Carrier s Facilities and Switching Systems

Carrier's Facilities and Switching Systems

This section provides you with more information about LECs, IXCs, their facilities, areas of operation, and their switching systems.

First-Tier Exchange Carriers

Since the Telecommunications Act of 1996 in the U.S., a service provider can be any provider of telecommunications services that is certified by individual states to become a LEC. The newly certified companies acquired the name of competitive LEC (CLEC), and the existing (former) companies are called incumbent LECs (ILECs). There are several other LECs (OLECs) and some other abbreviations, but for simplicity, only first-layer carriers are referred to as LECs. Recently, more than 1300 LECs were in operation in different parts of the U.S. The country is divided into 240 areas that are called local access and transport areas (LATAs or LATs), which are based on approximately an equal number of calls in each area. A LEC refers to any company that operates in the LATA boundaries, providing Intra-LATA services and functions. LECs can operate in many LATAs, but can't carry traffic between separate LATAs.

Inside the CO

Access lines from CPEs and trunks to other switches enter the local exchange, otherwise called central office (CO), usually with rare exceptions below ground level. This sub- basement is known as a cable vault, and in some metropolitan areas their number can exceed tens of thousands of lines and trunks. The lines and trunks eventually make their way up to the main distribution frame (MDF), which is a facility usually called a wire center. An IXC uses the same term differently, where the MDF is a large, two-dimensional patch panel, and every line or trunk has a unique horizontal and vertical number.

The number and variety of switches in the U.S. and other countries is enormous , and it is practically impossible to provide any kind of precise classification. It is important to remember that the purpose of patching is to adequately identify each connection; this identification is required to assign each connection to the port of the switch. The switching in the CO is always port-to-port, bandwidth-to-bandwidth, and is transparent to any of the bits or packets exchanged over the lines and trunks.

In general, there are two ways to switch data in the CO:

• Leased line

• Switched

The term "nailed-up" refers to a situation when the lines are patched in such a way that they always transfer data from port A to port B. This is leased-line terminology, which is typical for Frame Relay solutions using FT1 and T1/E1. The data can be transferred through the PSTN or redirected to equipment called a Digital Crossconnect System (DCS). This facility, which is also known as private line (non-switched) service, bypasses the PSTN switch, and transfers data from the access lines straight to the trunk system.

A private line solution ensures privacy of the data exchange and is one of the provider-dependant Virtual Private Network (VPN) solutions. DCS is a digital type of equipment that operates over digital access lines, but not over analog local loops . To provide analog local loop conversion to DCS, the signals must first be converted to digital form.

Data that is switched through the CO uses a local switch(s) that is based on the dialup phone number from the call originator. This solution is related to dialup, ISDN, and other services that require an initial call setup phase before proceeding any further.

Each circuit that must be switched must first have a port assigned on the switch. As soon as the originator issues a call, the call is redirected to the requested destination, without adding anything to the content of the user data.

Second Layer Exchange CarriersIXC

In the U.S., segmentation of the telecommunication network is related to the area in which they operate, and is governed by a single carrier (LATA). To distinguish between different areas and services, there is another set of terms related to the interconnection of different carriers and their zone of responsibility. As previously mentioned, the first layer carriers operate in the LATAs. If calls cross LATA boundaries, they need to be handled by an IXC, sometimes called IECs or long-distance carriers. To provide this long-distance service, IXCs need to maintain a point of presence (POP), or switching office within the LATA. Based on FCC issued licenses, more than 700 companies operate as IXCs today. AT&T and Sprint hold the largest market presence in the U.S. Interestingly enough, Sprint still maintains a few LECs in areas such as North Carolina and Nevada, which creates an unusual mix between LEC and IXC. If, in any particular area, the IXC does not have a POP, based on the rules of equal access, the long-distance can be handled by another IXC, which charges back for the service.

