PAN network speed issues: PAN download speed around 30 KB/s, much lower than expected

熬夜的 · December 16, 2025, 11:22am

- **Chip Model**: SF32LB58

- **SDK Version**: SiFli SDK 2.4.4

- **Reference Example**: SiFli-SDK/example/bt/pan

Causes Already Ruled Out

TCP window too small (increased to 8KB)
Sniff mode impact (tested with disabled sniff)
Application layer buffer too small (increased to 8KB)
Excessive CPU yielding (tested without yielding)

Tried many configurations and tested different platform Bluetooth PAN (Personal Area Network) setups, but Bluetooth network speed remains capped at ~20 KB/s

Download complete: 2.58 MB in 206 seconds (avg 12.8 KB/s)

CONFIG_RT_LWIP_TCP_SND_BUF=8196    # 8KB transmit buffer
CONFIG_RT_LWIP_TCP_WND=8196        # 8KB receive window
CONFIG_RT_LWIP_PBUF_NUM=16         # Number of pbufs
CONFIG_RT_LWIP_TCP_SEG_NUM=40      # Number of TCP segments
CONFIG_RT_LWIP_TCPTHREAD_MBOX_SIZE=48
CONFIG_RT_LWIP_TCPTHREAD_STACKSIZE=8196
CONFIG_RT_LWIP_ETHTHREAD_STACKSIZE=8196
CONFIG_RT_LWIP_ETHTHREAD_MBOX_SIZE=48
CONFIG_RT_LWIP_USING_MQTT=y
CONFIG_RT_LWIP_USING_WEBSOCKET=y
CONFIG_LWIP_ALTCP=y
CONFIG_LWIP_ALTCP_TLS=y
CONFIG_PKG_USING_MBEDTLS=y
CONFIG_PKG_USING_MBEDTLS_USE_ALL_CERTS=y
CONFIG_PKG_USING_MBEDTLS_USER_CERTS=y
CONFIG_PKG_USING_WEBRTC=y
CONFIG_PKG_USING_WEBCLIENT=y

System DEBUG logs enabled — observing peer window size of 64154, receiver gets 1432 bytes per packet, and there’s a significant ~400ms delay between packets, which is limiting throughput

