Fibre channel switched arbitrated loop

Details Fibre channel switched arbitrated loop Fibre channel switched arbitrated loop

1998-04-16

2001-11-27

Olms, Douglas (Department: 2661)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S254000, C370S455000, C370S462000

Reexamination Certificate

active

06324181

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to fibre channel arbitrated loop network architecture and more particularly to an adaptation of fibre channel architecture to switchable star architectures. The purpose of the invention is to increase the throughput of fibre channel arbitrated loop networks.
Fibre channel networks are a particular class of high-speed networks defined by ANSI standard X3T11. Fibre channel networks can be constructed in three different topologies: point-to-point, arbitrated loop and fabric switch. Point-to-point networks consist of two stations connected directly to (and only to) each other. Fabric switches can connect networks of up to 16 million stations and provide multiple classes of service (connection-oriented, connection-less with acknowledgement, connection-less without acknowledgement, or intermix).
Referring to
FIG. 1
, an arbitrated loop topology
10
is shown, which is a shared-bandwidth, logical ring topology designed for low-cost attachment of such stations
12
as servers and disk array devices. Up to 126 stations can communicate on an arbitrated loop
14
, but only two stations can communicate interactively at any one time (with an exception for broadcast or multicast conversations). The arbitrated loop standard also allows for a single fabric port
16
to be resident on the loop for connection into the larger network.
In an arbitrated loop topology, stations
12
,
16
,
18
,
20
,
22
that intend to communicate on the loop
14
must “arbitrate” for access. Stations signal their intent to arbitrate by issuing a primitive signal for circulation around the loop to all nodes or stations, called an ARB. The ARB is a special ordered set signal which contains the identity of the station that requests access. When multiple stations request access simultaneously, the station with the lowest physical address prevails. (Physical addresses are assigned cooperatively by the stations each and every time the loop topology changes.) A station
12
that wins arbitration then “opens” the station
18
with which it wants to communicate by sending an OPN primitive, which is a special ordered set signal including the address of the target station as an argument. The two stations
12
,
18
then communicate in either half-duplex or full-duplex fashion, until both stations agree that the conversation is finished.
Arbitrated loops are typically wired together using hub devices
15
in which the loop is formed among the connected stations. A hub device
15
allows multiple stations to be connected together, although only one conversation can take place at any instant in time. This is analogous to networking topologies such as Token Ring (ISO/IEEE 802.5) and Ethernet (ISO/IEEE 802.3).
Referring to
FIG. 2
, in networking topologies, such as loop networks
014
,
114
,
214
,
314
, the conventional prior art approach to increasing the available bandwidth is to provide passthrough switches
24
,
26
,
28
which allow multiple concurrent conversations to take place. The disadvantage is a need for extra switching logic and buffer memory to support connections. In fibre channel technology it is conventional to use the fabric switch topology when greater bandwidth is needed. However, fabric switches tend to be complex and expensive, particularly given that they must also implement a large number of standardized features, such as address space resolution, and support for multiple classes of service.
Any modification to the arbitrated loop must behave such that devices connected to it are unaware that they are not on a normal loop when they are communicating. This is important for interoperability with any existing fibre channel arbitrated loop hardware on the market. What is needed is a mechanism whereby any switching element presents an interface equivalent to a loop.
SUMMARY OF THE INVENTION
According to the invention, in a fibre channel network environment wherein an arbitrated loop is provided a switchable connection with another arbitrated loop, a technique and corresponding apparatus are provided for controlling signal paths through the hubs stack between any two devices such that routing of a signal and interactive communication can be carried out transparently to the fibre channel network without excessive overhead, and wherein control is changed in an orderly manner such that end-to-end device connections are unaware that they are not connected to the same hub. A switched arbitrated loop (SAL) according to the invention provides the concurrent bandwidth resource of a fabric switch without the extra features which would increase design cost and operational overhead. In a specific embodiment, a switched arbitrated loop supports only 126 nodes for stations (plus one fabric port) in a single loop topology. The invention provides the concurrent connections and high bandwidth capabilities of a conventional fabric switch, but at a cost close to that of a conventional hub while behaving such that devices connected to it are unaware that they are not on a normal loop when they are communicating.
The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.


REFERENCES:
patent: 5383191 (1995-01-01), Hobgood et al.
patent: 5751715 (1998-05-01), Chan et al.
patent: 5784557 (1998-07-01), Oprescu
patent: 6000020 (1999-12-01), Chin et al.
patent: 6118776 (2000-09-01), Berman

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