WANs III: “Pack-It” In: Frame Relay Networks

Sample Frame-Relay Network

As you can observe by the evolution of cellular phones and the Internet, technology never stands still; rather, it continues to morph, change, and improve, and often at a dizzying pace.  Such was the case with leased lines, especially with the introduction of the personal computer.  Even before the advent of the public Internet, business entities needed efficient connectivity between multiple locations, but disliked the mileage-based charges involved with private line based WANs.  To complicate matters, these links were not in constant use, meaning that when they sat idle (such as during the night), they were paying for bandwidth they were not using.  Well, why not share bandwidth, and make the whole process less costly and more efficient?  The solution was packet-switched networks, including such technologies as SMDS, ATM, and of course Frame-Relay; due to becoming obsolete, we will skip over the first two and concentrate just on Frame-Relay.

When I started out in the industry, Frame-Relay was “all the rage” because it solved several problems with private lines right away.  First, the only charge for mileage was between the customer premise and the service provider Point of Presence (or POP).  Usually this was less than thirty miles (at least in my area), which trimmed the cost substantially.  In addition, you no longer had to be restricted to only joining pairs of sites, instead you could put many sites on the network and only consume one physical port on equipment).  It also allowed customers to share bandwidth in the network, which also drove down costs.  For a time, this was considered cutting-edge connectivity.

Now on to the mechanics of how all of this works.  First, connections across the network are logical rather than physical, which is how the flexibility is achieved to begin with.  Equipment connects to the service provider, to a logical entry point called a port, sold at speeds in increments of T1 (1.544 Mbps).  Every site needs a port to communicate, but the logical point-to-point connections are created using Permanent Virtual Circuits, or PVC’s, and identified using Layer 2 WAN addresses called Data Link Connection Identifiers, abbreviated DLCI. Multiple PVC’s can terminate on a port, such as in the diagram above.  Notice that the Denver location has three PVC’s defined between itself and the other three sites, and the DLCI’s identify the specific PVC’s in use.  All of this information is sent by service provider equipment using a protocol called the Local Management Interface, which acts as a keepalive mechanism, as well as DLCI and other information.

Frame-Relay is a complex topic with many more nuances than this, but it gives you a good start.

Next time we will look at the ultimate WAN, the Internet!

– Joe

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