Software Specifications Traffic Management
Centillion 1200N ATM Switch User Manual 5-7
NTP 297-8103-903
Congestion
Control In the ATM Switch, congestion is controlled by applying a back pressure in the
direction of line output unit » switch » line input unit and restricting th e number o f
incoming cells. The ATM Switch also supports congestion control using Explicit
Forward Congestion Indicator (EFCI) and RM (Resource Management) cell.
Furthermore, the ATM Switch controls congestion from ABR/UBR class traffic
by using EPD (Early Packet Discard), or a function that discards cells in frame
increments when the input buffer is congested.
Priority Control The ATM Switch performs the five classes of controls below on traffic delay:
■CBR (Constant Bit Rate service)
■rt-VBR (Variable Bit Rate service (real time))
■nrt-VBR (Variable Bit Rate service (non real time))
■ABR (Available Bit Rate service)
■UBR (Unspecified Bit Rate service)
In this list, the higher the control, the better the delay c haracteristics. Priority is
given to CBR and rt-VBR traffic as they are transmitted ahead of other classes of
traffic.
These delay priorities can be selected for each connection when establishing the
connection.
In the switch, a cell delay of no less than 40 µsec occurs depending on the degree
of passing cell congestion and the degree of priority.
The ATM Switch performs two classes of controls on cell loss. Cell loss priority
control is performed in cell increments based on the CLP bit value in the header of
each cell. The definition of CLP bit value is as follows:
■CLP bit "0" (high priority)
��CLP bit "1" (low priority)
In the ATM Switch, to ensure traffic quality, every line card shares one input
buffer (32,000 cell capacity) and one output buffer (32,000 cell capacity).
Shaping Shaping is a function that schedules cell transfer so it complies with the
communication rate and traffic parameters declared by each connection. This
function ensures that even bursty data are transferred at a consistent rate without
being discarded and that the network bands are used efficiently.
The ATM Switch allows one of eight stages of shaping to be set for each port.