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• ATM layer—Resides over the physical layer, and implements cell-based communication with
its peer layer by invoking the services provided by the physical layer. It is independent of
physical media, implementation of the physical layer, and types of services being carried. Data
passed to this layer takes the form of 48-byte payloads, which are 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 five-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 forwards information between the ATM layer and upper layer protocols. 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—Responsible for WAN interconnection, voice interconnection,
Layer 3 interconnection, encapsulation, LAN emulation, multiprotocol over ATM, and traditional
IP.
IPoA, IPoEoA, PPPoA, and PPPoEoA
An ATM interface can implement IPoA, IPoEoA, PPPoA, and PPPoEoA to carry data encapsulated
in IP, PPP, or Ethernet across an ATM network.
IPoA
IP over ATM (IPoA) enables ATM to carry IP packets. In an IPoA implementation, ATM serves as the
data link layer protocol for the IP hosts on the same network. To enable these hosts to communicate
across an ATM network, IP packets must be encapsulated in ATM cells.
By making full use of the advantages of ATM, IPoA delivers excellent network performance and
ubiquitous mature QoS assurance.
IPoEoA
IP over Ethernet over ATM (IPoEoA) adopts a three-layer architecture, with IP encapsulation at the
top layer, IP over Ethernet (IPoE) in the middle, and IPoEoA at the bottom.
IPoEoA is suitable where Ethernet packets are to be forwarded through ATM interfaces. In IPoEoA
applications, you can associate multiple PVCs with one Layer 3 virtual Ethernet (VE) interface and
PVCs associated with the same VE interface are interconnected at Layer 2.
PPPoA
PPP over ATM (PPPoA) enables ATM to carry PPP protocol packets. With PPPoA, PPP packets, in
which IP packets or other protocols' packets are encapsulated, are encapsulated in ATM cells. In this
approach, ATM is the carrier of PPP packets and the communication process of PPPoA is managed
by PPP, which allows for flexibility and rich applications. To transmit PPP packets across ATM, a
virtual-template (VT) interface is required.
PPPoEoA
PPPoE over ATM (PPPoEoA) enables ATM to carry PPPoE protocol packets. With PPPoEoA,
Ethernet packets are encapsulated in ATM cells, through which you can use a PVC to implement the
functionality of Ethernet. To carry Ethernet frames over ATM, the virtual Ethernet (VE) interface was
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