Overview
The movement of data is an integral function of any PC. Looking at it internally, data flows to and from the CPU, chipset, hard drive, expansion slots, and RAM, all through a range of interconnects. HyperTransport; DMI; QPI and PCI-Express are just a few forms of internal data transmission used by most PCs.
However, the modern PC, be it a notebook or desktop computer, also provides a number of means by which peripherals and storage can be connected without needing to install the devices directly into the machine. External connectivity takes many forms, and in this HEXUS.help we examine some of the more common interfaces present on modern systems.
The background
The development of the PC has been such that new technologies have been introduced that cater for a particular (or future) need. With little or no standardisation through the last 30 years or so, various external interfaces have been created, often in relative isolation from the hardware community at large, that address key issues such as networking, peripheral connection, video-editing, and so forth.
Isolationist design has also led to multifarious ports that offer the same intrinsic features – high-speed data transfer, for example – and modern machines tend to ship with feature-overlapping connectors which are located on the I/O section of a desktop board and all around the chassis for a laptop. Appreciating this, we will group the connections via their intended purpose.
High-speed peripheral connectivity - USB
Universal Serial Bus (USB) is perhaps the most ubiquitous method of connecting external devices to a PC. Created in 1996 by a consortium of big-name technology companies (Intel, IBM, Microsoft, et al), it was designed to replace a gaggle of serial and parallel ports and to provide simpler usage by adopting hot-swapping capabilities – the ability to disconnect and reconnect a USB-powered device without rebooting the computer.
Conservative estimates reckon that more than six billion USB devices have been manufactured in the last 10 years, making it the most popular external interconnect.
Topology
The topology of USB is such that the host controller, usually integrated into the motherboard and physically wired to a port, is able to connect up to 127 devices through the use of tiered hubs. In practice, most systems use no more than four USB devices from a single controller/port, via a single hub.
Connection
Connecting to the PC via a type-A plug and to peripherals via a type-B, micro-sized or custom connectors, USB is able to pull power from the host machine for devices that require less than 500mA and 5V (2.5W). USB data sticks, keyboards, mice, and certain hard drives fall into this category. All other higher-power devices need their own external power source. The power envelope extends to devices connected to non-powered hubs, such that a number of devices must share the 500ma/5V if they’re to be host-powered.
Speed
The serial connection, dubbed USB 1.1, was originally launched with a full-speed transfer of 12Mbit/s (1.5MB/s) together with a reduced-speed, low-power transfer (USB 1.0) of 1.5Mbit/s (187.5KB/s) for devices that didn’t require huge bandwidth – keyboards and mice, for example.
USB 2.0, known as Hi-Speed, was launched in April 2000 and purported a maximum transfer speed of 480Mbit/s (60MB/s) – a 12x improvement over USB 1.1. Independent tests have shown that the vast majority of controllers are able to transfer no more than 40MB/s, however, due in part to protocol overheads. What’s important to note is that USB 2.0 ports are identical and backwardly-compatible with USB1.1/1.0.
USB 2.0 also adds a number of features such as USB On-The-Go - where devices can communicate without requiring a dedicated PC as a bridge – and battery-charging protocol.
USB 3.0, aka SuperSpeed, to be commercially released in 2010, offers a potential 4,800Mbit/s (600MB/s) transfer rate but requires a newer, larger connector to accommodate the extra bandwidth. Generally backwardly-compatible with previous USB modes, it will become the de facto standard when launched.
High-speed peripheral connectivity – FireWire
IEEE 1394 is an Apple-developed interconnect that does much the same job as USB. Launched one year earlier, in 1995, as a replacement for SCSI, the high-speed bus is known is generally known by the Apple brand-name of ‘FireWire’. Sony markets the standard as I.LINK.
FireWire can support up to 63 other devices per controller and provide far greater power – up to a maximum 45W at up to 30V - to peripherals without the need for auxiliary power. That’s why it’s been particularly suited to A/V equipment such as camcorders and external hard drives holding video data.
