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Network Access Services
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
Network access services provide businesses with communication links to carrier and service provider wide area networks. A telephone is connected via twisted-pair copper wire (the local loop) to the public telephone network where switches connect calls. Internet users can connect to the Internet over the same local loop or use a variety of other services, including cable TV connections, wireless connections, and fiber-optic connections. This topic surveys traditional and emerging network access methods.
The traditional access method is the public-switched telephone network. The earliest IBM mainframe systems communicated over standard phone lines using protocols such as BSC (Binary Synchronous Communication) and later SDLC (Synchronous Data Link Control). The early Internet (then called ARPANET) was connected with a mesh of AT&T long-distance telephone lines that provided 50-Kbit/sec throughput.
The PSTN and its TDM (time division multiplexing) architecture have defined network access for years. Only recently has this changed as cable TV companies and CLECs (competitive local exchange carriers) provide alternative services that are not restricted to the time slotting of the TDM system and the restrictions of a backbone transport network designed and built by the world's telephone companies. Alternative services include metropolitan optical networks running Ethernet and broadband wireless systems (radio, microwave, or infrared). A central theme with new network access services is convergence of voice and data networks into an all-packet-switched network that interconnects with the older PSTN for backward compatibility. This is the so-called NPN (new public network). The concept is to improve the quality of service on the Internet to support voice and other real-time traffic with the same reliability as the circuit-switched PSTN.
As mentioned, the telephone network was designed to carry multiple digitized voice phone calls over a single trunk. In the case of the North American T1 trunk, an analog voice call is digitized into a 64-Kbit/sec stream and inserted into a continuous series of interleaved time slots on the trunk. TDM is good for voice because it supports the real-time streaming nature of voice, but it is not so good for data, which is usually transmitted in bursts. Allocating burst data to repeating time slots is inefficient.
The incumbent telephone carriers still hold a monopoly on access services in many areas, and they are not about to give up a service that is profitable to them in order to build a new costly network. One telephone company executive said that telephone carriers would continue to push TDM circuits until telephone switching equipment is fully depreciated (2010 in some cases), even though more efficient data services could be established.
New Service Providers and Networks
Common service providers are the ILECs (incumbent local exchange carriers), CLECs (competitive local exchange carriers), and ICPs (integrated communications providers). The ILECs are the phone companies that formerly had a monopoly in most service areas. They used to be called the RBOCs (regional Bell operating companies), but due to mergers and acquisitions, this title no longer applies. The Telecom Act unbundled the local loop, allowing hundreds of new service providers to compete in the access market. Under the Act, ILECs must offer network interconnections, co-location of equipment at central offices (on a lease basis and space permitting), access to unbundled elements, and resale of bandwidth at wholesale prices.
The CLECs appeared in the latter half of the 1990s, after the Telecom Act of 1996 opened up the local market to competition. The original CLECs modeled themselves after the ILECs, wholesaling some ILEC services and providing bandwidth and services of their own. The original CLECs had to deploy costly and complex class 5 switches to provide voice services in order to compete with the ILECs. Later, the CLECs went into the high-bandwidth access business by offering DSL services to home users and fiber-optic access to buildings and business parks. One of the defining features of current CLECs is that they have installed integrated access switches from vendors like Santera, Redback Networks, and others. These switches integrate class 5 voice switch functionality with support for any type of data service, including dial-up, DSL, frame relay, ATM, wireless, and others.
ICPs are basically CLECs and other competitive providers that offer a full range of high-speed access services to support the requirements of home and business users. Most have installed the integrated access switches.
An MSPP (multiservice provisioning platform) is a complex and expensive device that sits between customers and the core network. It consolidates diverse traffic streams coming from customers and forwards those streams to the appropriate backbones. Some traffic will go to the PSTN, some will go to the Internet, and some will go to DWDM optical networks that make up the backbone of the carrier or service providers' networks. Traffic with high priority or a need for QoS may go into DWDM optical circuits (lambdas) that are provisioned to provide a high quality of service and a direct path to a specific destination. See "Network Core Technologies."
Data arriving from customer premises equipment does so in a variety of formats such as TDM, DSL, ATM, frame relay, IP, Ethernet, and wireless. The MSPP must process these protocols to ensure that traffic is routed to the appropriate destinations.
The Tachion Networks Fusion 5000 Broadband Services Switch is essentially a "collapsed central office" that integrates class 5 business voice features, packet-based voice and data, and other key ingredients of a traditional central office, such as SS7. It merges transport, switching, routing, and signaling into one highly compact system. A single Fusion 5000 can replace more than a half dozen disparate systems in a traditional central office.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.