N-linked glycosylation begins in the endoplasmic reticulum. A core oligosaccharide is synthesized attached to a molecule of the lipid dolichol. The oligosaccharide is transferred from dolichol to an asparagine residue of the polypeptide chain. The oligosaccharide is first trimmed and then modified by the addition of monosaccharide units as the protein molecule is processed through the endoplasmic reticulum and the Golgi apparatus. Ref : Biochemistry by U. Satyanarayana 3rd edition Pgno : 562