"Comparative Analysis of DSL and Cable Modem"
Comparative Analysis of DSL and Cable Modem CSCI6704 project report Instructor: Dr. Sampalli Srinivas Submitted by Jie Ou B00163043 Qiufen Qi B00153797 Contents Abstract………….…………………………………………. 2 1 Objective ………………………………………………… 3 2 Introduction ……………………………………………… 3 3 Literature Survey……………………………………..…. 4 3.1 DSL Technology…………………………..…………………….... 4 3.1.1 DSL Network Infrastructure…………………………………4 3.1.2 DSL Modem Functions…………………………………….. 6 3.1.3 Other DSL Technologies………………………………….... 7 3.2 Cable Modem Technology……………………………...………… 9 3.2.1 Cable Modem Network Infrastructure………………………. 9 3.2.2 DOCSIS Standard……………………………………….…. 11 4 Comparative Analyses………………..…………………13 4.1 Speed………………………………………………….…………..13 4.2 Security………………………………………………………….. 14 4.3 Cost……………………………………………………………… 15 4.4 Availability and Others………………………………………….. 16 4.5 The Future……………………………………………………….. 17 5 Summary……..…………………………………..…….…18 References………………………………..…...……………19 1 Abstract Digital Subscriber Line (DSL) and cable modem are two of current major broadband access services. Both offer advantages to the home and small business customers. This report gives their basic concepts, provides literature survey for these two technologies, and performs detail comparative analyses between them on speed, cost, security, availability, and other elements as well as their future markets. As a conclusion, it is difficult to say which one is better. It really depends on many things, such as provider, the type of the services, etc. 2 1 Objective    Thanks to the dramatic increasing availability of information from the Internet, people now more and more rely on the Internet for communications, business and entertainment. As a result, the high-speed Internet access market has gained a significant growth for the past few years. At the end of year 2001, global DSL subscription has grown to a range between 16 and 19 million, and the number of cable modem users has increased to the  value between 12 and 13 million . It’s estimated that by the end of year 2005, the  subscribers to broadband Internet access service will roughly grow to 84 million . Meanwhile, the broadband access technology has experienced explosive development as well. Currently, the major broadband access services include Digital subscriber line (DSL), cable modem (CM), satellite and wireless. It’s reported that cable modems will still remain the most widely used form of broadband Internet access in North America. But in the worldwide area, total DSL subscribers will exceed the cable users by some time next year. So what are the DSL and CM technologies about, and which one outperforms the other? What is the attitude of users toward them? In the rest of this paper, we will introduce the basics of these two technologies, and have a comparison on them with aspects of speed, cost, security, availability, and other elements. 2 Introduction    Broadband usually refers a type of Internet connection that allows data, voice and video information transmission with extremely high speed. Broadband connections are typically always on, so that the users don't have to waste time dialing in to a service provider. As soon as the computer connected to the service is turned on, the connection is there. In the broadband connection, there are two direction of data streams travel on the medium and equipments. One is the downstream, which refers to the information going form the Internet to the user’s computer; the inverse is the upstream, which means the 3 information travels from the user’s computer to the Internet. Usually upstream data rate can be much lower than the downstream rate and consequently the bandwidth required by the downstream is higher. Digital Subscriber Line (DSL) achieves the broadband high speed via the existing telephone twisted-pair networks. It was first developed in 1989. At that time it was primarily designed for video. One of the reasons that DSL becomes one of the favor choices of broadband access service is that it provides high data rate for the streaming video. DSL typically provides a data rate of 128 Kbps to 6 Mbps on the downstream feed, and 128 to 640 Kbps upstream. Cable modem connection, as the name implies, is based on the same wire network set up by the cable TV service provider. CATV was introduced more than 50 years ago, as a unidirectional medium designed to carry broadcast analog video channels to the maximum number of customers at the lowest possible cost. To survive and complete in the market place, key multiple system operators (MSOs) decide to define a system capable providing bi-directional high-speed transmission. Hence come into the being of cable modem access technology. Unlike DSL, the downstream data rate for a cable modem is typically 4 to 8 Mbps, with an