tcp_receive: window update 64154
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001c4fc
lwip_recv_tcp: buflen=72 recv_left=4018 off=1360
lwip_recv_tcp: deleting pbuf=0x2001c4fc
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=-7, pbuf=0x0
tcp_recved: received 1432 bytes, wnd 8196 (0).
lwip_recvfrom(0):  addr=23.225.33.154 port=80 len=1432
lwip_recvfrom(0, 0x20042246, 3946, 0x0, ..)
lwip_recv_tcp: top while sock->lastdata=0x0
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001c4fc
lwip_recv_tcp: buflen=1288 recv_left=3946 off=0
lwip_recv_tcp: deleting pbuf=0x2001c4fc
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=-7, pbuf=0x0
tcp_recved: received 1288 bytes, wnd 8196 (0).
lwip_recvfrom(0):  addr=23.225.33.154 port=80 len=1288
lwip_recvfrom(0, 0x2004274e, 2658, 0x0, ..)
lwip_recv_tcp: top while sock->lastdata=0x0
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001c4fc
lwip_recv_tcp: buflen=1360 recv_left=2658 off=0
lwip_recv_tcp: deleting pbuf=0x2001c4fc
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=-7, pbuf=0x0
tcp_recved: received 1360 bytes, wnd 8196 (0).
lwip_recvfrom(0):  addr=23.225.33.154 port=80 len=1360
lwip_recvfrom(0, 0x20042c9e, 1298, 0x0, ..)
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001c4fc
lwip_recv_tcp: buflen=1360 recv_left=1298 off=0
lwip_recv_tcp: lastdata now pbuf=0x2001c4fc
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
tcp_recved: received 1298 bytes, wnd 6774 (1422).
lwip_recvfrom(0):  addr=tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
23.225.33.154 port=80 len=1298
lwip_recvfrom(0, 0x200411b0, 5861, 0x0, ..)
lwip_recv_tcp: top while sock->lastdata=0x2001c4fc
lwip_recv_tcp: buflen=62 recv_left=5861 off=0
lwip_recv_tcp: deleting pbuf=0x2001c4fc
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001ac5c
lwip_recv_tcp: buflen=1360 recv_left=5799 off=62
lwip_recv_tcp: deleting pbuf=0x2001ac5c
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001bed4
lwip_recv_tcp: buflen=1360 recv_left=4439 off=1422
lwip_recv_tcp: deleting pbuf=0x2001bed4
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=-7, pbuf=0x0
tcp_recved: received 2782 bytes, wnd 8196 (0).
lwip_recvfrom(0):  addr=23.225.33.154 port=80 len=2782
lwip_recvfrom(0, 0x20041c8e, 3079, 0x0, ..)
lwip_recv_tcp: top while sock->lastdata=0x0
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001bed4
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: buflen=1360 recv_left=3079 off=0
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: deleting pbuf=0x2001bed4
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001ac5c
lwip_recv_tcp: buflen=108 recv_left=1719 off=1360
lwip_recv_tcp: deleting pbuf=0x2001ac5c
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001c4fc
lwip_recv_tcp: buflen=1252 recv_left=1611 off=1468
lwip_recv_tcp: deleting pbuf=0x2001c4fc
lwip_recv_tcp: top while sock->lastdata=0x0
lwip_recv_tcp: netconn_recv err=-7, pbuf=0x0
tcp_recved: received 2720 bytes, wnd 8196 (0).
lwip_recvfrom(0):  addr=23.225.33.154 port=80 len=2720
lwip_recvfrom(0, 0x2004272e, 359, 0x0, ..)
lwip_recv_tcp: top while sock->lastdata=0x0
tcp_slowtmr: processing active pcb
tcp_slowtmr: polling application
tcpip_thread: PACKET 0x2001eefc
tcp_receive: window update 64154
lwip_recv_tcp: netconn_recv err=0, pbuf=0x2001c4fc
lwip_recv_tcp: buflen=359 recv_left=359 off=0
lwip_recv_tcp: deleting pbuf=0x2001c4fc
tcp_recved: received 359 bytes, wnd 8196 (0).
lwip_recvfrom(0):  addr=23.225.33.154 port=80 len=359
[Delay Stats] Avg: 616 ms, Min: 292 ms, Max: 2544 ms, Count: 330

=== Final Delay Statistics ===
Total reads: 330
Average delay: 616 ms
Min delay: 292 ms
Max delay: 2544 ms
==============================

Is 20 KB/s already the maximum possible speed?

static int pan_download(int argc, char **argv) {
    const char *url = argv[1];
    const char *output = argv[2];
    
    // Disable Sniff mode
    bt_interface_set_sniff_enable(0);
    
    // Create session (8KB buffer)
    struct webclient_session *session = webclient_session_create(8192);
    
    // Send GET request
    int resp_status = webclient_get(session, url);
    if (resp_status != 200) {
        goto cleanup;
    }
    
    // Open output file
    int fd = open(output, O_WRONLY | O_CREAT | O_TRUNC, 0666);
    
    // Allocate buffer (8KB)
    char *buffer = rt_malloc(8192);
    
    // Download data (without yielding CPU)
    while (1) {
        int bytes_read = webclient_read(session, buffer, 8192);
        if (bytes_read <= 0) break;
        
        write(fd, buffer, bytes_read);
    }
    
cleanup:
    bt_interface_set_sniff_enable(1);  // Re-enable Sniff
    rt_free(buffer);
    close(fd);
    webclient_close(session);
    return 0;
}

chaotaoyang · December 22, 2025, 8:02am

Is it the same when you test with another phone?

In theory, it’s also related to the size of the package you sent.

熬夜的 · December 23, 2025, 8:48am

I originally thought the test website shouldn’t have speed limiting, since downloads via browser were very fast, almost instant. Later, after disabling browser cache, I discovered the site actually does have throttling. I’ll run more tests later. Thanks for the clarification.