Cable Speeds
It is not cut and dry - read whyDepending on the provider, the throughput that cable modem users will receive has been advertised to the public to be somewhere between 10 times to 1,000 times faster than dial-up connections to the Internet. Many times, providers will determine the throughput by comparing the maximum data rate support provided by the cable modem with dial-up data rates. For example, if a cable modem system has been developed to support up to 30 Mbps (bits not bytes) downstream and 1.5 Mbps upstream, then the provider will calculate the difference based on a 33.6 or 56k modem. When providers use this approach to pitch their services, they absolutely deceive their potential customers because cable modem throughput NEVER reaches its technological cap. This is true for a user's connection to content accessed from the Internet and from content accessed from the Internet servers located at the provider's headend facilities.
The speed bumps begin at the customer premises. In many cases, the client PC inhibits throughput due to an inability to process fast data rates. For example, a 90-MHz machine may not be able to support incoming data at 5 Mbps. Also on the customer end, the Ethernet 10Base-T connection restricts users to 10 Mbps. Therefore, as long as 10BaseT is the interface, it is impossible to gain throughput exceeding 10 Mbps - no exceptions.
The inhibitors continue onto the network. The network within your neighborhood may slow throughput. The data has to pass through coax, taps, and amps. If those network components are old and shabby, they may affect data transfer. There may exist loose connections or be old and worn and as a result be more susceptible to ingress. Ingress can not only slow throughput but kill a connection altogether. In most cases, providers replace the traditional standard RG-59 coax with RG-6 and replace taps and sometimes add filters before introducing data services. However, this is certainly not going to be a universal practice considering there are more than 20,000 cable operators in the world.
Throughput can also be affected by the number of cable modem users in a given neighborhood and the types of activities those users are engaged in on the network. Cable modem technology utilizes a shared medium in which all of the users served by a node (200-2000 homes depending on the provider) share bandwidth. Therefore, if Joe is the only cable modem user served by his node, then he has access to all of the bandwidth and his throughput to the Internet servers and CMTS (cable modem termination system) should be fairly consistent. On the flip side, if all of Joe's neighbors have a cable modem, then he will see slower data rates. Just as highway traffic moves slower at rush hour because of the number of cars on the road, volumes of users reduce throughput. The scenario becomes worse if all of Joe's neighbors are gobbling bandwidth by viewing streaming media or using their home PCs as servers. Many cable providers watch for traffic within nodes and if a particular node becomes congested, they will add another node in the neighborhood to solve the problem. Obviously, some providers will be lethargic.
The emergence of tiered services will also help control neighborhood congestion. Providers will be able to allocate bandwidth to users, which will place a cap on bandwidth access. This will keep the bandwidth hogs from shafting the neighborhood.
Once the data hits the node, it jumps onto an optical fiber backbone, which carries traffic to the CMTS. While riding the fiber, there are no speed bumps.
When the data arrives at the CMTS, Joe may be subject to misconfigurations or configurations of the CMTS or headend Internet servers that might not provide optimal throughput. There is no documentation that indicates that throughput has ever been affected by misconfigurations at the headend, but the potential exists.
If Joe is accessing content on the Internet that is not cached at the provider's headend, then the access request must travel beyond the cable modem network to the traditional Internet. Whether connected via a dial-up or a cable modem, Joe is going to be subject to all of the speed bumps on the Internet, which include slow and congested routers and servers and the T-1 lines and worse that might be between Joe and his content. Cable modems do not make the Internet a faster network. Instead, they make the connection between the user's PC and the backbone quicker. As the Internet infrastructure is improved accessed content outside of the cable infrastructure will improve. Until then, users will only find the full benefit of Internet access via cable modems through cached content and that content that is served over a private network.
In addition to all of the aforementioned inhibitors, data throughput is bursty. Unlike telco technology, cable modem technology does not support CBR (constant bit rate). Cable modem users are not connected to the backbone at 10 Mbps or 1 Mbps. The data rate is going to bounce up and down during a transfer. For a couple of seconds, a user may be getting 2 Mbps and then suddenly drop to 70 Kbps then jump up to 400Kbps and so on and so on.
The average throughput of this bursty connectivity to the cable headend will depend on the cable modem technology utilized and all of the factors mentioned above. Since all cable plants are different and all activity within a neighborhood node is going to be unique not only in general, but at any given moment, there is absolutely no way to provide a fixed data throughput rate to cable modem service. Testimonials range from users claiming throughputs in excess of 2 Mbps, while others have claimed that their 56k modem provided greater connectivity speeds.
In 99.9% of the cases, cable modem users are getting throughput to the cable headend and to the traditional Internet at speeds greater than that provided by an ISDN connection. A ballpark average (provided reluctantly) for throughput to the Internet would be 500Kbps down and 128Kbps up. For accesses within the cable network, the ballpark average throughput would be 1.5 Mbps downstream and 500Kbps upstream.
While the reference numbers provided may be far below those advertised by a provider, the difference between a 56k or ISDN connection and a cable modem connection at the above mentioned data rates is dramatic. Web pages pop up instantly. Large file downloads are executed in seconds or minutes rather than hours. Streaming audio and video and multi-player gaming is adequately supported. The mean time spent on Internet activities is reduced, allowing the user to do much more in a given time. Despite whether or not the throughput matches what is advertised, for the money, cable modem service is almost always the best value an Internet user can get for a high-speed connectivity to the Internet.

