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Received: by delta.eecs.nwu.edu (4.1/SMI-4.0-proxy) id AA24641; Tue, 21 Mar 95 00:48:10 CST Date: Tue, 21 Mar 95 00:48:10 CST From: email@example.com (TELECOM Digest (Patrick Townson)) Message-Id: <9503210648.AA24641@delta.eecs.nwu.edu> 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. PAT From: firstname.lastname@example.org (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 Overview 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 document. 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 audits. 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 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 1996. 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 etc. Cost 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 month. 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 month. 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 month. * The Circuit Cellular cost per user would range from $37 to $128 per month. 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 month. 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. Interoperability 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 AVAILABILITY: 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 established. Summary 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 used. CDPD: 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!