A legacy public switched telephone network (PSTN) is comprised of the following individual components that provide end-to-end call functionality:

Figure 1.1 illustrates a legacy PSTN.

Figure 1.1 Legacy PSTN network

The two types of business phone systems are private branch exchanges (PBXs) and key systems.

A PBX provides a common connection to the CO for many users. The sizing of the CO connection depends on the call volume, both incoming to the PBX and outgoing from the PBX. For example, based on call volume, a single T1 circuit that supports up to 24 calls could be used in a PBX environment to support 100 users. The CO routes all calls to the PBX in the customer’s Direct Inward Dial (DID) block, such as 773-555-0100 through 773-555-0199, to the T1 connection, and the PBX redirects the incoming calls to the appropriate extension. Typically users dial an access code, such as 9, to gain access to a PSTN circuit.

A key system places all CO connections on everybody’s phone. A key system implementation for a small office with 20 people would have 20 phones with eight direct line connections to the CO on each phone. Users select an available line on their phone to access the PSTN directly.

Legacy telephony uses the following three types of signaling:

The following sections describe each signaling type.

Call setup in the PSTN requires coordination between the COs involved with the calls, as shown in Figure 1.2.

Figure 1.2 Call setup in the PSTN

After the CO collects the subscriber-dialed digits, the CO switch uses SS7 to map the circuit between COs to complete a telephone call across the PSTN. The call usually makes several hops, for example, from the local CO to a long-distance company to the remote CO. At each hop along the way, the CO or service provider telephone switches need to map the incoming call path to the outgoing call path. SS7 is responsible for lining up all the component links to create an end-to-end call.

A numbering plan is used in telecommunications to allocate telephone number lengths and ranges to countries, regions, areas, and exchanges and to nonfixed telephone networks such as mobile phone networks. The numbering plan defines the rules for assigning numbers to end devices.

A typical dialed telephone number is comprised of two types of digits:

A standards governing body, such as the North American Numbering Plan organization, typically administers numbering plans. The governing body is responsible for regulating the distribution and formatting of numbers. Many regional and national numbering plans exist, including the following:

Using the NANP as our first example, telephone numbers are represented as a standard 10-digit pattern, NXX-NXX-XXXX, where N is a digit from 2 through 9 and X is a digit from 0 through 9. The three sections are typically referred to as area code, exchange, and station or line.

Another example that operates in parallel with the individual European countries, such as the U.K. National Numbering Scheme, is the European Telephony Numbering Space (ETNS). ETNS is a European numbering space that is parallel to the existing national numbering spaces and is used to provision pan-European services, which is an international service that can be invoked from at least two European countries. The main objective is to allow effective numbering for European international services for which national numbers might not be adequate and global numbers might not be available. The designation of a new European country code, 388, allows European international companies, services, and individuals to obtain a single European number to access their services.

Four ETNS services are now available: Public Service Application, Customer Service Application, Corporate Networks, and Personal Numbering. A European service identification (ESI) code is designated for each ETNS service. The one-digit service type code follows the European country code 388 and European service code 3 (3883), as shown in Table 1.1.

Table 1.1 ETNS Service and ESI Codes

There are three parts to an ETNS subscriber number:

The maximum length of a European subscriber number is 15 digits; for example, 3883 S XXXXXXXXXX, where S is 1, 3, 5, or 7.

E.164 is an international numbering plan for public telephone systems in which each assigned number contains a country code, a national destination code, and a subscriber number. An E.164 number can have up to 15 digits. The International Telecommunication Union (ITU) originally developed the E.164 plan.

In the E.164 plan, each address is unique worldwide. With up to 15 digits possible in a number, there are 100 trillion possible E.164 phone numbers. This makes it possible, in theory, to direct-dial from any conventional phone set to any other conventional phone set in the world by inputting no more than 15 single digits.

Most telephone numbers belong to the E.164 numbering plan, although this does not necessarily extend to internal PBX extensions.

The E.164 numbering plan for telephone numbers includes the following plans: