Sharing an Internet connection is one of the main motivations of building a home network. Typically when a single computer is connected to the Internet, there are long periods while the connection is idle. This pattern of heavy use and idle periods means that four or five computers can use a single Internet connection and all five computers seem to have sole use of the connection. But before we show how to share an internet connection we first will review the myriad of connection options available for your home Internet connection.
Generally, which choice you have will depend on where you live. The list of available options includes:
To better understand how the analog modems work, we can examine an early modem.
Picture of an Early Modem (010)
When a modem initial connects and the speaker is turned on, you hear a series of tones. These tones are used so that the calling modems can determine the type of of the receiving modem. Each tone is sent for a few seconds and the receiving modem responds to its particular tone and a connection is established. Most modern modems can communicate with a wide range of older modems. It can take nearly a minute until the sending modem "guesses" the right tone of an older receiving modem.
This works well, but telephones have limited fidelity when it comes to the quality of sound which is transmitted from one phone to the other. Because phones are intended to portray human voice, they are limited to a frequency range of 3000 Hz. This is why modems are limited to 2400 baud. Using sophisticated digital signal processing, we can send up to 56000 bits per second through this audio channel. Unfortunately, achieving this maximum rate depends on a very clean telephone line and good connection.
Figure of POTS (020)
Even with these limitations, standard telephone modems are by far the most common Internet connection from homes. The nominal cost for a phone line is $16.00 and the cost of Internet service is about $20.00 for a total cost of $36.00 per month.
Plain old telephone service (POTS) dates back to the origins of the telephone at the beginning of the twentieth century. In the 1970's (Rich check this one), the telephone companies began to design the new "telephone service" to replace plain old telephone service. This new service was called "Integrated Services Digital Network" or ISDN. Instead of sending sound, ISDN deals with data. The basic ISDN service provided two 64kbit data channels and one 16kbit signaling channel. Each of the data channels could carry a digitized phone conversation.
The ISDN connection essentially extended the functionality of the telephone company switching equipment into the home. Each of the data channels can either carry a phone conversation or a data connection. To carry a phone conversation, the sound was digitized by special "packet telephones" which were to be installed in the customer's homes. The architecture of ISDN envisioned a wider range of digital telephony "gadgets" - but unfortunately, that never really came to pass. Some people would claim that ISDN was priced too high while others might feel that in 1980, we did not yet possess enough technology to develop and manufacture packet telephones at a reasonable price. Regardless of the reasons, ISDN was not readily available until the mid-1990's and by then, the Internet revolution had occurred when data connections were "packet oriented" rather than "connection oriented".
In many locations, because of limitations of ADSL or Cable Modems, ISDN is the only option to go faster than a telephone modem without resorting to wireless. Even thought ISDN is somewhat past the peak of its technology curve, there are two basic solutions to use ISDN in a home network.
When you pick up an analog phone and dial another phone, the ISDN modem or router actually is generating the dial-tone that you hear in the handset. As you press keys, the router interprets the tones and once you have dialed the phone number it requests a connection over the 16Kb control channel. When the connection is established, the router is notified on the control channel and digitized sound begins to flow over one of the 64Kb channels. The ISDN router converts the sounds to and from digital form and acts as a bridge between the analog handset and the digital connection to the telephone company. When calling from an ISDN connected phone, you can dial any other phone. If the destination phone is connected via a POTS connection, the phone company makes the necessary conversions between analog and digital.
Picture of the ISDN router and analog phone
When you have an ISDN modem or router in your home, many of the functions that were formerly done in the telephone company central office are actually done in your ISDN equipment. This allows significant flexibility in several ways.
When the ISDN router or ISDN modem is used for data, at least one of the 64Kb connections is dialed to your Internet Service Provider (who also has an ISDN connection). This connection is a direct digital connection which yeilds an error-free 64Kbs. Generally, the PPP (Point-to-Point) protocols operates over this connection. PPP handes both the initial authentication and the data transfer over the link.
There are two ways to get up to 128Kb/sec on your Internet connection.
With one ISDN connection into your home, you get the following:
One of the primary downsides of using ISDN is the cost. An ISDN line may cost about $50 per month (they are more expensive than POTS). In addition, your Internet Service Provider may charge a premium to allow you to use ISDN - it is typical to pay $30 or more to an ISP for unlimited ISDN service. This takes the overall cost to use ISDN to nearly $80.00 per month.
