Free Tutors on Fibre Chanel cables, plugs and signal encoding

Tutors on Fibre Chanel cables, plugs and signal encoding

Fibre Chanel defines the physical transmission medium (cable, plug) and specifies which physical signals are used to transmit the bits '0' and '1'. In contrast to the SCSI bus, in which each bit has its own data line plus additional control lines. Fibre Channel transmits the bits sequentially via a single line. In general, buses come up against the problem that the signals have a different transit time on the different data lines (skew), which means tha the speed can only be increased to a limited degree in buses. The different signal transit times can be visualized as the hand rail in an escalator that runs faster or slower than the escalator stairs themselves. Fibre Channel therefore transmits the bits serially. This means that, in contrast to the parallel bus, a high transfer rate is possible even over long distances. The high transfer rate of serial transmission more than compensates for the parallel lines of a bus. Transfer rates of 200 MByte/s are currently (2003) standard; we expect that in 2004 the firs products will support 400 MByte/s and 1 GByte/s. When considering the transfer rate i should be noted that in the fabric and point-to-point topologies the transfer is bi-directional and full-duplex, which means that today the transfer rate of 200 MByte/s is available in

each direction. Fibre Channel defines various cable types (Table 3.2) for copper and fibre-optic cable where the higher speeds only support fiber-optic. Various plug types are defined both for copper cable and for fiber-optic cable. Figure 3.10 shows various plug types for fiber-optic cable. Apart from their different dimensions, no technical advantages are associated with the various types. Copper cables are subdivided into 'intracabinet' cables and 'intercabinet' cables. Intra-cabinet cables are designed for cabling within an enclosure, they are less well shielded against electromagnetic interference – and thus cheaper – than inter cabinet cable, which can be used to connect up devices outside the limits of enclosures. Fiber-optic cables are more expensive than copper cables. They do, however, have some advantages:

• greater distances possible than with copper cable;

• insensitivity to electromagnetic interference;

• no electromagnetic radiation;

• no electrical connection between the devices;

• no danger of 'cross-talking'.

Different cable and plug types are also defined for fiber-optic cable. Cables for long distances are more expensive than those for short distances. The dentitions of various cables makes it possible to choose the most economical technology for each distance to be bridged. With 1 GByte Fibre Channel there will be some innovations in fiber-optic cables. First, a new cable type has been introduced – the 50 micron high bandwidth cable – with which greater distances can be spanned than with a conventional 50 micron cable. Second, it will be possible to multiplex the data stream over four connections. This may occur first

by distributing the data over four fiber-optic pairs (4 lines). In another variant, Coarse Wavelength Division Multiplexing (CWDM), these four physical lines are replaced by four signals in different frequency ranges, so that one physical pair of lines is sufficient. In practice, we will have to wait and see which of these different cable variants the industry will actually support with real products for 1 GByte Fibre Channel. For all media, the Fibre Channel standard demands that a single bit error may occur at most once in every 1012 transmitted bits. On average, this means that for a 100 Mbit/s connection under full load a bit error may occur only every 16.6 minutes. The error recognition and handling mechanisms of the higher protocol layers are optimized for the maintenance of this error rate. Therefore, when installing a Fibre Channel network it is recommended that the cable is properly laid so that the bit error rate of 1012 is, where

possible, also achieved for connections from end device to end device, i.e. including all components connected in between such as repeaters and switches.

The distance information in Table 3.2 specifies the minimum distances at which the error rate can reliably be kept within the stipulated figure, given the current state of technology and proper laying of the cable during the timeframe the standard was ratified. Technical improvements and proper laying of the cable make it possible for even greater distances to be bridged in actual installations. Today (2003), distances up to several 10 kilometres are supported for 200 MByte/s. The reduction in the supported cable lengths could represent a problem when upgrading the equipment of an existing Fibre Channel SAN to a higher speed, thus it should be checked in advance, whether a given distance can be bridged at the higher speed as well.