Index


Telecommunications access technologues page

    ISDN

    ISDN is a digital telephone line which allows normal telephoneoperation and data communications at speeds of 64 kbit/s and128 kbit/s using normal home telephone wire. Using the same copper phone lines that modems use, ISDN delivers a considerable speed improvement (up to 128 Kbps) and provides essentially perfect transmission reliability. And ISDN can mesh into other digital technologies, such as Frame Relay and ATM, making possible future speeds several times higher even than 128 Kbps.The "Integrated" part of ISDN's name refers to the combining of voice and data services over the same wires (so computers can connect directly to the telephone network without first converting their signals to an analog audio signal, as modems do). This integration brings with it a host of new capabilities combining voice, data, fax, and sophisticated switching. And because ISDN uses the existing local telephone wiring, it's equally available to home and business customers.

    Most important for Internet users, however, is that ISDN provides a huge improvement in access speed at only a fractional increase in cost (at the time it was introduced). ISDN service is available today in most major metropolitan areas and almost throughout the country in USA and in most European countries. Many Internet Service Providers (ISPs) sell ISDN access (at least have solce). The ISDN connection price depends on your local telephone company, equipment budget, and ISP. An Internet ISDN connection consists of three componenents: the ISDN line itself, the equipment (an ISDN TA and possibly an NT1), and the ISP's fees.

    ISDN provides a raw data rate of 144 Kbps on a single telephone company (called telco in the business) twisted pair. To better suit voice applications, this 144 Kbps channel is partitioned into subchannels: two 64 Kbps B (for bearer) channels and one 16 Kbps D (for data) channel. Each B channel can carry a separate telephone call and usually has its own telephone number, called a Directory Number (DN). You can combine the two B channels together to form a single 128 Kbps data channel through a process called bonding (more on that later). The B channels carry customer voice or data signals. The D channel carries signals between your ISDN equipment and the phone company's central office. The two bearer plus one data channel is called the Basic Rate Interface (BRI) in telco lingo, or sometimes just 2B+D for short.

    To connect to an ISDN network, you need a terminal adapter (TA). A typical TA for data-only applications might simply emulate a pair of ordinary (albeit very fast) Hayes-compatible modems, translating standard modem setup and dialing commands into ISDN call-setup commands. Depending on the type of TA this can be connected to RS-232 port, USB port or your computer bus (PCI bus). An example of a more sophisticated TA is the ISDN router, which connects to an ISDN line on one side and your office or home LAN on the other. An ISDN router can carry your network traffic through ISDN network to another place in ISDN network (for example to your ISP or other office etc.).

    Because ISDN is purely digital, the telco can more easily deliver data intact from end to end, largely eliminating the effects of noise. And because the 64 Kbps channel is essentially a pure "bit pipe," with no rate negotiation or handshaking involved, there are no modem speed or protocol differences to cause conflicts. In fact, because the negotiation phase with ISDN is so simple, ISDN takes only a second or two to dial and establish a connection (modems may take as long as a minute to accomplish the same thing). These benefits alone are worth the cost of two high-speed modems, which is about what a bare-bones TA costs.

    Some protocols related to ISDN:

    • V.110 Signaling: V.110 signaling is a form of ISDN rate adaptation. V.110 is a fixed-frame based rate adaptation standard that subdivides the ISDN channel capacity so it can carry one lower speed (sub-rate) data channel. V.110 signaling supports asyncronous speeds of 9600, 19200, and 38400 Kbps. This standard is rarely used in North America.
    • V.120 Protocol: This protocol allows ISDN modems to transfer files using familiar protocols such as X-, Y-, and Zmodem. V.120 can only make a connection on one of the two data-carrying channels at a time over ISDN, so it is limited to a maximum throughput of 64 kbps. It is mostly used to connect to ISDN-capable BBSs. Connecting to an Internet service provider over ISDN uses PPP or MPPP rather than V.120 to establish communications.
    • MPPP: MPPP (multilink point-to-point protocol) is a standard communications protocol used over ISDN to bond separate data-carrying B channels together to transfer data effectively through a larger "pipe." Just as they can under PPP, dissimilar devices can communicate over multilink PPP connections to access the Internet. MPPP also allows both channels to be used for either voice or data transmissions and supports dynamic bandwidth allocation. This means that one of the two channels can be automatically dropped and reallocated for a phone call when calls come in. Once a call has been completed, the channel can be reconnected to continue data transfer over MPPP.
    • PPP: PPP (point-to-point protocol ) is the Internet standard for serial communications. PPP defines how your modem connection exchanges data packets with other systems on the Internet.

    ISDN system defines ISDN reference points, labeled R, S/T, and U. Each interface point requires an electrically different device connection and cabling. The U reference point is the incoming unshielded twisted pair (UTP); the S/T reference point is a four-wire UTP cable used inside buildings.

    What is meant by a "point to point" ISDN line? The U interface is a good example of a point to point link. The exchange and NT1 are the only devices connected to the pair of wires. This can also be said for the S interface, where the NT1 and only 1 TE are connected. What is meant by a "point to multi-point" ISDN line? This is where one device is connected to many devices. For example the single NT1 is connected to several TE's (Telephone, fax machine, and computer for example).

    Why does the U interface only require 2 wires and the S interface 4 wires? The U interface is design for point to point connection only (from the local exchange to the NT1). Full duplex data transmission and reception is made possible by the use of advanced echo cancellation. The S interface has been designed to allow multiple TE's to connect at the S interface up to a maximum of 8 devices. This requires the use of a pair of wires for transferring data from the NT1 to the TE's, and another pair of wires to transfer the data from the TE's to the NT1. The wiring used for S interface generally has four wire pair, and those spare pairs are generally used for supplying power to ISDN devices that need more operating power than what is available through the main data pairs suppled through the ISDN wiring.

