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 Comm Corner

Telecommunications Connections
Real World DSL/Direct Cable Comparison

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

Michael Espinoza is owner of Technology Coaching, a training and consulting firm that specializes in the PDA market. He co-chairs the PDA SIG with John Woody.


This article continues a series that I started in 1998 and 1999 about broadband high speed communications connections. A two part article done for this column in August and September 1998 contained an overview of bandwidth as it relates to high speed telecommunications connections. The August article, Part 1, defined bandwidth, discussed technologies that existed then, and covered costs being offered at the time. Analog or POTS (Plain Old Telephone Service), ISDN (Integrated Services Digital Network), and the commercial direct T-1 digital services were covered. The September 1998 article, Part 2, looked at the, in 1998, proposed broadband capabilities offered by Direct Cable and ADSL (Asymmetric Digital Subscriber Line) services to be offered by the cable TV and telephone companies in San Antonio. Then in separate columns in 1998 and 1999, I covered the Paragon (Time Warner) direct cable technology approach, and, the Southwest Bell ADSL technology approach. Finally, in June 1999, I covered a broadband bandwidth comparison between the two San Antonio providers, Time Warner and Southwest Bell.

This column will begin a real-world DSL/Direct Cable comparison from my experience using both services. I currently have both a direct cable (Roadrunner) and a DSL connection up and running. This report will be a summary of my subjective feel for each of the services.

Some review of the overall technologies should be covered here before forging ahead. Remember that our bandwidth definition concerns both the amount of data as well as the speed at which the data is moved. The true definition of bandwidth therefore concerns both size and speed and is measured as bps (Bits-Per-Second). The current analog POTS dial-up modem technology delivers data at a rate of approximately 52,000 bps, depending on the condition of the POTS telephone line being used. ISDN service, also a dial-up service, is the first digital capability available over the POTS telephone line twisted pairs. ISDN service operates at 128,000 bps, both up and downstream. T-1 service is a dedicated direct connection that delivers 1,540,000 bps. T-1 service has been used as a dedicated commercial service that large business accessed between its own connections. T-1 was known as a leased line service. There are connections in the T-1 range at multiples of that speed such as T-3 which contains multiple T-1 lines. There are higher speed connections even than these in the SONET (Synchronous Optical Network) connections that operate at multiples of 51,840,000 bps (STS-1 etc.). Remember that all of these connections above the ISDN level are digital and direct meaning that they move the data in parallel and are open or hot all the time.

The ADSL connection, for example operates at the lower end of the SONET capabilities. It functions over existing twisted pair phone cable and operates in a frequency range outside that of the audio phone frequency, i.e., the phone audio and the data can function at the same time. Its data flow is all digital. ADSL speeds are in the 1 to 8 Mbps range downstream and in the 100 to 800 Kbps range upstream. It can be a dedicated connection, i.e., one with a dedicated IP address or it can be dynamically assigned. Dedicated connections operate at the higher speeds and cost more to use. A broadband modem is used to receive and send data over the connection in either mode. The ADSL connection is provided over an existing POTS telephone line. The broadband modem receives itís signal from the POTS twisted pair and then feeds it to the computer NIC (Network Interface Card) or router via an Ethernet cable. The receiving computer or router either accepts the signal dynamically or as a static IP address. In either case the ISP connection is direct or open to the outside Internet world.

There is a law of physics involved with providing ADSL connections. The distance that can be supported by the signal on the POTS telephone line is 18,000 feet. In the real world this distance is shorter. SW Bell uses something under 17,500 feet. This means that the telephone infrastructure must be modified to accommodate data users in any given telephone subscriber area. The digital portion of the telephone infrastructure must be moved out from the CO (Central Offices) normally handling the service. SW Bell has spent an enormous amount of capital in adding these extensions to their system infrastructure.

The Direct Cable connection uses the CATV (Cable TV) video spectrum to provide its broadband capability. The video spectrum ranges from 6 MHZ to 750 MHZ. The TV video uses from 55 MHZ to 550 MHZ and is the majority user for this medium. Cable has one way access downstream within the 550 MHZ to 750 MHZ frequency spectrum. Within this upper frequency spectrum, there is a clean RF (Radio Frequency) capable of providing 20 to 30 Mbps downstream. Up stream access in the 6 MHZ to 55 MHZ frequency spectrum. This spectrum provides a clean RF channel capable of 2.5 Mbps upstream. The cable company usually limits the downstream connection to 10 Mbps and the upstream to 2 Mbps.

