Specifications

Escala Tower PL & S, E, T System Hardware
Chapter 3: External Storage and SAN Subsystems 111/116
9es3s1c3.doc
Rev 5.9
02/12/2003
The Bitstrip TW can be cascaded to other FC-AL hubs for the expansion of the loop. Port expansion is
supported to the FC-AL limit of 126 nodes. Any port may be used for cascading.
The Bitstrip TW implements the most complete suite of intelligent data detection tests to ensure the
highest availability of the Fibre Channel Arbitrated Loop. This test suite includes:
•
Valid 8B/10B Data Detection
•
Oscillation Detection
•
Open Cable Detection
These three tests allow the Bitstrip TW to differentiate between valid Fibre Channel data and
erroneous data, preventing corruption on the loop. Multiple detection circuits ensure that nodes will be
bypassed only on real line failures, allowing transient events such as bit failures to be handled by the
Fibre Channel frame level protocol. In the event that a node is bypassed, front panel LEDs provide
convenient indications.
7.9.2. SAN Topologies & Interoperability
Despite the advanced features of the hub to retime and regenerate the signal allowing hub cascading,
hub interconnections must be avoided because of the risk of performance bottleneck. The hub shares
the 100 MB/s bandwidth between the connected devices. Thus with 8 servers connected to a single
hub, the average bandwidth per server is only 12,5 MB/s (100/8), i.e. much less than SCSI.
Interconnecting hubs does not increase the available bandwidth, but at the opposite share the 100 MB/s
between more devices. The single case where hubs interconnection is tolerated (but should not exceed
3) is when there is no more than 9 devices (host bus adapters or storage subsystems) and the
interconnection simplify the cabling. Typical examples are the disaster recovery configurations. Please
refer to the SAN configuration guide for more information.
Another consequence of the bandwidth sharing concerns servers with multiple adapters: attempting to
connect multiple adapters to a single hub is a design error. It will not increase the bandwidth available
for the server, and furthermore, a failure of the hub (like a power outage) will impact all the connected
adapters.
Notes:
•
The interconnection of hubs and switches is not supported.
•
The mix of DAS, Symmetrix and libraries (with either native Fibre Channel tape connection or
through bridges) is not supported.
•
The mix of Escala servers and Express 5800 servers is authorized, through fabric connection is
preferred for heterogeneous environments.
7.9.3. Packaging
The hub is tabletop equipment.
It can also be mounted into a rack. It is designed as a precise half rack chassis so that two hubs can be
installed in a single, 1U-height rack space. There is a double rack mount kit designed for mounting two
hubs, side by side into a rack.