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From: (TELECOM Digest (Patrick Townson))
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To: telecom
Subject: CDPD vs Ckt Cell Data UPDATE

Too large for a regular issue of the Digest, and submitted here FYI.
(No, that does not mean "For Your Insomnia")   Although what we have
come to call an Infomercial, a sort of interesting one in my opinion.
Please send your feedback along for a followup in the Digest.

  From: (AIRCOM1)
  Subject: CDPD vs Ckt Cell Data UPDATE
  Date: 17 Mar 1995 22:32:54 -0500
  Organization: America Online, Inc. (1-800-827-6364)

Here is the updated paper originally published in November and posted all
over the net. Many people have E-mailed me about this update, so finally,
here it is - Whew!

I received great input from many vendors and carriers and I believe it
resulted in a good, in fact much better, factual paper. Interestingly
enough, I received some E-mail stating that the paper was unfair to CDPD,
and other E-mail stating it was unfair to Circuit Switched. I have
incorporated all of the factual and verifiable inputs into a more accurate
article on both sides of the fence. I hope that you find it informative
and useful.

I am sure I will get the usual barrage of E-mail from each camps
proponents, but unless you are going to correct me on a fact that is just
plain wrong, please don't waste your time. I have heard every opinion in
the book on both sides, so you probably won't be saying anything new.
However, if you have some constructive way to improve the article, by all
means E-mail me. I will respond, and I do incorporate every good input I
get. Also, if you find this article useful, please let me know.

By the way, before I posted this, I had it reviewed by vendors and
carriers of both technologies and received agreement from both sides that
this is a fair and unbiased article. I hope you agree.

Thanks for the patience and happy reading!

Cellular Data
A comparison of CDPD and Circuit Switched Data
Kevin J. Surace
November 1994
Revision 1.2 March 1995

 While much talk has surrounded CDPD and circuit switched data, many myths
and misunderstandings have been spread about both services. This article
attempts to describe in detail the operation of both technologies. In
addition, clear comparisons are drawn between both service offerings in
terms of cost, interoperability, and usage in an attempt to help users
decide which service will best meet his or her needs.

Quick Primer

 Before we begin to dig into the information too deeply, a quick primer on
bits, bytes, bauds and compression is in order.

A bit is a single 1 or 0, and a baud is the "raw" number of bits that can
be sent per second. For instance, a 1200 baud modem can send up to 1200
raw bits per second. We use the term "raw" here to not include error
correction or compression, as these will change the users perceived number
of bits per second transferred.

 While all of this is interesting, what does it all mean to a real user? 

 Well, a byte is equivalent to 8 bits, which roughly equals 1 character. A
character can be a single letter or number or other "types" character, as
well as special and control characters.

 How does this turn into something useful for real users?

 As was just stated, a character is roughly equivalent to 8 bits. In
actuality though, when we are calculating actual throughput, it takes a
little more than 8 bits to represent one character. This is because
different protocols have more or less overhead (additional bits) which add
on to the original 8 which represented the character. So in a real data
transfer between two modems, we approximate 9 to 10 bits per character for
throughput considerations.

 Ultimately, it is the users perceived throughput that matters most. And
in standard modems today, errors (and subsequent error correction) reduces
throughput, and standard compression can increase throughput over the raw
baud rate. This is how a modem can advertise a throughput of up to 57,600
bits per second (even though the baud rate is 14,400 bits per second), as
they are showing the highest throughput a modem will support with a highly
compressible file. The most important characteristic in evaluating
throughput is a characters per second analysis. This can quickly tell the
user how long a given amount of data will take to send. For example, this
document contains about 27,000 characters, so if my throughput were 2000
characters per second, it would take about 14 seconds to send this

 CDPD (optionally in specification 1.1) and circuit switched modems
usually have V.42bis, which provides up to 4X compression on data files.
However, since many files are precompressed using ZIP or other utilities,
compression in the modem will not compress those any further.

Circuit Switched Data

 Data sent over standard "land lines" is sent today using modems in a
circuit switched fashion. These modems translate digital information into
a complex waveform which can be sent over the standard analog phone
system. Since most modems are based upon international standards (CCITT or
ITU), a modem on the other end can decode the complex waveform back into
the digital data it represented.

 There are generally two types of standards required for modems to
inter-operate. One is a modulation standard, which defines how a modem
will modulate the line. The other is a protocol standard, which defines
how the modems will deal with errors, how they will compress data, and
which format they will communicate with each other.

