This section will talk about what is modem and the different types of modems.
What is a Modem
A modem is an abbreviated form of Modulator and demodulator. In data communication, when a signal is transmitted from the transmitting station, it is modulated with some carrier signal. The modulated signal is passed through communication media and when received by the distant station is demodulated back to its original signal.
Thus, a modem is a network device used for modulation and demodulation. When it sends data, it is modulated and when it receives the data, it then demodulates.
Generally, telephone lines carry analog signals. Hence, the modem converts digital data into analog form during modulation and it converts back analog to digital form during demodulation.
Types of modems
There are many different types of modems.
The modem has a long history since its origin. Earlier, 56 kbps dial-up modems were used for a slow Internet connection. Nowadays, DSL broadband modems are popular for accessing high-speed Internet.
The different standards of modems are listed below:
- Dial-up modem
- Digital Subscriber Line (DSL)
- Cable modem
Dial-up modem
During the early days of the Internet, dial-up modems were used to access the data services through the public switched telephone (PSTN).
It establishes a data communication by dialing the telephone numbers to Internet Service Provider (ISP) using plain old telephones systems (POTS).
The conventional telephones line uses the bandwidth of 3000 Hz covering the range from 300 Hz to 3500 Hz to carry voice signals. But, the entire range not only carries voice but also lots of distortion and unwanted noise. However, analog voice signals can tolerate this spurious noise and interference up to some extent. But, the data signals cannot accept heavy noise and interference. Hence, the edge frequencies are filtered out to restrict the noise and distortion. The actual bandwidth of 2400 Hz ranging from 600 Hz to 3000 Hz is used for data communication.
The maximum speed offered by the dial-up modem is up to 56 kbps.
The following V-series standards are available for dial-up internet access:
- V.32 : It offers data rates up to 9600 bps.
- V.32bis: It offers data speed of up to 14,400 bps.
- V.34 : data rate up to 28,800 bps.
- V.90 : provides data speed of 56 kbps for downstream data and 33.6 kbps for upstream data.
- V.92 : offers upstream data rate up to 48 kbps and downstream rate of up to 56 kbps.
The dial-up modem offers a maximum bandwidth of 56 kbps. Hence it is also termed as 56 kbps modem.
Digital Subscriber Line (DSL)
Dial-up modems, earlier used for Internet access, have many limitations. The most annoying factor was the maximum data speed of 56 kbps which was not enough for the internet service. Digital Subscriber Line (DSL) later on introduced with higher speed using the same conventional telephone lines.
DSL technology employs various standards such as:
- ADSL
- HDSL
- SDSL
- VDSL
The different first letters in all the standards, sometimes collectively known as xDSL. ( x may be A, H, S or V).
ADSL
It stands for Asymmetric DSL and is the most popularly used among Internet users in the home and small office. It widely replaces the old slow speed dial-up modem. It is called asymmetric because the data speed is different for upstream and downstream. The maximum downloading and uploading speed is 1.5 MHz and 512 kbps respectively.
HDSL
It stands for a High-bit-rate Digital Subscriber Line. It is one of the earlier versions of DSL technology and is an alternative to the T-1 line operating at 1.544 Mbps. The transmission speed of the T-1 line is limited to 3200 ft or 1 Km due to higher attenuation at longer distances. Therefore, the repeater has to be installed at an extra cost to extend the data transmission range.
HDSL replaces the T-1 line with the same data rate of 1.544 Mbps. The range of data transmission is achieved up to 12,000 ft or 3.6 Km without using the repeater. The full-duplex communication is possible using two pairs of twisted cables.
SDSL
Acronym for Symmetric Digital Subscriber Line. It is a DSL technology that offers a data rate up to 768 kbps in full-duplex mode using a single twisted pair cable. It differs from ADSL, that SDSL provides the identical data transfer rate for both upstream and downstream data.
VDSL
It stands for Very High-data-rate DSL. It is faster than ADSL and offers a very high-speed data rate through twisted pair cable, coaxial cable, or fiber optic cable. It provides upstream and downstream data rates up to 16 Mbps and 52 Mbps respectively. VDSL supports high-speed data applications such as high definition TV streaming, Voice over IP telephony ( VOIP), etc.
Cable modem
It is a high-speed modem used by cable broadband service. It provides a high-speed bandwidth of HFC ( Hybrid Fibre Coaxial)and RFoG(Radio Frequency over Glass) network.