HOW CABLE MODEMS WORK

HOW CABLE MODEMS WORK

For millions of people, television brings news, entertainment and educational programs into their homes. Many people get their TV signal from cable television (CATV) because cable TV provides a clearer picture and more channels. See How Cable TV Works for details. Many people who have cable TV can now get a high-speed connection to the Internet from their cable provider. Cable modems compete with technologies like asymmetrical digital subscriber lines (ADSL). If you have ever wondered what the differences between DSL and cable modems are, or if you have ever wondered how a computer network can share a cable with dozens of television channels, then read on.Now, we'll look at how a cable modem works and see how 100 cable television channels and stuff.dewsoftoverseas.com can flow over a single coaxial cable into your home. Extra SpaceYou might think that a television channel would take up quite a bit of electrical "space," or bandwidth, on a cable. In reality, each television signal is given a 6-megahertz (MHz, millions of cycles per second) channel on the cable. The coaxial cable used to carry cable television can carry hundreds of megahertz of signals -- all the channels you could want to watch and more. (For more information, see How Television Works.) In a cable TV system, signals from the various channels are each given a 6-MHz slice of the cable's available bandwidth and then sent down the cable to your house. In some systems, coaxial cable is the only medium used for distributing signals. In other systems, fiber-optic cable goes from the cable company to different neighborhoods or areas. Then the fiber is terminated and the signals move onto coaxial cable for distribution to individual houses. When a cable company offers Internet access over the cable, Internet information can use the same cables because the cable modem system puts downstream data -- data sent from the Internet to an individual computer -- into a 6-MHz channel. On the cable, the data looks just like a TV channel. So Internet downstream data takes up the same amount of cable space as any single channel of programming. Upstream data -- information sent from an individual back to the Internet -- requires even less of the cable's bandwidth, just 2 MHz, since the assumption is that most people download far more information than they upload. Putting both upstream and downstream data on the cable television system requires two types of equipment: a cable modem on the customer end and a Cable-Modem Termination System (CMTS) at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place.

