Last week I submitted a question about SONET connectivity. The message reached a much wider audience than I had anticipated and many people took the time to provide very detailed data. All of the information was beneficial and I very much appreciate the efforts. This message summarizes the information received. The beginning of the message is my attempt at summarizing the responses to each point. Excerpts from the individual responses are included after the summary to provide additional details and an opportunity for enthusiastic readers to draw their own conclusions. Thanks, Peter Polasek

This is my first posting to the NANOG list. I don't think this is off-topic, but if so, please send replies (or flames) directly to me (rather than to the list) and I will issue a detailed summary.

We are in the process of implementing an OC-12 SONET ring connection between two sites in New Jersey that span a distance of 15 miles. The SONET ring will be provided by Bell Atlantic and is composed of fully redundant hardware (there are no single points of within the telecom equipment) and redundant rings. We are splitting the OC-12 into pairs of OC-3's on two routers in each location (running ATM on the WAN). This interface is extremely mission critical to the point that a 99.9% uptime will not be acceptable. I have the following questions:

1) Bell Atlantic assures us that, because of the redundancy, we can expect 100% uptime from the OC-12. I would like feedback as to whether this is a realistic portrayal of the SONET environment.

The overwhelming majority felt that the reliability of the SONET will be very high. The real world switch times between primary and secondary rings are well under 1 second. Nobody believes there will be 100% uptime, but, by all accounts, there should be lots of 9's following the 99.9% figure. All of the respondent's reliability statements were predicated with the condition that all telecom equipment and fiber is redundant and non-shared. Even Bell represents SONET as 'virtually' 100% uptime. When asked about SONET failures, the BELL representative leaned back in the chair, convincingly straining to recall an incident, then finally described an episode in Alabama where connectivity was lost during simultaneous floods and earthquakes while, I believe, JR was being shot. Actually, I may not have all the details correct - but it was either an academy award performance or a sincere portrayal of a reliable medium. The response from the group seems to support the latter. Many suggested that 'guarantee of diversity' and 'downtime penalty' clauses should be written into the Bell Atlantic contract. Howard Berkowitz suggested URL 'http://www.bell-atl.atd.net/s-wpaper' describing a Bell Atlantic SONET deployment for military organizations. Several people pointed out that the sample period is extremely important when talking about uptime percentages. This is a very valid point. Thirty minutes of downtime in a year represents 99.994 percent uptime. If the 30 minutes is consecutive, then this would be a very large impact on service. If there were 30 one minute interruptions, the impact would be far less significant for our application (though it would represent vastly disappointing performance). Several people questioned the use of ATM on WAN. We are not running ATM anywhere within the internal LAN and do not need voice or video. The ATM interface is being deployed on the WAN because it is the only option for 155Mbps connectivity - almost. Cisco provides a relatively new 'Packet over Sonet' (POS) option that more effectively uses the OC-3 bandwidth because it eliminates the ATM encapsulation overhead. We are considering this option but are a little hesitant because it is not terribly mature at this point. I would be interested in hearing about any real-world experience with POS from those who are using it in a mission critical production environment.

2) We have the option of using either single-mode or multi-mode fiber OC-3 connections - what factors should be considered in selecting the fiber media type.

The primary difference between the two is distance. Single mode fiber has much longer range than multimode. Single mode also has a higher theoretical bandwidth limit - I didn't get any specific limits for each fiber type, but I suspect it is not an issue for OC-3 and OC-12 data rates. Virtually all phone company fiber is single mode and they typically provide an on-site converter to hand off the signal in multi-mode. 3com and Cisco support both single and multi mode (with different cards). The multi-mode cards are less expensive. Cisco offers the single mode in two flavors - 'long range' and 'intermediate'. A few people suggested avoiding the long range flavor. One also suggested that it is very easy to damage the eyes through exposure to open single mode fiber cables. Surprisingly, nobody provided the specific range limits, but I was able to get these from the Cisco rep: Type Distance Aprox. List Price ------------------------ -------- ----------------- Multi-Mode 2Km $8000.00 Single-Mode-Intermediate 15Km $10000.00 Single-Mode-Long 45Km $12000.00 Austin Schutz suggested the following URL to help select the correct fiber mode: http://www.cisco.com/univercd/cc/td/doc/product/core/cis7505/ipicg

3) Which routers should be used. The options are 3com NB-II's DPE+ (dual CPU), Cisco 7200 series, or Cisco 7500 (each router will have at least two 100Base-T LAN ports). OK, I know the 3com suggestion is a loaded question for this list, but has anybody used the 3com's in this capacity? We are a 3com shop that is considering switching to Cisco - this is a significant decision because switching will require us to continue to maintain the existing 3com environment (~500 routers) and the new Cisco routers.

