Talk:ALOHAnet
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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] Agreed
I'm going to rewrite this later... Notanotheridiot 18:26, 24 April 2007 (UTC)
[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)
[edit] Updated
I have made a few changes. I removed the erroneous statement that stations listen before send, which is not the case for vanilla Aloha. I'm not sure if any variant tried that. It might work on short distance wireless networks, and does of course work on short wired LANS. Hidden station problems for wireless operation make this problematic.
I also added more about the historical information, such as we have it. Metcalfe did apparently get the system working much more effectively, but this very likely used protocols more advanced than either Aloha or slotted Aloha. It may have included packet reservation, or packet scheduling. It may also have used additional knowledge of traffic patterns, as for quite a while I think it was a centralised system. I am unaware of the details. Initially the network would have had only a few internal nodes and connected units, so traffic would have been relatively light. Did the number of nodes and connnected units ever increase, say to 100 or more? Did they ever change the signalling rate to use higher transfer rates?
Also, was all communication on Aloha node to node, or did they ever use broadcast modes, which might be useful for software updates etc.?
How long was the basic network running before it was substantially overhauled? Was it ever replaced, or did it just gradually evolve into a significantly different system?
Indeed, what system are they using in Hawaii now? David Martland 09:48, 15 October 2007 (UTC)
Additionally - quoting from the article:
"This shared transmission medium system generated interest by others. ALOHA's scheme was very simple. Because data was sent via a teletype the data rate usually did not go beyond 80 characters per second. When two stations tried to talk at the same time, both transmissions were garbled."
Even if teletypes were used this might only have restricted the mean transfer rate. The data transfer rate across the wireless links could have been higher, as surely the data was buffered up into frames before sending. In those days it was common for local echo to be used with transmission only on pressing the RETURN key.
Additionally, the University of Hawaii was relatively well endowed with modern equipment for the time, and it's documented that they used VDUS, which were probably operating at 9600 baud - approx 1 kbyte/second. The return rate for outbound content from Oahu, for example if requests for Unix help files were made across the network, could have been close to 1000 characters/second. Then, as now, it is possible that the network was used in an asymmetric fashion. David Martland 21:09, 15 October 2007 (UTC)
[edit] possible mistake
I have never cotributed to wikipedia before, so I won't change the article itself, since I don't really know how. But, as I am currently doing a project concerning ALOHANET, while searching for the actual bitrate of ALOHANET I found this document: http://ethernethistory.typepad.com/papers/Alohanet.pdf On page 12 it says that ALOHANET was run on two [u]24000 baud[/u] channels, not 9600 as it is written in this article. Someone visiting this place - please verify my information.
Dariusz Wawer [scyth*at*tenbit.pl], 02.12.2007 18:26 —Preceding unsigned comment added by 82.210.137.42 (talk) 17:28, 2 December 2007 (UTC)
