At its simplest, the new Thunderbolt technology is essential the previous Light Peak interface that was being worked on in collaboration between Intel and Apple. There have been a number of changes made to the interface from its proposed technology to what has actually made it into products. For instance, Light Peak was originally intended to be an optical interface standard but Thunderbolt has dropped that in favor of more traditional electrical cabling. This does put in a number of limitations to how the cabling works but it made it much easier to implement.
Video and the Interface Connector
The big reason for the change in the Thunderbolt interface had to do with selecting an interface connector. Rather than relying on a new connector, the Thunderbolt technology relies instead upon the DisplayPort technology and its mini-connector design. The reason for doing this was so that a single combined cable could carry a video signal in addition to the data signal. DisplayPort was a logical choice among the video connector interfaces because it already had an auxiliary data channel built into its specification. The other two digital display connectors, HDMI and DVI, lack this capability.
So what makes this feature so compelling? A good example is a small ultraportable laptop such as the MacBook Air. It has very limited space for external peripheral connectors. By using Thunderbolt on the device, Apple was able to combine both data and video signals into a single connector. When combined with Apple Thunderbolt Display, the monitor also acts as a base station for the laptop. The data signal portion of the Thunderbolt cable allows for the display to utilize USB ports, a FireWire Port and a Gigabit Ethernet over the one cable. This goes a long way to reducing the overall clutter of cables coming out of the laptop and expands the overall capabilities as both physical Ethernet and FireWire ports are not featured on the ultrathin laptop.
In order to maintain compatibility with traditional DisplayPort monitors, the Thunderbolt ports are fully compatible with the DisplayPort standards. This means that any DisplayPort display can be attached to a Thunderbolt peripheral port. It is important to note that this effectively will render the Thunderbolt data link on the cable inoperable along that cable. Because of this, companies such as Matrox and Belkin are designing Thunderbolt base stations that will connect to a computer that allow for a DisplayPort pass through to connect to a traditional monitor and still utilize the data capabilities of that Thunderbolt port for Ethernet and other peripheral ports via the base station.
Using More Than One Device Per Interface Port
Another feature that made its way into the Thunderbolt specification is the ability to utilize multiple devices from a single peripheral port. This saves from the need to have multiple ports that were common to many computers. As the computers get smaller, there is less space for connectors. Many ultrathin laptops such as a the MacBook Air and ultrabooks may only have room for two or three connectors. There are a large number of different peripheral ports, more than can fit on such a device.
To achieve the ability to use multiple peripherals on a single port, Thunderbolt takes the daisy chain functionality that was introduced with FireWire. In order for this to function, the Thunderbolt peripherals have both an inbound and outbound connector port. The first device on the chain is connected to the computer. The next device in the chain would connect its inbound port to the first's outbound port. Each subsequent device would be connected similarly to the previous item in the chain.
Now, there are some limits to the number of devices that can be put on a single Thunderbolt port. Currently, the standards allow for up to six devices to be put in a chain. Obviously, much of this has to do with the limitations of the data bandwidth that is supported. If you put too many devices, it can saturate that bandwidth and reduce the overall performance of the peripherals. This is most apparent with the current standard when multiple displays are attached to a single chain.
To achieve the data link portion of the Thunderbolt interface, Intel decided to use the standard PCI-Express specifications. Essentially, Thunderbolt merges together a PCI-Express 3.0 x4 interface to the processor and combines this with the DisplayPort video and puts it over a single cable. Using the PCI-Express interface is a logical move as this is already used as a standard connector interface on the processors for connecting to internal components.
With the PCI-Expess data bandwidths, a single Thunderbolt port should be able to carry up to 10Gbps in both directions. This is more than enough for most current peripheral devices that a computer would connect to. Most storage devices run well below the current SATA specifications and even solid state drives can't achieve near these speeds. Additional, most local area networking is based upon Gigabit Ethernet which is just a tenth of this overall bandwidth. This is why the Thunderbolt displays and base stations are typically able to provide the networking, USB peripheral ports and still be able to pass through data for external storage devices.
How It Compares To USB 3 and eSATA
USB 3.0 is the most prevalent of the current high speed peripheral interfaces. It has the advantage of being compatible with all the backward USB 2.0 peripherals which makes it extremely useful but has the limitation of being one port per device unless a hub device is used. It does offer full bi-directional data transfers but the speeds are roughly half that of Thunderbolt at 4.8Gbps. While it does not specifically carry a video signal the way that Thunderbolt does for DisplayPort, it can be used for video signals either though a direct USB monitor or via a base station device which can break out the signal to a standard monitor. The downside is that the video signal has higher latency than Thunderbolt with DisplayPort monitors.
Thunderbolt is obviously much more flexible than the eSATA peripheral interface as it is much more flexible. External SATA is only functional for use with external storage devices, In addition, it is really only functional for connecting to a single storage device. Now, that can be a drive array which can be extremely fast and hold lots of data. Thunderbolt just has the advantage of being able to connect to multiple devices. Similarly, the current eSATA standards max out at 6Gbps compared to the 10Gbps of Thunderbolt.
While Thunderbolt has been fairly slow to be adopted by manufactures outside of Apple, it is starting to finally see a number of serious peripherals make it to market. After all, USB 3.0 was released nearly a year before it started to make it into many PCs. The flexibility of the interface connector for smaller computing devices is extremely compelling for many manufacturers to start implementing into their ultrathin laptops. In fact, the new Ultrabook 2.0 specifications from Intel call for either a Thunderbolt or USB 3.0 interface to be required on the systems. This requirement will likely spur the adoption of the interface port greatly in the coming years.