What are the duties of a presales Engg in networking feild?

Pre-sales engineers are subject matter experts with specialized technology experience in network solutions, storage, servers and security. Assigned to work within a defined sales territory, these engineers respond to customer requests for proposals, develop designs and bills of materials for cost-effective solutions, and provide shoulder-to-shoulder knowledge transfer or training to implementation teams bringing the technology solutions online. The pre-sales engineer must be well-versed in the selling company's portfolio of products and services and keep up with new technologies entering the market.

My response is more related to skills what a presales engineer must polish in order to be successful.

Presales Engineer irrespective of which domain in technology requires expertise of his job first of all. These expertise are expressed in the artifacts he or she creates in order to support sales team. Most of the time, this support extends to customers, in explaining them the solution's technical details or training them to be aware of what they are buying. 

Therefore, two most important skills a presales engineer must polish is the ability of writing technical documents in layman's term and be able to explain them as a versatile trainer.