Hitachi GR2000 Series Installation Manual page 262

4.6.1 Bandwidth Planning Considerations
4.6.1.1 Routing Capacity of GR2000 on ATM Interface
The routing capacity of GR2000 on ATM interface is 50 kpps transmission per line.
Plan the network so that the traffic exceeding the router's capacity will not rush into
the router.
4.6.1.2 Cellularization Overhead
When relaying packets from a network of different medium such as Ethernet to an
ATM network, the output traffic becomes higher than the input traffic because of the
cellularization overhead. Figure 4-9 illustrates a sample cellularization process that
converts an Ethernet frame (IP packet) to ATM cells.
For example, if a 64-byte frame is received from an Ethernet network, the IP packet
length is 46 bytes. When transmitting this frame to an ATM network, PAD is added to
normalize the data length on 48-byte boundary as shown below.
Header (8 bytes) + IP packet (46 bytes) + PAD and Trailer (42 bytes) = 96 bytes