Technical Comparison between ADSL and CABLE MODEM

In the evolution from the existing narrowband Access Technology to the final optical access solution, there will be many very vital transitional technologies, among which, ADSL technology based on telephone lines and Cable Modem technology based on wired cables are the most promising.
ADSL features asymmetric uplink and downlink speeds. Therefore, its applications are mainly applicable to providing users with Internet services and VOD services, rather than lan interconnection services. Internet access services are provided using the "ADSL + ATM/Ethernet" method. Clients are configured with ADSL Remote devices, and local clients are configured with DSLAMADSL local devices). They are connected by a pair of wires over a common phone. The remote ADSL device provides an Ethernet interface for the user's PC. DSLAM is connected to the ISP through ATM or fast Ethernet. With this network, you can achieve broadband access to the Internet. DSLAM can be placed in the ISP data center and connected to the ISP through the access network. It can also be directly placed in the ISP data center, and directly connected to the ISP access platform using the LAN. The "ADSL + ATM" method can be used to provide VOD services using ADSL.
Cable Modem is also a technology with asymmetric upstream and downstream bandwidth. It is suitable for Internet access and VOD services. For Internet access services, you can use the "HFC + Cable Modem + Ethernet/ATM" method. The local terminal must be equipped with an hfc-head device that is connected to the Internet through ATM or fast Ethernet, and the signal modulation and mixing functions are completed. Data signals are transmitted to users' homes through fiber-optic coaxial hybrid network (HFCs). Cable Modem completes signal decoding, demodulation, and other functions, and transmits digital signals to PCs through Ethernet ports. In turn, the Cable Modem receives upstream signals from the PC and sends them to the head end device through the HFCs After encoding and modulation.
Both ADSL and Cable Modem technology groups can provide a variety of services and meet the needs of current broadband services. The bandwidth of ADSL is lower than that of Cable Modem. However, the stability, reliability, power supply, and operation and maintenance systems of the Network Using Cable Modem and HFCs all have some problems. In addition, because the bandwidth of the network line is shared, after a user reaches a certain scale, it is actually unable to provide broadband data services, and the bandwidth shared by the user is very limited. The following is a comparison between the two technologies.

-- Security. Because all user signals of Cable Modem are transmitted on the same coaxial Cable, there is a risk of wire eavesdropping. To solve the cabling listening problem, we must first protect the cables. Secondly, we need to understand the setting parameters established during the line initialization process, which is technically difficult to solve. But ADSL does not have this problem.
-- Reliability. Because CATV is a tree-like network, it is very easy to cause single point of failure, such as cable damage, amplifier failure, and Transmitter failure, which will cause the interruption of user services on the entire node. ADSL uses a star network. The fault of an ADSL device only affects one user.
-- Stability. Early users of Cable Modem can enjoy excellent services, because the bandwidth and frequency band of the line are abundant when there are few users. However, the addition of each Cable Modem user will increase noise, occupy channels, reduce reliability, and affect the existing user service quality on the road. This will be a major challenge that Cable Modem urgently needs to solve. ADSL will not be affected by the number of users and traffic in the access network. Of course, if the egress bandwidth of DSLAM is smaller than the total bandwidth that all users may need, there will be congestion during peak hours, but this problem can be solved by increasing the egress bandwidth.
-- Compatibility. Although both Cable Modem and ADSL have introduced technical specifications and standards, the products of different manufacturers are not compatible yet, which brings difficulties to the market expansion.
-- Networking cost. In terms of networking costs, the cost of ADSL equipment is obviously higher than that of Cable Modem, but the latter must be applied only after the transformation of HFCs is completed. At present, most of China's HFCs can only meet the bandwidth requirements of 450MHz, while the two-way service provided by HFCs requires at least MHz of bandwidth. This clearly requires replacement of all non-conforming coaxial cables. At the same time, to achieve two-way HFCs, You need to replace the one-way amplifier currently used on the cable TV network. This part of the transformation cost is also quite high.