Previous | Table of Contents | Next |
Questions Answered in This Chapter
What is the ATM protocol?
What are ATM function layers?
What is a cell?
What are UNIs and NNIs?
What are virtual paths, virtual channels, PVCs, and cell headers?
What are the ATM service classes?
What is Quality of Service?
Now is a good time to revisit the OSI protocol model. See Figure 6-1. The Open Systems Interconnection (OSI) model was developed in 1984 by the International Standardization Organization (ISO). The OSI model specifies a standard telecommunications frame of reference for developers, operating companies, and other interested parties. Today, descriptions of architectural protocols are generally espoused in OSI model terms. The seven OSI layers are arranged in a functional manner. That is, all functionality associated with related activities is defined by a particular layer. The model is an idealized conception of how the world would be, if we lived in a perfect world. While most companies follow the model fairly close—after all it does lend itself adequately to the task—the demarcation from one layer to another frequently is not a cut-and-dried issue. However, we are not concerned with splitting hairs and debating gray areas. The OSI model will be used just as it was intended, as a model to present appropriate material in a more understandable form.
Figure 6-1. Open Systems Interconnection layering model
At the heart of the OSI model is the idea of peer-to-peer communications. Peer-to-peer communications means only those layers on the same level will exchange data. That is, the layers of Application A will only exchange data with the same layers of Application B. Put in terms of the everyday work environment, vice presidents only talk to vice presidents, directors only talk to directors, and senior managers only talk to senior managers. Ever known any such workplaces? See Figure 6-2. Remember, the OSI model is an idealized representation and in the real world layers usually must exchange information with the layers at least immediately above and below them. More like the real world of the workplace, too. While the OSI model is comprised of seven layers, the ATM model is only three layers whose functionality closely follows the bottom three OSI model layers.
Figure 6-2. OSI layering model peer-to=peer communications
A graphical representation of the relationships of ATM protocol components is shown in Figure 6-3. Notice in the figure the components nearer the bottom front are closer to the fundamental transmission elements. The functions of the three ATM protocol layers are shown in Figure 6-4.
The three ATM protocol layers are depicted in Figure 6-4 with each layer’s task and sublayers identified. The AAL layer is responsible for adding/removing the ATM header from the payload and consists of five AAL layer types: AAL1, AAL2, AAL3/4, and AAL5.
Figure 6-3. ATM protocol cube
The ATM physical layer converts the data into the appropriate electrical or optical signal format and the corresponding voltage levels necessary to drive the electrical or optical transmission circuits.
The ATM layer performs switching, multiplexing, transmission, and control functions. The ATM layer is responsible for congestion control including tagging cells that exceed the user’s maximum allowable transmission rate as Discard Enable (DE) by setting the Cell Loss Priority (CLP) bit to a binary 1. The ATM layer congestion control mechanism includes management of incoming data buffers. The ATM layer adds the cell header at the source and removes the cell header at the destination. To get the cell from source to destination, the ATM layer translates the cell address. Also, this layer is responsible for placing the cells onto the transmission media in the sequence in which they arrived.
Figure 6-4. Functions of ATM protocol layers
AAL Type | User Services | Connection Type | Example |
AAL 1 | CBR | Connectionless Oriented | voice services at DS-0, DS-1, and DS-3 rates, video |
AAL 2 | VBR-rt | Connectionless Oriented | compressed packet mode video |
AAL 3/4 | VBR-nrt | Connection or Connectionless Oriented | delay tolerant data with sequencing and error detection support |
AAL5 | VBR | Connection Oriented | delay tolerant data requiring minimal sequencing and error detection support |
Table 6-1. ATM AAL layers
The Adaptation Layer (AAL) inserts and extracts data into the 48-byte payload and adds and removes data into the 5-byte header. There are four types of AAL layers as depicted in Table 6-1. Each AAL layer type is designed to cope with a specific user service type.
AAL Type 1
AAL Type 1 functions are used for constant bit rate (CBR) services. CBR services are time dependent which include real-time audio and video.
AAL Type 2
AAL Type 2 functions are used for variable bit rate video transmission. AAL Type 2 is not fully defined at this writing.
AAL Type 3/4
Originally two separate types, AAL 3 (connection oriented) and AAL 4 (connectionless oriented), AAL Type 3/4 functions are used to support variable bit rate, delay-tolerant services requiring sequencing and error detection functionality. Data traffic in this category includes such activities as large file transfers.
Previous | Table of Contents | Next |