PDF of this tutorialFocusing on the Unbundled Loop
Before turning to the different approaches telcos take to managing the local loop, it is necessary to have a clear understanding of what a local loop is and how it connects to other parts of the phone system. A local loop is the transmission path between a central office and a subscriber's premises. The diagrams and descriptions that follow illustrate how other parts of the phone system connect to that loop.
Before local loops were unbundled, they always ran through the provider's switch, making them easily accessible for testing and repairs, as shown in Figure 1.
Figure 1.
Switches and DLC Systems
In a typical scenario, a CLEC establishes a regional switch in an area some distance from a city in order to take advantage of lower property costs in more remote locations. The CLEC then uses fiber-optic cable or T1 lines to connect that switch with digital loop carrier systems (DLCs) that are located in a point of termination (POT) bay in the ILEC's central office in the city.
A DLC, which might be described as a miniature central office in a 10-foot cage, could support as many as 5,000 of the CLEC's lines. It is at this DLC in the POT bay that the copper circuits begin, and that dial tone for the CLEC's lines originates.
The Main Distribution Frame
The copper cable emerging from the DLC continues to the main distribution frame (MDF). The MDF is a metal structure that may be quite large—as much as 1,000 feet in length. The MDF has two sides: the horizontal side (the side that faces the switch) and the vertical side (the side that faces the cable). Jumper wires connect the two sides of the MDF.
In some cases, to accommodate the new CLEC lines they are installing from the POT bay, ILECs will have to build new blocks on the MDF. These are installed at vacant spots on the frame. As the number of new CLEC lines and associated jumpers increases, occupying more space, growth and maintenance problems will arise.
The Intermediate Distribution Frame
Some ILECs may install an additional structure, an intermediate distribution frame (IDF), between the POT Bay and the MDF. An IDF is used when a CLEC has a variety of services—tier 1 (T1), integrated services digital network (ISDN), and plain old telephone service (POTS)—that need to be groomed by the ILEC before being sent to the CLEC's subscribers.
After passing through the MDF, and possibly an IDF as well, the copper cables go out to telephone poles or manholes. Next, they go to a cross-connect box in the outside plant and then to a demarcation point at a subscriber's premises. The demarcation point is a junction box at the subscriber's premises where ILEC maintenance stops. Finally, the cables go to the subscriber's phone or fax machine. The cable up to the demarcation point is the ILEC's responsibility. The cable from the demarcation point to the phone or fax machine is the responsibility of the CLEC or the customer.
In the unbundled loop, cables do not go through the provider's switch. Without direct access to these cables, traditional testing equipment becomes unusable. The shaded portion of Figure 2 is under the control of the CLEC.
Figure 2.
Cables That Do Not Go Through the Switch
The most important point about the transmission path discussed above is that the copper cable pairs do not go through the ILEC switch. Because they do not, there is no connectivity between the switch and the local loop. In this situation, an ILEC's existing service tools, which connect with the switch, are not useable.
To understand the significance of this lack of connectivity, imagine a field technician dispatched to repair a fault at a cross box, trying to find a pair or "work the trouble." Normally, a technician would use DATU line conditioning in this process. The technician might encounter a situation where one pair is connected to the technician's company's switch and can be conditioned, while the next pair down in the box is connected to another telco's switch and therefore cannot be conditioned. The DATU system is connected to the switch, but unfortunately for the technician, not all pairs are. As a consequence, the technician cannot finish the job, and the telco has dispatched a highly paid expert but no fault was fixed.
