The term modem is derived from Modulator/Demodulator. A modem is a hardware device that is used to convert digital signals from a computer to analog signals (modulation) in order to transmit them over analog lines. At the receiving end, it converts the analog signals back to digital signals (demodulation) so that a computer can understand them. In their typical usage, modems are connected to a computer in order to provide remote access (or Internet connectivity) using analog telephone lines. It can be built onto the motherboard of the computer, can be installed as an extension card, or can be an external device. External modems can either be connected to one of the serial ports or to the USB port of the computer.
When used as an internal device, modems must be configured to use system resources such as an I/O address or IRQ. Modems use the serial communication (COM) ports in a computer, and resources used by these ports must be available in order to correctly configure the modem. Table 8 provides a summary of the COM ports and resources used by them.
When used as an internal device, modems must be configured to use system resources such as an I/O address or IRQ. Modems use the serial communication (COM) ports in a computer, and resources used by these ports must be available in order to correctly configure the modem. Table 8 provides a summary of the COM ports and resources used by them.
Modems are available in different sizes, speed capabilities, and costs. The data transmission speed of a modem depends mainly on the type of Universal Asynchronous Receiver/Transmitter (UART) chip used and varies from 9.6 Kbps to over 900 Kbps. Modems with up to 115 Kbps speeds are commonly used for dialup networking. Figure show how a modem works.
Classification of Modems
Modems can be of the following types:
(a) Landline Modems
(b) Wireless Modems
(c) LAN Modems
Modems can be of the following types:
(a) Landline Modems
(b) Wireless Modems
(c) LAN Modems
(a) Landline Modems
Landline modems are modems which connect to the public switched telephone network(PSTN). To connect to PSTN, these modems have a jack known as RJ-11 jack, or regular phone jack. Landline modems can be further classified into the following types:
(i) Internal modems
(ii) External modems
(iii) PCMCIA modems
(iv) Voice/data/fax modems
Internal Modems
Internal modems are installed within the computer, as interface cards. They use the computer’s CPU power for encoding and decoding.
Landline modems are modems which connect to the public switched telephone network(PSTN). To connect to PSTN, these modems have a jack known as RJ-11 jack, or regular phone jack. Landline modems can be further classified into the following types:
(i) Internal modems
(ii) External modems
(iii) PCMCIA modems
(iv) Voice/data/fax modems
Internal Modems
Internal modems are installed within the computer, as interface cards. They use the computer’s CPU power for encoding and decoding.
External Modems
External modems are installed as a separate hardware device, outside the computer. They are more expensive than internal modems. They connect to the serial port on the computer using a DB9 or DB25 connector. External modems are useful when several users need to share a single modem.
PCMCIA Modems
PCMCIA modems are credit-card modems used in laptop computers. PCMCIA stands for Personal Computer Memory Card International Association.
Voice/data/fax Modems
Voice/data/fax modems are modems which are used for transferring files, sending and receiving faxes and voice mail using associated software.
External modems are installed as a separate hardware device, outside the computer. They are more expensive than internal modems. They connect to the serial port on the computer using a DB9 or DB25 connector. External modems are useful when several users need to share a single modem.
PCMCIA Modems
PCMCIA modems are credit-card modems used in laptop computers. PCMCIA stands for Personal Computer Memory Card International Association.
Voice/data/fax Modems
Voice/data/fax modems are modems which are used for transferring files, sending and receiving faxes and voice mail using associated software.
(b) Wireless Modems
Wireless modems are radio transmitters/receivers installed into mobile computing devices. Using wireless modems, one can connect to a network while being mobile. Unlike landline modems, wireless modems do not plug into an RJ-11 jack. Wireless modems are designed to communicate to these private radio transmission networks. A wireless modem out of range of a wireless transmission network is of no use. As of now, there are very few manufacturers of wireless modems.
(c) LAN Modems
LAN modems allow shared remote access to LAN resources. LAN modem comes fully preconfigured for a single particular network architecture such as Ethernet or token ring and/or a particular network software such as IPX, NetBIOS, NetBEUI etc. LAN modems are of various types depending upon the number of ports, network architecture(s) supported, network protocols supported, client platforms supported, memory requirements, security etc.
Wireless modems are radio transmitters/receivers installed into mobile computing devices. Using wireless modems, one can connect to a network while being mobile. Unlike landline modems, wireless modems do not plug into an RJ-11 jack. Wireless modems are designed to communicate to these private radio transmission networks. A wireless modem out of range of a wireless transmission network is of no use. As of now, there are very few manufacturers of wireless modems.
