NAVIGATION
My Investigation on Cable Modems
Investigation of CM's operation & architecture
Alternative Broadband Technologies
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1. Introductory study of the cable modem technology 1.1 Modem’s History 1.2 Computer communications 1.2.1 Analogue transmission 1.2.2 Digital transmission 1.3 Definition of the Modem device. 1.4 Definition Of Cable Modem 1.5 Installation Of Cable Modem 1.6 Operation Of Cable Modem 1.7 Cable Modem’s Speed 1.8 Shared Network 1.9 Cable Applications
1. Introductory study of the cable modem technology
The early nineties a revolutionary new device emerged that changed our every life for ever. This device was the modem. For the first time computers and other devices were able to communicate with each other. This changed the way we view computers for ever. Now computers became a communication tool supporting a variety of applications.
1.1 Modem’s History Although internet has only the recent years came into our lives (only 2000 host computers in 1986) it has expanded rapidly all over the globe for a very large number of reasons. Nowadays there are more than 50 million hosts worldwide! The early 90’s were the beginning of the big expansion of the internet since software developers begun creating user friendly access tools available to anyone. This was a big step since it meant that anyone with a computer, a phone line and a modem could connect to the internet if he signed up with an Internet Service Provider (ISP). The ISP provides the user with internet access and an e-mail address through a typical phone line. In order to achieve access in the internet the user’s computer dials the ISP's computer which connects the user to the internet. These modems run up to 56 kbps and whenever the user was connected to the internet the phone line was busy. Cable modem engineering began in 1995. The need to offer internet services to their subscribers made cable companies team up with ISPs in order to provide these services using their already installed cable network. The outcome of their efforts became a very successful product - The Cable modem.
1.2 Computer communications Computers operations are based on the binary system, this means that they can identify two states: `0` and `1`. Communication between two computers can be achieved in two ways: The analogue and the digital
1.2.3 Analogue transmission The analogue method of computer communication requires two devices, one in each end, in order to achieve communication between the two computers. These devices are called modems. Their name comes from the words MOdulators / DEModulatorS. Their job is to `convert`, in one end, the bits to analogue signal so it can be transmitted over an analogue communication channel like the telephone network. Also the modem on the other end has to transform the analogue signal received back to digital bits so the data can be identified by the computer. This method is used widely over the last years. This is due to its complexity since it is much simpler than digital transmission and hence cheaper. Furthermore it will continue being used for communication over the next years since analogue networks are located everywhere. The only difference in this type of computer communication over the last years is the way data is transformed to analogue signal. This is because the modem manufacturers are trying to achieve faster and more reliable communication links between computers. The most well known methods for transforming data into analogue signal and visa versa are:
1.2.4 Digital transmission On the other hand in the digital transmission method the bits are transformed into digital pulses instead of analogue signal in order to be transmitted through a digital communication channel. In this digital level the terms coding and decoding replace the terms modulating and demodulating. There are specific methods for this transformation. These methods for coding and decoding of bits into digital pulses are known as:
1.3 Definition of the Modem device.
The modem is a device that transforms the digital signal of computers into the corresponding transmission signals so that communication can be achieved. There are three main categories of modems depending on the technology they use. These three categories are: · Voiceband ( 300 – 3400 Hz) · Baseband ( 0 – 100 KHz) · Broadband ( 37 – 51.4 GHz) Some of these operate only with leased lines and do not operate on dial-up connections. Most modems used to be dial-up but the need for faster services as well as the need of cable TV networks to provide fast and always-on Internet services produced a new generation of modems, the Cable Modems that are going to be the main topic of this project.
1.4 Definition Of Cable Modem Cable modem is a device that allows high-speed internet access via a cable TV network. A cable modem typically has two connections. One goes to the cable wall outlet and the other to a computer. There are two types of cable modems: internal modems and external. Most cable modems in the market are external since most users lease the modem from the cable company instead of buying them. The external modems are connected to the PC through a standard 10Base-T Ethernet card and twisted-pair UTP wiring. There are also USB and PCI card modems available in the market.
1.5 Installation Of Cable Modem
When someone signs up for a cable Internet service it is usually through the same company they receive their cable television services. The cable service provider may also be the Internet Service Provider (ISP) but sometimes they just partner with an ISP in order to provide the user with Internet services.
Pic. 0: Architecture of a cable modem network
To install the cable internet service in one’s home or business, a technician comes out and hooks up their service by splitting the coaxial cable so that one end goes to the cable box and the other to the cable modem that is connected to a computer (Pic. 1).
