Cable Internet and Leased Line

CABLE Internet

CABLE Internet is another possibility for local access to data network through the internet. Most CABLE Internets operate by taking some of the bandwidth available on a cable television system away from television signals and dedicating it instead to data traffic. On some older cable television networks, data signals are modulated onto sine waves, much like traditional modems do and placed on some of the existing analogue system channels. The transmission speed of these modems is much higher than that of conventional modems, because they are not limited by the 3000 Hz bandwidth of telephone line. Newer digital cable television systems, with digital cable converter boxes, actually send the television signals, and the user's data, on separate digital channels on the system. These digital CABLE Internets are not truly modems at all, but more like the Digital service units.

Using these various techniques, competing technologies offer man different speeds. In most cases, the transmission speed to the subscriber is much higher than the transmission speed to the network. In many cases, the bandwidth in a neighbourhood is shared, just like in an office LAN, so the user never has access to all of the bandwidth. There is always the risk that a neighbour may be downloading huge amounts of data, slowing down the entire neighbourhood network, but cable system providers can monitor this usage and place reasonable restrictions on customers to prevent abuse. Similarly like on a LAN, anyone electronically monitoring the cable with test equipment can see the data sent by anyone else on this particular network. To maintain privacy, conventional encryption technology used for any sensitive data.

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

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, Fibre optics goes from the cable company to different neighbourhoods or areas. Then the fibre 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 Internet 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 Internet on the customer end and a CABLE Internet 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

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, DHTP for assigning and administering the IP addresses of all the cable system's users, and control servers for a protocol called Cable Labs Certified CABLE Internets

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 Internets.

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 Internet 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 Internets is programmed with the distance from the head-end, and provides the proper signal strength for accurate transmission.

Leased Line

A leased line is a fast permanent connection to the internet from the office or network, giving access to the internet instantly. Leased lines come in different sizes dependant upon the needs with bandwidth from 64 Kbps to 2764 Mbps.

An analogue leased service offers customers the opportunity to lease a line, sometimes called dedicated line that is permanently connected to another customer. Although the connection still passes through the switches in the telephone network, subscribers experience it as a single line because the switch is always closed; no dialling is needed.

Telephone company offer another service called digital leased line service for the subscribers. One advantage is that digital services are less sensitive than analogue services to noise and other forms of interference. There is different type of service available like: switched/56, DDS, DS

Swithced/56 Service

Switched/56 is the digital version of an analogue switched line. It is a switched digital service that allows data rate of up to 56Kbps. To communicate through this service, both parties must subscribe. A caller with normal telephone service cannot connect to a telephone or computer with switched/56 even if using a modem. Because the line in a swithched/56 service is already digital, subscribers do not need modems to transmit digital data. However, they do need another device called a digital service unit (DSU). This device changes the rate of the digital data created by the subscriber's device to 56Kbps and encodes it in the format used by the service provider. The DSU is often included in the dialling process.

Digital data service (DDS)

Digital data service is the digital version of an analogue leased line; it is a digital leased line with a maximum data rate of 64Kbps. Like switched/56, DDS requires the use of a DSU. The DSU for this service is cheaper than that required for switched/56, however it does not need a dial pad.

Data Signal (DS) services

After offering switched/56 and DDS services, the telephone companies say a need to develop a hierarchy of digital services much like that used for analogue services. The next step is digital signal (DS). DS is a hierarchy of digital signal. DS-0, DS-1 and so on are the names of services. To implement those services, the telephone companies use T line (T-1 to T-4). There are lines with capacities precisely matched to the data rate of the DS-1 to DS-4 services.

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