Again, a broadband cable modem converts the RF signals into digital information and vice versa. The signal comes into the modem via a RC59 BNC coaxial cable. The modem connects to the computer NIC or router via an Ethernet cable.

This connection is on a shared access network, i.e., each cable data node (connection) is part of a computer subnetwork in the cable grid. This is the reason for the 10 Mbps downstream and the 2 Mbps upstream limitation. This insures that everyone on the subnetwork will have access. As noted in the previous article, there are two issues that need to be covered about shared access networks. First, everyone on the subnetwork 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 the network. Second, direct access on a shared network means the same thing it means on any LAN (Local Area Network), shared resources at the host computer can be seen by others on the same subnetwork. This is a security issue. The cable company insures that everyone on the subnetwork has access by limiting each subnetwork. In the case of Time Warner Road Runner (RR), each subnetwork contains no more than 450 subscribers. As with ADSL, the direct cable connections can be provided both dynamically and as static IP addresses. The connection is direct or open all the time as with ADSL.

A Real World Subjective Comparison
As noted above, I have both a ADSL and Direct Cable Internet connection for test and work comparison use. Both are dynamically provided by the respective ISP, Sbcglobal.net and Roadrunner. The RR connection has been in use longer than the ADSL connection because SW Bell took longer to build the required infrastructure to service my home area, i.e., the final 17,500 feet. As a cable TV subscriber, I already had the direct cable connection to my house.

The RR connection was simple to setup. Once I had subscribed to RR, an account was established providing me with dynamic access to the RR ISP. The RR technician installed the data splitter onto my cable TV line and ran a BNC cable to my office. I installed the provided direct cable modem and the connection signal was there. Everything worked well with RR.

Until, after about one and one half years of service, I started getting down time when it rained. The service signal would start and stop suddenly. It took about 60 days and several service calls to find out the problem or problems. There were two. One concerned the sudden down time. It seems that the TV cable from the line on the utility pole in my back yard was run under a limb of an oak tree there. The TV cable had become taught due to limb growth and when the wind moved it, the cable would slip out of the utility pole connection cutting the connection. By the way, my TV service also stopped when this happened, as I discovered well into the 60 day problem period. A new line to house cable was installed with plenty of slack in it. Then, the second part of the problem started to manifest itself. After a rain, my RR service would stop or slow down. Mainly it stopped and any number of modem reboots, (unplug the modem power for about 30 seconds) would not get it started. Then, about an hour or two later, the system would be back in operation with no outside help. During these events, the first five TV channels and the last four channels would be grainy or out of focus. After more technical service calls, the main line to house connection was swapped out. When opened, it was corroded. Together, these two fixes have gotten the direct cable service back on continuous operation.

There are periods when the subnetwork is actually slower. It is because the bandwidth is being used by most of the subscribers in the subnetwork for this area. An occupational hazard.

The ADSL service, once SW Bell got its infrastructure in place has been operating without any problems at all. It seemed to be a bit harder or more complex to install than the RR service. There were far too many steps to jump through to get it up and running.

Both function as computer to modem connections and through broadband routers without problems. I use both connections on my SOHO network. Both access the Internet equally well via individual computers and the network.

Performance, i.e., throughput is also consistent. The direct cable connection, either from a single computer or from the network, is faster both upstream and downstream. The www.satx.rr.com site has a speed test link on it. There are two local tests that can be conducted from this site. One sends and receives data. The other is a up/download of a 1.8 GB graphic. I did an informal test on each connection over the past few days and have come up with these numbers.

The Direct Cable RR connection has consistently provided faster throughput as different times of the day. An average of nine data speed tests done at different times of the day was 2723.4 kbps downstream and 332.9 Kbps upstream. The speed test graphic average was 1903.3 downstream and 238.0 upstream.

The ADSL connection done at the same time provided an average of 1143.3 Kbps downstream and 140.2 Kbps upstream on the data test from the same site. The graphic test averaged 1145.7 downstream and 142.7 upstream.

The costs of the two services are nearly equal. The RR connection costs $44.95 per month. The DSL connection costs $49.95 per month.

Conclusion
My conclusion is that either broadband connection will work for the home user/SOHO setup. Faster is better and both provide more than enough capability. I will touch on my security arrangements in up coming articles concerning these connections.


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