    ISDN U-loop wiring: ISDN Basic Rate Interface (BRI) is provided by a carrier from a central office (CO) switch to the customer premise with a two wire U-loop RJ-45 connector on the center pins 4-5 (or through other kind of traditional telephone outlet through same pins as normal PSTN signal used to be wired to).

             RJ45 Plug
             =========
             1  N/C
             2  N/C
             3  N/C
             4  U-loop network connection
             5  U-loop network connection
             6  N/C
             7  N/C
             8  N/C
    

             RJ45 Plug for U+PS2
             ===================
             1  N/C
             2  N/C
             3  N/C
             4  U-loop network connection
             5  U-loop network connection
             6  N/C
             7  -48 VDC
             8  -48 VDC Return
    
    

    ISDN S/T Bus: The S/T bus connects the NT1 with the terminal equipment. 4-wires are needed on the S/T bus to carry the signals between equipment.

    There are several ways to wire this bus:

    • ISDN S/T Bus (Point-to-Point): One logical terminal is on the S/T bus which can be 1km long.
    • ISDN S/T Bus (Short Passive): Up to eight terminals on the S/T bus which can be within 100 to 200m.
    • ISDN S/T Bus (Extended Passive): Up to eight terminals on the S/T bus which can be up to 500m.
    • ISDN S/T Bus (NT1 Star) Up to eight terminals on the S/T bus which are wired from a central NT1 and can be up to 1km in length each.

             RJ45 Plug for ISDN S/T bus
             ==========================
             1  N/C
             2  N/C
             3  White/Green .....  Receive +
             4  Blue ............  Transmit+
             5  White/Blue ......  Transmit-
             6  Green ...........  Receive -
             7  White/Brown .....  -48VDC (option)
             8  Brown ...........  -48VDC Return (option)
    
    
    Note, if power is not required an RJ11 (6-pin) plug could be used.

    The ISDN cables can be silver satin patch cables (the kind that make 10Base-T Ethernet installers cringe). The S/T bus can also be silver satin but most installers use CAT 3 or CAT 5 with one drop per terminal equipment. It is true that only 4-wires are needed on the S/T bus but see below for optional power needs.

    There are three ways to power ISDN devices through the S interface:

    • PS1: Power source 1 is the power that is normally fed as phantom power across the S interface from the NT1 to TE's. It is fed across pins 3, 4, 5 and 6 of the RJ45 connection.
    • PS2: Power source 2 is the power that can be fed directly across the S interface from the NT1 to TE's. It is fed across pins 7 and 8 of the RJ45 connection. It is generated by feeding the NT1 with local mains derived 40V DC power, which the NT1 then distributes out to the TE's.
    • PS3: Power source 3 is the power that can be fed directly across the S interface from TE to TE's OR from TE to NT1. It is fed across pins 1 and 2 of the RJ45 connection.

    Normally powered S interfaces are able to supply up to 8W, and the voltage on the S interface can vary between 24V and 42V. Restricted powered S interfaces are able to supply up to 420mW, and the voltage on the S interface can vary between 32V and 42V. Some NT1 devices have a switch to turn off power if it is not required by the terminal equipment. For safety reasons the power should not be put on the S/T bus if it is not required and your equipment has option to turn it off. Typically, ISDN PC cards do not require power from the S/T bus, but ISDN telephones do require power from the S/T bus. Check your vendor equipment specifications carefully.

    The ISDN devices on the same bus are identified with a special TEI code. Terminal Endpoint Identifier (TEI) is a number assigned to a TE so that the local exchange can differentiate between different TE's which are connected at the S interface. The typical TEI number range os 0-63. Automatic TEI refers to the way in which a TEI is assigned by the local exchange to a particular TE (Terminal Equipment) that is requesting a TEI. The TE sends a management message to the local exchange requesting a TEI, the local exchange allocates a TEI to the TE, then the TE checks communication with the local exchange on the allocated TEI. Auto TEI is usually used on point to mutli-point links, but also can be used on point to point links. In fixed TEI system the TEI (0-63) is programmed into the TE. The corresponding TEI needs to be programmed into the local exchange for it to cummunicate with the TE. Fixed TEI is usually used on point to point links, but can also be used on point to mutli-point links.

    xDSL (digital subscriber loop)

    DSL (Digital Subscriber Line) is a new, digital data-connection method that allows high-speed Internet connections over standard telephone lines. A standard telephone infrastructure around the world consists of a pair of copper wires that the phone company installs in your home. A pair of copper wires has plenty of bandwidth for carrying data in addition to voice conversations. Voice signals use only a fraction of the available capacity on the wires. DSL exploits this remaining capacity to carry information on the wire without disturbing the line's ability to carry conversations. To use DSL, a DSL modem or DSL router is required. They work with the same single-copper-wire pair that telephone services use, but they contain sophisticated digital signal processors that take advantage of a much greater range in the frequency spectrum. The result is much higher bandwidth capability than standard telephone service and modem combinations.

    ADSL is a DSP-based communications technology that can dramatically increase the speeds of data communications over the typical copper wiring that connects most homes and businesses to the public telephone network. DSL is implemented using high frequency signals "piggybacked" on one of your existing telephone lines (those can coexist in same wire at the same time).ADSL can transfer daya up to megabits per seconds speed and is particularly well suited to Internet-related communications.DSL normally only works within a certain distance (typically 16000 feet) of the telephone company central office and is therefore unusable for many rural and semi rural customers. Is usually provided as "always on" service. ADSL systems employ a discrete multi-tone (DMT) modulation technique that features carrier tone frequencies located at multiples of the baud frequency: n * 4.3125 kHz. In ADSL systems, downstream data (from a central office to a remote terminal) may use tones n = 6,..., through 256. Upstream data, on the other hand, may use tones, n = 6,..., through 32. As specified by the standards bodies, an ADSL modem's bandwidth is 25.875 to 1,104 kHz downstream and 25.875 to 138 kHz upstream. The overlap of upstream and downstream bands is left to the discretion of the modem vendor.