upstream rate of 200 Kbps to 2 Mbps. 3 Literature Survey 3.1 DSL Technology      3.1.1 DSL Network Infrastructure DSL uses the existing twisted-pair telephone line to transform high bandwidth data by setting up a dedicated point-to-point connection between the network service provider's (NSP) central office (CO) and the end user site. Figure 1 shows the picture of the DSL infrastructure. Here, RT means Remote Terminal, DLC means Digital Loop Carrier, MDF means Main Distribution Frame. The original telephone network structure was primarily designed for voice transmission. Taking the advantage of advanced transmitting and switching technologies, fiber optic 4 facilities currently exist in early every telephone company backbone network. The use of fiber optic bring into advantages like: • Improve the quality of the services. • Increase the capacity of traffic. • Reduce operational expenses for network operators. Figure 1: DSL Network Infrastructure  Central offices are interconnected through an inter-CO network, which consists of Digital Access and Cross-connect Systems (DACS) and T/E-carrier transmission equipment. These inner networks have been upgraded with the latest fiber optic ring technology (SONET or SDH). But as to the access network side, situations are very different. The access network consists of the twisted-pair copper wire local loops and associated equipment, which connects the end users to the central offices. Some end users may locate far away from the central office so that it makes local loop very long. The problem of attenuation will come up when the signal has to traverse the long distance and make the signal very weak. The telephone company has two ways to solve this problem: • Use loading coils to modify the electrical characteristics of the local loop, allowing better quality voice-frequency transmission over extended distances. 5 • Set up remote terminals where the signals could be terminated at an intermediate point, aggregated and backhauled to the central office. 3.1.2 DSL Modem Function There are a few forms of DSL technologies, which can be covered by the term “xDSL”. By replacing the “x” with different characters, it turns out ADSL, SDSL, HDSL, HDSL- 2, G.SHDL, IDSL and VDSL. Currently the most popular deployment of DSL technology is ADSL. ADSL stands for Asymmetric Digital Subscriber Line. The term “asymmetric” means there is more bandwidth of the downstream than of the upstream. In this section, most of the context will focus on the ADSL. Each end of an ADSL twisted-pair telephone line connects an ADSL modem. The modem creates three information channels: • A high-speed downstream channel, data rate ranges from 1.5Mbps to 9Mbps. • A medium-speed duplex channel, data rate ranges from 16kbps to 640kbps. • A basic telephone service channel. The basic telephone service channel is split off from the digital modem by filters, so that it is guaranteed that the basic telephone service will not be interrupted, even ADSL fails. To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways: frequency-division multiplexing (FDM) or echo cancellation. Each channel can be submultiplexed to generate multiple lower-rate channels. FDM assigns two bands to the upstream and the downstream separately. The downstream path is then divided by time-division multiplexing into one or more high- speed channels and one or more low-speed channels. In the same way, the upstream path is also multiplexed into corresponding low-speed channels. Echo cancellation assigns the Figure 2: FDM and Echo Cancellation  P P 6 upstream band to overlap the downstream, and separates the two by means of local echo cancellation. The working of FDM and echo cancellation is shown in Figure 2. The ADSL modem with minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16-kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps for duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are also available. ADSL modems accommodate Asynchronous Transfer Mode (ATM) transport with variable rates and compensation for ATM overhead, as well as IP protocols. Downstream data rates depend on a number of factors, such as the length of the copper line, its wire gauge, the presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. 