Segmentation - 3Com 3C63100-AC-C - PathBuilder S600 Bridge/router Reference Manual

86 C 4: P B S600 M A O
HAPTER ATH UILDER ODULE AND PPLICATION VERVIEW
that are permanent and are accessed through the ATM network frequently, such
as servers.
The example in Figure 47 shows only one transmission path: the DS3 UNI between
the two PathBuilder S600s. As a result, cells leaving PathBuilder S600 A on VPI/VCI
1/33 arrive at PathBuilder S600 B on VPI/VCI 1/33. If the DS3 trunk were switched
through a standard telephone switching system to interconnect many PathBuilder
S600s, each PathBuilder S600 would have to have the same VPI/VCI designations
assigned. The ATM switch removes this restriction.
Figure 48 shows several PathBuilder S600s in an ATM-switched network. The
switch maps one VPI/VCI into another based on the destination of the packets in
the cell, in the process learning the complete topology of the network. Each
PathBuilder S600 connected to the switch can be configured independently.
Figure 48 Virtual Circuits in a Switched Network
PathBuilder S600 A
VPI/VCI 1/3
PathBuilder S600 B
VPI/VCI 4/6
ATM
Switch
PathBuilder S600 C
VPI/VCI 10/23

Segmentation

Once the CPU determines that a packet should go across the bridge, it
encapsulates the packet per RFC 1483 and adds a pad and trailer conforming to
AAL5 (ATM Adaption Layer 5) at the end of the packet (see Figure 49). 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 49 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 Module based on the bandwidth and quality of service