In 1980, Gottfried Ungerboeck from IBM Zurich Research Laboratory applied channel coding techniques to search for new ways to increase the speed of modems. His results were astonishing but only conveyed to a few colleagues.[2] In 1982, he agreed to publish what is now a landmark paper in the theory of information coding.[citation needed] By applying parity check coding to the bits in each symbol, and mapping the encoded bits into a two-dimensional diamond pattern, Ungerboeck showed that it was possible to increase the speed by a factor of two with the same error rate. The new technique was called mapping by set partitions, now known as trellis modulation.
Error correcting codes, which encode code words (sets of bits) in such a way that they are far from each other, so that in case of error they are still closest to the original word (and not confused with another) can be thought of as analogous to sphere packing or packing pennies on a surface: the further two bit sequences are from one another, the easier it is to correct minor errors.
V.32bis was so successful that the older high-speed standards had little to recommend them. USR fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off 19,200 bit/s method they referred to as V.32ter, but neither non-standard modem sold well.
V.34/28.8k and 33.6k[edit source | editbeta]
An ISA modem manufactured to conform to the V.34 protocol.
Any interest in these systems was destroyed during the lengthy introduction of the 28,800 bit/s V.34 standard. While waiting, several companies decided to release hardware and introduced modems they referred to as V.FAST. In order to guarantee compatibility with V.34 modems once the standard was ratified (1994), the manufacturers were forced to use more flexible parts, generally a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips.
Today, the ITU standard V.34 represents the culmination of the joint efforts. It employs the most powerful coding techniques including channel encoding and shape encoding. From the mere 4 bits per symbol (9.6 kbit/s), the new standards used the functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and 33.6 kbit/s modems. This rate is near the theoretical Shannon limit. When calculated, the Shannon capacity of a narrowband line is \text{bandwidth} \times \log_2 (1 + P_u/P_n), with P_u/P_n the (linear) signal-to-noise ratio. Narrowband phone lines have a bandwidth of 3000 Hz so using P_u/P_n=1000 (SNR = 24 dB), the capacity is approximately 24 kbit/s.[3]
Without the discovery and eventual application of trellis modulation, maximum telephone rates using voice-bandwidth channels would have been limited to 3,429 baud × 4 bit/symbol = approximately 14 kbit/s using traditional QAM
The V.61 Standard introduced Analog Simultaneous Voice and Data (ASVD). This technology allowed users of v.61 modems to engage in point-to-point voice conversations with each other while their respective modems communicated.
In 1995, the first DSVD (Digital Simultaneous Voice and Data) modems became available to consumers, and the standard was ratified as v.70 by the International Telecommunication Union (ITU) in 1996.
Two DSVD modems can establish a completely digital link between each other over standard phone lines. Sometimes referred to as "the poor man's ISDN", and employing a similar technology, v.70 compatible modems allow for a maximum speed of 33.6 kbit/s between peers. By using a majority of the bandwidth for data and reserving part for voice transmission, DSVD modems allow users to pick up a telephone handset interfaced with the modem, and initiate a call to the other peer.
One practical use for this technology was realized by early two-player video gamers, who could hold voice communication with each other over the phone while playing.
Using digital lines and PCM (V.90/92)[edit source | editbeta]
A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally handled in hardware with software. In this case the modem is a simple interface designed to act as a digital-to-analog and an analog-to-digital converter. Softmodems are cheaper than traditional modems because they have fewer hardware components. However, the software generating and interpreting the modem tones to be sent to the softmodem uses many system resources. For online gaming, this can be a real concern. Another problem is the lack of cross-platform compatibility, meaning that non-Windows operating systems (such as Linux) often do not have an equivalent driver to operate the modem.
List of dialup speeds[edit source | editbeta]
These values are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines).[7] For a complete list see the companion article list of device bandwidths. A baud is one symbol per second; each symbol may encode one or more data bits.
Modem bank at an ISP
In the late 1990s Rockwell/Lucent and USRobotics introduced new competing technologies based upon the digital transmission used in modern telephony networks. The standard digital transmission in modern networks is 64 kbit/s but some networks use a part of the bandwidth for remote office signaling (e.g. to hang up the phone), limiting the effective rate to 56 kbit/s DS0. This new technology was adopted into ITU standards V.90 and is common in modern computers. The 56 kbit/s rate is only possible from the central office to the user site (downlink). In the United States, government regulation limits the maximum power output, resulting in a maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34 technology at 33.6 kbit/s.