Connection
FireWire is hot-swappable and usually connects to the host system via a six-pin connector that’s terminated on the other end via either four-pin or six-pin interfaces. FireWire, however, can also run through a range of different connectors, including coaxial, fibre-optic, and Ethernet.
Speed
Launched as IEEE 1394 with a maximum transfer speed of 400Mbit/s (50MB/s), the pragmatic, real-world transfer speed is faster than even USB 2.0’s, as there’s less protocol overhead when moving data. Independent tests have verified FireWire’s bandwidth superiority over USB 2.0.
A revised specification, IEEE 1394a, was launched in 2000 and introduced the smaller four-pin connector. A larger connector was debuted in 2002 under the IEEE 1394b standard. The nine-pin connector, backwardly-compatible with previous modes, increased speed to 800Mbit/s (100MB/s). FireWire S1600 and S3200 standards are now in place and will compete against USB 3.0 in 2010, and beyond.
Fundamentally a better technology than USB on many fronts, the uptake of FireWire hasn’t been as pronounced. Apple initially charged considerable patent fees for the adoption of the standard, stifling interest, which have now been dropped. Further, USB’s support from Intel has seen the necessary controllers integrated directly into the motherboard’s chipset, whereas FireWire still requires separate ASICs that increase the total bill of materials (BOM). For this reason, FireWire controllers and connectors tend to be present on higher-end laptops and desktop computers.
High-speed peripheral connectivity – eSATA
SATA (Serial ATA), released in 2003, was designed to replace ATA (aka IDE), the established interconnect between motherboard and internal storage. Advantages such as speed, reduced cable size, hot-swap functionality, and command queueing make SATA a better choice for connecting hard drives and even optical drives to chipsets on motherboards. Now, virtually all shipping motherboard core-logics have built-in SATA controllers, with ATA support by way of third-party ASICs.
Connection
L-shaped SATA connectors are used to couple the drive(s) to the controllers, and only one drive can be attached per controller port unless a specific port-multiplier is used. The connection is robust enough for an internal environment, but some manufacturers provide latching connectors that ensure they don’t come loose when transporting the system.
Speed
The first generation of SATA operated at a maximum 1.5Gbit/s (187MB/s) before protocol overheads. Whilst the speed is faster than the 133MB/s for PATA, newer, larger hard drives can transmit at 200MB/s-plus. Second-generation controllers negotiate at 3Gbit/s (375MB/s) which is enough bandwidth for all but the most esoteric of RAIDed drive setups. Third-generation SATA will provide double the speed, which should be enough for the very fastest multi-channel SSDs.
eSATA
Regular SATA is designed for internal use, but a derivation, eSATA, provides the same functionality to external devices – usually hard drives. Using a different-shaped connector that’s incompatible with internal SATA, eSATA can provide USB 2.0- and FireWire 400-beating bandwidth with the added benefit of a direct connection, without the need for a bridging chip. The connector is more robust than internal SATA, as well.
eSATA, however, doesn’t usually provide host power to the drive and, by intimation, is limited to servicing hard-drives alone. Naturally, a lack of widespread host-based power curtails its effectiveness as a high-speed external interconnect.
Summary
The need to connect external peripherals, at high speed, to a host computer has seen the advent of a number of technologies in the last 15 years. USB and FireWire are two of the most common, servicing a wide variety of peripherals from camcorders to storage devices, and provide key features such as hot-swap ability, host power, multiple devices per port, and plug-and-play usage. Both interconnect standards will launch faster iterations in 2010.
eSATA, on the other hand, isn’t as versatile but does suit users looking for a simple, cheap and speedy method of attaching storage to a single computer.
USB will continue to dominate the mass market in 2010 due to its integration into the majority of shipping chipsets, and all formats promise a sustained transfer speed in excess of 300MB/s.
