Aspects of Cable Modem Technology

 Video has true broad bandwidth within its medium. Video is a real bandwidth hog. The video spectrum uses 4.2 MHz of bandwidth. The video pipe reaches from 6 MHz to 750 MHz, with TV using the bandwidth from 55 MHz to 550 MHz. Cable has two way access within this spectrum. Within the upper 200 MHz, there is 6 MHz channel of clean radio frequency (RF), providing 20 to 30 Mbps downstream (download) capacity, and within the lower 50 MHz; and, providing 3 MHz return (cable plant management signals), up to 2 Mbps upstream (upload) capacity. This connection capacity is readily available to large numbers of subscribers via the installed cable TV media. 

All cable signals start as digital data at the cable company Central Office and are transmitted to universal broadband routers where the digital data is converted into RF signals. The digital data is carried over radio frequency (RF) carrier signals on the cable distribution network. The cable medium infrastructure is a combination of fiber optics/coax (hybrid fiber/coax) capable of carrying many RF signals at the same time. TV and data signals are carried together from the cable Central Office to network node distribution points (Headend) in the cable grid. From the headend, the signals are routed into fiber node sub- networks of 500 to 1,500 subscribers. The data signals are separated from the TV signals at a ground block located at each subscriber premise. The data signals are then routed into the cable modem for conversion back into computer understood digital data. 

 The cable data modem converts the RF signals into digital information and back from digital information to RF signals. A cable modem functions nearly just like an analog modem by converting digital data signals from one form to another for transmission. The cable data modem is located on the subscribers home or office. The upstream RF signal from the subscriber computer is also converted at the headend equipment by the cable company into digital data for further routing into the Internet or other locations. 

 Cable data services are deployed on a shared access network. The available 20 to 30 Mbps service is shared by multiple subscribers simultaneously. There are two issues, which need to be mentioned about shared access network. First, everyone on the network gets a piece of the available bandwidth just like any network. The more subscribers on the network, the less bandwidth each subscriber will have available at any given time when everyone is simultaneously using it. This is the case with LAN networks and the Internet networks, the more users on-line, the slower or less bandwidth is available. I will address the cable industry explanation later. 

Second, direct access on a shared network means the same thing it does with any LAN network. With share turned on in your computer, others on the same cable network segment can potentially view your data files located on the shared HDD partition. This is the case if your computer is on a LAN within your premises. In the case of a single computer, the problem can potentially be solved by not turning the SHARE on in your operating system. 

 The CATV industry under the auspice of its Cable Television Laboratories, Inc., (CableLabs), has run extensive laboratory and field tests to examine and refine deployment of cable modem technology to ensure reliability and security to subscribers. CableLabs testing has focused on two areas, network configuration, and the medium access control (MAC) protocol found within the lower part of the second layer of the Open Systems Interconnection (OSI) standard for networks. 

 The CableLabs model is that of shared access medium configuration vice dedicated circuit connections (voice/data telephone (POTS) service). In this model, “a single high bandwidth ‘pipe’ is shared by many users” in the network. The more users are on-line at a given time, the less of the total bandwidth is available for each user, in general. This is not completely the case with users as they go about the routine of using the cable network for Internet and other uses. 

 There are two mitigating factors, which intercede. One is the network design, which will be limited to 500 to 1,500 subscribers per cable network node. The other is the “bursty” nature of the data communications traffic that is transmitted on the network. Typical data communications involves a search sequence with the associated data flow in the pipe and then there is a wait period as the data is digested. The higher level of bandwidth is available to all users at a given time because very few of the 500 to 1,500 subscribers on-line on a cable node are not doing exactly the same thing at the same moment. 

 The OSI standard level two MAC protocol is used to control the timing and coordination of data to and from each subscriber in the cable network node. This is in the Data Link layer, which resides in the network interface card (NIC). The Data Link layer is made of two sub layer protocols, the upper sub- layer is the Logical Link Control protocol, which governs the link between the two endpoints. The lower sub-layer, the MAC protocol, assembles and disassembles the data frame, the packet carrying the information to and from the computer. The MAC protocol allocates bandwidth to users, arbitrates among users, and keeps track of all user’s activities so that each user receives the throughput from the network. 