3.1.3 Other DSL Technologies As mentioned above, there are several forms of DSL: 1) HDSL HDSL stands for high bit rate DSL. It was originally developed by Bellcore. This technology has been standardized by ANSI in the United States and by ETSI in Europe. The reason that HDSL became popular lies in that it is a better way to provide T1 or E1 over twisted-pair copper lines than the long-used technique known as Alternative Mark Inversion (AMI). HDSL uses less bandwidth and requires no repeaters up to the CSA range. Also T1 service can be installed in a day for less than $1,000 by installing HDSL modems at each end of the line. Whereas by comparison, it takes longer and costs more to install T1 via AMI. This is because of the requirement to add repeaters between the subscriber and the CO. HDSL is heavily used in cellular telephone buildouts. Traffic from the base station is backhauled to the CO using HDSL in more than 50 percent of installations. Currently, the vast majority of new T1 lines are provisioned with HDSL. However, because of the 7 embedded base of AMI, less than 30 percent of existing T1 lines are provisioned with HDSL. Though HDSL has advantages over AMI, it does have drawbacks. First, it uses the voice band, so there is no provision existing for analog. Also unlike ADSL, HDSL is a symmetric system, and it has crosstalk at both ends of the line. 2) SDSL Symmetric Digital Subscriber Line (SDSL) is a rate-adaptive version of HDSL. Both SDSL and HDSL are symmetric. SDSL allows equal bandwidth for the downstream and the upstream. The other common of SDSL and HDSL is that it does not support analog calls. SDSL supports data only on a single line, using 2B1Q line coding. SDSL can transmit up to 1.54 Mbps to and from a subscriber, or can be configured to offer a variable range of bandwidth up to 1.45 Mbps. SDSL is suitable for WAN technology for small to medium businesses and branch offices. The symmetry offered by SDSL, combined with always-on access, makes it a favorable and affordable alternative to dedicated leased lines and Frame Relay services. Because traffic is symmetrical, file transfer, web hosting, and distance-learning applications can effectively be implemented with SDSL. 3) HDSL-2 HDSL-2 is an emerging standard and a promising alternative to HDSL. HDSL-2 intends to offer a symmetric service at T1 speeds by using a single-wire pair rather than two pairs. This technical change will enable the system to operate for a larger potential audience. But at the same time, it will require more aggressive modulation, shorter distances, and better phone lines. HDSL-2 is developed to enable the interoperation of equipments by different vendors. The most outstanding advantage of HDSL-2 is that it is designed not to interfere with other services. However, HDSL-2 is full rate only, offering services only at 1.5 Mbps. 8 4) ISDL ISDL stands for ISDN digital subscriber line. It is a cross between ISDN and xDSL. ISDL uses only a single-wire pair to transmit full-duplex data at 128 kbps and at distances of up to RRD range. This is similar to the ISDN. The users can continues to use existing CPE (ISDN BRI terminal adapters, bridges, and routers) to make the CO connections. The difference between ISDN and ISDL is that ISDL does not connect through the voice switch. The limitation of ISDL is that the customer no longer has access to ISDN signaling or voice services if they use ISDN. 5) VDSL VDSL stands for Very High Data Rate Digital Subscriber Line. VDSL transmits high- speed data over short reaches of twisted-pair copper telephone lines, with a range of speeds depending on actual line length. Both upstream and downstream channels will be separated in frequency from bands used for basic telephone service and Integrated Services Digital Network (ISDN), enabling service providers to overlay VDSL on existing services. Currently, the two high-speed channels are also separated in frequency. As needs arise for higher-speed upstream channels or symmetric rates, VDSL systems may need to use echo cancellation. 3.2 Cable Modem Technology       3.2.1 Cable Modem Network Infrastructure Cable modem Internet access service is based on the CATV network. A CATV network consists of a head-end, serves as the local data network operations center. In head-end all incoming signals are received. Next the signals are multiplexed with the frequency- division multiplexing (FDM) technique, amplified, and transmitted downstream for distribution to the complete cable plant. The original CATV networks were exclusively one-way, which consisted of diverse amplifiers in a cascade form to compensate for the intrinsic signal loss of the coaxial. 9 This long amplifier cascades brought high noise into the transmission, and made the performance of the whole network unreliable and failure-prone. This problem was solved with the introduction of the fiber optic technology, and the hybrid fiber coax (HFC) plant. In the network architecture shown in Figure 3, portions of the coaxial cable and supporting amplification elements are replaced with ring-structure multifiber optic cable from a head-end or distribution hub location. Obviously the introduction of the optic fiber can significantly reduce the number of cascaded amplifiers and consequently improve the Figure 3: Cable Modem Network Infrastructure  system reliability, the signal-to-noise ratio (SNR) of the downstream video signal, and support of greater system bandwidth. In addition, this makes the system eligible for the bi-directional data transmission. The HFC structure also give another benefit that reduces operational and maintenance costs, and improves the immunity of the system to ingress noises. Network robustness, scalability and flexibility are achieved by the fiber ring topology. 