High-Speed at Home: Cable vs. Telco's xDSL Network Systems (CSE 588) April 30, 1997 I. Introduction A network connection is only as fast as its slowest link. For home networking, this has historically been the connection between the personal computer and the network service provider. This connection is generally through a phone line using either an analog or digital modem. Today the state of the art for analog modems is at 56.6 kbps. These modems are just hitting the streets and their actual performance is largely dependent on how clean the phone line connection is. Digital modems are rated at 128 kbps using ISDN. These modems require ISDN service from the telephone service provider and have been known to be very difficult to set up and manage. As demand for bandwidth continues to increase with the growing use of the Internet and more demanding applications, subscribers will have one fundamental choice on selecting their service provider. This is whether to receive wire network service over a cable modem or a DSL modem. This choice is positioning the phone companies in direct competition with the cable companies. Both industries are eagerly trying to provide a service which is technologically superior to the other, competitively priced, and most importantly, utilizes the enormous infrastructure both industries already have in place (that is, copper cable and Hybrid Fiber/Coax HFC). This also positions the equipment manufacturers against one another to deliver competitive technologically advanced products. This paper examines these two fundamental approaches. It describes the underlying capabilities and technology associated to both approaches. Issues and standards will also be addressed. The choice of wireless technology using direct broadcast satellites is not covered by this paper. II. Cable Modems Cable modems appear to be more challenged by the cost of upgrading their cable plants than they are by cable modem technology. These plants require substantial investment in equipment to support 2 way communication with the end user. In order to understand more about cable modems and cable routers, several equipment manufacturers and their products are looked at. Hewlett-Packard's QuickBurst Cable Modem offers speeds which are 100's of times faster than current analog modems. They are also more sophisticated than dial up modems by functioning like a network bridge, router or Ethernet hub. HP's modem connects to a PC through an Ethernet 10BaseT LAN. Multiple PCs can communicate using a single modem using a network hub. TCP/IP support offers access to the many Internet applications. Network management support is offered using the Simple Network Management Protocol (SNMP). The HP modem connects to a standard CATV wire and communicates over a dedicated band of frequencies to an HP Link Router. This Link Router connects to the network via ATM or a ThinLan Ethernet Link. Communication between the Link Router and modem uses an HP protocol called Multimedia Transmission Link (MXL). It supports privacy using DES encryption. It is designed to eliminate packet segmentation and to support future constant bit rate (CBR) services to support applications such as real-time video and voice. Download transmission speeds from the Link Router to the modem are at 30 Mbps. The Router's transmissions are split to Fiber Service Areas (FSA) which are typically groups of 500 to 2500 subscribers. Each FSA is treated as an IP subnet. Communication is via broadcast. Each modem filters out only those transmissions which are directed at it, by accepting only those on a pre-assigned frequency channel. This supports Ethernet addressing schemes such as unicast, broadcast and multicast. Upper level IP packet filtering, encryption, and decryption provide added insurance that packets are only read by the appropriate modem. Because upstream cable transmissions use a frequency range which is much more susceptible to noise called "ingress", these transmissions require much more handling. This is magnified by the number of modems connected to the same Link Router. Different modulation techniques are used by the different vendors to handle this. HP uses Quadrature phase-shift keying (QPSK) for uploads at 3 Mbps and Binary phase-shift keying (BPSK) for uploads at 1.5 Mbps. The approach is to find the cleanest available channel and direct the communication along that frequency. Multiple upstream transmissions are handled by using collision backoff and time slotting given the size of the packet. Time slots are allocated to a specific modem. Only one modem can transmit during a time slot. If a slot is not allocated, any modem can transmit. The Link Router will send an Ack if the transmission was successful. If no Ack is received the modem can assume that a collision occurred with another modem's transmission. The modem will back off using some algorithm and then retransmit. At the headend of the system resides a server complex which connects to the Link Router via ATM or Ethernet. These servers support Domain Name Services, user authentication, manages levels of service for each user, firewall services, and Internet applications such as e-mail, Telnet, and FTP. Motorola's CyberSURFR modem offers speeds comparable to HP. It too can act as an Ethernet hub supporting several PCs with IP addresses. The CyberSURFR modem