What is External SATA (eSATA)?

PC External Storage Interface Based Off the SATA Standards

SATA and eSATA Cables
SATA (left) and eSATA (right) Cables.

Introduction

USB and FireWire have both been a huge boon to external storage, but their performance compared to desktop drives has always lagged behind. With the development of the new Serial ATA standards, a new external storage format, external Serial ATA, is now starting to enter the marketplace. This article will look into the new interface, how it compares to the existing formats as well as what it can mean in terms of external storage.

USB and FireWire

Before looking at the external Serial ATA or eSATA interface, it is important to look at the USB and FireWire interfaces. Both of these interfaces were designed as high-speed serial interfaces between the computer system and external peripherals. USB is more general and used for a wider range of peripherals such as keyboards, mice, scanners and printers while FireWire is almost exclusively used as an external storage interface.

Even though these interfaces are used for external storage, the actual drives used in these devices are still using the SATA interface. What this means is the external enclosure that houses the hard or optical drive has a bridge that converts signals from the USB or FireWire interface into the SATA interface used by the drive. This translation causes some degradation in the overall performance of the drive.

One of the big advantages that both of these interfaces implemented was the hot swappable ability.

Previous generations of storage interfaces typically did not support the ability to have drives dynamically added or removed from a system. This feature alone is what made the external storage market explode.

Another interesting feature that can be found with eSATA is the port multiplier. This allows a single eSATA connector to be used to connect an external eSATA chassis that provides multiple drives in an array.

This can provide expandable storage in a single chassis and the ability to develop redundant storage via a RAID array.

eSATA vs. SATA

External Serial ATA is actually a subset of the additional specifications for the Serial ATA interface standard. It is not a required function, but an extension that can be added to both controller and devices. In order for eSATA to properly function both must support the necessary SATA features. This is particularly important as many early generation SATA controllers and drives do not support the Hot Plug capability that is critical for the function of the external interface.

Even though eSATA is part of the SATA interface specifications, it uses a very different physical connector from the internal SATA connectors. The reason for this is to better shield the high-speed serial lines used to transfer the signals from EMI protection. It also provides a 2m overall cable length compared to the 1m for internal cables. As a result, the two cable types can not be used interchangeably.

Speed Differences

One of the key advantages that eSATA offers over USB and FireWire is speed. While the other two have overhead from converting the signal between the external interface and the internal based drives, SATA does not have this problem.

Because SATA is the standard interface used on many new hard drives, a simple converter between the internal and external connectors is required in the housing. This means that the external device should run at the same speed as an internal SATA drive.

So, here are the speeds for the various interfaces:

  • USB 1.1 – 15 Mbps
  • FireWire (1394a) – 400 Mbps
  • USB 2.0 – 480 Mbps
  • FireWire 800 (1394b) – 800 Mbps
  • SATA 1.5 – 1.5 Gbps
  • SATA 3.0 – 3.0 Gbps
  • USB 3.0 – 4.8 Gbps
  • USB 3.1 – 10Gbps

It should be noted that the newer USB standards are now faster in theory than the SATA interface that the drives in the external enclosures use.

The thing is that because of the overhead of converting the signals, the newer USB will still be slightly slower but for most consumers, there is almost no difference. Because of this, eSATA connectors are much less common now as using the USB-based enclosures are much more convenient.

Conclusions

External SATA was a great idea when it first came out. The problem is that the SATA interface has essentially not been changed in many years. As a result, the external interfaces have become much faster than the storage drives. This means that eSATA is much less common and in fact not really used on many computers at all anymore. This may change if the SATA Express catches on but this is not likely meaning that USB will probably be the dominate external storage interface for many years to come.

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