The age of SCSI, modem ports, serial ports, parallel ports (you may include seaports and airports if you wish. Hee Hee!!) are numbered. People want Speed - blazing speed. These ports cannot support such high speeds. If these can't then who can? Well, the answer is pretty straight forward and it is provided by none other than my personal favorite - Apple Computer. But whatzit ? It is as Apple calls it - FireWire.
People are sharing video, still images, and audio, and are constantly searching for faster, easier ways of transferring such information. This phenomenon is driving the convergence of computers, consumer equipment, and communications. Communication is the force that draws these separate market segments together.
Convergence will happen when seamless, high-speed communication becomes readily available. The IEEE 1394 protocol appears to be a strong contender for the communications channel that will make this happen.
The IEEE 1394-1995 protocol had its genesis at Apple Computer, which still retains the Firewire trademark. The goal of the protocol is to provide easy-to-use, low-cost, high-speed communications. The protocol is also very scaleable, provides for both asynchronous and isochronous applications, allows for access to vast amounts of memory mapped address space, and—perhaps most important for the aforementioned convergence - allows peer-to-peer communication.
Some people see 1394 and USB as competitors for the communications channel of the future, but in reality they are more complementary than competitive. USB is a lower-speed, lower-cost, host-based protocol. The proposed upgrade of USB to 120Mbps and 240Mbps may alter this situation slightly, but not as much as some have predicted.
Confusion sometimes surrounds the alphabet soup that seems to envelop the 1394 protocol. The only currently approved specification is the IEEE 1394-1995 specification, which will be the basis for future extensions and enhancements. IEEE 1394-1995 supports transfer rates of 100, 200, and 400Mbps. As with many first cuts at a standard, 1394-1995 left some things up to the interpretation of the specification’s implementers, which caused some interoperability problems and has led to the work being done on the 1394a specification. This revision provides some clarification on the original specification, changes some optional portions of the spec to mandatory, and adds some performance enhancements. The 1394a specification is nearing completion and should be approved in the near future; some semiconductor vendors, in fact, are already claiming compliance to the new specification. In addition to the 1394a specification, work is progressing on the 1394b specification. 1394b will provide for additional data rates of 800, 1,600, and 3,200Mbps. It will also provide for long-haul transmissions via both twisted pair and fiber optics, and offer backward compatibility with the existing standard.
Topology
The 1394 protocol is a peer-to-peer network with a point-to-point signaling environment. Nodes on the bus may have several ports on them. Each of these ports acts as a repeater, retransmitting any packets received by other ports within the node.
Because 1394 is a peer-to-peer protocol, a specific host isn’t required, such as the PC in USB.
Configuration of the bus occurs automatically whenever a new device is plugged in. Configuration proceeds from leaf nodes (those with only one other device attached to them) up through the branch nodes. A bus that has three or more devices attached will typically, but not always, have a branch node become the root node. I’ll discuss configuration in more detail later in this article.
A 1394 bus appears as a large memory-mapped space with each node occupying a certain address range. The memory space is based to the IEEE 1212 Control and Status Register (CSR) Architecture with some extensions specific to the 1394 standard. Each node supports up to 48 bits of address space (256 TeraBytes). In addition, each bus can support up to 64 nodes, and the 1394 serial bus specification supports up to 1,024 buses. This gives a grand total of 64 address bits, or support for a whopping total of 16 ExaBytes of memory space—enough for the latest version of your favorite word processor and perhaps even a file or two!
Transfers and transactions
The 1394 protocol supports both asynchronous and isochronous data transfers, as I’ll discuss in the following paragraphs.
Isochronous transfers. Isochronous transfers are always broadcast in a one-to-one or one-to-many fashion. No error correction nor retransmission is available for isochronous transfers. Up to 80% of the available bus bandwidth can be used for isochronous transfers. The delegation of bandwidth is tracked by a node on the bus that occupies the role of isochronous resource manager. This may or may not be the root node or the bus manager. The maximum amount of bandwidth an isochronous device can obtain is only limited by the number of other isochronous devices that have already obtained bandwidth from the isochronous resource manager.
Asynchronous transfers. Asynchronous transfers are targeted to a specific node with an explicit address. They are not guaranteed a specific amount of bandwidth on the bus, but they are guaranteed a fair shot at gaining access to the bus when asynchronous transfers are permitted. The maximum data block size for an asynchronous packet is determined by the transfer rate of the device.
Asynchronous transfers are acknowledged and responded to. This allows error-checking and retransmission mechanisms to take place.
The bottom line is that if you’re sending time-critical, error-tolerant data, such as a video or audio stream, isochronous transfers are the way to go. If the data isn’t error-tolerant, such as a disk drive, then asynchronous transfers are preferable.
Physical Layer
The 1394 specification defines four protocol layers, known as the physical layer, the link layer, the transaction layer, and the serial bus management layer.
The physical layer of the 1394 protocol includes the electrical signaling, the mechanical connectors and cabling, the arbitration mechanisms, and the serial coding and decoding of the data being transferred or received. The cable media is defined as a three-pair shielded cable. Two of the pairs are used to transfer data, while the third pair provides power on the bus. The connectors are small six-pin devices, although the 1394a also defines a four-pin connector for self- powered leaf nodes. The power signals aren’t provided on the four-pin connector. The baseline cables are limited to 4.5m in length. Thicker cables allow for longer distances.
The two twisted pairs used for signaling, called out as TPA and TPB, are bidirectional and are tri-state capable. TPA is used to transmit the strobe signal and receive data, while TPB is used to receive the strobe signal and transmit data. The signaling mechanism uses data strobe encoding, a rather clever technique that allows easy extraction of a clock signal with much better jitter tolerance than a standard clock/data mechanism. With data strobe encoding, either the data or the strobe signal (but not both of them) change in a bit cell.
Configuration
The physical layer plays a major role in the bus configuration and normal arbitration phases of the protocol. Configuration consists of taking a relatively flat physical topology and turning it into a logical tree structure with a root node at its focal point. A bus is reset and reconfigured whenever a device is added or removed. A reset can also be initiated via software. Configuration consists of bus reset and initialization, tree identification, and self identification.
Reset. Reset is signaled by a node driving both TPA and TPB to logic 1. Because of the “dominant 1s” electrical definition of the drivers, a logic 1 will always be detected by a port, even if its bidirectional driver is in the transmit state. When a node detects a reset condition on its drivers, it will propagate this signal to all of the other ports that this node supports. The node then enters the idle state for a given period of time to allow the reset indication to propagate to all other nodes on the bus. Reset clears any topology information within the node, although isochronous resources are “sticky” and will tend to remain the same during resets.
Tree identification. The tree identification process defines the bus topology. After reset, but before tree identification, the bus has a flat logical topology that maps directly to the physical topology. After tree identification is complete, a single node has gained the status of root node. The tree identification proceeds as follows.
Both I/O technologies offer incredible convenience through their �hot plug� capability, eliminating any need to turn off or restart the computer when attaching a new peripheral. For additional ease of use, they also feature automatic configuration, no device IDs or terminators, and simple-to-use cables. USB can support up to 127 devices per computer, and FireWire up to 63 devices. Both technologies provide their own bus power, enabling peripherals to be even simpler. And both technologies are cross-platform industry standards.
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