Asymmetric digital subscriber line (Asymmetrical Digital Subscriber Loop ADSL) technology uses advanced modulation technology, it can provide two channels at the same time: continuous data and voice (0 ~ 4kHz band for audio transmission, telephone 30kHz ~ 1.1MHz band data transmission), make it can send broadband data without affecting the normal call at the same time.
Based on the characteristics of Internet users without taking the telephone line to dial, not only convenient for individual users and is not affected by the operator information transmission channel blocking, for business, education, medical and other activities through the real-time video transmission, so it is widely used.
The ADSL uses an existing telecommunications network, which separates the wideband digital signal from the analog voice channel by a splitter, and then enters the modem. ADSL modem is divided into ADSL (ATU-C) and ADSL (ATU-R), used to deal with carriers and end users upload and downlink ADSL signal.

As shown in figure 1: Because the existing telecommunication network based on copper, and connected ADSL devices vulnerable to electrical interference from outside, such as AC power line, electric induction, touch ESD (Electrostatic Discharge) electrostatic discharge and lightning surge, resulting in excessive current and voltage.
So we must take effective measures to protect against damage on the possible end and user end equipment. Usually, a protection element is used to carry out overvoltage protection and series current limiting device.
Commonly used current limiting devices are: fuse, metal film fuse, CPTC, PPTC, etc.. In practical application, because the PPTC device set advantage since the compound function, the action time of surface temperature, power consumption is smaller and faster fault conditions is more low, make it become a popular solution. The use of PPTC devices in the ADSL device or the associated circuit for overcurrent protection has become the trend of development.
The fuse device belongs to a disposable, low heat capacity, the protection scheme, to use the fuse has a very high rated current value, usually ten times the actual value of the working current is even greater.
The surface temperature of the fuse fault conditions increased, if the fault equipment continued in the super normal working current, and the current can not reach the level of the quick fuse fuse, temperature fuse so it is possible to exceed the allowable range, the equipment will be in a very unsafe state.
At the same time, most of the local security management agencies in the telecommunications network equipment requirements clearly put forward in the event of excessive current or lightning strikes, must meet the required level of tolerance.
The common method is to use the fuse fuse a longer reaction time to improve the tolerance, but to respond to the fuse failure time, through energy (I T) more, resulting in the possibility of damage of electronic components is greater.
As a result, the use of anti lightning fuse there are more restrictions. PPTC devices have the advantages of self recovery, high precision, high reliability, high heat capacity, small volume etc.. Experimental data show that the same rated voltage and current resistance of the PPTC device over current protection element and fuse compared with the action time is fast, the action of the surface temperature is lower, less power loss and so on.