Most recommended Cisco (which is not terribly surprising given the list membership) citing support as the primary discriminator. Nobody discouraged the 3com implementation and several are running similar environments trouble-free with 3com equipment. For processing power, the 7500 is recommended over the 7200 because a 7200 can only support 3 Fast-Ethernet cards at wire-speed (we will probably have 2 fast-E, 2 OC3's in each router). The Cisco 12000 series router can handle a single OC-12 interface (this is not practical for us because we need router diversity). A few recommended using 3com's CoreBuilder ATM switch products (7000/9000 series) to eliminate the need for routers. The excerpts from individual responses are included below: =========================================================== From: "Howard C. Berkowitz" <hcb@clark.net> ------------------------------------------- There's an interesting white paper about a Bell Atlantic SONET deployment for military organizations at: http://www.bell-atl.atd.net/s-wpaper From: Austin Schutz <tex@shrubbery.net> --------------------------------------- Multimode fiber has the disadvantage that it it not capable of as high bandwidth as single mode - a consideration if you are digging a trench for it. If you want to make a good ball-park guess as to whether or not multimode will work for you it would probably be worthwhile to see: http://www.cisco.com/univercd/cc/td/doc/product/core/cis7505/ipicg/ipicgpos.... From: Hal Murray <murray@pa.dec.com> ------------------------------------ I get very suspicious when anybody says "100% uptime". I would go over the routing of the fibers very carefully. There have been all too many tales of one backhoe cutting all supposedly redundant connections between points X and Y. You should probably even review it yearly. You might consider going with 2 sets of routers from different vendors. The idea being that software bugs in one may not kill the others. From: scott w <scott@digisle.net> --------------------------------- Make sure each 'side' of the ring is run along *physically* different paths. From: ilazar@rpm.com: --------------------- Make sure the fiber rings are geographically diverse, i.e. they run along separate paths. I've seen cases where one cut can take down both parts of a ring due their being run in the same conduit. You might also want to have a third backup of some sort. Single-mode allows for greater distances for most protocols. I don't know the specifics for ATM, but for gigabit ethernet, the distance difference is huge i'm a Cisco guy, so I'm biased, but for OC-12 I would be looking at the 12000's, not the 7xxx series. Best bet is to call Cisco and talk to an SE. I really don't know anything about 3-com's product line so I can't comment. You might also want to look at the Ascend GSR. From: David Lesher <wb8foz@nrk.com> ----------------------------------- Do you have a written guarantee of diversity routing of the fiber? From: Robert Gibson <wa3pxx@pimmitrun.com> ------------------------------------------ Cisco would be the safest option. Cisco has people available 24hrs per day who REALLY know hardware, and 4 hour turnaround for failed hardware. Remember with even two routers you have software issues that might not be easy to fix with anyone other than Cisco. There is no reason to think the 3COM would not work, but I doubt they could respond/fix things ANY time. From: bmanning@vacation.karoshi.com: ------------------------------------ You won't get 100%. 3com gear is fine, esp. if you are comfortable w/ the UI. From: Erik <00199740@bigred.unl.edu>: ------------------------------------- I work for an ILEC/LEC/CLEC here in the midwest, we use SONET equipment for data and voice traffic. Bell Atl was not wrong in their portrayal of SONET. But redundancy really depends on what they are using for switching, such as are they using Bi-directional Line Switched Rings, or Unidirectional Line Switched rings? Most LECs use Bi-directional Line Switched rings for heavy traffic applications, and it is used by 90% or so carriers out there. Based on our experiences, we have never noticed a failure which caused both the working ring, and protection ring to go down, and have experienced no downtime whatsoever. I work primarily with WAN applications and we had a cut last year and we didn't see any downtime associated with the cut, but, our carrier dept notified us that we were on our protection ring, and we were on our working ring within a few hours, again, with no downtime. One of the Good Things (tm) about SONET is you can split your traffic up, and use what is called "virtual rings." (i.e. running some traffic over the protection ring) But as far as my experience and our experience with SONET, has been a positive one, and Bell Atl is correct, as far as I can tell. The