There are basically two types of connection:
- Analogue: Modem
- Digital: ISDN, Cable, ADSL
Analogue:
- So that we have a connection between two computers via telephone line, we must use the modems;
- They are responsible for
converting the digital signals used by computers into analogue signals - used in analog telephone lines.
- The
modulation occurs when the computer sends the digital signal to the modem to convert into analog signal;
- The
demodulation occurs in the opposite direction when the line sends analog signal to be converted to digital.
-
Modems can be used in service lines connected where the computer needs to make a connection for each communication session - connect, transmit and finally detach.
- Its speed, can reach to 56.000 bps without the data compression.
- In
private lines, the computer establishes the connection as soon as it switches on and the speed in this type of line varies according to the contract made with the company that provides the service.
ISDN:- ISDN (Integrated Services Digital Network) exists since the 80s in some European countries.
- All information that leaves the equipment, instead of being transmitted in analog, will be
transmitted digitally.
- Its equipment transform analog signal (sound/voice) and make the transmission.
- Therefore, the distance and the connection type cease to have an effect on the quality of the signals.
- Plus, ISDN uses
protocols for transmission errors.
- The
B channels, or useful channels, only transport data packages with a constant speed and effective 64000 bit/s bidirectional.
- For two equipment be activated at the same time, there are two B channels per basic access.
-
Channel D, or data channel, works with 16000 bit/s in order to have a reserve of 8000 bit/s.
- With channel D, there is all the necessary information of the two B channels, including the
transmission data protocol,
type of equipment and the
information service (por example: impulse, date, time, call on hold).
- The channel D supported by the hardware, can, if necessary, add the two B channels to transmit data more quickly (
channel bundling).
In the ISDN connection there are two ways of communication:- The first way is the basic access and is intended for the particular user or the small and medium enterprises.
- You can
connect multiple terminal equipment because the basic access connection has always available two channels, thereby enabling the use of a maximum of two equipment or connections simultaneously.
- The second option is the primary acess (
Primary Multiplex).
- This allows the use of, up to, 30 channels and is connected through a
switchboard, and not through a basic access.
- Primary access does not have a common connection therefore is dependent on the switchboard.
Cable:- The cable TV supports the TV signal and the data that flows on the Internet.
- In your home or office, a
splitter separates the TV signals from data, and a hardware called
cable modem (although not technically a modem) connects your computer to the Internet.
- It is possible to achieve speed up to 24Mbps in both directions, but the
performance is degraded when the increasing numbers of users are under the same head-end, in other words, from the same of TV signal carrier to a number of residences.
- The access does not depend on the phone line, it starts immediately after you turn on the computer,
24 hours a day.
A cable modem has
two connections:
- One for the cable output that stays outside the subscriber's house and another one for a computer.
- Although a cable modem makes the modulation signals between analog and digital, is a much more complex device than a telephone modem.
- The cable modem needs a branch of the sign that divides the signs that goes to the television and to the computer.
- So, after the request of the service, a technician visits the place to dock a device called splitter, if he finds the line in appropriate quality conditions.
- This is the engine that allows the use of Internet access without interfering the TV programming.
- Another difference is the need for a network interface Ethernet 10BaseT to use the coaxial system that the service of the supplier company provides.
- The access services via cable modem has a problem of deployment still big, because the loop of cable TV operators doesn't have much penetration and investment to expand it is not happening in a way very intense.
- Access by cable modem only compensates for whom it is user of cable TV, because the investments for hiring the service aren't as high.
- Besides, in the case of cable modem, despite the speed of transmission declared to be similar to the ADSL, the physical environment is shared, with a margin of speed guaranteed.
- So if one cable that comes to the house of the subscriber go through many other homes, there may be degradation in the access speed.
- In ADSL, this sharing also exists, but is limited to 10% - today, the ADSL subscribers doesn't receive less than 10% of the rate of transfer agreed, which is called the speed guaranteed.
- In cable, there's still much idleness of the network, the deterioration may not happen very often for a while, but as the network strangles, inevitably there will be problems of performance.
xDSL:ADSL - Asymmetric Digital Subscriber Line has been developed to complete the connection with the domestic consumers.
- Leads two separate streams of data having more bandwidth, so that the flow reaches the local terminal (downstream).
- It is efficient, because symmetrical signs in many twisted pairs cable (such as a Telephone Central), limits the rate of transmission and the size of the line.
- In a simplest way, ADSL is a new technology based on modems that convert lines phones of common twisted-pairs, in paths of access to multimedia and data communications to high-speed.
ADSL - Asymmetric Digital Subscriber Line. 
Full Duplex
Router
Switch
Hub
Bridge