 The common modulation standards include V.22bis (2400 baud), V32bis (up
to 14,400 baud), and the newest V.34 (up to 28,800 baud). These rates are
"raw" modulation rates in each direction, and do not take into account
compression or errors which require resending. All of the major modulation
standards are full duplex, that is data can travel in both directions at
the same time. As stated earlier, these are all international standards,
so complete interoperability is maintained provided modems on both ends
support the same standards. Since often a modem on one end may have a
later standard than the other, the modems will negotiate with each other
in order to find a common standard.

 The common protocol standards include MNP2,3, 4 and V.42 for error
correction and MNP5 and V.42bis for data compression. The combination of
V.42 and V.42bis offer the best error correction and data compression (4
to 1) standards available today. If you take the raw modulation (say
14,400/V.32bis) times the average compression rate (say 4:1 with V.42bis),
you obtain the throughput factor, in this case 57,600 bits per second,
which is over 5,000 characters per second. Although this is the rated
throughput, a typical user will generally average 3,000 to 4,000
characters per second on a clean (with no noise) landline connection.

Data over Cellular

 Sending data over cellular has several differences from landline. In
cellular, there is limited bandwidth, poor frequency response, defined
cellular events and noise and distortion. While some of these are
attributable to the network, the biggest bottleneck has been with the
cellular voice phones. They were, and still are, designed to send and
receive voice calls. The demand of data on a transceiver are dramatically
different than the human voice requires. So, while the modem technology
can attempt to make up for network and transceiver problems, only limited
success can be achieved by the modem alone.

 Until a few years ago, sending data over circuit cellular (the same
channel that cellular voice phones use) was often slow and unreliable.
While landline modems were whisking along at 14,400 baud, data sent over
cellular was usually limited to 1200 baud (and still is with many
products). In addition, the standard modem protocols (like V.42) were not
robust enough to handle the harsh cellular environment which includes
noise and cellular events such as cell hops, power changes, and system

 In 1992, a proprietary two-sided protocol called MNP10 from Microcom became
available. While this protocol offered more robustness and often enhanced
connect speeds to 4800 baud (and even 9600 later), it was not a standard
and not widely available. It, like other proprietary two-sided protocols,
required that both modems (the remote modem and the host modem) have MNP10
in order to gain any benefit. Also, MNP10 treated cellular events as
noise, causing long retrain cycles after each event. (A modem retrain is
when the modems test and probe the line to set their equalizers for best
performance. This occurs because the modems lose synchronization with each
other, and can not send data until a retrain occurs. Retrain cycles can
take between 6 and 25 seconds depending on the protocol and the number of
attempts to achieve success.) This resulted in low throughput for the
user, since the modems were tied up training with each other much of the
time. This, and the incompatibility with CCITT standards, limited cellular
calls only to a few modem types,  so it never became widely popular.

 Recently, Rockwell released an updated version called MNP-10EC in their
modem chipsets. It includes some "ETC like" enhancements, and adds some
benefit even if the landline modem doesn't support MNP-10EC. While
extensive testing has yet to be done, early results show  performance
similar to ETC.

 In 1993, another proprietary two-sided protocol called ETC became
available from AT&T/Paradyne. While ETC addressed some of the problems
of MNP10, it was still required on both ends in order to obtain the
maximum benefit (however, as a step in the right direction, some
significant gain could be achieved with it only on one side). Without
ETC on the host end, data calls still could result in hang-ups due to
cellular events, as this protocol still treated cellular events as
random noise. Since the ETC protocol is not a modem standard, it is
not clear whether it will perpetuate itself throughout the market and
survive long term.

 In late 1994, a cellular-side-only protocol was introduced as AirTrue
from Air Communications. This was the first protocol to allow complete
interoperability since it is fully compatible with (and is optimized to
work with) industry standard V.42 on the host end, thus requiring no
special modems or protocols to get the maximum benefit. In addition, the
technology was the first to address the transceiver noise/distortion and
the first to build in (read and interpret) cellular events and system
messages as part of the modem protocol. This allows AirTrue to operate as
an extension of the cellular network, rather than an isolated modem. 

 With the latest technology such as AirTrue, average throughputs can range
from 3000 to 4000 characters per second with compression. As an example,
the 27,000 characters of text in this document take about 7 seconds to
send over circuit cellular with AirTrue, but could take as long as 24
seconds with other protocols or poor network conditions. However, with a
97% call success rate and typical connections of 9600 to 14,400 baud
(depending on product), circuit cellular data technology is now
approaching the convenience and reliability of landline.


 CDPD was announced in 1992 as an alternative way to send and receive data
over the existing cellular network. The intention was to develop a method
where short messages and data could be sent in between voice calls using
much of the same infrastructure. At the time of its initial development,
circuit cellular data calls were slower and less reliable, so virtually
anything would be an improvement.

