Channel extension is a whole topic by itself. This is not usually a trivial task. The first thing to do is to get a handle on the total amount of changed data (writes) on a daily basis for the primary side. Also monitor your primary side for peak write times. Your link must be able to handle peak writes for the solution to be viable. Determine whether you will use Sync or Async copy over the extenders. Any connection not using DWDM or direct Escon or Fibre should be Async, especially if the distance is greater than 10-20KM. Copy latency due to the speed of light over Fibre will impact an application using Sync copy. Some vendors can do Async, others cannot (semi-sync comes to mind). Some vendors use sequencing and time stamping of the writes during Async, others do not. This is mandatory for transaction consistency across the link. (Compaq uses write logging and reverse replay, which is just as good). Make sure you check with your storage vendor for information on this.
CNT is currently installed in most of the shops I have seen doing channel extension. They have a lot of experience doing this, and should be fine for any solution you choose. INRANGE is coming up fast though. They have a very fast and elegant box that is compatible with most remote copy solutions. Both are not cheap, but you get what you pay for. CNT just bought Articulent, and that gave them a stable of very good consultants to help you install and configure your solution.
Both INRANGE and CNT excel at one thing or the other. CNT is great for ESCON to ATM connections. INRANGE is great for Fibre-to-Fibre connections. Most enterprise storage vendors usually recommend a solution compatible with their subsystems, and can configure it for you, so my advise would be to go with the recommendation of your particular storage vendor. They will select what fits best for the connections you are trying to make.
This was first published in June 2001