154 lines
8.2 KiB
HTML
154 lines
8.2 KiB
HTML
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<TITLE> Modem-HOWTO: Modem Pools, RAS</TITLE>
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<A HREF="Modem-HOWTO.html#toc3">Contents</A>
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<H2><A NAME="s3">3.</A> <A HREF="Modem-HOWTO.html#toc3">Modem Pools, RAS</A></H2>
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<H2><A NAME="ss3.1">3.1</A> <A HREF="Modem-HOWTO.html#toc3.1">Introduction</A>
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</H2>
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<P> A "modem pool" is a group of modems which are normally used to
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receive incoming calls. Today, many such modems may be on a single
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card. ISPs once used modem pools so that customers could call in to
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the ISP, but today, the RAS (Remote Access Server) has replaced modem
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pools for ISPs. RAS works for incoming calls at near 56k (V.90 and
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V.92) and uses what amounts to "digital modems". Modem pools use the
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older analog modems and can only go to 33.6 kbps for incoming calls.
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Thus analog modem pools are more likely to be used by small
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organizations that don't want to use the more expensive RAS. A RAS is
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in a sense a digital modem pool. </P>
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<P>An analog modem pool may be provided by an analog multi-port modem card.
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In olden days it was many modems in an external chassis (something
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like an external modem). Such modems could be analog modems similar to
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modems used for home/office PCs (can't send above 33.6k even if they
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are labeled "56k modems"). A RAS will use "digital modems" which
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can send at nearly 56k (if you have a good line). The "digital
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modems" require a digital connection to the telephone line and don't
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use any serial ports at all. All of these modem pools (or RAS's) will
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require that you install special drivers for them.</P>
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<H2><A NAME="ss3.2">3.2</A> <A HREF="Modem-HOWTO.html#toc3.2">Analog Modem Pools, Multi-modem Cards </A>
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</H2>
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<P> A "multimodem card" is short for "multiport modem card". Some put
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a hyphen after "multi": multi-modem or multi-port. An analog modem
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pool is just many analog modems (the common home/office modem)
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provided either on an internal plug-in card or in an external chassis.
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Each modem comes with a built-in serial port. There is usually a
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system of sharing interrupts or of handling interrupts by their own
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electronics, thus removing much of this burden from the CPU. Note
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that these modems are not "digital modems" and will thus not be able
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to use 56k for people who dial-in.</P>
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<P>Here is a list of some companies that make analog multiport modem
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cards which plug into slots in a PC. 8 modems/card is common. The
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cards listed claim to work with Linux and the websites should point
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you to a driver for them.</P>
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<P>Multi-modem Cards (analog, not digital):
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<UL>
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<LI> Equinox SST Multi-modem. PCI, 56k, 4 or 8 ports<BR>
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<A HREF="http://www.equinox.com/product/multi-modem.htm">http://www.equinox.com/product/multi-modem.htm</A>
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</LI>
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<LI> MultiModemISI by Multi-Tech Systems. 56k or 33.6k, PCI,
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4 or 8 ports. ISDN/56k hybrids.<BR>
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<A HREF="http://www.multitech.com/PRODUCTS/MultiModemISI/">http://www.multitech.com/PRODUCTS/MultiModemISI/</A>
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</LI>
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<LI> PCI-RAS cards by Perle. 56k, 4 or 8 ports.<BR>
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<A HREF=" http://www.perle.com/products/default.asp?cat=C007"> http://www.perle.com/products/default.asp?cat=C007</A>
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</LI>
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<LI> RocketModem by Comtrol. ISA 33.6k, 4 or 8 port.<BR>
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<A HREF="http://www.comtrol.com/sales/specs/rm.htm">http://www.comtrol.com/sales/specs/rm.htm</A>
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</LI>
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<LI> RocketModem II by Comtrol. PCI 56k, 4 or 6 port<BR>
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<A HREF="http://www.comtrol.com/sales/specs/rmii.htm">http://www.comtrol.com/sales/specs/rmii.htm</A>
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</LI>
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<LI> RockForce. 56k, 2 or 4 port Two port V.92/V.44<BR>
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<A HREF="http://www.mainpine.com/">http://www.mainpine.com/</A>
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#RockForce+ Two port V.90 (www.mainpine.com/prodrockplus.html)
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#RockForceDUO Two port V.92/V.44 (www.mainpine.com/prodduo.html)
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#RockForceQUATRO Four port V.92/V.44 (www.mainpine.com/prodquatro.html)
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#RockForceDUO+ Two port V.92/V.44/V.34 SuperG3 Fax =
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#(www.mainpine.com/prodduoplus.html)
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#RockForceQUATRO+ Four port V.92/V.44/V.34 SuperG3 Fax =
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#(www.mainpine.com/prodquatroplus.html)
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#</LI>
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<LI> Multi-modem communication adapters by Digi.
