess. I'd suggest that to master protocol theory and reality you need to start by looking at the big picture, ... again back to the problem to be solved, and in that context roles and responsibilities. The OSI seven layer model helps ... but it's just a model. Some history and politics helps too. In the context of NRM and command/responses that initiated this thread you need to recall that when SDLC/HDLC were being defined IBM ruled the computing world. SNA was the advanced network architecture of the day, IP had only been heard of by a very few. Naturally the protocols defined revolved around the SNA and IBM model, huge mainframe computers (masters) central to all processing, managing and controlling thousands and thousands of slave devices. Naturally the protocols defined for this task contained the concepts of commands/responses and master/slave roles. Concepts that may seem foreign today in peer to peer communications. Even 30 odd years ago there were steps away from that with HDLC and "asynchronous balanced mode". In ABM, each party in the communication acted as both a master and a slave depending on the context. In fact the concept of master/slave was meaningless but the roles/commands/responses remained in the lexicon for historical reasons and as a compromise to those forces trying to control the protocol definitions to favor their computing models. ISO, CCITT, and even our beloved IAB and IETF are organizations that defined standards to appease their hundreds of members, naturally there are compromises. The resulting protocol definitions often have extraneous baggage that will make you scratch your head and go "huh"? The address byte in HDLC (0x01 or 0x03) and the concept of a command/response in ABM is one such piece of baggage. You still have to get it right to interoperate though. :)Ragnar Quote Link to comment Share on other sites More sharing options...
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