Later in V.92, the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s, but at the expense of download rates. A 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo on the telephone line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a 33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher.[4] V.92 also adds two other features. The first is the ability for users who have call waiting to put their dial-up Internet connection on hold for extended periods[vague] of time while they answer a call. The second feature is the ability to quickly connect to one's ISP. This is achieved by remembering the analog and digital characteristics of the telephone line, and using this saved information when reconnecting.
Using compression to exceed 56k[edit source | editbeta]
Today's V.42, V.42bis and V.44 standards allow the modem to transmit data faster than its basic rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3*6 == 320 kbit/s using pure text.[original research?] However, the compression ratio tends to vary due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video).[5] At some points the modem will be sending compressed files at approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.[6]
In such situations a small amount of memory in the modem, a buffer, is used to hold the data while it is being compressed and sent across the phone line, but in order to prevent overflow of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream. This is accomplished through hardware flow control using extra lines on the modem–computer connection. The computer is then set to supply the modem at some higher rate, such as 320 kbit/s, and the modem will tell the computer when to start or stop sending data.
Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fibre is not economical.
Even where a cable is installed, it is often possible to get better performance or make other parts of the system simpler by using radio frequencies and modulation techniques through a cable. Coaxial cable has a very large bandwidth, however signal attenuation becomes a major problem at high data rates if a baseband digital signal is used. By using a modem, a much larger amount of digital data can be transmitted through a single wire. Digital cable television and cable Internet services use radio frequency modems to provide the increasing bandwidth needs of modern households. Using a modem also allows for frequency-division multiple access to be used, making full-duplex digital communication with many users possible using a single wire.
Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies.
Transparent modems operate in a manner similar to their phone line modem cousins. Typically, they were half duplex, meaning that they could not send and receive data at the same time. Typically transparent modems are polled in a round robin manner to collect small amounts of data from scattered locations that do not have easy access to wired infrastructure. Transparent modems are most commonly used by utility companies for data collection.
Smart modems come with media access controllers inside, which prevents random data from colliding and resends data that is not correctly received. Smart modems typically require more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE 802.11 standard defines a short range modulation scheme that is used on a large scale throughout the world.a
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.[13] 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.[14][15] 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.[16]
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'[17]
Broadband[edit source | editbeta]
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.
Error correcting codes, which encode code words (sets of bits) in such a way that they are far from each other, so that in case of error they are still closest to the original word (and not confused with another) can be thought of as analogous to sphere packing or packing pennies on a surface: the further two bit sequences are from one another, the easier it is to correct minor errors.
V.32bis was so successful that the older high-speed standards had little to recommend them. USR fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off 19,200 bit/s method they referred to as V.32ter, but neither non-standard modem sold well.
V.34/28.8k and 33.6k[edit source | editbeta]
An ISA modem manufactured to conform to the V.34 protocol.
Any interest in these systems was destroyed during the lengthy introduction of the 28,800 bit/s V.34 standard. While waiting, several companies decided to release hardware and introduced modems they referred to as V.FAST. In order to guarantee compatibility with V.34 modems once the standard was ratified (1994), the manufacturers were forced to use more flexible parts, generally a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips.
Today, the ITU standard V.34 represents the culmination of the joint efforts. It employs the most powerful coding techniques including channel encoding and shape encoding. From the mere 4 bits per symbol (9.6 kbit/s), the new standards used the functional equivalent of 6 to 10 bits per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and 33.6 kbit/s modems. This rate is near the theoretical Shannon limit. When calculated, the Shannon capacity of a narrowband line is \text{bandwidth} \times \log_2 (1 + P_u/P_n), with P_u/P_n the (linear) signal-to-noise ratio. Narrowband phone lines have a bandwidth of 3000 Hz so using P_u/P_n=1000 (SNR = 24 dB), the capacity is approximately 24 kbit/s.[3]
Without the discovery and eventual application of trellis modulation, maximum telephone rates using voice-bandwidth channels would have been limited to 3,429 baud × 4 bit/symbol = approximately 14 kbit/s using traditional QAM
The V.61 Standard introduced Analog Simultaneous Voice and Data (ASVD). This technology allowed users of v.61 modems to engage in point-to-point voice conversations with each other while their respective modems communicated.
