Installing and Administering HP FDDI/9000 Software

26 Chapter 1
FDDI Resources
FDDI Concepts
HP S700 workstations connect to the dual ring network through a device
called a concentrator which is directly attached to both rings via A ports
and B ports. The workstations connect to the concentrator via M ports.
Stations connected to a ring via a concentrator are referred to as
single-attached stations (SAS). The FDDI concentrator provides
another type of fault tolerance. When a station is removed from the
network via a concentrator, the station is bypassed within the
concentrator and the topology of the FDDI network is not affected.
Ring access is controlled by a Token, a frame comprised of a unique
sequence of symbols, which circulates according to the value of a Target
Token Rotation Time (TTRT). The TTRT is negotiated by all stations
during ring initialization. During the initialization process, each station
on the ring submits a Requested Token Rotation Time (T_Req). All
active stations then negotiate a TTRT value (T_Neg) that becomes the
final TTRT value upon successful ring initialization.
When the ring is operational, the token rotates continuously and
sequentially around the ring. The token is the means by which the right
to transmit is passed from one station to another. If a station has data to
transmit, it must first capture the token from the ring. The station may
then transmit data until all its data has been sent or the Token
Holding Timer (THT) expires. The THT limits the length of time a
station may have the token before issuing a new one. After a
transmission is complete, the station creates and transmits a new token
onto the ring. The token continues to circulate, providing other stations
the chance to gain access to the ring, until it is captured by another (or
same) station waiting to transmit data.
The FDDI Dual Ring During Normal Operation
The portion of the network map shown in Figure 1-3 illustrates the data
path through the primary ring and dual-attached stations during normal
operation. In this example, the value of the Ring Management State
(RMT), (the function of FDDI Station Management (SMT) which
manages MAC layer components) is RingOp for every MAC station on
the ring, indicating that every station is part of an operational FDDI
ring. (Refer to the fddistat manpage for more information.) Data enters
each dual-attached station or concentrator on the primary ring via Port
A and leaves via Port B. Data is transmitted to the single-attached
stations via the concentrator M ports. The secondary ring remains
inactive during normal operation.