factors to consider are: What type of applications would be running over this fibre? If it is bandwidth intensive, you would want to go with single-mode fibre. If it is bursty traffic, and not a constant bit-rate application such as video, or multimedia applications, or any application that is bandwidth intensive, then multi-mode would be fine. Another factor is, single-mode fiber is a bit more expensive than multi-mode because of the end terminating equipment. The 7200 would work depending on the application, we use a 75xx series router with our SONET rings. We have about 7 of them, each connected to a 7xxx or higher series router. I haven't had any experience with 3com fiber equipment, so I can't really comment on that. As far as redundancy, and reliability Cisco would be the way to go just based on my experience. If it is going to be bandwidth intensive the way to go would be with 75xx series, they have the backplane and CPU horsepower to handle streaming video, and medical imaging and such, which is what this sounds like to me. From: Leo Bicknell <bicknell@ufp.org> ------------------------------------- Unfortunately, no. Draw your redundant counter-rotating rings, so both sides travel down the same fiber bundle: _______ ______ / \____________________/ \ station ____________________ station \_______/ \______/ A cut of the fiber bundle in the middle will take out both rings, downing the circuit. You need to make sure both rings have redundancy physical paths from end to end for true redundancy, two different building entrances, two different long haul paths. Anywhere they are within 50' of each other one backhoe might take them both out. If you have an active mux that can run Single Mode over the long haul and multimode locally the multimode will be cheaper. If you have a passive mux you'll need single mode interfaces that have a 15km throw length. These will be a little more than multimode. There is no (uptime) advantage to one or the other. One would hope any router would work to balance between two circuits. Personally I would recommend using 7500 class routers, with OC-3 POSIP to the sonet side, and appropriate (fast-e, fddi, ATM, or gig-e) to the lan side. In particular with redundant RSP's the deliver excellent uptime. From: noc@nso.org (Network Operations Center) --------------------------------------------- of what consists the assurance ? contractual agreement that they pay X-sum, if their word wasn't good ?  A solid  guaranteed100% up time cant be reached at any event, as you depend on a third party... One might reach 99.9999999%, but there's always that risk.... and then there are lawyers... I didn't use 3Com.  I'd also look into the Cisco 12000 as it supports OC12. as in most cases Peter, it all depends on the 'doshies'. From: Ron_Johnson@enron.net --------------------------- This holds true if the ring does not share a single path. I.E. The ring runs down a right of way on a railroad on both inbound and outbound runs. SONET works roughly like FDDI in it's ability to self heal and route around failures. Of course your connection is a single point of failure. If your local router dies, or your onsite SONET gear loses power, you are down no matter what. Also your choice of ATM, is arguable. SONET is optical "T" carrier service. Running ATM over SONET is at a overhead price. I guess to make this more clear would be to ask; Would you run ATM over a T-1? Cisco makes Packet over SONET cards (POS) for 7500 series routers. Consider running in native modes. You will also get better signaling from the SONET rings about ring conditions. We use single mode, But that is only because our SONET gear prefers single mode. Single mode fiber is dangerous. The power behind the laser on single mode will definitely cause damage to human optical devices (eyes). Single mode fiber has advantages of being able to run longer distances without repeaters. Long haul SONET is always single mode. Multimode conversion is handled by your SONET ADM box that Bell Atlantic will supply. From: Steve_Blanchard@3com.com ------------------------------ 3Com has some of the largest ATM networks in the world, in highly mission critical environments. Cisco equipment is not bad either. Converting to cisco will minimally cost you much more and require an additional management platform as well as the requisite learning curve. Hopefully, you have discussed your requirements with 3Com and also cisco. From: "Bill St. Arnaud" <bill.st.arnaud@canarie.ca> --------------------------------------------------- Yes, for the SONET rings themselves. Depending on the architecture, there is not likely to be same redundancy afer the ADM. If it is just a short pig tail of a couple of meters it should not make any difference (whether you use single or multi mode fiber). It is how the routers are