CELLULAR DIGITAL PACKET DATA(CDPD):
WHAT MAKES IT RELIABLE?

By Vasilis Koudounas vk5@doc.ic.ac.uk



CONTENTS

INTRODUCTION

The Cellular Digital Packet Data (CDPD) network is a little over one year old and already is proving to be a hot digital enhancement to the existing phone network. CDPD transmits digital packet data at 19.2Kbps , using idle times between cellular voice calls on the cellular telephone network.

In this so-called Age of Information , no one need be reminded of the importance not only of speed but also of accuracy in the storage , retrieval and transmission of data.

SOURCES OF ERRORS

Machines do make errors, and their non-man-made mistakes can turn otherwise flawless programming into worthless, even dangerous, trash. Just as architects design buildings that will remain standing even through an earthquake, their computer counterparts have come up with sophisticated techniques capable of counteracting the digital manifestations of Murphy's Law.

Digital information , by definition, consists of strings of "bits" -- 0's and 1's -- and a physical device , no matter how capably manufactured , may occasionally confuse the two. The most probable reason for this is the low powered transmission of data over long distances.

Mobile communications use radio signals which are subject to eavesdropping. The mobile network is also vulnerable to other unwanted security breaches.

WHAT HAS TO BE DONE TO MAKE A NETWORK RELIABLE?


Fig.1 Data over GSM network

There are some actions that are necessary in order to obtain reliability over a network.

The information needed for the above actions are stored in data bases. The Home Location Register (HLR) stores information relating the subscriber to its network. This includes information for each subscriber on subscription levels , supplementary services and the current or most recently used network and location area. The Authentication Centre (AUC) provides the information to authenticate subscribers using the network , in order to guard against possible fraud , stolen subsciber cards , or unpaid bills. The Visitor Location Register (VLR) stores information about subscription levels , supplementary services and location for a subscriber who is currently in, or has very recently been ,in that area. It may also record whether a subscriber is currently active , thus avoiding delay and unnecessary use of the network in trying to call a switched off terminal.

OVERHEAD ENSURES RELIABILITY

Although the raw data rate for CDPD is 19.2 kbps the actual throughput rate is more in the vicinity of 9.6 kbps . The reason is that CDPD adds a large amount of overhead to each block of transmitted data for reliability. The CDPD encodes each block using a systematic Reed-Solomon forward-error-correcting code. In addition , the information and parity bits in each block are exclusive-ORed with a pseudorandom sequence after Reed-Solomon encoding to assist the MDBS (Mobile Data Base Station) and M-ES (Mobile End System) modems in maintaining bit synchronization. A color code is added to each block to detect cochannel interference from a remote MDBS or cell site. Encryption and decryption are also part of the specification , which ensures that a customer's data is private and which protects the service from fraud.


Fig.2 Typical mobile data schematic including mobile data terminal.

You can't always get what you want -- but if you try , sometimes, you just might find you get what you need. Reed and Solomon managed to get a coding system that was based on groups of bits (bytes) rather than individual 0s and 1s.That feature makes Reed-Solomon code particularly good at dealing with "bursts" of errors.

Mathematically, Reed-Solomon codes are based on the arithmetic of finite fields. Indeed, the 1960 paper begins by defining a code as "a mapping from a vector space of dimension m over a finite field K into a vector space of higher dimension over the same field." Starting from a "message" $(a_0, a_1, . . .,a_{m-1})$, where each $a_k$ is an element of the field K, a Reed-Solomon code produces $(P(0),P(g), P(g^2), . . ., P(g^{N-1}))$, where N is the number of elements in K, g is a generator of the (cyclic) group of nonzero elements in K, and P(x) is the polynomial $a_0 + a_1x + . . . + a_{m-1} x^{m-1}$. If N is greater than m, then the values of P overdetermine the polynomial, and the properties of finite fields guarantee that the coefficients of P--i.e., the original message--can be recovered from any m of the values.

Conceptually, the Reed-Solomon code specifies a polynomial by "plotting" a large number of points. And just as the eye can recognize and correct for a couple of "bad" points in what is otherwise clearly a smooth parabola, the Reed-Solomon code can spot incorrect values of P and still recover the original message. A modicum of combinatorial reasoning (and a bit of linear algebra) establishes that this approach can cope with up to s errors, as long as m, the message length, is strictly less than N - 2s.

In today's byte-sized world, for example, it might make sense to let K be the field of degree 8 over $Z_2$, so that each element of K corresponds to a single byte (in computerese, there are four bits to a nibble and two nibbles to a byte). In that case, $N = 2^8 = 256$, and hence messages up to 251 bytes long can be recovered even if two errors occur in transmitting the values $P(0), P(g), . . ., P(g^{255})$. That's a lot better than the 1255 bytes required by the say-everything-five- times approach.

CDPD vs. OTHER TECHNOLOGIES

It is crucial to develop a secure personal mobile communication environment as secure information flow and privacy are amongst users' top priorities today. CDPD , at 19.2 kbps is not only the fastest available technology, but also the most secure due to its built in RC4 encryption algorithm.

Cellular Digital Packet Data (CDPD) Circuit Switched Cellular Specialized Mobile Radio (Extended) Propietary Wireless Data Networks
Speed best best good good
Security best better good better
Ubiquity best best good better
Cost of Service best better better good
Cost of Deployment best best better good
Mobility best good better good
Interoperability best good good better

References:

Interview with Dr Zinonas Ioannou, Mobile Services, Cyprus Telecommunications Authority

"The basics of the GSM technology platform"
GSM World focus 1996 , published by "Mobile Communications International"

"Wireless Data"
IEEE Communications Magazine - January 1995

"Tellabs Wireless"
CDPD vs. Other technologies
http://steinbrecher.com/compare.html

The CDPD Network
John Gallant, Technical Editor, PCSI


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