Types of Leased Lines – Your Options Explained

There are several types of leased lines.

Fibre Leased Lines

Fibre Leased Lines provide a dedicated symmetric data connection by sending light over fibre optic cables. In reality, almost all leased lines use fibre for a major portion of route, particular the 'backhaul' element. However some leased lines use fibre all the way.

If you've got a choice between Fibre and DSL, choose Fibre.

Of all the types of leased line, Fibre leased lines offer the higest speeds.

DSL Leased Lines

DSL is used to provide other types of low-bandwidth leased lines. Leased lines are non-contended and symmetric, whereas most DSL connections are contended and asymmetric. However, it is possible to use a symmetric alternative to ADSL, called SDSL. This offers a higher upload speed than standard ADSL, allowing a symmetric connection to be formed.

There's also a more recent version of ADSL called ADSL2+Annex M. This offers faster upload speed than traditional ADSL.It's not symmetric. However, by deliberately limiting your downstream connection, it's possible to provide symmetric connections of up to 3.5Mbps or below.

ADSL and SDSL speeds drop the further you get from your exchange, so you may not be able to get the speed you want via a DSL leased line. Usually it's only used if a fibre leased line is not available.

ADSL2+ Annex M and a download speed far below the capacity of the line. Most DSL services are contended. In other words, your traffic has to fight it out with other customers traffic when travelling between your local telephone exchange and your ISP's network. However, this is just a cost-driven business decision designed to reduce backhaul and IP transit costs. It is possible to get dedicated DSL connections.

DSL speeds drop the further you are from your local telephone exchange. To counter this, it is possible to bond several DSL connections together to increase the maximum speeds available.

MPLS Leased Lines

These types of leased lines are becoming ever more popular.

MPLS stands for Multi-Protocol Label Switching. It's a technology that's used for getting data from A to B by wrapping up the data (encapsulating it), sticking one or more labels on it, then deciding where stuff should be sent based on those labels.

That all seems rather pointless until you consider how easily a corporate WAN could suffer from congestion.

Just imagine that you had a 12 site WAN, with 11 regional sites, each with 2Mbps connections, and a head office that has a 10Mbps connection (including 4Mbps of Internet access). In theory, 22Mbps of traffic could go from the regional sites to the Head Office. Another 4Mbps would go from the Internet to Head Office. That's 26Mbps in total, a LOT more than a 10Mbps connection to Head Office could cope with.

Now clearly the links aren't likely to be 100% full. But at 50% utilisation you still have 13Mbps of data trying to travel over a 10Mbps connection, and one in four packets being dropped. How should the network decide which packets to drop? Which packets should be given priority? How do you ensure that time-sensitive applications such as VoIP telephony don't suffer service degradation as a result of this network congestion?

MPLS enables you to label the different types of data on your WAN so that time-sensitive data is given priority over delay-tolerant data. To move into more geeky terms, it allows you to apply different Classes of Service to different types of traffic, ensuring that the levels of latency, jitter and packet-loss experienced by each type of traffic is appropriate.

MPLS is something that's delivered on top of a leased line circuit. In other words you can get an MPLS leased line that's build upon cable leased lines, or upon DSL ones, or upon a combination of them.

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