Previous | Table of Contents | Next |
Multicasting is nothing more than a single source, referred to as the root, sending ATM cells to two or more destinations, called a leaf. Sometimes, multicasting is referred to as a point-to-multipoint connection. Multicasting allows one location in the network to broadcast data to as many other locations as desirable. A typical application would be the corporate headquarters broadcasting new product information to a sales force located in many cities across the country, continent, or globe.
ATM classifies three kinds of physical interfaces. The physical interfaces are, in the real world, ATM switches, CSU/DSUs and/or routers. The interface between the customer premises and the service provider’s backbone network is called the User Network Interface (UNI) to Network Node Interface (NNI), or UNI-to-NNI. See Figure 6-14.
Figure 6-15. Off to Grandma's house
The customer premises interface has two types of UNI interfaces. They are:
The service provider’s backbone network interface has two types of interfaces. They are:
Data Exchange Interface (DXI) is the interface between an ATM CSU/DSU and another topology’s router. A CSU/DSU is a Channel Service Unit/Data Service Unit. CSU/DSUs are devices that terminate PVCs/ports on customer premises and provide signal conditioning. DXI is responsible for converting variable length, frame-based data to fixed length ATM cells.
Perhaps the interrelationships of the three ATM layers still escape the casual reader. Let’s take a Cell Family Vacation to Grandma’s and see for ourselves how the three layers interwork. Like all journeys, this one will begin at the beginning as depicted in Figure 6-15.
The members of our ATM Cell family, named John Audio, Marsha Video, and Baby Data, arrive at the source AAL. Here they get into the same vehicle (ATM cell payload) in the AAL parking lot, open the road map, and plan their route to their destination (assemble VPI/VCI). The source ATM layer directs the Cell family vehicle to the correct parking lot exit. Now, the source physical layer is the type of road that is the route between source and public ATM network. The route the family must take might be a dirt road (twisted copper pairs), an improved road (coaxial), or a freeway (optical). Today, they are lucky to find themselves speeding down a superhighway at 155 Mbps (OC-3). Now, the members of the ATM Cell family are seasoned travelers who know that a good rest is needed after traveling so far so fast. The ATM public network acts as a rest stop (cell buffers) in the middle of a major interchange (backbone ATM switch) for the family. When the family reaches the Public ATM network (physical layer), the family receives a much needed rest as they wait their turn to enter the busy interchange (switch). The family finally reaches the interchange (ATM public network switch) and must decide which direction to go in. One wrong turn and they are lost forever. John finds an exit and quickly drives off the crowded expressway (OC-3 light wave) onto a dusty dirt road (twisted copper pairs). Slowly driving along (64 Kbps) trying to miss all the chuckholes, John almost misses Grandma’s driveway (destination ATM layer). Marsha quickly shouts out to make a turn and the Cell family enters Grandma’s driveway (destination physical layer). Turning off the engine, all the family members exit (destination AAL layer) the rental car (ATM cell payload).
ATM is fundamentally a transmission technology responsible for assembling data received from a variety of sources into payloads. After attaching the proper addressing and error control information into the cell header, ATM is responsible for placing the bit stream representing the cell-based data onto the transmission media that may be any one of the various media types, such as copper wire, coaxial, or fiber. To perform the tasks required, ATM must be compatible with a wide range of legacy technologies.
Previous | Table of Contents | Next |