29-12-2009, 03:44 PM
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ABSTRACT
FireWire is one of the fastest peripheral standards ever developed, which makes it great for use with multimedia peripherals such as digital video cameras and other high speed devices like the latest hard disk drives and printers. This new technology enables isochronous service while providing the bandwidth needed for audio, imaging, video, and other streaming data. FireWire promises to revolutionize the transport of digital data for computers and for professional and consumer electronics products. By providing an inexpensive, high speed method of interconnecting digital devices.
This paper focuses on the theory and thought behind FireWire as well as technical information, raising issues that need to be addressed.
1. INTRODUCTION
FireWire, originally developed by Apple Computer, Inc is a cross platform implementation of the high speed serial data bus “define by the IEEE 1394-1995 [FireWire 400],IEEE 1394a-2000 [FireWire 800] and IEEE 1394b standards-that move large amounts of data between computers and peripheral devices. Its features simplified cabling, hot swapping and transfer speeds of upto 800 megabits per second. FireWire is a high-speed serial input/output (I/O) technology for connecting peripheral devices to a computer or to each other. It is one of the fastest peripheral standards ever developed and now, at 800 megabits per second (Mbps), its even faster .Based on Apple-developed technology, FireWire was adopted in 1995 as an official industry standard (IEEE 1394) for cross-platform peripheral connectivity. By providing a high-bandwidth, easy-to-use I/O technology, FireWire inspired a new generation of consumer electronics devices from many companies, including Canon, Epson, HP, Iomega, JVC, LaCie, Maxtor, Mitsubishi, Matsushita (Panasonic), Pioneer, Samsung, Sony and Texas Instruments. Products such as DV camcorders, portable external disk drives and MP3 players like the Apple iPod would not be as popular as they are today with-out FireWire. FireWire has also been a boon to professional users because of the high-speed connectivity it has brought to audio and video production systems. In 2001, the Academy of Television Arts & Sciences presented Apple with an Emmy award in recognition of the contributions made by FireWire to the television industry. Now FireWire 800, the next generation of FireWire technology, promises to spur the development of more innovative high-performance devices and applications. FireWire800 (an implementation of the IEEE 1394b standard approved in 2002) doubles the throughput of the original technology, dramatically increases the maximum distance of FireWire connections, and supports many new types of cabling. This technology brief describes the advantages of FireWire 800 and some of the applications for which it is ideally suited.
2. 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. Figure 1 shows what a typical consumer may have attached to their 1394 bus.
Fig 1: A FireWire bus
Because 1394 is a peer-to-peer protocol, a specific host isnâ„¢t required, such as the PC in USB. In Figure 1, the digital camera could easily stream data to both the digital VCR and the DVD-RAM without any assistance from other devices on the bus
FireWire uses 64-bit fixed addressing, based on the IEEE 1212 standard. There are three parts to each packet of information sent by a device over FireWire:
¢ A 10-bit bus ID that is used to determine which FireWire bus the data came from
¢ A 6-bit physical ID that identifies which device on the bus sent the data
¢ A 48-bit storage area that is capable of addressing 256 terabytes of information for each node!
The bus ID and physical ID together comprise the 16-bit node ID, which allows for 64,000 nodes on a system. Individual FireWire cables can run
as long as 4.5 meters. Data can be sent through up to 16 hops for a total maximum distance of 72 meters. Hops occur when devices are daisy-chained together. Look at the example below. The camcorder is connected to the external hard drive connected to Computer A. Computer A is connected to Computer B, which in turn is connected to Computer C. It takes four hops for Computer C to access camera.
3. TRANSFERS AND TRANSACTIONS
The 1394 protocol supports both asynchronous and isochronous data transfers.
Isochronous transfers: Isochronous transfers are always broadcast in a one-to-one or one-to-many fashion. No error correction or 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. Asynchronous transfers are acknowledged and responded to. This allows error-checking and retransmission mechanisms to take place.
Physical layer
The 1394 specification defines four protocol layers, known as the physical layer, the page link layer, the transaction layer, and the serial bus management layer. The layers are illustrated in Figure 2.
Fig 2: IEEE-1394 Protocol 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 arenâ„¢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.
4. CONFIGURATIONS
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. Letâ„¢s take the example of our sample home consumer network shown in Figure 1 . 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.
After reset, all leaf nodes present a Parent_Notify signaling state on their data and strobe pairs. Note that this is a signaling state, not a transmitted packet. The whole tree identification process occurs in a matter of microseconds. In our example, the digital camera will signal the set-top box, the printer will signal the digital VCR, and the DVD-RAM will signal the PC. When a branch node receives the Parent_Notify signal on one of its ports, it marks that port as containing a child, and outputs a Child_Notify signaling state on that portâ„¢s data and strobe pairs. Upon detecting this state, the leaf node marks its port as a parent port and removes the signaling, thereby confirming that the leaf node has accepted the child designation. At this point our bus appears as shown in Figure 3 . The ports marked with a P indicate that a device which is closer to the root node is attached to that port, while a port marked with a C indicates that a node farther away from the root node is attached. The port numbers are arbitrarily assigned during design of the device and play an important part in the self identification process.
Fig 3: Bus after leaf node identification
After the leaf nodes have identified themselves, the digital VCR still has two ports that have not received a Parent_Notify, while the set-top box and the PC branch node both have only one port with an attached device that has not received a Parent_Notify. Therefore, both the set-top box and the PC start to signal a Parent_Notify on the one port that has not yet received one. In this case, the VCR receives the Parent_Notify on both of its remaining ports, which it acknowledges with a Child_Notify condition. Because the VCR has marked all of its ports as children, the VCR becomes the root node. The final configuration is shown in Figure 4.
Fig 4: Bus after tree identification is complete
Note that two nodes can be in contention for root node status at the end of the process. In this case, a random back-off timer is used to eventually settle on a root node. A node can also force itself to become root node by delaying its participation in the tree identification process for a while. See References 1 and 2 for more details.
Self Identification. Once the tree topology is defined, the self identification phase begins. Self identification consists of assigning physical IDs to each node on the bus, having neighboring nodes exchange transmission speed capabilities, and making all of the nodes on the bus aware of the topology that exists. The self identification phase begins with the root node sending an arbitration grant signal to its lowest numbered port. In our example, the digital VCR is the root node and it
signals the set-top box. Since the set-top box is a branch node, it will propagate the Arbitration Grant signal to its lowest numbered port with a child node attached. In our case, this port is the digital camera. Because the digital camera is a leaf node, it cannot propagate the arbitration grant signal downstream any farther, so it assigns itself physical ID 0 and transmits a self ID packet upstream. The branch node (set-top box) repeats the self ID packet to all of its ports with attached devices. Eventually the self ID packet makes its way back up to the root node, which proceeds to transmit the self ID packet down to all devices on its higher-numbered ports. In this manner, all attached devices receive the self ID packet that was transmitted by the digital camera. Upon receiving this packet, all of the other devices increment their self ID counter. The digital camera then signals a self ID done indication upstream to the set-top box, which indicates that all nodes attached downstream on this port have gone through the self ID process. Note that the set-top box does not propagate this signal upstream toward the root node because it hasnâ„¢t completed the self ID process.
