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Chapter 7Performance optimisation for the support of TCIS pFor the upstream volumetric data (Figure 7.18a), the DVB stepped curve is caused bythe encapsulation of PDU messages into ATM cells (of 53 bytes), which are thentransmitted throughout the upstream channel as 64-byte frame slots. Therefore, a usermessage of 1450 bytes (without protocol overhead) causes the DVB protocol to requestthe following data: (1450-payload + 20-TCP + 20-IP + 3-LLC + 18-MAC + 8AAL5) /48 AAL5 PDUS ≈ 32-slots = 2048 bytes. This results in 71 % upstream efficiency asindicated in Figure 7.18b. Here, every upstream slot transmitted includes 6 bytes of FECinformation to correct up to 3 bytes (T=3) over each ATM cell. Figure 7.19 presents theupstream PDU structures of this protocol.Volumetric Data (bytes)2250200017501500125010007505002500a) Upstream Volumetric Data204815901520DOCSIS, no FECDVBDOCSIS, with FEC0 250 500 750 1000 1250 1500User Message Length (byes)Efficiency (%)1009080706050403020100b) Upstream EfficiencyDOCSIS->95%, no FECDVB->71%DOCSIS->91%, with FEC0 250 500 750 1000 1250 1500User Message Length (bytes)Figure 7.18 – Upstream volumetric data and efficiency.1450-byte user dataApplication byte streamTCP-Hdr.20-byte1450-byte user dataTCP Segment (TCP-PDU)IP-Hdr.20-byte1470-byte TCP PDUIP Datagram (IP-PDU)LLC-Hdr.3-byte1490-byte IP PDULLC Segment (LLC-PDU)MAC-Hdr.14-byte1493-byte IP PDUMAC-FCS.4-byteMAC Frame (MAC-PDU)DVB-Hdr.9-byteATM payload48-byte64-byte Upstrem Slot (1)DVB-Tr.7-byte1511-byte MAC PDUDVB-Hdr.9-byteCPCS-Tr.8-bytesATM payload31-bytePAD17-byte64-byte Upstrem Slot (32)ALL5-CPCS (CPCS-PDU)DVB-Tr.7-byteUpstream SlotDVB/DAVICMAC&PHYFigure 7.19 – DVB/DAVIC upstream PDU structures.7-29