configured that is more critical. It sounds like you want to run IP over ATM. Your ATM switch could give you more problems than the routers. That has been our experience on an SONET based carrier network. At a NANOG meeting last spring SPRINT outlined a good cross-linked architecture of how your routers should be connected to the SONET ADM. From: "John A. Tamplin" <jat@Traveller.COM> ------------------------------------------- You also mention ATM but don't say anything about the switches involved. If you are using a public ATM network, then I doubt you will get better than 99.9% uptime. Otherwise, you need to be at least as careful about the ATM switches as you are the routers. You mention that you are a 3COM shop, so you might be using the LinkBuilder 7000 ATM switch (that name may not be exact, they went through so many names on it) -- if so, you need to worry about cell loss in high load conditions, especially when you have links that aren't all the same speed. We had one here with OC3 connections to routers and hosts, and a DS3 to one of our other POPs. It dropped cells like crazy on the DS3 because it couldn't properly rate-limit the cell stream, and I would suspect you would have the same problem mixing OC12 and OC3 interfaces. We swapped it out with a Cisco ATM switches and all of the problems went away. Have them show you a fiber map of the path your fiber actually takes. Too many times both sides of the ring are in the same place somewhere along the path, a perfect target for a backhoe. Other than that, you obviously can't get 100% uptime (there is always the chance that the last working hardware fails before the redundant hardware is replaced), but you can get arbitrarily close with sufficient levels of redundancy. In our experience (not with BellAtl), we have not had a failure of any sort on our OC12 ring in over a year, and we don't have redundant muxes on this end, and there is about 20 feet of non-diverse fiber path in the loop. Now telco operator error when adding circuits over the OC12 is another story entirely :). Simply what your equipment can take and the distances you need to go. If you have a short distance, you have to have an attenuator on the SM fiber to avoid overdriving the receiver. Most telco equipment only support SM. We are all Cisco here, using 7500s for the core routers and 7200s and 4500s for the borders. If you stick to stable IOS releases, they just work. Our core routers are only down for IOS upgrades, period -- I can't remember the last failure. I've never used 3COM routers, but have had only lackluster results with their ATM and Ethernet switches. I don't know if 3COM has anything similar, but with Cisco you can setup two routers on each end and use HSRP for redundancy. You probably also want to get dual RSPs and power supplies since you are so concerned about downtime. From: Ron_Johnson@enron.net --------------------------- OC-3(c) POS is a released product from Cisco. It is no longer Beta. But it still is pretty fresh. While OC-3 ATM has been around in Cisco's for 2+ years. Cisco offers the OC-3 POS card in channelized and unchannelized forms, and in single and multimode configurations. You will likely want to look at OC-3c multimode. Here is some Cisco blurbage about POS from Cisco's web site. http://www.cisco.com/warp/customer/733/adap/pos/literature.shtml From: Nikki Gupta Mehta <nmehta@cisco.com> ------------------------------------------- How are you splitting the OC12 into OC3? Why are you running ATM? You might want to consider the 12008. 100% uptime? depends on how intelligently they have built their sonet rings. Mostly this is true. I think a ring wraps in less than 25ns. distance and cost. SM is used for long distances and is more expensive. Depending on the manufacturer there are 2 types of SM: long reach and intermediate reach. MM is for shorter distances and is less expensive. From: Niels Bakker <niels@euro.net> ------------------------------------ 12000 series are fully redundant. One mainboard can go up in flames, the other will take over with no downtime. 7507 and 7513 are not-so-fully redundant: you can equip them with two power supplies and two main processing boards, but a problem with one of them (not sure about the PSU's though) requires an (automatic) reboot. 