The user enables FRF.12 e2e fragmentation on a per FR SAP basis. A fragmentation header is added between the standard Q.922 header and the payload. This header consists of a 2-byte Network Layer Protocol ID (NLPID) of value 0xB1 to indicate e2e fragmentation payload and a 2-byte containing the Beginning bit (B-bit), the End-bit (E-bit), the Control bit (C-bit), and the Sequence Number field.
The following is the mode of operation for the fragmentation in the transmit direction of the FR SAP. Frames of all the FR SAP forwarding class queues are subject to fragmentation. The fragmentation header is, however, not included when the frame size is smaller than the user configured fragmentation size. The SAP transmits all fragments of a frame before sending the next full or fragmented frame. The fragmentation threshold is configurable in the range 128 to 512 bytes. In the receive direction, the SAP accepts a full frame interleaved with fragments of another frame to interoperate with other vendor implementations.
An FR SAP with FRF.12 e2e fragmentation enabled can be part of a VPLS service, an IES service, a VPRN service, an Ethernet VLL service, or an IP VLL service. This SAP cannot be part of a FR VLL service or an FRF.5 VLL service. However, fragmented frames received on such VLLs are passed transparently as in current implementation.