Understanding the network.pdf - Back to Home

Historically, due to ATM's faster operating rates, ATM switch
backplanes had significantly higher bandwidth ratings than traditional
LAN switches. This is really not the case today. Like a LAN switch, the
ATM switch backplane needs to have enough bandwidth to support
the simultaneous operation of its ports at the highest operating rate.
ATM interfaces do not support full-duplex operation, so the required
switch backplane can be calculated quite easily by adding the number
of ports, multiplying the result by their highest operating rate, and
then dividing the sum by two. If the result is less than the switch's
rated backplane, blocking will result when all the switch ports are
active.
An ATM switch logic controller consists of the same basic elements
found in a LAN switch logic controller: a CPU, a switch logic module,
DRAM, and Flash. Due to ATM's fast operating rate and guaranteed
QoS demands, cellprocessing is ideally performed in hardware.
Therefore, the use of ASIC-based switch logic modules is quite
common. However, where most ATM switches tend to fall short is in
the area of buffer space. ATM switches commonly use a shared DRAM
buffer to store cells they cannot process. If the switch comes under
heavy load and the buffers overflow, the switch will drop cells. This, in
turn, forces the transmitting applications to retransmit the lost
packets, which contributes even further to the switch's overloaded
condition. With ATM switches, buffer sizes often dictate how well the
switch will perform. The general rule is "bigger is better." In the end,
the speed of the ASIC and the backplane are irrelevant if the switch
cannot actually process the cells.
ATM Cell Forwarding
ATM cells are forwarded using a technique derived from X.25 and
Frame Relay known as label swapping. With label swapping, each cell
has a logical connection identifier (LCID). In ATM's case, the Virtual
Path Identifier (VPI) and the Virtual Channel Identifier (VCI) fields in
the cell header are the cell's LCID. The VPI/VCI are used individually
or collectively to determine which port the cell exits from. Each ATM
switch in the connection path has its own mapping table that contains
the local VPI/VCI mappings for each port and their relations to PVC
and SVC connections handled by the switch. Cells are then forwarded
between ATM switch ports based on their VPI/VCI. Once the cell
reaches the ATM switch output port, a new VIP/VCI is inserted into the
cell header and the cell is forwarded on to the next switch. This
process is repeated until the cell reaches its final destination.