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For purposes of illustration, at least one example merits further discussion. A remote worker might desire to access an application residing on a LAN-connected server. An ISDN call to a local LAN site would save on long distance charges; that LAN site would connect the user to a remote site through another ISDN link. When the remote client workstation is idle, ISDN would disconnect the LAN-to-LAN link to save on long distance charges. Through a process known as spoofing, the application would remain alive, as it continues to see a logical link over the B-channel. The interactive data conversation can quickly be reinitiated due to the fast call setup time of SS7, which is an integral element of ISDN. The remote worker in this scenario might be a telecommuter, working from home several days a week.
In terms of vertical markets, ISDN is of particular interest in the healthcare and education sectors, largely because of its ability to support imaging and video through rate adaption. TeleMedicine, in particular, is of interest, as it allows specialists to diagnose and treat patients in remote areas based on video examination and transmission of X-ray images across error-free and high-speed ISDN links.
The applications for ISDN are virtually unlimited, at least in terms of the network services that can be accessed. Through a single ISDN local loop, voice, facsimile, data, video and image information can be accommodated. Additionally, simultaneous access to multiple networks and network services can be accomplished, perhaps including circuit-switched voice, X.25 packet, and Frame Relay. From a user perspective, ISDN is highly flexible. From a carrier perspective, ISDN offers the advantage of consolidating access to multiple networks; relieving the strain on local loop, switching, and transport facilities. Additionally, ISDN offers the LECs the promise of putting the telephone company world back together much like it was before deregulation, divestiture, and competition. In other words, the LECs can market ISDN as a single network access solutionsort of a one-stop shop.
ISDN has experienced differing levels of success around the world due to various marketing approaches, pricing strategies and, in some cases, aggressive government support. For instance, the Japanese government has lent strong support to the development and deployment of high-technology networks and network services.
Telecom Australia was very successful in marketing PRI to large user organizations, in part as an alternative to leased lines. An unusual offering is that of semi-permanent circuits within PRI, priced at approximately 50% of the cost of a dedicated circuit. In the competitive Australian telecom environment, this approach was successful in countering leased-line networks offered by alternative carriers such as Optus.
Deutsche Bundespost Telekom offers ISDN on a widely available and low cost basis. In excess of 80% of Germanys population has access to ISDN within six weeks of placing an order. Pricing is very attractive compared to leased lines. For application development, CAPI (Common Application Programming Interface) is supported.
For years, carriers in Western Europe have offered a service known as 0B+D. That offering provides access to a solo 16 Kbps D-channel for low-speed data transmission. Packet data is supported at speeds up to 9.6 Kbps, with signaling and control consuming the balance of the capacityno B-channels are involved. This service effectively challenges X.25 packet networking for transaction-oriented applications such as credit card authorization.
Although Broadband Data Networking will be discussed in detail in a later section, it is worth a quick overview of certain key technologies and service offerings, including Frame Relay and Cell Relay.
A relative newcomer, Frame Relay was first deployed in the mid-1990s. Much like packet switching, each frame is addressed individually. Frame relay also makes use of special switches and a shared network, of very high speed. Unlike packet switching, frame relay supports the transmission of virtually any computer data stream and frames are variable in length (up to 4,096 bytes). Rapidly gaining in popularity, frame relay is widely available in many highly developed nations. International frame relay service is also becoming widely available. A service which is primarily data-oriented, Frame Relay does not support voice or video effectively. As is the case with X.25 packet switching, Frame Relay overhead can be relatively high, delays (latency) in transmission are expected, and network congestion can result in lost data; the responsibility for error detection and correction is shifted to the user in a Frame Relay environment.
Clearly the future of communications networking, cell switching encompasses both Switched Multimegabit Data Service (SMDS) and Asynchronous Transfer Mode (ATM), data is organized into cells of fixed length (53 octets), shipped across very high-speed facilities and switched through very high speed, specialized switches. While SMDS has proved to be very effective for data communications ATM will be pervasive in the future; ATM will be the platform for Broadband ISDN (B-ISDN). ATM is primarily data-oriented, although it is ultimately intended to support voice and video. Standards are still developing and availability is limited. Current disadvantages of ATM include its relatively high cost, high overhead, and lack of fully developed standards.
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