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Navas 28800-56K Modem FAQTM 

(Answers to Frequently Asked Questions)

[Modem Picture]

Cable modem/DSL users: see Navas Cable Modem/DSL Tuning GuideTM

C. MODEM FEATURES

Copyright 1999-2008 The Navas GroupSM, All Rights Reserved.
Permission is granted to copy for private non-commercial use only.
Send mirror and commercial license inquiries to John Navas.

Posted as <http://modemfaq.navasgroup.com/faq_c.htm>.

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What's the difference between V.FC and V.34? What's next?

V.FC
A proprietary specification for speeds up to 28800 bps, based on an early draft of the V.34 standard, that was rushed to market by modem chipset leader Rockwell International. It works reasonably well, but lacks the sophistication and robustness of V.34; for example, V.FC does not support split speeds. Many (but not all) V.FC modems are upgradable to V.34, but the modem typically has to be returned to the manufacturer for a "datapump" change. V.FC-only modems will not connect above 14400 bps to V.34 modems that lack V.FC support. With the advent of V.34, V.FC is rapidly fading as modems are upgraded. In the opinion of the author there is now little or no reason to get a modem that has V.FC but not V.34, although until V.FC fades completely there will still be a significant advantage to a modem that supports both V.34 and V.FC.
V.34
A true international standard for speeds up to 33600 bps (more realistically 28800 bps) that is more sophisticated and robust than V.FC. Some but not all V.34 implementations support optional features like 31200 and 33600 speeds and/or split speeds for transmit and receive. V.34 is rapidly replacing V.FC in the marketplace. V.34-only modems will not connect above 14400 bps to V.FC-only modems. In the opinion of the author V.34 is clearly the modem standard of choice. It may well prove to be the last widely-supported analog modem standard.
56 Kbps
Rockwell Semiconductor Systems (along with a number of its customers), USRobotics, and Lucent Technologies have announced plans to introduce modems that work over conventional phone lines at speeds up to 56 Kbps. These proprietary protocols are now being replaced by V.90, a true international standard. See "What are 56K modems?"
28800-33600 fax
Fax is currently limited to 14400 bps over dialup analog phone lines (Group III V.17). Efforts are underway to define a 28800-33600 bps fax specification, which would probably be widely adopted. However, general availability is probably a few years away.
Voice
See "What are 'voice' modems?"
Plug and Play
See "What is Plug and Play"
ISDN
See "What is ISDN?"
"Cable modems"
See What are "cable modems?"
ADSL/SDSL/HDSL
See "What are ADSL, SDSL, and HDSL?"
Universal Serial Bus
A better way to connect a modem to your computer than the conventional UART or parallel port. (See "What is a UART?") Expected to become a standard feature of many chipsets and motherboards (e.g., those manufactured by Intel) and operating systems (e.g., Windows 95/98), although USB is not yet available. (See "Universal Serial Bus Home Page" and Intel's "USB Interactive Tour")
See also "Modems: the make-it-happen machines for the ultimately wired of the 90's"
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What are 56K modems?

Rockwell Semiconductor Systems (along with a number of its OEM customers), USRobotics, Lucent Technologies, and Motorola Information Systems Group, are introducing modems that can work over conventional phone lines at speeds up to 56 Kbps under certain conditions (as noted below). The industry has aligned itself around two incompatible technologies:
K56 (also known as K56Plus and K56flex)
Supported by Rockwell, Lucent, Motorola, and others (including 3Com).

How to find a K56 POP (point of presence):
Ascend Directory
Epoch Internet
x2
Supported by USRobotics, Texas Instruments, and others.

How to find an x2 POP (point of presence):
x2 Directory
V.90
The new international standard
For more information see:
Class action nemesis to go after 56K modem claims
PCWorld New Radio interviews John Navas on 56K modems [new 4/23/97] "ISPs take sides" "USR may join 56-kbps group" So-called "Open 56K Forum" [new 2/27/97]
[The name notwithstanding, this appears to be a "K56flex" promotional organization.]
One of the members of the "Open 56K Forum," 3Com, is acquiring USRobotics. It's unclear what impact the acquisition will have on the competition with "x2" from USRobotics. Motorola and Rockwell Announce Plans to Cooperate on K56 Modem Technology[new 2/14/97]
[It is unclear if Motorola will also interoperate with Lucent]Rockwell And Lucent Announce Their Intention To Interoperate At 56KbpsRockwell Semiconductor Systems: Rockwell OEM customers: USRobotics: x2 licensees: Lucent Technologies: Motorola Information Systems Group: Other material: As the competitive battle between Rockwell/Lucent and USRobotics heats up, the latter appears to be in the stronger position for two reasons:
  1. USR has a substantial market position among major Internet Service Providers (ISPs), with a substantial installed base that can be easily upgraded to 56K ("x2") technology.
  2. USR claims that all of its modems currently in distribution will be easily upgradable to 56K technology. (While choosing such a modem would seem to be a good way to get ready for 56K deployment, the higher speed may well only be possible when connecting to an ISP that installs USR-brand digital "x2" technology.)
An interesting possible fallout is that the new 56K technology may finally force down the price of ISDN. (See "What is ISDN?")
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What are "voice" modems?