7200 series can't be made redundant (unless you buy two and have a little devil on standby forever, I guess). Also, try asking on cisco-nsp@qual.net or comp.dcom.sdh-sonet; probably a small overlap in audience, but you may get something better out of it than I am able to provide. :) If your telco is willing to put 100.0% on paper, I think I'll move. I certainly wouldn't count on it... From: Dave Israel <davei@biohazard.demon.digex.net> --------------------------------------------------- Assuming the primary and backup paths are not in the same cable anywhere (don't laugh, it happens), the chances of the OC-12 itself failing inside Bell land are tiny. (Of course, if your local loop gets assaulted by a backhoe, you're in trouble.) Single mode goes farther. Multi-mode is cheaper. If you've got relatively short cables, save some money and use multi-mode. If you're dragging fiber all the way across the building, or between buildings on a campus, use single-mode. If you go Cisco, I'd recommend the 7500 series. The 7200 series won't handle two OC-3's and two fast ethers. You'll drop packets. From: "Mark Evans" <evansm@cerf.net> ------------------------------------ Having come from Bell Atlantic NJ in a 'previous life', I'll take a swag at point #1. 1) Bell Atlantic assures us that, because of the redundancy, we can expect 100% uptime from the OC-12. I would like feedback as to whether this is a realistic portrayal of the SONET environment. It depends on who is telling you this, but it sounds like it is tinged with sales-speak. SONET is a good thing, and I am not looking to diminish it - but SONET is only as good as the implementation it is riding on. Some of the things that would need to be in place to make the 100% claim more supportable are: a) dual entrances to the facilities in question, preferably coming into separate sides of the building from different streets b) going into separate fiber bundles when it leaves the building (versus multiple strands in the same bundle (aka a collapsed ring), which would be vulnerable to pole knock-down or backhoe fade) c) fully redundant electronics - this gets into whether you have interface diversity on the same SONET mux (vulnerable to chassis failure - which is infrequent) versus having redundant electronics. This gets into the dual entrance discussion as well - having 2 telephone rooms in the facility (on separate power sources) allows for dual electronics -- if you have both entrance facilities terminating in the same equipment room, you may have both strands going into the same mux (albeit on different interfaces). d) the type of SONET configuration being utilized (ex. bi-directional line switched ring) 2) Depends on what BA NJ is offering to hand off. We frequently utilize multi-mode into the 72xx and the 75xx, and things operate quite nicely. If you are dealing with very short lengths (patch cable distances) the optical characteristics are not terribly different between single- versus multi-. However, since optic performance characteristics are not my specialty, I'll defer to what other respondents to your message suggest. 3) Can't give you advise on this one. We're a Cisco powered network, and like that flavor of hardware. I can't say much about 3com's ATM abilities, with no large base of experience to draw on. From: "Bruce R. Babcock" <bbabcock@cisco.com> --------------------------------------------- Even Sonet takes a small amount of time to self heal. Depending on what they do with the facility and how you have the routers configured, the router may or may not 'see'`an interface flap. If the interface flaps, the route table will clear for that interface and traffic can reroute assuming that alternate paths exist. A few packets will be lost in any kind of link event regardless of how you are configured. During a reroute, there is the chance of mis-sequence of small number of packets. This is a non-issue for TCP and impact varies for UDP but usually it is minimal. This is a very short duration event/possibility. Our routers can load share on up to 6 paths. We can do this for any IP routing protocol (IGP or EGP/BGP), even static routes and are not limited to OSPF. Load sharing can be sequenced delivery of packets between each pair of IP endstations. We also have link-by-link / packet-by-packet load sharing (sequence not guaranteed) There is more about this technology on our web page under "CEF - Cisco Express Forwarding". This is newer switching technology that most of the ISPs are implementing now. It also supports rate shaping and IP QOS features at up to OC3 line rates. MM is usually less expensive. If the Sonet ADM is co-lo on your facility or a Km or 2 away, MM would be fine. Most carriers default to SM but this [adds a per-port cost you may not need. The primary deciding factor between MM and SM is length (assuming you need to install fiber in either case). If you are within MM limits, verify that it is less expensive than SM and go with whatever costs less. SM supports longer differences than MM and is available in various power output levels. The Sonet ADM's will usually run SM on the ring for increased distance but use either SM or MM for connection between the DTE and the ADM. Given that you will run two 100BT interfaces and probably a few OC3 links, 750X is the best choice. From: Tony Li <tli@juniper.net> ------------------------------- How many significant digits do you consider acceptable? Even in an ideal APS environment, link failure detection and protection switching does take finite time. You might get 99.999% uptime, but probably not 99.9999999%. Methinks that you've been subjected to Marketing. ;-) From: Ken_King@3com.com ------------------------ I thought I would ask you why you wanted to route at all when you have the entire OC3 at your disposal. I have installed several switched connections just like the one that you described in Las Vegas & Albuquerque, and have had no downtime in over 12 months.