modems@telebit.UUCP (Modem Mail Account) (03/07/90)
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Telebit Corporation Revision 1.01 01 DECEMBER 1989
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A BRIEF TECHNICAL OVERVIEW OF TELEBIT MODEMS
By Michael Ballard, Director, Network Systems, Telebit Corp.
Edited by C. E. Castillo, UNIX Communications Specialist, Telebit Corp.
Before starting on this document, a caveat: this document is intended
to address many of the questions and comments about TELEBIT
modems that have appeared from the user community. We are
striving to provide as much information as possible, in such a
way that will be useful to the widest group of readers. This is
NOT intended to be a Marketing Article, but rather a technical
overview for the more sophisticated reader. Its purpose is to
inform, not to sell product. If anyone is offended by Telebit
taking this action, please mail directly to us first, to avoid
cluttering the newsgroup. Thank you.
I would like to provide some background for Unix users considering
the use of a TELEBIT high-speed dialup modem.
I served as project manager and principal programmer for
Telebit's protocol support developement. The UUCP "g", Kermit,
Xmodem and Ymodem protocols are directly supported in the firmware
of all TELEBIT modems. Peter Honeyman, co-developer of ATT's
HoneyDanBer/BNU UUCP, coded those portions of the firmware which
support the "g" protocol.
TELEBIT modems employs a patented multicarrier modulation scheme
coined DAMQAM (Dynamically Adaptive Multicarrier Quadrature Amplitude
Modulation). A CRC-16 based sliding window protocol with selective
retransmission runs on top of this modulation scheme insuring data
integrity across the phone line. This telephone line protocol is
known as the Packetized Ensemble Protocol or PEP. PEP is the
trademark by which all modems employing this technique can be
recognized.
This technique (DAMQAM) divides the voice bandwidth into 511
individual channels each capable of passing 2, 4, or 6 bits per
baud based on the measured characteristics of the individual
frequencies associated with each channel. On a typical phone
connection, the modem uses a subset of about 400 of those channels.
Each time the modem connects to a circuit established on the dialup
Public Switched Telephone Network (PSTN), the TELEBIT modem measures the
quality of the connection, and determines the usable subset of the 511
carriers. The aggregate sum of bits modulated on this subset of
carriers multiplied times the baud rate yields a bit per second
rate that on a local telephone connection (i.e. round trip through
your local telco) is 18031 bps. This 18031 bps is then reduced by
about 20% to allow for the CRC overhead, to about 14400 bps of data
throughput.
Long distance line quality varies with location and carrier, but you
can expect this number to be in the 10000 to 17000 bps range under
most conditions domestically. By choosing a high quality long
distance carrier, you will ensure the best throughput overall.
The modem operates at 7.35 and 88.26 baud, transparently changing
baud rates to accomodate the pace and quantity of data traffic.
When in "interactive mode" the modem sends data using 11 msec
packets (which run at 88.26 baud). Each packet contains 15 bytes
of data. In "file transfer mode" the modem uses 136 msec packets
(that transfer at 7.35 baud) that contain 256 bytes of data.
The TrailBlazer decides which packet size to use on an ongoing
dynamic basis. No intervention from the user is required.
At lower speeds, such as 300, 1200, and 2400 bps, TELEBIT modems
provide emulation (performed in the DSP section, not by a "chip"
modem) to support these standards. The 300 bps standard is called
Bell 103C. At 1200 bps, two standards exist, Bell 212A and CCITT
V.22. Both are supported. At 2400 bps, the standard is called
CCITT V.22 bis. These speeds are all available with or without
MNP Class 4 Error Correction and MNP 5 Data Compression.
The TELEBIT T2500 modem provides the CCITT V.32 modulation standard
for 9600 bps communication. This is accomplished by the use of the
Rockwell V.32 modem "chip set". The addition of this "daughter"
module allows the T2500 to use V.32 for asynchronous and synchronous
communications. V.32 allows for 9600 bps, full-duplex communications.
TELEBIT modems employs a Motorola 68000 and a Texas Instuments
TMS32010 digital signal processor to accomplish this performance.
Because of this substantial computer horsepower (about 7.5 MIPS),
TELEBIT modmes are really communications processors, rather than
conventional modems.
The software defined architecture produces a flexible product platform
that allows broad feature development capabilities while allowing the
product's installed base to benefit from those developments by installing
upgrade EPROM sets.
