R3303-HP HSR6800 Routers Layer 2 - WAN Configuration Guide

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them. Meanwhile, continuous bit streams received from physical media are restored to cells, which
are then passed to the ATM layer.
• 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 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 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.