In 1995, the first DSVD (Digital Simultaneous Voice and Data) modems became available to consumers, and the standard was ratified as v.70 by the International Telecommunication Union (ITU) in 1996.
Two DSVD modems can establish a completely digital link between each other over standard phone lines. Sometimes referred to as "the poor man's ISDN", and employing a similar technology, v.70 compatible modems allow for a maximum speed of 33.6 kbit/s between peers. By using a majority of the bandwidth for data and reserving part for voice transmission, DSVD modems allow users to pick up a telephone handset interfaced with the modem, and initiate a call to the other peer.
One practical use for this technology was realized by early two-player video gamers, who could hold voice communication with each other over the phone while playing.
Using digital lines and PCM (V.90/92)[edit source | editbeta]
A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally handled in hardware with software. In this case the modem is a simple interface designed to act as a digital-to-analog and an analog-to-digital converter. Softmodems are cheaper than traditional modems because they have fewer hardware components. However, the software generating and interpreting the modem tones to be sent to the softmodem uses many system resources. For online gaming, this can be a real concern. Another problem is the lack of cross-platform compatibility, meaning that non-Windows operating systems (such as Linux) often do not have an equivalent driver to operate the modem.
List of dialup speeds[edit source | editbeta]
These values are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines).[7] For a complete list see the companion article list of device bandwidths. A baud is one symbol per second; each symbol may encode one or more data bits.
Modem bank at an ISP
In the late 1990s Rockwell/Lucent and USRobotics introduced new competing technologies based upon the digital transmission used in modern telephony networks. The standard digital transmission in modern networks is 64 kbit/s but some networks use a part of the bandwidth for remote office signaling (e.g. to hang up the phone), limiting the effective rate to 56 kbit/s DS0. This new technology was adopted into ITU standards V.90 and is common in modern computers. The 56 kbit/s rate is only possible from the central office to the user site (downlink). In the United States, government regulation limits the maximum power output, resulting in a maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34 technology at 33.6 kbit/s.
Later in V.92, the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s, but at the expense of download rates. A 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo on the telephone line. To avoid this problem, V.92 modems offer the option to turn off the digital upstream and instead use a 33.6 kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher.[4] V.92 also adds two other features. The first is the ability for users who have call waiting to put their dial-up Internet connection on hold for extended periods[vague] of time while they answer a call. The second feature is the ability to quickly connect to one's ISP. This is achieved by remembering the analog and digital characteristics of the telephone line, and using this saved information when reconnecting.
Using compression to exceed 56k[edit source | editbeta]
Today's V.42, V.42bis and V.44 standards allow the modem to transmit data faster than its basic rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3*6 == 320 kbit/s using pure text.[original research?] However, the compression ratio tends to vary due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video).[5] At some points the modem will be sending compressed files at approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.[6]
In such situations a small amount of memory in the modem, a buffer, is used to hold the data while it is being compressed and sent across the phone line, but in order to prevent overflow of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream. This is accomplished through hardware flow control using extra lines on the modem–computer connection. The computer is then set to supply the modem at some higher rate, such as 320 kbit/s, and the modem will tell the computer when to start or stop sending data.
Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fibre is not economical.
Even where a cable is installed, it is often possible to get better performance or make other parts of the system simpler by using radio frequencies and modulation techniques through a cable. Coaxial cable has a very large bandwidth, however signal attenuation becomes a major problem at high data rates if a baseband digital signal is used. By using a modem, a much larger amount of digital data can be transmitted through a single wire. Digital cable television and cable Internet services use radio frequency modems to provide the increasing bandwidth needs of modern households. Using a modem also allows for frequency-division multiple access to be used, making full-duplex digital communication with many users possible using a single wire.
Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies.
Transparent modems operate in a manner similar to their phone line modem cousins. Typically, they were half duplex, meaning that they could not send and receive data at the same time. Typically transparent modems are polled in a round robin manner to collect small amounts of data from scattered locations that do not have easy access to wired infrastructure. Transparent modems are most commonly used by utility companies for data collection.
Smart modems come with media access controllers inside, which prevents random data from colliding and resends data that is not correctly received. Smart modems typically require more bandwidth than transparent modems, and typically achieve higher data rates. The IEEE 802.11 standard defines a short range modulation scheme that is used on a large scale throughout the world.a
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.[13] 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.[14][15] 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.[16]
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'[17]
Broadband[edit source | editbeta]
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.
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Unknown - Wednesday, September 5, 2012
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