What is often advertised as "high-speed dial-up Internet" or "accelerated dial-up" by service providers such as Earthlink and NetZero in the United States is a form of dial-up access that utilizes the newer modem standard v.92 to shorten the log-on (or handshake) process, and then once a connection has been established the provider will selectively compress, filter, and cache data being sent to the users home with the overall effect of increasing the speed of browsing most standard web pages (see also proxy server).
The term high speed is misleading as these processes do not increase the overall throughput of the line, only making more efficient use of the bandwidth that is already there. Certain applications cannot be accelerated, such as SHTTP, streaming media, or file transfers. The compression of certain files such as pictures can have a negative affect on the browsing experience of the user, due to the lower quality that it imposes.
Wednesday, January 10, 2007
Dial-Up Access
Dial-up access uses a modem connected to a computer and a telephone line to dial into an Internet service provider's node to establish a modem-to-modem link, which is then routed to the Internet.
Despite the advent of widely available broadband Internet access in most parts of the Western world, many people worldwide still connect via dial-up simply because there is no high speed Internet in their area.
Dial-up requires time to establish a telephone connection (approximately several seconds, depending on the location) and perform handshaking before data transfers can take place. In locales with telephone connection charges, each connection incurs an incremental cost. If calls are time-charged, the duration of the connection incurs costs.
Dial-up access is a transient connection, because either the user or the ISP terminates the connection. Internet service providers will often set a limit on connection durations to prevent hogging of access, and will disconnect the user — requiring reconnection and the costs and delays associated with that.
Dial-up requires no additional infrastructure on top of the telephone network. As telephone points are available throughout the world, dial-up remains useful to travelers. Dial-up is usually the only choice available for most rural or remote areas where getting a broadband connection is impossible due to low population and demand. Sometimes dial-up access may also be an alternative to people who have limited budgets as it is offered for free by some, though broadband is now increasingly available at lower prices in countries such as the United States and Canada due to market competition.
Despite the advent of widely available broadband Internet access in most parts of the Western world, many people worldwide still connect via dial-up simply because there is no high speed Internet in their area.
Dial-up requires time to establish a telephone connection (approximately several seconds, depending on the location) and perform handshaking before data transfers can take place. In locales with telephone connection charges, each connection incurs an incremental cost. If calls are time-charged, the duration of the connection incurs costs.
Dial-up access is a transient connection, because either the user or the ISP terminates the connection. Internet service providers will often set a limit on connection durations to prevent hogging of access, and will disconnect the user — requiring reconnection and the costs and delays associated with that.
Dial-up requires no additional infrastructure on top of the telephone network. As telephone points are available throughout the world, dial-up remains useful to travelers. Dial-up is usually the only choice available for most rural or remote areas where getting a broadband connection is impossible due to low population and demand. Sometimes dial-up access may also be an alternative to people who have limited budgets as it is offered for free by some, though broadband is now increasingly available at lower prices in countries such as the United States and Canada due to market competition.
Monday, January 8, 2007
Operation Theory of DSL
The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voiceband or commercial bandwidth.
At the local telephone exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).
The laws of physics - specifically, the Shannon limit - caps the speed of data transmission. For a long time, it was believed that a conventional phone line couldn't be pushed beyond the low speed limits (typically under 9600 bps). However, in the 1930s techniques were developed for broadband communications that allowed the limit to be greatly pushed.
The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether or not they are usable.
ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where transmission errors are impermissible, even though resending packets may increase latency.
Because DSL operates at above the 3.4kHz voice limit it can not be passed through a load coil. Load coils are in essence filters that block out any non-voice frequency. They're commonly set at regular intervals in lines placed only for POTS service. A DSL signal can not pass through a properly installed and working load coil nor can voice service be maintained past a certain distance without them. Some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this phone companies are efforting to remove load coils on copper loops that can operate without them and conditioning lines to not need them through the use of FTTN
At the local telephone exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).
The laws of physics - specifically, the Shannon limit - caps the speed of data transmission. For a long time, it was believed that a conventional phone line couldn't be pushed beyond the low speed limits (typically under 9600 bps). However, in the 1930s techniques were developed for broadband communications that allowed the limit to be greatly pushed.
The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether or not they are usable.
ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where transmission errors are impermissible, even though resending packets may increase latency.
Because DSL operates at above the 3.4kHz voice limit it can not be passed through a load coil. Load coils are in essence filters that block out any non-voice frequency. They're commonly set at regular intervals in lines placed only for POTS service. A DSL signal can not pass through a properly installed and working load coil nor can voice service be maintained past a certain distance without them. Some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this phone companies are efforting to remove load coils on copper loops that can operate without them and conditioning lines to not need them through the use of FTTN
History of DSL
Digital subscriber line technology was originally implemented as part of the ISDN specification, thus can operate on a BRI ISDN line as well as an analog phone line.
Joe Lechleider at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling.
US telephone companies promote DSL to compete with cable modems. DSL service was first provided over a dedicated "dry loop", but when the FCC required ILECs to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over UNE), this allows a single pair to carry data (via a DSLAM) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephones. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.
Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km (1.25 miles) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. Some customers, however, are located farther than 2 km (1.25 miles) from the central office, which significantly reduces the amount of bandwidth available (thereby reducing the data rate) on the wires.
Joe Lechleider at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling.
US telephone companies promote DSL to compete with cable modems. DSL service was first provided over a dedicated "dry loop", but when the FCC required ILECs to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over UNE), this allows a single pair to carry data (via a DSLAM) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephones. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.
Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km (1.25 miles) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. Some customers, however, are located farther than 2 km (1.25 miles) from the central office, which significantly reduces the amount of bandwidth available (thereby reducing the data rate) on the wires.
DSL (Digital Subscriber Line)
DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, although in recent years, many have adopted digital subscriber line as a more marketing-friendly term for the most popular version of DSL, ADSL.
Typically, the download speed of DSL ranges from 640 kilobits per second (kbit/s) to 3,000, or exceptionally from 128 to 24,000 kbit/s depending on DSL technology and service level implemented. Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for the rarer Symmetric Digital Subscriber Line (SDSL).
Virtual ISP
A Virtual ISP (vISP) purchases services from another ISP (sometimes called a wholesale ISP or similar within this context) that allow the vISP's customers to access the Internet via one or more Points of Presence (PoPs) that are owned and operated by the wholesale ISP. There are various models for the delivery of this type of service, for example, the wholesale ISP could provide network access to end users via its dial-up modem PoPs or DSLAMs installed in telephone exchanges, and route, switch, and/or tunnel the end user traffic to the vISP's network, whereupon they may route the traffic toward its destination. In another model, the wholesale ISP does not route any end user traffic, and needs only provide AAA (Authentication, Authorization and Accounting) functions, as well as any "value-add" services like email or web hosting. Any given ISP may use their own PoPs to deliver one service, and use a vISP model to deliver another service, or, use a combination to deliver a service in different areas. The service provided by a wholesale ISP in a vISP model is distinct from that of an upstream ISP, even though in some cases, they may both be one and the same company. The former provides connectivity from the end user's premises to the Internet or to the end user's ISP, the latter provides connectivity from the end user's ISP to all or parts of the rest of the Internet.
A vISP can also refer to a completely automated white label service offered to anyone at no cost or for a minimal set-up fee. The actual ISP providing the service generates revenue from the calls and may also share a percentage of that revenue with the owner of the vISP. All technical aspects are dealt with leaving the owner of vISP with the task of promoting the service. This sort of service is however declining due to the popularity of unmetered internet access also known as flatrate.
A vISP can also refer to a completely automated white label service offered to anyone at no cost or for a minimal set-up fee. The actual ISP providing the service generates revenue from the calls and may also share a percentage of that revenue with the owner of the vISP. All technical aspects are dealt with leaving the owner of vISP with the task of promoting the service. This sort of service is however declining due to the popularity of unmetered internet access also known as flatrate.
How ISPs Connect to the Internet
Just as their customers pay them for Internet access, ISPs themselves pay upstream ISPs for Internet access. In the simplest case, a single connection is established to an upstream ISP using one of the technologies described above, and the ISP uses this connection to send or receive any data to or from parts of the Internet beyond its own network; in turn, the upstream ISP uses its own upstream connections, or connections to its other customers (usually other ISPs) to allow the data to travel from source to destination.
In reality, the situation is often more complicated. For example, ISPs with more than one Point of Presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and have connections to each one at one or more of their PoPs. ISPs may engage in peering, where multiple ISPs interconnect with one another at a peering point or Internet exchange point (IX), allowing the routing of data between their networks, without charging one another for that data - data that would otherwise have passed through their upstream ISPs, incurring charges from the upstream ISP. ISPs who require no upstream, and have only customers and/or peers, are called Tier 1 ISPs, indicating their status as ISPs at the top of the Internet hierarchy. Routers, switches, Internet routing protocols, and the expertise of network administrators all have a role to play in ensuring that data follows the best available route and that ISPs can "see" one another on the Internet.
In reality, the situation is often more complicated. For example, ISPs with more than one Point of Presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and have connections to each one at one or more of their PoPs. ISPs may engage in peering, where multiple ISPs interconnect with one another at a peering point or Internet exchange point (IX), allowing the routing of data between their networks, without charging one another for that data - data that would otherwise have passed through their upstream ISPs, incurring charges from the upstream ISP. ISPs who require no upstream, and have only customers and/or peers, are called Tier 1 ISPs, indicating their status as ISPs at the top of the Internet hierarchy. Routers, switches, Internet routing protocols, and the expertise of network administrators all have a role to play in ensuring that data follows the best available route and that ISPs can "see" one another on the Internet.
Sunday, January 7, 2007
An Internet service provider
An Internet service provider (abbr. ISP, also called Internet access provider or IAP) is a business or organization that provides to consumers access to the Internet and related services. In the past, most ISPs were run by the phone companies. Now, ISPs can be started by just about any individual or group with sufficient money and expertise. In addition to Internet access via various technologies such as dial-up and DSL, they may provide a combination of services including Internet transit, domain name registration and hosting, web hosting, and colocation.
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