10 The distribution hub is the interchange point between the regional fiber network and the HFC plant. At the hub, the cable modem termination system (CMTS) coverts data from a wide area network (WAN) protocol into digital signals that are modulated for transmission over HFC plant. The signal will be later on demodulated by the cable modem in the home or business. In the optical node, optical signals are converted into electrical signal that can then be propagated downstream to the entire end user site. At the end user site, a splitter locating at the end of the coaxial cable will split the data into one stream that sends to the TV set and the other stream that goes into the cable modem. 3.2.2 DOCSIS Standard The Data Over Cable Service Interface Specification (DOCSIS) standard is resulted from the Multimedia Cable Network System Partners, Ltd. (MCNS). It prescribes multi-vendor interoperability and promotes a retail model for the consumer's purchase choice of a CM. All products are rigorously tested by the CableLabs. Only those passing all the tests will be CableLabs Qualified for head-end Cable Modem Terminating System (CMTS), and CableLabs Certified for CM devices. This kind of process ensures the multi-vendor interoperability. Currently, there are 3 different version specifications for DOCSIS. DOCSIS 1.0, 1.1 and 2.0. The most common implementation is DOCSIS 1.0, while 2.0 is still in the development stages. The DOCSIS interface specifications enable the Internet Protocol (IP) traffic data traversal over nonproprietary, multi-vendor, interoperable cable systems in bi-direction, between the cable system head-end and customer CMs over an all-coaxial or HFC network. DOCSIS layer definitions include the IP network layer, the data link layer and the physical layer. Under the data link layer, there are several sub-layers: • Logical Link Control (LLC) sub-layer that conforms to Ethernet standards. • Link security sub-layer that works for basic privacy, authorization, and authentication. • Media Access Control (MAC) sub-layer that supports the operation of variable length protocol data units (PDU). 11 The physical layer is comprised of the following sub-layers: • Downstream convergence sub-layer that conforms to MPEG-2. • Physical Media Dependent (PMD) sub-layer that supports downstream and upstream modulation and formatting. The DOCSIS physical layer allows considerable flexibility such that the quality of transmission can be achieved over cable plants of varying kinds. It provides optional choices for the upstream channel bandwidths and modulation available for both the upstream and downstream signal flows. Figure 4 shows the comparison of DOCSIS OSI DOCSIS Data over Cable Higher layers Application DOCSIS control Transport TCP/UDP message Network IP Data link DOCSIS MAC Upstream TDMA Downstream TDMA Physical Digital IF modulation Digital RF modulation HFC Figure 4: OSI and DOCSIS layers  protocol layers and the OSI standard layers. DOCSIS further specifies the mandatory servers that must interface the CMTS and CM, to make the system functional and operational. The servers include: • Dynamic Host Configuration Protocol (DHCP) server, which provides needed IP addresses for both the CM and subsequent PC devices. • Time of Day (TOD) server, which is used for the purpose of time-stamping operational system events. • Trivial File Transfer Protocol (TFTP) server, which is used to register and download CM configuration files for individual customer service. These configurations could include quality of service (QoS) parameters, baseline privacy (BPI) implementation, operating frequency assignments, the number of host devices etc. 12 The DOCSIS specifications also dictate a CM registration process. 