Overview Of Ipoa, Ipoeoa, Pppoa And Pppoeoa Applications - 3Com MSR 50 Series Configuration Manual

128 C 5: ATM C
HAPTER
Overview of IPoA,
IPoEoA, PPPoA and
PPPoEoA Applications
ONFIGURATION
Figure 6 ATM protocol model
Management plane Management plane
Control plane Control plane
Upper layer Upper layer Upper layer Upper layer
protocol protocol protocol protocol
ATM adaptation layer ATM adaptation layer
ATM layer ATM layer
Physical layer Physical layer
The specific functions of various layers are as follows:
The physical layer mainly provides transmission channels for ATM cells, forming
■
continuous bit streams by adding the transmission overheads onto the cells
from the ATM layer. AT the same time, upon receiving continuous bit streams
from the physical media, the physical layer takes out the effective cells and
transfers them to the ATM layer.
The ATM layer, residing over the physical layer, implements cell-based
■
communication with peer layers by leveraging the service provided by the
physical layer. The ATM layer relies on the types of the physical media and the
specific implementation of the physical layer, as well as the types of services
being transmitted. What is input into the ATM layer are 48-byte payloads,
which are called segmentation and reassembly protocol data units (SAR-PDUs),
and what the ATM layer outputs are 53-byte cells, which are also transferred to
the physical layer for transmission. What is input into the ATM layer are 48-byte
payloads, which are called segmentation and reassembly protocol data units
(SAR-PDUs), and what the ATM layer outputs are 53-byte cells, which are also
transferred to the physical layer for transmission. The ATM layer is responsible
for generating a 5-bytes cell header, which will be inserted in front of a
payload. Other functions of the ATM layer include VPI/VCI transmission, cell
multiplexing/demultiplexing, and generic flow control.
As the interface between upper layer protocol and ATM Layer, ATM Adaptation
■
Layer (AAL) is responsible for forwarding the information between ATM Layer
and upper layer protocols. At present, four types of AAL have been put
forward -- AAL1, AAL2, AAL3/4 and AAL5, each of which supports some
special services. Most ATM equipment manufacturers' products use AAL5 to
support the data communication service.
ATM upper layer protocols provide functions such as WAN interconnection,
■
voice interconnection, interconnection with existing Layer 3 protocols,
encapsulation mode, LAN emulation, multi-protocol over ATM, and classical IP.
ATM interfaces support the IPoA, IPoEoA, PPPoA and PPPoEoA applications.
User pl a ne User pl a ne