WANs II: A New “Lease” on Life!

Private/Leased Line Network

One of the sayings I tend to use when explaining network concepts is a twist on the opening words of the book of Genesis, “In the beginning was the mainframe…” because so many things we take for granted today started with that piece of technology.  In terms of WAN connectivity, the original form of connectivity took place across specialized telephone lines that carried data rather than voice conversations.  Because the end-user/customer paid the IXC for the exclusive use of the line, they were referred to as private lines or point-to-point leased lines.  As mainframes were replaced by personal computers (and networks) these lines connected to devices that convert bits of data to electrical impulses that can be transmitted across these lines for long distances.  The technical term for this device is a Channel Service Unit/Data Service Unit, or CSU/DSU, which used to be an external device but are now integrated on interface modules on routers.  This is actually a good time to introduce two more terms, namely DCE and DTE.  A Data Terminal Equipment (DTE) device is the terminal or end-point sending and receiving information, usually a computer or router.  DCE devices, on the other hand, perform the conversion between raw data and the format needed for transmission, such as a CSU/DSU or modem.  The acronym stands for either Data Communications Equipment or Data Circuit-Terminating Equipment, depending on the publisher.  Cisco prefers the latter term as a general rule.

Private lines utilize electrical circuits to create the pathway between locations, as illustrated in the diagram above; this is in contrast to packet switched networks (which we will deal with later).  Depending on the part of the world you live in, there will be differing names and capacities, such as T1 (1.544 Mbps) or E1 (2.048 mbps), and even higher speeds.  These are typically copper connections, with very high speeds delivered on fiber optic connections (OC-X).  In North America these are the T1/DS1 and T3/DS3 standards, while the rest of the world utilizes the E1/E3 standards.  These lines are charged by mileage and often very expensive as a result, although very secure since a customer has private and full-time use of the connection, although if it ever sits idle that ends up being wasted bandwidth.  The structure of the framing and line coding ca be rather complex, and something you should definitely be familiar with as you pursue your certification studies.

Next time we will look at packet-switched WANs! Don’t you just love this stuff?

– Joe


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