interface broadcasts counter on Cisco Serail

One of my friend ask me, what is the interface broadcasts mean on the output from show interface wilt encap fr ??

First at all, I only know it's a broadcastscounter for WAN interface. and Why will it increase ?

R8(config-if)#do sh int s1/0
Serial1/0 is up, line protocol is up
Hardware is M4T
Internet address is 1.1.88.8/24
MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation FRAME-RELAY, crc 16, loopback not set
Keepalive set (10 sec)
Restart-Delay is 0 secs
LMI enq sent 45, LMI stat recvd 46, LMI upd recvd 0, DTE LMI up
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
Broadcast queue 0/64, broadcasts sent/dropped 0/0, interface broadcasts 0
Last input 00:00:02, output 00:00:02, output hang never
Last clearing of "show interface" counters 2w2d
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
46 packets input, 742 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

R8(config-if)#

1, I clear all counter on the interface

R8(config-if)#do clear count s1/0

2, then I repeat ping 1000 times with des ip = 1.1.88.255 (boardcast ip address)

R8(config-if)#do pi 1.1.88.255 re 1000 t 0

Type escape sequence to abort.
Sending 1000, 100-byte ICMP Echos to 1.1.88.255, timeout is 0 seconds:
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R8(config-if)#do sh int s1/0
Serial1/0 is up, line protocol is up
Hardware is M4T
Internet address is 1.1.88.8/24
MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation FRAME-RELAY, crc 16, loopback not set
Keepalive set (10 sec)
Restart-Delay is 0 secs
LMI enq sent 2, LMI stat recvd 2, LMI upd recvd 0, DTE LMI up
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
Broadcast queue 0/64, broadcasts sent/dropped 0/0, interface broadcasts 1000
Last input 00:00:03, output 00:00:03, output hang never
Last clearing of "show interface" counters 00:00:18
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
2 packets input, 26 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

R8(config-if)#

that's why the interface broadcast is.