2.1.6. The Presentation Layer
Unlike the lower layers, which are concerned with getting the bits from the sender to the receiver reliably and efficiently, the presentation layer is concerned with the meaning of the bits. Most messages do not consist of random bit strings, but more structured information such as people's names, addresses, amounts of money, and so on. In the presentation layer it is possible to define records containing fields like these and then have the sender notify the receiver that a message contains a particular record in a certain format. This makes it easier for machines with different internal representations to communicate.
2.1.7. The Application Layer
The application layer is really just a collection of miscellaneous protocols for common activities such as electronic mail, file transfer, and connecting remote terminals to computers over a network. The best known of these are the X.400 electronic mail protocol and the X.500 directory server. Neither this layer nor the two layers directly under it will be of interest to us in this book.
2.2. ASYNCHRONOUS TRANSFER MODE NETWORKS
The OSI world sketched in the previous section was developed in the 1970s and implemented (to some extent) in the 1980s. New developments in the 1990s are overtaking OSI, certainly in the technology-driven lower layers. In this section we will touch just briefly on some of these advances in networking, since future distributed systems will very likely be built on them, and it is important for operating system designers to be aware of them. For a more complete treatment of the state-of-the-art in network technology, see (Kleinrock, 1992; and Partridge, 1993, 1994).
In the past quarter century, computers have improved in performance by many orders of magnitude. Networks have not. When the ARPANET, the predecessor to the Internet, was inaugurated in 1969, it used 56 Kbps communication lines between the nodes. This was state-of-the-art communication then. In the late 1970s and early 1980s, many of these lines were replaced by T1 lines running at 1.5 Mbps. Eventually, the main backbone evolved into a T3 network at 45 Mbps, but most lines on the Internet are still T1 or slower.
New developments are suddenly about to make 155 Mbps the low-end standard, with major trunks running at 1 gigabit/sec and up (Catlett, 1992; Cheung, 1992; and Lyles and Swinehart, 1992). This rapid change will have an enormous impact on distributed systems, making possible all kinds of applications that were previously unthinkable, but it also brings new challenges. It is this new technology that we will describe below.
2.2.1. What Is Asynchronous Transfer Mode?
In the late 1980s, the world's telephone companies finally began to realize that there was more to telecommunications than transmitting voice in 4 KHz analog channels. It is true that data networks, such as X.25, existed for years, but they were clearly stepchildren and frequently ran at 56 or 64 Kbps. Systems like the Internet were regarded as academic curiosities, akin to a two-headed cow in a circus sideshow. Analog voice was where the action (and money) was.
When the telephone companies decided to build networks for the 21st Century, they faced a dilemma: voice traffic is smooth, needing a low, but constant bandwidth, whereas data traffic is bursty, usually needing no bandwidth (when there is no traffic), but sometimes needing a great deal for very short periods of time. Neither traditional circuit switching (used in the Public Switched Telephone Network) nor packet switching (used in the Internet) was suitable for both kinds of traffic.
After much study, a hybrid form using fixed-size blocks over virtual circuits was chosen as a compromise that gave reasonably good performance for both types of traffic. This scheme, called ATM (Asynchronous Transfer Mode) has become an international standard and is likely to play a major role in future distributed systems, both local-area ones and wide-area ones. For tutorials on ATM, see (Le Boudec, 1992; Minzer, 1989; and Newman, 1994).
The ATM model is that a sender first establishes a connection (i.e., a virtual circuit) to the receiver or receivers. During connection establishment, a route is determined from the sender to the receiver(s) and routing information is stored in the switches along the way. Using this connection, packets can be sent, but they are chopped up by the hardware into small, fixed-sized units called cells. The cells for a given virtual circuit all follow the path stored in the switches. When the connection is no longer needed, it is released and the routing information purged from the switches.
This scheme has a number of advantages over traditional packet and circuit switching. The most important one is that a single network can now be used to transport an arbitrary mix of voice, data, broadcast television, videotapes, radio, and other information efficiently, replacing what were previously separate networks (telephone, X.25, cable TV, etc.). New services, such as video conferencing for businesses, will also use it. In all cases, what the network sees is cells; it does not care what is in them. This integration represents an enormous cost saving and simplification that will make it possible for each home and business to have a single wire (or fiber) coming in for all its communication and information needs. It will also make possible new applications, such as video-on-demand, teleconferencing, and access to thousands of remote data bases.
Cell switching lends itself well to multicasting (one cell going to many destinations), a technique needed for transmitting broadcast television to thousands of houses at the same time. Conventional circuit switching, as used in the telephone system, cannot handle this. Broadcast media, such as cable TV can, but they cannot handle point-to-point traffic without wasting bandwidth (effectively broadcasting every message). The advantage of cell switching is that it can handle both point-to-point and multicasting efficiently.
Fixed-size cells allow rapid switching, something much harder to achieve with current store-and-forward packet switches. They also eliminate the danger of a small packet being delayed because a big one is hogging a needed line. With cell switching, after each cell is transmitted , a new one can be sent, even a new one belonging to a different packet.
ATM has its own protocol hierarchy, as shown in Fig. 2-4. The physical layer has the same functionality as layer 1 in the OSI model. The ATM layer deals with cells and cell transport, including routing, so it covers OSI layer 2 and part of layer 3. However, unlike OSI layer 2, the ATM layer does not recover lost or damaged cells. The adaptation layer handles breaking packets into cells and reassembling them at the other end, which does not appear explicitly in the OSI model until layer 4. The service offered by the adaptation layer is not a perfectly reliable end-to-end service, so transport connections must be implemented in the upper layers, for example, by using ATM cells to carry TCP/IP traffic.
Fig. 2-4. The ATM reference model.
In the following sections, we will examine the lowest three layers of Fig. 2-4 in turn, starting at the bottom and working our way up.
2.2.2. The ATM Physical Layer
An ATM adaptor board plugged into a computer can put out a stream of cells onto a wire or fiber. The transmission stream must be continuous. When there are no data to be sent, empty cells are transmitted, which means that in the physical layer, ATM is really synchronous, not asynchronous. Within a virtual circuit, however, it is asynchronous.