Telecommunications Fundamentals, Chapter 2: The Local Loop

The Local Loop

Objectives

List and describe functions of component functions of the traditional local loop.

Local Loop

The purpose of the local loop is to provide the customer with access to the public switched network (Figure 2.1). The customer's call travels across the local loop to the switch, which either:

• the customer to another customer (intra-office call), or
• Routes the call across trunks to another switch and another local loop to its destination.

There are six main component parts of the traditional local loop (Figure 2.2):

• Feeder cable - F1
  • Underground
  • Conduit
• Serving Area Interface (SAI) - B-Box
  • Feeder cable meets distribution cable
• Distribution Cable - F2
  • Aerial
  • Buried
• Distribution Cable and Drop Wire Cross-Connect Point
• Aerial
  • Pedastel
  • Buried Plant
• Drop wire
  • Buried
  • Aerial
• Subscriber or Standard Network Interface (SNI)

Lee mentions five kinds of plans for outside plant. But only the serving area concept and the modified serving area concept seem to be in favor. I'll let another expert describe it, " In order to standardize the way loop distribution plants are set up in the U.S. (and to prevent chaos) the Bell System created a standard reference design. For urban and suburban areas, this plan was called the Serving Areas Concept (SAC) plan. Basically, in the SAC plan, each city is divided into one or more Wire Centers which are each handled by a local central office switch. A typical WC will handle 41,000 subscriber lines. Each WC is divided into about 10 or so Serving Areas (depending on the size and population of the city), with an average size of 12 square miles . . . each Serving Area may handle around 500 to 1,000 lines or more for maybe 200 to 400 housing units, typically a tract of homes."]

Feeder Cable

Feeder (or F1) cable is the largest cable used in the local loop, usually 3600 pair copper cable (Figure 2.3). Demand from the distribution cables is aggregated to determine the proper size for the feeder cable. Feeder cable comes out of the Central Office and goes to the SAI.

• It is usually placed in conduit, accessed by manholes and sometimes referred to as underground cable.

Editor's note: As I mentioned in the first chapter, a perhaps clearer explanation of outside plant as a whole is provided in this article.]

Serving Area Interface (SAI)

A cross connect point is used to distribute the larger feeder cable into smaller distribution cables (Figure 2.4). The feeder cable is terminated on pins mounted on the back of the terminal panel inside the SAI. The distribution cables are terminated on adjacent panels. To accomplish the cross-connection between the two cables a jumper is run on the front side of the panel.

Wire Termination

There are three different ways to terminate the cables and jumpers (Figure 2.5). A binding post is a threaded stud with a nut. The insulation is stripped off and the wire is wrapped once around the stud. Then the nut is screwed down on top of the wire. A wire wrap is performed by inserting a wire into a wire wrap gun, which will wrap the wire very tightly around a pin. A punch down is performed by inserting a wire into the punch down pins. It requires a special tool that forces the wire between the pins.

• The wire insulation is cut and the wire is crimped between the pins.

Distribution Cable

Distribution (or F2) cable is a smaller version of feeder cable. It contains a smaller number of twisted pair wires.

• Its sheathing varies based on its placement (i.e., buried versus aerial).

Distribution Cable Termination

Depending upon the existing facilities in a given area, and city ordinances, distribution cable, drop wire cross-connect device can be either an aerial terminal, pedestal, or handhole.

Drop Wire

The drop wire terminates at the Subscriber Network Interface (SNI).

Subscriber Network Interface (SNI)

The Subscriber Network Interface is the device which serves as the demarcation point between local exchange carrier (LEC) responsibility and customer responsibility for telephone service. It is usually a gray box with modular telephone jacks inside (Figure 2.6) The SNI provides the LEC with a place to troubleshoot service problems.

Central Office Termination

Now let's see how the feeder cable gets connected to the switch which will provide the dial tone for the customer.

Cable Vault

The feeder cable, in conduit, travels underground from the first manhole into the cable vault or cable entrance facility. Inside the cable vault the feeder cable enters a splice case where it is di vided into 100-pair riser cables (Figure 2.7). At this point, the (black) feeder cable is spliced to a (gray) cable called a riser cable. Riser cable is fire retardant; feeder cable is not. Because the cable vault is usually below ground, the feeder cable travels horizontally into the vault from the street. Once feeder cable is spliced to riser cable, it usually will travel vertically up the walls of the cable vault through the ceiling/floor above to the protector blocks (either on an MDF or protector frame).

Conventional Main Distribution Frame (MDF)

The conventional main distribution frame (MDF) provides the point of cross-connection between the OSP cable pairs and the office equipment (switch).

• One side of a conventional main distributing frame is known as the vertical side or outside plant (OSP) cable termination (Figure 2.8).

[Ed. Note: for a clearer diagram of Figure 2.8, click here.]

The cables coming from the local loop are terminated on protector blocks on the vertical side of the MDF. The MDF protector blocks terminate 100-cable pairs.

• Each line is separately identified by cable number and pair number representing the Fl cable.

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