The root node will then continue to signal an Arbitration Grant signal to its lowest numbered port which in this case is still the set-top box. Because the set-top box has no other attached devices, it assigns itself physical ID 1 and transmits a self ID packet back upstream. This process continues until all ports on the root node have indicated a self ID done condition. The root node then assigns itself the next physical ID. The root node will always be the highest-numbered device on the bus. If we follow through with our example, we come up with the following physical IDs: digital camera = 0; set-top box = 1; printer = 2; DVD-RAM = 3; PC = 4; and the digital VCR, which is the root node, 5.
Note that during the self ID process, parent and children nodes are also exchanging their maximum speed capabilities. This process also exposes the Achilles heel of the 1394 protocol. Nodes can only transmit as fast as the slowest device between the transmitting node and the receiving node. For example, if the digital camera and the digital VCR are both capable of transmitting at 400Mbps, but the set-top box is only capable of transmitting
at 100Mbps, the high-speed devices cannot use the maximum rate to communicate amongst themselves. The only way around this problem is for the end user to reconfigure the cabling so the low-speed set-top box is not physically between the two high-speed devices.
Also during the self ID process, all nodes wishing to become the isochronous resource manager will indicate this fact in their self ID packet. The highest numbered node that wishes to become resource manager will receive the honor.
5. NORMAL ARBITRATION
Once the configuration process is complete, normal bus operations can begin. To fully understand arbitration, a knowledge of the cycle structure of 1394 is necessary.
A 1394 cycle is a time slice with a nominal 125µs period. The 8kHz cycle clock is kept by the cycle master, which is also the root node. To begin a cycle, the cycle master broadcasts a cycle start packet, which all other devices on the bus use to synchronize their timebases.
Immediately following the cycle start packet, devices that wish to broadcast their isochronous data may arbitrate for the bus. Arbitration consists of signaling your parent node that you wish to gain access to the bus. The parent nodes in turn signal their parents and so on, until the request reaches the root node. In our previous example, suppose the digital camera and the PC wish to stream data over the bus. They both signal their parents that they wish to gain access to the bus. Since the PCâ„¢s parent is the root node, its request is received first and it is granted the bus. From this scenario, it is evident that the closest device to the root node wins the arbitration.
Because isochronous channels can only be used once per cycle, when the next isochronous gap occurs, the PC will no longer participate in the arbitration. This condition allows the digital camera to win the next arbitration. Note that the PC could have more than one isochronous channel, in which case it would win the arbitration until it had no more channels left. This points out the important role of the isochronous resource manager: it will not allow the allotted isochronous channels to require more bandwidth than available.
When the last isochronous channel has transmitted its data, the bus becomes idle waiting for another isochronous channel to begin arbitration. Because there are no more isochronous devices left waiting to transmit, the idle time extends longer than the isochronous gap until it reaches the duration defined as the subaction (or asynchronous) gap. At this time, asynchronous devices may begin to arbitrate for the bus. Arbitration proceeds in the same manner, with the closest device to the root node winning arbitration.
This point brings up an interesting scenario: because asynchronous devices can send more than one packet per cycle, the device closest to the root node (or the root node itself) might be able to hog the bus by always winning the arbitration. This scenario is dealt with using what is called the fairness interval and the arbitration rest gap. The concept is simpleâ€once a node wins the asynchronous arbitration and delivers its packet, it clears its arbitration enable bit. When this bit is cleared, the physical layer no longer participates in the arbitration process, giving devices farther away from the root node a fair shot at gaining access to the bus. When all devices wishing to gain access to the bus have had their fair shot, they all wind up having their arbitration enable bits cleared, meaning no one is trying to gain access to the bus. This causes the idle time on the bus to go longer than the 10µs subaction gap until it finally reaches 20µs, which is called the arbitration reset gap. When the idle time
reaches this point, all devices may reset their arbitration enable bits and arbitration can begin all over again.
Link layer
The page link layer is the interface between the physical layer and the transaction layer. The page link layer is responsible for checking received CRCs and calculating and appending the CRC to transmitted packets. In addition, because isochronous transfers do not use the transaction layer, the page link layer is directly responsible for sending and receiving isochronous data. The page link layer also examines the packet header information and determines the type of transaction that is in progress. This information is then passed up to the transaction layer.
Transaction layer
The transaction layer is used for asynchronous transactions. The 1394 protocol uses a request-response mechanism, with confirmations typically generated within each phase. Several types of transactions are allowed. They are listed as follows:
¢ Simple quadlet (four-byte) read
¢ Simple quadlet write
¢ Variable-length read
¢ Variable-length write
¢ Lock transactions
Lock transactions allow for atomic swap and compare and swap operations to be performed.Transactions can be split, concatenated, or unified. Figure 3 illustrates a split transaction. The split transaction occurs when a device cannot respond fast enough to the transaction request. When a request is received, the node responds with an acknowledge packet. An acknowledge packet is sent after every asynchronous packet.
In fact, the acknowledging device doesnâ„¢t even have to arbitrate for the bus; control of the bus is automatic after receiving an incoming request or response packet
Fig 5: A Split Transaction
In Figure 5, the responder node sends the acknowledge back and then prepares the data that was requested. While this is going on, other devices may be using the bus. Once the responder node has the data ready, it begins to arbitrate for the bus, to send out its response packet containing the desired data. The requester node receives this data and returns an acknowledge packet (also without needing to re-arbitrate for the bus).
If the responder node can prepare the requested data quickly enough, the entire transaction can be concatenated. This removes the need for the responding node to arbitrate for the bus after the acknowledge packet is sent.
For data writes, the acknowledgement can also be the response to the write, which is the case in a unified transaction. If the responder can accept the data fast enough, its acknowledge packet can have a transaction code of complete instead of pending . This eliminates the need for a separate response transaction altogether. Note that unified read and lock transactions arenâ„¢t possible, and the acknowledge packet canâ„¢t return data.
6. 1394a ARBITRATION ENHANCEMENT
The 1394a addendum adds three new types of arbitration to be used with asynchronous nodes: acknowledged accelerated arbitration, fly-by arbitration, and token-style arbitration.
Acknowledged accelerated arbitration. When a responding node also has a request packet to transmit, the responding node can immediately transmit its request without arbitrating for the bus. Normally the responding node would have to go through the standard arbitration process.
Fly-by arbitration. A node that contains several ports must act as a repeater on its active ports. A multiport node may use fly-by arbitration on packets that donâ„¢t require acknowledgement (isochronous packets and acknowledge packets). When a node using this technique is repeating a packet upstream toward the root node, it may concatenate an identical speed packet to the end of the current packet. Note that asynchronous packets may not be added to isochronous packets.
Token-style arbitration. Token-style arbitration requires a group of cooperating nodes. When the cooperating node closest to the root node wins a normal arbitration, it can pass the arbitration grant down to the node farthest from the root. This node sends a normal packet, and all of
the cooperating nodes can use fly-by arbitration to add their packets to the original packet as it heads upstream.
Bus Management
Bus management on a 1394 bus involves several different responsibilities that may be distributed among more than one node. Nodes on the bus must assume the roles of cycle master, isochronous resource manager, and bus manager.
Cycle master. The cycle master initiates the 125µs cycles. The root node must be the cycle master; if a node that is not cycle master capable becomes root node, the bus is reset and a node that is cycle master capable is forced to be the root. The cycle master broadcasts a cycle start packet every 125µs. Note that a cycle start can be delayed while an asynchronous packet is being transmitted or acknowledged. The cycle master deals with this by including the amount of time that the cycle was delayed in the cycle start packet.