(Not due to our equipment at any rate) I also have colleagues in the Phoenix office that have had NBII routers doing ATM for a couple of years. Is there a reason why you wish to route rather than switch? Are you planning on implementing voice and/or video? From: Dan Martin <Dan.Martin@anixter.com> ------------------------------------------ 1. the ring they give you will live in a single ddm2000 or fujitsu sonet mux. they generally don't break, but stuff happens. the way to manage their guarantee is to give them a chance to give you back a month's payment for every hour the circuit is down. see how much money they are willing to bet on "100%" up time. you may even want to look at an atm hand off from bell atlantic into a broadband access device to leverage voice and FUNI services. 2. if you use single mode it will be easier for bell atlantic to trouble shoot connections, they will only need test equipment with single mode interfaces. if you use multimode your atm interfaces will be a few thousand dollars cheaper. we have one customer with multimode and one with single mode. if you get single mode make sure you don't get any more than intermediate reach. you need to be careful not to over drive their interfaces. as bell atlantic would say "you don't want your stuff too hot" they speak a secret language. 3. i have 4 netbuilder IIs with atm interfaces that have been in production for over a year in an atm wan environment that have run fine. the 3com routers may not say cisco on them anywhere but they do work. if you have a bunch of frame relay connections and are getting atm you should look into funi. 3com did it at olsten on long island and i did it at EHS on long island. we've worked on a number of projects like yours and might be of some material assistance in getting this thing up and running. From: Sean Donelan <SEAN@SDG.DRA.COM> -------------------------------------- 100% uptime meaning Bell Atlantic will pay you some small token amount when it fails? or 100% uptime meaning it will never fail? Figure out how much Bell Atlantic will pay you, and compute your risk factor. However, you are getting into the insanity region of availability vs. disaster planning, category 5 hurricane, regional firestorm, sabotage or terrorist attack, etc. Yes, all have happened, and will happen again. But paying a bigger insurance premium may be cheaper for these extremes. Even a perfectly functioning SONET ring will have some delay as the APS switches to the protect circuit. So make sure your application can tolerate whatever the maximum switching delay. Most carriers consider SONET outages of less than 60ms 'normal' and an overall SONET end-user restoral of 2 seconds 'acceptable.' SONET also has its own set of hardware failures (e.g. APS controller failure), user errors (e.g. improper provisioning, improper maintenance), and the dreaded multiple failure modes (e.g. a fiber cut during a Forced Switched, double Forced Switches in the same ring at the same time, and other types of ring partitions), and software/firmware bugs. Humans are always a single point of failure in any system. In the last carrier summary report I saw the number of SONET failures was very small, but was greater than zero. The number of carriers reporting was also small, making any statistical relevance of the numbers virtually nill. On average less than 5.5 SONET outages a year were reported across 7 carriers. 3com is more likely to jump when when a customer with 500 3com routers has a problem than cisco will jump for someone with only a couple of cisco routers. Assuming cisco will even agree to sell their favored products to anyone without their seal of approval. And you already know the capabilities and operation of 3com. On the other hand, cisco's SONET products have had the extensive testing and feedback from a number of noted luminaries/customers who haven't been shy about encouraging cisco to rectify any apparent shortcomings. So you won't be the leading edge trailblazing customer. And since its a good bet Bell Atlantic will blame any problems you encounter on your CPE equipment, using the CPE equipment which Bell Atlantic 'related subsidiaries' resells may cut short at least one round of finger-pointing. From: "Roeland M.J. Meyer" <rmeyer@mhsc.com> --------------------------------------------- For total system uptime 90.0% (one nine or less) Desktop systems. 