Motorola MPC8260 - Manual (Page 822)

29-42 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
use ABR, VBR or UBR+ services. Each connection table entry resides in a 32-byte space.
Table 29-15 lists sizes for RCT, TCT, and TCTE.
Note that an ATM channel is considered internal if its tables are in an internal dual-port
RAM; it is considered external if its tables are in external memory.
Notes:
To improve performance, store parameters for fast channels in
internal dual-port RAM and parameters for slower channels in
external memory. Connection tables for external channels are
read and written from external memory each time the CP
processes a cell. The CP does, however, minimize memory
access time by burst fetching the 32-byte entry and writing
back only the Þrst 24 bytes.
In all connection tables, Þelds which are not used must be
cleared.
29.10.2.1 ATM Channel Code
Each ATM channel has a channel code used as an index to the channelÕs connection table
entry. The Þrst channel in the table has channel code one, the second has channel code two,
and so on. Codes of 255 or less indicate internal channels; codes greater than 255 indicate
external channels. Channel code one is reserved as the raw cell queue and cannot be used
for another purpose. The channel code is used to specify a VC when sending a
ATM
TRANSMIT command, initiating the external CAM or address compression tables, and when
the CP sends an interrupt to an interrupt queue.
Example:
Suppose a conÞguration supports 1,024 regular ATM channels. To allocate 4 Kbytes of
dual-port RAM space to the internal connection table, determine that channel codes 0Ð63
are internal (64 VCs
´ 64 bytes (RCT and TCT) = 4 K). Channels 0Ð1 are reserved. The
remaining 962 (1024 - 62) external channels are assigned channel codes 256Ð1217. See
Figure 29-24.
Table 29-15. Receive and Transmit Connection Table Sizes
ATM Service Class RCT TCT TCTE
CBR, UBR service 32 bytes 32 bytes Ñ
ABR, VBR, UBR+ service 32 bytes 32 bytes 32 bytes