    ADSL (asymmetrical DSL) is the most commonly technology used to provide broadband Internet connections to homes and small offices. This technology offers potential to up to 6..8 Mbit/s downlink (256k or 512k being typical speeds used) and uplink speed up to around 700 bkit/s (typically 256k or 368k). ADSL service is typically provided as always-on service. The ADSL modem box connects to your computer via USB port or Ethernet Network Interface Card (NIC). For some systems you have also an option to buy an ADSL modem in a form of a PCI card which plugs inside your PC.

    ADSL technology can coexist in the same line with normal analogue telephone line (PSTN). Normally ADSL system cannot coexist with digital technologied like ISDN on the same line (there are versions called Annex B and Annex C that can do that). When the same line is used for both ADSL and PSTN, you typically need to add some special filters between the line and PSTN devices to filter out the ADSL high frequency signals from entering those devices. ADSL can not coexist on the same line with ISDN system because they operate at same frequency range (there are some other DSL systems which can do this). ADSL connections are always built between the terminal device on the user premises (ADSL modem) and the central office device (ADSL DSLAM). ADSL modems can't be directly connected to eahc other. Generally when the same line is shared with normal telephone service and ADSL, there is need to use ADSL splitters that isolate ADSL signals and telephone signals from disturbing each other. The ADSL splitter stops the ADSL actually breaking into the voice part of your phone line - this sounds a bit like fast clicks when it happens. The second reason is to ensure that the ADSL line is terminated correctly at the frequencies ADSL operates. If the line is not correctly terminated, this can cause transmission errors and communication reliabity problems. ADSL splitters are needed only when the same line is shared with ADSL and normal telephone line. Splitters are not needed when one line carriers only ADSL signals.

    Besides ADLS, there are many related technologues like HDSL, HDSL2, ADSL, RADSL, VDSL and many other which are commonly usually called as DSL or xDSL technologies. The performance (data rate and error rate) of DSL depend on used wiring conditions (line length, line noise etc.) and the used DSL technology. Symmetrical DSL (SDSL) is a technique which transports fast digital signals through telephone line wiring. SDLS system transfers the data at the same speed to both directions (typically from 256 kbps to few megabits per second) through telephone line wiring. Two suitabe SDLS modems can be connected to each other through some kilometers (typically below 5-10 km) of telephone line (leased line).The newest competitor in the field is VDSL. VDSL stands for very high bit-rate DSL. It is seen by many as the next step in providing a complete home-communications/entertainment package. VDSL provides an incredible amount of bandwidth, with speeds up to about 52 megabits per second (Mbps). VDSL's amazing performance comes at a price: It can only operate over the copper line for a short distance, about 4,000 feet (1,200 m). The current (year 2002 end) products seem to be still limited to around 15 Mbit/s speeds.There are also many other DSL technologies. Here is a short list of techologies you might encounter:

    • Asymmetric DSL (ADSL): This is called "asymmetric" because the download speed is greater than the upload speed. ADSL can provide up to 8 Mbps downstream and 800 Kbps (kilobits per second) upstream. This technology works up to distance of few kilometers (but not full speed there). Typical ADSL services sold by operators for home are 256 kbit/s, 512 kbit/s or 1 Mbit/s downstream (and usually somewhat less upstream). ADSL can work at the same time with normal telephone on the same wire pair. The maximum supported distance is 18 000 feet (5500 meters).
    • G.Lite Technology is is similar to full rate ADSL but operates at a lower data rate of up to 1.5Mbps downstream and 512Kbps upstream, depending on line conditions and lengths. This ADSL version will eliminate the need for telecom to install and maintain a premises based POTS splitter, meaning it can coexist with existing phones without special filtering, at least in theory. In some cases low pass filters in series with the POTS terminals are needed in order to reliably achieve maximum data rates.
    • High bit-rate DSL (HDSL): This technology provides transfer rates comparable to a T1 line. This technology uses two pairs of wire separate from normal telephone line. The operating speed is 1.54 Mbps to both directions. Maximum supported distance is 12 000 feet (3650 meters).
    • ISDN DSL (ISDL): This based on ISDN technology and is operating at fixed rate of 144 Kbps in both directions. The supported line lenght is up to 35 000 feet (10700 meters)
    • Multirate Symmetric DSL (MSDSL): This is Symmetric DSL that is capable of more than one transfer rate (rate usually defined by service provider). The supoorted speed is up to 2 Mbps to both direction. Distances are supported up to 29 000 feet (8800 meters). This technology does not support telephone on the same wire pair.
    • Rate Adaptive DSL (RADSL): This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line. The supported speed is up to 7 Mbps download and 1 Mbps upload. Distances are supported up to 18 000 feet (5500 meters)
    • Symmetric DSL (SDSL): This system receives and sends data at the same speed. This system works using one wire pair (but does not allow telephone on the same line). This is often used for high speed data links using leased lines. SDSL supports data speed of 2.3 Mbps to both directions. Distances are supported up to 22 000 feet (6700 meters).
    • Very high bit-rate DSL (VDSL): An extremely fast connection technology with speed up to 52 Mbit/s. VDSL is asymmetric. The supported speed is up to 52 Mbps download and 16 Mbps upload. VDSL can only operate to quite short distances, up to about 4,000 feet (1,200 m). VDSL support normal telephone on the same line. VDSL is used in connection with fibre to the neighborhood.
    • VDSLPlus: An extension to VDSL to use also higher frequencies (frequencies over 12 MHz) for data transfer. This new technology is developed by Infineon and Metalink. It can provide theoretically transmission speeds up to 150 Mbit/s. VDSLPlus is claimed to be backward compatible with existing VDSL products (works with them with the speeds that they support).
    • Universal ADSL (ADSL): This ADSL technology provides a data rate up to 1.5 Mbps downstream and upstream up to 384 kbps. The essential difference between ADSL is that DSL Lite doesn't require a splitter at the user end.