(c) LAN Modems
LAN modems allow shared remote access to LAN resources. LAN modem comes fully preconfigured for a single particular network architecture such as Ethernet or token ring and/or a particular network software such as IPX, NetBIOS, NetBEUI etc. LAN modems are of various types depending upon the number of ports, network architecture(s) supported, network protocols supported, client platforms supported, memory requirements, security etc.
Modem Standards
There are two types of modem standards. These are
(a) Bell Modems
(b) ITU-T Modems
The first commercially available modems were developed by Bell Telephone company in the early 1970s. Being the first modem manufacturer, they defined the development of the technology and provided the standard which subsequent manufacturers followed. Some major Bell modems include the 103/113 series, 202 series, 212 series, 201 series, 208 series and 209 series.
Many of the popular modems are based on the standards published by ITU-T. V.21, V.22, V.23, V.22bis, V.32, V.32bis, V.33 and V.34 modems are ITU-T modems.
There are two types of modem standards. These are
(a) Bell Modems
(b) ITU-T Modems
The first commercially available modems were developed by Bell Telephone company in the early 1970s. Being the first modem manufacturer, they defined the development of the technology and provided the standard which subsequent manufacturers followed. Some major Bell modems include the 103/113 series, 202 series, 212 series, 201 series, 208 series and 209 series.
Many of the popular modems are based on the standards published by ITU-T. V.21, V.22, V.23, V.22bis, V.32, V.32bis, V.33 and V.34 modems are ITU-T modems.
Modem Protocols
(a) X.25 Protocol
X.25 is an end to end protocol. It acts as an interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). X.25 defines how the user’s DTE communicates with the network and how packets are sent over the network using DCEs (see figure) . It also provides a set of other services such as reverse charge, call direct and delay control. Figure
(a) X.25 Protocol
X.25 is an end to end protocol. It acts as an interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). X.25 defines how the user’s DTE communicates with the network and how packets are sent over the network using DCEs (see figure) . It also provides a set of other services such as reverse charge, call direct and delay control. Figure
X.25 is commonly used in wide area communications with multiple communicating devices. X.25 packet switching networks allow remote devices to communicate with each other without the expense of individual leased lines.
(b) Triple-X Protocols
X.3, X.28 and X.29 protocols are collectively known as Triple-X protocols. Triple-X protocols are used to connect a dumb terminal to an X.25 network. A dumb terminal is any terminal that does not understand X.25 protocol. X.3 defines a packet assembler/disassembler (PAD). PAD is required for connecting a dump terminal to an X.25 network. PAD buffers the characters and assembles them into X.25 packets. When a packet arrives, PAD disassembles the packets into the original characters.
X.28 defines the rules for communication between a dumb terminal and a PAD. It describes the commands that can be used for communication.
X.29 defines relationship between a PAD and a remote terminal. Using X.29, one can set some of the parameters in PAD.
X.3, X.28 and X.29 protocols are collectively known as Triple-X protocols. Triple-X protocols are used to connect a dumb terminal to an X.25 network. A dumb terminal is any terminal that does not understand X.25 protocol. X.3 defines a packet assembler/disassembler (PAD). PAD is required for connecting a dump terminal to an X.25 network. PAD buffers the characters and assembles them into X.25 packets. When a packet arrives, PAD disassembles the packets into the original characters.
X.28 defines the rules for communication between a dumb terminal and a PAD. It describes the commands that can be used for communication.
X.29 defines relationship between a PAD and a remote terminal. Using X.29, one can set some of the parameters in PAD.
Transceivers and media converters
As the name indicates, a transceiver is a device that combines the functions of a transmitter and a receiver. It does not refer to any standalone or separate hardware device but is normally built into devices such as network cards, modems, hubs, switches, or routers. Depending on the type of network cabling in use, you may find fiber optic transceivers used in fiber optic networks; RF transceivers used in wireless networks, and Ethernet transceivers.
Media converters are used to enable interconnection of one type of media (usually cabling) to another type. For example, you may want to connect a network segment wired with a fiber optic cable to another segment wired with UTP/STP cables. In another example, you may wish to connect a coaxial cable segment to a UTP/STP network segment.
As the name indicates, a transceiver is a device that combines the functions of a transmitter and a receiver. It does not refer to any standalone or separate hardware device but is normally built into devices such as network cards, modems, hubs, switches, or routers. Depending on the type of network cabling in use, you may find fiber optic transceivers used in fiber optic networks; RF transceivers used in wireless networks, and Ethernet transceivers.
Media converters are used to enable interconnection of one type of media (usually cabling) to another type. For example, you may want to connect a network segment wired with a fiber optic cable to another segment wired with UTP/STP cables. In another example, you may wish to connect a coaxial cable segment to a UTP/STP network segment.
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