Another aspect of the Link layer that should be considered is the amount of data-buffer memory supported. Typically, the more bandwidth an application requires, or the more simultaneous isochronous/asynchronous traffic that needs to be supported, the larger the buffer memories must be.
Per the IEC 61883-4 specification for MPEG-2 transmission, the minimum amount of buffer memory necessary in a 1394 receiver is 3264 bytes—based on a single 60-Mbit/s transport stream. Figure 4 gives the details on how this buffer size is derived. The buffer is needed to remove the jitter induced by the 1394 transmitter due to variable delays in bus arbitration and access-grant, variable amounts of 1394 traffic on the bus, and the lack of sync be-tween the MPEG stream input rate and the 1394 cycle timer.
As the number of simultaneous isochronous channels present goes up, or the bit rate of an individual stream increases, this receive buffer needs to be larger. System designers should analyze the various applications that require support to determine both the maximum MPEG bit rate and maximum number of isochronous channels to be simultaneously present. Then, designers can select a 1394 Link layer to best meet the receive-buffer memory requirements.
The IEC 61883-4 and ITU-R BO.1294 over 1394 specifications cover additional buffer requirements for smoothing of MPEG network jitter. This buffer amount should be considered too, especially in cases where a full transport stream will be decimated to a single program transport stream either at the 1394 transmit node prior to being transmitted or at the 1394 receiving node at reception.
Copy protection: The proliferation of digital media has created a need to safeguard the rights of content owners with regard to protected A/V data (movies on DVD, movies purchased via pay per view, and so on). Basically, all digital I/O types will require some form of copy protection technology for these media.
The 1394 protection method, titled Digital Transmission Content Protection (DTCP), is sometimes referred to as "5C" for the originating five companies' merged proposals. DTCP requires data to be encrypted prior to transmission. Only authorized equipment can successfully participate in any DTCP transaction. DTCP requires that nodes encrypt outgoing data and decrypt incoming data via a special cipher, called "M6."
Designers should select 1394 silicon that provides support for DTCP authentication and key exchange algorithms and encrypt/decrypt functions. The chip set should additionally support as many individual decrypt/encrypt ciphers as the maximum number of simultaneous isochronous channels required to be supported.
In the offing: Anyone interested in 1394 applications in consumer electronics should watch certain areas closely, including digital audio, content protection, SBP-3 ongoing work, and DVD Forum actions.
Firewire is a natural for digital audio. As DVD and CD players move to DVD-Audio and Super-Audio CD playback capability, support for carrying high-quality, multichannel audio across a networked digital interface will drive future requirements into the 1394 interface. The DVD Forum has developed a draft specification aimed at defining the 1394 interface for DVD players, specifying the format and transport protocols for both audio and video output.
SBP-3 work has begun inside Technical Committee T10 of the NCITS. This ongoing work seeks, among other things, to add isochronous services to the already robust asynchronous support in SBP-2.
The recent licensing agreement be-tween the DTCP and Sony and Warner Brothers is an important milestone, paving the way for these two content owners to begin supplying digital content to 5C-enabled digital consumer equipment. Look for other content owners to license this technology soon. Additionally, designers should be aware of the "robustness" rules as dictated by the DTCP license agreement.
Table 2 provides a list of those standards and specifications that deal with 1394 and its use in consumer electronics applications. Standards listed but not freely available at the indicated Web site can be purchased from Global Engineering Documents at http://global.ihs.com.