Isochronous resource manager. The isochronous resource manager must be isochronous transaction capable. The isochronous resource manager must also implement several additional registers. These registers include the Bus Manager ID Register, the Bus Bandwidth Allocation Register, and the Channel Allocation Register. Isochronous channel allocation is performed by a node that wishes to transmit isochronous packets. These nodes must allocate a channel from the Channel Allocation Register by reading the bits in the 64-bit register. Each channel has one bit associated with it. A channel is available if its bit is set to a logic 1. The requesting node sets the first available channel bit to a logic 0 and uses this bit number as the channel ID.
In addition, the requesting node must examine the Bandwidth Available Register to determine how much bandwidth it can consume. The total amount of bandwidth available is 6,144 allocation units. One allocation unit is the time required to transfer one quadlet at 1,600Mbps. A total of
4,915 allocation units are available for isochronous transfers if any asynchronous transfers are used. Nodes wishing to use isochronous bandwidth must subtract the amount of bandwidth needed from the Bandwidth Available Register.
Bus manager. A bus manager has several functions, including publishing the topology and speed maps, managing power, and optimizing bus traffic. The topology map may be used by nodes with a sophisticated user interface that could instruct the end user on the optimum connection topology to enable the highest throughput between nodes. The speed map is used by nodes to determine what speed it can use to communicate with other nodes.
The bus manager is also responsible for determining whether the node that has become root node is cycle master capable. If it isnâ„¢t, the bus manager searches for a node that is cycle master capable and forces a bus reset that will select that node as root node. The bus manager might not always find a capable node; in this case, at least some of the bus management functions are performed by the isochronous resource manager.
7. KEY FEATURES
Data transfer speeds up to 800 Mbps
Distances up to 100 meters
Plug-and-play connectivity
Highly efficient architecture
Compatibility with current FireWire products
Real-time data delivery
On-bus power
More advanced than USB 2.0
Support for a wide range of devices
Data Transfer Speeds Up to 800 Mbps
FireWire 800 is capable of transferring data at 800 Mbps, twice the speed of the original FireWire. This performance increase has been achieved primarily by using the same highly efficient encoding scheme used by Gigabit Ethernet and Fibre Channel. In fact, the FireWire roadmap outlined in the IEEE 1394b standard will eventually take the theoretical bit rate to 1600 Mbps and then up to a staggering 3200 Mbps. Thatâ„¢s 3.2 gigabits per second, which will make FireWire indispensable for transferring massive data files and for even the most demanding video applications, such as working with uncompressed high-definition (HD) video or multiple standard-definition (SD) video streams.
Distances Up to 100 Meters
Not only is FireWire 800 twice as fast as before, but it can be used over much longer distances. The 1394b specification allows the use of various types of cabling, each offering different speed/distance capabilities, as shown in the following table.
Plug-and-Play Connectivity
FireWire allows for true hot-swappable, plug-and-play connection of peripheral devices. There is no need to shut down the computer before adding or removing a FireWire device. Nor do you need to install drivers, assign unique ID numbers, or connect terminators. You can connect a few devices in a simple chain or add hubs to attach as many as 63 devices to a single FireWire bus. The number of available FireWire buses can be increased via PCI and CardBus cards. FireWire is a true peer-to-peer technology. Using a FireWire hub, multiple computers and FireWire peripherals can be connected at the same time. Such an arrangement would, for instance, enable two computers to share a single FireWire camera.
Highly Efficient Architecture
The added speed and longer cable distance of FireWire 800 come from two major improvements to the underlying technology. First is a new, highly efficient arbitration scheme for all the devices sharing a given FireWire bus. For data to be sent and received successfully (such as from a camcorder to a computer, or from a computer to a disk drive), the various FireWire devices must determine which device gets to transmit when. Otherwise, the data packets would collide and reduce the data transfer rate. In FireWire 400, arbitration is performed after each transaction, so thereâ„¢s a delay before a device wins the arbitration and is allowed to start sending its data. In FireWire 800, the arbitration for the next transaction is done while the current data packet is being sent. This way, the device chosen by the arbitration is ready to send its data as soon as the current transmission is completed. The second improvement brings even greater efficiency. FireWire 800 uses advanced data encoding based on codes used by Gigabit Ethernet and Fibre Channel. This encoding, which is called 8B10B, results in less signal distortion than the original FireWire data/strobe (D/S) encoding. Together, these arbitration and data
encoding improvements are called the beta mode of FireWire operation. Beta mode distinguishes FireWire 800 (1394b) from the 1394-1995 and 1394a legacy modes.
Compatibility with Current FireWire Products
FireWire 800 provides two modes of transmission: the pure beta mode (1394b) and the backward-compatible legacy mode that works with FireWire 400 devices (1394-1995 and 1394a). FireWire 400 devices use a 6-pin or 4-pin connector; FireWire 800 devices use a 9-pin connector. The FireWire 400 ports on Apple systems have 6 pins, while the FireWire 800 ports have 9 pins. Existing FireWire 400 devices can be plugged into either type of port, although in some cases an adapter cable (4-pin to 6-pin, 6-pin to 9-pin, and so on) is required. FireWire 800 devices can achieve FireWire 800 speeds only on the FireWire 800 port.
Real-Time Data Delivery
Unlike many other data transfer technologies, FireWire can guarantee real-time delivery of data. This is critical for streaming media applications such as audio and video, where delayed or out-of-order frames are unacceptable. The data traffic between FireWire nodes is divided into isochronous and asynchronous transfers. Isochronous transfers provide guaranteed transmission opportunities at defined intervals; if a packet is not received successfully, it is not resent. In asynchronous transfers, the intervals between transmissions can vary, and data can be resent if it is missed. For example, sending a live TV broadcast requires isochronous transmission to ensure that each frame arrives on time and in the correct order. By contrast, storing data on a hard disk drive can be done asynchronously. It is okay for dropped data to be resent, and it does not matter in what order the data arrives, because each packet is tagged with an address or sequence number to reliably identify it. FireWire is one of very few interfaces that combine both isochronous and asynchronous
capabilities. FireWire can reserve up to 80 percent of its bandwidth for one or more isochronous channels, making it an excellent interface for applications that require real-time data transmission.
On-Bus Power
Like its predecessor, FireWire 800 provides significant amounts of power on its bus (up to 45 watts, with a maximum of 1.5 amps and 30 volts). This means that many devices can be powered through the FireWire cable and will not need their own power cables and adapters. For example, Appleâ„¢s iPod digital music player uses FireWire as its sole data and power connection. The player can recharge its built-in battery while itâ„¢s downloading new music from your computer.
(Note that some manufacturers offer products that use 4-pin FireWire connectors; these devices cannot receive power from a FireWire bus and must be powered separately. The 9-pin and 6-pin connectors do carry power.) FireWire also includes an aggressive power management scheme; power is used only when actually needed.