 CDPD is a packet technology, that is, it sends small packets (usually up
to about 1,500 bytes) of information for small bursts of time. While
technically, files of virtually any length may be sent, the network is
optimized for fast, low cost transmission of smaller files. Since the data
(such as messages) is often sent in small amounts, users aren't as
concerned with throughput, as they would be with circuit switched data
(where you are paying for time, not data). 

 CDPD is designed as an IP network. It does not use phone numbers
directly, rather it uses addresses for everyone on the network. As such,
you would not directly dial the modem on your desktop as you would with
circuit switched data, rather you would send a message to an address which
could go through a gateway to your LAN, then to your desktop as another
node on the network. 

 CDPD uses a modulation technique known as Gaussian Minimum Shift Keying
(GMSK) to modulate the carrier in a full duplex mode (forward and reverse
channels). It also uses a forward error correction technique known as Reed
Solomon coding. Due to network and protocol  requirements (including the
forward error correction), the raw modulation rate of 19,200 baud is
reduced to about 9,600 bits per second of actual user data on an unloaded
system. This is because the overhead requirements of the protocol are
close to 50%. This data rate is relatively constant while the CDPD user
has grabbed a channel for single or multiple packets. However, depending
upon the network, the actual implementation, and the user load (including
voice),  actual CDPD channel control by a single user (channels are shared
by multiple users) can be as low as 10% (often referred to as the access
duty cycle, which in this case is 10% on, 90% off). Thus, true user
throughput in actual use with multiple users can range from around 960
bits per second to 9600 bits per second depending on system load. This is
equivalent to about 100 characters per second at the lowest throughput,
and 1,000 characters per second at the top end. These figures are raw and
do not include compression. As an example, the 27,000 characters of text
in this document take between 10 seconds and 100 seconds to send using
CDPD (with optional compression) depending on which carrier, who's
product, and how much traffic is on the network. So far, however, the CDPD
networks have not had significant traffic on them, and the throughput
characteristics tend toward the faster side.

 Currently, CDPD is available in a few metropolitan areas. You cannot yet
send messages to other CDPD users in other areas directly as some systems
are not directly connected (but carriers will work through this over
time). Also, you cannot use your CDPD system in another area without
setting up another address (and monthly bill). However, you can send
messages to addresses (such as a mailbox) which can be retrieved by
another CDPD user in another area provided he has gateway access to the
mailbox (and you do to).

 Since CDPD is billed on a per packet and/or per byte basis, short
messages and small inquiries can be very cost effective in comparison to
circuit switched cellular. Due to its "addressing" nature, CDPD easily
meets the needs of two-way messaging today. While availability is
currently limited, it should be available in many metropolitan areas by

 It should be briefly noted here that two other packet radio networks
already exist in the US. They are RAM and ARDIS, and each has been
available for several years. While it is beyond the scope of this article
to descirbe these systems, they each have their own advantages and
dis-advantages as all networks do. While the cellular network in general
offers some clear advantages over private radio networks, a user
considering CDPD should also investigate these two networks as well.
Which System Should I Choose?
 The proper choice for any user will certainly depend on how they work on
the road and what they would like to accomplish. Several factors come into
play here including cost, type of data, interoperability, ubiquity, access


 The cost of use of both technologies (CDPD and Circuit) can be high or
low depending upon your need and how you use the network. It is not true
that Circuit Cellular is more expensive than packet, or that packet is
more expensive than circuit. The only true statement that can be made is
that the actual cost of each service greatly depends upon your usage
patterns and the type of data that is sent. In general, CDPD is cheaper as
you get closer to a short messaging service usage, that is a high message
count with very little data in each message. Circuit Cellular becomes the
most cost efficient with fewer connections, but larger amounts of data in
each connection.

 The general pricing model used is the published Bell Atlantic Mobile CDPD
pricing, and SF Bay Area peak airtime pricing ($0.45/min) for circuit
cellular (both on the high end) and raw characters per second of 800 on
circuit cellular. Actual prices however vary greatly for corporate
accounts and different locations. On the low cost end, a flat cost of
$50.00 per megabyte is used for CDPD and 1500 raw characters per second at
$0.15/min for circuit cellular.

 In order to show the different costs, several examples will be given.
These examples do not include the monthly access charge which can range
from $20 to $65 per month for either service. Since both CDPD and circuit
cellular prices vary nationwide and by pricing plan and usage, these
examples will show the minimum and maximum known (though not necessarily
published) nationwide pricing for these services. For the purpose of this
paper, it is assumed that all data is precompressed prior to being sent,
so the byte count is the actual amount of data sent over the air.

