Telecom or common carrier networks have evolved over many years. The basic elements include switching, transmission, and network management. Telecom network operations run 24/7 and require very similar levels of flexibility and reliability as broadcast operations. Telecom networks are said to be ubiquitous and are built to reach the widest possible market for their services.
A key characteristic of communications networks is the concept of channelization or use of one or a group of channels between two points to support movement of information. Channelization first appeared when Edison and a now nameless technician accidentally discovered movement of sound waves over a pair of wires. That first pair of wires eventually turned into two pair to facilitate a two-way talk path. Over time through the magic of technological evolution ways and means to enable multiple channels on a single talk path, or four-wire facility were realized. For many years the jellybean of telephone technology was and still is the voice grade channel. The channel, be it wire or a virtual channel has certain capabilities and limitations bound by the laws of physics. Attempting to send a 30-Mbs payload through a 64-Kbs channel doesn’t result in any more success than an attempt to pump 30 barrels of oil per hour through a 1-inch pipe. Expecting networks to carry voice, data, and video without some way to match each with a unique part of a communications channel is the equivalent of mixing oil, water, and orange juice into the same pipe and expecting each to arrive intact at the other end. The challenge is not so much in the mixing as in the separation at the receive end.
Telecom network architecture is standards based. Most telecom network operators standardize and base their designs on a limited number of manufacturers and suppliers, generally constrained by maintaining sound, competitive procurement practices. Equipment and software suppliers active in the market generally participate in standards development and tend to comply in most, if not all, aspects of applicable standards.
Telecom networks interoperate across business entity and physical boundaries that are local, regional, national, and international.
Telecom networks are built using a layered architecture. This architecture is based on international standards and consists of a physical layer, facilities, and service layers. Although this layering characteristic is related, it should not be confused with the seven-layer ISO model commonly used in data communications and information technology documentation. It is also related and should not be confused with the four-layer approach sometimes seen or referred to in Internet or Internet protocol (IP) documentation.
One of the main objectives of this book is to bring clarity to this picture for the reader. Clarity comes from understanding a greater level of detail of the telecom entity. For purposes here, the Internet is another facilities or service layer in the overall architecture. The Internet relies on classical telecom physical or transmission layer infrastructure. It is likely to remain that way for the foreseeable future.
Many, if not most Internet people, netizens as they sometimes refer to themselves in a third-person way, lack appreciation and understanding of the physical layer or the entire classical telecom infrastructure. To many it’s old, outdated, obsolete, and subject to complete disregard, yet it is critical to successful operation of the Internet. That’s because the original Department of Defense Advanced Research Projects Agency project included an assumption that the physical layer would always be 100% available. That general approach and attitude remains today and is reflected in such things as contemporary certification training and testing by router equipment manufacturers. There has always been a similar attitude and approach in classical data communications network design and operations. The simple message in this point is ‘‘don’t forget the layer 1 and 2.’’
Another area of misunderstanding and common misuse is the term private network. Networks are made up of individual elements of hardware and software, supported and managed by a sophisticated network and equipment management infrastructure capable of monitoring, detecting, and reporting network behavior and performance outside predetermined limits. Strictly speaking, a private network is built using the same or similar kinds of equipment, software, and supporting infrastructure. It requires capital investment and ongoing operations expense. In the same context, private networks are used exclusively by their owners and don’t provide services or facilities to others. Satellite facilities are generally thought of as private networks; however, regardless of the term, they are built using shared and nonshared equipment.
Public networks, also called common carrier networks, are shared among a community of enterprise and residential customers. They operate under rules and regulations promulgated by the Federal Communications Commission (FCC) and state Public Utilities Commissions (PUCs), based on state and federal law in the United States, and similar government bodies in other countries. This includes physical aspects of orbital platforms, radio frequency spectrum, and, to a lesser degree, the equipment and services in ground station facilities.
Virtual private network (VPN) is a term used to describe a network designed and created from public network facilities and services. In some designs, the customer purchases or otherwise acquires the right to use equipment and software making up a VPN. Typically, this equipment is located on customer premises, although it may be colocated in service provider facilities with other customers or carrier owned equipment. VPNs didn’t just show up yesterday. The incarnation in the mid-1980s was software-defined network (SDN). These terms are another source of confusion. At a high level, both mean the same thing. Both evolved from private networks, which are built using point-to-point private line facilities and premises based multiplexing and switching equipment. The use of the terms virtual private and software-defined came into vogue around the time of the divestiture of AT&T and the deregulation of the long distance industry. Early examples of SDNs include AT&T and the Bell Operating Companies, who shared central office switching as an alternative to a private branch exchange and enhanced private switched communications services.
VPN is often used to describe a data network, but it is nothing more than a combination of point-to-point private leased lines and shared core routers or, in the old days, multiplexers. Some will claim that VPNs include encryption, firewalls, and so-called tunneling and that they are capable of carrying voice traffic. These claims and attributes are all true; however, the concept of shared faculties is the same as used in the SDNs that arose in the late 1970s and early 1980s to support multiple locations private dial plans for telephone service.
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