itself does not have an IP address or require an IP subnet for the connected PCs. PCs are connected to the modem using 10BaseT Ethernet. The modem communicates over a CATV line to a Cable Router. This router connects to network interfaces such as 10BaseT and 100BaseT Ethernet and FDDI. The router is responsible for allocating modems equal access to the shared bandwidth. Modems are automatically discovered by the router and the connected computers can be assigned IP addresses statically or using a DHCP server. The router will detect congestion on the HFC network and move data traffic from a congested channel to a less utilized channel. Communication between the modem and router uses DES encryption. Authorization and registration is performed when the modem is first turned on. Network management is performed using an SNMP proxy agent. Both upstream and downstream traffic use the Motorola Cable Data Link Protocol. Download transmission is across a 30 Mbps shared data channel with each modem able to use up to 10 Mbps. Upstream traffic uses a 768 kbps shared data channel. LANcity Cable Modems from Bay Networks offer similar functionality. Unlike HP and Motorola, these modems offer symmetrical transmission rates of 10 Mbps. The modems are "filtering MAC layer bridges that prevent unauthorized information into the user premises" [8] by forwarding only to known destination addresses. In order to address the Quality of Service issue, Fore and General Instruments (GI) are introducing an ATM-Based Cable Modem system. This will offer subscribers guaranteed transmission rates necessary to support applications such as real-time video. Of course this does not alleviate the requirement of having enough overall bandwidth available for the number of potential subscribers. Using switched based ATM provides added security by not risking that a packet be received or illegally intercepted by another user of the shared Ethernet resource. GI's ATM-based modem offers 27 Mbps downstream transmission rates and 2 Mbps upstream rates. At the headend is an ATM switch capable of 2.5 Gbps or 10 Gbps. Cable modems appear to have a head start as compared to the DSL technology. Several new Cable ventures are already underway to provide high-speed network access. One of the most notable is the @Home network. @Home uses cable modems from several vendors including HP, Motorola and Bay Networks. One unique characteristic of the @Home network is that it is providing a very large ATM backbone network that is parallel to the Internet. This is to offer a certain Quality of Service currently unavailable with the Internet. The cable industry is moving toward a cable-modem specification issued by a working group of the Multimedia Cable Network Systems (MCNS) consortium. The specification defines the modulation to be used between the Cable Modems and the Routers. It calls for two quadrature-amplitude-modulation formats-64-QAM and 256-QAM-for downstream transmissions and quadrature phase-shift keying (QPSK) and 16-QAM would be used for upstream transmissions. Equipment makers are also supporting an MCNS-compliant silicon platform developed by Broadcom Corp. These chips will provide common solutions for both the physical and MAC layers. Above these layers "there is a whole gray area that will still need to be sorted out, including security and network-management issues." [12] III. xDSL Modems DSL Modem manufacturers believe they offer competitive advantages over Cable Modems with respect to speed, availability, interactivity and security. Judging by the information available on DSL it appears Cable Modem technology is further ahead. Even so, DSL availability like Cable modems is more constrained by the cost of upgrading the infrastructure than is by the technology itself. DSL stands for Digital Subscriber Line. What is attractive about DSL technology is that it implements high speed transmissions on most of the telephone companies existing copper wire infrastructure. It does this by using sophisticated algorithms to adjust the signal to account for the particular copper line characteristics. DSL comes in many varieties. HDSL is the most popular, supporting symmetric transmissions of 1.544 Mbps and 2.048 Mbps. It requires two twisted pair lines for the former and three twisted pair lines for the latter. It supports line lengths up to 12000 feet. ADSL is intended for the last leg of the phone line into the customer's home. It supports asymmetric transmissions depending on line lengths. At 18000 feet it can download at 1.544 Mbps. At 9000 feet it can download at 8.448 Mbps. Upstream transmissions range from 16 kbps to 640 kbps. VDSL is the fastest of all the DSL technologies but also supports the shortest line lengths. At 4500 feet of wire it can support downloads of 12.96 Mbps. and at 1000 feet it can transmit at 51.84 Mbps. Uploads range from 1.6 Mbps to 2.3 Mbps. No general standards exist for VDSL yet. Other DSL technologies include SDSL (symmetric) and RADSL (rate adaptive) which adjusts its transmission speed based on the length and quality of the line. DSL modems do not rely on the current voice switching network. The central office modem can be connected directly to routers, bridges, Ethernet and ATM switches. The end user modem in ADSL systems provides an integrated 10Base-T Ethernet interface. These modems are