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Tip:  SCSI write over FC requires two round trips per I/O operation, as shown in the following example: 

2 (round trips) x 2 (operations) x 5 microsec/km = 20 microsec/km 

At 50 km, you have another latency, as shown in the following example: 

20 microsec/km x 50 km = 1000 microsec = 1 msec (msec represents millisecond) 

Each SCSI I/O has 1 ms of more service time. At 100 km, it becomes 2 ms for more service time. 

Link bandwidth is the network capacity to move data as measured in millions of bits per second (Mbps) or billions of bits per second (Gbps). 

The term bandwidth  is also used in the following context: 

GLYPH<SM590000> Storage bandwidth: The ability of the back-end storage to process I/O. Measures the amount of data (in bytes) that can be sent in a specified amount of time. 

GLYPH<SM590000> Remote copy partnership bandwidth (parameter): The rate at which background write synchronization is attempted (unit of MBps). 

Intersystem connectivity supports mirrored foreground and background I/O. A portion of the link is also used to carry traffic that is associated with the exchange of low-level messaging between the nodes of the local and remote systems. A dedicated amount  of the link bandwidth is required for the exchange of heartbeat messages and the initial configuration of intersystem partnerships. 

Interlink bandwidth must support the following traffic: 

GLYPH<SM590000> Mirrored foreground writes, as generated by foreground processes at peak times 

GLYPH<SM590000> Background write synchronization, as defined by the Global Mirror bandwidth parameter 

GLYPH<SM590000> Intersystem communication ( heartbeat messaging ) 

Fibre Channel  connectivity is the standard connectivity that is used for the remote copy intersystem networks. It uses the Fibre Channel protocol and SAN infrastructures to interconnect the systems. 

Native IP connectivity has been introduced with IBM Spectrum Virtualize version 7.2 to implement intersystem networks by using standard TPC/IP infrastructures. 

## Network latency considerations 

The maximum supported round-trip latency between sites depends on the type of partnership between systems. Table 6-6 on page 180 lists the maximum round-trip latency. This restriction applies to all variants of remote mirroring. 

<img src='content_image/976260.jpg'>

More configuration requirements and guidelines apply to systems that perform remote mirroring over extended distances, where the round-trip time is greater than 80 ms. If you use remote mirroring between systems with 80 - 250 ms round-trip latency, you must meet the following additional requirements: 