TODO: SPIDS and Ordering and Synchronous (always transmistting zeros)
In the mid 1990's, cable television companies began to experiment with and deploy networking solutions based on the CATV connections into many urban and suburban homes. Unlike POTS and ISDN, cable modems were developed from the ground up to do packet-oriented data networking. The basic idea was to dedicate one or more television channels to data networking.
A cable modem connection is permanently on - the service includes your internet connection, so no fees are necessary for the ISP. Often you can purchase a service which wil allow you to have a web or file server permanently connected to the net in your home. It is the closest thing to having a connection which functions like a local area network which is connected via a high-speed link.
While this seems simple and clever, there are a number of challenges which must be faced to use CATV for data networking. The first challenge is the fact that cable television distribution had always been designed to be a broadcast from an origination point (called the "head-end") to the subscribers. In order to keep from running thosands of individual cable connections from each house to the head-end, the cable system is designed as a multi-level tree.
Between the head-end and the subscriber,
the signal is split several times. Each time it is split, the signal is
cleaned-up, adjusted and amplified. A cable company needed to spend signiifcant
effort making sure that their amplifiers were properly adjusted to insure that
each subscriber had a high-quality signal.
While this seems complex enough, using this structure to implement two-way high-speed data communications is very challenging. The first task is to convert the cable plant to be a two-way system. In a two-way system, the amplifiers must take a subset of the channels coming back from the subscriber's homes and pass that channel back up the tree. At a minimum, this requires the replacement of all the amplifiers in the cable television system. Furthermore, it is necessary to make sure that the cable system is very carefully adjusted and monitored. A bad signal or badly tuned amplifier in a one-way system may affect as few as ten homes. On the other hand, when there is a bad signal in the uplink channel, it corrupts the channel as it is propogated up the cable system.
Once the cable plant has been upgraded to allow one or more television channels to be two-way, there is still the detail as to how we move network data across television channels. At its most basic, a cable modem is indeed a "modem" - that is that the data is converted from digital to analog (in this case video rather than audio) before being transmitted over an analog medium. In a way, the cable modem in your home is a small "television station". Once the basic conversion from digital to analog has handles, we also must arbitrate between the different cable modems so that only one cable modem transmits at any given moment. If two cable modems transmit at the same time, their signals would collide and no data would be transmitted. The collision might even produce noise on other channels on the cable - so two cable modems transmitting at the same time could cause your neighbor to see a bad signal while watching their soccer game.
This arbitration is resolved using a protocol called "token bus". No station is allowed to transmit until it has been told that it has permission - the permission is the "token" - if you have the token, you can transmit, otherwise you must hold onto your data and remain silent. The network insures that the token will be fairly passed around, so that even under heavy load from several modems, each modem will have an opportunity to transmit many times per second.
All this technical detail, is not meant to scare you from using cable modems - a well-built and well-maintained two-way cable system provides extremely high reliability. But the idea is to give you some idea as to the significant engineering which goes making a cable modem work. And perhaps when the technical support person from the cable company fixes your connection, you can tell them an extra-special special "thank you".
You should feel fortunate if cable modems are in your area. They provide high speed, always-on service for somewhere between $30 and $50. If you consider the cost of an ISP and a second phone line, a cable modem provides a much better experience at about the same cost.
By running all of the wires all the way
back to the central office, the telephone company can deploy new services to
your home simply by changing the equipment which your wires are plugged into
(and changing how much you are billed each month). In the above example, homes A
and B have POTS, while C has ISDN, and D has ADSL. Of course, C and D need
special equipment to make use of their advanced services.
While having a direct connection from your home to the phone office is a seeming advantage when deploying network services, making ADSL work has taken a significant amount of effort because of the limitations of a long, unshielded pair of copper wires when used for data communications. The copper wiring installed in most of the country to support POTS is effectively long speaker wire. An analog telephone is basically a microphone and a speaker.
As we move data across these copper wires, as the speeds increase, the wires become radio antennae. The signals begin to "leak out" of the wires - this has two bad side effects - with too much leakage, a high frequency signal which is sent through the wires does not come out the other end because it has all leaked out. The second problem is where all this leakage goes after it leaks out of the wires. It end up in all of the other nearby wires causing interference.
