Overview of IPoA, IPoEoA, PPPoA and PPPoEoA Applications 129IPoA IP over AAL5 (IPoA) carries IP packets over AAL5. AAL5 provides the IP hosts onthe same network with the data link layer for communications. In addition, toallow these hosts to communicate on the same ATM network, IP packets must betuned somewhat.As the bearer network of IP services, ATM provides high speed point-to-pointconnections which considerably improve the bandwidth performance of IPnetwork. On the other hand, ATM also provides excellent network performanceand perfect QoS.IPoEoA IPoE over AAL5 (IPoEoA) adopts a three-layer architecture, with IP encapsulation atthe uppermost layer, IP over Ethernet (IPoE) in the middle, and IPoEoA at thebottom.When a device is connected to a remote access server at high speed to access anexternal network, PVC over ATM is used because of the long distance. In this case,it is required for the ATM port of the server to carry Ethernet packets, which isknown as IPoEoA.For IPoEoA, the device can implement the following basic functions:In the application of IPoEoA, one virtual Ethernet (VE) interface can be associatedwith multiple PVCs.PVCs associated with the same VE interface are interconnected at layer 2.PPPoA PPP over AAL5 (PPPoA) means that AAL5 bears the PPP protocol packets: Itsessence is that ATM cells are used to encapsulate PPP packets, while IP or otherpackets are encapsulated in PPP packets. In this way, AAL5 may be simply viewedas the bearer layer of PPP packets. PPPoA is important because the communicationprocess of PPPoA is managed by PPP, and thus it can make use of PPP’s flexibilityand extensive applications. Before transmitting PPP packets over AAL5, users mustcreate a virtual template (VT) interface. For more information about virtualtemplate interfaces, refer to “VT and VA Interface” on page 534.PPPoEoA PPPoE over AAL5 (PPPoEoA) carries PPPoE packets over AAL5. This is toencapsulate Ethernet frames in ATM cells. In this case, a PVC to simulate allfunctions of Ethernet. To allow AAL5 carry Ethernet frames, the interfacemanagement module provides the virtual Ethernet (VE) interface. This VE interfacehas Ethernet characteristics and can be dynamically created through configurationcommands. The following is the protocol stack for the VE interface:ATM PVC at the bottom layerEthernet at the link layerProtocols the same as those for the Ethernet interface at the network layer andupper layersFor more information about the VE interface, please refer to “Introduction to VE”on page 536.
