Nortel Networks 8000 - Manual (Page 389)

Nortel Secure Router 8000 Series
Configuration -WAN Access 10 Low-speed ATM configuration
10.1 Introduction
This section covers the following topics that you need to know before you configure ATM:
ATM protocol stack
ATM interface
ATM OAM
ATM applications
references
10.1.1 ATM protocol stack
Asynchronous Transfer Mode (ATM) is the transmission and switching mode for broadband
ISDN services by the International Telecommunication Union - Telecommunication
Standardization Sector (ITU-T) in June, 1992. Because of its high flexibility and its capability
for supporting the multi-media service, ATM is considered as the key to realize broadband
communications.
As defined by ITU-T, ATM implements transmission and switching of data based on cells. An
ATM cell has a fixed length of 53 bytes. The first 5 bytes make up the cell header that
contains the cell routing and priority information. The remaining 48 bytes are used for
payloads.
ATM is connection-oriented. Each virtual circuit (V C) is identified by a Virtual Path Identifier
(VPI) and a Virtual Channel Identifier (VCI) together. One VPI/VCI value has local
significance on a segment of the link between ATM nodes. When you release a connection,
the relevant VPI/VCI values are also released.
The ATM protocol architecture has the following three planes:
• User Plane
• Control Plane
• Management Plane
User Plane and Control Plane are divided into four layers respectively:
• Physical Layer
• ATM Layer
• ATM Adaptation Layer (AAL)
• Upper Layer
Each layer is further divided into several sub-layers.
The Control Plane uses signaling protocols to establish and release connections.
Management Plane is divided into Layer Management and Plane Management.
• Layer Management: manages all the layers in each plane. It has a layered structure
corresponding to other planes.
• Plane Management: performs system management and communication between different
planes.
The relationships among layers and planes are shown in Figure 10-1.
Issue 5.3 (30 March 2009) Nortel Networks Inc. 10-3