3Com S330 manual Segmentation, Reassembly

228APPENDIX B: PATHBUILDER S330/S310 MODULE AND APPLICATION OVERVIEW

Segmentation

Once it is determined that a packet should go across the bridge, the packet is encapsulated per RFC 1483 and a pad and trailer conforming to AAL5 (ATM Adaption Layer 5) is added at the end of the packet. See Figure 166. The trailer is fixed at eight bytes and contains information such as the new length of the packet and cyclic redundancy check bytes (CRC facilitates error checking at the receive end). The pad is set to 0 to 47 bytes to make the full packet length including the trailer divisible by 48 bytes. The packet with the RFC 1483 encapsulation is then segmented into 48-byte (384-bit) cells by the SAR function.

Figure 166 AAL5 Pad and Trailer

			Trailer
Ethernet Packet	PAD	Reserved	Length	CRC
	(0-47 Bytes)	2 Bytes	2 Bytes	4 Bytes

The cells are assigned to a virtual circuit defined between the incoming and outgoing ports based on the destination address of the original packet. A 5-byte header containing the virtual circuit assignment, along with other information, is added to each cell. The cells are queued in the output FIFO (First In First Out memory) of the Ethernet based on the bandwidth and quality of service requirements assigned to the virtual circuit at configuration. The FIFO provides elastic storage between the Ethernet and the CTX.

Reassembly

When the CTX receives a cell from the T1/E1 UNI interface, it broadcasts it to the Ethernet port. As cells are received by the reassembler, their headers are read to determine if they belong to the port. If so, the header is stripped and the cell is stored in memory, appended to the previous ones for that virtual circuit. The system also looks for the AAL5 trailer that signifies the end of a packet. When it finds a trailer, it performs the CRC calculation, checks the length of the cells since the previous trailer to be sure no cells were missed, strips the trailer, appends the cell to the others to reform the original packet, and sends the packet through the Ethernet interface to the LAN.

Typical Ethernet and A typical Ethernet and voice application configuration involves multiple Voice Application PathBuilder S330s connected through an ATM network that could consist of

Configurations simply an ATM switch or of an ATM network consisting of multiple switches. Figure 167 shows the latter configuration.