Serial is Faster

Parallel ATA, better known as the IDE (Integrated Drive Electronics) interface, or simply ATA (Advanced Technology Attachment), has been around for time immemorial. CPU speeds have increased manifold, RAM and storage capacities have shot through the roof, but ATA has, by and large, stood the test of time. But like the cliché goes, all good things must come to an end; and ATA is beginning to show its age. 

Despite having reached its capacity speed of 133 MB/s with the Ultra ATA/133 specification in hard drives, ATA (having started at 3.3 MB/s in 1989) is struggling to keep pace with the fast increasing speeds of storage devices. It is also characterized by a high pin count (40) and high voltage requirements (5V) that are proving to be major headaches for modern day designers. 

Step in Serial ATA. As the name suggests, Serial ATA transfers data serially unlike the current IDE interface, which does it in parallel. In Serial ATA, only one channel is used for transmission for each direction in which data is to be transmitted. So for bi-directional serial communication between devices A and B, two channels would be needed. Thus to transfer eight blocks of data from A to B would require eight clock pulses. On the other hand, parallel data transfer of eight data blocks would require 16 channels in all (eight for each direction). So the same amount of data could be transferred on a single clock pulse–clearly a marked increase in performance. But this is at a cost–not only the cost of extra channels needed, but also an increase in the signaling requirements for transferring all that data simultaneously. In other words, serial transfer technology can be slower, but it provides considerable cost benefits.

Serial ATA reduces the pin count to seven (power, data, control and ground, all included), which enables a thinner cable, which, in turn, means improved airflow in your cabinet. Serial ATA also needs only 250mV to function. This reduced power requirement makes it ideal for use in mobile devices. Also supported is an increased maximum cable length of 1 meter, up

from the 18 inches of Parallel ATA. 

But, most importantly, Serial ATA is set to break the speed barrier of ATA–it starts off at 150 MB/s and if things move as expected, it will double its speed every three years and reach 600 MB/s by 2008. This means that serial is no longer slow, like your COM port, but “quick as a gazelle” compared to the USB. It also means that Serial ATA can give serious competition to SCSI and other similar technologies which have till now been preferred for enterprise use due to their speed and reliability. The fact that Serial ATA has built-in advanced reliability techniques can only help its cause. This is in sharp contrast with Parallel ATA, which having been designed at a time when designers had more pressing issues than offering built-in reliability techniques, offers little reliability by itself. 

Increased reliability is also ensured, in a way, by allowing hot pluggable Serial ATA devices. So you can add/remove your hard-drives while your system is running. No, we wouldn’t recommend you trying that with your current hard disk, and definitely not the one hosting your OS partition! Whether or not Serial ATA replaces SCSI as RAID’s trusted friend in the future remains to be seen.

A lot of the advantages of Serial ATA over traditional ATA are due to the way it connects devices–by using a point-to-point approach. This means that each device is connected to the Serial ATA controller directly through its independent port. 

This is in contrast with parallel ATA, which uses the master-slave configuration that allows multiple devices to share a common bus. Your motherboard, for example, has Primary Master/ Slave and Secondary Master/ Slave devices, for a total of four ATA devices supported. The two primary devices share one communication channel, as do the secondary ones. This, obviously, introduces certain delays and performance issues when the devices on the same communication channel need to talk to the controller simultaneously. 

Thus, adding a new disk in a Parallel ATA system doesn’t ensure increasing its performance by the maximum throughput of the disk. This is due to the overheads involved in making these devices talk to the controller. For example, having three disks, each capable of performing at 100 Mb/s, connected using Parallel ATA would result in the total performance slightly less than the expected

300 Mb/s due to the reasons mentioned. 

Clearly, such a situation would never arise in Serial ATA as the point-to-point approach used ensures no sharing of communication channels and thus a maximum throughput of 3 X 100 = 300 Mb/s. Serial ATA also increases the scalability of systems, as the number of devices that can be linked up to a system is now dependent only upon the number of point-to-point connections available of the disk controller. 

Thus, adding a new device is as simple as plugging in the device and a cable–no annoying jumper settings and the likes to be taken care of.

With so many things in its favor, Serial ATA is poised to take off in a big manner. The work on the specification (more details at www.serialata.org) has involved everybody that counts in the industry, big and small, and first Serial ATA products could be out as soon as early 2003. Expect not many adoption issues–Serial ATA controllers will have built in support for existing Parallel ATA drivers and bridges for parallel-to-serial conversion are also on the anvil. Major OSs already support it because it is designed to be backward compatible. 

Kunal Dua