Understanding the network.pdf - Back to Home

• Connection management—Connection establishment, link testing, and error
monitoring between adjacent PHYs. MAC-to-PHY configuration, bypass
control, and medium availability acknowledgment are part of this function.
• Ring management—Beacon detection and generation, transmission problem
resolution, address management, duplicate address detection, and MAC
availability monitoring.
• Control frame generation—Generation and processing of SMT control frames,
which come in the following flavors:
o Neighborhood information frames—These frames determine
duplicate addresses and the development of a local ring address map
and ring path.
o Status information frames—Two types of status information frames,
configuration and operation, are used to glean additional status
information about the stations connected on the ring.
o Parameter management frames—These frames are used in
conjunction with the station's management information base
attributes to facilitate management information collection and station
configuration.
o Status report frames—These frames are used to send station and ring
status announcements to other stations on the ring.
The Physical Layer Protocol
The Physical Layer Protocol (PHY) is independent of the PMD. This enables it to
interface with a variety of PMD interfaces. The PHY is responsible for encoding and
decoding data, using the 4B/5B encoding scheme. The MAC-to-PHY interface is a
4-bit data path operating at 25MHz. The PHY converts the MAC datastream into
4-bit symbols and then, using a 4B/5B encoder, converts them into a 5-bit symbol
code. The 5-bit code is then serialized and transmitted through a NRZI converter at
a line rate of 125MHz. The NRZI encoded stream is then handed to the PMD for
transmission across the medium, also transmitted at a clock rate of 125MHz, which
yields a 100Mbps transmission rate. FDDI gets its speed from the encodfting
method. The PHY is also responsible for managing timing, and with FDDI, this is a
distributed process. Each station has its own clock and gets its clocking signal from
the incoming datastream. The data is buffered and transmitted back out to the ring.
The internal station buffer expands or contracts the inter-packet gap to make up for
timing errors. This avoids line jitter and affords a much more efficient data flow,
because the inter-packet gap is optimized to a minimum.
The Physical Layer Medium Standard
The Physical Layer Medium (PMD) interface describes the electromechanical
interfaces, media and transmission specifications, and station attachment