OK... Let me try to answer these....
<REX> Speaking of engineering, it is great, David, to have you participating on this message board to clear up questions we might have.
Well, Thanks! It's nice to feel welcome!
REX>I was wondering if you could explain what exactly is keeping the user's transmission speed down in the 28.8 kbps to 38 kbps range?
Well, let me do my best with this one...(I work mostly with the RF part of the network..)
There are different things that contribute to the "overhead" of moving the data. For example, there are a few bits associated with every byte of information determining when the byte starts and stops and so on. There is also addressing and routing information in every packet, after all, we have to know where to route the data to!
Even though the RF transmission is fast, if the data has to take a few "hops" from one radio to another the time adds up and effectively slows the data rate. We go to great pain to try and make sure that people are usually only 1 hop from a WAP. Then there is the fact that if a packet fails, it has to be retried, this will effectively lower the baud rate. So, even though the RF data rate over the network is 100 kbps. The user winds up with around 28 to 30 kbps. Just FYI, the modem in my apartment happens to "see" a WAP directly and I typically get around 35 to 40 kbps in my apartment!
<REX> If the WAP has a T-1 line, and the radio transmission is quick, then why can't access be at 128 kbps?
Well, the new network we are working on will offer these speeds.
You just have to hang in there. I have seen some of the test data. It is pretty impressive!
<REX> Is it a spectrum problem?
NO, NO, NO... Not at all. The 26MHz of spectrum we are using gives us plenty of capacity in the current network.
<REX> Is that why MCOM had to buy frequency?
The current plan is to use the licensed spectrum as a backup for the unlicensed spectrum. Remember.... being fewer hops to the WAP makes a difference in the speed you get. Well, if we cannot get to you in one or two hops from the WAP with the new system, we will have the option of getting the data to you over the licensed spectrum, where we can transmit much higher power. Hopefully then we can get the data to you with fewer hops between you and the WAP. We still intend on using the unlicensed spectrum in the 2.4GHz band whenever possible because so much spectrum is available there to be used! Again, the licensed spectrum is a backup.
<REX> Or is it a radio modem problem? I assume that in order to step up to the higher speeds, one has to trade in the current modem to obtain the speedier one?
I believe that this will be the case, but to be honest I am not sure how we will handle that in terms of upgrading. That is for the marketing types upstairs :-)
<REX>Or is it a software problem in keeping all the data packets in order?
Nope. There is plenty of computing power in the radios. Most of them do not even know they are working! (Meaning the current requirements are not very taxing!) The computational power that we are hanging on streetlights is amazing!
<REX>Another question I have is with all these 900 mhz cordless phones coming out on the market. Will these phones have any impact on MCOM's use of that frequency range?
Not really... the frequency hopping spread spectrum technology that we employ is very robust and very immune to interference. When Metricom went into this, we went into it knowing the spectrum we were going to be "playing in" is "messy". The FCC part 15 rules that we operate under say we have to accept any interference that is present in the band, meaning we cannot tell someone else to get off the air just because they are interfering with us. We have had this in mind all along and therefore have developed a very robust network. We did not start out thinking it was going to be a clean environment!
<REX> Are there any capacity worries?
I have never heard any mention of a capacity problem. I do not believe we are even close! So, at least for the moment, no, there are no worries. HOWEVER, if an area did start to get "bogged down", adding a few poletop radios to that area and maybe installing another WAP will increase the capacity of that area! Problem Solved!
Of course, we would LOVE to have so many customers such that capacity DID become an issue, we would put in more WAPS and have a big party :-)
Interference concerns?
There have been some small interference issues in the past, but they have all been resolved with relatively little effort. Outside interference is generally not a big problem
Thanks in advance for your help!
Your welcome!
David
david@metricom.com |