99.0% (two nines) Intermediate business systems 99.9% (three nines) Most business data systems and workgroup servers 99.99% (four nines) High-end business systems and your friendly neighborhood telco 99.999% (five nines) Bank Data Centers and Telco Data Centers, some ISPs 99.9999% (six nines) Only God and Norad live here. 99.99999% (seven nines) Even God doesn't have pockets this deep. There is a matching exponential cost increment with each step. From: Alex Bligh <amb@gxn.net> ------------------------------- Don't just buy from one carrier. Even if they give you dig plans, and diversity warranties, they are likely to reroute things without telling you. Even if you get around this one, and all the other problems, and are satisfied you have permanent, true, diversity which will protect you against any one fiber break, it doesn't protect you against a procedure break, like someone terminating the wrong circuit on misreading a circuit ID. The above is certainly true in the UK, and from my experience in the US I'd think it's doubly or quadruply true. Also, use the lowest level routing redundancy you can find. I may get flamed for this, but if bandwidth is not an issue, you might consider ATM switches instead especially if cutover time is critical. From: "David Greer" <david-greer@gnc-hq.com> -------------------------------------------- If you are going to use Cisco which is a good choice, do not go cheap and get a 7200 for the job. A 7200 can only handle three fast ethernet ports. I would look at the 7500 series or possibly there gigabit router series. That will give you enough muscle to handle the job. I would call in your local Cisco office to check out which model is most appropriate. Nobody gets 100% uptime from anything. Can't happen! Look at the distance, I believe multi mode is only good for a couple of miles, then you have to go single mode, which will drive your costs up. From: "Dave Cooper" <dave_cooper@eli.net> ----------------------------------------- In addition to what Alex has stated, if you are purchasing this OC-12c or OC-3c from a single carrier, you might want to check that the carrier has a 'dual-entrance' into your building. Although the "main" fiber backbone may be truly ringed and redundant, it is common practice for RBOCs and CLECs to spur off the backbone and bring the fibers into the building via SINGLE sheath. This subjects the spur to backhoes or augers that might be digging up the sidewalks in front of your building. Most large data centers require the telco/clec to enter the facility (via fiber) from two diverse entry points. This literally brings their main backbone "through" the facility, thereby, truly preventing a fiber-cut that will take down your OC-12/OC-48 Sonet gear. Good dual-entries will even land the two diverse fiber runs on two separate FODUs in the event that someone is moving fibers or reterminating. (However, this kind of fiber build usually requires a revenue commitment from the customer since it costs three times as much a s a standard spur build.) Might be a good thing to check out if your applications are very mission critical. From: Ken_King@3com.com ------------------------ You might want to take a look at the PathBuilder s600/700(formerly known as the Accessbuilder 9600) These ATM access devices are being used in RBOCs all over the company and they are very solid. I will enclose a couple of PDF files for you. From: john heasley <heas@shrubbery.net> --------------------------------------- fwiw, we found it easier to use SM for all applications. a wee bit more expensive, but it is easier to spare parts and distance is hardly ever an issue. From: Dave Cooper <dave_cooper@eli.net> ----------------------------------------

More seriously - SLA's that specify a sampling period then also give an indication what is considered too long an outage. If you get just under the .1% downtime allowed per year all in one go you may well be pretty pissed at being told the 8 hour outage was within the SLA.

The quasi Engineering guidelines for many CLECs when calculating average downtime over a year's span is 52 minutes (meaning .0001% downtime over the year). Anything above and beyond this estimate would be suspect. Obviously, these Engineering baselines vary from carrier to carrier. Also, this 52 minute guideline relates to the SONET ring and the muxes and not the tributaries (OC-3 or OC-12) or the optical/electrical hand-offs that might fail due to bad terminations/bad wiring/or misconfigured nodes. A common failure for OC-3c or OC-12c is the 2-fiber optical handoff to the customer which has nothing to do with the SONET ring itself or the associated SONET gear.