Cisco Systems 8600 Series manual ATM Cell Addressing, ATM Adaptation Layer, Aal, Smds

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-1	AAL-2	AAL-3/4	AAL-3/4
(AAL)				AAL-5
Connection Mode		Connection-oriented	Connection-oriented	Connection-oriented	Connectionless
End-to-End Timing		Yes	Yes	No	No
Relationship
Bit Rate		Constant	Variable	Variable	Variable
Examples		Uncompressed	Compressed voice	Frame relay, SNA,	SMDS
		voice, constant	and video	TCP-IP, E-mail
		bit-rate video

General Description 2-11