Cable Modems vs. ADSL, Technology, Applications and Myths

In the US, the two primary broadband technologies competing for the home market are Cable Modems and Asymmetric Digital Subscriber Loop (ADSL). There are a number of myths associated with each of these technologies and this seminar provides an understanding of the technologies and the strengths and weakness of each.

In today's markets, ADSL and Cable Modem service can be purchased for between $29.95 and $49.95 depending on the region of the country, speeds being offered, and whether you own the modems or lease them from the operator. As of the first quarter of 2006, there are roughly three times as many cable modems in service compared to ADSL and the trend appears to growing in favor of Cable Modems.

Cable Modems are being offered by the Cable System operators and the perceived reliability of the network is poor. Telephone Operators offer ADSL and their reputation for ease of installation and customer service is poor. There are many examples of both good and bad data services for each technology depending on how the networks are constructed.

Cable Modem Technology

A typical Cable system shown in Figure 1, has a Headend, sometimes called the Master Head End (MHE), that contains the major equipment for the network. It assembles the program material from the many different video sources. It also has the main link funneling data from the cable modem to the Internet.

Figure 1. A typical Cable TV system

The Headend serves all customers in a large geographical area and may typically be 100,000 subscribers. The headend sends the assembled data and video to branch locations around the geographic area to a facility known as a Distribution Hub. A Distribution Hub contains the modulators to put a video channel on a particular TV channel. (e.g. NBC goes on TV channel 12). It also contains the receivers for data that is sent from the homes.

The headend uses lasers to feed optical fibers that spread out across the area. Each fiber will feed approximately 2,000 homes. When the fiber reaches a neighborhood, it is converted from an optical signal to an electrical signal and split into four different paths feeding 500 homes. This combination of optical (fiber) and electrical (coaxial) is called Hybrid Fiber Coax or HFC for short.

The data signal is carried just like any other video signal through the system. Unlike the video signal, the data system also requires a channel that goes back to the head end since Internet users are interactive. The way the system carries both the downstream (head end to subscriber) and upstream (subscriber to head end) signals is by using separate frequencies as shown in Figure 2.

Figure 2. Upstream and downstream data use different frequencies on the same cable.

The lower frequencies are used to send data from the Cable Modem to the Headend and the higher frequencies are used to send data from the Headend to the Cable Modem. There are many TV channels in the higher frequencies and there can be multiple channels dedicated to data transmission verses video transmission. Also each geographic area fed by a fiber can reuse the data channel in a manner analogous to reusing a frequency in cell phone network.

ADSL Technology

The acronym ADSL stands for Asymmetrical Digital Subscriber Loop and this technology uses your existing telephone line to transmit data between the Central Office and your computer. Figure 3 shows the major components for this type of system.

Figure 3. The major components of the ADSL network

There are two types of ADSL technology, a full rate system and a "lite" system that is used primarily for residential data services. In the home, all the phones need to have a filter located at each phone so that these phones don't cause interference with the higher frequencies being used by the ADSL modem.

At the telephone Central Office, there is a frequency splitter that separates the voice signal and sends it the voice switch. A unit called a DSLAM (DSL Access Multiplexer) is the unit that houses the data card that combines data from many ADSL modems and forwards them to the Internet.

A standard voice signal on your telephone line uses frequencies between 300 Hz. to 4,000 Hz. The phone line can actually handle higher frequencies than that and these and these higher frequencies are used to handle the upstream and downstream data between the DSLAM and the ADSL modem. This is shown graphically in Figure 4.

Figure 4. Frequency usage for an ADSL system.

As shown, a large packet overlaps the time interval of the priority packet. The CMTS when controls this process by informing the CM of the first packet that it is to transmit part the first part of the first packet and then to stop transmitting. The CM for the second packet is told to transmit its priority packet and then the CM for the first packet is told to continue transmitting.

Myths

The battle between broadband service providers have made claims against the other that are usually slightly true but not exactly what it appears. Here are a few of these myths that have been presented to the public.

Myth #1 - ADSL has dedicated bandwidth

This is partially true in that the link between the central office and the ADSL modem is used only by that customer. After it arrives at the central office, it is immediately combined with data from all the other ADSL subscribers in that region and from that point on, all the links are shared amongst many users.

Typically there are more links in an ADSL network compared to a Cable Modem network because of restrictions imposed on the phone company. So the myth may be true but total irrelevant since it leads up to believe you'll have better performance.

The truth is either network can be designed well or poorly in terms of how many subscribers share the links. In order to get a network people can afford to use, the cost of the links must be shared. Performance is an economic decision that your broadband provider decides for you.