Paragon (Time Warner) RoadRunner Deployment

The installation service is being accomplished in advance of deploying the cable modems to get a jump on setup for subscribers. The Paragon Installation Technician inspects the existing wiring for RF leakage, isolation, and location, including cable bending to route the connection to the subscriber computer. As a sidelight, if anyone is building a new home or remodeling, it would be a good idea to pre-install the data quality cable for later use. The cable modem has not been selected yet as of this column. I am told that the selection is to made within the next month. Paragon is going slowly to make sure the cable modem selected is Data Over Cable System Integration Specification (DOCSIS) complaint. This by the way, is the specification under which all cable transmission services are undertaken. The cable modem is an external device with the cable input connection and an Ethernet RJ45 connection on the output side. An Ethernet Network Interface Card (NIC) will be required in the subscriber computer to complete the connection. Up to three computers may be connected to the cable modem. The cable modem does not have to be completely adjacent to the subscriber computer as the Ethernet cable jumper cable is handled like any network cable within 100 ft of the cable modem using CAT 5 UTP RJ45 connectors. Paragon is selecting the NIC type, which can be ISA, EISA, PCI, PC Card, or built-in Ethernet AUI or 10Base-T. 

 Paragon’s price list includes a monthly charge based on current cable TV service to the extent that it is installed. Cable subscribers can obtain the cable modem data service for $39.95 additional charge to the current TV service charges. Non-subscribers can obtain cable modem data service for #52.95 per month. Basic TV subscribers are charged $8.79 per month. There is an installation charge of #109.95 for the wiring additions and cable modem. The NIC can be obtained for an additional charge. 

Wall drop charges are $17.31. A “Wall Drop” is a hole that must be run through an interior wall. A maximum of three computers may be connected to each cable modem. Up to two cable modems may be installed at the address. The charge for two setups is double one setup. The cable modem data services are in addition to the current TV cable service charges already in place. The cable modem data service includes 5 e-mail addresses and 5 MB of personal WWW space. 

 A self install class will be offered at least weekly and will be restricted to small groups to ensure everyone is qualified at the end of the class. The cable modem, NIC, software, and manuals are furnished at this class. The cable technician will check any installed cable for the above mentioned problems and determine whether it can be used. Paragon cannot guarantee self installed (no install class) cable setups. 

 Paragon cannot guarantee the bandwidth availability in the RoadRunner service. Remember the SHARED access notion of the cable service. Each sub-node may have varied subscribers, which limits the available bandwidth to each subscriber in the sub-node who may be on-line simultaneously. In fact, the only guarantee any provider, including POTS, ISDN, or ADSL , can make is to the first point of aggregation. The first point of aggregation is the point from the subscriber computer to the next switching device such as a terminal server for POTS, access server for ISDN, DSLAM for ADSL, or router for T-1/T-3. The first point of aggregation for the cable modem service is from the computer to the cable node and is rated at 10 Mbps downstream and 2 Mbps upstream. Aggregation is related to throughput and latency of the data transmission after it leaves the first in-line data stream connection. This will be the subject of another article this year. 

 One other point must be made about cable modem data service. The computer that is connected to the service is on a shared network with other computers. The connection is a open direct connection to the network. There is a potential security issue when using this service. If your operating system, such as Windows 95/98 has SHARES turned on for some reason, others on the network segment may be able to look at or manipulate your personal data. It is also potentially the issue with ADSL, as you remember, is being offered by SW Bell as a “direct” connection. There is room for some development of personal security firewalls as a result of these services. 

 Paragon Cable has just completed deployment testing in Austin for its RoadRunner service. The Austin service has had good and poor performance. Data throughput and latency are the main problems and are similar to the problems found in any network or Internet connection. Both are related to the amount of data and the route through which the data must travel. But these are the subject of still another column this year. Austin subscribers seem to be relatively happy with their service. 

Conclusion

 John Woody is a networking communications consultant specializing in small office, home office networks, training setup, and internet connectivity.