 While these price ranges may seem confusing to some, they are
representative of the actual costs users are currently paying, and can
expect to pay for the next 18 to 24 months. In several examples, the
monthly usage costs for CDPD and circuit switched overlap, in which case
other factors (such as service availability and future uses) should be
used in determining the proper choice for your application.

Example 1 - Trucking/Messaging Application:

 A trucking company has trucks that will each make 35 deliveries a day. At
every stop, an address will be sent to the truck showing the next stop.
The average message length is 150 characters. Assuming there are 22
workdays a month, there is a total of 770 messages per truck per month.

 * The CDPD usage cost per user would range from $6 to $46 per month. 
 * The Circuit Cellular cost per user would range from $115 to $347 per

Example 2 - Sales Automation Application:

 A sales company has a number of salespeople who will be equipped with
wireless communications to check inventory, enter orders, check email, and
send faxes to customers. The average salesperson accesses the order
management system 1 time per day  to review inventory in which 20K of data
flows, 2 times per day to get email including attachments (average 10K
each time), 4 times each day to enter orders which are 1K each, and 3
faxes per day equal to 20K each.

 * The CDPD usage cost per user would range from $114 to $850 per month.
 * The Circuit Cellular cost per user would range from $30 to $92 per

Example 3 - Insurance Application:

 An insurance company has decided to send digital photographs of claims
directly to headquarters. An average of four photos per day will be sent
averaging 250K per photo.

 * The CDPD usage cost per user would range from $1,100 to $8,140 per
 * The Circuit Cellular cost per user would range from  $37 to $128 per

Example 4 - Mobile Executive Application:

 These persons check their Email at the office four times per day
(averaging 15K per time), send three faxes at 20K each, login to
on-line services for ten minutes per day (averaging 50K of data sent
and received/reviewed), and use the LAN access to accounting/inventory
and sales systems reviewing 50K of data each day.

 * The CDPD usage cost per user would range from $240 to $1,790 per month.
 * The Circuit Cellular cost per user would range from $41 to $139 per

Example 5 - Field Service Application:

 These persons connect six times per day to inquire a parts database, review
parts lists, check delivery availability, and obtain address status. Each
inquiry is 500 bytes in each direction or 1 kilobyte total data exchanged.

 * The average CDPD airtime/packet usage cost per user would be $7 to $50
per month.
 * The average Circuit Cellular airtime usage cost per user would be $20
to $59 per month.

 The charts below illustrate the actual costs of sending data on both the
CDPD and circuit cellular networks. The CDPD costs are from Bell Atlantic
Mobile. The circuit cellular costs are based upon the nationwide averages
per minute of $0.35/min.


 Besides cost, interoperability will be a major consideration in choosing
the correct system for your needs. The following table illustrates the
basic connectivity difference between circuit switched and packet
services. As the table illustrates, circuit switched service allows you to
dial a phone number and access any modem or fax machine. CDPD is designed
to converse only to other addresses on the network such as mailboxes or
gateway addresses. For example, if you wanted to have a remote session to
your desktop computer, you would probably choose circuit switched as you
could not dial directly into the modem on your desk with CDPD, and even if
you could, the cost would be prohibitive. On the other hand, if you only
wanted to send very brief messages back to a network address at your
office, CDPD might be a better choice.

Function                          Circuit Cellular            CDPD

*Dial phone numbers and modems          Yes                    No
*Call Fax Machines                      Yes                    No
*Compatible with LAN's
 and user software networks             Yes               Limited to IP
 with an IP connection
Need gateway to modems                  No                     Yes
Talk only to other addresses            No                     Yes


 In general, it is possible to use circuit switched cellular data in most
of the US today (about 98% population coverage), as well as Canada, South
America, and others for a total of 73 AMPS compatible countries. Products
are widely available by mail order, retail, and carrier outlets. While
some products significantly outperform others, users have a wide range of
price and performance to choose from. Airtime rates are kept reasonably
competitive due to two carriers competing in most markets.

 CDPD is still in its infancy, and has coverage in less than 10% of the
population today. Since most carriers are not yet ready to support
individual users,  the 10% coverage is generally limited to specialized
applications or corporate accounts. The coverage is expected to grow to
over 60% by 1996. It will be around the year 2002 before CDPD will reach
98% (if all other carriers deploy it), and there is no scheduled
availability in most other countries. Additionally, CDPD will be available
in most markets from only a single carrier in each martket for some time,
limiting service choice options.