    Here is a comparision of the maximum speeds available with most popular xDSL technologies:
    TechnologySpeed to customerSpeed from customer
    ADSL8 Mbit/s1 Mbit/s
    ADSL212 Mbit/s1 Mbit/s
    ADSL2+24 Mbit/s3 Mbit/s
    VDSL52 Mbit/s30 Mbit/s

    DSL is a distance-sensitive technology: As the connection's length increases, the signal quality and connection speed decrease. ADSL service has a maximum distance of 18,000 feet (5,460 m) between the DSL modem and the DSLAM, though for speed and quality of service reasons, many ADSL providers place an even lower limit on the distance. The maximum distance limit is set as the designated maximum distance for the system, the distance where the system woudl work on "normal lines". In some special cases it could be possible to get even somewhat longer distances when line is good, but the connection reliabity would not be the best possible (because we are pushing the limits). At the upper extreme of the distance limit, ADSL customers may experience speeds far below the promised maximums, whereas customers close the central office or DSL termination point may experience speeds approaching the maximum, and even beyond the current limit in the future. You might wonder why, if distance is a limitation for DSL, it's not a limitation for voice telephone calls, too. The reason for this is that DSL technologies use much higher bandwidth (higher frequency) signals which attenuate more than voice signals.

    With ADSLs systems quite often devices called ADSL filters needs to be installed. The filter at home should filter the phones so that they dont short-circuit the ADSL signal. So, the ADSL adaptershould be directly connected to the incoming POTS line, and the old phones through the filter. The typical installations have one filter for each phone.It is always possible to use only one filter for all phones, but then the filter has to be installed before any branching of the line.The ADSL filter is not compulsory in all cases. If the system seems to work without, then fine. In some cases the ADSL signal can be heard as audible noise. Also, if the filteris omitted, the ADSL line may not work when the handset is nothooked. This may be checked by looking at the ADSL modem signallevels. Especially Ethernet-connected modems tend to offerthe signal level information. If the signal levelsdo not rise when the handset is lifted, then everything isfine. The reason that ADSL signals can be heard on some phones as noise, even though ADSL and POTS operate at different frequencies is the following: The ADSL signals need to be rather close to the POTS signals, as higher frequencies attenuate faster in the phone line. Most normal telephones are not much designed how they react to the freequencie above normal POTS frequescies, and some phones can pass through / demodulate those higher frequencies as signal that canb be heard from phone. Installing the ADSL filter will stop those high frequency signal going to phone, so will stop this problem completely or reduce it so much that it does not do any harm.

    On the ADSL line there is a large number of settings that need to be correct for the things to work. For many parameters the operator has the choise to select the settings bets suit for situation. The ADSL connection generally works so that the operator configures the DSLAM to scertain settings and standards (doe example ADSL standard, maximum speed, etc.). The customer end ADSL modem then makes the connection and auto-configures to the setting suitable to communicate with DSLAM (some settins might need to be set manually to ADSL modem). Sometimes you might hear terms 'FastPath' and 'Interleaving' related to ADSL settings. Interleaving is an error correction protocol that is implemented for your line at the DSLAM. With Interleaving enabled, the DSLAM can correct errors in the data stream it receives before passing that data to your gateway router. It is usually implemented on noisy or marginal lines and can greatly increase sync stability and effectively eliminate "first hop" packet loss. The largest drawback to Interleaving is that it will significantly increase your ping time, specifically to your first hop gateway router. If Interleaving is not enabled on your line, it is configured as FastPath. FastPath allows the DSLAM to pass the data received from you to the first hop router without performing any error correction. As a result, marginal lines could experience an increase in packet loss and decrease in sync stability (i.e. frequent sync loss). However, FastPath does produce decreased ping times, especially to the first hop router. Your access prodiver company is responsible for DSL from FastPath to Interleaved if that is needed.

    Cable TV data

    Many traditionally one-way cable TV networks have nowadays converterto two-day data highways by adding two way operation to the cable TVnetwork amplifiers and connecting cable modems to the cable TV network.Cable modems are devices that allow high-speed access to the Internet via a cable television network. While similar in some respects to a traditional analog modem, a cable modem is significantly faster. There are different cable modem systems in use.

    Some cable companies have "one-way" cable modem service. In this system, communications in the down direction is by cable but the return path is by conventional telephone line and telephone modem (33 Kbps).

    Two way cable systems transmit data in both directions via cable and therefore do not need a telephone line. Two way systems need two way cabel TV network to work on. In two way cable network the cable networks is designed to work in direction fro headend to to customer at normal TV and radio frquencies (typically from 65 MHz to 600..900 MHz frequencies). The downstream data is transfered on this frequency range. The lower frequencies (typically from 5 Mhz up to 40..65 MHz) are used fro communications from the subscibers to head end.

    In cable TV data system uplink speeds are typically higher than 56K modem but not as high as downlink speeds. Downlink speeds are typically at least several hundred kilobits per second. Cable modem service is typically provided as always-on service. The modem box connects to your computer via USB port or Ethernet Network Interface Card (NIC).

    A "Cable Modem" is a device that allows high-speed data access (such as to the Internet) via a cable TV network. A cable modem will typically have two connections, one to the cable wall outlet and the other to a computer (PC). Most cable modems are external devices that connect to the PC through a standard 10Base-T Ethernet card and twisted-pair wiring. External Universal Serial Bus (USB) modems and internal PCI modem cards are also available. Nowadays it seems that 10/100Base-T Ethernet is the most commonly used connection method.

    Cable modem speeds vary, depending on the cable modem system, cable network architecture, and traffic load. An asymmetric cable modem scheme is most common and is specified in the DOCSIS, EuroDOCSIS and DVB EuroModem standards. The downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream.The dominant service offered by cable modem is high-speed Internet access.

    Many cable TV operators are packaging high-speed data services much like they do basic cable television service: Internet service package that includes software, unlimited Internet access, specialized content and rental of a cable modem.