More Advanced Than USB 2.0
FireWire and USB have both found their place in the computer and consumer electronics industries. USB is the technology of choice for most computer mice, keyboards, and other lower-bandwidth input devices. FireWire with its higher bandwidth, longer distances, and much higher-powered bus is more suitable for devices such as high- speed external disk drives, digital video (DV), professional audio, high-end digital still cameras, and home entertainment components. The extra speed of FireWire 800 compared with USB 2.0 makes FireWire much more
suitable for bandwidth-intensive applications like video and graphics, which often consume hundreds or even thousands of megabytes of data per file. The short cable distance of USB 2.0 (about five meters) limits its
usefulness in deployments that require long-haul cabling and multiple sources of data, such as sound stages and studios. Another differentiator of FireWire is that it is a true peer-to-peer technology. USB 2.0 works in a master/slave arrangement, meaning that data must be copied to a central computer, then back again. Because FireWire does not require a central host, two or more FireWire peripherals can communicate with each other directly as peers, sending each piece of data across the bus only once, directly to its destination. So at the same speed in Mbps, FireWire can be used to move data much faster than USB. This feature makes FireWire suitable for connecting home entertainment components such as game consoles, home stereos, and TVs. Finally, FireWire is designed to transport useful amounts of power as well as data, so itâ„¢s possible to power and even charge the battery of many FireWire peripherals directly from your computer. While USB 2.0 allows at most 2.5W of power, enough for a simple device like a mouse, FireWire devices can provide or consume up to 45W of power, plenty for high-performance disk drives and rapid battery charging. This feature is of special benefit to users of portable computers, who can take important peripherals on the road without carrying a lot of wall-mount power adapters.
8. SUPPORT FOR A WIDE RANGE OF DEVICES
Mass storage
Storage devices especially portable ones are being radically transformed by the adoption of FireWire. Not only does FireWire permit an external hard disk drive to be mounted by simply connecting a single plug, it can even provide enough electricity to power the drive. FireWire mass storage devices include hard disk drives, magneto-optical drives, high-capacity removable drives, tape drives, and CD/DVD products, including both read-only and read/write drives.
Video
Digital video (DV) camcorders capture video and audio and can send a perfect copy to a computer for editing, adding special effects, and making other modifications to create a finished video. FireWire provides the high-speed connection required to download digital video quickly. FireWire 800 even has the necessary throughput for bandwidth-intensive applications that were not possible over the original FireWire, such as multiple-stream, uncompressed, standard-definition video. The long-distance capability of FireWire 800 also gives production studios and similar businesses more flexibility to locate each piece of equipment where itâ„¢s most appropriate, rather than having to put everything adjacent to the computer.
Digital audio
FireWire delivers the bandwidth required for high-quality digital audio. Even FireWire 400 has enough bandwidth over a single connector for hundreds of channels of noise free, high-resolution digital audio and up to 256 channels of MIDI. FireWire 800 can handle twice as many simultaneous real-time streams. Support for cabling up to 100 meters gives you more configuration options with FireWire 800 than solutions such as USB, enabling you to use a Macintosh system as a virtual patch bay that connects audio devices in situations ranging from a personal studio to a huge multiroom production facility. You can even hot-swap devices in and out of the audio processing chain as your needs change.
Digital recording, processing, and storage bring many advantages to todays musicians, producers, engineers and of course, listeners. These advantages include clean transmission and audio integrity even with multiple generations of copying. FireWire offers a high-speed, flexible bridge between professional digital audio components. In addition, because FireWire has been widely embraced by consumer electronics manufacturers, you can easily integrate video cameras, receivers, and other consumer gear with professional audio equipment.
Digital still cameras
Digital cameras are one of the fastest-growing peripheral segments in computing. They allow you to capture high-quality still images and transfer them digitally to your Macintosh system, eliminating the need for traditional film developing and scanning. FireWire provides a means for transferring images from the camera to the computer that is much faster and more convenient than serial, parallel, or even USB connections especially for high-resolution images that can be hundreds of megabytes in size. The high bandwidth of FireWire 800 will be increasingly important as consumer-product manufacturers offer cameras with higher and higher megapixel ratings.
Printers and Scanners
The benefits of FireWire printers and scanners include faster direct connections for high-quality imaging applications and the ability to free up Ethernet bandwidth by sharing printers within small FireWire-equipped workgroups. Printers and scanners also benefit from the built-in power capabilities of FireWire. Portable printers donâ„¢t need a separate battery, and consumer and film scanners. Donâ„¢t need wall-mounted power supplies.
Home entertainment
Set-top boxes, personal video recorders (PVRs), game consoles, home stereo equipment, DVD players, digital TVs, interactive TVs, and computers all have a need to communicate using a common plug-and-play, high-speed interconnection thatâ„¢s capable of efficiently transmitting video, audio, graphics, and Internet data. More and more of these devices are already available with FireWire ports, so they can easily be integrated into the modern home entertainment system.
Networking
Workgroup computers that have FireWire or other 1394 ports can be linked via FireWire and communicate using standard IP networking protocols. FireWire can also be used to cost-effectively share a printer, scanner, camera, or other device. Networking with FireWire offers several advantages over Ethernet. FireWire 400 is faster than 100BASE-T, the most common Ethernet speed, and FireWire 800 comes close to the speed of 1000BASE-T. FireWire 800, with its ability to guarantee the timing interval of data packets, can also deliver smooth real-time video.
9. HARDWARE AND SOFTWARE SUPPORT
Hardware. Several manufacturers offer components for engineers designing systems to support IEEE 1394. Integrated circuit providers typically provide a chipset that includes a page link layer controller and a physical layer controller. One of the goals of the 1394a addendum is to provide interoperability among the various page link layer and physical layer controllers. Complete PCI-based cards that plug into a PC backplane are available from companies such as Adaptec, Sony, and Texas Instruments.
Software. IEEE 1394 is directly supported in the new Windows Driver Model (WDM), which is used in Windows 98 and will be available in Windows NT 5.0. For chipsets and devices to support the drivers provided in the new versions of Windows, several members of the 1394 Trade Association have banded together to create the 1394 Open Host Controller Interface (OHCI) Specification. The OHCI provides a page link layer controller, as well as bus management functionality. In addition, the OHCI defines several DMA controllers for asynchronous and isochronous transactions. These controllers provide registers that a standard 1394 driver, provided by Microsoft, can use to configure the controller and schedule transactions.
Microsoft provides WDM streaming drivers to support audio and video devices such as DVD players, MPEG decoders, tuners, and audio codecs. These streaming drivers permit low-latency support for isochronous channels. The drivers minimize the transitions between user mode and kernel mode, which significantly reduces the overhead for driver calls and data movement.
For storage devices, printers, and scanners, Windows NT 5.0 supports the Serial Block Protocol (SBP-2). Microsoft recommends that devices be written to support the SCSI command set so the device can use the existing SCSI class driver that sits on top of the SBP-2 driver. If the vendor doesnâ„¢t support the SCSI protocol, they will need to write their own class driver to support their own command set.
In addition to the SBP-2 specification for storage devices, other standard data formats that ride on top of 1394 are in various stages of completion. These include the Tailgate specification, which defines a method for adapting ATA/ATAPI controllers to 1394, a digital video (DV) standard, and a printer protocol. The Digital Still Image Working Group and an industrial control and instrumentation group are also working on related standards.
10. CONCLUSION
FireWire is one of the fastest peripherals ever developed, which makes it great for use with multimedia peripherals such as digital video cameras and other high speed devices like the latest hard disk drives and printers. It provides an inexpensive, high speed method of interconnecting digital devices. FireWire800 doubles the throughput of the original technology dramatically increases the maximum distance of FireWire connections, and supports many new types of cabling. The FireWire roadmap outlined is assumed to take upto a staggering rate of 3200Mbps from the theoretical 1660Mbps.that is 3.2 gigabits per second that will make the FireWire indispensable for transferring massive data files and for even the most demanding video applications, such as working with uncompressed high definition video or multiple standard definition video streams.
