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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 (FR SMs)
ATM to Frame Relay Direction
The least-significant bit of the CPC S - U U is m apped to the C/R bit of the F r am e R elay frame.

Translation and Transparent M odes

Each service interworking (SIW) connection can exist in either translation or transparent mode. In
translation mode, the FRSM transl ates protocols between the FR NLPID encap sulation (RFC 1490) and
the ATM LCC encapsulation (RFC 1483). In t r an spa re nt m o de , t he FRS M do es n o t t r an s late.
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 “Fr ame Forwarding” on a port-by-port basis.
Frame forwarding operates the sam e a s sta nd ar d Fr am e Re lay ex cep t
•The 2-byte Q.922 header is not ass u med /i nt er pret ed .
•All frames received are mapped to a sp eci fic co nnec tion if it exists. Otherwise, the f ra mes 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-Networ k Interface (FUNI)
All FRSMs support the ATM Frame-based User-to-Network Interface (FUNI). When a frame arrives
from the FUNI interface, the FRS M removes the 2-byte FUNI header and s egments the frame into ATM
cells by using AAL5. In th e reverse direction, the FRSM as sembles 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 Indica t io n mapping is provided i n 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 hea de r is a lways 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.