    A cable modem can generally provide Intenet access to multiple PCs, assuming they are connected via a local area network (Ethernet LAN). In this configuration each PC must have an assigned IP address, which the cable ISP ususally sells. Other alternative is to connect all PCs to cable modem through a connection sharing device/software (NAT technology). If you have two or more computers in your home or small office you can ask the cable company to wire up the additional computers with cable modems for an additional (but smaller) monthly fee. Many cable modem companies offer this kind of service. Other option is to share the connections yourself by using special network connection sharing box or using a computer for this (Windows 98 SE or later Windows version you can share a single cable modem connection, also Linux can do this job very well). To be able to use this network connection sharing, your computer need to be networked together.

    Cable modem systems are generally implemented as a very asymmetrical system. There is fast download and slower upload. All the network traffic is controlled by the operator system (it gives different modems rights to transmit and control what they receive). The cable modem systems are generally built so that the user cable modem can only communicate with the access router in the operator premises (this device then forward yout traffic to where-ever you want to communicate and the operator policy allows).

    Nowadys a modern typical cable TV data connection looks like an Ethernet LAN connection to the end user. But not all connections are like that. There are many cable companies are offering ppp connections via the TV cable connection. Unfortunately many of the cable companies have bought onto a bastardisation called PPP over Ethernet (PPPoE). Why one should want to layer a ppp connection over something which already provides a layer which can handle any of the higher layers (Ethernet) is mysterious, but as consumers with monopoly cable suppliers we often have no say in the matter.

    Powerline communications

    Powerline Carrier (PLC) is a communication technique thatuses the existing power wiring (120 Volts, 240, etc) to carry information. It is kind of "wireless" means of communication, because PLC technology can supersede the installation of dedicated wiring in some applications. Various applications use PLC technology. PLC applications range from power company equipment controlling to computer networking.

    The two main reasons there will be communications problems with PLC transmissions are LOW SIGNAL LEVEL and NOISE. Power lines and their associated networks are not designed for communications use. They are a hostile environment that make the accurate propagation of communication signals difficult. Two of the biggest problems faced in using power lines for comunications are excessive noise levels and cable attenuation. Noise levels are often excessive, and cable attenuation at the frequencies of interest is often very large. The most common causes of excessive noise in a domestic situation are the various household devices and office equipment connected to the network. Noise and disturbances on the power network include overvoltages, undervoltages, frequency variations and so on. A lot of superimposed noise is often caused by switching devices such as light dimmers, the electric motors in many common apppliances, and high-frequency noise caused by computer monitors and televisions. For example vacuum cleaners, hand-held drilling machines etcetera which use univeral series wound motors generate a lot of impulse noise to power line. TV-sets are a very common source of distortion. Light dimmers are also a source of mains noise. Signal attenuation in the power line environment is often great, and unpredictable. Attenuation has been measured at up to 100dB/km. Furthermore, it is very difficult to obtain a meaningful model of the communications channel, due to the time-variance of this channel. As devices are connected and disconnected from the power network, network characteristics change drastically. For a power line carrier communications system to perform reliably it must be able to avoid, or cope with, the different types of noise encountered on its communications channel. These different types of noise all exist at different frequencies, and occur at unpredictable times. The system must also cope with the changing characteristics of the communications path. Everything can change over the frequency range, both the attenuation, phase response and noise level. The powerline enviroment is hard enviroment for any communication.

    Typical frequency ranges used in powerline communication is from 30 Khz to 150 KHz. In Europe mains power line communication is standardized at 1991 in EN 50065-1 standard.EN 50065-1 is defined to standardize signaling on low-voltage electrical installations in the frequency range 3 kHz to 148,5 kHz. It gives general requirements, frequency bands and electromagnetic disturbances. The frequency range of EN 50065-1 is split to four different frequency bands. In the A-band, the carrier signal can be from 9KHz to 95KHz where electricity suppliers and their licensees are permitted to communicate.Powerlines can also be used for other applications,The C-band is for consumer use with an access protocol. This band goes from 125KHz to 140KHz. Between the A-band and C-band is the B-band, used for consumer use without an access protocol so this band has some freedom of communication. Devices can interfere with one another and baby alarms use this band. Above 150KHz, communication is prohibited in most parts of Europe.

    The situation in USA is some what different. Traditional powerline-communication systems (both narrowband and spread-spectrum) use carrier frequencies below the medium-wave (MW) broadcast band (that is, below ;500 kHz). The oldest commercial powerline-communication system, X-10 USA's X-10 home-control system (also sold under several other brand names) uses an amplitude-modulated, 120-kHz carrier. Popular "wireless" intercom systems and some data communications devices for examaple operate at around 150kHz..500 Khz frequency range. There are also systems that use higher frequencies. Research by many companies has shown that somewhere above the MW band, there is an area of the spectrum is relatively free of powerline noise, but still low enough to keep the power line from acting as a good antenna. Elcom Technologies has for example used the following frequenciers: FM at 3.58 or 4.5 MHz to route audio from a stereo system to loudspeakers in different rooms, telephone modules use FM at frequencies from 5.5 to 6.5 MHz, TV channels 3 and 4 (60 to 72 MHz) for video signals and a proprietary modulation scheme operating from 120 to 450 kHz, lets you use your home's ac wiring as the transmission medium of a LAN.

