How to Test Your T1? 488
lawpoop asks: "We have a T1 line for our building with a local ISP. Right now, we're looking for competitive bids from different companies. The local guy is offering a good price, but the larger guys are saying he may be overselling the T1 service through a DS-3. He swears he's not. So, how do I tell? The sales guys say 'There's bandwidth meters on the web,' but they fail to mention exactly how I can tell if I have a true T1. I've tried a half-dozen bandwith meters on various websites, and the results are highly variable. We've gotten 300-900 Kbps. Each site has disclaimers as to internet traffic, time of day, etc. Furthermore, we split the T1 out over a hub with two other tenants in the building. I'm coming through from behind that hub. How can I tell for certain that I'm getting a full T1? A service tech with a line tester? Any dead-on bandwith meters? What would an oversold T1 read out to be as compared to a true T1? If the larger guys are trying to scare me to their service with stories of oversold T1s, I need to know that they aren't doing it also!"
Not sure I understand your question...do you? (Score:5, Interesting)
I suspect that you're asking how you can tell whether or not your ISP is selling 50 million T-1 lines when he himself only has a T-3 connection with the rest of the world.
I think the simplest way is to ask. Talk to the sales engineers who work for the larger guys - tell them "Ok, you're trying to scare me away from a smaller vendor...how can I prove for myself how he's configured?" Ask the small guy "This looks like a really good deal...can you demonstrate to me I'll get X level of performance?"
You can't. (Score:3, Interesting)
However, as other people have noted, after installation it's fairly easy to measure the bandwidth you're actually seeing. Telltale signs of oversold uplinks are things like vastly better performance at 3am than 3pm.
All in all, it's not necessarily a bad thing to have a (slightly) oversold uplink, as long as it is constantly monitored and upgraded if/when end users' aggregate usage is more than 75% for any length of time.
Cheers
-b
TTCP (Score:1, Interesting)
You're behind a hub? You ain't got a T1. (Score:3, Interesting)
Ahem. From this, it looks like you're really just buying Internet access with a "T1" rate. 900kbps is almost as good as you're going to get on a T1. Maybe upto 1.1Mbps or so, tops. You have to allow for protocol overhead, latency of all equipment between you and the "other side", and congestion that may or may not exist.
3 letters (Score:5, Interesting)
If said small provider is telling the truth, then he won't have a problem signing one. I've found in my area that the big guys are the bullshiters when it comes to SLA's.
Re:Welp. (Score:3, Interesting)
Actually, why dont I ask the slashdot crowd. I've asked three techs smarter than me the same question and I got a few different answers back. And the other people here might appreciate this info, as a search shows nothing yet.
T1s. How can you get them?
-ISDN PRI (Primary Rate ISDN)- this is 23 64 bit channels plus 1 64 bit controller channel, the delta channel I think. And the others are B (bearer?) channels. So B and D? (man I am going to get clobbered with a cluestick)
-Point to Point. This is straight copper from you to your endpoint. But how does the technology work? Is this the one that uses Time Division Multiplexing?
-Frame Relay. A packetized protocol running on layer 2, I think.. You get a line from you into a "frame cloud", which is an abstraction of the telephone company's network. Then your other end gets a similar line into the frame network. The phone company makes a Permanent Virtual Circuit (PVC) from you to them. A Data Link Connection Identifier (DLCI) on each end plus a circuit indentification defines a PVC (plus maybe some other stuff?) The bandwidth is shared! There's at least two settings- CIR and Burst. CIR is your Commited Information Rate- this is what you are PROMISED to get. Note this is just from the phone company. Your ISP can still screw you if they dont have the counterpart backbone bandwidth available. Burst is how fast the phone company will let your pipe flood through to when the network isn't busy... it's like a bonus. I believe both are negotiated. anyways Please please, would a telephone guru jump all over me and tear me apart? I'd love to get these straight.
Re:Over selling not inherently evil... (Score:3, Interesting)
Numbers are so much fun. I'm sure you already know this, but it's worth pointing out:
Network links (and CPU's, for that matter) at any given instant in time are either at 0% utilization or 100% utilization. Anything in the middle is an average, so you need to ask what it's an average of. For stock MRTG or Cricket setups, it's a five minute average - a spike to 100% means the line was pegged solid for at least five minutes. If the 80% threshold you mentioned is more like an hour average, the line may well have been fully saturated for 48 minutes of that hour.
Oh, and it's useful to ask questions about this stuff before signing any service contract that includes burst traffic fees.
Re:A Full T1 is ... (Score:5, Interesting)
BZZT. Remind me to never buy any bandwidth from you.
