Thus, fault tolerance was high on their priority list; billing customers was not. This approach led to a connectionless design in which every packet is routed independently of every other packet. As a consequence, if some routers go down during a session, no harm is done as long as the system can reconfigure itself dynamically so that subsequent packets can find some route to the destination, even if it is different from that which previous packets used.
The connection-oriented camp comes from the world of telephone companies. In the telephone system, a caller must dial the called party's number and wait for a connection before talking or sending data. This connection setup establishes a route through the telephone system that is maintained until the call is terminated. All words or packets follow the same route. If a line or switch on the path goes down, the call is aborted. This property is precisely what the DoD did not like about it.
Why do the telephone companies like it then? There are two reasons:
- Quality of service.
- Billing.
The second reason the telephone companies like connection-oriented service is that they are accustomed to charging for connect time. When you make a long distance call (or even a local call outside North America) you are charged by the minute. When networks came around, they just automatically gravitated toward a model in which charging by the minute was easy to do. If you have to set up a connection before sending data, that is when the billing clock starts running. If there is no connection, they cannot charge for it.
Ironically, maintaining billing records is very expensive. If a telephone company were to adopt a flat monthly rate with unlimited calling and no billing or record keeping, it would probably save a huge amount of money, despite the increased calling this policy would generate. Political, regulatory, and other factors weigh against doing this, however. Interestingly enough, flat rate service exists in other sectors. For example, cable TV is billed at a flat rate per month, no matter how many programs you watch. It could have been designed with pay-per-view as the basic concept, but it was not, due in part to the expense of billing (and given the quality of most television, the embarrassment factor cannot be totally discounted either). Also, many theme parks charge a daily admission fee for unlimited rides, in contrast to traveling carnivals, which charge by the ride.
That said, it should come as no surprise that all networks designed by the telephone industry have had connection-oriented subnets. What is perhaps surprising, is that the Internet is also drifting in that direction, in order to provide a better quality of service for audio and video, a subject we will return to in Chap. 5. But now let us examine some connection-oriented networks.
X.25 and Frame Relay
Our first example of a connection-oriented network is X.25, which was the first public data network. It was deployed in the 1970s at a time when telephone service was a monopoly everywhere and the telephone company in each country expected there to be one data network per country—theirs. To use X.25, a computer first established a connection to the remote computer, that is, placed a telephone call. This connection was given a connection number to be used in data transfer packets (because multiple connections could be open at the same time). Data packets were very simple, consisting of a 3-byte header and up to 128 bytes of data. The header consisted of a 12-bit connection number, a packet sequence number, an acknowledgement number, and a few miscellaneous bits. X.25 networks operated for about a decade with mixed success.In the 1980s, the X.25 networks were largely replaced by a new kind of network called frame relay. The essence of frame relay is that it is a connection-oriented network with no error control and no flow control. Because it was connection-oriented, packets were delivered in order (if they were delivered at all). The properties of in-order delivery, no error control, and no flow control make frame relay akin to a wide area LAN. Its most important application is interconnecting LANs at multiple company offices. Frame relay enjoyed a modest success and is still in use in places today.
Asynchronous Transfer Mode
Yet another, and far more important, connection-oriented network is ATM (Asynchronous Transfer Mode). The reason for the somewhat strange name is that in the telephone system, most transmission is synchronous (closely tied to a clock), and ATM is not.ATM was designed in the early 1990s and launched amid truly incredible hype (Ginsburg, 1996; Goralski, 1995; Ibe, 1997; Kim et al., 1994; and Stallings, 2000). ATM was going to solve all the world's networking and telecommunications problems by merging voice, data, cable television, telex, telegraph, carrier pigeon, tin cans connected by strings, tom-toms, smoke signals, and everything else into a single integrated system that could do everything for everyone. It did not happen. In large part, the problems were similar to those we described earlier concerning OSI, that is, bad timing, technology, implementation, and politics. Having just beaten back the telephone companies in round 1, many in the Internet community saw ATM as Internet versus the Telcos: the Sequel. But it really was not, and this time around even diehard datagram fanatics were aware that the Internet's quality of service left a lot to be desired. To make a long story short, ATM was much more successful than OSI, and it is now widely used deep within the telephone system, often for moving IP packets. Because it is now mostly used by carriers for internal transport, users are often unaware of its existence, but it is definitely alive and well.