REFERENCE
¢ WEBSITES
¢ 1394ta.org
¢ embedded1999/9906/9906feat2.htm
¢ FireWire.com
¢ tisc/docs/msp/1394.htm
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ABSTRACT
FireWire is one of the fastest peripheral standards ever developed, which makes it great for use with multimedia peripherals such as digital video cameras and other high speed devices like the latest hard disk drives and printers. This new technology enables isochronous service while providing the bandwidth needed for audio, imaging, video, and other streaming data. FireWire promises to revolutionize the transport of digital data for computers and for professional and consumer electronics products. By providing an inexpensive, high speed method of interconnecting digital devices.
This paper focuses on the theory and thought behind FireWire as well as technical information, raising issues that need to be addressed.
1. INTRODUCTION
FireWire, originally developed by Apple Computer, Inc is a cross platform implementation of the high speed serial data bus “define by the IEEE 1394-1995 [FireWire 400],IEEE 1394a-2000 [FireWire 800] and IEEE 1394b standards-that move large amounts of data between computers and peripheral devices. Its features simplified cabling, hot swapping and transfer speeds of upto 800 megabits per second. FireWire is a high-speed serial input/output (I/O) technology for connecting peripheral devices to a computer or to each other. It is one of the fastest peripheral standards ever developed and now, at 800 megabits per second (Mbps), its even faster .Based on Apple-developed technology, FireWire was adopted in 1995 as an official industry standard (IEEE 1394) for cross-platform peripheral connectivity. By providing a high-bandwidth, easy-to-use I/O technology, FireWire inspired a new generation of consumer electronics devices from many companies, including Canon, Epson, HP, Iomega, JVC, LaCie, Maxtor, Mitsubishi, Matsushita (Panasonic), Pioneer, Samsung, Sony and Texas Instruments. Products such as DV camcorders, portable external disk drives and MP3 players like the Apple iPod would not be as popular as they are today with-out FireWire. FireWire has also been a boon to professional users because of the high-speed connectivity it has brought to audio and video production systems. In 2001, the Academy of Television Arts & Sciences presented Apple with an Emmy award in recognition of the contributions made by FireWire to the television industry. Now FireWire 800, the next generation of FireWire technology, promises to spur the development of more innovative high-performance devices and applications. FireWire800 (an implementation of the IEEE 1394b standard approved in 2002) doubles the throughput of the original technology, dramatically increases the maximum distance of FireWire connections, and supports many new types of cabling. This technology brief describes the advantages of FireWire 800 and some of the applications for which it is ideally suited.
2. 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. Figure 1 shows what a typical consumer may have attached to their 1394 bus.
Fig 1: A FireWire bus
Because 1394 is a peer-to-peer protocol, a specific host isnâ„¢t required, such as the PC in USB. In Figure 1, the digital camera could easily stream data to both the digital VCR and the DVD-RAM without any assistance from other devices on the bus
FireWire uses 64-bit fixed addressing, based on the IEEE 1212 standard. There are three parts to each packet of information sent by a device over FireWire:
¢ A 10-bit bus ID that is used to determine which FireWire bus the data came from
¢ A 6-bit physical ID that identifies which device on the bus sent the data
¢ A 48-bit storage area that is capable of addressing 256 terabytes of information for each node!
The bus ID and physical ID together comprise the 16-bit node ID, which allows for 64,000 nodes on a system. Individual FireWire cables can run
as long as 4.5 meters. Data can be sent through up to 16 hops for a total maximum distance of 72 meters. Hops occur when devices are daisy-chained together. Look at the example below. The camcorder is connected to the external hard drive connected to Computer A. Computer A is connected to Computer B, which in turn is connected to Computer C. It takes four hops for Computer C to access camera.
3. TRANSFERS AND TRANSACTIONS
The 1394 protocol supports both asynchronous and isochronous data transfers.
Isochronous transfers: Isochronous transfers are always broadcast in a one-to-one or one-to-many fashion. No error correction or 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. Asynchronous transfers are acknowledged and responded to. This allows error-checking and retransmission mechanisms to take place.
Physical layer
The 1394 specification defines four protocol layers, known as the physical layer, the page link layer, the transaction layer, and the serial bus management layer. The layers are illustrated in Figure 2.
Fig 2: IEEE-1394 Protocol 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 arenâ„¢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.
4. CONFIGURATIONS
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. Letâ„¢s take the example of our sample home consumer network shown in Figure 1 . 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.
After reset, all leaf nodes present a Parent_Notify signaling state on their data and strobe pairs. Note that this is a signaling state, not a transmitted packet. The whole tree identification process occurs in a matter of microseconds. In our example, the digital camera will signal the set-top box, the printer will signal the digital VCR, and the DVD-RAM will signal the PC. When a branch node receives the Parent_Notify signal on one of its ports, it marks that port as containing a child, and outputs a Child_Notify signaling state on that portâ„¢s data and strobe pairs. Upon detecting this state, the leaf node marks its port as a parent port and removes the signaling, thereby confirming that the leaf node has accepted the child designation. At this point our bus appears as shown in Figure 3 . The ports marked with a P indicate that a device which is closer to the root node is attached to that port, while a port marked with a C indicates that a node farther away from the root node is attached. The port numbers are arbitrarily assigned during design of the device and play an important part in the self identification process.
Fig 3: Bus after leaf node identification
After the leaf nodes have identified themselves, the digital VCR still has two ports that have not received a Parent_Notify, while the set-top box and the PC branch node both have only one port with an attached device that has not received a Parent_Notify. Therefore, both the set-top box and the PC start to signal a Parent_Notify on the one port that has not yet received one. In this case, the VCR receives the Parent_Notify on both of its remaining ports, which it acknowledges with a Child_Notify condition. Because the VCR has marked all of its ports as children, the VCR becomes the root node. The final configuration is shown in Figure 4.
Fig 4: Bus after tree identification is complete
Note that two nodes can be in contention for root node status at the end of the process. In this case, a random back-off timer is used to eventually settle on a root node. A node can also force itself to become root node by delaying its participation in the tree identification process for a while. See References 1 and 2 for more details.
Self Identification. Once the tree topology is defined, the self identification phase begins. Self identification consists of assigning physical IDs to each node on the bus, having neighboring nodes exchange transmission speed capabilities, and making all of the nodes on the bus aware of the topology that exists. The self identification phase begins with the root node sending an arbitration grant signal to its lowest numbered port. In our example, the digital VCR is the root node and it
signals the set-top box. Since the set-top box is a branch node, it will propagate the Arbitration Grant signal to its lowest numbered port with a child node attached. In our case, this port is the digital camera. Because the digital camera is a leaf node, it cannot propagate the arbitration grant signal downstream any farther, so it assigns itself physical ID 0 and transmits a self ID packet upstream. The branch node (set-top box) repeats the self ID packet to all of its ports with attached devices. Eventually the self ID packet makes its way back up to the root node, which proceeds to transmit the self ID packet down to all devices on its higher-numbered ports. In this manner, all attached devices receive the self ID packet that was transmitted by the digital camera. Upon receiving this packet, all of the other devices increment their self ID counter. The digital camera then signals a self ID done indication upstream to the set-top box, which indicates that all nodes attached downstream on this port have gone through the self ID process. Note that the set-top box does not propagate this signal upstream toward the root node because it hasnâ„¢t completed the self ID process.
The root node will then continue to signal an Arbitration Grant signal to its lowest numbered port which in this case is still the set-top box. Because the set-top box has no other attached devices, it assigns itself physical ID 1 and transmits a self ID packet back upstream. This process continues until all ports on the root node have indicated a self ID done condition. The root node then assigns itself the next physical ID. The root node will always be the highest-numbered device on the bus. If we follow through with our example, we come up with the following physical IDs: digital camera = 0; set-top box = 1; printer = 2; DVD-RAM = 3; PC = 4; and the digital VCR, which is the root node, 5.
