222 APPENDIX B: P ATH BUILDER S330/S310 MODULE AND A PPLICATION O VERVIEWn The PathBuilder S330/S310 performs all the required translation andmanagement functions between these two networks, implementing the stacksas indicated in Figure 162 and performing the following key functions:n Translating from Q.922 Frame to FR-SSCS/ATM AAL5 PDU and fromFR-SSCS/ATM AAL5 PDU to Q.922 Framen Supporting 2-octet—not 3-octet or 4-octet—FR header (10-bit DLCI)n Allowing you to configure DLCI to VPI/VCI mappingn Providing one-to-one mapping: maps one FR-SSCS DLCI (default FR-SSCSDLCI (1022) or 16-991) for one FR DLCI to one ATM VPI/VCI. (The FR-SSCSDLCI(s) for the two end systems should be configured to be the same.)n Managing trafficn Handling congestionEthernet Interface The PathBuilder S330/S310 Ethernet LAN modules provide multiple connections toEthernet segments over ATM WAN.When the Ethernet interface receives data from the LAN, any packets appearingon the 10BASE-T ports are checked for packet integrity and stored in the sharedmemory area. The system then performs a bridge operation to determine if thepacket will be sent on to any other interface. See “Bridge Operation” below, fordetails.n If the packet is to go on the HDLC port, it is bridged directly after beingencapsulated properly for Frame Relay.n If the packet is to go on an ATM interface, the bridge determines the VCIconnection for the packet destination. The packet is then encapsulatedaccording to RFC 1483, and the descriptor is handed off to the SAR, togetherwith the associated ATM header descriptor. The SAR appends the AAL5convergence sublayer to the packets and queues them according to the shaperyou specify for that connection. The packet is then segmented according to therate of the traffic descriptor, and the cells are sent to the CTX where ATMswitching and queueing is performed.The SAR contains 15 traffic shapers, each of which can be programmed forsustained cell rate of transmission (SCR), peak cell rate (PCR), and maximum burstsize (MBS).The traffic shapers work as follows:n The packet is segmented using a dual leaky buffer algorithm, whereby the cellsare transmitted from each connection in the shaper at an average rate until thebucket of token fills up (a token is given to the connection at an average rate ifit has no cells to transmit at that moment).n The shaper then turns the burst mode on and transmits at the peak rate for aburst length. Note that the shaper serves every connection independently.n Cells received from the ATM WAN are switched via the CTX chip to the SARqueues. You can enable shaping on the SAR queues to slow down theincoming traffic on the SAR.
