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Rather than employing circuit switching, which is far too expensive for intensive, interactive computer communications, ARPAnet, and its successors such as the Internet, make use of packet switching. Packet switching involves the transmission of data in packets of fixed length across a shared network. Each packet is individually addressed, in order that the packet switches can route each packet over the most appropriate and available circuit. In this manner, packets offered to the network by large numbers of users can make use of the same switches and transmission facilities, thereby dramatically lowering the cost of data transmission to the individual user organization.

Traditional packet switching offers the advantage of being based on mature and stable technologies. Additionally, it is widely available on an international basis and is low in cost. Its disadvantages include the fact that it is intended to support only relatively low-speed data transmission. As the switches assume a 1960-vintage analog network environment of twisted pair, each switch is responsible for examining each individual packet for errors created in transmission. Further, each switch is responsible for resolving identified errors through a request for retransmission. These factors, in combination, result in unpredictable, variable levels of delay in packet transmission. Therefore, packet switching generally is considered to be unsuitable for stream-oriented communications such as real-time voice and video.

X.25 (an international standard packet switching interface which will be discussed in Chapter 8) offers great advantage in terms of its ability to support the connection of virtually any computer system through its ability to accomplish protocol conversion. This highly desirable feature classified X.25-based packet networks as the first value-added networks.

Frame Switching (Frame Relay)

A relative newcomer, frame relay was first offered commercially in 1992 by Wiltel (U.S.). 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 in its native form—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. Disadvantages include the fact that frame relay, like packet switching, is oriented towards data transmission. Further, transmission delays are variable and uncertain in duration. While increasingly satisfactory technologies have been added for support of voice and video, frame relay is not designed with those applications in mind.

Cell Switching

Clearly, cell switching is fundamental to the future of communications. Encompassing 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 is primarily data-oriented, although it is ultimately intended to support voice and video as well. Standards are still being developed and availability is limited. Current disadvantages include its relatively high cost and data communications orientation.

Photonic Switches

Still in development, photonic switches are yet another dimension in the evolution of switch technology. Capable of supporting circuit-, packet- frame- and cell switching, photonic switches will eliminate the requirement for optoelectric conversion when connected to a fiber optic transmission system. Clearly, they also will offer advantages in terms of speed and error performance. While prototype photonic switches currently are in use in testbed environments, it likely will be a decade or so before they are commercially viable in CO applications; it is highly unlikely that they will find application in the PBX world, at least not in the foreseeable future.

Signaling and Control

Signaling and control comprises a set of functions which must take place within any network in order ensure that it operates smoothly. In this context, various elements within the network must identify themselves, communicate their status and pass instructions. Fundamental examples include on-hook and off-hook indication, dialtone provision, call routing control, busy signal, and billing instructions. Further examples include dialed digits, route availability, routing preference, carrier preference, and originating number or circuit [2-2].

In more sophisticated, contemporary networks, the responsibility for overall signaling and control functions resides within a separate common channel signaling (CCS) and control network. Such a sophisticated CCS network involves highly intelligent devices which are capable of monitoring and managing large numbers of lower order devices in the communications network which it controls. From a centralized Network Control Center (NCC), the network can be monitored, and faults or performance failures can be identified, diagnosed, and isolated. Finally, the lower order devices in the communications network oftentimes can be addressed and commanded to correct the condition.

References

[2-1] Doll, Dixon R. Data Communications: Facilities, Networks and Systems Design. NY: John Wiley & Sons. 1978.
[2-2] Engineering and Operations in the Bell System. NJ: Bell Telephone Laboratories, Inc. 1977.
[2-3] Bates, Bud. Introduction to T1/T3 Networking. Norwood MA: Artech House. 1992.
[2-4] Augarten, Stan. Bit by Bit. NY: Ticknor & Fields. 1984.
[2-5] Wonders of the Universe. NY: The Werner Company. 1899.
[2-6] Brooks, John. Telephone: The First Hundred Years. NY: Harper & Row, 1976.
[2-7] AG Communications Systems. Introduction to Telecommunications. AZ: AG Communications Systems Corporation, 1990.


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