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Arbitrated Loop Disclosure Number: IPCOM000111917D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 4 page(s) / 144K

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Related People

Sanderson, W: AUTHOR [+2]


Described is an alternative to the Fibre Channel switched network by allowing Fibre Channel ports to be connected in a lower-cost loop.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 39% of the total text.

Arbitrated Loop

      Described is an alternative to the Fibre Channel switched
network by allowing Fibre Channel ports to be connected in a
lower-cost loop.

      The ANSI Fibre Channel Standards (FCS) committee has been
working on a lower-cost version of a Fabric (which is usually
conceived to be made up of switches or hubs).  To provide
connectivity without additional hardware, the committee looked at
attaching devices/systems in a loop (requires half the number of
active transmitter/receiver pairs) using the buffer-insertion
technique of transferring data.

      However, the FCS allows for 2.1KByte frames which requires
considerable on-chip high-speed buffering, making this interface more
expensive than some of the other emerging interfaces.

      To provide low-cost connectivity, the following Arbitrated Loop
was developed.  Fig. 1 describes the loop model that is used
throughout this article.

      This article describes a low-cost alternative for serially
connected devices by using an Arbitrated Loop (i.e., before sending
any frames, the port must arbitrate (obtain ownership) for the loop).
It assumes some knowledge of the ANSI Fibre Channel Standard (FCS)
and references FC-PH revision 2.2, January 24, 1992.

      In the Arbitrated Loop environment, an FC-defined N_Port or
F_Port requires a small amount of arbitration logic such that a port
may operate as both a loop-port or as an end-port.  An N_Port or
F_Port that has the Arbitrated Loop capability is called an L_Port
for the remainder of this article.  Fig. 2 is a high-level model of
the L_Port.

      The L_Port FC-2 level functions operate as currently defined by
FC-PH version 2.2 (i.e., it performs Login, sends and receives
frames, and manages credit counts).  When on a loop, the Loop Port
Logic (LPL) will do the following:

1.  Loop Initialization Primitive

2.  ARBitrate for the loop resources

3.  OPeN the other L_Port

4.  CLoSe the loop

The LPL does this by transmitting LIP, ARB, OPN, and CLS primitives,
and checking for these primitives such that the loop is transparent
to the port.

Currently there are four special Loop primitives defined:

1.  LIP (Loop Initialization Primitive) - resets the loop - used
    during initialization of an L_Port on the loop.

2.  ARB (Arbitration) - may be repeated contiguously many times

3.  OPN (Open) - one before the first frame is sent

4.  CLS (Close) - one after the last frame is sent

      During initialization (power-on) the L_Port recognizes its
attachment environment.  If it determines that it is on a
point-to-point link, the LPL logic becomes transparent to the
operation of the port.  However, if it determines that it is in a
loop topology, the LPL logic is activated.  Then in order for any
L_Port to transmit a frame, it must first obtain ownership of the

Ownership of the loop is obtained through arbitration as follows:

1.  The arbitration primitive sequence (ARBx - where x i...