    HomePlug Powerline Alliance efforts for HomePlug BPL, a to-the-home specification that standardizes the technology used for delivering Internet access to homes through Broadband Powerline (BPL) access, and HomePlug Command and Control, which enables advanced, whole-house control of lighting, appliances, climate control, security and other devices. HomePlug 1.0 is the specification for a technology that connects devices to each other through the power lines in a home. HomePlug-certified products connect PCs and other devices that use Ethernet, USB and 802.11 "Wi-Fi" technologies to the power line via a HomePlug "bridge" or "adapter". The HomePlug alliance validated its HomePlug 1.0 powerline networking technology through an extensive field trial of 500 homes throughout North America. HomePlug 1.0 promises to give around 10 Mbps data rate (about 8.2 Mbps real maximu), whole-house coverage, robustness and ease of implementation. HomePlug 1.0 products overcome power line noise and channel quality challenges by using an adaptive approach that uses robust transmission technique combined with sophisticated forward error correction (FEC), error detection, data interleaving, and automatic repeat request (ARQ). The OFDM used by HomePlug is specially tailored for powerline environments. It uses 84 equally spaced subcarriers in the frequency band between 4.5MHz and 21MHz. Cyclic prefix and differential modulation techniques (DBPSK, DQPSK) are used to completely eliminate the need for any equalization. Impulsive noise events are overcome by means of forward error correction and data interleaving. HomePlug payload uses a concatenation of Viterbi and Reed-Solomon FEC. Sensitive frame control data is encoded using turbo product codes. HomePlug AV is the next generation of powerline technology. HomePlug AV will use frequencies in the range of 2 to 28 MHz. HomePlug AV uses an OFDM PHY with advanced FEC, channel estimation and adaptation. HomePlug BPL is the Alliance's specification for to-the-home powerline communications technologies.

    Three different techniques have been traditionally used for mains power line communications.

    • With a PLL (Phase Looked Loop), the transmission could be performed with Amplitude Shift Keying, Frequency Shift Keying or Phase Shift keying. This is a well known technology that is inexpensive, but it's performance is limited. In short, the two best options from those modulations are frequency shift keying (FSK) and phase shift keying (PSK).
    • A second technique is based upon Spread Spectrum with a correlator. Here a transmitted signal occupies a bandwidth considerably bigger than the minimum necessary to send the information so it has to be spread and modulated. The receiver has to know the transmitted pattern and the signal is sampled regularly. In practical applications the bandwidth available is a threshold because of the Cenelec regulations. Low bandwidth increases susceptibility to signal distortion so it needs correction (TV-sets are a very common source of distortion).
    • The third technique incorporates a DSP working in a narrow band and using dual carrier frequency operation mode with impulse noise cancellation and an adaptive distortion correction mechanism.

    A unique challenge of power line carrier communication systems is the method used to couple the communications signal onto the power network. In the receive direction we wish this coupling network to possess strong band-reject properties, blocking the mains power (230V 50 Hz or 110V 60 Hz) signal, but passing the high-frequency communications signal unattenuated. In the transmit direction we wish the coupling network to have wide-pass properties, passing the transmitted signal unattenuated. For low attenuation we wish the coupling network to be approximately impedance-matched to the power line. The general approach is to use a coupling transformer to isolate the mains system from the communications system. In many systems the coupling transformer is combined with a capacitor and resitor network to get a "tank circuit" of suitable frequency response.

    Powerline communication technology, which is the ability to transfer data overstandard AC wiring, has existed for many years. However, the technology has not yet been widely adopted for data networking in homes and small businesses due to high cost, low speed, low functionality, and other barriers. The currently commercially available PLC communication systems are typically low capacity, relatively simple systems designed primarily for home automation. The most popular low speed PLC communication systems are:

    • CEBus, or Consumer Electronics Bus has a PLC communications option in the standard. A binary digit in CEBus is represented by how long a tone is applied to the channel. CEBus uses a language of object oriented such as volume up/down, and so on. It is a commercially owned protocol.
    • X-10 is the most often used in terms of sheer popularity. X-10 is the de-facto world standard for home automation, mainly used in USA. Originally proposed in 1978, X-10 is a standard that provides certain specifications of how a device should place a signal onto the power line. Using the zero crossing point of the mains carrier for synchronisation, the presence of a 120 kHz signal burst at the zero crossing indicates the transmission of a binary one, whilst the absence of the 120 kHZ signal indicates a binary zero transmission. X-10 contains a detailed addressing scheme, to prevent device clash. The X-10 system is designed only for one way communications in mind (central controller commands different devices around house) and it is very slow, although adequate for simple home automation tasks.

    There has also been ssytems that have tried to use higher frequencies to cope with some of the problems related to lower frequencies and allow higher communication speed. There are numerous incompatible technologies trying to use the same AC wiring in a home or office in the same frequency range: the 2-30Mhz sweet spot, each targeting a specific application space.

    Broadband over Power Line (BPL) is a form of power line carrier (PLC) technology that uses existing low and medium-voltage power lines to deliver broadband services to homes and businesses. BPL proponents, primarily electric power utilities, are testing BPL systems in several markets. BPL uses frequencies between 2 MHz and 80 MHz. Radio amaterus fear that BPL could affect HF and low-VHF amateur allocations wherever it's deployed. The problem in using the open wire medium voltage lines is potential radio signals radiationg out of the wires. Electrical transmission lines were designed to conduct 50- or 60-Hz power from point to point. At those frequencies, power lines are excellent transmission lines and little of that 50- or 60-Hz power is radiated. The electric-utility industry also uses those lines at frequencies below 490 kHz to transmit Power Line Carrier signals to control utility equipment (some system use frequencies formk few hindred Hz up to few kHz for slow communications). Those power lines, however, were not designed to carry radio frequency energy. As the frequency of carrier-current signals conducted on powerlines is raised, the amount of signal radiated from the line increases rapidly. The gain of the power-line as a radiator increases rapidly with frequency. Aradiating conductor with relatively low emissions at 0.1 MHz can have emissions tens of dB higher at HF.

