The System Design And Implementation Of Embedded ADSL Gateway

The services provided by the network are becoming more and more, as a result, the bandwidth needed is becoming broader and broader. The normal user must go through the access network and backbone to access the Internet via ICP There are always high-rate and broadband lines connected to the backbone, The backbone of telecom can provide wide enough bandwidth for all the data transmission as more and more optic fiber with WDM and DWDM technologies have been paved .But "the last mile" from user's home to central office is still narrow-band and analog, which becomes bottom-neck of the communication subnet preventing it from being widen. ADSL, is the most cost-effective technology recently, because it can utilize the existing loop line of PSTN.lt is also the most popular one of the RBB(Residential BroadBand network) technologies.The paper provides a scheme for user-side ADSL access------embedded ADSLgateway. This system ,the combination of ADSL and embedded technology, consists of hardware and software, the paper will describe the platform and firewall as well as proxy , the transplant of Linux and the encode and decode in the physical layer is beyond this paper.The gateway is a especial-purpose computer and based on x86 architecture for software-compliance with PC. So most software can be test on the PC and can be transplanted to gateway after all is OK.The platform is designed from top to bottom, at first, the functionalities and interface of the three module should be specified clearly, and then go into the details of it, came back to modify the interface made before .The tree modules are connected through PCI bus.ADSL module implements the ADSL modern as the ATU-R; the network interface consists of two part,wileless LAN(it is optional) and LAN ,can connect with two different LAN.The main control module controls the action of the others which is made of SDRAM, microcontroller, FlashROM and serial port.The firewall and proxy ,parts of the software of the system, based on the IP chians,is a packet filtering firewall.First,it provides transparent-proxy to the user on the user's LAN via IP forwarding and IP masquerading in which all the hosts share on IP address to access the Internet. And then the firewall provides protection to the host behind it via IP filtering preventing invalid access such as IP spoofing .After that, the firewall can let the hosts behind it to provide Internet services to the outside hosts via IP port forwarding, for example WWW and SMTP.The whole paper consists of three part: the first part(including chapter 1 and 2)discusses the advantages of ADSL compared with other access technologies and analyses the principle of ADSL,the second part(including chapter 3 and 4) tells how to design the system and hardware platform; the third part(including chapter 5)describes the implementation of firewall and ip forwarding and proxy. After all, the last chapter summarize the paper and give a prospect of the gateway.