So-called voice modems fall into one (or sometimes more) of three categories:
  1. Modems that can function as an answering machine or voice-mail system. Some can also function as a speakerphone.
  2. Modems that can transmit data or voice over the same connection. (Radish "VoiceView")
  3. Modems that can transmit data and voice simultaneously over the same connection. (DSVD) Note that when voice is active, the speed of data transmission drops substantially.
Modems in the category 1 have been on the market for some time. Modems in categories 2 and 3 are just coming onto the market -- expect a continuing stream of new product announcements.

Note: None of these (current) voice modems permit you to:

See "Modems: the make-it-happen machines for the ultimately wired of the 90's"
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What are "PCI" modems?

PCI modems are internal modems that use a PCI slot rather than a legacy ISA slot. Many (but not all) PCI modems are either host-based (see "What are 'software' or 'soft' modems?") or controllerless (see "What are 'Windows' modems?")

Pros:

Cons:

PCI modems based on standard controllers include:

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What are "software" or "soft" modems?

So-called "software" modems use the main system processor for both modem control and modem datapump functions, thereby putting a heavy load on the system processor (e.g.., about 22% of a Pentium 300). In the opinion of this author, "software" modems are only suitable for higher-end systems (Pentium 166 and above) with a lot of extra main processor power to burn. This technology is sometimes known as "Host Signal Processing."

Pros:

Cons:

More information:

(See also "What are 'Windows' modems?" and "What are 'RPI' or 'WinRPI' modems?")
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What are "Windows" modems?

So-called "Windows" modems use the main system processor for modem control (but not modem datapump) functions, thereby putting some load on the system processor (e.g., about 4% of a Pentium 300).

Pros:

Cons:

More information:

(See also "What are 'software' or 'soft' modems?" and "What are 'RPI' or 'WinRPI' modems?")
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What are "RPI" or "WinRPI" modems?

So-called "RPI" (Rockwell Protocol Interface) modems use the main system processor for modem control (but not modem datapump) functions, thereby putting some load on the system processor.

Pros:

Cons:

More information:

(See also "What are 'software' or 'soft' modems?" and "What are 'Windows' modems?")
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What are "cellular" modems?

Standard modems have a difficult time transmitting data over cellular phone connections. Certain modems (typically in PC Card or PCMCIA form) have special protocols designed to enhance cellular communications, and may also have various adapters that connect them to different cellular phones. However, even with such special support, data transmission over a cellular connection is problematic, typically in the range of only 9600 bps, and disconnections are not uncommon.
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What are split/asymmetric speeds?

In the past, most standard modems were only capable of transmitting and receiving at the same speed. (The exceptions were proprietary modems such as the USR HST that used greatly different transmit and receive speeds.) Since most connections were made at the maximum speed, there was little reason to support different transmit and receive speeds.

With the advent of speeds up to 28800 bps that is no longer true. It's now quite common to have a connection where at least one (and often both) speeds must be limited to less than 28800 bps, and it's not uncommon to find that a connection will support faster speed in one direction than the other. (One reason is that send and receive channels are separated for transmission between telephone switching offices.)

Having been designed to optimize performance over a wide variety of conditions, V.34 includes an optional specification for asymmetric (differing or split) transmit and receive speeds. For example, a connection might support a transmit speed of 28800 bps but a receive speed of only 26400 bps; without split speed, the speed in both directions would have to be limited to 26400 bps. For this reason split speed capability is a worthwhile and desirable feature.

Not all V.34-compliant modems support split speeds. For example, at the time of this writing most "glue 'n go" Rockwell V.34 clones, do not support split speeds. On the other hand, others, including all USR V.34 modems (Sportster 28800/33600 as well as the Courier V.34), Diamond/Supra, and Motorola, do support split speeds.
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What is Selective Reject (SREJ)?

Selective Reject (SREJ) is an optional, advanced LAPM (V.42) error correction capability that allows the receiving modem to request retransmission of a given block while continuing to receive later blocks; i.e., to receive the retransmission late (out of order). Without it, the entire transmission has to start over at the retransmitted block, which can result in later blocks being resent unnecessarily. Hence, Selective Reject can improve throughput where there is a significant error rate, particularly over links with long delays (e.g., satellite links).

Not all modems support Selective Reject, which is typically found only in high-end products.
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