DSL modem
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.
New types of broadband modems are beginning to appear, such as doubleway[disambiguation needed] satellite and power line modems.
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 either capable of modulating/demodulating hundreds of channels simultaneously and/or are capable of using much wider channels than dial-up modems.
Many broadband modems include the functions of a router, such as Ethernet and WiFi, and other features such as DHCP, NAT and firewalls.
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."
Bridged mode[edit source | editbeta]
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 upstream router is expected to use Point-to-point protocol over Ethernet (PPPoE) or Multilink PPP in order to establish a connection on the DSL phone line.
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.
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.
New types of broadband modems are beginning to appear, such as doubleway[disambiguation needed] satellite and power line modems.
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 either capable of modulating/demodulating hundreds of channels simultaneously and/or are capable of using much wider channels than dial-up modems.
Many broadband modems include the functions of a router, such as Ethernet and WiFi, and other features such as DHCP, NAT and firewalls.
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."
Bridged mode[edit source | editbeta]
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 upstream router is expected to use Point-to-point protocol over Ethernet (PPPoE) or Multilink PPP in order to establish a connection on the DSL phone line.
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.
0 comments:
Post a Comment