Talk:ALOHAnet
From Wikipedia, the free encyclopedia
Surely the protocol in which a sender listens to the "network" IS NOT the original Aloha protocol? I believe that such listening was not a feature of the original protocol, and this is why it had such a low bandwidth utilisation - around 18-19%.
Later modifications included using clock pulses - (Slotted Aloha), and very probably listening before transmiting - as suggested here. I have seen it written that Metcalfe's modifications brought the efficiency of the system up to around 90% channel utilisation - though how many tweaks were needed to get this I'm not sure.
It's difficult to get all the details - there is much which is not easily accessible, or interpretable, at the current time. David Martland 14:56, 22 Oct 2003 (UTC)
There are other details which it would be good to know about for the original implementation. For example, was the transmitter on Oahu (and indeed those at the other islands) "on" all the time? Obviously the receivers would have been on permanently. It might have been possible to somehow idle the transmitter, and then only apply power when there was a packet to send. This might have meant that there was not always a carrier to detect. With that sort of technology, the presence or absence of an FM carrier could have been used to determine whether the "channel" was active or not. However, if the carrier was always on, then what distinguished packet data from idling? Was there some form of idling bit sequence - alternating 0s and 1s perhaps, so that detecting "silence" would have been done by detecting a sufficient number of bits from this bit pattern? Where most data was character data, it would probably only have been necessary to detect a couple of bytes worth to be reasonably sure that it wasn't data. In the case where this pattern actually did occur within a packet, it would simply register as a collision if another station tried to transmit over it.
The situation with inbound (towards Menehune/Oahu) signals would also have been different, as there would have been several transmitters all capable of transmitting on the same frequency. For that situation it would seem necessary to reduce power, or switch off each transmitter when not sending a packet, in order to not mask out the other stations. Perhaps it was this realisation which led the developers eventually to suggest that their decision to use two frequencies - one for outbound and one for inbound data, was in fact the wrong decision - and that a single frequency network should have been developed.
What was the effect of capture ratio on the signals? Since FM was used, and since the stations were quite widely separated, it would actually have been possible for two stations to try to communicate simultaneously, and for only one to fail, due to one having a stronger signal at the receiver. A few dB difference in signal might have rendered this quite feasible. If indeed the acknowledgements were done by echoing the message, then the stronger message could have been echoed back, and then the other station could retry. Was this significant at all? It would clearly have improved the overall capacity, though it could also have meant that some stations tended to mask out others - perhaps consistently, and hence unfairly.
Does anyone know? David Martland 18:43, 22 Oct 2003 (UTC)
Further question: It would also be good to know something about how Aloha was "really" developed - if anyone knows, and/or is willing to spill the beans. Was the system really carefully worked out, or was it put together by a "go down to Radio Shack, buy it, and try it" approach, and then discover the problems which arise. I suspect that the real developement was a combination of "discovery" and predictive design - nothing wrong with that really - lots of systems get developed this way. Most people would probably be concerned to get wireless communication between remote locations working first, and then worry about problems with collisions etc. later. Is that what happened? David Martland 18:50, 22 Oct 2003 (UTC)
Contents |
[edit] Two reasons?
First section said it was important for two reasons, but only listed one. Was the other removed? Removed "two reasons". Tualha 16:30, 30 Nov 2003 (UTC)
I know the answers to lots of the questions here, after doing a lot of digging. I am writing a paper/talk about this, and will write up a summary for this place, but please be a bit patient. [Ignatios Souvatzis]
[edit] Menehune
I figure the menehune needs a mention (and some explanation). The page at http://research.microsoft.com/~gbell/Computer_Structures_Principles_and_Examples/csp0432.htm explains it a bit (with a uselessly small diagram). And I think we should say something like "...this network concentrator was named the MENEHUNE, after a mischievous type of polynesian fairy (see Menehune)". I'd add it in myself, but I can't really figure out where it belongs in the article. -- Finlay McWalter | Talk 21:37, 16 Feb 2004 (UTC)
[edit] Alohanet did not have CS!
It seems clear to me that the article is incorrect in stating that the Alohanet network was using CSMA.
With one frequency being used for 'multiple access' and the other for the acknowledgements 'broadcast' it is clear that this cannot be the case.
Stations would not be able to 'listen' (detect carrier) since they just transmitted on MA channel and listened only to the 'broadcast' channel for the acknowledgements of their messages.
The system would therefore be best described as MA/CD, but even the 'CD' is with a twist. The stations did not really detect collissions, however, they "knew" there had been a collission (or some other problem) when they did not get their acknowledgement on the broadcast channel.
CS was only 'invented' by Metcalfe around 1976 and he also made CD a feature of every station ...
[edit] The ALOHA protocol
I removed: (like a grade school classroom at recess) from the end of: This means that 81.6% of the total available bandwidth is basically being wasted due to stations trying to talk at the same time.
[edit] Ouch
People, this article is getting worse with every edit. It's now completely disorganized, filled with jargon, and has mixed tenses and styles. There's several sections on how the protocol works, two on the history, and an intro that is much too short. I'm a little overwhelmed with other articles right now to jump in though. Maury 00:51, 23 January 2007 (UTC)