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<A HREF="http:/www.dgii.com/solutions/mmcommadapters/index.html">http:/www.dgii.com/solutions/mmcommadapters/index.html</A></LI>
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</UL>
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</P>
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<H2><A NAME="digital_modem"></A> <A NAME="ss3.3">3.3</A> <A HREF="Modem-HOWTO.html#toc3.3">Digital Modems, RAS </A>
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</H2>
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<P> "Digital modems" are much different than the analog modems that
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most people use in their PCs. But they can communicate with analog
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modems at the other end of the phone line. ISP's use "digital modems"
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to send out data at almost 56k bps to 56k modems in homes and offices.
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The "digital modem" requires a digital connection to the telephone
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line (such as T1, EI, ISDN PRI, etc.). Except for some serial ISDN
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"modems", they don't use serial ports for the interface to the
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computer. Instead, they interface directly to a computer bus via a
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special card(s) (which may also contain the "digital modems"). They
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are often a component of "remote access servers" (RASs) or "digital
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modem pools"</P>
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<P>You may already know that each time you make a telephone call from an
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analog device (a telephone or a modem) it gets converted by the
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telephone company to a digital signal. Then it's transmitted to near
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its destination as a digital signal and finally converted back into an
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analog signal just before it reaches it's destination. But it's also
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possible to receive this digital signal directly from the telephone
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company if you have what is called a "T1" line, etc.</P>
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<P>The cables from the phone company that carry digital signals have been
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designed for high bandwidth so that the same cable carries many
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telephone calls. It's done by what's called "time-division
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multiplexing". A single phone call in a cable is carried on two
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different channels, one for each direction. So the RAS must connect
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each such channel-pair to the appropriate "digital modem" that
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services that phone call. Such tasks are done by what is sometimes
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called a "... concentrator".</P>
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<P>Now the digital signal received by a "digital modem" may really
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represent an analog signal which has been sent to it by an analog
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modem. This is because when you send an analog signal (including
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ordinary voice) to the telephone company, it gets converted into
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digital by the phone company. One way for the digital modem to deal
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with this digital signal would be to convert it to an analog signal
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and then put that thru an analog modem to get the digital data sent by
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the analog modem. But why do all this work? Since the signal is
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already in digital form, why not process it digitally? That's how
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it's done. The digital signal is processed and converted to another
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digital stream of bytes which represents data bytes sent by the analog
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modem. A "digital signal processor" (DSP) is commonly used for this
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task. A CPU could also handle it but it would be heavily loaded.</P>
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<P>Likewise, a "digital modem" must handle sending digital signals in the
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opposite direction from a RAS to a digital telephone line. Thus it
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only makes digital-to-digital conversions and doesn't deal in analog
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at all. It thus is not really a modem at all since it doesn't
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modulate any analog carrier. So the name "digital modem" is a
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misnomer but it does do the job formerly done by modems. Thus some
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"digital modems" call themselves "digital signal processors", or
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"remote access servers", etc. and may not even mention the word
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"modem".</P>
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<P>Such a RAS system may be a stand-alone proprietary server, a chassis
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containing digital modems that connects to a PC via a special
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interface card, or just a card itself. Digi calls one such card a
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"remote access server concentrator adapter". One incomplete
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description of what is needed to become an ISP is: See
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<A HREF="/http://www.cyclades.com/solutions/techtalk/techtalk0l.php">What do I need to be an ISP?</A>. Cyclades promotes their own
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products here so please do comparison shopping before buying anything.</P>
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