Category: VoIP

While data and telephony networks share much in common, one of the major ways in which they differ is with respect to the terminology used to describe network elements such as the links used to interconnect different types of equipment. For example, you’ve already learned that the link between a telecommunication carrier’s central office (CO) and a user’s home is known as the local loop. In most locations worldwide, the local loop is still implemented using a pair of copper wires sometimes referred to as the “tip and ring”. When two telephone systems are interconnected, such as two PSTN switches or a PSTN switch and a PBX, the link is described as a trunk link. The points below outline the different terms used to describe the various links on the telephone network.

Local loop. A link that interconnects a CO switch to a user home, typically implemented using a pair of copper wires.

Tie trunk. A link that interconnects PBXs.

CO trunks. A link that interconnects a CO switch and a PBX.

PSTN switch trunks. A link that interconnects switches on a carrier network.

The local loop connection between the CO switch and a user’s home is often referred to more generically as a phone line, and this is also correct. However, in an office environment where user telephones connect to a PBX, the correct term used to describe the connection is “station line”.

Much like in the world of data networking, specialized equipment is required in order to process and properly route telephone calls to the correct switch, and ultimately the recipient. In the world of telephony, calls are routed through the PSTN using high-end switching equipment located at the central offices and facilities of one or more telecommunications carriers. In larger organizations, these same functions are typically handled by specialized hardware systems known as a private branch exchange, more commonly known as a PBX. While a PBX is used to handle the routing of calls within an organization, public telephone switches are used to route calls across the PSTN.

Although the two devices are very similar in terms of the core function they provide (routing calls), they differ greatly in terms of scale and services offered. A public telephone switch at a CO will typically be capable of handle hundreds of thousands of phone lines, while a PBX may support anywhere from a few hundred to a few thousand. Where the public phone switch is primarily focused on the routing of phone calls, a PBX typically provides a variety of value-added features required by businesses including the voice mail, conference calling, and more. When a company implements a PBX, they are effectively setting up their own personal phone switch, which can be configured and administered according to their needs. While this provides organizations with a higher degree of flexibility, it also adds administrative responsibilities. Furthermore, to reach the outside world (lines external to their organization), a connection to the PSTN and ultimately the phone switch of the carrier’s local CO is still required. In most cases, this connection is provided by a dedicated circuit (T1 or T3 for example), depending upon the number of “outside” lines that the company requires.

Note: Although the implementation of a PBX allows an organization to add services or change the internal configuration of their phone system as their needs dictate, the investment in a PBX needs to be closely scrutinized. In most cases, the implementation of a PBX from a particular vendor requires that all associated equipment (such as handsets) be purchased from the same vendor. This can significantly increase the overall cost of the system.

Although almost all high-end telecommunications equipment produced today uses digital transmission in some form or another, the most common element that home users come into contact with still usually functions in analog – namely their telephone handset. With analog communication, a user’s voice is represented as an analog waveform that is transmitted across the wires connecting their phone to the telecommunication carrier’s local central office (CO). Many years ago, the telecommunication carrier’s entire network was comprised of analog equipment, and that waveform was ultimately sent across a dedicated circuit all the way to the receiver’s phone.

Although analog transmission works, it is not particularly efficiently, and is susceptible to noise interference during the transmission process. As such, almost all carrier networks are now entirely digital, with the exception of the last portion, from the user’s home to the central office (the local loop). In the local loop, analog still reigns supreme, and likely will for many years to come.

Because the local loop is largely still analog, and the rest of the carrier network digital, a conversion needs to take place. Specifically, the analog signal that travels from the user telephone to the CO needs to be converted to a digital signal to cross the carrier network, and then back to analog from the recipient’s CO to their phone (and ultimately their ear, which is able to comprehend the analog waveform – your voice). The process of converting the originating analog signal to digital requires the local CO telephone switch to “sample” the analog waveform, and then produce a digital representation of it. At the receiving end, this process is reversed, with the switch taking the digital signal and converting it back into a waveform. In the world of telephony, this is usually accomplished using a technique known as pulse code modulation (PCM).

While you aren’t required to be familiar with all of the details of how PCM works for the CCDA exam, the steps below are provided to give you a better sense of the conversion process for your own understanding.

1. The original analog signal is filtered by a coder-decoder (codec) so that only the range of frequencies that can be heard by the human ear (approximately 300hz to 3400hz) are sampled.

2. The filtered signal is sampled approximately 8000 times per second using a technique known as pulse amplitude modulation (PAM).

3. Each sample is converted into binary using PCM, and then transferred across the digital network. At receiving CO switch, this process is reversed to convert the signal from digital back into an analog waveform.