Inside the Cable ModemCable
modems can be either internal or external to the computer. In some cases, the cable modem can be part of a set-top cable box, requiring that only a keyboard and mouse be added for Internet access. In fact, if your cable system has upgraded to digital cable, the new set-top box the cable company provides will be capable of connecting to the Internet, whether or not you receive Internet access through your CATV connection. Regardless of their outward appearance, all cable modems contain certain key components: ·
A tuner · A demodulator ·
A modulator ·
A media access control (MAC) device ·
A microprocessor Tuner
The tuner connects to the cable outlet, sometimes with the addition of a splitter that separates the Internet data channel from normal CATV programming. Since the Internet data comes through an otherwise unused cable channel, the tuner simply receives the modulated digital signal and passes it to the demodulator.
In some cases, the tuner will contain a diplexer, which allows the tuner to make use of one set of frequencies (generally between 42 and 850 MHz) for downstream traffic, and another set of frequencies (between 5 and 42 MHz) for the upstream data. Other systems, most often those with more limited capacity for channels, will use the cable modem tuner for downstream data and a dial-up telephone modem for upstream traffic. In either case, after the tuner receives a signal, it is passed to the demodulator. DemodulatorThe most common demodulators have four functions. A quadrature amplitude modulation (QAM) demodulator takes a radio-frequency signal that has had information encoded in it by varying both the amplitude and phase of the wave, and turns it into a simple signal that can be processed by the analog-to-digital (A/D) converter. The A/D converter takes the signal, which varies in voltage, and turns it into a series of digital 1s and 0s. An error correction module then checks the received information against a known standard, so that problems in transmission can be found and fixed. In most cases, the network frames, or groups of data, are in MPEG format, so an MPEG synchronizer is used to make sure the data groups stay in line and in order. ModulatorIn cable modems that use the cable system for upstream traffic, a modulator is used to convert the digital computer network data into radio-frequency signals for transmission. This component is sometimes called a burst modulator, because of the irregular nature of most traffic between a user and the Internet, and consists of three parts: · A section to insert information used for error correction on the receiving end · A QAM modulator · A digital-to-analog (D/A) converter Media Access Control (MAC)The MAC sits between the upstream and downstream portions of the cable modem, and acts as the interface between the hardware and software portions of the various network protocols. All computer network devices have MACs, but in the case of a cable modem the tasks are more complex than those of a normal network interface card. For this reason, in most cases, some of the MAC functions will be assigned to a central processing unit (CPU) -- either the CPU in the cable modem or the CPU of the user's system. MicroprocessorThe microprocessor's job depends somewhat on whether the cable modem is designed to be part of a larger computer system or to provide Internet access with no additional computer support. In situations calling for an attached computer, the internal microprocessor still picks up much of the MAC function from the dedicated MAC module. In systems where the cable modem is the sole unit required for Internet access, the microprocessor picks up MAC slack and much more. In either case, Motorola's PowerPC processor is one of the common choices for system designers.

Cable Modem Termination System

Cable Modem Termination System
At the cable provider's head-end, the CMTS provides many of the same functions provided by the DSLAM in a DSL system. The CMTS takes the traffic coming in from a group of customers on a single channel and routes it to an Internet Service Provider (ISP) for connection to the Internet. At the head-end, the cable providers will have, or lease space for a third-party ISP to have, servers for accounting and logging, dynamic host configuration protocol (DHCP) for assigning and administering the IP addresses of all the cable system's users, and control The downstream information flows to all connected users, just like in an Ethernet network -- it's up to the individual network connection to decide whether a particular block of data is intended for it or not. On the upstream side, information is sent from the user to the CMTS -- other users don't see that data at all. The narrower upstream bandwidth is divided into slices of time, measured in milliseconds, in which users can transmit one "burst" at a time to the Internet. The division by time works well for the very short commands, queries and addresses that form the bulk of most users' traffic back to the Internet. A CMTS will enable as many as 1,000 users to connect to the Internet through a single 6-MHz channel. Since a single channel is capable of 30 to 40 megabits per second (Mbps) of total throughput, this means that users may see far better performance than is available with standard dial-up modems. The single channel aspect, though, can also lead to one of the issues some users experience with cable modems. If you are one of the first users to connect to the Internet through a particular cable channel, then you may have nearly the entire bandwidth of the channel available for your use. As new users, especially heavy-access users, are connected to the channel, you will have to share that bandwidth, and may see your performance degrade as a result. It is possible that, in times of heavy usage with many connected users, performance will be far below the theoretical maximums. The good news is that this particular performance issue can be resolved by the cable company adding a new channel and splitting the base of users. Another benefit of the cable modem for Internet access is that, unlike ADSL, its performance doesn't depend on distance from the central cable office. A digital CATV system is designed to provide digital signals at a particular quality to customer households. On the upstream side, the burst modulator in cable modems is programmed with the distance from the head-end, and provides the proper signal strength for accurate transmission.