Note that during the self ID process, parent and children nodes are also exchanging their maximum speed capabilities. This process also exposes the Achilles heel of the 1394 protocol. Nodes can only transmit as fast as the slowest device between the transmitting node and the receiving node. For example, if the digital camera and the digital VCR are both capable of transmitting at 400Mbps, but the set-top box is only capable of transmitting
at 100Mbps, the high-speed devices cannot use the maximum rate to communicate amongst themselves. The only way around this problem is for the end user to reconfigure the cabling so the low-speed set-top box is not physically between the two high-speed devices.
Also during the self ID process, all nodes wishing to become the isochronous resource manager will indicate this fact in their self ID packet. The highest numbered node that wishes to become resource manager will receive the honor.
5. NORMAL ARBITRATION
Once the configuration process is complete, normal bus operations can begin. To fully understand arbitration, a knowledge of the cycle structure of 1394 is necessary.
A 1394 cycle is a time slice with a nominal 125µs period. The 8kHz cycle clock is kept by the cycle master, which is also the root node. To begin a cycle, the cycle master broadcasts a cycle start packet, which all other devices on the bus use to synchronize their timebases.
Immediately following the cycle start packet, devices that wish to broadcast their isochronous data may arbitrate for the bus. Arbitration consists of signaling your parent node that you wish to gain access to the bus. The parent nodes in turn signal their parents and so on, until the request reaches the root node. In our previous example, suppose the digital camera and the PC wish to stream data over the bus. They both signal their parents that they wish to gain access to the bus. Since the PCâ„¢s parent is the root node, its request is received first and it is granted the bus. From this scenario, it is evident that the closest device to the root node wins the arbitration.
Because isochronous channels can only be used once per cycle, when the next isochronous gap occurs, the PC will no longer participate in the arbitration. This condition allows the digital camera to win the next arbitration. Note that the PC could have more than one isochronous channel, in which case it would win the arbitration until it had no more channels left. This points out the important role of the isochronous resource manager: it will not allow the allotted isochronous channels to require more bandwidth than available.
When the last isochronous channel has transmitted its data, the bus becomes idle waiting for another isochronous channel to begin arbitration. Because there are no more isochronous devices left waiting to transmit, the idle time extends longer than the isochronous gap until it reaches the duration defined as the subaction (or asynchronous) gap. At this time, asynchronous devices may begin to arbitrate for the bus. Arbitration proceeds in the same manner, with the closest device to the root node winning arbitration.
This point brings up an interesting scenario: because asynchronous devices can send more than one packet per cycle, the device closest to the root node (or the root node itself) might be able to hog the bus by always winning the arbitration. This scenario is dealt with using what is called the fairness interval and the arbitration rest gap. The concept is simpleâ€once a node wins the asynchronous arbitration and delivers its packet, it clears its arbitration enable bit. When this bit is cleared, the physical layer no longer participates in the arbitration process, giving devices farther away from the root node a fair shot at gaining access to the bus. When all devices wishing to gain access to the bus have had their fair shot, they all wind up having their arbitration enable bits cleared, meaning no one is trying to gain access to the bus. This causes the idle time on the bus to go longer than the 10µs subaction gap until it finally reaches 20µs, which is called the arbitration reset gap. When the idle time
reaches this point, all devices may reset their arbitration enable bits and arbitration can begin all over again.
Link layer
The page link layer is the interface between the physical layer and the transaction layer. The page link layer is responsible for checking received CRCs and calculating and appending the CRC to transmitted packets. In addition, because isochronous transfers do not use the transaction layer, the page link layer is directly responsible for sending and receiving isochronous data. The page link layer also examines the packet header information and determines the type of transaction that is in progress. This information is then passed up to the transaction layer.
Transaction layer
The transaction layer is used for asynchronous transactions. The 1394 protocol uses a request-response mechanism, with confirmations typically generated within each phase. Several types of transactions are allowed. They are listed as follows:
¢ Simple quadlet (four-byte) read
¢ Simple quadlet write
¢ Variable-length read
¢ Variable-length write
¢ Lock transactions
Lock transactions allow for atomic swap and compare and swap operations to be performed.Transactions can be split, concatenated, or unified. Figure 3 illustrates a split transaction. The split transaction occurs when a device cannot respond fast enough to the transaction request. When a request is received, the node responds with an acknowledge packet. An acknowledge packet is sent after every asynchronous packet.
In fact, the acknowledging device doesnâ„¢t even have to arbitrate for the bus; control of the bus is automatic after receiving an incoming request or response packet
Fig 5: A Split Transaction
In Figure 5, the responder node sends the acknowledge back and then prepares the data that was requested. While this is going on, other devices may be using the bus. Once the responder node has the data ready, it begins to arbitrate for the bus, to send out its response packet containing the desired data. The requester node receives this data and returns an acknowledge packet (also without needing to re-arbitrate for the bus).
If the responder node can prepare the requested data quickly enough, the entire transaction can be concatenated. This removes the need for the responding node to arbitrate for the bus after the acknowledge packet is sent.
For data writes, the acknowledgement can also be the response to the write, which is the case in a unified transaction. If the responder can accept the data fast enough, its acknowledge packet can have a transaction code of complete instead of pending . This eliminates the need for a separate response transaction altogether. Note that unified read and lock transactions arenâ„¢t possible, and the acknowledge packet canâ„¢t return data.
6. 1394a ARBITRATION ENHANCEMENT
The 1394a addendum adds three new types of arbitration to be used with asynchronous nodes: acknowledged accelerated arbitration, fly-by arbitration, and token-style arbitration.
Acknowledged accelerated arbitration. When a responding node also has a request packet to transmit, the responding node can immediately transmit its request without arbitrating for the bus. Normally the responding node would have to go through the standard arbitration process.
Fly-by arbitration. A node that contains several ports must act as a repeater on its active ports. A multiport node may use fly-by arbitration on packets that donâ„¢t require acknowledgement (isochronous packets and acknowledge packets). When a node using this technique is repeating a packet upstream toward the root node, it may concatenate an identical speed packet to the end of the current packet. Note that asynchronous packets may not be added to isochronous packets.
Token-style arbitration. Token-style arbitration requires a group of cooperating nodes. When the cooperating node closest to the root node wins a normal arbitration, it can pass the arbitration grant down to the node farthest from the root. This node sends a normal packet, and all of
the cooperating nodes can use fly-by arbitration to add their packets to the original packet as it heads upstream.
Bus Management
Bus management on a 1394 bus involves several different responsibilities that may be distributed among more than one node. Nodes on the bus must assume the roles of cycle master, isochronous resource manager, and bus manager.
Cycle master. The cycle master initiates the 125µs cycles. The root node must be the cycle master; if a node that is not cycle master capable becomes root node, the bus is reset and a node that is cycle master capable is forced to be the root. The cycle master broadcasts a cycle start packet every 125µs. Note that a cycle start can be delayed while an asynchronous packet is being transmitted or acknowledged. The cycle master deals with this by including the amount of time that the cycle was delayed in the cycle start packet.
