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254
A
B: P
B
S330/S310 M
A
O
PPENDIX
ATH
UILDER
ODULE AND
PPLICATION
VERVIEW
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 182. 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 182 AAL5 Pad and Trailer
Trailer
PAD
Reserved
Length
CRC
Ethernet Packet
(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 with IMA 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 S330 switches connected through an ATM network that could consist
Configurations
of simply an ATM switch or of an ATM network consisting of multiple switches.
Figure 183 shows the latter configuration.
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