IBM z13s Technical Manual page 191

Forward Error Correction
Forward Error Correction (FEC) is a technique used for reducing data errors when
transmitting over unreliable or noisy communication channels (improving signal to noise
ratio). By adding redundancy error-correction code (ECC) to the transmitted information, the
receiver can detect and correct a number of errors, without requiring retransmission. This
process improves the signal reliability and bandwidth utilization by reducing retransmissions
due to bit errors, especially for connections across long distance, like an inter-switch link (ISL)
in a GDPS Metro Mirror environment.
The FICON Express16S is designed to support FEC coding on top of its 64b/66b data
encoding for 16Gbps connections. This design can correct up to 11 bit errors per 2112 bits
transmitted. While connected to devices that support FEC at 16Gbps connections, the FEC
design allows FICON Express16S channels to operate at higher speeds, over longer
distances, with reduced power and higher throughput, while retaining the same reliability and
robustness that FICON channels have traditionally been known for.
With the IBM DS8870, z13s servers can extend the use of FEC to the fabric N_Ports for a
complete end-to-end coverage of 16Gbps FC links. For more information, see IBM DS8884
and z13s: A new cost optimized solution, REDP-5327.
FICON dynamic routing
With the IBM z13 and IBM z13s servers, FICON channels are no longer restricted to the use
of static SAN routing policies for ISLs for cascaded FICON directors. The z Systems now
supports dynamic routing in the SAN with the FICON Dynamic Routing (FIDR) feature. It is
designed to support the dynamic routing policies provided by the FICON director
manufacturers, for example, Brocade's Exchange Based Routing (EBR) and Cisco's Open
Exchange ID Routing (OxID). Check with the switch provider for their support statement.
A static SAN routing policy normally assigns the ISL routes according to the incoming port
and its destination domain (port based routing), or the source and destination ports pairing
(device based routing):
Port based routing (PBR) assigns the ISL routes statically based on first come, first
served, when a port starts a fabric login (FLOGI) to a destination domain. The ISL is round
robin selected for assignment. I/O flow from same incoming port to same destination
domain will always be assigned the same ISL route, regardless of the destination port of
each I/O. This system can result in some ISLs being overloaded while others are
underutilized. The ISL routing table is changed every time a z Systems server undergoes
a power-on-reset (POR), so the ISL assignment is somehow unpredictable.
Device based routing (DBR) assigns the ISL routes statically based on a hash of the
source and destination port. That I/O flow from same incoming port to same destination is
assigned to the same ISL route. Compared to PBR, the DBR is more capable of spreading
load across ISLs for I/O flow from same incoming port to different destination ports within
a same destination domain.
When using a static SAN routing policy, the FICON director is limited to assigning ISL routes
based on workload, and ISL can be overloaded or underused.
With dynamic routing, ISL routes are dynamically changed based on the Fibre Channel
exchange ID, which is unique for each I/O operation. ISL is assigned at I/O request time, so
different I/Os from same incoming port to same destination port are assigned different ISLs.
With FIDR, z13s servers have advantages for performance and management in
configurations with ISL and cascaded FICON directors:
Support sharing of ISLs between FICON and FCP (PPRC or distributed)
I/O traffics is better balanced between all available ISLs
Chapter 4. Central processor complex I/O system structure
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