228 APPENDIX 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
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
Voice Application
Configurations
A typical Ethernet and voice application configuration involves multiple
PathBuilder S330s connected through an ATM network that could consist of
simply an ATM switch or of an ATM network consisting of multiple switches.
Figure 167 shows the latter configuration.
Ethernet Packet
PAD
(0-47 Bytes)
Reserved
2 Bytes
Length
2 Bytes
CRC
4 Bytes
Trailer
