3Com S330 Ethernet Interface, Managing traffic Handling congestion, Ethernet segments over ATM WAN

222APPENDIX B: PATHBUILDER S330/S310 MODULE AND APPLICATION OVERVIEW

■The PathBuilder S330/S310 performs all the required translation and management functions between these two networks, implementing the stacks as indicated in Figure 162 and performing the following key functions:

■Translating from Q.922 Frame to FR-SSCS/ATM AAL5 PDU and from FR-SSCS/ATM AAL5 PDU to Q.922 Frame

■Supporting 2-octet—not 3-octet or 4-octet—FR header (10-bit DLCI)

■Allowing you to configure DLCI to VPI/VCI mapping

■Providing one-to-one mapping: maps one FR-SSCS DLCI (default FR-SSCS DLCI (1022) or 16-991) for one FR DLCI to one ATM VPI/VCI. (The FR-SSCS DLCI(s) for the two end systems should be configured to be the same.)

■Managing traffic

■Handling congestion

Ethernet Interface	The PathBuilder S330/S310 Ethernet LAN modules provide multiple connections to
	Ethernet segments over ATM WAN.

When the Ethernet interface receives data from the LAN, any packets appearing on the 10BASE-T ports are checked for packet integrity and stored in the shared memory area. The system then performs a bridge operation to determine if the packet will be sent on to any other interface. See “Bridge Operation” below, for details.

■If the packet is to go on the HDLC port, it is bridged directly after being encapsulated properly for Frame Relay.

■If the packet is to go on an ATM interface, the bridge determines the VCI connection for the packet destination. The packet is then encapsulated according to RFC 1483, and the descriptor is handed off to the SAR, together with the associated ATM header descriptor. The SAR appends the AAL5 convergence sublayer to the packets and queues them according to the shaper you specify for that connection. The packet is then segmented according to the rate of the traffic descriptor, and the cells are sent to the CTX where ATM switching and queueing is performed.

The SAR contains 15 traffic shapers, each of which can be programmed for sustained cell rate of transmission (SCR), peak cell rate (PCR), and maximum burst size (MBS).

The traffic shapers work as follows:

■The packet is segmented using a dual leaky buffer algorithm, whereby the cells are transmitted from each connection in the shaper at an average rate until the bucket of token fills up (a token is given to the connection at an average rate if it has no cells to transmit at that moment).

■The shaper then turns the burst mode on and transmits at the peak rate for a burst length. Note that the shaper serves every connection independently.

■Cells received from the ATM WAN are switched via the CTX chip to the SAR queues. You can enable shaping on the SAR queues to slow down the incoming traffic on the SAR.