Monday, February 07, 2011

Leased Line Vs ADSL & SDSL Technologies

What are the differences between ADSL and SDSL vs a leased line? Would it solve downtime and slowdown issues, and is it worth the extra premium it costs?

ADSL is asymmetric - since the tunnel flows across both links, your best throughput each way depends on the slowest "hop" - the upstream part of the ADSL. Since you have 2 links, and possibly different speeds on each, 1 direction is likely slower than the other.

Typically ADSL Internet feeds come with a "worst case" contention ratio, but give much lower rates in the typical case. In any case the underlying "plumbing" that gives rise to contention is symmetric, so contention normally doesnt bite in the upstream direction.

If you have consumer ADSL feeds, then the provider may be rate limiting them on top of any contention. This is put in place to control traffic flows for files sharing, but depending on who you use and how they implement it, it could decide your tunnel needs to be limited.

If you share the Internet feed for the tunnel with other general Internet traffic, you could be contending with yourself - the simplest fix for that on consumer circuits is just to have 2 links per site and split the traffic by function.

Finally consumer ADSL feeds come with a fairly slack availablility SLA due to price tradeoffs, and a 24 or more hour repair time due to the copper / DSLAM infrastructure being mainly there for consumer stuff. By default it is based on business hours - any / all may be an issue for your downtime.

SDSL is normally a business service, so you can expect ....

- a higher price
- a better SLA
- some control over price vs contention ratio.
- the higher upstream / lower downstream may improve your tunnel performance.

Both ADSL / SDSL could be used as access lines to other services - another option is MPLS.

A leased line is the classic symmetric "bit pipe" for telecomms services - here it could be a point to point link between the sites, or 1 at each site as an Internet access line. They come in different flavours and speeds - likely option here is T1 at approx 2 Mbps.

You will get higher availability and a fixed SLA of 4 or 5 hours, but higher charges.

In many countries the new high speed, higher reliability access line is a Ethernet link (lots of names for these depending on the provider and service flavour). Again - a bit pipe for access to different services like a leased line.

These come in different speeds 10 / 100 / 1000, and most providers will give some sort of "soft limit" for some services in 2Mbps steps say.

So 1 option not mentioned is an Ethernet direct llink between the sites. This will be economic if the sites are with in a few Km. If they are further apart then Ethernet access into an MPLS service will probably work out cheaper. Or get Ethernet based Internet access at each site.

If you go for any sort of private link between the sites, then keep the tunnel as a backup and you can use that if your main link is down.

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Blogger Владимир said...

- ADSL usually cheaper but the higher capacity direction points towards DSLAM and, therefore, the client side will have low capacity uplink which is a major problem for some applications that require trasporting data in the opposite direction. Somewhat normal data rates achieved using ADSL2+ (24mbit downstream 1.4mbit upstream ideally) but those are not available in areas with old infrastructure. Have flexible ATM QoS which usually not used by the provider but have great potential.
1.HDSL Brings T1/E1 data rates (1.5/2Mbit; symmetric) with granulation n*64kbit (aka timeslot) but distance to DSALAM is limited to 4,5km and pairs should have ideal parameters.
2. SHDSL gives data rates of 15mbit per pair with distance 3,5km and gives some flexibility (two pirs can be used to achieve 30mbit distance can be extended to 6km but than the speed will be significantly lower.
- Ethernet/IP/MPLS Access depends on the technology (usually MW or FO, can be Fixed WiMax) can be delivered with much higher rates and so the price can vary. Data plans can be provided with granularity and flexibility in terms of data rates, QoS, dozen of L2/L3 traffic management features. The last mile does not exist everywhere so can be delivered by lazers/wimax and than many restrictions apply due to these technologies' limitations and specific conditions.

As for reserving service critical sites, major service providers would use any technology and combinations with use of many equipment types (even Satellite links if it is critical enough) for uninteraptable continious service and different access types unification into a single network.
I heard of some line providers offer dataplans based on PayAsYouGo for redundancy lines (when you don't use you pay a small line fee, but when you do - a fortune per mbit/s)

All these varies and is depended on the goals.


6:48 PM  

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