Data Over Voice Networks
The next step in the evolution of networking was that of data transmission over voice networks. In the late 1950s large organizations in North America wanted to move data over telephone lines. This first was accomplished with a variation of the IBM 729 tape drive, which interfaced with the analog PSTN through a Bell dataset (1957) [8-2], or DataPhone (1961) [8-1], via an acoustic coupler and telephone set; an exact replica reconstituted the digital data stream on the receive end. Datasets quickly spread around the world. The telephone companies and PTTs rented the datasets to end users; deregulation let users acquire and interconnect their own equipment. The original datasets connected to the PSTN through a Direct Access Arrangement (DAA) device which served as a coupler, or protector, to protect the network from high signal levels, out-of-band frequencies, and aberrant voltages. This protection is incorporated into contemporary modems and other devices and is standardized and regulated by the ITU-T on an international basis and by the FCC (US) and other national regulatory bodies.
The telephone companies began to digitize their networks in the 1960s, as digital technology became reliable and inexpensive enough to support telecommunications applications and as the requirement surfaced for increased bandwidth in the carrier networks. Digital transmission facilities, in the form of T-carrier (North America) and E-carrier (Europe), increased the traffic capacity of existing facilities. In the 1970s, analog Electronic Common Control (ECC) switches began to be replaced with fully digital switches. Data transmission at relatively high speeds and over fully digital networks became a reality, although it would be a number of years until such capability became widely available.
Dataphone Digital Service (DDS) was the first publicly available service that was digital from end-to-end. Although still in wide use, it now competes with E/T-carrier, X.25, ISDN, Frame Relay, SMDS, and ATM for data transmission.
Data Networking Over the WAN (and MAN): Digital Data Networking
The evolution of data communications progressed at a snail’s pace for over a hundred years. For much of that time, neither the applications nor the enabling technologies existed. DDS and X.25 were major steps forward in the 1960s; T-carrier added to the solutions suite in the 1970s. ISDN began to appear in the 1980s, but never developed into a viable service offering. Only very recently has ISDN emerged from 25 years of obscurity to offer cost-effective solutions in certain applications.
Recently the evolution of digital data networking over Wide Area Networks (WANs) and Metropolitan Area Networks (MANs) has progressed with truly blinding speed. Frame Relay and SMDS are now widely deployed both domestically and internationally. ATM, while currently deployed on a limited basis, is viewed by most industry experts to be the network technology of choice far into the future. Broadband ISDN, the ultimate service offering of ATM, is touted as the culmination of network technologies in a broadband service scenario.
Clearly, digital transmission offers significant advantages, especially for data transmission. The advantages include increased bandwidth and bandwidth utilization, improved error performance and increased throughput, and enhanced management and control. This chapter focuses on conventional digital data networking options including dedicated leased lines, circuit switched connections, and packet switching; specifically Dataphone Digital Service (DDS), Switched 56, T-carrier, Packet Switching (X.25), and ISDN. An introduction into emerging data networking options will be presented in much greater detail in a later section including Frame Relay and Cell Relay (SMDS, ATM, and B-ISDN).
Dataphone Digital Service (DDS)
Dataphone Digital Service (DDS), also known as Digital Data System and SubRate Digital Loop (SRDL), was introduced by AT&T in 1974 [8-2], but the term now is used generically to describe an end-to-end, fully digital, dedicated service provided by most carriers. DDS is widely deployed in the United States and Canada and many other developed countries and is intended for relatively high-speed data transport between purely digital devices (computers). Employing specially-conditioned, dedicated, leased-line circuits provided to user organizations by the carriers, a DDS configuration may be either point-to-point or multipoint. In either case, all network control is the responsibility of a designated head-end system. The head-end, usually in the form of a Front-End Processor (FEP), controls all access to the network through a process of polling the remote devices. Additionally, all communications must pass through the head end; in other words, devices cannot communicate directly, as would be the case in a mesh network, where all locations are interconnected directly.
DDS is intended for full-duplex (FDX) synchronous communication provided over 4-wire circuits between devices of significance, (e.g., mainframes) and which communicate intensively (frequently and passing significant volumes of data). The DDS network provides network timing and synchronization through a master clock, to ensure that all clocks in all slaved network nodes operate at the same rate, or clock speed. These same timing signals are provided to end user Data Communications Equipment (DCE) for transmission synchronization. DCE is in the form of a DSU/CSU, as described in previous chapters. The DSU/CSU generally will operate at the full line rate, or on a subrate basis (lower speed), as required. While transmission generally is full-duplex (FDX), half-duplex (HDX) and simplex transmission is also served.
Transmission rates vary, within limits, according to the user organization’s requirements. Bandwidth generally is available at line rates of 2400 bps, 4800 bps, 9600 bps, 19.2 Kbps, and 56 Kbps (64 Kbps service is available in some areas). DDS signals actually are carried inside T1 channels in the carriers’ internal networks (T1 will be discussed later in this chapter).
While the cost equation of DDS circuits varies according to specific carrier tariffs and pricing strategies, cost is sensitive to distance between the points of termination and the level of bandwidth-such is the case with all dedicated leased-line services. A traditional rule of thumb is that DDS generally is cost-effective in applications that require communications between two locations greater than one hour per day at 56 Kbps.
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