Specifications

2-4 Cisco MGX 8220 Reference, Release 4.1
System Overview
Figure2-2 MGX 8220 Shelf Top Level Block Diagram
Traffic Management
Traffic management is provided by the MGX 8220 Connection Congestion Management (ACCM)
feature. This feature is a virtual source/virtual destination rate based closed loop feedback scheme
between nodes based upon ForeSight and the ATM Forum Available Rate specification. Periodically
the round trip delay (RTD) of a cell to the far end and back is calculated for each actual connection.
The RTD is used to determine how frequently ForeSight rate adjustment cells are sent to the far end.
Based upon available bandwidth and current access rate, every few microseconds a ForeSight cell is
transmitted to the far end with any rate adjustments it may be necessary to make better use of the
available bandwidth (rate is adjusted up, down or fast down). The MGX 8220 MIB includes rate
(MIR, PIR, and QIR) and adjustment (Up%, Down%, and Fast-Down%) objects.
ATM Local Management Interface (ATM LMI)
A Local Management Interface (ATM LMI) operates between the MGX 8220 shelf and the
BPX 8620. The ATM LMI is used to communicate change of status of semi-permanent connections.
The ATM LMI protocol conforms to the protocol specified in ITU-TS Recommendation Q.2931,
Sept. 1994. The ATM LMI also supports optional proprietary extensions to the ATM LMI for the
reporting of node information and BNI queue configurations.
The two end points of the ATM LMI signaling are an ATM LMI process in the BCC in the BPX 8620
and an ATM LMI process in the ASC in the MGX 8220 shelf. The ATM LMI PDUs are transported
over an AAL5 connection (VPI=3/VCI=31) between the BNI on the BPX 8620 side and the BNM
on the MGX 8220 side. The ATM LMI process in the BPX 8620 can support ATM LMI connections
for up to 16 MGX 8220 shelves.