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Digital Data Services
When networks must span more than a few kilometre's, new categories of technology come into play. Before considering WAN standards, it is useful to take a look at options that might be used by an organisation that wants to build a private WAN. Not all options are examined. A Leased line is a dedicated communication line between two points. It’s usually used by organisations to connect computers over a dedicated telephone line. Communication providers offer dedicated, leased lines at a variety of capacities. A dedicated line is a communication channel between two points that is leased by an organisation for its exclusive use. The dedicated line almost certainly does not consist of a pair of wires that stretches continuously between the end-points, and a customer's signal can pass through any combination of copper and optical fibre cables as well as terrestrial and satellite microwaves. The appearance to the customer, however, is of a directly wired channel. Dedicated lines may be analogue or digital in nature. T1 is an example of a digital leased-line technology. T1 supports full-duplex communication between two points. Originally intended for digital voice communication, T1 adapts as well to data communication, supporting data rates up to 1.544 Mbps. T1 circuits can utilise combinations of cables and microwave links. A T1 line supports 24 multiplexed 64 Kbps channels. Fractional T1 enables organisations to lease part of a T1 line in 64 Kbps increments. Other standards include T2 (6.312 Mbps), T3 (44.736 Mbps) and T4 (274.176 Mbps).
Figure 64 shows connecting remote sites with a Digital Leased Circuit. The interface to the leased line consists of a bridge or router to forward frames to the leased circuit. A Channel Service Unit/Digital Service Unit (CSU/DSU) to translate between LAN and the Digital Data Service (DDS) signal formats. A network interface provided by the communication service vendor. Leased lines can be used to construct quite large networks. The Internet is a world wide network that consists of thousands of host. Most connected by leased lines. The participant in the Internet share the cost of operating the Internet by bearing the cost of one or more leased lines to connect to other host sites. The downside of leased lines is that an organisation bears the full cost of the capacity they have leased. Some allowance must be made for peak traffic periods, and a portion of the channel capacity being paid for may be idle a great deal of the time. Dedicated lines ensure an organisation of a specified communication capacity, but come at a high cost. Switched lines provide an alternative to dedicated lines. When remote hosts need to communicate, one dials the other to establish a temporary connection. Switched connection can be configured using conventional modems and voice-grade lines, enabling organisations that have very limited bandwidth needs to avoid the cost of a digital service. A technology of switched digital communication is the Integrated Services Digital Network (ISDN). A variety of ISDN services are possible, providing different amounts of bandwidth. A common basic rate service consists of two 64 Kbps digital channels. Although the potential bandwidth of this service is 128 Kbps, the 64 Kbps channels function separately. Equipment at the customer site must be capable of aggregating the separate 64 Kbps channels into a 128 Kbps logical channel. ISDN has the potential to make switched digital communication widely available at low cost.
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