All four protocols (Kermit, Xmodem/Ymodem, UUCP), V.22bis support, MNP at
low speeds, multiple releases to improve the interactive performance
(earlier TELEBIT modems utilized only one baud rate), a multitude of
RS-232 behavior related features, leased line capabilities, remote
command processor access, echo suppressor compensation, increased
data rates, and a myriad of user requested features have found their
way into current production modems and are available to earlier
revisioned modems via the EPROM uprgrade kits.
PEP modems provide a full duplex serial interface to an attached computer,
however they employ a half duplex implementation on the telephone line.
TELEBIT refers to this half duplex technique as "Adaptive Duplex".
As the name implies, the ownership of the line (i.e. the ability
to transmit) adapts to the quantity of data available to send at
any single moment. Maximum efficiency is achieved by sending data
in a nonstop data stream at 19.2Kbps relying on serial interface
flow control to moderate the data flow into and out of the modem.
This allows the maximum amount of data to be available every time
a transmitting modem takes ownership of the line. In this way the
modem, not the DTE, controls the line turnarounds. The protocol
provides a ceiling at about 3k of sent data before a transmitting
modem must give up its turn and allow the other modem an opportunity
to send. A continuous 19.2Kbps data flow into the modem is required
to ensure that there is always 3k of data to send each time a
transmitting modem takes its turn. The serial interface speed must
exceed the telephone line speed, potentially 18,031 bps, or the maximum
efficiency of the modems can not be reached).
UUCP's "g" protocol behavior on dialup lines was a clear contradiction
of the desired behavior with the PEP protocol. "g" sends 3 small data
packets at time and then waits for the remote UUCP to ACK or NAK their
receipt. The resulting throughput when using UUCP and "g" with TELEBIT
modems was only a little better than a standard 1200 bps modem.
This was unacceptable. TELEBIT decided to improve UUCP performance.
TELEBIT modems can be configured to "spoof" the protocol by setting
a register (S111) to one of several values. The spoof can support four
different protocols: UUCP "g", Xmodem, Ymodem, and Kermit.
"Spoof" means to fool the various protocols into thinking that they
are getting their acknowledgment packets from the remote computer,
when in reality they are getting them from the modem.
All of these protocols are what are commonly referred to as
"send and wait" protocols. This type of protocol builds a packet
in computer A, sends it out through the modems, where it is received by
computer B. Next, computer B looks at the packet to determine whether
or not it arrived intact. If it did, it sends an ACK (acknowledgement)
packet back to computer A. If it did not arrive intact, it sends a NAK
(non-acknowledgement) packet. In either case, computer A can't send the
next packet out until it gets the ACK from the first packet. This is slow!
Since our modems are error-free between the modems, the only place data
could get "broken" is between the modems and their respective computers.
Let me draw the connection diagram below:
Ca <======> Ta <----------------> Tb <======> Cb
Ca = Computer A Cb = Computer B
Ta = TELEBIT Modem A Tb = TELEBIT Modem B
====== RS-232 Cable
------ Phone Line
When we are running our protocol support, we look at the packet coming
from Ca. Ta checks the packet for validity and sends the ACK or NAK.
Ca can begin building the next packet immediatly upon receipt
of Ta's ACK. This results in Ca building and sending packets as fast as it
can. Many packets are now forwarded to Tb. Tb now delivers the packets
to Cb, observing the rules of the protocol. Tb will deliver the next packet
or retransmit the previous packet based on the ACK or NAK received from Cb.
Cb ACKs and NAKs are then thrown away so as not to return to Ca.
Protocol support can be configured to run in parallel with data compression
enabled. The real world result of this is to increase protocol transfers
from 2-3 Kbps to 10-19.2 Kbps.
This covers most of the commonly asked questions about TELEBIT modems.
If any of the above information is unclear, or you have questions
regarding other aspects of modem technology or performance, send mail to:
Michael Ballard/Cerafin E. Castillo
Telebit Corporation
1315 Chesapeake Terrace
Sunnyvale, CA 94089
1-800-TELEBIT
UUCP: {ames, uunet, sun, pyramid, decwrl}!telebit!modems
INTERNET: modems@telebit.com
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Cerafin E. Castillo |\\\\\\\|
UNIX Communications Specialist |\\\\\\\|
Telebit Corporation |\\\\\\\|
1315 Chesapeake Terrace T E L E B I T
Sunnyvale, CA 94089
(408) 734-4333 UUCP: {sun,pyramid,uunet,ames}!telebit!cec
(408) 745-3104 INTERNET: cec@telebit.com
"...A kinder modem for a gentler UNIX community..."
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