      General articles

      • Alliance Leverages Home Power Outlets To Network PCs And Consumer Appliances    Rate this link
      • ARRL Comments on ET 03-104, the FCC Notice of Inquiry on Broadband Over Power Line - This page has several technical documents on how power lines act like antennas when they darry high frequency signals.    Rate this link
      • BPL is "Spectrum Pollution," ARRL President Says - ARRL President Jim Haynie, W5JBP, says Broadband over Power Line (BPL)--if widely deployed--would represent "spectrum pollution" on a level that is "difficult to imagine." A form of power line carrier (PLC) technology, BPL would use existing low and medium-voltage power lines to deliver broadband services to homes and businesses. It uses frequencies between 2 MHz and 80 MHz.    Rate this link
      • Compatibility Between Radio Communications Services and Power Line Communication Systems    Rate this link
      • Communications via Domestic Mains Wiring Investigation for RA1/ERU - This system consists of a pair of units. The base unit is connected to the domestic mains supply via a normal 13A plug and the input line. The extension unit connects to the mains via a 13A plug and the terminal apparatus is plugged into this unit. The system uses a pair of frequencies to provide full duplex communication between the two units. The base station transmits on 8.2 MHz and the extension unit transmits on 3.33 MHz. The units only transmit when in use, although we were made aware of a test mode to assist measurements.    Rate this link
      • Home Automation and the Power Line Carrier - Today, lights and appliances are controlled by home security systems, desktop PCs and mini control modules that can be plugged into an AC outlet and left to run a pre-programmed schedule. Regardless of the controlling host panel, most of the devices used today rely on power line carrier technology. Power line carrier (PLC) uses existing power lines that supply AC to a home's receptacles and light fixtures. This article introduces you to this technology.    Rate this link
      • HomePlug Standard Brings Networking to the Home - Ethernet-class networks over standard home power links are coming, thanks to ASIC-based signal processing advances that keep a lid on the interference and transfer function degradations that compromise the power line transmission medium.    Rate this link
      • HomePlug: Every outlet a network port? - If power-line-networking vendors do it right, connecting your PC, printer, or stereo together will just "happen" when you plug the power cord in the wall. The idea of using your home's ac-power lines to exchange data between computers has been around for a while. The difference this time is that technology has caught up to the task. HomePlug is promising data rates as fast as 14 Mbps over your home's power lines.    Rate this link
      • HomePlug Standard Brings Networking to the Home - Ethernet-class networks over standard home power links are coming, thanks to ASIC-based signal processing advances that keep a lid on the interference and transfer function degradations that compromise the power line transmission medium.    Rate this link
      • Powerline Coexistence - Powerline communication technology, which is the ability to transfer data over standard AC wiring, has existed for many years. However, the technology has not yet been widely adopted for data networking in homes and small businesses due to high cost, low speed, low functionality, and other barriers. Those barriers are quickly being overcome but new challenges threaten the multifaceted PLC data networking platform, namely numerous incompatible technologies    Rate this link
      • Power Line Communications (PLC) and Amateur Radio - This web page or CD contains files and links of information about PLC and related broadband technologies and how they may adversely affect Amateur Radio and other HF radio operation. This represents the technical work of many of the International Amateur Radio Union (http://www.iaru.org) Amateur Radio societies. trying to use the same AC wiring in a home or office in the same frequency range: the 2 - 30 Mhz sweet spot, each targeting a specific application space.    Rate this link
      • Powerline communication: wireless technology - using AC power lines to transmit data eliminates the cost of installing special wiring between distributed system elements but still powerline communication doesn't suit every application    Rate this link
      • Powerline Technology From Echelon - To communicate on the mains, a very sophisticated modem is needed. There are three main problems areas that have to be resolved namely noise, attenuation and a signal distortion.    Rate this link
      • Power Line Carrier Communications Theory - This page provides a brief summary of the problems facing PLC communications, an overview of current systems, and explores the modern communications methods and research currently taking place in PLC technologies. The report focuses in particular on low (less than 1 kV) PLC technologies, that is, those that are applicable for domestic use.    Rate this link
      • Powerline Ethernet: When WiFi won't and CAT-5 Can't - Powerline Ethernet runs over residential power lines using a Carrier Sense Multiple Access with collision Avoidance (CSMA/CA) protocol to arbitrate the shared medium; a Physical layer designed for transmission over electrical wiring; and Orthogonal Frequency Division Multiplexing (OFDM) in a bursty transmission mode of operation appropriate for a shared Ethernet medium.    Rate this link

      Circuits

      • Digital Remote Thermometer - Remote sensor sends data via mains supply, temperature range 00.0 to 99.9 ?C    Rate this link
      • Passive filter cleans up power-line communication - The application for this design required simple and low-cost I/O filters for PLC (power-line communications), where low power consumption is a crucial factor.    Rate this link
      • Power-line Appliance Controller - student report and circuit    Rate this link
      • FM Remote Speaker System - A high quality, noise free, wireless FM transmitter/receiver may be made using the LM566 VCO and LM565 PLL Detector. The LM566 VCO is used to convert the program material into FM format, which is then transformer coupled to standard power lines. At the receiver end the material is detected from the power lines and demodulated by the LM565. The complete system is suitable for high-quality transmission of speech or music, and will operate from any AC outlet anywhere on a one-acre homesite. Frequency response is 20?20,000 Hz and THD is under 0.5% for speech and music program material. This circuit uses 200 kHz carrier frequency.    Rate this link

    Modems

    A telephone line modem is a device whicl allows to transfer digitaldata over analogue telephone network also known as PSTN.The takes in the digital data stream from computer(typically through RS-232 port, USB port or computer bus)and converts this to an analogue signal which can be sentover telephone network (300-3400Hz frequency response). On the other end of the line the modem receives those analoguesignals and covert this information back to digital data.This works in the same way to another direction.At slow sped this conversion can be made qite easily(just modulate some audio tone), but at higher speedsthis gets very complicated because of the need of complex modulation schemes, error coorrection andthe compensation of telephone line on-idealities (echo cancelation, line equalization etc.).

