Tham khảo tài liệu 'advances in satellite communications part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 64 Advances in Satellite Communications VJHC scheme stated that TCP IP header fields can be grouped into several categories which are constant inferred and dynamic Jacobson 1990 . Constant fields are those field values that remained unchanged between consecutive packets hence can be eliminated. Inferred fields are those fields that can be recalculated at the receiving end. For example total length and header checksum field. The transmission efficiency can be improved significantly by suppressing inferred fields at the compressor and restoring them at the decompressor. The third group is dynamic fields which do no change frequently at the same time or change slightly thus it can be omitted in most cases. VJHC is proven to be effective towards header compression as it can reduce TCP IPv4 header from 40B to 4B which is 10 of its original size Tye Fairhurst 2003 . However the main disadvantage of VJHC scheme is it may lead to error propagation throughout the transmission when a compressed packet is lost on the link. This is due to the inconsistent context which will cause a series of packets to be discarded at the receiver end. Thus VJHC scheme is not applicable under satellite link with high bit error rate as this will lead to higher packet drop which will cause the satellite link performance to become even worse. RObust Header Compression ROHC Besides VJHC Robust Header Compression ROHC scheme is another well known header compression scheme. It is developed by ROHC working group of the IEFT Tye Fairhurst 2003 . ROHC is used for compressing IP packet headers and it is particularly suitable for wireless network. ROHC scheme allows bandwidth savings up to 60 in VOIP and multimedia communication applications JCP-Consult 2008 . In this scheme compression and decompression are treated as a series of states. Fig. 8. Compressor state diagram Effnet 2004 . As shown in Figure 8 ROHC compressor operates in 3 states which are Initialization and Refresh IR First Order FO
