You can utilize QOS on the Compellent to alleviate network contention.  When you setup the QOS, you pick your link speed and then if you enable Bandwidth Limiting you can choose down to the hour what percentage of that link replication can utilize.  It will utilize as much as you give it so you should have an idea how much data "regular" traffic consumes on the link and throttle below that if you want to ensure you do not impact production traffic. BUT you need to know that you can actually complete the replications in a reasonable amount of time. If not, then you'd have to increase bandwidth or look at utilizing a bandwidth optimizer that'll compress/dedupe the data before it goes over the wire. Choosing between Async/Sync is both a bandwidth issue but more importantly Sync requires low latency since the ack must be done first on the remote SAN then the local SAN before it goes back to the server.
I have Compellent LUNs in one city and want to replicate these to a Compellent Storage Array in another city. We only have a 1Gb link and 500Gb of data to replicate. I assume an Asynchronous replication would be needed, but need to know how this will affect production traffic on that link when the replication is fired off. Should these be done in off peak hours?
Replication across 1Gb link to remote data center