The speed at which data can be transmitted using telephone-style copper pairs of wires depends on two factors: (1) the quality of the wiring and (2) the length of the wires. As such ADSL is not available in every single home in an area - and the farther away from the central office one gets, the slower the maximum data rate that you will get. That is why you must provide your precise street address when ordering ADSL - they are computing the distance and looking up how your home is wired to the central office.
Distance (feet) Maximum Speed (upstream+downstream) 18000 1.544 Mbps 9000 6.1 MbpsGenerally, to be conservative, most ADSL installations will operate at 768Kbps (about 1/2 of 1.54 Mbps) downstream and somewhere between 128-384Kbps upstream. It is not generally practical to get ADSL if you are farther than 18000 feet from the central office without adding apmplifiers. And it is difficult to add the amplifiers because they must be installed in a way that no segment is greater than 18000 feet.
Even if your neighborhood has brand new wiring and is less than a mile from the central office, not all central offices have the apropriate equipment installed in the central office. But hopefully over time (much like ISDN) as demand (and revenue) for ADSL increases, it will be more widely available.
One of the slickest engineering achievements in ADSL is the fact that a single copper pair can both be used as POTS line and as an ADSL line at the same time. This is done using different frequencies for the POTS signal and the ADSL signal. The POTS signal is sound and operates at below 5Khz. The ADSL data is modulated so that it starts well above the voice signals. Both signals are combined onto the wire and at the receiving end, a simple analog filter (an inductor and a capacitor) is used to separate the signals.
Another advantage of ADSL over ISDN is the fact that ADSL is always on - because ADSL is oriented towards Internet-style traffic and not continuous synchronous data (like ISDN), if you are not sending or receiving any data using ADSL, no data is being forwarded. In a sense, this makes ADSL, less resource intensive for the phone company than ISDN. In most cases, your ADSL service includes the Internet connectivity, so there is no need to pay an extra $20 per month for Internet service.
In most locations, ADSL services ranges from $30 to $60 depending of competition and the options that you choose.
Fixed high speed wireless solutions are available is a small number of communities. These services typically are using the 2.4Ghz or 5.4Ghz frequencies. These frequencies are unlicensed, but equipment operating in these frequencies must follow certain power rules and tolerate interference from other equipment operating in the same frequency. These solutions can provide from 0.5 - 20Mb/sec connectivity at a cost comparable to DSL or cable modem.
It will take some time to see if fixed wireless expands beyond its core markets of:
Satellite based services come in two forms:
The two-way satellite based services are problemtic but coming up on the horizon. These services have extremely large up-front costs (see how much the last rocket lanugh sold for on ebay!). Once two-way satelite is well-deployed, they will be a true competitor for DSL and cable modems if they can be cost effectively deployed. These systems have the potential for nearly universal geographic coverage and should be a boon to rural environments.
A more exciting cellular technology is Cellular Digital Packet Data (CDPD). In CDPD, data is transmitted as packets in unused channels in the cellular telephone spectrum. Because CDPD is packet based, it can tolerate increased latency and moderate data loss. This allows the cellular network to give "best effort" delivery of data, knowing that if some data is lost, higher level network protocols will be able to retransmit or otherwise recover from the lost data.
Ultimately by providing "best effort" delivery, CDPD should be able to provide higher bandwidth at a lower cost. However most CDPD service coverage is concentrated in the major metropolitan areas. There are efforts (especially in Japan and Finland) to significantly increase the performance of this type of access. Once the bandwidth increases, this service may become a strong overall competitor to the cable, DSL, satellite, and fixed wireless solutions. Because CDPD is tied to the cellular infrastructure, its deployment will tend to be in moderately dense population areas and along well-travelled highways. But as the cellular telephone usage increases in general, more infrastructure capable of delivering CDPD services will be deployed.
Your Internet connection choices range from the mundane to the exotic. It would be nice if everything were simple and all these technologies were available to every consumer. In the short run, you simply need to assess which technologies are avialable for your home and pick the best one for your needs.
The "classic" choices are a standard telephone line, DSL connection, or a cable modem. Fixed Wireless and two-way satellite are a more exotic but very intriguing solution. ISDN is a good choice if the cost is relatively low. One way cable may be a good fit in some situations - it is a reasonable compromise if the price is competitive - especially in rural areas.
A good place to research internet service choices is the web site www.isp.com. Another good source of information is to talk to your neighbors and others who have tried the various services.
Now that we have data into our home, we will explore how to distribute it around the home.