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The cost equation changes where multiple locations are connected in a multipoint network configuration. A multipoint circuit also is known as a multi-drop circuit in telephone company parlance, as local loop connections historically are dropped from poles where they connect to larger cables. Multipoint circuits are also referred to as fantail circuits, as they fan out like the tail of a fish on the distant end. As shown in Figure 8.1, a headquarters data center in New York might be connected to a regional data center in Seattle. Field offices in Everett and Tacoma can cost-effectively take advantage of the same coast-to-coast circuit, as the incremental circuit mileage is low and the associated cost of those additional drops, therefore, is low.


Figure 8.1  DDS leased lines connecting data centers in New York and Seattle, with drops to Everett and Tacoma.

In such a multipoint network, the head-end addresses each remote system connected to the circuit, on the basis of a unique logical address and in a user-definable and variable polling sequence. The target device recognizes its address and responds across the network—all other devices remain silent. At that point, the two computer systems conduct a dialogue until such time as either the data transfer is complete or until the head-end truncates the communication in order to address other devices according to its programmed polling schedule. ATM (Automatic Teller Machines) typically are connected to the central bank in such a manner. In a typical ATM scenario, the central computer polls the individual ATMs, downloads the user’s request for a cash withdrawal, matches the account number and PIN for authentication purposes, queries the centralized database to determine the assigned level of withdrawal privileges and the current account balance, and either authorizes or denies the cash withdrawal.

DDS provides excellent reliability, which generally is stated as being 99.5%. All dedicated services, however, are susceptible to catastrophic failure from such causes as cable-seeking backhoes. Therefore, network redundancy must be considered in the form of either back-up DDS circuits or some alternative network service.

From an applications perspective, DDS is used for relatively intensive data-only communications applications between fixed physical addresses. In such an environment, it can be highly cost-effective, as usage charges do not apply to network traffic. Typical applications include connecting data centers for purposes of file transfer or data backup. Image transfer and other bandwidth-intensive applications such as CAD (Computer-Aided Design) can make cost-effective use of DDS circuits, also benefiting from the bandwidth and excellent error performance offered by the dedicated digital circuit. DDS also serves to connect e-mail and Group IV facsimile servers in a messaging network.

Switched 56 Kbps

Switched 56 is the generic name for Digital Switched Access (DSA), although 64 Kbps service is available in some areas. Switched 56 is a circuit-switched digital service intended generally for the same applications as DDS, although it is more cost-effective for less intensive communications. Although the service is switched, rather than dedicated, most of the general characteristics and all of the components are the same as DDS; the sole exception being that digital carrier exchanges are involved in setting up the DSA connections. DTE is in the form of computer systems that connect to digital local loops through DCE in the form of a DSU/CSU. Digital exchanges serve to switch the connection (see Figure 8.2), which is provided through digital carrier transmission facilities on the basis of special routing logic.


Figure 8.2  Switched 56 Kbps service.

The key difference between DDS and Switched 56 is that the calls are switched between physical locations on the basis of a logical address, which is the computer equivalent of a voice telephone number. In fact, Switched 56 is the digital data equivalent of a circuit-switched voice call through the PSTN. Based on specific routing instructions, the digital network switches establish the end-to-end circuit over entirely digital circuits. The call is set up, maintained, and torn down much like a voice call and it is priced in much the same way; the cost of the call is sensitive to distance, duration, time of day, and day of year. The cost of the call and of the local loop ($30 to $75 per month), of course, are dependent on specific carrier tariffs and pricing strategies. As the carriers’ Switched 56 service networks typically are not interconnected, calling generally is limited to each specific carrier domain unless the user has made other arrangements.

While DDS is more cost-effective for applications in which communications are intensive between specific physical locations, Switched 56 Kbps service is more cost-effective for communications between locations that communicate less frequently or that communicate lesser amounts of data. Switched 56 services often are employed as a back-up to DDS facilities; calls are switched through the highly redundant carrier networks, rather than relying on vulnerable dedicated circuits as is the case with DDS.

Digital Carrier Systems and Networks: T-carrier

Carrier systems are defined as systems that derive multiple logical channels from a single physical communications path, thereby supporting multiple communications. Initially developed for use within public carrier (LEC and IXC) networks, the systems provided increased traffic capacity between exchanges without the requirement for additional transmission facilities. As voice traffic grew in the post-war (W.W.II) period, the Central Office (CO) exchanges were strained, as were the transmission facilities connecting them—carrier systems solved that problem [8-1].

The original N-carrier (N = Number of channels) system, introduced in 1950, employed twisted-pair cable for connecting CO exchanges on an analog basis. A 2-wire or 4-wire analog connection employed multiplexers to deliver groups of 12 and, later, 24 Frequency Division Multiplexed (FDM) voice-grade channels. Technology was developed to provide supergroups of 60 channels and master-groups of 600 channels [8-2], [8-3], and [8-4].


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