EVENT_HANDLER(physical_ready)
{
    FRAME        f;
    size_t       len;
    int          link, checksum;

    len         = sizeof(FRAME);
    CHECK(CNET_read_physical(&link, &f, &len));

    checksum    = f.checksum;
    f.checksum  = 0;
    if(CNET_ccitt((unsigned char *)&f, (int)len) != checksum) {
        printf("\t\t\t\tBAD checksum - frame ignored\n");
        return;           // bad checksum, ignore frame
    }

    switch(f.kind) {
      case DL_ACK : {
        if(f.seq == ackexpected) {
            printf("\t\t\t\tACK received, seq=%d\n", f.seq);
            CNET_stop_timer(lasttimer);
            ackexpected = 1-ackexpected;
            CNET_enable_application(ALLNODES);
        }
        break;
      }
      case DL_DATA : {
        printf("\t\t\t\tDATA received, seq=%d, ", f.seq);
        if(f.seq == dataexpected) {
            printf("up to application\n");
            len = f.len;
            CHECK(CNET_write_application(&f.msg, &len));
            dataexpected = 1-dataexpected;
        }
        else
            printf("ignored\n");

        transmit_frame((MSG *)NULL, DL_ACK, 0, f.seq);
        break;
      }
    }
}