4 Comparative Analyses Both DSL and cable modem services have similar features in many aspects. They both offer high-speed Internet access. The broadband transmission techniques that their services rely on make many services function nicely, such as video-on-demand, multimedia conferencing, and online gaming, etc. These services usually don’t work well at 56 kbps using traditional dial-up access. So they provide a new realm of possibilities for network connectivity and applications to the homes and small businesses. Because of the history of the technologies that they built upon and different approaches of the service providers, DSL and cable modem differ from each other in some fundamental ways. They can be compared and contrasted from the following angles: • Speed • Cost • Security • Reliability and availability • Usability and quality of service 4.1 Speed In theory, cable modem can achieve networking speeds of approximately 30 Mbps when a 100 Mbps network interface card is used, while most type of DSL cannot reach 10 Mbps. But in practice, the speed advantage of cable over DSL is much lower than the one that might first appear. Both the performances of cable modem and DSL vary from time to time. It depends on the pattern of use and traffic on the Internet. Both services rarely reach their peak performances. Moreover, cable modem technology delivers shared bandwidth within the local neighborhood while DSL delivers dedicated local bandwidth. With cable, the speed of 13 the individual connection is determined by how many cable subscribers in the neighborhood, and how many of those subscribers are actually using the system as well as the load on the Internet generally. Sometimes the service will be very fast, at other times, it could be very slow. These make the speed performance of cable modem unpredictable. The results of the test conducted by Keynote Systems confirmed the above assertion. The results showed that a very modest ADSL service, which offers 384 kbps downstream and 128 kbps upstream, actually outperformed cable during the "peak personal hours" of 5pmto 11pm, and underperformed cable during the “business hours” of 8am to 5pm. DSL services typically offered today typically range in performance from 128 kbps to 1.544 Mbps. It can be hard to specify exact speed numbers for DSL because of the many variations in equipment. DSL is also a distance-sensitive technology. The bandwidth available to a home user, for example, depends significantly on the length of the line running from the home to the telephone company central office as well as the electrical quality of that line. Today, cable modem has a slight speed performance advantage over DSL. But this advantage seems to be a short-term one. Technologies like dedicated-bandwidth VDSL threatens to erase cable's speed advantage. 4.2 Security A unique security issue with cable is that the line is shared with others in your area, which makes it easy for a neighbor to snoop around your computer. Essentially, all cable customers in the region belong to the same LAN. If there were no security measures in place, anybody in the neighborhood might technically be able to click on their Windows Network Neighborhood icon and actually see the computer names and addresses of their neighbors on the service. If a customer enables file sharing on any drives, neighbors could even download copies of their data. DSL uses dedicated rather than shared cabling, and DSL users in a given neighborhood do not appear as nodes on a LAN. From this point of view, there exist some conclusions 14 that DSL services provide better security than cable modem. This is an oversimplified argument. DSL opens your computer to similar security risks “if your DSL provider does not have its own security measures to prevent hacking and listening to traffic on the network, you are vulnerable, too." Although some cable users encountered some problems in the past, many providers avoid it today by bundling security features in the cable modem hardware. For example, basic network firewall capabilities will prevent files from being viewed or downloaded. Most cable modems today comply with DOCSIS standard, which supports for cable network security features including authentication and packet filtering . In general speaking, both DSL and cable provide reasonably safe Internet access as long as one follows reasonable security precautions. No matter what form of Internet access is used, these precautions should be followed, which include use of a broadband router, firewall software, or proxy server software. One , if possible, should also disable network file sharing on the internal LAN. 4.3 Cost xDSL is likely to be more expensive than cable modems. You will have to pay for the DSL line as well as for interned access. But the cost of DSL varies widely. In some areas, you may be able to get xDSL service and ISP account as a package for about the same price as cable modem access. Both cable and DSL monthly access fees is getting lower and lower, and you can find some decent deals by shopping around. Many providers offer incentives such as a free modem or installation. Higher speed DSL service can easily cost $80 or more per month, with some kinds of DSL for small businesses costing several hundred dollars per month. So far, cable Internet service is often a bit cheaper than DSL: $39.95 per month or less. Figure 5 shows some DSL and Cable statistics on costs and speeds. 