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# Overview

Frame Relay is a public, connection-oriented packet service based on the core aspects of the Integrated Services Digital Network (ISDN). Frame Relay eliminates all processing at the network layer and greatly restricts data-link layer processing. Such simplified processing is possible because of the availability of virtually error-free physical connections and the presence of intelligent protocols at the end-user site, which can detect and retransmit faulty or discarded packets.

Frame Relay shifts responsibility for error recovery and flow control to the end user, thereby reducing protocol complexity and allowing high-speed packet delivery with low transit delay.

For a list of the modules on which you can configure Frame Relay, see ERX Module Guide, Appendix A, Module Protocol Support.

## Framing

E-series routers support the following framing features:

- HDLC for data-link framing
- 2-byte addresses only
- 8188-byte information field size (8192 minus 2 bytes for the address and a 16-bit CRC) or 8186-byte information field size (8192 minus 2 bytes for the address and a 32-bit CRC)

The router does not support:

- Protocol-dependent fragmentation
- Autodetection of the Local Management Interface (LMI) protocol type

## Error Frames

The router relies on higher-layer protocols to detect and recover from Frame Relay data loss. All Frame Relay error frames are discarded.

## Unicast and Multicast Addressing

Most Frame Relay services support both unicast (individual) and multicast (group) addressing. Under the most common implementation of multicasting, the Frame Relay network maps multiple individual addresses to a single multicast data-link connection identifier (DLCI) and delivers copies of a single Frame Relay packet to each member of the group.

| Note: The E-series router supports only unicast addressing.

## User-to-Network and Network-to-Network Interfaces

The Frame Relay User-to-Network Interface (UNI) is a protocol that permits users to access private or public Frame Relay networks and to establish a communications path to another user within the same network.

The Network-to-Network Interface (NNI) makes connections possible between users connected to different Frame Relay networks. These separate Frame Relay networks can be considered as subnetworks within a complete network service.

Figure 4 shows the interconnection of these types of subnetworks and the location of NNI between them.

Figure 4: Interconnection and Relationship of NNIs and Subnetworks

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