September 2008
Intermediate to advanced
550 pages
18h 10m
English
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11.4 VoIP VOICE PACKETS ON A DSL INTERFACE
In DSL interface-based VoIP, IP packets are sent as ATM cells as one of the popular options. The ATM cell size is 53 bytes with 48 bytes of payload and 5 bytes of ATM header error control (HEC) as given in reference [URL (Protocols-ATM)]. In the process of creating ATM cells, ATM adaptation layer 5 (AAL5) protocol data unit (PDU), segmentation and reassembly (SAR), and 8 bytes of AAL5 trailer are used. AAL5 will use headers depending on the supported protocols such as PPPoE, PPPoA, IPoA, and 8-byte trailer. The packet is then zero padded such that the packet size consists of integer multiples of 48 bytes. This process is required to split the packet into an integer number of ATM cells.
In this chapter, calculations for PPPoE are given, as PPPoE is popular in actual deployments. It is most popularly used with the DSL interface. Voice packets sent on PPPoE on a DSL interface will have more overhead bytes compared with PPPoA and IPoA. In general, many other supporting standards and RFCs exist for work on DSL and supported protocols. In this chapter, only header sizes are considered without going through the details of bit fields. Refer to the RFCs—RFC2516 [Mamakos et al. (1999)], RFC2364 [Gross et al. (1998)], RFC2225 [Laubach and Halpern (1998)] for additional details on the protocols and headers.
11.4.1 VoIP on a DSL Interface with PPPoE
In G.711 of a 10-ms packet interval, the VoIP packet (compressed voice payload + RTP/UDP/IP) size is ...
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