How DSL Works

How DSL Works

When you connect to the Internet, you might connect through a regular modem, through a local-area network connection in your office, through a cable modem or through a digital subscriber line (DSL) connection. DSL is a very high-speed connection that uses the same wires as a regular telephone line.
Here are some advantages of DSL:
* You can leave your Internet connection open and still use the phone line for voice calls.
* The speed is much higher than a regular modem (1.5 Mbps vs. 56 Kbps)
* DSL doesn't necessarily require new wiring; it can use the phone line you already have.
* The company that offers DSL will usually provide the modem as part of the installation.

But there are disadvantages:
* A DSL connection works better when you are closer to the provider's central office.
* The connection is faster for receiving data than it is for sending data over the Internet.
* The service is not available everywhere.
Now, we explain how a DSL connection manages to squeeze more information through a standard phone line -- and lets you make regular telephone calls even when you're online!

Skinny Voice, Broad Band
If you have read How Telephones Work, then you know that a standard telephone installation in the United States consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice. DSL exploits this "extra capacity" to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.

To understand DSL, you first need to know a couple of things about a normal telephone line -- the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second -- see How Telephones Work for a great demonstration of this). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases. The use of such a small portion of the wire's total bandwidth is historical -- remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity. DSL does just that.

Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit (most of the time).

Skinny Voice, Broad Band

If you have read How Telephones Work, then you know that a standard telephone installation in the United States consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice. DSL exploits this "extra capacity" to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.
To understand DSL, you first need to know a couple of things about a normal telephone line -- the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second -- see How Telephones Work for a great demonstration of this). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases. The use of such a small portion of the wire's total bandwidth is historical -- remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity. DSL does just that.
Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit (most of the time).
Voice and Data
Precisely how much benefit you see will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (Kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps.
You might wonder, if distance is a limitation for DSL, why it's not also a limitation for voice telephone calls. The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:
• Bridge taps - These are extensions, between you and the central office, that extend service to other customers. While you wouldn't notice these bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
• Fiber-optic cables - ADSL signals can't pass through the conversion from analog to digital and back to analog that occurs if a portion of your telephone circuit comes through fiber-optic cables.
• Distance - Even if you know where your central office is (don't be surprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal must travel between your house and the office.

Splitting the Signal

Splitting the Signal

There are two competing and incompatible standards for ADSL. The official ANSI standard for ADSL is a system called discrete multitone, or DMT. According to equipment manufacturers, most of the ADSL equipment installed today uses DMT. An earlier and more easily implemented standard was the carrierless amplitude/phase (CAP) system, which was used on many of the early installations of ADSL.

CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the 0 to 4 KHz (kilohertz) band, as they are in all POTS circuits. The upstream channel (from the user back to the server) is carried in a band between 25 and 160 KHz. The downstream channel (from the server to the user) begins at 240 KHz and goes up to a point that varies depending on a number of conditions (line length, line noise, number of users in a particular telephone company switch) but has a maximum of about 1.5 MHz (megahertz). This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.

DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 KHz wide. One way to think about it is to imagine that the phone company divides your copper line into 247 different 4-KHz lines and then attaches a modem to each one. You get the equivalent of 247 modems connected to your computer at once! Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. In addition, some of the lower channels (those starting at about 8 KHz), are used as bidirectional channels, for upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, makes DMT more complex to implement than CAP, but gives it more flexibility on lines of differing quality.

CAP and DMT are similar in one way that you can see as a DSL user. If you have ADSL installed, you were almost certainly given small filters to attach to the outlets that don't provide the signal to your ADSL modem. These filters are low-pass filters -- simple filters that block all signals above a certain frequency. Since all voice conversations take place below 4 KHz, the low-pass (LP) filters are built to block everything above 4 KHz, preventing the data signals from interfering with standard telephone calls.