Modems

MODEM

A modem (modulator-demodulator) is a device that modulates signals to encode digital information and demodulates signals to decode the transmitted information.

The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. 

Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. 

A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data..

In order for devices on the network to connect to the Internet, the router must be connected to a modem. 

Therefore, most routers have a specific Ethernet port that is designed to connect to the Ethernet port of a cable or DSL modem. A modem is a device that provides access to the Internet.

FEATURES OF MODEM:

1.  The speed of modem is measured in bps that is bits per second.

2. Modems has 2 modes - 

   Voice mode - Modem acts like a telephone.

   Data modem - Acts like a modem.

FUNCTIONS OF MODEM-

1. Error Correction: 

• In this process the device checks if the information they receive is undamaged. 
• It divides the information into packets called frames. Before sending this information, it tags each of the frames with checksums.
• Checksum is a method of checking redundancy in the data present on the computer. 
• The device that receives the information, verifies if the information matches with checksums, sent by the error-correcting modem. 
• If it fails to match with the checksum, the information is sent back.

2. Compressing the Data: 

• For compressing the data, it is sent together in many bits. 
• The bits are grouped together by the device in order to compress them.

3. Flow Control: 

• Different devices vary in their speed of sending signals. Thus, it creates problems in receiving the signals if either one of them is slow. 
• In the flow control mechanism, the slower one signals the faster one to pause, by sending a 'character'. When it is ready to catch up with the faster one, a different character is sent, which in turn resumes the flow of signals.

WORKING OF MODEM -

The sending modem modulatesthe data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data.

Wireless modems convert digital data into radio signals and back.

TYPES OF MODEM :

1. DSL - DIGITAL SUBSCRIBER LINE :

A digital subscriber line (DSL) modem is a device used to connect a computer or router to a telephone line which provides the digital subscriber lineservice for connectivity to the Internet, which is often called DSL broadband.

The term DSL modem is technically used to describe a modem which connects to a single computer, through an Ethernet Port, USB port, or is installed in a computer PCI slot. 

The more common DSL router is a standalone device that combines the function of a DSL modem and a router, and can connect multiple computers through multiple Ethernet ports or an integral wireless access point. 

Also called a residential gateway, a DSL router usually manages the connection and sharing of the DSL service in a home or small office network.

It usually has a series of LED status lights which show the status of parts of the DSL communications link:

• Power light - indicates that the modem is turned on and has power.
• Ethernet lights - There is usually a light over each Ethernet jack. A steady (or sometimes flashing) light indicates that the Ethernet link to that computer or device is functioning
• DSL light - a steady light indicates that the modem has established contact with the equipment in the local telephone exchange (DSLAM) so the DSL link over the telephone line is functioning. Newer modems that support ADSL2+ bonding will have one light for each line.
• Internet light - a steady light indicates that the IP address and DHCP protocol are initialized and working, so the system is connected to the Internet
• Wireless light - only in wireless DSL modems, this indicates that the wireless network is initialized and working

Many routers provide an internal web page to the local network for device configuration and status reporting. 

Most DSL routers are designed to be installed by the customer for which a CD or DVD containing an installation program is supplied. 

The program may also activate the DSL service. Upon powering the router it may take several minutes for the local network and DSL link to initialize, usually indicated by the status lights turning green.

ADVANTAGES -

1. High bandwidth .

2. Integration of DSL is easy with ATM  and WAN technology.

3. Cheap line charges from the phone company.

DISADVANTAGES-

1. It is distance dependent i.e the farther one lives from DSLAM , the lower the data rate. 

2. ADSL- ASYMMETRIC DISGITAL SUBSCRIBER LINE

Itis a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide.  

ADSL differs from the less common symmetric digital subscriber line (SDSL). Bandwidth (and bit rate) is greater toward the customer premises (known as downstream) than the reverse (known as upstream). This is why it is called asymmetric. 

Providers usually market ADSL as a service for consumers to receive Internet access in a relatively passive mode: able to use the higher speed direction for the download from the Internet but not needing to run servers that would require high speed in the other direction.

3. SDSL :Symmetric Digital subscribr line

A technology that allows more data to be sent over existing copper telephone lines (POTS).

SDSL supports data rates up to 3 Mbps.

SDSL works by sending digital pulses in the high-frequency area of telephone wires and can not operate simultaneously with voice connections over the same wires.

SDSL requires a special SDSL modem. SDSL is called symmetricbecause it supports the same data rates for upstream and downstream traffic. 

4. CABLE MODEM

It is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a hybrid fibre-coaxial (HFC) and RFoG infrastructure. 

Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high bandwidth of a HFC and RFoG network. 

A modem designed to operate over cable TV lines. Because thecoaxial cable used by cable TV provides much greater bandwidththan telephone lines, a cable modem can be used to achieve extremely fast access to the World Wide Web. This, combined with the fact that millions of homes are already wired for cable TV, has made the cable modem something of a holy grail for Internetand cable TV companies.

ADVANTAGES -              

1. Faster speed of connection

2. The telephone line is not used. This allows customers to save money by not buying a separate phone line for Internet connecting. 

3. The cable modem user will not experience busy signals when trying to connect to the Internet.

DISADVANTAGES -

1.Costly , expensive .

2. Not available widely.

5. INTERNAL AND EXTERNAL MODEM

The names "external modem" and "internal modem" refer to the location of the modem with respect to a computer. 

Internal modems are installed inside computers. 

An internal modem is a network device that is contained on an expansion board that plugs into the motherboard. 

Unlike an external modem, an internal modem contains no lights to inform the user of its current function or changing modem states. Instead, the user must rely on the software that came with the modem.

Internal modems are also known as on-board modems.

Internal modems come in two types: dial-up and wireless. Dial-up modems require a connection with a telephone line, network access phone number and username and login ID.

ADVANTAGES -

1. Internal modems cost less.

2. Easier to maintain.

3. More Portable.

External modems are stand-alone devices that computers can connect to. 

External modems tend to be slightly more expensive than internal modems. 

Many experts consider them superior because they contain lights that indicate how the modem is functioning. 

In addition, they can easily be moved from one computer to another. However, they do use up one COM port.

Some types of external modems include: 

• USB 
• Cable 
• DSL
• External wireless modems

ADVANTAGES -

1. Easier to set up.

2. Helps computer last long and perform better.