Options for coding include scrambling, 8B/10B coding, 16B/18B coding, and multilevel analog signaling (MAS). There's also MB810 coding, 64B/66B coding, or combinations of two or more, as in Cisco's implementation. As outlined previously, however, problems do reside within Cisco's proposal. The proposed busy-idle character is defined in addition to the normal idle character. It also uses HP's 64B/66B code to provide frame delimiting without needing to know the frame length or overwriting the frame preamble with length information. This gives the 3% overhead for the 64B/66B coding, and HP hasn't yet disclosed 64B/66B licensing terms. These terms need to be ironed out if progress is to be made. Also, 64B/66B coding is a new, unproven protocol unlike 8B/10B coding, which is robust and well known.
As a result, and in keeping with the idea that a unified LAN/WAN design is better for all concerned in terms of cost and ease of implementation, Nortel and Vitesse Semiconductor are each supporting a unified alternative (Fig. 5). Their proposal incorporates the word-hold, null-word-insertion scheme proposed by Nortel, wherein the PHY asserts the word-hold line when the PHY's FIFO is nearly full. This causes the MAC to send a null word in the data stream out to the PHY. The PHY will discard the null word, lowering the actual data rate to 9.58 Gbits/s—the preferred data payload rate for OC-192 compatibility.
Avoiding Proprietary Technology
This approach yields a 3% improvement in data throughput compared to the 64B/66B scheme. It also avoids the potential issue of proprietary technology licensing, albeit with the necessity of overwriting the preamble with frame-length information.
Regardless of its shortcomings, though, Rick Walker and Richard Dugan of Agilent Technologies are getting a lot of support for their proposed 64B/10B implementation (Fig. 6). Much of this positive response is due to its balanced coding and relatively low overhead. It's also particularly suited to the Hari proposal for two reasons: Only a limited number of characters are needed, and data is transmitted in contiguous blocks of at least 64 octets.
Of the other coding schemes, MAS seems to be garnering the most interest. It's a narrowband, low-dispersion, low-EMI coding scheme that uses amplitude modulation at 2.5 GHz and 10 Gbits/s. MAS takes an 8B/10B precoded input and is compatible with GMII extensions and Gigabit Ethernet PMDs. Plus, it can use existing MMF and SMF cable. It also can utilize 2.5-Gbit/s optics and possibly GbE optics, but it requires linear lasers and high-speed digital-to-analog and analog-to-digital converters (DACs and ADCs).
The MB810 scheme seems to have all of the right characteristics. Its implementation is so complicated, though, that many voters are having a hard time understanding it. Its validity is to be determined.
So while this is being written, voters are gearing up for the next 802.3ae Committee meeting in Albuquerque, N.M., scheduled for March 6-9. At that meeting, the brainstorming phase will be officially closed and voting will begin to narrow down the proposals to a manageable few, from 17 to about five or six. It's expected that at least four serial links will be left, with one parallel (wide WDM) proposal. By July of this year, no more will be accepted. In theory, the standard should be completed by March 2002. If that's to become a reality, a lot of issues—both technical and political—must be put to bed. For now, though, the potential financial rewards seem to be motivating the various factions to resolve their differences.