    There are many different modem standards. Here is a list of most common modem standards designed for operation over normal telephone lines:

    Modulation Transfer rate (in bits/sec)
    V.21 300
    V.22 1200
    V.22bis 2400
    V.27 4800
    V.32 9600
    V.32bis 14400
    V.32ter 19200
    V.34 21400, 24600, 28800
    V.34+ 28800, 33600
    V.90 56000 (down), 33600 (up)
    V.92 56000 (down), 48000 (up)

    Modem manufacturers like to advertise apparent speed, regardless of how fast their products actually pump data. When encountering these claims, keep in mind the following fact: for dial up modems, the fastest practical speed is about 28,000 bps. The 28,000 bps limit is a function of the public switched telephone network's (PSTN) signal-to-noise (S/N) ratio. Most of the U.S. PSTN has an S/N ratio of about 1000:1 for voice-grade lines, which (according to Shannon's law) yields a maximum data rate of about 30,000 bps. How, then, does one explain the 38,400, 57,600, and even 112,000 bps claims made by vendors? The answer is Data compression and some modems can work faster in ideal conditions. For example so called 56k modems can go beyond 30 kbps on one direction (download) when telephone line is very good, but the uplink is always limited to below 30 kbps.The ITU recommendation for 28,000 bps modems, called V.34, specifies a signaling standard designed to work reliably on most PSTN voice-grade lines.

    A separate recommendation, V.42, defines an error detection and correction protocol for modems that lets the modems themselves ensure reliable, error-free data transport. V.90 and V.92 are modem standard for newer 56k modems (capable of speeds up to 56 kbps in ideal line conditions). Please note that 56k is the advetised maximum speed that is very rerely reached. 46000 bps isn't bad for a V90. Remember that the 56000 bps advertised is atheoretical maximum depending on your modem, the quality of the line and onlocal legislation (in some places so ISPs and telephone operators cap the speed).

    Controlling the modem using software is sometimes necessary. Computers use AT commands to communicate with modems. You may issue AT commands via your communications application to control your modem. When the software inside modem has received an AT command, it responds with a message that is displayed on the screen of thedevice you are using to communicate with the modem. The name AT command come from the fact that the "AT" prefix must be included at the beginning of each command. The AT commands can be used to command the modem to do differentthigns (dial to specified number, disconnect line, make modem answer) and to change the modem configuration parameters.Standardized basic commands are found in V.25ter standard. Many modem manufacturers have manufacturer specific extra commands for controlling the manufacturer / modem model specific features of the modem. Most communications applications have user friendly interface that hides those AT command from the user.

    The external modems (those that connect to PC serial port)have a microcontroller inside them. It received the serial data coming to modem, the software running in it parses the AT commands and sends the response back to PC. This is how modems generally work. Older modems that plug inside PC also used to workexactly in the same way. Many modern cheap internal PC modems do not have anymore that modem microcontroller in them, andthe functions it normally does is implemented withthe modem driver. A WinModem is basically a modem which has been stripped of its key components. In actual fact, what you have is a crappy sound card with a phone jack on it. The Windows drivers supplied with it get your mainprocessor to do the job that the other components, were they present,should be doing. Remember that a couple of bucks saved on hardwaremultiplied by the number of units sold means huge savings which easily pay for the development of the Windows drivers. Since this job includes real-time signal analysis, the process doing it is time-critical. This adds an extra load on your processor, which already has enough processing power to do. The end result is a device which is only usable in Windows, which is going to be much more sensitive to the quality of your phone line and the audio signal being transmitted than a hardware modem would be, andwhich can very easily bring the operating system down in the event of asignal being particularly difficult to analyse. WinModems are pretty much limited to be used only with Windows operating system (some have even limited to certain Windows versions). Some intrepid Linux device driver developers have managed to produce Linux drivers for these devices, but those drivers have their limitations because the manufacturers of WinModem products don't generally disclose the technical specifications of their devices. Some people refrain from calling WinModems "modems" because they're not doing any signal modulation/demodulation, the system CPU isdoing that.

    Modems are often used by people to access Internet. The idea is that the user calls using modem to the ISP modem pool that connects them to Internet. The speed of modem based Internet access is always limites. Modem-speeding packages--and hype--have been around since Internet use became widespread. Web accelerators first came around years ago, and they didn't live up to the hype. Now TV commercials are advertising accelerators that speed up your dial-up connection by up to 5 times, they say. The new offerings work by storing Web pages and/or by compressing graphics and text, via a collaboration between their desktop software on your computer and smart caching of the Web pages on their own servers for quick page display. This kind of modem speeding services can speed up web browsing somewhat, but those services definitely can't compete with the raw capacity of cable or DSL connections when you're doing bandwidth-hogging tasks such as swapping music files or connecting to multiplayer games. You need to consider what you want, because using this kind of speeding service costs money.

    Satellite Internet

    In a similar way, satellite communications is dramatically changing the way we access the Internet. Areas with unreliable or non-existent terrestrial infrastructure can now be online in a matter of days, opening entire new markets to service providers. Virtually all major satellite operators either already offer or will soon offer Internet access services via existing hubs and GEO satellites. Satellite networks are a good media to deliver bulk data, anywhere and anytime. The way to use typical digital satellite broadcast stream to carry IP traffic is to use Multi-Protocol Encapsulation (MPE) standard (EN 301 192) to place the data to MPEG transport packets per ISO/IEC 13818-1 (MPEG Systems) and multiplexed together into an aggregate transport stream. The transport stream is encoded and modulated onto an IF carrier in accordance with the DVB-S standard, ETS 300 421. With this kind of system the same technology as used in digital satellite TV receivers can be used to receive the data. Satellite broadcasting in this way can easily carry the downlink, meaning the data from the Internet to the user computer. The data to opposite direction (web page requests, TCP protocol control data etc.) needs also be transported in some day. Here the different satellite system have large difference. Some system use a normal modem connection to handle the data transmission from user to Internet, some other systems use special satellite uplink systems. The downside of satellite Internet is the quite long delay from the Internet to use. This can be problematic to interactive services. Satellite internet has also soemwhat limited capacity, beacuse the same limited downlink bandwidth is shared between a large number of users in a large geographical area.

    • Skysecure White Paper - This paper explains in relatively non-technical terms why conventional VPN performance over satellite is sluggish when compared to the use of other applications across a space platform.    Rate this link


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