 While a few products currently exist, essentially none are available to
the individual user through conventional means (retail, mail order etc.).
Product availability should improve by 1996 ~1997, as more markets come
online with CDPD, and more vendors offer products.

 Both services actually have relatively low data usage today (in
comparison to 25 million circuit cellular voice customers). While numbers
vary widely, research studies estimate between 100,000 and 300,000 users
use circuit switched data regularly. This market has grown from virtually
no users in 1991, and carries more wide area wireless data than any other
commercial network today. Its growth has occurred in conjunction with the
dramatic growth in laptop sales.  As computers have become more mobile,
the need for wireless communications has increased.

 Since CDPD first became commercially available, the service has attracted
about 1000 paying customers in its first 12 months. However, this is a
very early market for CDPD, with significant growth predicted in coming
years as product and service availability widens.

Type of Communications

 The third item to consider is the type of data communications you will
likely do. If you are likely to only send and receive short messages to a
limited number of addresses, then CDPD is the right choice as it will be
less costly and quite reliable. However, CDPD is a connectionless packet
data system. That is it could take several seconds for your data to be
received by another address. In some cases, this might be less convenient
to use in a remote access application where high data rates and quick
response are required. On the other hand, CDPD has no connection time
waiting, as in circuit cellulars 15 to 45 seconds. So connections can be
made instantaneously.

 Circuit switched data is a real time full duplex system. As you type a
key, the other end can receive your key stroke instantaneously (actually,
it takes about 1/10 of a second depending upon the distance). So for high
data content applications such as remote access, file transfers, two way
interactive, and faxes, circuit cellular is the right choice. Also for
large file transfers, retrieving Email with attachments, logging into your
LAN network, online services (Compuserve etc.), remote access to your
desktop, BBS services, and ability to contact any modem at any time.

 CDPD is best suited for applications which use a large number of one or
two way messages which are short (less than 1/2 page) in nature. This can
include on-line terminal applications (in which many short messages are
continuously received), remote monitoring, remote control, and short
E-mail traffic.

 Essentially, if you want to do everything you can do at your desktop
remotely, circuit cellular data will provide this full functionality.
However, if your need is for short bursty transmissions and two-way
messaging, then CDPD is a much better choice.

 Also, a primary difference is that CDPD is essentially continuously
"online", so a short message can be sent literally in seconds (almost
always less than 10 seconds in field trials). Circuit Switched Data must
first dial the host and then connect. This connection time varies by
product, but ranges from 15 seconds to 45 seconds just to connect. So
obviously, a very short message would be better sent using CDPD, since no
data could be sent over circuit switched until the connection was


 Circuit cellular data and CDPD are different from each other in many
ways. Each service has its drawbacks and advantages. Contrary to popular
belief, one service is not cheaper than the other. In fact, both CDPD and
circuit cellular can be quite inexpensive depending upon how they are


  Packet data system designed for short bursty messages;
  System is optimized for wireless messaging;
  Cannot dial phone numbers directly, but can send messages to addresses;
  Faxes limited to text only (through a fax service gateway);
  Very cost effective for large numbers of messages of short content;
  More expensive for email attachments, faxes, large files, high
information flow;
  Average throughput ranges from 100 to 1000 characters per second (x4
with compression);
  Good availability by mid-1996 in many parts of the US;
  Cost is 2 to 20 cents per short message and $50 to $580 per megabyte;

 Circuit Cellular:

  Analogous to the standard landline phone system and modem;
  Dial virtually any modem in the world and connect;
  Complete faxing capability to any fax machine or fax modem;
  Cost effective for large content, high information flow applications;
  Expensive for large numbers of very short messages;
  Average throughput ranges from 500 to 1500 characters per second (x4
with compression);
  Complete availability in US and 72 other AMPS countries today;
  Cost is 15 to 45 cents per short message and $2 to $10 per megabyte;

The Fine Print:

About the author:

 Kevin Surace co-founded Air Communications in 1992 which develops
solutions for both circuit switched and packet networks. His background
includes engineering, sales, and marketing in wireless technologies,
semiconductors, and multichip modules. He has been a speaker at a number
of industry conferences including CES and CTIA conventions. He is
currently Vice President of Sales at Air Communications in Sunnyvale CA.

All trademarks are property of their respective owners.

Many thanks to the many, many others (including customers, developers,
vendors, and carriers) who also contributed to (and reviewed) this
article. Your generous time and effort has resulted in (I hope!) a clean,
factual, unbiased, and informative article for all users.

This article may be retransmitted or republished in any form on any
network provided the article remains un-modified and in its entirety. No
other permission is given.

Kevin Surace - Goin Wireless!