Demands on Today's Data Communications Technologies
1.1 -The Evolution of Data Transmission Technologies
For decades the development of data transmission technologies has not kept pace with the explosive increase in the capabilities of individual computer systems. Processor performance and data storage capacity in PCs, for example, increased a hundredfold in the 1980s, while data speeds in wide area networks increased only tenfold over the same period. Data speeds in local area networks even remained unchanged for long intervals, mainly due to the lengthy standardization processes involved in introducing new technologies. In the mid-1990s, however, this situation began to change, as high-speed technologies such as Asynchronous Transfer Mode (ATM) and Gigabit Ethernet were finally standardized and a wide range of affordable products for use with these technologies became available.
1.1.1 Local Area Networks
The majority of the local area networks (LANs) in use today are still based on transmission principles developed in the early to mid-1980s: 10/100 Mbit/s Ethernet (IEEE 802.3), and 4 or 16 Mbit/s Token Ring (IEEE 802.5). Since a substantial increase in network bandwidth was not feasible for many years, the number of network nodes per segment had to be reduced in order to accommodate the increasing use of multimedia and network-oriented applications made possible by faster processors. In the 1980s, for example, 802.3 Ethernet networks with more than 300 stations were not unusual, whereas today the average number of stations per segment is between 10 and 20 and falling.
A new LAN standard, called Fiber Distributed Digital Interface (FDDI), was introduced at the end of the 1980s. Based on fiber optic media, FDDI technology provided a data speed of 100 Mbit/s. FDDI was the first technology that made it possible to build high-performance backbone structures, but it gained acceptance only slowly because it required expensive hardware components, such as lasers on network cards and optical fiber cabling. Furthermore, it was soon apparent that even its bandwidth of 100 Mbit/s, shared by all nodes, would not be sufficient for emerging multimedia applications; in other words, FDDI was a medium-term solution at best. Nonetheless, for lack of alternative network technologies, many backbones were converted to FDDI in the years following its introduction. This all changed once again when LAN switching, 100 Mbit/s Ethernet, and 155 Mbit/s ATM technologies became marketable, providing more flexible and economical backbone solutions than those based on FDDI. Since then, the use of FDDI has been declining, and it is rarely considered as an option when new networks are designed. At the beginning of the first decade of the third millennium, further improvements in speed with technologies such as Gigabit Ethernet and 10 Gigabit Ethernet are starting to become widely deployed.
Although ATM is often mentioned in the same context as other high-speed technologies such as Fast Ethernet, Gigabit Ethernet, and LAN switching, there is a significant difference between ATM and all other communication technologies used in local area networks. Unlike the connectionless transmission mechanisms used by other LAN topologies, ATM uses connection-oriented data communication. Before transmission starts, a signaling process sets up a channel with the required bandwidth, delay, and other characteristics. User data is sent over this channel until a command on the signaling channel ends the connection. Insufficient bandwidth is not the only limitation that makes traditional network topologies unable to handle today's multimedia applications, since the effective bandwidth can be increased by limiting the network to one station per segment, as seen in segment-switching topologies. In addition to bandwidth, the transmission of multimedia applications over networks also requires real-time behavior that simply cannot be provided by the connectionless LAN data transmission technologies of the 1980s. ATM permits unrestricted use of multimedia applications in LANs. A connection between two ATM stations is not affected by the number of other stations in the network, since each station is supplied with transmission paths of fixed bandwidth and guaranteed communication characteristics, which are set for each connection in a "traffic contract."
Similar quality of service (QoS) mechanisms have been implemented in the IP protocol family over the past years. They enable the transmission of real-time IP services such as audio and video over broadband connectionless infrastructures such as Gigabit Ethernet. This has lead to the situation that today in local area networks ATM is primarily implemented for demanding, high performance backbones with specific needs, whereas the majority of LANs are migrating toward Gigabit Ethernet as a backbone infrastructure.
1.1.2 Wide Area Networks
While the transmission capabilities of LANs have been evolving in few step functions over the past decades, the available data speeds in wide area networks (WANs) have increased steadily over the years. X.25 connections, for example, widely used in the 1970s with data speeds of 2.4 kbit/s and 4.8 kbit/s, were replaced by Frame Relay links with speeds of 1.5 Mbit/s and 45 Mbit/s in North America, and 2 Mbit/s and 34 Mbit/s elsewhere (Figure 1.1). The Integrated Services Digital Network (ISDN), introduced in the mid-1980s, permitted more efficient use of communication lines by bundling analog and digital services. Especially in Europe, ISDN became an important medium for telephony and data transmission, until in 2000 xDSL technologies started to provide even higher data rates over the same telephony infrastructure. While ISDN provides transmission capacities of between 56 kbit/s (Basic Rate ISDN, or BRI) to 2 Mbit/s (Primary Rate ISDN, or PRI), the various xDSL services deliver data speeds from 128 bit/s to 8Mbit/s.
Figure 1.1 The evolution of data transmission technologies, 19802000.
When the ISDN specifications were developed in the early 1980s, it was assumed that bandwidths of 128 kbit/s (BRI) and 2 Mbit/s (PRI) would be sufficient for years to come. Soon after the protracted standardization process for ISDN was concluded, however, it was already apparent that the payload bandwidth of n x 64 kbit/s on which ISDN is based would not meet the rapidly increasing demands placed on data communications technology. The next step had to be the development of what were termed "broadband transmission systems," with bandwidths far beyond 2 Mbit/s. In the second half of the 1980s, standardization work was begun on a broadband ISDN (B-ISDN) specification, intended to be the future universal wide area network technology. The Asynchronous Transfer Mode, or ATM, was selected by the ITU in 1988 as the transport mechanism for B-ISDN. Since then, major telecommunications providers around the world have been setting up and operating B-ISDN (mostly referred to as ATM) communication networks.