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Cisco MGX 8220 Installation and Configuration
Release 5.0, Part Number 78-6430-03 Rev. D0, November 2003
Chapter
Frame Relay Service Modules (FRSMs)
ATM to Frame Relay Direction
The least-significant bit of the CPCS-UU is mapped to the C/R bit of the Frame Relay frame.
Translation and Transparent Modes
Each service interworking (SIW) connection can exist in either translation or transparent mode. In
translation mode, the FRSM translates protocols between the FR NLPID encapsulation (RFC 1490) and
the ATM LCC encapsulation (RFC 1483). In transparent mode, the FRSM does not translate.
Translation mode support includes address resolution by transforming address resolution protocol
(ARP, RFC 826) and inverse ARP (in ARP, RFC 1293) between the Frame Relay and ATM formats.
Frame Forwarding
The FRSM card can be configured as “Frame Forwarding” on a port-by-port basis.
Frame forwarding operates the same as standard Frame Relay except
• The 2-byte Q.922 header is not assumed/interpreted.
• All frames received are mapped to a specific connection if it exists. Otherwise, the frames are
dropped.
• No DE/CLP or FECN/EFI mapping is performed.
• “Illegal header count” and “Invalid DLCI” statistics are not kept.
• “Discarded frame count due to no connection” statistic is kept.
Frame-based User-to-Network Interface (FUNI)
All FRSMs support the ATM Frame-based User-to-Network Interface (FUNI). When a frame arrives
from the FUNI interface, the FRSM removes the 2-byte FUNI header and segments the frame into ATM
cells by using AAL5. In the reverse direction, the FRSM assembles ATM cells from the network into a
frame by using AAL5, adds a FUNI header to the frame, and sends it to the FUNI port.
Loss Priority Indication
Loss Priority Indication mapping is provided in both directions.
FUNI-to-ATM Direction
The CLP bit on the FUNI header is mapped to the CLP bit of every ATM cell that is generated for the
FUNI frame.
ATM-to-FUNI Direction
CLP bit in the FUNI header is always set to 0.
Congestion Indication
Congestion Indication mapping is provided in both directions.
FUNI-to-ATM Direction
EFCI is set to 0 for every ATM cell generated by the segmentation process.