COMNET III CACI

The former is responsible for the provision of operations, administration and maintenance
(OAM) functions. These include categories such as:
• fault management
• performance management
• security management
• accounting management
• configuration management
Basically, the layer management functions are responsible for the collection of
management statistics to be stored in management information bases (MIB’s). The plane
management functions are responsible for the coordination of all planes.
The layers in the B-ISDN model illustrate the protocol stack adopted by ATM. As
indicated in Figure 21, the ATM protocol is generally considered a layer 2 protocol within
the OSI reference model. It is located on top of the physical layer and requires the
services of higher layer protocols. However, strictly speaking there is no direct
correspondence between the two models, since ATM also provides some services which
are provided by layer 3 protocols in the OSI model.
Since the functionality implemented by the different layers provide a good understanding
about the operation of ATM, we will now look at the lower three layers of the B-ISDN
reference model in detail.
A.3.1
The ATM Adaptation Layer
The higher layers in Figure 21 represent all the protocols which are located on top of
ATM. These typically include protocols such as TCP/IP, IPX or ftp in the user plane,
SNMP or CMIP in the management plane or Q.2931 in the control plane. Before the
higher layer protocol data units (PDU’s) are handed to the ATM layer, they are required
to pass through the ATM Adaptation Layer (AAL). This layer adapts the transfer process
of the upper layer services to the ATM layer services. In particular, the AAL is
responsible for the segmentation and reassembly (SAR) of higher layer PDU’s and the
execution of convergence functions(CS). It takes the higher layer PDU and segments it
into a 48 byte AAL-PDU and provides some error checking functions. The ATM Forum
currently distinguishes between four different types of AAL’s to cater for the five ATM
service categories. Table 23 outlines this relationship and summarizes the main
characteristics of the different AAL’s.
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