H3C Technologies H3C SR8800 - Manual (Page 10)

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ATM architecture
ATM has a three-dimensional architecture. It consists of three planes: user plane, control plane, and
management plane. Both the user plane and the control plane are broken down into four layers, namely,
physical layer, ATM layer, ATM Adaptation Layer (AAL), and upper layer, each of which are further
divided into sub-layers.
• The control plane takes charge of establishing and tearing down connections using signaling
protocols.
• The management plane consists of layer management and plane management. The former takes
charge of managing the layers in each plane and has a layered structure corresponding to other
planes. The latter is responsible for system management and communications between different
planes.
The following figure illustrates the relationships between the layers and the planes in ATM.
Figure 2 ATM architecture
The functions of the four ATM layers are as follows:
• The physical layer mainly provides transmission channels for ATM cells. At this layer, cells passed
from the ATM layer become continuous bit stream after transmission overheads are added to them.
In addition, continuous bit streams received from the physical media are restored to cells on this,
which are then passed to the ATM layer.
• The ATM layer, residing over the physical layer, implements cell-based communication with peer
layers by invoking the services provided by the physical layer. It is independent of physical media
and the implementation of the physical layer, as well as the types of the services being carried. Data
passed to this layer takes the form of 48-byte payloads, known as segmentation and reassembly
protocol data units (SAR-PDUs); and data passed from this layer to the physical layer is 53-byte cells,
with the 48-byte payload being encapsulated in a 5-byte header. Other functions of the ATM layer
include VPI/VCI transmission, cell multiplexing/demultiplexing, and generic flow control.
• ATM Adaptation Layer (AAL) provides interfaces between high-level protocols and the ATM Layer.
It is responsible for forwarding the information between ATM layer and upper layer protocols. At
present, four types of AAL are available: AAL1, AAL2, AAL3/4, and AAL5, each of which supports
specific services provided in an ATM network. Most ATM equipment vendors adopt AAL5 for data
communication services.
• ATM upper layer protocols take charge of WAN interconnection, voice interconnection, Layer 3
interconnection, encapsulation, LAN emulation, multi-protocol over ATM, and traditional IP.