Myth #2 - Cable Modem systems can't provide guaranteed performance

Because the cable TV wire is shared by many people, there is a perception that you can't get guaranteed performance because your neighbors can "hog" the bandwidth. The truth is that the cable operators individually set each modem for some maximum level of performance and the user cannot exceed their fair share.

In addition to the peak bandwidth limitation, the cable modem standard can support telephony and other data streams that require guaranteed performance. It is completely up to cable operators on the way they design their networks and these networks can have excellent performance or performance that depends on the number of users.

Myth #3 - My Neighbor can read my Cable Modem data

There is no truth to this at all. Privacy is very important and the system has been designed with good security. There are two paths that need to be protected. The downstream path is received by all authorized users. The control information is necessary to keep the system synchronized. The data itself is protected by a three layers of encryption and the encryption keys are constantly changed.

The upstream path is almost impossible to be received by your neighbor because of the way the cable system is designed. First, your data is encrypted before being sent. Also, your neighbor's modem only receives information in the 50-750 MHz. band while your modem only transmits in the 5- 40 MHz. band. Assuming your neighbor got a special receiver the power level received would be very small.

There is a 12 dB signal reducer from the main distribution line to your home. To send to your neighbor means the cable modem signal is reduced by 12 dB to get to the main line and another 12 dB by your neighbors tap. This makes it difficult to receive since it's a very weak signal. After a short distance, the signal is converted to a fiber optic signal and a neighbor on a different fiber would receive absolutely no signal from your modem.

Myth #4 - The link costs of both technologies is the same

There cost of the modems for both technologies are similar but for other elements of the system there is a cost associated with having a dedicated connection. The discrepancy is shown in Table 1 below.

Table 1. Equipment needed to Service 1,000 Customers

Besides having more components, ADSL also requires more links before it can be connected to the Internet. The reason is that the regulated phone companies are not allowed to process data above layer 2. The data must go from the DSLAM central office (CO) to another (CO) where the ATM switch is located. It moves to another location to have the links for your Internet Service Provider concentrated to a single connection and then onto another link to your ISP. The ISP then goes to the Internet.

Cable Operators are the ISP and can avoid several steps. In addition, the ISPs can cache the data so that frequently used data is sent from local storage which speeds things up. Again the phone company by law cannot cache the data.

Applications

ADSL does a good job of providing a reliable service and can offer higher security of the links since it is based on a very reliable backbone that is generally well managed. Because of its higher cost structure and combining this with reliability concerns, small business are a good target for this technology.

Cable System cover the residential areas fairly well but there are many concentrated business areas that do not normally have good cable TV connections. The reliability of the network is not as good because of the philosophy of providing a low cost, non critical, entertainment service. Also when a single cable is cut it tends to affect far more people then when a telephone cable is cut. This noticeably decreases the reliability of the cable network. For residential Internet browsing, the performance and cost tend to make this a more successful technology.

Impediments to Success

ADSL technology is not available in all areas because of several technical issues.

• ADSL data rates get slower the further the customer is from the central office and generally can't be deployed in distances greater than 15,000 feet.

• Inside the big telephone cables, the presence of multiple ADSL, ISDN or T1 connections causes mutual interference limiting speed or availability of service.

• Many geographic areas are beyond 15, 000 feet and are served by optical fibers running to boxes (Digital Loop Carrier boxes) located in the area needing service. Optical fiber cannot transport ADSL and therefor the DSLAMs must be located in these boxes. Many of the boxes are full and do not have room for DSLAMs. These boxes must first be enlarged before ADSL can be deployed.

• DSLAMs are expensive and if only a few customers in an area take the service, the cost per user becomes very high.

Cable System also have challenges for that technology.

• The cable plant must be modernized to be Hybrid Fiber Coax (HFC) with an active return path. Rural areas will find it difficult to meet the economics for this upgrade. It is also very expensive to do this upgrade but most of the plants that are going to be upgraded, they are completed or will be completed in the near future.

• There are many quirks in the installation because many home owners have put signal splitters and amplifiers into their home wiring.

• Electrical noise sources in the home can get into the cable system and cause interference. Local radio transmitters and CB radios are also sources of interference if the plant is not properly isolated.

• Cable operators have a poor reputation for service and reliability of their networks.

• The cable plant is not located in many business parks.

In Summary:

• ADSL generally is a more costly system but the telephone companies have more reliable networks.

• Cable Modem systems are generally cheaper and faster than ADSL.

• There are many myths that each side in the broadband war are propagating.