1.6. Example Data Communication Services

Telephone companies and others have begun to offer networking services to any organization that wishes to subscribe. The subnet is owned by the network operator, providing communication service for the customers' hosts and terminals. Such a system is called a public network. It is analogous to, and often a part of, the public telephone system.

1.6.1. X.25 Networks

Many older network follow a standard called X.25 developed during the 1970s by CCITT to provide an interface between public packet-switched networks and their customers.

The physical layer protocol, called X.21, specifies the physical, electrical, and procedural interface between the host and the network.

The network layer protocol allows the user to establish virtual circuits and then send packets of up to 128 bytes on them. These packets are delivered reliably and in order. Most X.25 networks work at speeds up to 64 kbps. They are obsolete but still widespread.

X.25 is connection-oriented and supports two kinds of virtual circuits:

1. Switched virtual circuit is created when one computer sends a packet to the network asking to make a call to a remote computer. Once established, it can be used for sending packets.
2. Permanent virtual circuit is set up in advance by agreement between the customer and the carrier. It is always present and no call setup is required to use it. It is analogous to a leased line.

X.25 networks make possible to connect also ordinary (nonintelligent) terminals. It is realized by means of PADs (Packet Assembler Disassembler) whose function is described in a document known as X.3. A standard protocol between the terminal and PAD is called X.28, the protocol between the PAD and the network is called X.29.

1.6.2. Frame Relay

Frame relay can best be thought of as a virtual leased line. The customer leases a permanent virtual circuit between two points and can send frames (i.e. packets) of up to 1600 bytes between them.

The difference between an actual leased line and a virtual leased line is that with an actual one, the user can send traffic all day long at the maximum speed. With a virtual one, data burst may be sent at full speed, but the long-term average usage must be below a predetermined level. In return, the carrier charges much less for a virtual line than a physical one.

Frame relay provides a minimal service. For example, it is up to the user to discover that a frame is missing and to take the necessary action to recover.

1.6.3. Broadband ISDN and ATM

The telephone companies are aced with fundamental problem: multiple networks. Telephone and Telex use old circuit-switched networks. Each of the new data services as frame relay uses its own packet-switched network. DQDB (MAN) is different from these, and there is also the internal telephone call management network. Maintaining all these separate networks is a major headache, and there is another network, cable television, that the telephone companies do not control and would like to.

The solution of this problem is to invent a single new network for the future that will replace all the specialized networks with a single integrated network for all kinds of information transfer. This new network will have a huge data rate compared to all existing networks and services and will make it possible to offer a large variety of new services. This big project is now under way.

The new wide area service is called B-ISDN (Broadband Integrated Services Digital Networks). It will offer:

• video on demand,
• live television from many sources,
• multimedia electronic mail,
• CD-quality music,
• LAN interconnection,
• high-speed data transport for science and industry,
• many other services, all over the telephone line.

The underlying technology that makes B-ISDN possible is called ATM (Asynchronous Transfer Mode) because it is not synchronous (tied to a master clock).

A great deal of work has already been done on ATM and on B-ISDN system, although there is more ahead.

The basic idea behind ATM is to transmit all information in small, fixed-size packet called cells. The cells are 53 bytes long, of which 5 bytes are header and 48 bytes are data. ATM as a service is sometimes called cell relay.

ATM networks are connection-oriented.

ATM networks are organized like traditional WANs, with lines and switches. The intended speeds for ATM networks are 155 Mbps and 622 Mbps, with possible gigabit speeds later. The 155 Mbps speed was chosen because this is about what is needed to transmit high definition television. The exact choice of 155.52 Mbps was made for the compatibility with AT&T's SONET transmission system (the 622 Mbps are 4 155 Mbps channels).

It is worth pointing out that different organizations involved in ATM have different (financial) interests (the long-distance telephone carriers and PTTs vs. computer vendors). All these competing interests do not make the ongoing standardization process any easier, faster, or more coherent. Also, politics within the organization standardizing ATM (The ATM Forum) have considerable influence on where ATM is going.

1.6.4. The B-ISDN ATM Reference Model

• Physical layer. It deals with the issues of the physical medium. ATM cells may be sent on a wire or fiber by themselves, but they may be also be packaged inside the data of other carrier systems. In other words, ATM has been designed to be independent of the transmission medium.
• ATM layer. It deals with cells and cell transport. It defines the layout of a cell. It also deals with establishment and release of virtual circuits. Congestion control is also located here.
• AAL (ATM Adaptation Layer). It allows users to send packets larger than a cell. The ATM layer interface segments these packets, transmits the cells individually, and reassembles them at the other end.

ATM model is defined as being three-dimensional. The user plane deals with data transport, flow control, error correction, and other user functions. The control plane is concerned with connection management. The layer and plane management functions relate to resource management and interlayer coordination.

The physical and AAl layers are each divided into two sublayers (Fig. 1-31):

Fig. 1-31. The ATM layers and sublayers, and their functions.

• the bottom sublayer does the work,
• the top (convergence) sublayer provides the proper interface to the layer above it.

The PMD (Physical Medium Dependent) sublayer interfaces to the actual cable. It moves the bits on and off. For different carriers and cables, this layer will be different.

The TC (Transmission Convergence) sublayer sends the transmitted cells as a string to the PMD sublayer. At the other end, the TC converts a pure incoming bit stream from the PMD sublayer into a cell stream for the ATM layer (the task of data link layer of the OSI model).

ATM layer is a mixture of the OSI data link and network layers.

The SAR (Segmentation And Reassembly) sublayer breaks packets up into cells on the transmission side and puts them back together again at the destination.

The CS (Convergence Sublayer) makes it possible to have ATM systems offer different kinds of services to different applications (e.g. file transfer and video on demand have different requirements concerning error handling, timing , etc.).

1.6.5. Perspective on ATM

Some observations that will influence the future development on ATM:

• ATM is a project invented by the telephone industry, many computer vendors joined with the telephone companies to set up ATM Forum, that will guide the future of ATM.
• ATM is basically high-speed packet-switching, a technology the telephone companies have little experience with. They do have massive investment in a different technology (circuit switching). The transition will not happen quickly - it is a revolutionary change.
• Who will pay for the necessary replacement of the existing telephone system? How much will consumers be willing to pay to get a movie on demand electronically?
• Where the advanced services will be provided is crucial. If they are provided by the network, the telephone companies will profit from them. If they are provided by computers attached to the network, the manufacturer and operator of these devices make the profit.