The term “prerequites” implies “minimum” level requirement. That minimum is a smart developer who has access to the RFC library and some minimal platform to publish a web service. If, for example, in your company some external process needs to call SAP CRM to do a customer name/customer account number croass-reference lookup, minimal might be adequate or even exactly right.
For a full SOA environment to support multiple, complex web services calls and “orchestration”, the prequisites are higher and you have dozens of mix and match options, with Netweaver and XI being prime examples. In detween, what you have today (e.g., Business Connector) can be used as well.
For companies just starting out, sometimes minimal is actually best, because probably 80% of the business value/ROI of web services comes from 20% of the feasible web services calls. Also, in all cases, a “prerequisite” to success is a smart developer.