15 Installation Monthly Rates Connection Speeds Fee $100 to $200 $40 to $80 for home ADSL: 384-kbps to 9-mbps users download/128-kbps upload DSL SDSL: 1.5-mbps download/upload $80 to $320 for business users ISDL: Up to 144-kbps download/upload Cable $75 to $200 $39.95 to $49.95 1- to 2-mbps download/128- to 384-kbps Modem upload Figure 5: DSL and Cable Vital Statistics 4.4 Availability and Others In order to have cable modem service you have to order cable television (CATV) service as well. You need phone service for DSL. Almost everyone already has phone service but many people do not have CATV service. Compared to DSL service, cable modem service is not widely available. With DSL, you can be on the phone talking at the same time you are surfing the Internet and you don’t need to order another phone line. With cable modem, you must have cable television service to get cable modem service. DSL is on a closed, dedicated circuit enabling providers to offer a wide range of guaranteed speeds. Cable Modem exists on a shared network thereby making speed performance unpredictable. Cable Modem may also have multiple sources (or companies) providing local service and Internet access to consumers which takes away the convenience and accountability that comes with a single-source service provider. For DSL, you can use more than one computer on the Internet connection. Multiple computers on the same DSL connection won't hog the bandwidth of others in your 16 neighborhood. Cable modem companies might disconnect your service if you set up multiple computers on their connection. 4.5 The Future Despite the disadvantage of these two technologies, we will continue to see the wide- scale DSL and cable modem deployments in major metropolitan areas in US and the other countries. DSL is competing with cable modem. Currently, cable modem is in the lead, but DSL is racing to catch up. Figure 6 shows the results of market analysis in US from IDC. Figure 7 shows the results of market analysis in 2003 in Canada, Germany and Japan from the Stratdgis Group. 1999 2003 DSL 330 9,300 Cable Modem 1,350 8,980 Figure 6: Cable modem and DSL subscribers in 1999 and 2003 in US (in thousands) Canada Germany Japan DSL 1,808 1,428 1,095 Cable Modem 1,800 1,200 1,500 Figure 7: Residential Cable Modem and DSL subscribers in 2003 ( in thousands) 5 Summary Both cable modem and DSL offer high-speed Internet access using broadband transmission techniques. DSL uses the existing copper twisted-pair telephone line to transfer high bandwidth data by setting up a dedicated point-to-point connection between NSP central office and the end users. Cable modem Internet access service is based on 17 the CATV network. In both infrastructures, optic fiber is used to improve the quality of services, the capacity of traffic, and reduce operational expenses for network operators. There are several forms of DSL. Among of them, ASDL is the most popular one. The DOCSIS standard prescribes multi-vendor interoperability and promotes a retail model for the customer’s purchase choice of a cable modem. DSL and cable modem differ in several dimensions. The speed performance of cable modem is potentially better than DSL. It depends on the number of users shared on the system. The speed performance of DSL is geographically limited, i.e. depends on the length of the line from central office to the user. DSL and cable modem both provide reasonable security. But it seems that cable modem is less secure than DSL because of shared cabling for cable modem and dedicated line for DSL. However, DOCSIS standard enforces more security for cable modem. The cost of cable modem is likely less expensive than DSL. But cable modem is not as widely available as DSL, which uses existing phone line that almost every one has rather than CATV. Like speed and cost, quality of service and many other elements of DSL and cable modem service can vary significantly depending on the provider. As a conclusion, it is hard to say which one is better. It really depends on many things. Despite the disadvantages of these two technologies, they both have bright future in the market. 18 References  Ditch Your Dial-up. Retrieved on April 28, 2002 from <http://www.pcworld.com/features/article/0,aid,73865,pg,7,00.asp>.  The broadband marathon: access technologies jockey for subscribers. Retrieved on April 29, 2002 from <http://www.instat.com/catalog/Ncatalogue.asp?id=63#MB0104MI>.  Broadband Bob Report. Retrieved on April 28, 2002 from <http://www.catv.org/frame/bbb.html>.  Broadband Tutorial. Retrieved on April 28, 2002 from <http://www.proxim.com/learn/library/whitepapers/pdf/broadband.pdf>.  Short Sketch of the History of DSL. Retrieved on April 29, 2002 from <http://www.dslprime.com/Technology/DSL_s_History/dsl_s_history.html>.  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