General Description 2-11
Functional Description
In the STI header (see Figure 2-10), the Payload Class is used to indicate various classes of service
and BPX switch queues, e.g., Opticlass, the enhanced class of service feature of the BPX switch. The
ForeSight Forward Congestion Indication, the F bit, is used by ForeSight for congestion status.
The Cell Loss Priority (CLP) bit follows the PTI bits in all header types. When set, it indicates that
the cell is subject to discard if congestion is encountered in the network. For frame relay connections,
depending on mapping considerations, the frame Discard Eligibility status is carried by the CLP bit
in the ATM Cell. The CLP bit is also set at the ingress to the network for all cells carrying user data
transmitted above the minimum rate guaranteed to the user.
ATM Cell Addressing
Each ATM cell contains a two-part address, VPI/VCI, in the cell header. This address uniquely
identifies an individual ATM virtual connection on a physical interface. VCI bits are used to identify
the individual circuit or connection. Multiple virtual circuits that traverse the same physical layer
connection between nodes are grouped together in a virtual path. The virtual path address is given
by the VPI bits. The Virtual Path can be viewed as a trunk that carries multiple circuits all routed the
same between switches
The VPI and VCI addresses may be translated at each ATM switch in the network connection route.
They are unique only for a given physical link. Therefore, they may be reused in other parts of the
network as long as care is taken to avoid conflicts.
The VCI field is 16 bits wide with UNI and NNI header types described earlier. This allows for a
total possible 65, 535 unique circuit numbers. The UNI header reserves 8 bits for VPI (256 unique
paths) while the NNI reserves 12 bits (4,096 unique paths) as it is likely that more virtual paths will
be routed between networks than between a user and the network. The STI header reserves 8 bits for
VCI and 10 bits for VPI addresses.
ATM Adaptation Layer
The purpose of the ATM Adaptation Layer (AAL) is to receive the data from the various sources or
applications and convert, or adapt, it to 48-byte segments that will fit into the payload of an ATM
cell. Since ATM benefits from its ability to accommodate data from various sources with differing
characteristics, the Adaptation Layer must be flexible.
Traffic from the various sources have been categorized by the standards committees into four general
classifications, Class A through Class D, as indicated in Table 2-1. This categorization is somewhat
preliminary and initial developments have indicated that it may be desirable to have more than these
initial four classes of service.
Table 2-1 Classes of Traffic and Associated AAL Layers
Traffic Class Class A Class B Class C Class D
Adaptation Layer
(AAL)
AAL-1 AAL-2 AAL-3/4
AAL-5
AAL-3/4
Connection Mode Connection-oriented Connection-oriented Connection-oriented Connectionless
End-to-End Timing
Relationship
Yes Yes No No
Bit Rate Constant Variable Variable Variable
Examples Uncompressed
voice, constant
bit-rate video
Compressed voice
and video
Frame relay, SNA,
TCP-IP, E-mail
SMDS
