3Com MSR 50 Series, Руководство по настройке

Страница: 128 / 2443

128 C
HAPTER
5: ATM C
ONFIGURATION
Figure 6 ATM protocol model
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.
Overview of IPoA,
IPoEoA, PPPoA and
PPPoEoA Applications
ATM interfaces support the IPoA, IPoEoA, PPPoA and PPPoEoA applications.
Physical layer
ATM layer
ATM adaptation layer
Upper layer
protocol Upper layer
protocol
Control plane User plane
Management plane
Plane m
ana
gem
ent
H
ierarc
hic
al
m
anage
m
ent
Physical layer
ATM layer
ATM adaptation layer
Upper layer
protocol Upper layer
protocol
Control plane User plane
Management plane
Plane m
ana
gem
ent
H
ierarc
hic
al
m
anage
m
ent