retzerj at ohsu.edu
Mon Nov 18 18:06:40 UTC 2002
David Diaz replied to my comments
>>>Well the bingo latency number used a lot in voice is 50ms. Im simplifing without getting into all the details, but that's an important number. As far as VoIP goes, I think higher latency is ok, it's more important to have "consistent" latency. Fluctuating latency really affects VoIP more then a higher consistent latency. There are a lot of people doing VoIP and traditional voice on satellites and the latency there is huge.<<
Here's an example. Without naming networks, I recently subscribed to DSL at the Oregon coast because the local phone company, which is also a national network provider advertised that they use a particular ISP, who we have in the NWAX exchange in Portland. I thought, well I should be able to get a good connection back to Oregon Health and Sciences University (OHSU), and if so this will be a good path for the physicians in that coastal community who have wanted to particpate in our grand rounds and other continuing medical education programs. They also have wanted to let the public participate in our "healthy chats" program. These events are live and interactive. So, I was very optimistic and set up my connection. I was shocked to learn, however that the DSL provider routes all the bits from that location to Dallas/Fort Worth, Texas before letting them find their way to their eventual destination. Rather than a nice direct route to OHSU, the route was 19 hops via Texas and Silicon Valley (Palo Alto and San Francisco) before getting to Portland. The average latency, which I duplicated consistently with multiple destinations in the Portland area is 180 msec and I have seen packet loss hitting 30% every minute or two. There is absolutely no way that this connection would be able to handle an interactive application.
Yes, people have tolerated 500 msec latency on satellite links ― but only because they really had no choice.
Dave Diaz continued
>>>Fewer hops = less packet loss? There has been a lot of discussion on the list about that. I still dont see it although it does push latency up a bit. Truth is that there are a lot of tunnels or express routes build in, so we arent seeing all the hops nowadays. I think that's more for sales and marketing as people keep judging networks by hops in a traceroute.<<<
See above. Partly, I think it is just the odds of encountering congestion goes up exponentially with the number of hops. No engineering reason other than if you have 5% likelihood of hitting congestion on any one hop and then you have 19 your odds of hitting congestion are much higher. Combine that with a persistent connection for an interactive video session and you will find, as I did that every couple minutes you have a spike that causes fits with your video.
Dave Diaz continued
>>>An IP backbone is a bad place for live TV. Delayed or on demand tv yes. Live tv plays to the benefits of One to Many broadcast ability of satellite as Doug Humphrey will tell you. So a feed from a DSS dish into your local cache would work well. It still can be done at a per city peering point to better feed the broadband users. <<<
If we fix the IP backbones for interactive TV then broadcast should be a piece of cake. While I agree with a later post that questioned convergence for the sake of convergence, the benefits of IP+Ethernet are that it is an order of magnitude cheaper and you eliminate the need for any local "head end" equipment, manipulation by local stations, etc, etc. Ultimately, the only stuff that will originate locally is local news and content.
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