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AAL Type 5

AAL Type 5 was intended for variable bit rate, delay-tolerant data traffic that does not require sequencing and error detection to the extent of AAL Type 3/4. File transfers that are on high quality links might fall into this category.

AAL Type 5 has less overhead bits than AAL Type 3/4. Each cell length field is not used and neither is there a cell CRC. AAL Type 5 allows up to 64 Kbps of user data to be encapsulated in an ATM super-cell. The super-cell is identified by the Payload Type Indicator (PTI) portion of the ATM cell header.

ATM Planes

Boss, da planes! Da planes! (Sorry, could not resist the urge.) In the literature, ATM planes are usually defined in very general terms that make it difficult for an individual with little or no previous networking experience to understand just exactly what the planes are and what they do. I suppose the reason for such oblique attempts to define the planes has to do with the complexity of the underlying functionality.

Fundamentally, ATM planes are higher level software procedures that operate upon the ATM hardware, its associated software procedures, and the ATM data to provide a controlled and orderly process of establishing a communications link between two or more ATM users and transferring data between the users. Figure 6-5 shows the relationship among and between the planes. Well, trade one general explanation for another and you are still left with only a vague notion of the purpose of ATM planes. While each ATM plane can be a book-length topic itself, hopefully the following explanation will provide the reader with more substance and less fluff.


Figure 6-5.  ATM planes

User Plane

The User Plane is responsible for transferring user application information to network elements. Such information includes Quality of Service (QoS) parameters. This plane identifies to the network the particular ATM Adaptation Layer type used in the transmission. User Plane functionality permeates the physical layer, ATM layer, and AAL layer.

Control Plane

The Control Plane is responsible for establishing and releasing the connection in connection-oriented services, and providing other signaling information necessary to perform a switched service. The Control Plane functionality permeates the physical layer, ATM layer, and AAL layer.

Management Plane

The Management Plane is responsible for managing the User Plane and Control Plane. Through the Management Plane, the User Plane and the Control Plane are able to exchange information. The Management Plane is composed of two subplanes, the Plane Management and Layer Management subplanes.

Layer Management

The Layer Management subplane is responsible for managing each of the ATM layers, the physical layer, the ATM layer, and the AAL layer. Layer-specific management functions are grouped under the Layer Management umbrella.

Plane Management

The Plane Management subplane is responsible for managing and coordinating the functions of the complete ATM product.

The ATM Cell

ATM cell length is fixed at 53 bytes (octets) with 48 bytes (octets) for payload (user information) and 5 bytes (octets) reserved for the cell header. Cells can transport burst, voice, video, and image data. ATM cell format is simpler and requires less overhead than other standardized "packet" type technologies. Figure 6-7 depicts a frame relay frame format. Contrast Figure 6-7 with the ATM cell format of Figure 6-6.


Figure 6-6.  ATM cell format


Figure 6-7.  Frame relay frame format

Frame relay frame lengths are variable. Frame length can be up to 4,096 bytes. Frame relay was designed for bursty data with no intention of ever shipping real time-dependent traffic, such as audio and video, across the frame networks. Currently, several bodies are attempting to develop a Voice Over Frame Relay protocol.

ATM provides for four different flavors of ATM cells. The particular cell flavor of individual cells depends upon the usage of the link at the moment of interest. ATM cell types are: idle cells, unassigned cells, VP/VC traffic cells, and VP OAM traffic cells.

ATM Interfaces

There are two network interfaces encountered in ATM networks. There is the Network Node Interface (NNI) and the User Network Interface (UNI). An NNI is the interface between two network nodes and the UNI is the interface between the user and the network. ATM interfaces are discussed in detail on page 135.


Figure 6-8.  NNI-to-UNI ATM header format


Figure 6-9.  UNI-to-NNI ATM header format

ATM Cell Header

The ATM cell header is composed of 5 bytes (octets) of information that is used for signaling, routing, error detection, and Quality of Service purposes. The cell header is somewhat different depending upon the ATM interface under scrutiny. The NNI-to-UNI interface includes a Generic Flow Control field that is replaced with a Virtual Path Identifier in the UNI-to-NNI interface. Figures 6-8 and 6-9 depict the NNI-to-UNI and UNI-to-NNI header cell formats, respectively.

The NNI-to-UNI ATM header format includes the Virtual Path Identifier (VPI), Virtual Channel Identifier (VCI), Generic Flow Control (GFC), Payload Type Indicator (PTI), Cell Loss Priority (CLP), Header Error Control (HEC), and Reserved fields. The value of the bits, either a zero or a one, in each of the fields determines the destiny of the ATM cell.


Figure 6-10.  An ATM cable bundle

The UNI-to-NNI ATM header format includes the Virtual Path Identifier (VPI), Virtual Channel Identifier (VCI), Payload Type Indicator (PTI), Cell Loss Priority (CLP), Header Error Control (HEC), and Reserved fields. Of course, the value of the bits, either a zero or a one, in each of the fields determines the destiny of the ATM cell.


Figure 6-11.  ATM VPI and VCI signaling

Virtual Channels and Virtual Paths

Notice that the ATM cell header is comprised primarily of Virtual Path Identifiers (VPIs) and Virtual Channel Identifiers (VCIs).

Virtual channels (VCs) form bundles that comprise the virtual paths (VPs). The relationship between VCs and VPs is clearly depicted in Figure 6-10. Just imagine Figure 6-10 is a graphical representation of an actual transmission cable bundle. The individual wire pairs form the VCs and the bundles of VCs compose the VPs.


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