To connect a cable modem to a computer you need an Ethernet card since cable works on bus topology. The Ethernet card can be plugged in an empty motherboard slot. Furthermore there are other ways to connect the modem to the computer. One way is to connect the modem to a Universal Serial Bus (USB) slot and the other is by plugging a PCI cable modem (internal modem) straight to the motherboard slot. The bill for the internet service comes straight from the cable service provider and not the ISP unlike dial-up services. The installation is pretty easy from the user’s point of view but the cable companies must do more than simply install cable modem gear in order to provide high-speed internet access. They must build a very sophisticated end to end IP networking infrastructure in each community they serve, in order to support all their data subscribers. This infrastructure includes items like: · Internet Backbone Connectivity · Routers · Servers · Network Management Tools · Security Articles · Billing Items In other words cable operators have to build some of the world’s largest “intranets” in order to provide cable internet services. In these intranets a Head-End cable modem termination system (CMTS) communicates with cable modems located at the subscribers home in order to create a virtual local area network (LAN) connection. The reason that the cable operators have to do this is simple. The answer is that a network connection is only as fast as its slowest link. Moreover it is obvious that the benefit of a 10 Mbps cable link is lost if a subscriber tries to access content stored in a web server that is connected to the internet through a 56Kbps modem. To solve this problem, cable operators decided that they should push the content closer to the user. The way to do this is by “caching” or storing copies of popular Internet content on local servers. By doing this the user will be routed to the server in the local Head-End at top speed when he tries to access a web site, rather then accessing straight to the overcrowded internet.
1.6 Operation Of Cable Modem
Typical cable modems are something more than just a modem in its modulator/demodulator meaning of the term. Cable modems can do a variety of jobs. They are used as:
On a cable system digital data signals are carried over radiofrequency (RF) carrier signals. Cable modems operate by converting digital information into am modulated RF signal in order to be transmitted and also they convert the RF signals arriving back to digital information in order to be understand by the computer. Normally a cable modem sends and receives data in two slightly different ways. In the downstream direction, the digital information is modulated and then placed on a typical 6 MHz TV channel. The upstream channel, on the other hand, might be more complex. The upstream is transmitted in a frequency varying between 5 to 40 MHz. This environment tends to be very noisy, with RF interference and impulse noise. Moreover noise can be introduced in the subscriber’s location due to poor wiring or connections. To solve this problem, manufacturers nowadays use the Quadrature Phase Shift Keying (QPSK) modulation technique since it is “stronger” and less noise friendly than other techniques mentioned earlier. The only drawback is that it is slower than QAM.
1.7 Cable Modem’s Speed Cable modem’s speed varies, depending on the cable network architecture the cable modem system and the traffic load. In the downstream direction network speed can reach 27 Mbps. This bandwidth is then shared by the users in the local community. Only a few computers are able to connect to such high speeds. Most of computers are connected to a more realistic speed of 1 to 3 Mbps.
Then again the upstream is slower than downstream. In the upstream direction speeds can reach 10Mbps. However most of cable modems manufacturers decided, that an appropriate upstream speed should be less than 2.5 Mbps.
An asymmetric cable modem scheme has been adopted by the DOCSIS, EuroDOCSIS and the DVB EuroModem standards. The downstream channel has a much higher bandwidth allocation than upstream, mainly because Internet applications are asymmetric in nature. In the downstream direction is allocation almost 80% of the total bandwidth in a cable system. In the UK the cable ISP’s offer speeds varying from 128 Kbps to 1 Mbps with prices between £15 and £35 per month. Below is a table for the bandwidth needed for some internet applications.
Table 1: Applications & their bandwidth allocation
1.8 Shared Network
Although shared network might look promising, there are concerns that cable modem subscribers will see poor performance due to the increasing number of subscribers in the network. Unlike dial-up modems cable modem users do not occupy a fixed amount of bandwidth during their sessions. Instead they share the network. This means that after more and more subscribers sign up, bandwidth will eventually fall.
To overcome this problem a cable operator can simply add another 6 MHz Video channel for high-speed data, doubling the downstream bandwidth available to users. Although this would be an ideal way for things to be done for both users and the cable companies, some cable operators , like NTL, decided that instead of doing this to offer more bandwidth to their subscribers, they have just applied a 1 Gigabyte downloads limit to the users!!
1.9 Cable Applications Cable modems offer a new range of high-speed applications for their users, at speeds hundreds of times faster than dial up services. Cable subscribers have the luxury to be fully connected all day (always-on internet) without any interference. Some very popular applications are:
A few years earlier some of these applications could only be fantasised but nowadays with all this big development of modems and services they became a fact of our everyday lives.
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