equipped with a plain-old-telephone-service (POTS) splitter to separate the voice frequencies from the data-carrying frequencies which are carried on the same line. Westell's FlexCap2 is a RADSL system which operates at 640 kbps to 2.24 Mbps for downloads and 272 kbps to 1.088 Mbps upload. SNMP network management systems are supported. GlobeSpan's RADSL modem delivers even higher performance with download transmissions ranging from 600 Kbps to 7 Mbps and upload rates of 128 Kbps to 1 Mbps. Copper Mountain Networks is introducing DSL products which appear to be more for small businesses. They feel this is where DSL technology will first take hold and then move to the home. Their system includes a DSL Multiplexer for the Central Office which supports full duplex nonblocking SDSL throughput from 160 kbps to 1.168 Mbps for line lengths up to 22770 feet. Their speeds are lower than some competitors but they feel the higher end throughputs are not realistic at this time. The Multiplexer can connect to backbone networks using 10Base-T and 100Base-T Ethernet. On the user end, Copper Mountain Networks offers Red Rocket access routers. The phone line acts as a virtual LAN without dialing. These routers are automatically configured by the DSL Multiplexer when they are plugged into the phone line and turned on. Proxy agents in the Multiplexer handle packet forwarding and mulitiplexing for the Red Rocket router automatically. Both ends of the line are managed using SNMP support services. Sourcecom is offering the BANC system to support RADSL communication. At the Central Office end, the BANC 6000 offers access to Frame Relay, ATM, routed, and IP switched backbones. It can communicate with the BANC 1000 RADSL routers at up to 7 Mbps downstream and 1 Mbps upstream. On the user end, the BANC 1000 router offers support for IP, IPX, Frame Relay, PPP, HDLC, transparent bridging, SNMP, and Telnet. The prices point is such that this to is probably more feasible for small businesses than individual home users. With regards to xDSL standards, there are several different approaches to line coding. These include Carrierless Amplitude and Phase modulation (CAP), Discrete Multitone (DMT), and Quadrature Amplitude/Phase (QAM) modulation. In March 1993, the ANSI working group T1E1.4 selected DMT as the standard line code for ADSL. Several products are on the market that support or will support both DMT and CAP. IV. Other Comparisons One of the biggest issues facing Cable modems that DSL modems avoid is that all users compete for the same bandwidth. Many users share the same cable. This poses quality of service as well as security issues. As more subscribers are brought online, each subscribe continues to get less and less of the cables total bandwidth. This shared cable approach also introduces a security issue which is not present in the DSL strategy where every modem is allocated a separate (point to point) line to the network. Downtime of phone lines as compared to cable lines will be an issue. Clearly the phone company has a better track record here. Reliability of the connection will be very important to home business users. Speed does not appear to be a distinguishing factor unless high performance symmetric speeds are required for applications such as video conferencing. In this case DSL has the advantage. In comparing who will come on line first, the cable or phone companies, there is a third alternative which are the Local / Internet service providers. These providers may be able to offer wide spread use of DSL technology before either of the other two industries. V. Conclusion Rather than bandwidth speed, the bigger speed question is that of implementation. Both the cable companies and telephone companies are moving very slowly at providing wide spread access to these technologies. It could be that the first to market may become the de-facto standard rather than the best technology as has been the case many a time in the computing industry. Bibliography [1] ADSL Forum, General Introduction to Copper Access Technologies. Online. Available: http://www.adsl.com/adsl/general_tutorial.html [2] ADSL Tutorial. Twisted Pair Access to the Information Highway. ADSL Forum. Online. Available: http://www.adsl.com/adsl/adsl_tutorial.html [3] BANC 1000 and 6000 Designed To Provision Local xDSL Access Points. The Telechoice Report on xDSL. Telechoice Inc. April 1997. Online. Available: http://www.telechoice.com/xdslnewz/showDSL.cgi?862241242 [4] Cable Router. Motorola. Online. Available: http://www.mot.com/MIMS/Multimedia/prod/specs/routerSpec.html [5] CopperOptics Enhancing the Performance and Application of Copper Cable with HDSL : A Technology Brief from PairGain Technologies Inc. 1996. Online. Available: http://www.pairgain.com/copperop.htm [6] CyberSURFR Cable Modem. Motorola. Online. Available: http://www.mot.com/MIMS/Multimedia/prod/specs/modemSpec.html [7] D. Robertson. Motorola Announces Key New Features to CyberSURFR Cable Modem System. Motorola, March 1997. Online. Available: http://www.mot.com/MIMS/Multimedia/comp/PR/PR_3.1release.html [8] End-to-End Cable TV Data Networking Solution. 10,000,000 Bits Per Second - Data Over Cable TV. LANcity Products. Bay Networks Inc. Online. Available: http://sousaphone.lancity.com/products.html [9] FlexCap2 Rate Adaptive Digital Subscriber Line (RADSL), Westell