根本原因：網路環境比較差，需要延長echo-reply訊號的次數。

版本：Ubuntu 14.04.1 LTS

1. 開啟配置檔案

sudo gedit /etc/ppp/options 

lcp-echo-failure 4  改為 lcp-echo-failure 30 或者更大的數字

# If this option is given, pppd will send an LCP echo-request frame to t
he
# peer every n seconds. Normally the peer should respond to the echo-req
uest
# by sending an echo-reply. This option can be used with the
# lcp-echo-failure option to detect that the peer is no longer connected
.
lcp-echo-interval 30

# If this option is given, pppd will presume the peer to be dead if n
# LCP echo-requests are sent without receiving a valid LCP echo-reply.
# If this happens, pppd will terminate the connection.  Use of this
# option requires a non-zero value for the lcp-echo-interval parameter.
# This option can be used to enable pppd to terminate after the physical
# connection has been broken (e.g., the modem has hung up) in
# situations where no hardware modem control lines are available.
lcp-echo-failure 30
 

3. 修改原因

lcp-echo-failure次數被設為4，而lcp-echo-interval設為30秒，

即120秒鐘內，沒有echo-reply訊號，UBuntu便會斷開網路。

修改後，900s內沒有echo-reply訊號，UBuntu才會斷開網路。