Types of Glycosylation: There are several types of glycosylation  N- glycosylation O- glycosylation and C-linked glycosylation, glypiation and phosphoglycosylation as per the type of the sugar-peptide bond and the oligosaccharide attached.

N-glycosylation: In this type of glycosylation, glycans are covalently bound to the carboxamido nitrogen on asparagine (Asn or N) residues. This is the most common type of glycosylation- 90 percent of glycoproteins are N-glycosylated. It occurs as soon as protein is synthesised, rather almost simultaneously with the translation process.
N-glycosylation process and enzymes involved in the process are conserved across very wide range of species of eukaryotes and archae. N-glycosylation can be broken down into several events which include-

1. Precursor glycan assembly
2. Attachment
3. Trimming
4. Maturation

Initial steps of glycosylation are identical for all proteins but there is difference in trimming and maturation steps which generate diversity in glycosylated proteins.

Precursor glycan assembly: The purpose is to assemble 14 sugar molecules which consist of 3 Glucose (Glc), 2 N-acetylglucosamine (GlcNAc), and 9 Mannose sugar molecules on the ER membrane via dolichol. Dolichol is a polyisoprenoid lipid carrier rooted in the ER membrane via a pyrophosphate linkage (-PP-).  Firstly, first 7 sugar molecules obtained from sugar nucleotides (UDP- and GDP-sugars) in the cytoplasm, are added. After this assembly, complex flips to ER lumen side and seven more sugars are added to form Gcl₃Man9GlcNAc₂-PP-dolichol precursor glycan.