The CDPD network uses the concepts of CSMA/CD with a modification to address the dual split frequency nature of the channel. In the CDPD system, forward channel transmission (from the MDBS to the M-ES), is on a different frequency than reverse channel transmission (from the M-ESs to the MDBS). Within each cell, there is a single forward channel transmitter (the MDBS) and multiple reverse channel transmitters (M-ESs). Channel access contention only occurs among the M-ESs.
With the two frequency duplex channel, an M-ES is only receiving radio transmissions from the MDBS and is not receiving radio transmissions from other M-ESs. In other words, a reverse channel carrier sense mechanism is not possible or reliable. To address this aspect of the CDPD system, a digital indicator is provided on the forward channel in order to indicate reverse channel traffic status. This indicator, the BUSY/IDLE indicator is set whenever the MDBS senses reverse channel transmissions. This indicator is interspersed into the continuously transmitting forward channel and provides functionality similar to the carrier sense mechanism in CSMA/CD.
Another flag on the forward channel, the Decode Status, provides an indication as to whether the most recently transmitted reverse channel block has been successfully decoded by the MDBS. This provides a functionality similar to the collision detection mechanism within CSMA/CD5.7 . The use of these digitally encoded flags thus gives the mechanism the name Digital Sense Multiple Access (DSMA)5.8 , sometimes irreverently referred to as "dismay."
For the DSMA mechanism to operate efficiently, the MAC layer must exhibit the following characteristics.
¥ The transmissions must be segmented into blocks of fixed length.
¥ The transmitted blocks must include a mechanism for detection of reception errors.
¥ The mobile units must be able to quickly and reliably determine the Busy/Idle status of the channel.
¥ The start of transmissions for mobiles must be synchronized to reduce the collision window.
¥ The mobile units must be able to quickly determine the success of the most recent transmission.
To address these requirements, the CDPD MDBS continuously transmits on the forward channel. The transmission is block oriented with interspersed Busy/Idle flags and Decode Status flags. These flags are further encoded with the synchronization word. This is illustrated in Figure 5.6.
In the DSMA access scheme, data transmissions are grouped into fixed length blocks. In CDPD, this blocking requirement has been tied with the need for reliable transmissions. A forward error correcting Reed-Solomon (63,47) code block is used to provide for both needs.
A Reed-Solomon (63,47) code block consists of 63 symbols, each of 6 bits in length. Of these 63 symbols, 47 symbols are data, the remaining 16 symbols form the forward error correcting code. This means that each Reed-Solomon encoded data block can carry 282 = 47 * 6 bits of data link layer data. Detailed explanation of the performance of Reed-Solomon codes is beyond the scope of this book. Interested readers should examine the excellent discussion in [LIN-83].
Without delving into exact performance computations and proofs, we can state that the Reed-Solomon codes provide both error detection as well as error correction. However, there is a trade-off between error correction capability and error detection effectiveness. The selected Reed-Solomon (63,47) code allows development of algorithms that correct up to 8 symbol errors per block. However, in the interest of better undetected symbol error performance, the CDPD specifications suggest use of algorithms that correct only up to 7 symbol errors5.9. This brings the undetected symbol error rate to a vanishingly small 2.75 x 10-8.
The next requirement for DSMA is the ability for the mobile device to quickly and accurately determine the status of the reverse channel. The ineffectiveness of each M-ES detecting the transmission of other M-ESs directly has already been raised. The DSMA solution is for the reverse channel status to be indicated as a digital flag on the forward channel.
In CDPD, this Busy/Idle flag is transmitted once every 10 symbols of the forward channel transmission. The purpose of this frequency of transmission is to ensure that mobile devices have up-to-date information on the reverse channel status. This mechanism provides the reverse channel status every 3.125 milliseconds and is the basis for collision avoidance in CDPD channels.
The careful reader may have noticed that the Busy/Idle flag is a binary flag (either "busy" or "idle"), yet the indicator itself is 5 bits in length! This at first may seem at odds with the much repeated concern with the conservation of radio channel bandwidth. Surely, the designers could have used a single bit flag! Well, it is not an oversight. To ensure that mobile devices can make fast and accurate channel access decisions, the flags must be robust in the noisy RF environment. However, this robustness must not come at the expense of processing latency. This eliminated the protection of the Busy/Idle flag within the Reed-Solomon block5.10 . Instead the control flags are repeated and must be interpreted by the mobiles via a majority voting (or better) algorithm-three out of five bits in each flag for the reverse channel busy/idle determination. Analysis showed that under the expected channel characteristics a 5 bit encoding is adequate.
The Busy/Idle flag helps keep a mobile from transmitting while another unit is already operating on the channel. However, it does not prevent two or more units from starting their transmissions at the same moment. In such an instance, two or more units may have sensed the channel status as idle, and simultaneously decided that the channel was ready to accept its transmission. Another indicator must be used to efficiently recover from this collision condition.
The Decode Status flag is used for this purpose. On reception of a Reed-Solomon block, the MDBS executes the decoding algorithm. Typically during normal channel activity, the received block suffers less than 7 symbol errors and is successfully decoded. However, if a collision has occurred, there will be more than 8 symbol errors and the decoding process will fail. In CDPD, the MDBS transmits an indicator on the forward channel to announce the success or failure of decoding the most recently received block.
After transmitting a Reed-Solomon block, the mobile unit monitors the forward channel in search of the Decode Status flag even as it continues to transmit the next block. If the flag indicates successful reception of the transmitted block, the mobile is assured that it can continue to transmit. On the other hand, if the flag indicates a failure to decode the previous block, the mobile must assume that channel conditions were not favorable, perhaps due to a collision, and immediately cease transmission. The immediate cessation of transmission on decode failure is to ensure that a collision condition isn't allowed to persist.
Once again, while the Decode Status is a binary indicator, repetition encoding is used to increase the robustness of the indicator. The Decode Status flag is a five bit value that transmitted as five single bits, each separated by 59 bits. The Decode Status indicator is not contained within the forward channel Reed-Solomon blocks themselves to hasten the collision detection process of a transmitting mobile.
Figure 5.6 illustrates how the Busy/Idle flag and the Decode Status flags are bitwise exclusive-ORed with a forward channel synchronization word. The purpose of the synchronization word is to allow the mobiles to correctly determine where in the forward frame they are as they receive symbols from the system. Thus the forward channel synchronization word is evenly interspersed amongst forward channel Reed-Solomon blocks; this can be done because the forward channel is continuously transmitted. The reverse channel also needs a synchronization word, which occurs at the beginning of any (burst) transmission by a mobile.