Mobile broadband modems
Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax, etc.), are known as mobile broadband modems (sometimes also called wireless modems). Wireless modems can be embedded inside a laptop or appliance, or be external to it. External wireless modems are connect cards, USB modems for mobile broadband and cellular routers. A connect card is a PC card or ExpressCard which slides into a PCMCIA/PC card/ExpressCard slot on a computer. USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot. A USB modem used for mobile broadband Internet is also sometimes referred to as a dongle. A cellular router may have an external datacard (AirCard) that slides into it. Most cellular routers do allow such datacards or USB modems. Cellular routers may not be modems by definition, but they contain modems or allow modems to be slid into them. The difference between a cellular router and a wireless modem is that a cellular router normally allows multiple people to connect to it (since it can route data or support multipoint to multipoint connections), while a modem is designed for one connection.
Most of GSM wireless modems come with an integrated SIM cardholder (i.e., Huawei E220, Sierra 881, etc.) and some models are also provided with a microSD memory slot and/or jack for additional external antenna such as Huawei E1762 and Sierra Wireless Compass 885. The CDMA (EVDO) versions do not use R-UIM cards, but use Electronic Serial Number (ESN) instead.
The cost of using a wireless modem varies from country to country. Some carriers implement flat rate plans for unlimited data transfers. Some have caps (or maximum limits) on the amount of data that can be transferred per month. Other countries have plans that charge a fixed rate per data transferred—per megabyte or even kilobyte of data downloaded; this tends to add up quickly in today's content-filled world, which is why many people[who?] are pushing for flat data rates.
The faster data rates of the newest wireless modem technologies (UMTS, HSPA, EVDO, WiMax) are also considered to be broadband wireless modems and compete with other broadband modems below.
Until the end of April 2011, worldwide shipments of USB modems surpassed embedded 3G and 4G modules by 3:1 because USB modems can be easily discarded, but embedded modems could start to gain popularity as tablet sales grow and as the incremental cost of the modems shrinks, so by 2016 the ratio may change to 1:1.
Like mobile phones, mobile broadband modems can be SIM locked to a particular network provider. Unlocking a modem is achieved the same way as unlocking a phone, by using an 'unlock code'
ADSL (asymmetric digital subscriber line) modems, a more recent development, are not limited to the telephone's voiceband audio frequencies. Some ADSL modems use coded orthogonal frequency division modulation (DMT, for Discrete MultiTone; also called COFDM, for digital TV in much of the world).
DSL modems utilize a property that standard twisted-pair telephone cable can be used for short distances to carry much higher frequency signals than what the cable is actually rated to handle. This is also why DSL modems have a distance limitation. Standard voice and slower 56 kilobit modem communications are possible over many kilometers of cable, but the higher frequencies used by DSL are attenuated and DSL's maximum performance gradually declines as the cable length increases.
Cable modems use a range of frequencies originally intended to carry RF television channels, and can coexist on the same single cable alongside standard RF channel signals. Multiple cable modems attached to a single cable can use the same frequency band, using a low-level media access protocol to allow them to work together within the same channel. Typically, uplink and downlink signals are kept separate using frequency division multiple access.
For a single-cable distribution system, the return signals from customers require special bidirectional amplifiers or reverse path amplifiers that can send specific customer frequency bands upstream to the cable plant amongst the other downstream frequency bands.
Broadband modems should still be classified as modems, since they use complex waveforms to carry digital data. They are more advanced devices than traditional dial-up modems as they are capable of modulating/demodulating hundreds of channels simultaneously.
When broadband technology was introduced, networking and routers were unfamiliar to consumers. However, many people knew what a modem was because Internet access was still commonly done through dial-up. Due to this familiarity, companies started selling broadband modems using the familiar term "modem", rather than vaguer ones such as "adapter," "transceiver," or "bridge."
DSL modems without internal routing use what is known as bridge mode to connect to an upstream router device. DSL modems with built-in routing can also sometimes be set to bridged mode to disable the internal built-in router in order to use an upstream router instead, such as for Multiple-WAN load balancing, Multilink PPP, or to replace the built-in router with an external device with more capabilities.
In bridged mode, although Ethernet cabling is used to connect the modem to the upstream router, Internet Protocol is not used for communication between the devices. Instead the Ethernet cable is treated as a high speed serial Asynchronous Transfer Mode data connection according to RFC 1483. Multiple bridged DSL modems can all have the same configuration IP address without conflict or error, since the address is not used.
The static IP address assigned to the bridged DSL modem is only used when the modem is plugged into a client computer for directly configuring the modem, typically through a web interface.
Notice: This is part of a wikipedia entry: http://en.wikipedia.org/wiki/Modem
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