Isochronous resource manager. The isochronous resource manager must be isochronous transaction capable. The isochronous resource manager must also implement several additional registers. These registers include the Bus Manager ID Register, the Bus Bandwidth Allocation Register, and the Channel Allocation Register. Isochronous channel allocation is performed by a node that wishes to transmit isochronous packets. These nodes must allocate a channel from the Channel Allocation Register by reading the bits in the 64-bit register. Each channel has one bit associated with it. A channel is available if its bit is set to a logic 1. The requesting node sets the first available channel bit to a logic 0 and uses this bit number as the channel ID.
In addition, the requesting node must examine the Bandwidth Available Register to determine how much bandwidth it can consume. The total amount of bandwidth available is 6,144 allocation units. One allocation unit is the time required to transfer one quadlet at 1,600Mbps. A total of
4,915 allocation units are available for isochronous transfers if any asynchronous transfers are used. Nodes wishing to use isochronous bandwidth must subtract the amount of bandwidth needed from the Bandwidth Available Register.
Bus manager. A bus manager has several functions, including publishing the topology and speed maps, managing power, and optimizing bus traffic. The topology map may be used by nodes with a sophisticated user interface that could instruct the end user on the optimum connection topology to enable the highest throughput between nodes. The speed map is used by nodes to determine what speed it can use to communicate with other nodes.
The bus manager is also responsible for determining whether the node that has become root node is cycle master capable. If it isnâ„¢t, the bus manager searches for a node that is cycle master capable and forces a bus reset that will select that node as root node. The bus manager might not always find a capable node; in this case, at least some of the bus management functions are performed by the isochronous resource manager.
7. KEY FEATURES
Data transfer speeds up to 800 Mbps
Distances up to 100 meters
Plug-and-play connectivity
Highly efficient architecture
Compatibility with current FireWire products
Real-time data delivery
On-bus power
More advanced than USB 2.0
Support for a wide range of devices
Data Transfer Speeds Up to 800 Mbps
FireWire 800 is capable of transferring data at 800 Mbps, twice the speed of the original FireWire. This performance increase has been achieved primarily by using the same highly efficient encoding scheme used by Gigabit Ethernet and Fibre Channel. In fact, the FireWire roadmap outlined in the IEEE 1394b standard will eventually take the theoretical bit rate to 1600 Mbps and then up to a staggering 3200 Mbps. Thatâ„¢s 3.2 gigabits per second, which will make FireWire indispensable for transferring massive data files and for even the most demanding video applications, such as working with uncompressed high-definition (HD) video or multiple standard-definition (SD) video streams.
Distances Up to 100 Meters
Not only is FireWire 800 twice as fast as before, but it can be used over much longer distances. The 1394b specification allows the use of various types of cabling, each offering different speed/distance capabilities, as shown in the following table.
Plug-and-Play Connectivity
FireWire allows for true hot-swappable, plug-and-play connection of peripheral devices. There is no need to shut down the computer before adding or removing a FireWire device. Nor do you need to install drivers, assign unique ID numbers, or connect terminators. You can connect a few devices in a simple chain or add hubs to attach as many as 63 devices to a single FireWire bus. The number of available FireWire buses can be increased via PCI and CardBus cards. FireWire is a true peer-to-peer technology. Using a FireWire hub, multiple computers and FireWire peripherals can be connected at the same time. Such an arrangement would, for instance, enable two computers to share a single FireWire camera.
Highly Efficient Architecture
The added speed and longer cable distance of FireWire 800 come from two major improvements to the underlying technology. First is a new, highly efficient arbitration scheme for all the devices sharing a given FireWire bus. For data to be sent and received successfully (such as from a camcorder to a computer, or from a computer to a disk drive), the various FireWire devices must determine which device gets to transmit when. Otherwise, the data packets would collide and reduce the data transfer rate. In FireWire 400, arbitration is performed after each transaction, so thereâ„¢s a delay before a device wins the arbitration and is allowed to start sending its data. In FireWire 800, the arbitration for the next transaction is done while the current data packet is being sent. This way, the device chosen by the arbitration is ready to send its data as soon as the current transmission is completed. The second improvement brings even greater efficiency. FireWire 800 uses advanced data encoding based on codes used by Gigabit Ethernet and Fibre Channel. This encoding, which is called 8B10B, results in less signal distortion than the original FireWire data/strobe (D/S) encoding. Together, these arbitration and data
encoding improvements are called the beta mode of FireWire operation. Beta mode distinguishes FireWire 800 (1394b) from the 1394-1995 and 1394a legacy modes.
Compatibility with Current FireWire Products
FireWire 800 provides two modes of transmission: the pure beta mode (1394b) and the backward-compatible legacy mode that works with FireWire 400 devices (1394-1995 and 1394a). FireWire 400 devices use a 6-pin or 4-pin connector; FireWire 800 devices use a 9-pin connector. The FireWire 400 ports on Apple systems have 6 pins, while the FireWire 800 ports have 9 pins. Existing FireWire 400 devices can be plugged into either type of port, although in some cases an adapter cable (4-pin to 6-pin, 6-pin to 9-pin, and so on) is required. FireWire 800 devices can achieve FireWire 800 speeds only on the FireWire 800 port.
Real-Time Data Delivery
Unlike many other data transfer technologies, FireWire can guarantee real-time delivery of data. This is critical for streaming media applications such as audio and video, where delayed or out-of-order frames are unacceptable. The data traffic between FireWire nodes is divided into isochronous and asynchronous transfers. Isochronous transfers provide guaranteed transmission opportunities at defined intervals; if a packet is not received successfully, it is not resent. In asynchronous transfers, the intervals between transmissions can vary, and data can be resent if it is missed. For example, sending a live TV broadcast requires isochronous transmission to ensure that each frame arrives on time and in the correct order. By contrast, storing data on a hard disk drive can be done asynchronously. It is okay for dropped data to be resent, and it does not matter in what order the data arrives, because each packet is tagged with an address or sequence number to reliably identify it. FireWire is one of very few interfaces that combine both isochronous and asynchronous
capabilities. FireWire can reserve up to 80 percent of its bandwidth for one or more isochronous channels, making it an excellent interface for applications that require real-time data transmission.
On-Bus Power
Like its predecessor, FireWire 800 provides significant amounts of power on its bus (up to 45 watts, with a maximum of 1.5 amps and 30 volts). This means that many devices can be powered through the FireWire cable and will not need their own power cables and adapters. For example, Appleâ„¢s iPod digital music player uses FireWire as its sole data and power connection. The player can recharge its built-in battery while itâ„¢s downloading new music from your computer.
(Note that some manufacturers offer products that use 4-pin FireWire connectors; these devices cannot receive power from a FireWire bus and must be powered separately. The 9-pin and 6-pin connectors do carry power.) FireWire also includes an aggressive power management scheme; power is used only when actually needed.
More Advanced Than USB 2.0
FireWire and USB have both found their place in the computer and consumer electronics industries. USB is the technology of choice for most computer mice, keyboards, and other lower-bandwidth input devices. FireWire with its higher bandwidth, longer distances, and much higher-powered bus is more suitable for devices such as high- speed external disk drives, digital video (DV), professional audio, high-end digital still cameras, and home entertainment components. The extra speed of FireWire 800 compared with USB 2.0 makes FireWire much more
suitable for bandwidth-intensive applications like video and graphics, which often consume hundreds or even thousands of megabytes of data per file. The short cable distance of USB 2.0 (about five meters) limits its
usefulness in deployments that require long-haul cabling and multiple sources of data, such as sound stages and studios. Another differentiator of FireWire is that it is a true peer-to-peer technology. USB 2.0 works in a master/slave arrangement, meaning that data must be copied to a central computer, then back again. Because FireWire does not require a central host, two or more FireWire peripherals can communicate with each other directly as peers, sending each piece of data across the bus only once, directly to its destination. So at the same speed in Mbps, FireWire can be used to move data much faster than USB. This feature makes FireWire suitable for connecting home entertainment components such as game consoles, home stereos, and TVs. Finally, FireWire is designed to transport useful amounts of power as well as data, so itâ„¢s possible to power and even charge the battery of many FireWire peripherals directly from your computer. While USB 2.0 allows at most 2.5W of power, enough for a simple device like a mouse, FireWire devices can provide or consume up to 45W of power, plenty for high-performance disk drives and rapid battery charging. This feature is of special benefit to users of portable computers, who can take important peripherals on the road without carrying a lot of wall-mount power adapters.