Kids, this is what happens when you save a few bucks. You go with tier-19523 providers who are selling T1's off a SDSL circuit from a guy who splits colo space with a cleaning buisness.
From REAL ISPs (AT&T, Sprint, UUnet, etc) the story is quite a bit different. They DO oversubscribe to an extent, but to where? You and 500 other T1 customers go into a POP (Either on a frame cloud or through a bigass MUX) Coming out of that is at-minimum 3-4 OS3 links (155meg each) High-traffic nodes frequently run OC-12 or OC-48.
Now, if you all try to get to a single site, not only would the remote site not be able to handle the bandwidth but quite possibly you'd flood out the backbone links between you and them.
By the same token, however, all the OTHER backbone links would be unused.
So yes, you can say "I'm oversold", but you'd be wrong. Let's go into a true oversubscription example now:
Billy the Janitor decides he wants to be an ISP. So, after gunning down some of the druglords in his neighborhood, he gets a DS-3 (45 meg) into his hovel.
Finding that he can wire every other crackhouse in the neighborhood cheap since he knows people in the local telco monopoly, he starts selling "full T1s" for $400 a month. Wow, what a deal!
And they are "Full T1s", too... for the first 30 or so customers. After that, billy starts to oversubscribe. And, at $400/month, he sells HUNDREDS of T1s. Say, 200.
Now we have 300meg coming out of a router with 45 meg going in. Mmm, bottleneck. See how this is different from a multihomed POP in the case above?
Some real numbers:
Today sprint Peaked at 1523/1503 kb/s (in/out)
UUnet peaked at 1510/1508 (Delivered over frame, slightly lower peak bandwidth)
A frac T3 frame to bell only got 3313/3412 today, but it's pretty lightly loaded. I've done 5.5meg on it (and it's sold as a 4.5 meg CIR carried on a 6meg pipe)
The REAL answer is: Are you going to use the bandwidth, and if so, is it worth the premium it costs to get a tier-1 provider.
If you're just using corperate websurfing/email, HELL NO: buy the cheap one in a heartbeat. If you're reselling yourself, don't even THINK about doing anything BUT tier-1.
And don't forget latency. As the famous paper was titled: "It's the latency, stupid." You'd be amazed what a 10ms pingtime does for your effective bandwidth.
(Before I get flamed, I know bellsouth is technically tier-2. Especially since the twits don't know the difference between peer and transit BGP setups. YAY unreachability. They're working on it)
Re:A Full T1 is ... (Score:3, Interesting)
1. Your ISP cannot fix the entire internet.
2. If you are downloading across the globe without specifically tuning the tcp stack you are not going to get 192Kbytes. Ever. Your TCP window will not open enough.
So all you can do is chose an ISP with good connectivity in first place and follow up to make sure they keep it so. Bitch immediately if any peering links suck. Bitch immediately if you notice congestion anywhere on their own backbone. Bitch if they are routing traffic to the next village through a point on the other end of the globe (the way psi.com europe do). Ad naseum...
If everyone did that and made it public the Internet would have been much faster reliable.
Re:A Full T1 is ... (Score:2, Interesting)
Way off on point 2. Most places log calls and email. If someone starts bitching every time that effective througput drops below 190K, guess what? That ISP is going to stop listening, and quick. These are the sorts of customers ISP's hate hearing from, becuase most of the time customers that bitch like this think that the ISP actually can do point 1.
<Begin Rant>
Working for a regional ISP for the last 8 years, I can say that most of the time calls about bandwidth are from customers that we often call "Henny Penny" or "Boy crying wolf" type customers. If their ping time ever varies more than 3ms or their througput ever drops below what they think they should get, they call. More often than not, the problem is with the CPE, and not with the ISP. "Oh, you mean Kaaza on my other workstation will use bandwidth from this one?" I can't tell you how many times I've answered that question and had to bite my tounge trying not to call the caller on the other end of the phone a complete moron. Most of the time these are the sorts of speed complaints that come into an ISP call center, and you just have to be polite and explain to them in small words they will understand that their bandwidth, and the global Internet bandwidth is shared.
</End Rant>
The public Internet is the way it is (in wide spread use) because everyone does overbook. Broadband has made sure of that. There is no way an ISP can stay in business without overbooking, it just doesn't make financial sense. All ISP's now sell broadband connections -- including the Tier 1 ISP's. Where are these broadband customers getting their bandwidth? The same connections that they are providing to their business customers. Without heavy overbooking, $20-$30/month T1 speed DSL and Cable lines (not counting telco charges) would not be possible. The hard part of that equation for ISP's is making sure that you have enough aggregate bandwidth to enough other networks to satisfy peak traffic loads without maxing out all of your pipes.