Switches and Tandems

From the carrier's point of view, every carrier system can be broken down into switching systems, transmission facilities and CPE. The CPE (or station equipment) includes all available equipment on the customer side. The dividing line between provider access services and the CPE is called the network point of termination (POT), network interface (NI), Demarcation Point (D-mark), or minimum point of entry (MPOE). The transmission facilities include loop and interoffice. Loop transmission facilities provide clocking, switching, and signaling for the CPE. This part is often called a local loop, subscriber line, subscriber loop, or line. Interoffice transmission facilities connect the carrier's switching system. In this design, the term trunk refers to the communication path between two switches. The available options are to create full-mesh or partial mesh systems, with either no switching (leased lines), endpoint switching, or central switching, based on CO facilities. It is important to consider the carrier's telecommunication network as two segments working together: access services and transport services.

NOTE

When working with carriers, you can often see how they include different groups in the troubleshooting process, depending on the issue. When the local loop is down, you usually deal with technical CO personnel, but when your calls are not going through, the transport personnel must be involved in the troubleshooting process.

LEC/IXC Operations

When the CPE requires service, IXC involvement is not necessary if the problem deals with a LATA local phone call, a local long-distance voice call, data calls through dialup, and ISDN, or if a switched virtual circuit (SVC) is made to the local phone number. This reflects the tariffs of the service and the performance characteristics. When the same scenario includes different area codes in the connection (long distance), there is an interaction between carriers that must be considered in the design process.

The efficiency of interacting with different carriers depends on the way that their networks are structured, and their ability to provide immediate and high quality call transfer. Calls are metered by their number and duration. The fewer switches (hops) in the path of the call, the faster the call. At the same time, it is hard to maintain a full set of trunks from an IXC to every single LEC, creating a fully meshed structure not only within the IXC network, but also outside of it.

Tandem Office and Tool Office

Trunks are not directly run to another LEC's facilities, but to more centrally located LEs that are intermediate points equally convenient for all parties. These LEs are equipped with a secondary switch called a tandem that provides switching trunk-to-trunk, instead of local loop-to-local loop or local loop-to-trunk. It is provisioned for equal access from both parties and for providing services to them. Tandem is a telephony term that means "to connect in series." Thus, a tandem switch connects one trunk to another. A tandem switch is an intermediate switch or connection between an originating telephone call or location and the final destination of the call. The practice of trunk-to-trunk switching received the name of toll office, and a call routed through this office is called a toll call. Unlike long-distance calls, these calls stay within the LATA and do not cross LATA boundaries. This solution architecturally alleviates the challenges of an IXC, which instead of running trunks from anywhere-to- anywhere now runs trunks from their network to the nearest tandem office.

From an IXC's perspective, the toll office is called a service wire center, which provides services for POPs. Most IXCs maintain more than one center within the LATA, to provide redundancy for their services. LECs cannot provide Inter-LATA calls on their own because LEs are designed to provide service for all carriers within the LATA, and the LEC must route all Inter-LATA calls to trunks that are terminated by the IXC's POP (see Figure 3-4). Of the LEC's switches, the last switch pointing to the IXC is called an end office (EO), and it must be configured to point to the specific IXC (101010 for AT&T, 333 for Sprint, and so on). The IXC POP has access to more than one EO within the LATA. When the IXC's switch or LEC's switch is partitioned and certain trunks are designated for specific users, this is referred to as a VPN.

Figure 3-4. Design Example of Inter- and Intra-LATA Carriers Interaction

Figure 3-4 shows the LATA boundaries, IXC area of operation, the locations of IXC POPs, network points of termination (POTs), and network points of interface (POIs). In the lower end, LECs provide line-to-trunk connectivity for CPEs; the upper half shows the interaction between LECs and IXCs through tandem (trunk-to-trunk) switches. The book The New McGraw-Hill Telecom Factbook by Joseph A. Pecar and David A. Garbin (McGraw-Hill Professional Publishing, 2000) is an excellent reference for different designs and details, and conveys a complex technical subject in an easy-to-understand manner.

An example of interaction between the LEC and IXC for billing purposes is shown in Figure 3-5.

Figure 3-5. Example Showing an Interconnection Between SBC as a LEC and Sprint as an IXC in the San Jose, California Area (Courtesy of Cisco Provisioning)