8. SUPPORT FOR A WIDE RANGE OF DEVICES
Mass storage
Storage devices especially portable ones are being radically transformed by the adoption of FireWire. Not only does FireWire permit an external hard disk drive to be mounted by simply connecting a single plug, it can even provide enough electricity to power the drive. FireWire mass storage devices include hard disk drives, magneto-optical drives, high-capacity removable drives, tape drives, and CD/DVD products, including both read-only and read/write drives.
Video
Digital video (DV) camcorders capture video and audio and can send a perfect copy to a computer for editing, adding special effects, and making other modifications to create a finished video. FireWire provides the high-speed connection required to download digital video quickly. FireWire 800 even has the necessary throughput for bandwidth-intensive applications that were not possible over the original FireWire, such as multiple-stream, uncompressed, standard-definition video. The long-distance capability of FireWire 800 also gives production studios and similar businesses more flexibility to locate each piece of equipment where itâ„¢s most appropriate, rather than having to put everything adjacent to the computer.
Digital audio
FireWire delivers the bandwidth required for high-quality digital audio. Even FireWire 400 has enough bandwidth over a single connector for hundreds of channels of noise free, high-resolution digital audio and up to 256 channels of MIDI. FireWire 800 can handle twice as many simultaneous real-time streams. Support for cabling up to 100 meters gives you more configuration options with FireWire 800 than solutions such as USB, enabling you to use a Macintosh system as a virtual patch bay that connects audio devices in situations ranging from a personal studio to a huge multiroom production facility. You can even hot-swap devices in and out of the audio processing chain as your needs change.
Digital recording, processing, and storage bring many advantages to todays musicians, producers, engineers and of course, listeners. These advantages include clean transmission and audio integrity even with multiple generations of copying. FireWire offers a high-speed, flexible bridge between professional digital audio components. In addition, because FireWire has been widely embraced by consumer electronics manufacturers, you can easily integrate video cameras, receivers, and other consumer gear with professional audio equipment.
Digital still cameras
Digital cameras are one of the fastest-growing peripheral segments in computing. They allow you to capture high-quality still images and transfer them digitally to your Macintosh system, eliminating the need for traditional film developing and scanning. FireWire provides a means for transferring images from the camera to the computer that is much faster and more convenient than serial, parallel, or even USB connections especially for high-resolution images that can be hundreds of megabytes in size. The high bandwidth of FireWire 800 will be increasingly important as consumer-product manufacturers offer cameras with higher and higher megapixel ratings.
Printers and Scanners
The benefits of FireWire printers and scanners include faster direct connections for high-quality imaging applications and the ability to free up Ethernet bandwidth by sharing printers within small FireWire-equipped workgroups. Printers and scanners also benefit from the built-in power capabilities of FireWire. Portable printers donâ„¢t need a separate battery, and consumer and film scanners. Donâ„¢t need wall-mounted power supplies.
Home entertainment
Set-top boxes, personal video recorders (PVRs), game consoles, home stereo equipment, DVD players, digital TVs, interactive TVs, and computers all have a need to communicate using a common plug-and-play, high-speed interconnection thatâ„¢s capable of efficiently transmitting video, audio, graphics, and Internet data. More and more of these devices are already available with FireWire ports, so they can easily be integrated into the modern home entertainment system.
Networking
Workgroup computers that have FireWire or other 1394 ports can be linked via FireWire and communicate using standard IP networking protocols. FireWire can also be used to cost-effectively share a printer, scanner, camera, or other device. Networking with FireWire offers several advantages over Ethernet. FireWire 400 is faster than 100BASE-T, the most common Ethernet speed, and FireWire 800 comes close to the speed of 1000BASE-T. FireWire 800, with its ability to guarantee the timing interval of data packets, can also deliver smooth real-time video.
9. HARDWARE AND SOFTWARE SUPPORT
Hardware. Several manufacturers offer components for engineers designing systems to support IEEE 1394. Integrated circuit providers typically provide a chipset that includes a page link layer controller and a physical layer controller. One of the goals of the 1394a addendum is to provide interoperability among the various page link layer and physical layer controllers. Complete PCI-based cards that plug into a PC backplane are available from companies such as Adaptec, Sony, and Texas Instruments.
Software. IEEE 1394 is directly supported in the new Windows Driver Model (WDM), which is used in Windows 98 and will be available in Windows NT 5.0. For chipsets and devices to support the drivers provided in the new versions of Windows, several members of the 1394 Trade Association have banded together to create the 1394 Open Host Controller Interface (OHCI) Specification. The OHCI provides a page link layer controller, as well as bus management functionality. In addition, the OHCI defines several DMA controllers for asynchronous and isochronous transactions. These controllers provide registers that a standard 1394 driver, provided by Microsoft, can use to configure the controller and schedule transactions.
Microsoft provides WDM streaming drivers to support audio and video devices such as DVD players, MPEG decoders, tuners, and audio codecs. These streaming drivers permit low-latency support for isochronous channels. The drivers minimize the transitions between user mode and kernel mode, which significantly reduces the overhead for driver calls and data movement.
For storage devices, printers, and scanners, Windows NT 5.0 supports the Serial Block Protocol (SBP-2). Microsoft recommends that devices be written to support the SCSI command set so the device can use the existing SCSI class driver that sits on top of the SBP-2 driver. If the vendor doesnâ„¢t support the SCSI protocol, they will need to write their own class driver to support their own command set.
In addition to the SBP-2 specification for storage devices, other standard data formats that ride on top of 1394 are in various stages of completion. These include the Tailgate specification, which defines a method for adapting ATA/ATAPI controllers to 1394, a digital video (DV) standard, and a printer protocol. The Digital Still Image Working Group and an industrial control and instrumentation group are also working on related standards.
10. CONCLUSION
FireWire is one of the fastest peripherals ever developed, which makes it great for use with multimedia peripherals such as digital video cameras and other high speed devices like the latest hard disk drives and printers. It provides an inexpensive, high speed method of interconnecting digital devices. FireWire800 doubles the throughput of the original technology dramatically increases the maximum distance of FireWire connections, and supports many new types of cabling. The FireWire roadmap outlined is assumed to take upto a staggering rate of 3200Mbps from the theoretical 1660Mbps.that is 3.2 gigabits per second that will make the FireWire indispensable for transferring massive data files and for even the most demanding video applications, such as working with uncompressed high definition video or multiple standard definition video streams.
REFERENCE
¢ WEBSITES
¢ 1394ta.org
¢ embedded1999/9906/9906feat2.htm
¢ FireWire.com
¢ tisc/docs/msp/1394.htm
ata transfer speeds up to 800 Mbps
lug-and-play connectivity