ATM Virtual Circuits
Since ATM networks are connection-oriented, sending data requires first sending a packet to set up the connection. As the setup packet wends its way through the subnet, all the routers on the path make an entry in their internal tables noting the existence of the connection and reserving whatever resources are needed for it. Connections are often called virtual circuits, in analogy with the physical circuits used within the telephone system. Most ATM networks also support permanent virtual circuits, which are permanent connections between two (distant) hosts. They are similar to leased lines in the telephone world. Each connection, temporary or permanent, has a unique connection identifier.Once a connection has been established, either side can begin transmitting data. The basic idea behind ATM is to transmit all information in small, fixed-size packets called cells. The cells are 53 bytes long, of which 5 bytes are header and 48 bytes are payload, as shown in Fig. 1-31. Part of the header is the connection identifier, so the sending and receiving hosts and all the intermediate routers can tell which cells belong to which connections. This information allows each router to know how to route each incoming cell. Cell routing is done in hardware, at high speed. In fact, the main argument for having fixed-size cells is that it is easy to build hardware routers to handle short, fixed-length cells. Variable-length IP packets have to be routed by software, which is a slower process. Another plus of ATM is that the hardware can be set up to copy one incoming cell to multiple output lines, a property that is required for handling a television program that is being broadcast to many receivers. Finally, small cells do not block any line for very long, which makes guaranteeing quality of service easier.
All cells follow the same route to the destination. Cell delivery is not guaranteed, but their order is. If cells 1 and 2 are sent in that order, then if both arrive, they will arrive in that order, never first 2 then 1. But either or both of them can be lost along the way. It is up to higher protocol levels to recover from lost cells. Note that although this guarantee is not perfect, it is better than what the Internet provides. There packets can not only be lost, but delivered out of order as well. ATM, in contrast, guarantees never to deliver cells out of order.
ATM networks are organized like traditional WANs, with lines and switches (routers). The most common speeds for ATM networks are 155 Mbps and 622 Mbps, although higher speeds are also supported. 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 compatibility with AT&T's SONET transmission system, something we will study in Chap. 2. The 622 Mbps speed was chosen so that four 155-Mbps channels could be sent over it.
The ATM Reference Model
ATM has its own reference model, different from the OSI model and also different from the TCP/IP model. This model is shown in Fig. 1-32. It consists of three layers, the physical, ATM, and ATM adaptation layers, plus whatever users want to put on top of that.The physical layer deals with the physical medium: voltages, bit timing, and various other issues. ATM does not prescribe a particular set of rules but instead says that ATM cells can be sent on a wire or fiber by themselves, but they can also be packaged inside the payload of other carrier systems. In other words, ATM has been designed to be independent of the transmission medium. The ATM layer deals with cells and cell transport. It defines the layout of a cell and tells what the header fields mean. It also deals with establishment and release of virtual circuits. Congestion control is also located here.
Because most applications do not want to work directly with cells (although some may), a layer above the ATM layer has been defined to allow users to send packets larger than a cell. The ATM interface segments these packets, transmits the cells individually, and reassembles them at the other end. This layer is the AAL (ATM Adaptation Layer).
Unlike the earlier two-dimensional reference models, the ATM model is defined as being three-dimensional, as shown in Fig. 1-32. The user plane deals with data transport, flow control, error correction, and other user functions. In contrast, 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, one at the bottom that does the work and a convergence sublayer on top that 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 and handles the bit timing. For different carriers and cables, this layer will be different.
The other sublayer of the physical layer is the TC (Transmission Convergence) sublayer. When cells are transmitted, the TC layer sends them as a string of bits to the PMD layer. Doing this is easy. At the other end, the TC sublayer gets a pure incoming bit stream from the PMD sublayer. Its job is to convert this bit stream into a cell stream for the ATM layer. It handles all the issues related to telling where cells begin and end in the bit stream. In the ATM model, this functionality is in the physical layer. In the OSI model and in pretty much all other networks, the job of framing, that is, turning a raw bit stream into a sequence of frames or cells, is the data link layer's task.
As we mentioned earlier, the ATM layer manages cells, including their generation and transport. Most of the interesting aspects of ATM are located here. It is a mixture of the OSI data link and network layers; it is not split into sublayers.
The AAL layer is split into a SAR (Segmentation And Reassembly) sublayer and a CS (Convergence Sublayer). The lower sublayer breaks up packets into cells on the transmission side and puts them back together again at the destination. The upper 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.).
As it is probably mostly downhill for ATM from now on, we will not discuss it further in this book. Nevertheless, since it has a substantial installed base, it will probably be around for at least a few more years. For more information about ATM, see (Dobrowski and Grise, 2001; and Gadecki and Heckart, 1997).
Connection-Oriented Networks: X.25, Frame Relay, and ATM
4/
5
Oleh
Unknown
